PARAMETRIC INVESTIGATION ON SINGLE POINT INCREMENTAL ...... · x Thesis Approval Form The viva-voce of the PhD Thesis submitted by Shri Snehal Viranchibhai Trivedi (Enrollment No

PARAMETRIC INVESTIGATION ON SINGLE POINT INCREMENTAL

FORMING FOR DIFFICULT TO FORM MATERIAL

A Thesis submitted to Gujarat Technological University

for the Award of

Doctor of Philosophy

in

Mechanical Engineering

by Snehal Viranchibhai Trivedi

Enrollment No: 139997119015

under supervision of

Dr. Anishkumar Hasmukhlal Gandhi

GUJARAT TECHNOLOGICAL UNIVERSITY

AHMEDABAD

September – 2019

PARAMETRIC INVESTIGATION ON SINGLE POINT INCREMENTAL

FORMING FOR DIFFICULT TO FORM MATERIAL

A Thesis submitted to Gujarat Technological University

for the Award of

Doctor of Philosophy

in

Mechanical Engineering

by Snehal Viranchibhai Trivedi

Enrollment No: 139997119015

under supervision of

Dr. Anishkumar Hasmukhlal Gandhi

GUJARAT TECHNOLOGICAL UNIVERSITY

AHMEDABAD

September – 2019

ii

© Snehal Viranchibhai Trivedi

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x

Thesis Approval Form The viva-voce of the PhD Thesis submitted by Shri Snehal Viranchibhai Trivedi

(Enrollment No. 139997119015) entitled “Parametric investigation on single point

incremental forming for difficult to form material” was conducted on

…………………….………… (day and date) at Gujarat Technological University.

(Please tick any one of the following option)

The performance of the candidate was satisfactory. We recommend that he/she be

awarded the PhD degree.

Any further modifications in research work recommended by the panel after 3 months

from the date of first viva-voce upon request of the Supervisor or request of

Independent Research Scholar after which viva-voce can be re-conducted by the same

panel again.

(briefly specify the modifications suggested by the panel)

The performance of the candidate was unsatisfactory. We recommend that he/she

should not be awarded the PhD degree.

(The panel must give justifications for rejecting the research work)

--------------------------------------------------------- 2) (External Examiner 2) Name and Signature


------------------------------------------------------- Name and Signature of Supervisor with Seal


xi

ABSTRACT

Global demand of higher strength-to-weight ratio of structures leads progress in

development of variety of lightweight metals and its alloys. Metal forming processes are

preferable over range of manufacturing processes to get the lightweight products due to its

significant characteristics of obtaining homogeneous distribution of material for finished

product. Generally, high strength metals offer non uniform material distribution due to lower

nominal strain at fracture which limits the formability of material.

Hence for the proposed work Single Point Incremental Forming (SPIF) is identified

potential dieless forming process due to its characteristics to offer effective local deformation

resulting in greater formability. SPIF is flexible enough to produce customized formed

products of sheet metal. Present work focuses on investigation of formability of AMS4902

sheet using SPIF, which is having typical applications in industrial and aerospace

components, bellows, honeycomb, gaskets, aircraft skin, heat exchanger parts, medical and

dental devices, tubing, pickling baskets etc.

Methodology of the proposed work includes experimental investigation for SPIF of

square pyramid geometry ranging from 50o to 70o wall angle from AMS4902 sheet. Present

experimental investigation is an attempt to analyze the individual effect of various parameters

such as tool diameter, tool speed, tool feed rate, incremental depth of tool and their

interactions on thickness distribution, maximum formable angle, fracture depth and surface

roughness of part formed by SPIF. Based on results obtained from the experimental

investigation, tool diameter is found most significant parameter influencing percentage

thinning of pyramid wall. Tool diameter of 12 mm is offering reasonably uniform thickness

distribution compared to other diameters of tools used for forming of 50o wall angle pyramid

of AMS4902. Failure of pyramid wall is observed before designed depth due to pinning

effect of 8 mm diameter hemispherical tip tool. Incremental step depth is influencing most to

surface roughness of pyramid wall of AMS4902 formed by SPIF.

As the failure of 60o and 70o wall angle pyramid is observed during single-pass SPIF,

experimental investigation is extended to multiple pass SPIF to form60o and 70o wall angle

square pyramids out of already formed pyramids of 50o wall angle. Thinning of 87 percent of

pyramid wall is obtained in case of 60o wall angle pyramid and 13 mm failure depth is

observed in case of 70o wall angle pyramid during multi-pass SPIF.

xii

ACKNOWLEDGEMENT

With the radiant sentiment to express deep sense of gratitude from the bottom of my heart to

my respected supervisor, Dr. Anishkumar Hasmukhlal Gandhi, for his continuous

guidance, motivation, encouragement and support throughout this research work. His

guidance helped me a lot all the time during tenure of research and writing of this thesis. It

would not have been possible for me to constantly strive for better performance without his

extraordinary advises and vision.

Besides my advisor, I have been highly obliged by my Doctoral Progress Committee

Members Dr. H. K. Raval, Professor, SVNIT, Surat and Dr. K. P. Desai, Professor, SVNIT,

Surat for their rigorous reviews and precious comments during the journey of research. Their

ever valuable suggestions and constructive criticisms directed me towards progress of this

research work successfully.

I am thankful to Dr. Akshai Aggarwal, Ex. Vice Chancellor, Dr. Navin Sheth, Vice

Chancellor, Dr. K. N. Kher, Registrar and all staff members of Ph.D. Section, Gujarat

Technological University, Ahmedabad.

I acknowledge technical support provided by staff members of Central Institute of Plastics

Engineering and Technology (Centre for Skilling and Technical Support), Valsad to conduct

all experimental work as well as Indo German Tool Room, Ahmedabad to allow me to carry

out measurement using Coordinate Measuring Machine (CMM).

I must not forget to pay my sincere thanks to Dr. Devanshu Patel, President, Parul

University for believing in my potential. I extend my thanks to all teaching and non teaching

staff members of Department of Mechanical Engineering as well as Mr. Riteshbhai Patel,

Librarian of Parul Institute of Technology, Mr. Harsh Desai and who helped me directly or

indirectly for accomplishment of this research work.

It was next to impossible to progress for this research work without the moral support of my

family members. I dedicate this research work to my mother, Mrs. Geetaben Trivedi for her

blessings; to my father Dr. Viranchibhai Trivedi, my continuous source of inspiration; my

beloved wife, Mrs. Rinku Trivedi, for her caring nature as well as dedication towards social

responsibilities and my beloved son Dwij Trivedi, for his unconditional love forever. Finally,

I bow down to the Lord Almighty for providing me opportunities and favorable

circumstances in the life.

xiii

Table of Content

Chapter No. Title of Chapter Page

No. 1 Introduction 1

1.1 Metal forming processes 1 1.2 Properties influencing formability of metal 4 1.3 Properties of non-ferrous lightweight metals 6 1.4 Need of lightweight products 8 1.5 Organization of Thesis 9

References 11

2 Literature Review 13

2.1 Sheet and component attributes 14 2.1.1 Findings based on literature review on sheet and component attributes 18

2.2 Tool attributes and tool path strategies 18

2.2.1 Findings based on literature review on tool attributes and tool path strategies 25

2.3 Process attributes 25 2.3.1 Findings based on literature review on process attributes 31

2.4 Scope of Research 32 2.5 Objectives 33 2.6 Research Methodology 33

References 35 3 Experimental Investigations 40

3.1 Design of Experiments 40 3.2 Experimental set-up 43 3.3 Uni-axial tensile testing to determine properties of AMS4902 44 3.4 Circle-grid marking 45 3.5 Pilot experiments 47 3.6 Experimental set: 1 (Single-pass SPIF of 50o wall angle pyramids) 48 3.7 Experimental set: 2 (Single-pass SPIF of 50o wall angle pyramids) 50 3.8 Wall thickness measurement 51 3.9 Surface roughness measurement 52

3.10 Experimental Set: 3 (Single-pass SPIF of 60o and 70o wall angle pyramids) 54

3.11 Experimental Set: 4 (Multi-pass SPIF to form 60o and 70o wall angle pyramids from 50o wall angle pyramids) 54

3.12 Uncertainty in measurement 54

3.12.1 Wall thickness measurement 55

3.12.2 Surface Roughness measurement 56

References 57

xviii

Chapter No. Title of Chapter Page

No.

4 Results and Discussions 58

4.1 Effect of tool diameter, speed and feed (Experimental Set: 1) 58 4.2 Influence of tool diameter and incremental depth on average percentage thinning 59

4.2.1 Effect of tool diameter and incremental step depth interaction on average percentage thinning 59

4.2.2 Results of ANOVA for average percentage thinning 64 4.3 Influence of tool diameter and incremental depth on average surface roughness 66

4.3.1 Effect of tool diameter and incremental step depth interaction on average surface roughness (Ra) 66

4.3.2 Results of ANOVA for average surface roughness 68 4.4 Geometrical accuracy 71

4.5 Results and discussions to form 60o and 70o wall angle square pyramids by single-pass SPIF 75

4.6 Results and discussions on percentage thinning of 60o and 70o wall angle square pyramids formed out of 50o wall angle pyramids by multi-pass SPIF 75

4.6.1 Percentage thinning of 60o wall angle pyramid formed by multi-pass SPIF 76 4.6.2 Percentage thinning of 70o wall angle pyramid formed by multi-pass SPIF 77

5 Conclusions and Future Scope 79

5.1 Conclusions 79

5.1.1 Effect of tool diameter, speed, feed and incremental step depth on maximum formable depth of AMS4902 sheet during single-pass SPIF 79

5.1.2 Effect of parametric interactions between tool diameter and incremental step depth on average percentage thinning of pyramid walls formed out of AMS4902 sheet using single-pass SPIF

80

5.1.3 Effect of parametric interactions between tool diameter and incremental step depth on average surface roughness of pyramid walls formed out of AMS4902 sheet using single-pass SPIF

80

5.1.4 Effect of tool diameter on geometrical accuracy of pyramid wall angles formed out of AMS4902 sheet using single-pass SPIF 81

5.1.5 Effect of optimum process parameters to form 60o and 70o wall angle pyramids using single-pass SPIF 81

5.1.6 Effect of optimum process parameters to form 60o and 70o wall angle pyramids out of already formed pyramids of 50o wall angle using multi-pass SPIF

82

5.2 Future Scope 82

List of Publications 83

xv

List of Abbreviations

SHF Sheet Hydroforming SPF Superplastic Forming

HMGF Hot Metal Gas Forming EMF Electromagnetic Forming

IF Incremental Forming ISF Incremental Sheet Forming

SPIF Single Point Incremental Forming TPIF Two Point Incremental Forming CNC Computerized Numerically Control BCC Body Centered Cubic FCC Face Centered Cubic HCP Hexagonal Close Packed CP Ti Commercially Pure Titanium UV Ultraviolet

ASTM American Society for Testing and Materials ASME American Society of Mechanical Engineers AMS Aerospace Material Specifications SAE Society of Automotive Engineers AISF Asymmetric Incremental Sheet Forming DDQ Deep Drawing Quality HSS High Speed Steel AA Aluminum Alloy Al Aluminum

FLD Forming Limit Diagram FLC Forming Limit Curve

VWACF Variable Wall Angle Conical Frustum POM Polyoxymethylene PE Polyethylene PA Polyamide

PVC Polyvinylchloride PC Polycarbonate

EDD Extra Deep Drawing CMM Coordinate Measuring Machine GUM Guide to the Expression of Uncertainty in Measurement VIM International Vocabulary of Basic and General Terms in Metrology

ANOVA Analysis of Variance

xvi

List of Symbols

D Tool Diameter Z Incremental Step Depth S Tool Rotational Speed F Feed Rate σ0 Flow Stress K Strength Coefficient Ԑ Plastic Strain N Strain Hardening Exponent R Anisotropy Ratio ra Average value of anisotropy r0 Anisotropy in rolling direction r45 Anisotropy at 45o to the rolling direction r90 Anisotropy in transverse direction Δr Difference in planer anisotropy to Original thickness of sheet tf Final thickness of sheet

Mm Millimeters MPa Mega Pascal GPa Giga Pascal Ra Arithmetic Mean Roughness

Μm Micrometers

xvii

List of Figures

Figure No.

Title of Figure Chapter No.

Page No.

1.1 Types of Incremental Sheet Forming Processes (a) SPIF (b) TPIF 1 2 1.2 Various domains of applications of SPIF and TPIF 1 2 1.3 Application of incremental sheet forming processes in plastic

industries 1 3

1.4 Comparison of formability for various sheet metal forming processes 1 3 1.5 SPIF terminology with deformed part 1 4 1.6 Difference in Planer Anisotropy 1 6 2.1 Schematic diagram of Single Point Incremental Forming Process

with equipments 2 13

2.2 Various shapes used to demonstrate SPIF 2 15 2.3 Various generatrices of parts formed (a) Circular (b) Elliptical (c)

Parabolic (d) Exponential 2 16

2.4 Geometrical Errors in SPIF 2 18 2.5 Types of tool paths (a) Contour tool path (b) Spiral tool path 2 19 2.6 Various tool Profiles (a) Angular, (b) Flat, (c) Hemispherical,

(d) Parabolic 2 20

2.7 Fractography at the fracture initiation zone (left) and at its opposite section (right) for tools of Φ20 (above) and Φ 10 mm (below)

2 21

2.8 Various toolpath strategies (a) helical (b) alternating (c) inside-out 2 22 2.9 Flow chart of applied research methodology 2 34 3.1 Square pyramid as a part geometry 3 41 3.2 Experimental Set-up (a) SPIF Fixture (b) SPIF Set-up on CNC

Milling machine 3 43

3.3 (a) Tensile test set-up (b) Tested specimens of AMS4902 at 0o, 45o& 90o

3 44

3.4 Various patterns of Grid Marking 3 45 3.5 (a) 2D drawing of circle grid pattern

(b) Circle-grid printing set-up (c) Circle-grid marking on AMS4902 sheet

3 46

3.6 Failure of AMS4902 sheets marked using laser grid marking technique (a) Failure depth of 7 mm for 1 mm thick sheet (b) Failure depth of 8 mm for 1.5 mm thick sheet

3 47

3.7 Single point incremental forming of 1.5mm thick sheet of AMS4902 with good quality of deformed circle grid pattern printed using UV printing

3 48

3.8 Components formed using SPIF with parametric combinations of experimental set: 1

3 49

3.9 Components formed using SPIF with parametric combinations of experimental set: 2

3 50

3.10 Wall Thickness measurement of square pyramid using CMM 3 51

xviii

Figure No.

Title of Figure Chapter No.

Page No.

3.11 Deformed pyramid with locations for measurement of wall thickness using CMM

3 52

3.12 Fixture developed to hold the pyramid during surface roughness measurement.

3 53

3.13 Surface Roughness Measurement for Pyramid Wall (a) Calibration of Surface Roughness Tester (b) Roughness Measurement Set-up

3 53

3.14 Maximum range of permissible uncertainty of indication for CMM 3 56 4.1 Effect of tool diameter on Average Percentage Thinning for same

incremental step depth (a) Effect of tool diameters on average percentage thinning of pyramid wall at 0.25 mm step depth (b) Effect of tool diameters on average percentage thinning of pyramid wall at 0.50 mm step depth (c) Effect of tool diameters on average percentage thinning of pyramid wall at 0.75 mm step depth

4 62

4.2 Effect of incremental depths on Average Percentage Thinning for same diameter of tool (a) Effect of incremental depths on average percentage thinning of pyramid wall for 12 mm diameter tool (b) Effect of incremental depths on average percentage thinning of pyramid wall for 16 mm diameter tool

4 63

4.3 Main effects plots for means, SN ratios and standard deviations of average percentage thinning (a) Main effects plot for means of average percentage thinning (b) Main effects plot for signal to noise ratio of average percentage thinning (c) Main effects plot for standard deviations of average percentage thinning

4 64, 65

4.4 Effect of interaction of incremental depths and tool diameters on Average Surface Roughness

4 68

4.5 Main effects plots for means, SN ratios and standard deviations of average surface roughness (a) Main effects plot for means of average surface roughness (b) Main effects plot for signal to noise ratio of average surface roughness (c) Main effects plot for standard deviations of average surface roughness

4 69, 70

4.6 (a) Wall angle measurements for a pyramid formed using 12 mm diameter of tool and 0.25 mm incremental depth (b) Wall angle measurements for a pyramid formed using 16 mm diameter of tool and 0.25 mm incremental depth

4 72, 73

4.7 Average wall angles of pyramid walls formed using 12 mm and 16 mm diameter tools

4 74

4.8 Single-pass SPIF for 60o and 70o wall angle square pyramids (a) Failure of 60o wall angle square pyramid (b) Failure of 70o wall angle square pyramid

4 75

xix

Figure

No. Title of Figure Chapter No.

Page No.

4.9

Pyramid of 60o wall angle formed using multi-pass SPIF and average percentage thinning (a) 60o wall angle pyramid formed out of 50o wall angle pyramid using multi-pass SPIF (b) Measurement of wall thickness of 60o wall angle pyramid using point micrometer for the wall angle formed by multi-pass SPIF (c) Average percentage thinning Vs Component depth

4 77, 78

4.10

Pyramid of 70o wall angle formed using multi-pass SPIF and average percentage thinning (a) 70o wall angle pyramid formed out of 50o wall angle pyramid using multi-pass SPIF (b) Average percentage thinning Vs Component Depth

4 79

xx

List of Tables

Table No. Title of Table Chapter

No. Page No.

1.1 Comparison of properties of various sheet materials 1 7

2.1 Summary of findings reported for various grades of sheet blanks with component geometries 2 17

2.2 Various tool profiles and related dimensional specifications 2 20

2.3 Combination of tool materials, tool diameters, tool end geometries and tool paths in combination to form various sheet blank materials 2 24,

25 2.4 Design of experiments for some process parameters 2 26 2.5 Level of parameters to conduct experimentations using SPIF 2 29

2.6 Summary of parametric combinations reviewed for tool rotational speeds and feed rates to form specific sheet blank material using SPIF 2 30

2.7 Summary of parametric combinations reviewed for feed rates and incremental depth to form specific sheet blank material using SPIF 2 30

2.8 Summary of parametric combinations reviewed for tool rotational speeds and incremental depth to form specific sheet blank material using SPIF

2 31

2.9 Comparison of material properties of AMS4902 with other materials 2 32

3.1 DoE for Experimental Set: 1 to perform singlepass SPIF to form square pyramid of 50o wall angle 3 41

3.2 DoE for Experimental Set: 2 to perform singlepass SPIF to form square pyramid of 50o wall angle 3 42

3.3 Results of tensile testing of AMS4902 3 44 3.4 Other average property parameters of AMS4902 3 45

3.5 DoE for Experimental Set: 3 to perform singlepass SPIF to form square pyramid of 60o and 70o wall angle 3 54

3.6 Measurement uncertainty of CMM model PRISMO 5 HTG VAST prescribed by manufacturer 3 55

4.1 Results of average thickness measured using CMM 4 60

4.2 Average percentage thinning for corresponding average wall thickness after forming 4 61

4.3 Results of ANOVA for average percentage thinning of pyramid walls formed by SPIF 4 64

4.4 Results of average surface roughness for individual walls of pyramids 4 67

4.5 Results of average surface roughness for various pyramids formed using SPIF 4 68

4.6 Response table of signal to noise ratios for surface roughness 4 69

4.7 Results of wall angles measured for pyramids formed using SPIF during various experiments 4 74

4.8 Results of average wall angles measured for various pyramids formed using SPIF 4 74

Metal forming processes

1

CHAPTER: 1

Introduction

This chapter describes broad classification of metal forming processes and characteristics of

various modern metal forming processes. This chapter also discusses desired properties

influencing formability of metals and properties of non ferrous lightweight metals. This

chapter is concluded with the challenges to form non ferrous lightweight metals and aim to

takeover present research problem with respect to global need of lightweight products.

1.1 Metal forming processes

Metal forming is the process in which permanent change in shape and size can be

obtained with the application of force without causing failure of material. Metal forming

processes possess capability to control and improve the properties of material. Forming

processes can be broadly categorized in two groups; (i) Bulk Metal Forming and (ii) Sheet

Metal Forming. Bulk Metal Forming is a severe deformation process resulting in massive

shape change in which the surface area-to-volume of the work is relatively small and mostly

preferred to be done in hot working conditions. Sheet metal forming involves forming and

cutting operations performed on metal sheets, strips, plates and coils. The surface area-to-

volume ratio of the starting metal is relatively high in case of sheet metal forming.

Customized and functional demands of products forces technological advancement in basic

sheet metal forming operations performed either under tensile, compressive, bending or shear

conditions. Since past few years, considerable growth in shot peen forming, hydroforming,

electromagnetic forming, superplastic forming, laser assisted forming and incremental sheet

forming is noticed.

Shot peen forming is especially suitable for large-surface parts with a large curvature and

without sharp contour changes to endure a longer fatigue life in service without failure. Sheet

hydroforming (SHF) is a technology that uses hydraulic fluid taken to very high pressure as

an essential tool to form into complex parts with special curves even with unusual shapes out

of sheet metals and tubes fitted to a specially designed die. Superplasticity is the ability of

materials to undergo extreme elongation, and it occurs within a narrow range of temperatures

Introduction

2

and deformation rates. Integral structural parts can be produced by combining superplastic

forming (SPF) with diffusion bonding (SPF/DB). However, the SPF method is not

economically competitive because of its long cycle time. Therefore, in recent years, various

means of shortening the cycle time have been investigated with encouraging results. The

electromagnetic forming process (EMF) is a highly dynamic process using pulsed magnetic

fields to form metals with high electrical conductivity such as aluminum. In this process,

deformation of the workpiece is driven by the interaction of a current generated in the

workpiece with a magnetic field generated by a coil adjacent to it [1-3].

(a) (b)

FIGURE 1.1 Types of Incremental Sheet Forming Processes (a) SPIF (b) TPIF [4]

Incremental forming (IF), popularly known as dieless forming process, has great

potential to form sheet metal into complex three dimensional components with the use of

relatively simple and low cost tools. Sheet metal can be deformed progressively and locally

using spherical forming tool controlled by CNC machine during incremental forming process.

Negative dieless incremental forming is known as single point incremental forming (SPIF)

while positive die-less incremental forming is known as two point incremental sheet forming

(TPIF) as shown in Fig. 1.1(a) and (b).

FIGURE 1.2 Various domains of applications of SPIF and TPIF

Metal forming processes

3

The main advantages of incremental forming are high process flexibility, relatively

low hardware costs and enhanced formability compare to various other sheet metal forming

processes. In order to satisfy the global need of mass customization of recent era, potential

application domains of SPIF and TPIF include producing architectural and decorative items,

industrial items like miniatures in aerospace industry, automotive industry and biomedical

products and prototypes as shown in Fig. 1.2. As mentioned in Fig. 1.3, applications of

incremental sheet forming processes also extend to prepare moulds using blow moulding

operation useful for thermoforming process in plastic industries.

FIGURE 1.3 Application of incremental sheet forming processes in plastic industries

As depicted in Fig. 1.4, greater deformation of a sheet metal can be obtained in the

incremental forming compared to conventional forming even at room temperature due to its

ability to deform locally. The tool rotation and feed rate are two important parameters

contributing to the ability to form at higher rates of production. [2-9].

FIGURE 1.4 Comparison of formability for various sheet metal forming processes [9]

Introduction

4

Various process parameters influencing incremental forming process includes sheet blank

thickness, tool diameter, spindle rotational speed, feed rate, incremental step depth (Δz), tool

path strategy, tool end geometry, lubrication at tool-sheet interface as shown in Fig. 1.5.

FIGURE 1.5 SPIF terminology with deformed part [7]

Formability assessment of any sheet material is an interest to carry out in terms of sheet

thickness distribution after forming, maximum formable wall angle and maximum formable

depth of the component formed using SPIF. Effect of process parameters and material

property parameters on dimensional and geometrical accuracy of the component formed by

SPIF is also key interest of research.

1.2 Properties influencing formability of metal

Formability is the ability of material to withstand the stretch or draw stresses of forming

before failure in terms of necking or tearing. Formability of any material majorly depends on

its properties like yield strength, strain hardening, modulus of elasticity, anisotropy and

ductility. Yield strength is one of the important properties as it determines the force required

to start plastic deformation. Hard metals possess high yield strength and having reduced

stretch distribution characteristics, making them less stretchable and drawable means less

formable. Generally, forming of hard metals is preferable at elevated temperature but it is

challenging to control material properties while forming at room temperature. Hence, low

value of yield strength is preferred as less force and energy is needed for plastic deformation

of a sheet at room temperature. The motivation for present experimental work is to take over

Single Point Incremental Forming of hard sheet metal having higher yield strength at room

temperature.

Properties influencing formability of metal

5

Modulus of elasticity of a material plays vital role as it determines elastic springback

or recovery. It means that as modulus of elasticity of Aluminum is one-third than that of the

Steel, springback of Aluminum will be three times than that of the Steel. Hence it is desirable

to carry out the deformation beyond the desired point based on amount of springback.

Strain or work hardening is the phenomenon in which the moving dislocations interact

with each other and with the grain boundaries; therefore continuous yielding becomes more

difficult. This mechanism is called strain or work hardening. In cold forming the relation

between flow stress (σ0) and plastic strain (Ԑ) is given by;

σ0 = K Ԑn (1.1)

Where; K is strength coefficient and n is strain or work hardening rate

Higher the work hardening means need of higher load and energy, high tool wear and

higher cost. At the same time work hardening prevents local yielding and increases

elongation. Local necking takes place during the stretching of materials in the absence of

work hardening which results into non-uniform plastic deformation. Hence reasonable

amount of strain or work hardening is desired only in order to get parametric balance of

required force and stretching of sheet metal.

Anisotropy means difference in flow strength in thickness direction than that in the

plane of sheet which may result in excess wrinkling, local thinning or actual rupture. The

anisotropy ratio (r) is defined as;

r = εw / εt (1.2)

Where; εw is principal strain in width direction and εt is principal strain in the thickness

direction.

For evaluation of r, test specimens need to be cut in three directions, i.e. 0o, 45o and

90o with respect to the rolling direction. Three values of r, i.e. r0, r45 and r90 are to be

determined.

The average value of anisotropy ratio (ra) can be calculated as;

rୟ = ୰బାଶ୰రఱା୰వబସ

(1.3)

The difference (Δr) in r values is indicator of planer anisotropy as shown in Fig. 1.6 which is

responsible parameter for change in mechanical properties of metal with direction and can be

defined as;

Δr = rmax – rmin (1.4)

Introduction

6

FIGURE 1.6 Difference in Planer Anisotropy [10]

The value of ra more than unity is an indicator that the sheet is stronger in the thickness

direction. This reduces thinning and neck formation in the sheet at the highly stressed

locations during deep drawing and hence enhances drawability.

Ductility is an essential property of material for its formability but it is not an absolute

constant for any metal or alloy under all conditions. In fact, it may get modified by

optimizing the process parameters. Hence, the same material may show different formability

in different forming processes as it depends on various external factors including hydrostatic

pressure, plastic deformation already suffered, strain rate, temperature etc. Ductility of a

material also affected by some intrinsic factors like composition, grain size and crystal

structure of a material. Metals with BCC and FCC structure shows higher ductility compared

to those with HCP crystal structure [7, 10, 11, 12].

1.3 Properties of non-ferrous lightweight metals

Generally, ferrous metals have good formability as it possesses good ductility. But the

density of ferrous metal is high compared to non-ferrous lightweight metals like Aluminum,

Beryllium, Titanium and Magnesium alloys. The global industrial needs of lightweight

structures and construction lead to investigate about replacement of Steel with Aluminum,

Magnesium, Titanium, metal foams, composites and also some non-metallic materials like

polymers, elastomers and polymer matrix composites. Table 1.1 depicts about comparison of

important properties influencing formability of various materials.

Properties of non-ferrous lightweight metals

7

TABLE 1.1 Comparison of properties of various sheet materials

Properties

Various sheet materials

Steel

Alloys

Aluminum

Alloys

Magnesium

Alloys

Beryllium

Alloy

Commercially

Pure (CP)

Titanium

Density

(gm/cc) 7.75-8.05 2.7 1.77 1.84 4.51

Modulus of

Elasticity

(GPa)

195 - 215 68 – 73 45 110 – 330 105 - 120

Yield

Strength

(MPa)

150 35 – 75

(Annealed) 160 - 230 240 275 - 345

Ultimate

Tensile

Strength

(MPa)

310 90 – 185

(Annealed) 240 - 310 370 350 - 485

Percentage

Elongation in

50 mm

10 - 55 12 – 25 15 3 45

The expected properties of lightweight metals and alloys include low density, high

strength to weight ratio and low toxicity. Aluminum is good conductor of heat and electricity

and also useful as an alloying element in Steel, Titanium and Magnesium. Aluminum is

lighter than Titanium but not as strong. Aluminum alloys do not have ductile to brittle

transition. Beryllium possesses 30 percent less density than Aluminum and 50 percent greater

rigidity than Steel. Beryllium has high thermal conductivity, high specific heat dissipation

and corrosion resistant in normal ambient conditions and at elevated temperature too. The

specific rigidity of Beryllium is about four times greater than composites and six times

greater than other alloys or metals. Titanium and its alloys have higher strength to weight

ratio, good fatigue properties, excellent corrosion, heat and wear resistance. Titanium and its

alloys cover applications in marine and chemical sectors for condensers, evaporators, reaction

vessels for chemical processing, tubing and tube headers in desalinization plants, sea water

piping and cryogenic vessels due to its excellent corrosion resistance property. Titanium is

Introduction

8

also useful in biomedical applications for hip joint, knee joint and heart valve replacement

surgeries. Magnesium and its alloys are characterized by moderate strength, good ductility,

low density and excellent corrosion resistance. Magnesium based materials possess low

elastic modulus and high unit resilience [2, 5, 13, 14, 15].

1.4 Need of lightweight products Application of lightweight products plays crucial role in transport sector where

masses are subjected to motion. Reduced unsprung masses in a vehicle chassis improve

driving comfort and safety at even higher speeds. Selection of materials having maximum

strength and stiffness with lesser weight is a key criterion when selecting them for

automobile, train, ship, aircraft or defense manufacturing industries to meet requirement of

reduction in fuel consumption and greenhouse gases for improving fuel efficiency.

Improvement in fuel economy of 7 percent is estimated by every 10 percent of weight

reduction out of the total weight of a vehicle which also means that for every kilogram of

weight reduced in a vehicle, there is about 20 kg of carbon dioxide reduction. In order to

meet the recycling and recovery targets of 85 percent at the end-of-life of vehicles are driving

the automobile industry to adopt lightweight materials technology. Appropriate

manufacturing (rolling, extrusion), forming and joining technologies require development,

simulation and validation for the innovative materials and applications. Lightweight

construction deals with the use of light weight materials and with different design strategies

too. To design a body structure of trains, aircraft, ships or vehicles include design of frame

structure and shell structures both [13-15].

Metal forming processes are characterized to produce lightweight products of

improved properties by obtaining uniform distribution of material over entire volume of

product without compromising the rigidity of product. Higher strength to weight ratio of

quality sheet metal product is an integrative effort involving field of material science, design

and manufacturing too. The challenge with forming of higher strength metals like magnesium

and titanium includes non uniform distribution of material due to lower nominal strain at

fracture which ultimately limits the formability of metal. The present experimental work is an

attempt to assess formability of AMS4902 using Single Point Incremental Forming at room

temperature. AMS4902 is an unalloyed grade of titanium designated by SAE International

under Aerospace Material Specifications which contains 99-99.5 percent titanium with

balance being made up of iron and interstitial impurity elements hydrogen, nitrogen, carbon

Need of lightweight products

9

and oxygen. AMS4902 is available in the form of strip, sheet and plate. AMS4902 is of high

demand lightweight material grade for aerospace applications under the category of

commercially pure titanium grade 2 which is equivalent to ASME SB265 and ASTM B265

(Grade 2). Aerospace applications of AMS4902 include airframe skins in warm areas,

ductwork, brackets and galley equipments. Over and above exceptional strength-to-weight

ratio, AMS4902 possesses excellent corrosion resistance, good fatigue properties and low

toxicity which widens its application range in marine and chemical sectors for condensers,

evaporators, reaction vessels for chemical processing, tubing and tube headers in

desalinization plants, sea water piping and cryogenic vessels. The aim of presented

experimental work is to analyze the individual effect of various parameters such as tool

diameter, tool rotation, tool feed rate, incremental depth of tool and their interaction during

single-pass and multi-pass of hemispherical tool on thickness distribution, maximum

formable angle, maximum formable depth and surface roughness of end product.

1.5 Organization of Thesis Thesis contains five chapters to address objectives of research work. Outline of various

chapters is as follows;

CHAPTER 1 discusses about modern metal forming processes, properties influencing

formability of metals, properties of non ferrous lightweight metals and challenges to form it,

global need of lightweight products and concludes with the aim to address present research

problem.

CHAPTER 2 focuses on survey of specific literatures related to materials for sheet metals,

geometrical parameters including part geometries, process parameters influencing single

point incremental forming, tool material and tool geometries. Selection of sheet metal,

thickness of sheet, part geometry, tool diameter, tool material, tool geometry, range of

process parameters including tool speed, feed, incremental depth and lubrication is reported.

The chapter summarizes scope of research and objectives for selected range of attributes.

CHAPTER 3 addresses on design of experiments and experimentations conducted. It explains

about design of experiments, SPIF fixture and experimental set-up, pilot experiments

conducted on stainless steel and aluminum sheets, effect of grid marking on AMS4902 sheet,

experimental set: 1 and 2 of single pass SPIF for 50o, 60o and 70owall angle pyramids, multi

pass SPIF to form 50o wall angle pyramid to 60o and 70o wall angle pyramids.

Introduction

10

CHAPTER 4 reports about results and discussion on individual effects of tool diameter, tool

speed, tool feed and incremental depth on formability of AMS4902 sheet. It plots effect of

interaction of tool diameter and incremental depth on thickness distribution and surface

roughness during single pass SPIF of AMS4902 sheet. It includes results of maximum

formable depth during single pass SPIF into 50o, 60o, and 70o wall angle pyramids. It also

discusses about results of thickness distribution and maximum formable angle during multi

pass SPIF of AMS4902 sheet.

CHAPTER 5 summarizes important conclusions regarding individual effects of parameters

and parametric interactions on thickness distribution, maximum formable wall angle,

maximum formable depth and surface roughness during single pass and multi pass SPIF of

AMS4902 sheet derived from results of presented experimental work. The chapter extends

the scope of future work.

References

11

References

1. Markovina R, Blagojević B, Ban D (2008) Peen-Forming-The Possibility of Technology

Transfer from Aircraft Industry to the Production of High-Speed Ships, Brodogradnja

([email protected]); 59, 35-43.

2. Lihui L, Kangning L, Cai G, Yang X, Guo C, Bu G (2014) A critical review on special

forming processes and associated research for lightweight components based on sheet and

tube materials, Manufacturing Rev. 2014,1-9. 3. Trzepieciński T (2012) Advances in sheet metal forming technologies, Mechanika z. 84

(4/12) Rzeszow University of Technology, DOI: 10.7862/rm.2012.12, 59-70. 4. Jackson K, Allwood J (2009) The mechanics of incremental sheet forming, Journal of

Materials Processing Technology, 209, 1158–1174. 5. Jeswiet J, Geiger M, Engel U, Kleiner M, Schikorra M, Duflou J, Neugebauer R, Bariani

P, Bruschi S (2008) Metal forming progress since 2000, CIRP Journal of Manufacturing

Science and Technology, 1,2–17. 6. Park JJ, Yung HK (2003) Fundamental studies on the incremental sheet metal forming

technique, Journal of Materials Processing Technology, 140, 447–453. 7. Ham M, Jeswiet J (2006) Single Point Incremental Forming and the Forming Criteria for

AA3003, Annals of CIRP, 55/2, 241-245. 8. Nimbalkar DH, Nandedkar VM (2013) Review of Incremental Forming of Sheet Metal

Components, Intenation Journal of Engineering Research and Applications, 3/ 5,39-51. 9. Behera AK, Desousa RA, Ingarao G, Oleksik V (2017) Single point incremental forming:

An assessment of the progress and technology trends from 2005 to 2015, Journal of

Manufacturing Processes, 27,37–62. 10. Plastic deformation of metals and related properties,

ecampus.sriramanujar.ac.in/files/files_2015/Plasticity-related_properties_069bc.pdf

[Online] [Accessed 5 October 2015] 11. Fratini L, Ambrogio G, Lorenzo RD, Filice L, Micari F (2004) Influence of mechanical

properties of the sheet material on formability in single point incremental forming,

Annals of CIRP, 53/1, 207-210. 12. Billur E, Altan T (2006) Challenges in Forming Advanced High Strength Steels,

Engineering Research Center for Net Shape Manufacturing (ERC/NSM), 285-304.

Introduction

12

13. Kleiner M, Geiger M, Klaus A (2003) Manufacturing of Lightweight Components by

Metal Forming, CIRP Ann-Manuf. Technol., 52(2), 521–542. 14. Ghassemie E (2011) Materials in Automotive Application, State of the Art and Prospects,

New Trends and Developments in Automotive Industry, University of Sheffield UK,

ISBN 978-953-307-999-8, Publisher In Tech, 365-394. 15. Sivanandini M, Dhami SS, Pabla BS (2012) Formability of Magnesium Alloys,

International Journal of Modern Engineering Research (IJMER),2, Issue.4, 2464-2471,

ISSN: 2249-6645.

Literature Review

13

CHAPTER: 2

Literature Review

The present chapter emphasizes on review of literature specific to assessment of formability

of various sheet materials using SPIF within the range of parametric combinations prescribed

by researchers. The state of work is segregated mainly in three sub sections namely; (1)

literature review related to sheet and component attributes; (2) tool attributes and tool path

strategies; and (3) process attributes including tool rotational speed, feed rate and incremental

depth with lubrication at tool-blank interface.

Single Point Incremental Forming (SPIF) is a type of Asymmetric Incremental Sheet Forming

(AISF) described by Jeswiet et al. [1] as “dieless forming” of sheet metal using single point

tool patented by Edward Leszak in 1967. Emmens et al. [2] discussed about history of

development of incremental sheet forming in which distinct difference is mentioned about

patent claimed by team of Walter Berghahn of General Electric Company and Edward

Leszak. The modern AISF was first described and developed by Mason in 1978 as small

batch size sheet metal forming process. Asymmetric Single Point Incremental Forming

(AISF) can be performed by holding sheet blank rigidly against movement of tool

establishing contact with sheet blank as shown in Fig. 2.1.

FIGURE 2.1 Schematic diagram of Single Point Incremental Forming Process with equipments [3]

Single Point Incremental Forming became a key interest of research as an advance material

processing technique for researchers since more than a decade due to its characteristics of

enhanced formability compared to conventional sheet forming processes reported by Kim and

Yang, Kim and Park, Filice et al. [4-6].

Literature Review

14

2.1 Sheet and component attributes

Many researchers reported effect of sheet thickness, step down, speed, tool size on maximum

draw angle in relation with properties of various grades of sheet metals like AA1050-O,

AA6114-T4, AA3003-O, AA8008-O, Al3003-O, Al5754-O, Al5182-O, AA6111-T4P,

DC04, HSS, DDQ steel, Copper, Brass formed by SPIF. They obtained forming limit curve

of negative slop with much higher strains by forming sheets into variety of geometrical

shapes including dome, cone, hyperbola, pyramid using SPIF as shown in Fig. 2.2. They also

concluded that strain hardening exponent of the material as most influencing parameter

affecting formability followed by strength coefficient and percentage elongation. Ham and

Jeswiet [7, 8] conducted experiments using design of experiments for deforming a cone out

of AA3003-O of thickness ranging from 0.8 mm to 2.1 mm. Authors presented a

methodology to develop FLDs for forming AA6451, AA5182 and AA5754 using SPIF and

summarized that material with lower ultimate tensile strength offers more formability.

Ambrogio et al. [9] presented mathematical relation between various parameters influencing

accuracy using statistical analysis of experimental data determined for truncated pyramid of

50o and 60o wall angle formed out of 0.5 mm and 1.5 mm thick sheet of AA1050-O. Hussain

et al. [10] carried out experiments on CNC milling machine for CP Ti sheet of 0.99 mm at

room temperature by forming a variable wall angle conical frustum (VWACF). Franzen et al.

[11] evaluated the formability limit and characterization of 2 mm and 3 mm thick PVC sheets

at room temperature. Martins et al. [12] employed SPIF to form five different polymer sheets

including Polyoxymethylene (POM), Polyethylene (PE), Polyamide (PA), Polyvinylchloride

(PVC) and Polycarbonate (PC) incrementally into cones with an increasing wall angle on a

conventional CNC milling machine and confirmed potential of process to form deep complex

shape. Silva et al. [13] summarized that crack propagation at the junction of inclined wall and

the corner radius of cone with varying wall angle formed from PVC was due to tensile

meridional stresses acting under stretching modes of deformation rather than localized

necking. Hamilton and Jeswiet [14] examined a model to predict the orange peel effect in

SPIF using measured roughness values and forming parameters for 0.8128 mm thick sheets

of Al3003-H14. Bouffioux et al. [15] conducted experiments by forming AlMgSc sheet of 0.5

mm thickness into a straight wall cone angle ranging from 10o to 46o and also validated using

numerical simulation. The purpose was to study effect of wall angle on forming forces.

Sheet and component attributes

15

FIGURE 2.2 Various shapes used to demonstrate SPIF [1]

Hussain et al. [16] demonstrated forming of 2.6 mm thick sheet of AA1060 into conical,

square and hexagonal pyramidal geometry using SPIF. They optimized process parameters

for reducing defects of squeezed out wall formation, corner fold and bulge height. Based on

experimental and FEA results Malhotra et al. [17] confirmed the claim that both through

thickness shear and local bending of sheet around the tool play a role in fracture during SPIF

process of Al5052 into cone and funnel shape geometry. Ambrogio et al. [18] compared

workability of hot incremental sheet forming of 1 mm thick sheet of AA2024-T3, AZ31B-O

and Ti6Al4V (Grade5) into conical frustum with forming at room temperature. Palumbo and

Brandizzi [19] conducted a study to investigate combined effect of electric static heating with

high tool rotations during forming of 1 mm thick sheet of Ti6Al4V (Grade5) into scaled car

door shell using SPIF. Arfa et al. [20] performed Single Point Incremental Forming on 1.2

mm thick sheet of Al3003-O into truncated cones and pyramids experimentally and validated

results of equivalent plastic strain and final wall thickness obtained using numerical

simulation. Ambrogio et al. [21] conducted experiments for forming 1 mm thick sheet of

Titanium ASTM Grade2 and ASTM Grade5 (Ti6Al4V) into a cone of wall angle 30o and 25o

respectively on CNC Lathe using SPIF. Gomez-Lopez et al. [22] presented a case study of

Literature Review

16

forming DC-05 steel sheet into pyramidal shape using SPIF in Solidworks environment. Xu

et al. [23] investigated forming behavior of 1.27 mm thick sheet of AA5052-H32 into

truncated funnel shape. Kurra and Regalla [24] conducted experiments in order to assess

formability and thickness distribution. They deformed EDD steel sheet of 1 mm thickness

into Varying Wall Angle Conical Frustum (VWACF) with different generatrices of circular,

elliptical, parabolic and exponential as depicted in Fig. 2.3 using SPIF.

FIGURE 2.3 Various generatrices of parts formed (a) Circular (b) Elliptical (c) Parabolic (d) Exponential

[24] Desai et al. [25] conducted parametric investigations for Die-Less Rapid Prototyping (DLRP)

process on 0.91 mm thick Al1200-H14 sheet by forming it into 80o wall angle cone. Malwad

and Nandedkar [26] presented experiments on SPIF of AA8011 sheet into constant wall

truncated cone of 50 mm depth. Adams and Jeswiet [27] presented design guidelines for

single-pass SPIF and method of developing intermediate models for multi-pass SPIF with

case studies. Naranjo et al. [28] carried out numerical simulation of SPIF for commercially

pure titanium grade2 (ASTM B-265) of 0.8 mm sheet thickness using ANSYS workbench.

Behera et al. [29] compared accuracy of ellipsoidal shapes of medical implant formed out of

titanium grade 1 using SPIF with characterization models generated by Multivariate Adaptive

Regression Splines (MARS). They used predicated deviations to generate optimized tool

paths in order to minimize shape and dimensional inaccuracies. Uheida et al. [30] conducted

experimental study on 0.8 mm thick sheet of CP Ti Grade2 into varying wall angle conical

frustum (VWACF) of 25 mm height. Afonso et al. [31] formed tunnel and semi tunnel type

Sheet and component attributes

17

parts from 1050-H111 aluminum sheet of 2 mm thickness. Gupta and Jeswiet [32] presented

experiments to form 2.54 mm thick sheet of AA3003-O into scale version of C-channel

geometry of an airplane fuselage. Overall findings from the literature reviewed on various

sheet blank materials and component geometry is tabulated in Table 2.1. TABLE 2.1 Summary of findings reported for various grades of sheet blanks with component geometries

Part

Geometry

Grades of

Steel Alloy

Grades of Al

Alloy

Grades of Mg

Alloy

Grades of Ti

Alloy

Other

Special

Materials

Cup Shape DP600, DP800, DP1000, DP1200, DP1400.

-

AZ 31, ZK10, ZK41

- -

Truncated Cone with constant wall angle

(10o to 80o

Wall Angle)

DC04, DC05, DC06, HSS,

DDQ steel

Al1050, Al1200-H14, Al 3003-O,

Al3003-H14, Al5052,

Al5182-O, Al5754-O,

AA1050-H111, AA1050-O, AA 1100,

AA 3003-O , AA-2024-O, AA2024-T3,

AA5052, AA 5083,

AA6111-T4P, AA8008-O,

AA8011

AZ 31, AZ-31 B, AZ-

31 O, LZ61,

Yttrium- ZK10, ZK41,

CP TI Grade-2 (ASTM B265)

CP Titanium

ASTM Grade 5 (Ti6Al4V)

Polymers like

POM, PE, PA,

PVC, PC

C101

Brass

AlMgSc

Truncated Pyramid

with constant

wall angle

AISI 304 HSS,

DDQ steel,

AA1050-O, AA6114-T4, - - Copper,

Brass

Truncated Cone with

varying wall angle

EDD Steel - - CP TI Grade-2 (ASTM B265) -

Literature Review

18

2.1.1 Findings based on literature review on sheet and component

attributes Based on summary tabulated regarding sheet blank materials and component

geometries in table 2.1, it can be observed that the major work had been carried out for Steel

sheets, Aluminum sheets and its alloys. Similarly, major work on the component geometry of

constant wall or variable wall angle frustum of cone was found. Very less work had been

found on the hard sheet metal like Ti, Mg and its alloys to form it into pyramidal geometry.

From the literature survey, major research efforts were found in the direction to determine the

formability of various grades of Steel sheets, Aluminum sheets and their alloys in terms of

maximum formable wall angle or maximum forming depth during SPIF at room temperature,

hot incremental forming and multi pass forming. Potential research gap is identified to assess

formability in terms of wall thickness distribution after SPIF.

2.2 Tool attributes and tool path strategies

Ham and Jeswiet [7] performed experiments using 4.76 mm and 12.7 mm diameter of tool on

three different thicknesses of AA3003-O sheet. They concluded that the interaction of

material thickness and tool size have significant effect on formability. Ham and Jeswiet [8]

performed SPIF for forming various grades of aluminum sheets of different thicknesses using

4.76 mm, 6.35 mm and 9.52 mm tool diameters. Micari et al. [3] addressed factors

influencing various geometrical errors including pillow effect, sheet bending and springback

during SPIF as explained in Fig. 2.4. Authors suggested optimization of tool trajectories as a

promising strategy amongst suitable strategies discussed to improve geometrical accuracy of

formed component.

FIGURE 2.4 Geometrical Errors in SPIF [3]

Hussain et al. [10] carried out experiments using hemispherical tool of 8 mm, 12 mm and 16

mm diameter made of HSS in order to determine formability of CP Ti sheets. Authors

concluded that maximum formable angle decreases significantly with the increment in tool

Tool attributes and tool path strategies

19

diameter from 8 mm to 12 mm compared to 12 mm to 16 mm. Duflou et al. [31] explored

multi-step tool path strategy experimentally to compare it with simulation output in order to

contribute for better understanding of material relocation. Franzen et al. [11] evaluated

formability limit of PVC sheets by deforming at room temperature using 10 mm and 15 mm

tool diameters. Dejardin et al. [32] demonstrated SPIF through experiments and FEA for

forming a cone out of 1 mm thick sheet of AA1050 using 10 mm diameter of tool. They

reported that springback can be accurately predicted from numerical simulations based on

shell elements associated with a suitable forming tool path. Malhotra et al. [33] proposed tool

path generation strategy to obtain a smoother component base by applying in-to-out and out-

to-in tool paths for each intermediate shapes during multi pass single point incremental

forming of 1 mm thick sheet of AA5052. Authors validated the proposed strategy using 5 mm

and 10 mm diameter hemispherical tools to form a cone as initial shape out of flat sheet and

cap of sphere as final shape out of cone. Ambrogio et al. [17] conducted hot incremental

forming of AA2024-T3, AZ31B-O and Ti6Al4V using 12 mm diameter of HSS tool and

compared results of workability with results obtained at room temperature. Palumbo and

Brandizzi [19] performed experiments to form 1 mm thick sheet of Ti6Al4V using cemented

carbide tool of 16 mm diameter. Gomez-Lopez et al. [22] simulated SPIF for DC-05 sheet

using 12 mm hemispherical tool diameter of AISI420 steel. Ambrogio et al. [21] used 15 mm

diameter tool of hemispherical shape for single point incremental forming of Titanium Grade

2 and Grade 5 on CNC lathe. Kurra et al. [34] implemented tool path trajectories generated

using CAM packages into MatLab and Ls-Dyna for various geometries and found good

agreement for geometric and dimensional accuracy. Nimbalkar and Nandedkar [35] reviewed

procedure to generate the contour and spiral tool paths for incremental sheet forming as

shown in Fig. 2.5.

FIGURE 2.5 Types of tool paths (a) Contour tool path (b) Spiral tool path [20]

Hussain et al. [36] suggested guidelines for tool size selection for single point incremental forming of

AA2024-O and concluded that tool radius twice the sheet thickness offers good spifability. Cawley et

Literature Review

20

al. [37] tested results of formability and surface quality of components formed using angle,

flat and parabolic tool profiles shown in Fig. 2.6 and compared with results of hemispherical

tool profile. All the tools were machined from ASTM A681 tool steel.

(a) (b) (c) (d)

FIGURE 2.6 Various tool Profiles (a) Angular, (b) Flat, (c) Hemispherical, (d) Parabolic [37]

They compared the results for minor shape variation of proposed tool profiles tabulated in

Table 2.2 by forming components out of 1.59 mm thick sheet of Al3003-O. They concluded

that formability is highest with reduced contact area of parabolic tool head as it results in

higher localized stress which allows the sample to resist fracture more readily. TABLE 2.2 Various tool profiles and related dimensional specifications [37]

Tool Type Parameter/ Value

Angle (r=2.54mm) Φ=60o, Φ=70o, Φ=80o

Flat (D=12.7mm) r=5.08mm, r=2.54mm

Hemispherical D=5.08mm, D=10.16mm

Parabolic (D=12.7mm) y=x2, y=5x2, y=10x2

Gupta and Jeswiet [38] performed experiments to form 2.54 mm thick sheet of AA3003-O

into scaled version of C-channel geometry of an airplane fuselage using 9 mm diameter flat

tool having 3 mm corner radius. Kurra and Regalla [24] used 10 mm diameter hemispherical

head tool of EN36 to form of 1 mm thick EDD steel sheet into various component geometries

incrementally. Desai et al. [25] applied 6 mm diameter hemispherical end tool of EN08 with

out-to-in contour tool path to perform Die-Less Rapid Prototyping process. Malwad and

Nandedkar [26] used 6 mm and 12 mm diameter hemispherical head tool and concluded that

tool diameter affects both formability and surface finish. They also mentioned that 12 mm

tool diameter generates more force but it supports sheet better during forming while

formability decreases for tool diameter less than 6 mm as tool tends to penetrate inside the

sheet instead of offering uniform deformation. Centeno et al. [39] conducted experimental

analysis to compare influence of bending in SPIF and stretch-bending on 0.8 mm thick sheet

of AISI304. They used 6 mm, 10 mm and 20 mm diameters of tools with hemispherical head


21

to form conical frustum of circular generatrix. They reported enhancement of formability in

SPIF with decrease in tool diameter limiting up to 6 mm tool diameter as indentation mark of

forming tool was observed on the inner surface of metal sheet using 6 mm diameter tool. As

shown in Fig. 2.7, minor strain was observed closer to plane strain conditions due to small

zone of sheet placed under 10 mm diameter forming tool while in the case of 20 mm diameter

forming tool, the strain distribution slightly deviates towards biaxial conditions due to strain

distribution over more extended area. Authors concluded that although the punch radius is an

important factor for the bending effect induced in SPIF, it is not the only factor responsible to

obtain stable deformations well above the FLC. J. Jeswiet et al. [40] discussed design guide

integrating surface roughness of deformed component, tool end geometry, multi-pass

technique and SPIF at an elevated temperature. Authors recommended galvanized steel and

stainless steel flat-ended tool of 12.7 mm or larger diameter for aluminum sheet blank as it

offers best combination of good formability and very low surface roughness.

FIGURE 2.7 Fractography at the fracture initiation zone (left) and at its opposite section (right) for tools

of Φ20 (above) and Φ 10 mm (below) [39]

Bagudanch et al. [41] performed forming on 2 mm thick sheet of polycaprolactone (PLC) to

form into customized cranial geometry using 6 mm diameter hemispherical end tool of

Literature Review

22

Vanadis 23 steel by SPIF. They suggested that modification of tool path is one of the

important strategies in negative incremental sheet forming in order to improve the

geometrical accuracy of formed components. Gatea et al. [42] recommended scope for

application of new tool designs, tool materials and development of different algorithms to

generate appropriate tool paths capable to form product with good surface finish and

dimensional accuracy. McAnulty et al. [43] presented a review on interactions between

various parameters influencing SPIF and reported scope to work on the interaction between

tool material, blank material and lubrication as it has considerable influence on friction

conditions at tool-sheet interface. Salem et al. [44] investigated influence of tool path on

cumulative strain along the constant wall of cone. They performed SPIF to form the cone

from AA7075-O sheet of 1.6 mm thickness using 12.7 mm diameter tool of hemispherical tip

followed by spiral tool path. Uheida et al. [30] demonstrated SPIF to form CP Ti Grade 2

sheets using 10 mm diameter hemispherical forming tool of steel 2312 operated through out-

to-in spiral toolpath. Abbas [45] compared effect of elliptical profile tool on final product

profile, final thickness, strain and stress distribution of formed component over results by

hemispherical and flat profile tools using numerical simulation. Behera et al. [46]

summarized the maximum formable wall angle obtained by various researchers employing

various tool diameters to form constant or varying wall angle conical frustum out of variety

of sheet materials of various thicknesses. Authors suggested scope of research in the area of

simultaneous control of thickness variation with dimensional accuracy by incorporating real

time tool path correction strategies. Afonso et al. [47] used three different tool path strategies,

(i) helical toolpath, (ii) alternating strategy with stepdown at wall center and (iii) inside-out

strategy with air movements and stepdown at wall center as shown in Fig. 2.8 to form tunnel

shape component of 1050-H111 sheet.

FIGURE 2.8 Various toolpath strategies (a) helical (b) alternating (c) inside-out [47]

They found that the alternating toolpath with the side changing position from one tunnel wall

to other outside the part edge is most reliable strategy. Kumar et al. [48] conducted SPIF of


23

AA2024-O sheet into truncated cone using 7.52 mm, 11.60 mm and 15.66 mm diameters of

hemispherical tip tool and flat-end tool having smaller and bigger corner radii operated with

helical tool path. They found increment in formability with increase in tool diameter. They

concluded that flat-end tool with larger corner radius improved formability while flat-end

tools with lower corner radius and lower tool diameter experienced an earlier fracture of sheet

material. Yoganjaneyulu et al. [49] performed SPIF of 1 mm thick CP Ti Grade 2 sheet using

forming tool diameter of 8 mm, 10 mm and 12 mm of F6 tool steel with spiral tool path. They

observed maximum deformation fracture strain on the component deformed using 12 mm

diameter tool. Gupta and Jeswiet [38] demonstrated four different strategies of multi-stage

toolpath namely; (i) Conventional Downward strategy, (ii) Downward-Downward-

Downward-Up (DDDU) strategy, (iii) Inside Outside-Outside Inside (IO-OI) strategy, (iv)

Tunnel strategy to form vertical wall angles with complexities of C-channel geometry of

airplane fuselage. Flat profile forming tool of 9 mm diameter with 3 mm corner radius was

used in order to meet demand of tight tolerances of corner radius on formed component and

to eliminate pillow effect. They found the Conventional Downward strategy as the most

suitable strategy to form identified geometry of C-channel. Findings based on literature

review conducted regarding tool materials, tool diameters, tool end geometries and tool paths

in combination of sheet blank material is tabulated in Table 2.3;

Literature Review

24

TABLE 2.3 Combination of tool materials, tool diameters, tool end geometries and tool paths in combination to form various sheet blank materials

Tool

Materials

Sheet Blank Materials Grades of

Steel and its Alloy

Grades of Al and its Alloy

Grades of Mg and its

Alloy

Grades of Ti and its Alloy

Other Special

Materials

Cemented Carbide

Tool - - -

16 mm diameter

Hemispherical tip tool

-

HSS -

12 mm diameter Hemispherical

tip tool

12 mm diameter


8 mm, 12 mm, 16 mm

diameter Hemispherical

tip tool (Spiral path)

-

EN08 -

6 mm diameter Hemispherical

tip tool (Out-to-in

Contour path)

- - -

EN36

10 mm diameter


(Contour path)

- - - -

AISI 420 12 mm


tip tool - - - -

ASTM A681 tool

steel -

5.08 mm Angle, 12.7 mm Flat & Parabolic, 10.16 mm & 20.32 mm


tip tool

- - -

Vanadis 23 steel - - -

10 mm diameter


6 mm diameter


Steel 2312 - - -

10 mm diameter


(Out-to-in Spiral path)

-

Process attributes

25

Tool

Materials

Sheet Blank Materials Grades of

Steel and its Alloy

Grades of Al and its Alloy

Grades of Mg and its

Alloy

Grades of Ti and its Alloy

Other Special

Materials

F6 tool steel - - -

8 mm, 10 mm, 12 mm


tip tool (Spiral path)

-

Not Specified any Tool Material

6 mm, 10 mm, 12 mm


5 mm, 6 mm, 8 mm, 10 mm, 12 mm, 14 mm, 15

mm, 16 mm, 20mm diameter Hemispherical

tip tool

- -

15 mm diameter


2.2.1 Findings based on literature review on tool attributes and tool path

strategies

Combination of tool materials and sheet blank materials has been tabulated in table 2.3 for

which research had already been carried out including tool diameters, tool end geometries

and tool path strategies. Generally, contour and spiral strategies of tool paths were found

most common strategies of choice to apply for SPIF of various grades of sheet metals. Effect

of tool end geometries and tool diameters on formability as well as dimensional accuracy of

components formed out of variety of sheet materials was also addressed.

2.3 Process attributes

In order to develop process mechanics of Single Point Incremental Forming (SPIF) for

different sheet materials, combination of various process parameters including tool rotational

speed, feed, incremental depth and lubrication at tool-sheet interface plays vital role. Kim and

Park [5] performed straight groove test by operating tool at 0.1 mm, 0.3 mm and 0.5 mm feed

rates to investigate effect of process parameters on formability of Aluminum 1050 sheet in

rolling and transverse direction. Authors reported improvement in formability with ball tool

of 10 mm diameter operated at lower feed rate in the presence of little friction. Jeswiet et al.

Literature Review

26

[1] summarized effect of spindle speed and lubrication on surface roughness of various

shapes formed by SPIF using different sheet blank materials. They also presented challenges

to develop strategies to obtain dimensional accuracy of variety of product shapes to be

formed out of various sheet blank materials for the interest of applications of automotive,

aerospace, architectural and biomedical. Ham and Jeswiet [7] performed SPIF at the

parametric combination of 100 rpm and 600 rpm spindle speeds, 1270 mm/min and 2540

mm/min feeds with 0.0508 mm, 0.127 mm and 0.254 mm step size to determine effect on

formability of AA3003-O in terms of maximum formable angle and depth of component

formed. They concluded that the faster spindle speed improved the formability at lower feed

and also observed significant effect of material thickness, tool diameter and its interaction on

maximum formable wall angle. Hussain et al. [50] demonstrated SPIF at 2500 mm/min

horizontal feed and 0.15 mm/ revolution vertical feed to determine maximum formable angle

by forming 0.91 mm thick aluminum sheet into varying wall angle conical frustum

(VWACF). Hussain et al. [10] performed experiments to investigate effect of pitch, tool

diameter, feed rate and friction at tool-sheet interface on maximum formable wall angle of

VWACF formed using SPIF from CP Ti sheet of 0.99 mm thickness. 98.5 percent pure MoS2

(Molybdenum Disulphide) of small grain size was mixed with grease to apply as a lubricant

at tool sheet contact. Design of experiments for combination of process parameters used by

Hussain et al. is tabulated in Table 2.4 as;

TABLE 2.4 Design of experiments for some process parameters [10]

Experiment Feed Rate

(f) (in mm/min)

Tool Diameter

(d) (in mm)

Pitch

(p) (in mm )

1 2600 12 0.2

2 2600 12 0.75

3 2600 12 1.3

4 1200 12 0.75

5 4000 12 0.75

6 2600 8 0.75

7 2600 16 0.75

They concluded that drop in formability of CP Ti was higher when feed rate exceeded 2500

mm/min and friction at tool-blank interface did not play any significant role to increase

formability of CP Ti, rather higher friction offered poorer surface quality. Duflou et al. [31]

Process attributes

27

performed SPIF of 1.5 mm thick sheet of AA3103 at 2 m/min feed rate, 100 rotations/min

spindle speed and 1 mm stepdown with contour tool path to explore multi-step toolpath

strategy. Dejardin et al. [32] performed SPIF at 400 rpm rotational speed, 500 mm/min feed

rate and 0.2 mm step depth. Ambrogio et al. [51] demonstrated SPIF to form customized

ankle support out of Deep Drawing Quality (DDQ) Steel of 1 mm thick sheet.

Theyperformed SPIF using 11 mm diameter tool of hemispherical tip operated at 500 rpm

speed, 1000 mm/ min feed and 0.5 mm step depth.

Hussain et al. [16] performed SPIF at 2600 mm/min feed rate and 0.3 mm pitch to present

empirical model for optimization of process parameters. Palumbo and Brandizzi [19]

performed experiments to investigate contribution of tool rotational speed during SPIF of 1

mm thick Ti6Al4V combining with static heating. SPIF tests were performed by employing

16 mm diameter cemented carbide tool at 800-1600 rpm speed range, 1800 mm/min feed and

step depth ranging from 0.5-1 mm in the presence of OKS 280 solid lubricant to form into

scaled geometry of car door shell. They noticed that high rotational speed helped to stabilize

the necking, increase accuracy and surface quality of formed geometry. Ambrogio et al. [21]

concluded that increase in feed did not supply enough power to affect the significant change

in material microstructure for the selected feed values of 6 m/min, 60 m/min and 600 m/min

for Ti ASTM Grade2 and 5 m/min, 50 m/min and 500 m/min for Ti ASTM Grade5. They

performed experiments on 0.25 mm and 1 mm pitch for ASTM Grade2 while 0.1 mm, 0.3

mm and 0.5 mm were the pitch values selected for ASTM Grade5. Xu et al. [23] investigated

influence of low tool rotation range from 0-1000 rpm and high tool rotation range from 2000-

7000 rpm on formability mechanism by forming 1.27 mm thick sheet of AA5052-H32 into

truncated funnel shape at constant feed of 150 mm/min and step depth of 0.5 mm. They

reported that increase in through thickness shear was a key factor for increase in formability

which was observed in the range of 0-500 rpm while thermal effect became dominant reason

for enhanced formability between 2000-3000 rpm and active dynamic recrystallization due to

refinement of microstructure favored for improving formability beyond 3000 rpm. They also

observed that formability of material decreases with laser surface textured forming tool due

to reduction in friction and heat generation at tool-blank interface compared to regular SPIF

forming tool. Desai et al. [25] presented experiments on Die-Less Rapid Prototyping (DLRP)

process operated at 1250 rpm and with feed rate values of 25 mm/min, 50 mm/min and 75

mm/min with incremental depth of 0.5 mm and 0.8 mm in order to determine effect of feed

rate on formability. They also investigated effect of tool rotational speed ranging from 500-

2000 rpm at equal interval of 250 rpm on formability of Al1200-H14 for constant feed rate of

Literature Review

28

50 mm/min and 0.5 mm incremental depth. They noticed less forming time at higher feed rate

but concluded that forming of higher cone angle is possible at lower incremental depth and

optimum value of feed rate specific to blank material operated with contour tool path. They

observed no effect of tool rotation on forming time but recommended higher tool rotation for

good surface quality and lesser geometrical error. Malwad and Nandedkar [26] demonstrated

SPIF at 1000 rpm spindle speed, 1500 mm/min feed rate and 0.2 mm and 0.5 mm incremental

depth for formability assessment of AA8011. Bagudanch et al. [52] performed SPIF of 0.8

mm thick sheet of AISI 304 using 6 mm, 10 mm and 20 mm diameter tools operated at 1000

rpm speed, 3000 mm/ min feed and 0.2 mm and 0.5 mm values of step depths. They have

used Houghton TD-52 lubricant at tool-sheet interface. They observed increase in forming

force with increment in tool diameter and step depth while decrease in forming force with

increase in spindle speed due to increase in temperature caused by friction at tool-sheet

interface.

Gatea et al. [42] presented technological capabilities and limitations of Incremental Sheet

Forming (ISF) processes in detail with knowledge gap for integrated effect of process

parameters on formability, deformation and failure mechanics, geometrical and dimensional

accuracy and surface roughness for various hard to form materials including new techniques

of hot ISF and multi-pass ISF. Authors recommended that there is a need for research to

establish relations between step depth, tool rotation and feed rate with type of material, effect

of ratio of initial sheet thickness and tool radius on FLCF, development of algorithm for

prediction and improvement of springback, dimensional accuracy and surface finish with

respect to material properties, forming parameters, tool designs, tool paths and lubrication.

McAnulty et al. [43] conducted quantitative analysis on review of 35 research papers and

found lack of focus in parameter interactions for SPIF as they are highly interdependent and

material specific. Authors presented a theoretical framework for experimental parameters in

order to establish comparability of results of research in future. Uheida et al. [30] investigated

the influence of sliding velocity of hemispherical tip forming tool on thermomechanical loads

during SPIF test of CP Ti Grade2 sheets for the speed range of 450 rpm to 15000 rpm and

step size of 0.3 mm. The major conclusion drawn was about initiation of failure of sheet

above 4000 rpm due to escalation of material and reduction in forming angle was also

observed. They also found tool rotation conducive up to 2500 rpm to obtain deformation of

sheet with ease at reduced forming forces. They reported that increase in feed rate contributed

for slightly increment in in-plane forces than heat generation at tool-sheet interface. Echrif

and Hrairi [53] summarized research trend in forming methods, formed sheets, forming path

Process attributes

29

strategies, forming limits, forming tools and simulation for Incremental Sheet Forming (ISF).

Behera et al. [46] presented a progressive assessment for SPIF from 2005 to 2015 with its

current state of art in order to derive roadmap for investigation interest in future. Kumar et al.

[48] performed SPIF in order to determine impact of forming tool shape, tool diameter, wall

angle, step size, sheet thickness and tool rotation on formability of AA2024-O sheet. They

conducted experiments using hemispherical tip tool and flat tools operated at free spindle

speed, 1000 rpm and 2000 rpm with the step depth values of 0.2 mm, 0.5 mm and 0.8 mm.

They concluded that increase in wall angle and step size led to decrease in formability and

combination of higher tool diameter with higher step size was also responsible for loss of

formability due to fracture of component at lower depth. Yoganjaneyulu et al. [49] examined

variation of fracture behavior of 1 mm CP Ti Grade2 by performing SPIF for the speed

values of 300 rpm, 450 rpm and 600 rpm at constant feed of 300 mm/min and varying step

depth of 0.2 mm, 0.4 mm and 0.6 mm. They reported that limiting fracture strain values were

diminished for 12 mm diameter tool operated at speed of 600 rpm. Dwivedy and Kalluri [54]

determined effect of tool diameter, sheet thickness, feed, spindle speed, depth of indentation

(z-depth) on forming force and concluded that sheet thickness followed by z-depth had

significant effect on average axial and pick axial forces. They presented experiments for SPIF

of EDD steel using hemispherical tool of EN-36 with SAE 40 as a lubricant. Table 2.5 shows

various parameters used by Dwivedy and Kalluri [54] for performing experiments. TABLE 2.5 Level of parameters to conduct experimentations using SPIF [54]

Factor (unit) Level-1 Level-2 Level-3

Tool Diameter (mm) 10 12 14

Sheet Thickness (mm) 0.6 1.0 1.2

Feed (mm/min) 1300 1400 1500

Spindle Speed (rpm) 700 800 900

Depth of indentation (z-Depth) (mm) 0.2 0.3 0.4

Parametric combination of tool rotational speeds, feed rates and incremental depths used for

SPIF of specific sheet blank material is summarized in Table 2.6, 2.7 and 2.8.

Literature Review

30

TABLE 2.6 Summary of parametric combinations reviewed for tool rotational speeds and feed rates to form specific sheet blank material using SPIF

Range of Tool

Rotational Speed (rpm)

Range of Feed Rate (mm/min)

0-999 1000-1999 2000-2999 3000-

3999

4000-

4999

5000-

and

more

0-999

AA 1050, AA5052-H32, Al1200-H14, CP Ti Grade 2

AA3003-O, EDD Steel, Ti6Al4V,

CP Ti Grade 2

AA3003-O, AA3103,

CP Ti Grade 2 - CP Ti

Grade 2 CP Ti

Grade 2

1000-1999 AA5052-H32, Al1200-H14

AA8011 Ti6Al4V - - - -

2000-2999 AA5052-H32, Al1200-H14 - - - - -

3000-3999 AA5052-H32 - - - - - 4000-4999 AA5052-H32 - - - - -

5000-5999 AA5052-H32 - - - - -

6000-6999 AA5052-H32 - - - - - 7000 and

more AA5052-H32 - - - - -

TABLE 2.7 Summary of parametric combinations reviewed for feed rates and incremental depth to form

specific sheet blank material using SPIF Range of Feed

Rate (mm/min)

Range of Incremental Depth (mm)

0.01 - 0.49 0.5 – 0.89 0.9 – 1.29 1.3 and more

0-999 AA1050 CP Ti Grade 2

AA5052-H32, Al1200-H14,

CP Ti Grade 2 - -

1000-1999 AA3003-O,

AA8011, CP Ti Grade 2,

EDD Steel

C-101 AA8011,

CP Ti Grade 2, Ti6Al4V

Ti6Al4V -

2000-2999 AA3003-O,

AA1060, AlMgSc CP Ti Grade 2

CP Ti Grade 2 AA3103 CP Ti Grade 2

3000-3999 - - - - 4000-4999 CP Ti Grade 2 CP Ti Grade 2 - - 5000-5999 Ti ASTM Grade2 - Ti ASTM Grade2 - 6000-6999 Ti ASTM Grade5 Ti ASTM Grade5 - -

7000 and more Ti ASTM Grade2 Ti ASTM Grade5

CP Ti Grade 2 Ti ASTM Grade5 TI ASTM Grade2 -

Process attributes

31

TABLE 2.8 Summary of parametric combinations reviewed for tool rotational speeds and incremental depth to form specific sheet blank material using SPIF

Range of Tool Rotational

Speed (rpm)

Range of Incremental Depth (mm)

0.01 - 0.49 0.5 – 0.89 0.9 – 1.29

0-999 AA1050,

AA3003-O, CP Ti Grade 2

AA5052-H32, Al1200-H14, Ti6Al4V

CP Ti Grade 2

AA1050-O, AA6114-T4,

AA3103 HSS, DDQ steel, Copper, Brass,

Ti6Al4V

1000-1999 AA8011,

AA2024-O CP Ti Grade 2

AA5052-H32, Al1200-H14,

AA8011, Ti6Al4V -

2000-2999 CP Ti Grade 2 AA5052-H32, Al1200-H14, AA2024-O -

3000-3999 CP Ti Grade 2 AA5052-H32 -

4000-4999 CP Ti Grade 2 AA5052-H32 -

5000-5999 CP Ti Grade 2 AA5052-H32 - 6000-6999 CP Ti Grade 2 AA5052-H32 -

7000 and more CP Ti Grade 2 AA5052-H32 AA2024-O -

2.3.1 Findings based on literature review on process attributes

Based on tabulated combinations SPIF process parameters in Table 2.6, 2.7 and 2.8; it can be

observed that optimization of process parameters including spindle speed, feed rate,

incremental depth and lubrication at tool-sheet interface has been carried out with respect to

formability of specific sheet metals in terms of maximum formable angle and depth,

geometrical accuracy and surface roughness of components formed. Impact of interactions

between various process parameters to obtain quality of customized products formed using

SPIF is of interest of research which may also helpful database to develop dedicated

machines for Incremental Sheet Forming (ISF). In the recent era the research interest is also

to operate SPIF at higher tool rotational speed and feed rate in the benefit of industry as both

are contributing parameters in order to satisfy demand of higher production rate.

Literature Review

32

2.4 Scope of Research

The potential scope of present research work is identified to determine formability of

AMS4902 in terms of thickness distribution, maximum formable angle, fracture Depth

during single pass and multi pass SPIF at room temperature.

The scope of present work also includes determining effect of various parameters and

their interactions influencing formability, geometrical accuracy and surface quality of

components formed out of AMS4902 using SPIF.

AMS4902 possesses HCP structure having three slip systems namely prismatic plane slip,

pyramidal plane slip and basal plane slip. Although the HCP and FCC structures possess

highest atomic packing factor of 0.74 which signifies 74% of volume of unit cell

occupied by atoms, the HCP structured metals are difficult to deform due to limited

number of available active slip systems compare to BCC or FCC structured metals.

Comparing the average properties of AMS4902 with other metals; TABLE 2.9 Comparison of material properties of AMS4902 with other materials

Properties

Material

Steel

(DC-05)

Aluminum

(AA2024-O)

Magnesium

(AZ31) AMS4902

Density (gm/cc) 7.83 2.78 1.77 4.51

Modulus of Elasticity (GPa) 210 73.1 45 105

Yield Strength (MPa) 149.67 80.428 150 345

Ultimate Tensile Strength (MPa) 308.54 166.56 255 485

% Elongation in 50 mm 40 18.73 15 42.53

AMS4902 is having higher yield strength and ultimate strength; hence it is less

stretchable and difficult to form than Magnesium. At the same time, AMS4902 is 56

percent lighter than Steel and most useful in transport sectors like aerospace industries

and automobile industries. As Modulus of Elasticity of AMS4902 is 2.45 times higher

than Magnesium, the Springback for AMS4902 is expected 0.4 times lesser than

Magnesium which may offer more dimensional accuracy of formed component.

Hence, AMS4902 is having potential to develop material characteristics and process

mechanics for conducting parametric investigation using Single Point Incremental

Forming.

Objectives

33

2.5 Objectives

The following objectives are set in order to analyze formability of AMS4902 using Single

Point Incremental Forming.

(1) To decide range of speed, feed, incremental depth and tool diameter for single pass and

multi pass SPIF of AMS4902.

(2) To assess formability of AMS4902 in terms of thickness distribution, maximum formable

wall angle and failure depth (by deforming 1.5 mm thick sheet into 50°, 60° & 70° wall

angle square pyramid in a singlepass and 60° & 70° wall angle square pyramid in

multipass SPIF).

(3) To determine effect of parametric interaction between tool diameter with incremental step depth on formability of AMS4902.

(4) To determine effect of parametric interaction between tool diameter and incremental step depth for single pass SPIF on surface roughness of component formed out of AMS4902.

2.6 Research Methodology

On the basis of scope of presented research, systematic approach has been adapted to

determine formability of 1.5 mm thick sheet of AMS4902 in terms of wall thickness

distribution, maximum formable wall angle and maximum formable depth of pyramids

during single pass and multi pass SPIF. In order to perform SPIF of AMS4902, values of

tool diameter, tool rotational speed, feed rate and incremental depth have been decided

based on literature review. Design of experiments has been formulated to investigate

individual effect of process parameters on formability of AMS4902 sheet as discussed in

chapter: 3. Experimental work is extended to address the research gap identified to

investigate effect of interaction between tool diameter and incremental step depth on

percentage thinning and surface roughness of pyramid walls formed using SPIF. The

critical and interesting findings about geometrical accuracy of pyramid walls formed using

SPIF are discussed in chapter: 4 which are also highlighted in conclusions. Fig. 2.9 depicts

about the flow chart for methodology applied to address the presented research problem.

Literature Review

34

FIGURE 2.9 Flow chart of applied research methodology

Start

Selection of Process Parameters and Design of Experiments

Development of fixture for Single Point Incremental Forming

Experimental Set: 1 Single pass SPIF to form

50owall angle square

Possibility of forming

pyramid up to design

depth

No Do not consider the

parameters for Experimental Set: 2

for which the failure of sheet occurs before

forming up to design depth


50owall angle square

Yes

Measurement of thickness of pyramid walls formed during

Experimental Set: 2

Measurement of surface roughness of pyramid walls formed during Experimental Set: 2

using fixture fabricated to hold the pyramid

Selection of combination of optimum process parameters based on calculated values of minimum average percentage thinning and minimum average surface roughness of pyramid walls


60oand 70owall angle square pyramids

Experimental Set: 4 Multi pass SPIF to form 60oand 70owall angle square pyramids out of already formed pyramids of 50owall angle using optimum process parameters determined

Conclusion

Stop

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Processes, Materials and Manufacturing Processes, 26, 1404–1414.

54. Dwivedy M, Kalluri V (2019) The effect of process parameters on forming forces in

single point incremental forming, Procedia Manufacturing, 29, 120-128.

Experimental Investigations

40

CHAPTER: 3


This chapter discusses about design of experiments, experimental set-up, testing of material

property parameters, circle-grid marking and pilot experiments to demonstrate effect of circle

grid marking on AMS4902 sheet during SPIF. Discussion also includes methodology to

conduct experiments during experimental set: 1, 2 and 3 for single pass SPIF to form 50o, 60o

and 70o wall angle square pyramids of AMS4902 sheet and experimental set: 4 for multi pass

SPIF to form 60o and 70o wall angle pyramids out of already formed AMS4902 square

pyramids of 50o wall angle. Additionally, methodology used to measure thickness and

roughness of pyramid wall has also been explained.

3.1 Design of Experiments

Design of experiments is of prime importance to decide constant and variable input

parameters as well as range and level of variable parameters (factors) before performing

experiments. Based on extensive literature survey, it is decided to investigate the effect of

tool diameter, tool rotational speed, feed rate and incremental depth on formability of 1.5 mm

thick sheet of AMS4902. Other parameters such as sheet thickness, tool end geometry, tool

path, tool material and friction (lubrication) at tool-sheet interface are kept constant.

VALONA 7035 IN high performance neat oil is used as lubricant at tool-sheet interface for

all experiments including pilot experiments. The forming tool is made off AISI304 with

hemispherical end geometry. Contour tool path is selected for all SPIF experiments of

presented research work. Values of input variables such as tool diameters, tool rotational

speeds, feed rates and incremental depths are decided based on findings summarized from

literature review. In order to predict existence of any nonlinear relationship amongst input

variables and responses, three levels of each input variables are selected. This combination of

all four factors at three levels of Taguchi L9 array takes care of reliability of results

reasonably same as results by full factorial experimental set with the economy of material

expenditure and time to perform experiments as well. Table 3.1 shows the experimental

design for total 9 experiments tabulated based on L9 array of Taguchi Design of Experiments

Design of Experiments

41

as an experimental set: 1. As limited literature is found on pyramidal geometry; 50o, 60o and

70o wall angle square pyramids of 30 mm depth are decided as targeted part geometry as

shown in Fig. 3.1.

FIGURE 3.1 Square pyramid as a part geometry

TABLE 3.1

DoE for Experimental Set: 1 to perform singlepass SPIF to form square pyramid of 50o wall angle

Experiment

Tool

Diameter

(mm)

Speed

(rpm)

Feed

(mm/ min)

Incremental

Step Depth

(mm)

1 D1 = 8 S1 = 1250 F1 = 1200 z1 = 0.25

2 D1 = 8 S2 = 2250 F2 = 2600 z2 = 0.5

3 D1 = 8 S3 = 3250 F3 = 4000 z3 = 0.75

4 D2 = 12 S1 = 1250 F2 = 2600 z3 = 0.75

5 D2 = 12 S2 = 2250 F3 = 4000 z1 = 0.25

6 D2 = 12 S3 = 3250 F1 = 1200 z2 = 0.5

7 D3 = 16 S1 = 1250 F3 = 4000 z2 = 0.5

8 D3 = 16 S2 = 2250 F1 = 1200 z3 = 0.75

9 D3 = 16 S3 = 3250 F2 = 2600 z1 = 0.25


42

Based on observations and results obtained for square pyramids formed by SPIF using

parametric combination of experimental set: 1, the parametric study is narrowed down for

experimental set: 2 to derive optimum combination of parameters as mentioned in Table 3.2.

Following parameters are decided as response parameters for fulfillment of objectives of

presented research work with reference to findings concluded from literature survey;

(1) Formability in terms of thickness distribution of pyramid wall formed out of 1.5 mm

thick sheet of AMS4902 during single pass and multi pass SPIF.

(2) Formability in terms of maximum formable wall angle of pyramid formed out of 1.5 mm

thick sheet of AMS4902 during single pass and multi pass SPIF.

(3) Formability in terms of maximum formable depth of pyramid with reference to higher

degree of wall angles formed out of 1.5 mm thick sheet of AMS4902 during single pass

and multi pass SPIF.

(4) Surface roughness of pyramid walls formed out of 1.5 mm thick sheet of AMS4902

during single pass SPIF.

TABLE 3.2 DoE for Experimental Set: 2 to perform singlepass SPIF to form square pyramid of 50o wall angle

Experiment

Tool

Diameter

(mm)

Speed

(rpm)

Feed

(mm/min)

Incremental

Step Depth

(mm)

10 D1=12 S=1250 F=4000 z1=0.25

11 D1=12 S=1250 F=4000 z2=0.5

12 D1=12 S=1250 F=4000 z3=0.75

13 D2=16 S=1250 F=4000 z1=0.25

14 D2=16 S=1250 F=4000 z2=0.5

15 D2=16 S=1250 F=4000 z3=0.75

Next sub section describes about the experimental set-up and a fixture fabricated to hold the

sheet rigidly against movement of hemispherical tip tool during SPIF as there is no need of

dedicated die and punch.

Experimental Set-up

43

3.2 Experimental set-up The presented experiments are performed using fixture capable to hold 250 mm x 250 mm of

maximum size of sheet. The fixture is facilitated with dowel pin holes in order to locate even

small size sheets. The fixture consist of three major parts namely base plate, back plate and

top plate as shown in Fig. 3.2 (a). Back plate is equipped with through square pocket of size

105 mm x 105 mm. AMS4902 sheet of 1.5 mm is hold between top plate and back plate to

form a pyramid of largest square size of 100 mm x 100 mm and depth of 30 mm as depicted

in Fig. 3.1. All experiments are conducted at Central Institute of Plastics Engineering and

Technology (Centre for Skilling and Technical Support), Valsad.

(a) (b)

FIGURE 3.2 Experimental Set-up (a) SPIF Fixture (b) SPIF Set-up on CNC Milling machine

The fabricated SPIF fixture is clamped on CNC Milling machine for present experimental

work as depicted in Fig. 3.2 (b). Specification of CNC Milling machine is given as under.

Make: Ace Manufacturing Systems, Bangalore, India Model: mcv-350

Power: 5.5 kW

Controller: FANUC

Tool travel: X-axis – 1000 mm, Y-axis – 650 mm, Z-axis – 500 mm

Table length: 510 mm, Table width:1150 mm

Groove width:10 mm, Groove spacing: 300 mm


44

3.3 Uni-axial tensile testing to determine properties of AMS4902

(a) (b)

FIGURE 3.3 (a) Tensile test set-up (b) Tested specimens of AMS4902 at 0o, 45o& 90o

TABLE 3.3 Results of tensile testing of AMS4902

Sample No.

Yield Strength (MPa)

(0.2% Proof Stress)

Average Yield Strength (MPa)

(0.2% Proof Stress)

Ultimate Tensile

Strength (MPa)

Average Ultimate Tensile

Strength (MPa)

Test Results for Specimens at 0o of Rolling Direction 95-VN-2016 321.8

323.6

408.6

408.0 96-VN-2016 320.9 406.9

97-VN-2016 328.0 408.5


305.1

396.1

397.1 101-VN-2016 305.7 399.0

102-VN-2016 302.6 396.3


279.6

412.8

413.7 109-VN-2016 278.8 414.6

110-VN-2016 283.5 413.8

Material properties of AMS4902 are obtained by conducting uni-axial tensile test

according to ASTM E8-09 on three test specimens oriented in rolling direction (0o), three

specimens oriented at 45o of rolling direction and three specimens of transverse direction (90o

to the rolling direction). The tensile tests are performed with cross head speed of 5 mm/min

Circle-grid marking

45

and stress-strain graphs are obtained from computer controlled universal testing machine

equipped with extensometer. Fig. 3.3 (a) and (b) shows tensile test set-up with extensometer

and specimens tested respectively. The obtained value of average yield strength and ultimate

strength in each direction is tabulated in Table 3.3 while average values of other important

property parameters like strength co-efficient, strain hardening exponent, elastic modulus and

density are tabulated in Table 3.4 [1]. TABLE 3.4 Other average property parameters of AMS4902 [2, 3]

Sr. No. Property Parameter Calculated value of property parameter

1. Strain Hardening (n) 0.23

2. Strength Coefficient (K) 852.44 MPa

3. Modulus of Elasticity (E) 105x103 MPa

4. Density (ρ) 4.51gm/ cm3

3.4 Circle-grid marking

(a) 2.5mm circles (b) 2.5mm circles with center (c) 5mm circles in 12.5mm Square

(d) 5mm circles in 12.5mm Square (e) 5mm circles with 2.5mm square (f) 5mm square

FIGURE 3.4 Various patterns of Grid Marking [5] Circle-grid marking is one of the popularly used techniques for measurement of

deformation of sheet in terms of major and minor strains. The grid is an array of precisely

spaced gage points marked on a sheet metal before forming into final shape. Appropriate

combination of circle and square grid pattern is preferable in order to obtain minute

measurement of straining. Larger pattern of 6 mm to 125 mm are suggested to use for

measurement of low level strains on formed components but are not preferable to determine


46

Forming Limit Curve (FLC). According to ASTM 2218-02, 2.5 mm square sides or circle

diameters are suggested as a gage length within the range of ±0.025 mm. It is important to

ensure that the grid pattern marked must not be damaged during operating conditions like

friction or lubrication of selected forming process. Varieties of grid patterns are shown in Fig.

3.4 (a) to (f) [4, 5].

According to ASTM E2218-02, the pattern of 5 mm circle diameter with 2.5 mm

square as shown in Fig. 3.4 (e) is selected to apply on AMS4902 sheet. Fig. 3.5 (a) depicts

2D drawing of circle grid pattern prepared using CAD tool to be printed on the central area of

150 mm x 150 mm out of overall blank size of 200 mm x 200 mm x 1 mm and 200 mm x 200

mm x 1.5 mm AMS4902 sheet.

(a)

(b) (c)

FIGURE 3.5 (a) 2D drawing of circle grid pattern (b) Circle-grid printing set-up (c) Circle-grid marking

on AMS4902 sheet

Pilot experiments

47

Several methods of printing of circle-grid pattern on undeformed sheet include Screen

printing (silk-screen printing/ serigraphy), Electrochemical etching, Photochemical etching

(UV Printing) and Laser marking. Laser marking is applied initially for trial to conduct pilot

experiments during presented course of work looking to its accuracy, durability, quality and

resolution. Due to failure of laser marked AMS4902 sheet of 1 mm and 1.5 mm thickness

from circle-grid lines before forming depth during the process of SPIF, it is decided to carry

out circle-grid printing by photochemical etching (UV printing) method as depicted in Fig.

3.5. Fig. 3.5 (b) shows the UV printing set-up for circle-grid marking and 3.5 (c) shows the

circle-grid pattern printed on AMS4902 sheet using photochemical etching (UV printing) [6].

3.5 Pilot experiments

Pilot experiments are conducted to verify feasibility of performing SPIF process by

deforming AMS4902 sheets of 1 mm and 1.5 mm thickness into designed geometry of

truncated pyramid using developed fixture and generated tool path. The purpose of pilot

experiments is also to determine the effect of circle-grid applied by laser marking and

photochemical etching (UV printing) on AMS4902 sheets during SPIF in order to select

appropriate method of circle-grid marking for further experimentations. Initially circle grid

pattern shown in Fig. 3.5 (a) is applied using laser marking technique on 1 mm and 1.5 mm

thick sheet of AMS4902. Then both sheets are deformed into designed geometry of 50o wall

angle square pyramid of 30 mm depth using 8 mm diameter hemispherical tip tool by single-

pass SPIF. The said experiment is performed at the parametric combination of 1250 rpm

speed, 1200 mm/min feed and 0.5 mm incremental depth. The failure depth of 7 mm and 8

mm is observed respectively for 1 mm and 1.5 mm thick sheet of AMS4902, marked with

circle grid pattern using laser marking technique as shown in Fig. 3.6 (a) and (b).

(a) Failure depth of 7 mm for 1 mm thick sheet (b) Failure depth of 8 mm for 1.5 mm thick sheet

FIGURE 3.6 Failure of AMS4902 sheets marked using laser grid marking technique


48

One of the key reasons for failure of AMS4902 sheet at grid lines is high stress concentration

on the grid pattern during SPIF. The failure of sheet during SPIF can be avoided by

controlling higher penetration of laser applied during marking of circle-grid pattern. After the

failure of sheets observed from the laser grid pattern, the circle-grid printing is applied using

photochemical etching (UV printing) on 1.5 mm thick sheet and it is deformed by single-pass

SPIF as shown in Fig. 3.7.

FIGURE 3.7 Single point incremental forming of 1.5mm thick sheet of AMS4902 with good quality of

deformed circle grid pattern printed using UV printing

As depicted in Fig. 3.7, the single point incremental forming of 1.5 mm thick sheet of

AMS4902 is obtained up to 39 mm depth on which the circle grid pattern is printed using

photochemical etching (UV printing) technique. A good quality of deformed circle grid

pattern is observed with measurable gage points for strain measurement. Hence, it is decided

to apply circle grid marking using photochemical etching (UV printing) for further

experimentations as it does not affect the intrinsic properties of material during forming

process.

3.6 Experimental set: 1 (Single-pass SPIF of 50o wall angle pyramids)

Truncated square pyramid of 50o wall angle, 30 mm design depth and largest square of size

100 mm x 100 mm is formed out of 1.5 mm thick sheet of AMS4902 by operating

hemispherical tip tool at parametric combinations as given in Table 3.1 using single-pass

SPIF. Taguchi L9 array is used to get the optimum combination of tool diameter, tool

rotation, tool feed and incremental depth with each at three levels. Range of tool diameters

Experimental set: 1(Single-pass SPIF of 50o wall angle pyramids)

49

are varied from 8 mm to 16 mm with the increment of 4 mm at each level. Values of speed

are varied from 1250 rpm to 3250 rpm with the increment of 1000 rpm at each level.

Variation of feed rate is by 1400 mm/min at each level starting from 1200 mm/min. Variation

of incremental depth is by 0.25 mm at each level starting with 0.25 mm. In order to generate

contour tool path for present experimental work, the square pyramid of desired size (i.e. 100

mm x 100 mm largest size of square and 30 mm depth) and wall angle is modeled first in

MasterCAM software. The planner tool path is generated for defined combinations of speed

and feed starting from largest square of pyramid. After each planner movement of tool, an

incremental movement of tool is given by desired amount in the direction of depth till total

depth of pyramid. Identical setting of SPIF fixture for X-Y movement of table with reference

to Z movement of tool has been taken care using dial gauge before performing each

experiment.

FIGURE 3.8 Components formed using SPIF with parametric combinations of experimental set: 1


50

The forming depth of 30 mm of square pyramid is obtained for experiment no. 1, 4, 5 and 7

out of all nine experiments of experimental set: 1 as shown in Fig. 3.8. SPIF performed at

intermediate or higher tool rotational speed of selected range in combination with lower or

intermediate feeds leads to excessive heat generation and friction at tool-sheet interface

which ultimately results into damage of sheet and tool instead of forming the sheet during

experiment no. 6, 8 and 9. The pinning effect is observed during forming of sheet with 8 mm

hemispherical headed tool during experiment no. 2 and 3. Based on detail discussion on

results of experimental set: 1 reported in next section, the study is narrowed down to

optimum values of tool diameter, tool rotational speed and feed in order to determine

interaction of parameters on thickness distribution, maximum formable wall angle, maximum

formable depth and surface roughness of pyramid with 50o wall angle by single-pass SPIF for

the experimental set: 2 reported as Table 3.2.

3.7 Experimental set: 2 (Single-pass SPIF of 50o wall angle pyramids)

FIGURE 3.9 Components formed using SPIF with parametric combinations of experimental set: 2

Wall thickness measurement

51

As depicted in Fig. 3.9, pyramids of design depth are successfully formed by all presented

combinations of parameters during all six single-pass SPIF experiments of experimental set:

2. The pyramid wall formed at 0.25 mm incremental depth offers less roughness compared to

pyramid walls formed at 0.5 mm and 0.75 mm incremental depth. Hence, Experimental Set: 3

is decided to conduct experiment to deform truncated square pyramids of 60o and 70o wall

angles by employing single-pass SPIF for parametric combination of 12 mm tool diameter,

1250 rpm tool speed, 4000 mm/min feed and 0.25 mm incremental step depth which has

offered minimum thinning and minimum surface roughness during previous set of

experiments.

3.8 Wall thickness measurement

Measurement of wall thickness distribution is carried out using Coordinate Measuring

Machine at Indo German Tool Room, Ahmedabad as shown in Fig. 3.10. Specifications of

CMM used for wall thickness measurement are as under;

Make: Carl Zeiss, Germany Model: PRISMO 5 HTG VAST

Measuring Range: X - 700 mm, Y - 900 mm, Z - 500 mm

Permissible Work piece weight: 1200 Kg

FIGURE 3.10 Wall Thickness measurement of square pyramid using CMM

Wall thickness measurement for all pyramids is carried out by defining plane of sheet with

reference to hole of dowel pin using CMM scanning of fourteen points initially. Similarly,


52

planes of inner surface of walls of each pyramid are also defined by CMM scanning of nine

points with reference to hole provided for dowel pin.

FIGURE 3.11 Deformed pyramid with locations for measurement of wall thickness using CMM

Pyramid wall thickness is measured by CMM probe at four different depths predefined on the

outer surface of wall as shown in Fig. 3.11 with reference to predefined plane of inner surface

of pyramid wall. Average wall thickness for any pyramid is determined by averaging the wall

thickness measured at various depths on all four walls of that pyramid. The readings of

average wall thickness of a pyramid are plotted against depth of measurement.

3.9 Surface roughness measurement

In order to carry out surface roughness measurement of pyramid walls deformed out of

AMS4902 sheet, a dedicated fixture is developed which is capable to hold the pyramid

against the stylus movement of roughness tester as shown in Fig. 3.12. A fixture is also

capable to accommodate variation in pyramid wall angle to ensure contact between stylus and

surface of pyramid wall.

Surface roughness measurement

53

FIGURE 3.12 Fixture developed to hold the pyramid during surface roughness measurement

(a) Calibration of Surface Roughness Tester (b) Roughness Measurement Set-up

FIGURE 3.13 Surface Roughness Measurement for Pyramid Wall A drive unit of surface roughness tester can be used by inserting into display unit and also

can be used by detaching it from display unit. This detachable drive unit is capable to offer

flexible measurement of intricate surface too. Surface roughness measurement of pyramid

walls is followed by calibration of surface roughness tester using developed fixture as shown

in Fig. 3.13 (a) and (b). Average surface roughness for a pyramid wall is calculated from four

readings of Ra value measured for an individual wall of a pyramid. Similarly, average surface

roughness for a pyramid is determined by average of average roughness for all four walls of

that pyramid.


54

3.10 Experimental Set: 3 (Single-pass SPIF of 60o and 70o wall angle

pyramids)

The optimum parameters derived from experimental set 1 and 2 are tabulated in Table 3.5 in

order to deform 1.5 mm thick AMS4902 sheet into 60o and 70o wall angle pyramids by

single-pass SPIF. The detail discussion on failure of higher wall angle pyramids is reported in

next section. TABLE 3.5

DoE for Experimental Set: 3 to perform singlepass SPIF to form square pyramid of 60o and 70o wall angle

Experiment

Wall

Angle

(Degree)

Tool

Diameter

(mm)

Speed

(rpm)

Feed

(mm/min)

Incremental

Step Depth

(mm)

16 60 T=12 S=1250 F=4000 z=0.25

17 70 T=12 S=1250 F=4000 z=0.25

3.11 Experimental Set: 4 (Multi-pass SPIF to form 60o and 70o wall angle

pyramids from 50o wall angle pyramids)

Further the Experimental Set: 4 is conducted for multi-pass SPIF to form 60o and 70o wall

angle square pyramids out of already formed pyramids of 50o wall angle with the parametric

combination same as experimental set: 3.

3.12 Uncertainty in measurement

Any measurement results are incomplete if they are presented without consideration of

uncertainty in the measurement as every measurement is subjected to some uncertainty.

Consideration of uncertainty in measurement confirms the confidence for validity of

measurement results. According to ‘Guide to the Expression of Uncertainty in Measurement’

(GUM), an uncertainty is defined as “a parameter associated with the result of a measurement

that characterizes the dispersion of the value that could reasonably be attributed to the

measurand”. International Vocabulary of Basic and General Terms in Metrology (VIM) have

Uncertainty in measurement

55

defined the measurement uncertainty as “non-negative parameter characterizing the

dispersion of quantity values being attributed to measurand, based on the information used”.

Various sources of measurement uncertainties include measuring instruments, type of

measurand for which measurement need to be carried out, measurement process, sampling,

environmental conditions, skill of operator etc. It is difficult to obtain repeatability and

reproducibility of measurand due to inherent instability of measuring equipment. As per

recommended good practices to reduce measurement uncertainty, four readings of wall

thickness and surface roughness for each wall of pyramids are taken to obtain average value

of measurand for presented course of work. Calibration corrections of measuring instruments/

equipments are also taken care before employing it for measurement. Range of uncertainty

for Coordinate Measuring Machine (PRISMO 5 HTG VAST) used for wall thickness

measurement and Surface Roughness Tester (SURFTEST SJ-210) used for roughness

measurement is discussed in next sub sections [7, 8, 9].

3.12.1 Wall thickness measurement

The pyramid wall thickness measurement for presented course of work is carried out using

CMM model PRISMO 5 HTG VAST (Make: Carl Zeiss, Germany) at Indo German Tool

Room, Ahmedabad. It is important before performing measurement using CMM to establish

probing strategy which includes magnitude and direction of probe force, type of probe stylus

used and the measuring speed of probe. Measurement uncertainty prescribed by the

manufacturer for the said model of CMM is as mentioned in Table 3.6. TABLE 3.6 Measurement uncertainty of CMM model PRISMO 5 HTG VAST prescribed by

manufacturer

Sr. No. Description Measurement uncertainty

1. Uncertainty in linear measurement U1 = (1.5+L/350) µm

2. Uncertainty in volumetric measurement U3 = (2.0+L/300) µm

According to terminology defined in ISO 10360; L is the measured size in millimetres and A

is positive constant in micrometers supplied by manufacturer. The value of A is 1.5 µm in the


56

case of linear measurement and 2.0 µm in the case of volumetric measurement carried out

using CMM PRISMO 5 HTG VAST.

FIGURE 3.14 Maximum range of permissible uncertainty of indication for CMM [10]

The value of dimensionless positive constant (K) supplied by manufacturer is 350 in the case

of linear measurement and 300 in the case of volumetric measurement for CMM PRISMO 5

HTG VAST. Fig. 3.14 signifies the permissible range of uncertainty for the measurement of

pyramid wall thickness using CMM PRISMO 5 HTG VAST in presented work [10].

3.12.2 Surface roughness measurement

Measurement uncertainty of surface roughness tester, SURFTEST SJ-210 (Make: Mitutotyo),

is prescribed as (5.08+10L) µm by manufacturer wherein L is the nominal length in

micrometer. This uncertainty represents expanded uncertainty expressed at 95 percent

confidence level.

Results and discussion based on experiments conducted during all four experimental

sets are presented in Chapter: 4. Effect of parametric interactions observed on formability of

AMS4902 during experimentations are also discussed in detail in the section of results and

discussion.

.

References

57

References: 1. ASTM Designation: E8/ E8M – 09 (2009) Standard Test Methods for Tension Testing of

Metallic Materials, 1-27.

2. ASTM Designation: E646 – 00 (2000) Standard Test Method for Tensile Strain-

Hardening Exponents (n-Values) of Metallic Materials, 1-8.

3. NPL Report DEPC MPE 016 (2005) TESTAND - WP3 Final Report: Modulus

Measurement Methods, 1-41.

4. ASTM Designation: E2218 – 02 (2002) Standard Test Method for Determining Forming

Limit Curves, 1-15.

5. Strain Grid Whitepaper (2013) Universal Marking Systems Ltd. www.ums.co.uk 1-15.

6. Ozturk F, Dilmec M, Turkoz M, Ece R. E. and Halkaci H. S. (2009) Grid Marking and

Measurement Methods for Sheet Metal Formability. 5th International conference and

exhibition on design and production of machines and dies/molds, Kusadasi, Aydin,

Turkey, 1-10.

7. BIPM JCGM 100 (Joint Committee for Guides in Metrology - GUM 1995 with minor

corrections) (2008) Evaluation of measurement data — Guide to the expression of

uncertainty in measurement, 1-134.

8. Stephanie Bell (2001) Measurement Good Practice Guide No. 11 (Issue 2), A Beginner’s

Guide to Uncertainty of Measurement, ISSN 1368-6550, 1-41.

9. NABL 141 (2016) Guidelines for Estimation and Expression of Uncertainty in

Measurement (Issue 3), 1-50.

10. David Flack (2011) NPL: Measurement Good Practice Guide No. 42 – CMM

Verification, ISSN 1368-6550, 1-113.

Results and Discussions

58

CHAPTER: 4


This chapter discusses about individual effects of tool diameter, tool speed, tool feed and

incremental depth on formability of AMS4902 sheet. The results obtained for the effect of

interaction of tool diameter and incremental depth on thickness distribution and surface

roughness during single-pass SPIF of AMS4902 sheet is presented graphically. This chapter

also discusses about results of geometrical accuracy of wall angle formed by single-pass

SPIF, thickness distribution and maximum formable angle during multi-pass SPIF of

AMS4902 sheet.

4.1 Effect of tool diameter, speed and feed (Experimental Set: 1)

Out of nine experiments of Experimental Set: 1, four pyramids with full design depth

of 30 mm are obtained for experiment 1, 4, 5 and 7 as shown in Fig. 3.8. Single pyramid of

full design depth is formed during experiment 1, out of three pyramids formed using 8mm

tool diameter at various parametric combinations of speed, feed and incremental step depth of

selected ranges. The failures of pyramids before design depth are observed at 15 mm and 12

mm depth during experiment 2 and 3 respectively using 8mm diameter tool. Smaller diameter

of tool of 8 mm operated at the combination of higher and intermediate speeds with higher

and intermediate feeds of selected range respectively results into pinning effect on AMS4902

sheet which leads the failure of sheet.

Square pyramids up to design depth are formed successfully during experiment 4 and

5 out of all three experiments conducted with 12 mm diameter tool. Additionally, wavy wall

surface is observed for the pyramid obtained by parametric combination during experiment 5.

Higher speed at lower feed during experiment 6 generates excess local friction and heat

generation due to rubbing at a point of tool sheet interface ultimately results into welding of

tool with sheet blank which does not allow the tool further to carry forward the process of

SPIF even with the tool of 12 mm diameter.

A very good quality square pyramid with good quality of circle grid marking on the

formed pyramid of design depth is obtained for experiment 7 conducted using 16 mm

Influence of tool diameter and incremental depth on average percentage thinning

59

diameter tool. Experiment 8 and 9 performed using larger diameter tool operated at

intermediate and low tool speed in combination with comparatively low feed than experiment

7 results into excess local friction and heat generation due to rubbing. This excess local

friction and rubbing at a point of tool-sheet interface ultimately leads to welding of the tool

with sheet and doesn’t allow tool to further carry forward the process of SPIF.

Based on results of Experimental Set: 1 discussed, the parametric study is narrowed

down to operate SPIF with 12 mm and 16 mm tool diameter at the combination of 1250 rpm

with 4000 mm/min feed to get the uniform wall thickness distribution and minimum surface

roughness.

4.2 Influence of tool diameter and incremental depth on average percentage

thinning

4.2.1 Effect of tool diameter and incremental step depth interaction on

average percentage thinning

Pyramid wall thickness measurement is carried out using CMM for all six pyramids

formed during Experimental Set: 2 as shown in Fig. 3.9. Wall thicknesses of any pyramid are

measured at various depths of pyramid wall. In order to analyze percentage thinning by

stretching, thicknesses measured near top and bottom faces of square pyramids are not

considered as those readings of thickness measurement falls under bending zone. So, four

readings of measured thicknesses on straight zone of each wall are considered to calculate

average sheet thickness for a particular pyramid at specified depths of 7 mm, 12 mm, 17 mm

and 22 mm as shown in Fig. 3.11. Average percentage thinning at specified depth of pyramid

wall is calculated using equation given below based on average thickness results reported in

Table 4.1. The different pattern of metal flow is observed at 7 mm depth on wall A than wall

B, C and D because incremental depth after each planner movement of tool was applied at the

middle of the wall A instead of corner to avoid failure at corner of pyramid.

Average percentage thinning = ቂ୲ି୲୲ቃX 100 (4.1)

Where; (1) to = Original thickness of sheet = 1.5 mm

(2) tf= Average final wall thickness after forming (mm)


60

TABLE 4.1 Results of average thickness measured using CMM

Exp. No.

Readings of thickness measurement using CMM at specified depth Thickness at 7 mm

depth of wall-A (mm) Thickness at 12 mm

depth of wall-B (mm) Thickness at 17 mm

depth of wall-C (mm) Thickness at 22 mm

depth of wall-D (mm) Specific Reading

Avg. Thickness

Specific Reading

Avg. Thickness

Specific Reading

Avg. Thickness

Specific Reading

Avg. Thickness

10

1.2154

1.26

0.6234

1.06

0.5626

1.02

0.5931

1.03 1.3287 1.3019 1.2707 1.2835 1.2986 1.2967 1.2316 1.2122 1.1998 1.032 1.023 1.0323

11

1.0844

1.23

0.5148

1.00

0.8601

0.95

0.9131

0.97 1.3833 1.2909 1.0211 1.012 1.3668 1.238 0.9324 0.9424 1.0887 0.9901 1.003 1.021

12

1.3952

1.19

0.9185

0.91

0.849

0.87

0.649

0.89 1.2006 1.0384 1.0529 1.0852 1.0708 0.9294 0.848 0.9038 1.0939 0.7731 0.7153 0.9043

13

1.1562

1.14

0.9411

0.85

0.7543

0.73

1.0112

1.03 1.0997 0.8234 0.6644 0.9753 1.174 0.8184 0.7431 1.0224 1.1309 0.8315 0.7623 1.121

14

1.4533

1.21

0.9274

0.80

0.6958

0.74

0.6905

1.04 1.0738 0.7206 0.63 0.9418 1.2489 0.8225 0.8037 1.2199 1.0641 0.7079 0.8102 1.3004

15

1.0676

1.08

0.8268

0.73

0.4394

0.58

0.501

0.94 1.0701 0.6832 0.5749 0.9148 1.0259 0.7361 0.5641 1.2913 1.142 0.665 0.7382 1.0693

Average percentage thinning for corresponding depth and measured thickness of

pyramid wall is reported in table 4.2 and also plotted graphically as shown in Fig. 4.1 (a), (b),

(c) and Fig. 4.2 (a), (b) to determine effect of tool diameter and incremental depth interaction

on average percentage thinning of pyramid wall formed out of AMS4902 sheet. Fig. 4.1 (a),

(b) and (c) presents the effect of 12 mm and 16 mm diameters of tools on average percentage

thinning obtained at specified depths of pyramids formed when operated at incremental step

depths of 0.25 mm, 0.50 mm and 0.75 mm respectively. Looking to the graphs, it is observed

that 12 mm diameter tool offers less thinning and also reasonably


61

uniform thickness distribution compared to 16 mm diameter tool irrespective of values of the

incremental depths out of selected range due to surface contact of tools with sheet blank.

Looking to the values of average percentage thinning reported in Table 4.2,

significant increment in average percentage thinning is observed at 7 mm and 12 mm depth

of measurement compared to 17 mm depth of measurement for the pyramid walls formed

using 12 mm diameter tool of hemispherical tip for experiment 10, 11 and 12 respectively.

The increment of incremental step depth by an amount of 0.25 mm for subsequent

experiments of 10, 11 and 12 is the influencing parameter for increment in average

percentage thinning of pyramid walls formed using 12 mm diameter tool. The reduction rate

in increment of average percentage thinning at 17 mm depth of measurement for experiment

10, 11 and 12 is caused due to strain hardening. Further reduction in percentage thinning of

pyramid wall at 22 mm depth of measurement leads to increment in average thickness of

pyramid wall by small amount compared to average thickness reported at 17 mm depth of

measurement. TABLE 4.2 Average percentage thinning for corresponding average wall thickness after forming

Exp. No.

Average percentage thinning of pyramid wall at specified depth At 7 mm depth of pyramid wall-A

(mm)

At 12 mm depth of pyramid wall-B

(mm)

At 17 mm depth of pyramid wall-C

(mm)

At 22 mm depth of pyramid wall-D

(mm) Avg.

Thickness %

Thinning Avg.

Thickness %

Thinning Avg.

Thickness %

Thinning Avg.

Thickness %

Thinning 10 1.26 16.00 1.06 29.33 1.02 32.00 1.03 31.33

11 1.23 18.00 1.00 33.60 0.95 36.40 0.97 35.27

12 1.19 20.66 0.91 39.01 0.87 42.25 0.89 40.96

13 1.14 23.73 0.85 43.33 0.73 51.67 1.03 31.19

14 1.21 19.33 0.80 46.67 0.74 50.67 1.04 30.67

15 1.08 28.24 0.73 51.48 0.58 61.39 0.94 37.06

Effect of tool diameter can be observed by comparing the reduction in average wall

thickness of pyramids formed during experiments 13, 14 and 15 with average wall thickness

of pyramids formed during experiment 10, 11 and 12. It is observed that the reduction in

average wall thickness measured at 7 mm, 12 mm and 17 mm depths for pyramids formed

using 16 mm diameter tool is higher than the pyramids formed using 12 mm diameter tool

which ultimately results into more percentage thinning of pyramid walls formed using larger

diameter tool. The little increment in average thickness of pyramid wall at 22 mm dept


62

measurement is also observed for experiments 13, 14 and 15 same as experiments 10, 11 and

12. It is preferable to use 12 mm diameter tool over 16 mm diameter tool in order to obtain

less thinning of pyramid walls and comparatively uniform thickness distribution based on

overall observations reported in Table 4.2.

(a) Effect of tool diameters on average percentage thinning of pyramid wall at 0.25 mm step depth

(b) Effect of tool diameters on average percentage thinning of pyramid wall at 0.50 mm step depth

(c) Effect of tool diameters on average percentage thinning of pyramid wall at 0.75 mm step depth FIGURE 4.1Effect of tool diameter on Average Percentage Thinning for same incremental step depth


63

Fig. 4.2 (a) and (b) explains effect of incremental step depth on average percentage thinning

obtained at various depths of pyramids formed when operated with 12 mm and 16 mm

diameters of tools. The average percentage thinning is observed maximum for the SPIF

performed using 0.75 mm incremental step depth irrespective of tool diameters. In the case of

experiment 10 performed using 12 mm diameter tool in combination with 0.25 mm

incremental step depth offers minimum thinning compared to 0.50 mm and 0.75 mm

incremental step depths used for experiment 11 and 12 with 12 mm diameter tool. Looking to

Fig. 4.2 (b), it can be observed that SPIF performed using 16 mm diameter tool in

combination with 0.25 mm and 0.50 mm incremental depth during experiment 13 and 14

offers almost equal thinning.

(a) Effect of incremental depths on average percentage thinning of pyramid wall for 12 mm diameter tool

(b) Effect of incremental depths on average percentage thinning of pyramid wall for 16 mm diameter tool

FIGURE 4.2 Effect of incremental depths on Average Percentage Thinning for same diameter of tool


64

4.2.2 Results of ANOVA for average percentage thinning

Results of Analysis of Variance (ANOVA) for average percentage thinning of pyramid walls

are reported in Table 4.3. TABLE 4.3 Results of ANOVA for average percentage thinning of pyramid walls formed by SPIF

Factors DF Seq SS Adj SS Adj MS F-Value P-Value Percentage Contribution

Tool Diameter 1 105.462 105.462 105.462 44.31 0.022 58.96

Incremental Step Depth 2 68.661 68.661 34.33 14.42 0.065 38.38

Error 2 4.761 4.761 2.38 - - 2.66

Total 5 178.884 - - - - -

(a) Main effects plot for means of average percentage thinning

1612

40

38

36

34

32

300.750.500.25

Tool Diameter (mm)

Mea

n of

Mea

ns

Incremental Step Depth (mm)

Main Effects Plot for MeansData Means


65

(b) Main effects plot for signal to noise ratio of average percentage thinning

(c) Main effects plot for standard deviations of average percentage thinning

FIGURE 4.3 Main effects plots for means, SN ratios and standard deviations of average percentage

thinning

1612

-30.0

-30.5

-31.0

-31.5

-32.0

-32.50.750.500.25

Tool Diameter (mm)

Mea

n of

SN

rati

os


Main Effects Plot for SN ratiosData Means

Signal-to-noise: Smaller is better

1612

14

13

12

11

10

9

0.750.500.25

Tool Diameter (mm)

Mea

n of

StD

evs


Main Effects Plot for StDevsData Means


66

Percentage contribution of each factor influencing average percentage thinning is determined

based on factor sum and total sum of squares. Based on values of percentage contribution

obtained, tool diameter is found most significant parameter influencing average percentage

sheet thinning. As the P-value obtained for the incremental step depth is more than 0.05, it is

statistically less significant than tool diameter. The main effects plot for means, signal to

noise ratios and standard deviations shown in Fig. 4.3 (a), (b) and (c) also signify the effect of

tool diameter on average percentage thinning over incremental step depth. Based on the main

effects plot, the optimum value obtained for the tool diameter is 12 mm in order to obtain

uniform and minimum thinning of sheet. The larger surface contact of 16 mm diameter tool

with sheet blank generates higher amount of local heating which ultimately results into more

plastic flow of metal and more thinning compare to 12 mm diameter tool. Effect of

incremental depth is less compare to tool diameter on average percentage thinning because

incremental depth is applied at an instance of every cycle of tool movement while tool

diameter remains in touch with sheet material to form a wall throughout the cycle of tool

movement.

4.3 Influence of tool diameter and incremental depth on average surface

roughness

4.3.1 Effect of tool diameter and incremental step depth interaction on

average surface roughness (Ra)

Four readings of surface roughness for each pyramid wall are obtained using

Mitutoyo surface roughness tester, SURFTEST SJ-210 as shown in Fig. 3.13 (b). Average

surface roughness of each pyramid wall is calculated from four readings of surface roughness

obtained as reported in Table 4.4. Overall average roughness for a pyramid is determined by

averaging the average roughness calculated for all four walls of that pyramid as reported in

Table 4.5 and are also presented graphically in Fig. 4.4 (a) and (b). Graphs describe an

interactive effect of tool diameter and incremental step depth on surface roughness of three

pyramids formed using 12 mm diameter tool and three pyramids formed using 16 mm

diameter tool. From Fig. 4.4 (a) and (b), it is observed that irrespective of any of the tool

diameter operated at 0.25 mm incremental step depth offers less surface roughness compared

Influence of tool diameter and incremental depth on average surface roughness

67

to 0.50 mm and 0.75 mm incremental step depth. Surface roughness of the pyramid walls is

found highest while forming is performed using 12 mm diameter tool in combination with

incremental depth of 0.50 mm compared to 0.25 mm and 0.75 mm incremental depths as

depicted in Fig. 4.4 (a). While in the case of SPIF performed using 16 mm diameter tool,

surface roughness is observed highest for the forming condition at 0.75 mm incremental

depth compared to 0.25 mm and 0.50 mm incremental depths as shown in Fig. 4.4 (b). TABLE 4.4 Results of average surface roughness for individual walls of pyramids

Exp. No.

Readings of surface roughness measured using surface roughness tester Roughness readings on WallA (Ra-µm)

Roughness readings on WallB (Ra-µm)

Roughness readings on Wall C (Ra-µm)

Roughness readings on WallD (Ra-µm)

Specific Reading

Avg. Roughness

Specific Reading

Avg. Roughness

Specific Reading

Avg. Roughness

Specific Reading

Avg. Roughness

10

6.418

5.773

6.263

5.888

5.973

5.818

6.371

5.875 4.821 6.333 5.904 5.702 5.895 4.983 4.861 5.84 5.957 5.973 6.532 5.586

11

10.091

9.378

10.731

12.220

9.272

10.325

11.957

10.278 8.09 11.837 11.695 9.117 9.173 11.538 9.609 9.591

10.157 14.772 10.725 10.446

12

12.168

9.779

9.914

10.239

9.414

9.793

8.209

9.890 9.287 11.506 9.78 9.751 8.621 9.641 9.801 11.319 9.038 9.896 10.176 10.279

13

4.016

3.791

3.884

3.882

3.632

3.455

2.847

3.195 3.011 3.005 3.755 2.974 3.717 3.742 3.093 3.227 4.419 4.897 3.339 3.733

14

6.370

5.792

5.486

5.802

6.615

5.779

6.051

5.958 4.603 4.924 5.373 4.867 6.659 6.029 5.891 5.450 5.536 6.769 5.237 7.465

15

7.341

7.652

6.816

7.498

5.853

7.162

7.453

6.781 6.645 7.529 8.167 6.178 8.167 7.002 7.176 6.165 8.453 8.646 7.45 7.328

Looking to the values of average surface roughness reported in table 4.5 and plotted

in Fig. 4.4 (a) and (b); it is observed that irrespective of any of the values of incremental step


68

depths, average surface roughness of pyramid wall is less for the pyramids formed using 16

mm diameter tool compared to pyramids formed using 12 mm diameter tool. Less deflection

of larger diameter tool during operation due to its rigidity and larger contact area at tool-sheet

interface are the key reasons for the less surface roughness of pyramid walls formed using 16

mm diameter tool. TABLE 4.5 Results of average surface roughness for various pyramids formed using SPIF

Exp. No.

Average roughness of Wall-A (Ra-µm)

Average roughness of

Wall-B (Ra-µm)


Wall-C (Ra-µm)


Wall-D (Ra-µm)


pyramid (Ra-µm)

10 5.773 5.888 5.818 5.875 5.838

11 9.378 12.220 10.325 10.278 10.550

12 9.779 10.239 9.793 9.890 9.925

13 3.791 3.882 3.455 3.195 3.581

14 5.792 5.802 5.779 5.958 5.833

15 7.652 7.498 7.162 6.781 7.273

FIGURE 4.4 Effect of interaction of incremental depths and tool diameters on Average Surface

Roughness

4.3.2 Results of ANOVA for average surface roughness

Results of Analysis of Variance (ANOVA) for average surface roughness of pyramid walls

are reported in Table 4.6.

Influence of tool diameter and incremental depth on average surface roughness

69

TABLE 4.6 Response table of signal to noise ratios for surface roughness

Source DF Seq SS Adj SS Adj MS F-Value P-Value Percentage Contribution

Tool Diameter 1 15.447 15.447 15.447 17.7 0.052 43.55

Incremental Step Depth 2 18.279 18.279 9.14 10.47 0.087 51.53

Error 2 1.745 1.745 0.873 - - 4.92

Total 5 35.472 - - - - -

(a) Main effects plot for means of average surface roughness

1612

9

8

7

6

5

0.750.500.25

Tool Diameter (mm)

Mea

n of

Mea

ns


Main Effects Plot for MeansData Means


70

(b) Main effects plot for signal to noise ratio of average surface roughness

(c) Main effects plot for standard deviations of average surface roughness

FIGURE 4.5 Main effects plots for means, SN ratios and standard deviations of average surface

roughness

1612

-13

-14

-15

-16

-17

-18

-190.750.500.25

Tool Diameter (mm)M

ean

of S

N ra

tios


Main Effects Plot for SN ratiosData Means

Signal-to-noise: Smaller is better

1612

0.7

0.6

0.5

0.4

0.3

0.2

0.750.500.25

Tool Diameter (mm)

Mea

n of

StD

evs


Main Effects Plot for StDevsData Means

Geometrical accuracy

71

Percentage contribution of each factor influencing average surface roughness is determined

based on factor sum and total sum of squares. Based on values of percentage contribution

obtained, incremental step depth is found most significant parameter influencing average

surface roughness of pyramid walls. The main effects plot for means, signal to noise ratios

and standard deviations shown in Fig. 4.5 (a), (b) and (c) also signify the effect of

incremental step depth on average surface roughness over tool diameter. Based on the main

effects plot, the optimum value obtained for the incremental step depth is 0.25 mm in order to

obtain minimum surface roughness of pyramid walls. Lesser stair casing effect for small

amount of incremental depth results into less surface roughness of pyramid wall. 16 mm

diameter tool exhibits less surface roughness compare to 12 mm diameter tool for same

amount of incremental depth due to larger contact area at tool-sheet interface and rigidity of

tool.

4.4 Geometrical accuracy

Measurement of pyramid wall angle is carried out using data scanned for pyramid wall by

CMM. The IGES file of data scanned for pyramid walls is used to convert it into part file of

modeling software named as Creo 2.0. The measurement of all four walls (A, B, C and D) of

square pyramids formed using 12 mm and 16 mm diameter tools with 0.25 mm incremental

depth is as shown in Fig. 4.6 (a) and (b). A representative case for the measured values of

wall angles for pyramids formed using 12 mm and 16 mm diameters of tools in combination

with 0.25 mm incremental depth is only presented in Fig. 4.6 (a) and (b) respectively as the

measured values of wall angles reported in Table 4.7 are same for the combinations even with

0.50 mm and 0.75 mm incremental step depth. Wall A, B, C and D are defined with color

code of Blue, Green, Red and Yellow respectively and angle of each wall with horizontal is

measured. Results of angles measured for all four walls of each pyramid formed during

experimental set: 2 are reported in Table 4.7. Table 4.8 shows average value of wall angles

calculated for wall A, B, C and D of square pyramids formed using 12 mm and 16 mm

diameter tools and same has been plotted graphically in Fig. 4.7. Geometrical accuracy in

terms of pyramid wall angle is observed much closer to the targeted geometry of 50o wall

angle of pyramids formed using 16 mm diameter hemispherical tip tool compared to the

pyramid walls formed using 12 mm diameter tool. More springback is observed in pyramid

walls formed using smaller diameter tool compared to larger diameter tool of selected range.


72

Based on the measured and reported values of pyramid wall angles, it can be said that larger

diameter tool is capable to form geometrically accurate component compared to smaller

diameter tool.

FIGURE 4.6 (a) Wall angle measurements for a pyramid formed using 12 mm diameter of tool and 0.25

mm incremental depth

Geometrical accuracy

73

FIGURE 4.6 (b) Wall angle measurements for a pyramid formed using 16 mm diameter of tool and 0.25

mm incremental depth

Additionally, strain hardening and tool deflection can also be considerable factor for

geometrical deviation between target and actual wall angle of pyramid. Continuous yielding

of sheet becomes difficult at higher rate of strain hardening for hard material which requires

higher load and energy to deform the sheet. Deflection of smaller diameter tool during

operation of SPIF against hard sheet metal also leads to deviation of geometrical accuracy.


74

TABLE 4.7 Results of wall angles measured for pyramids formed using SPIF during various experiments

Exp. No.

Tool Diameter

(mm)

Incremental Step Depth

(mm)

Wall Angle of Wall-A (Degree)

Wall Angle of Wall-B (Degree)

Wall Angle of Wall-C (Degree)

Wall Angle of Wall-D (Degree)

10 12 0.25 37.99 37.70 37.29 38.00

11 12 0.5 37.99 37.70 37.29 38.00

12 12 0.75 37.99 37.70 37.29 38.00

13 16 0.25 49.10 49.40 49.50 49.00

14 16 0.5 49.10 49.40 49.96 49.00

15 16 0.75 49.10 49.40 49.96 49.00

TABLE 4.8 Results of average wall angles measured for various pyramids formed using SPIF

Exp. No.

Tool Diameter

(mm)

Incremental Step Depth

(mm)

Avg. wall angle of Wall-A

(Degree)

Avg. wall angle of Wall-B

(Degree)

Avg. wall angle of Wall-C

(Degree)

Avg. wall angle of Wall-D

(Degree) 10, 11, 12 12 0.25, 0.5, 0.75 37.99 37.70 37.29 38.00

13, 14, 15 16 0.25, 0.5, 0.75 49.10 49.40 49.81 49.00

FIGURE 4.7 Average wall angles of pyramid walls formed using 12 mm and 16 mm diameter tools

Results and discussions on forming of 60o and 70o wall angle square pyramids by single-pass SPIF

75

4.5 Results and discussions on forming of 60o and 70o wall angle square

pyramids by single-pass SPIF

Square pyramids of 60o and 70o wall angles are attempted to form using 12 mm diameter

hemispherical headed tool with 0.25 mm incremental step depth at 1250 rpm and 4000

mm/min by single-pass SPIF.

(a) Failure of 60o wall angle square pyramid (b) Failure of 70o wall angle square pyramid

FIGURE 4.8 Single-pass SPIF for 60o and 70o wall angle square pyramids

Failure of 60o wall angle pyramid is observed at 8 mm depth while failure of 70o wall angle

pyramid is observed at 7 mm depth as shown in Fig. 4.8 (a) and (b) respectively. Looking to

the results, it can be derived that wall angle is also an important limiting parameter for single

point incremental forming of difficult to form material like AMS4902.

4.6 Results and discussions on percentage thinning of 60o and 70o wall angle

square pyramids formed out of 50o wall angle pyramids by multi-pass SPIF

Square pyramids of 50o wall angle formed with 12 mm diameter hemispherical

headed tool operated at 1250 rpm, 4000 mm/min and 0.25 mm incremental step depth are

employed for multi-pass SPIF to get further forming of 60o and 70o wall angle pyramids

using the same parametric combination.


76

4.6.1 Percentage thinning of 60o wall angle pyramid formed by multi-pass

SPIF

Multi-pass SPIF with 12 mm diameter tool, 0.25 mm incremental depth, 1250 rpm speed and

4000 mm/min feed is employed to form 60o wall angle pyramid out of already formed

pyramid with 50owall angle. Pyramid with 60o wall angle is successfully formed up to initial

13 mm depth of 50o wall angle square pyramid as shown in Fig. 4.9 (a).

(a) 60o wall angle pyramid formed out of 50o wall angle pyramid using multi-pass SPIF

(b) Measurement of wall thickness of 60o wall angle pyramid using point micrometer for the wall

angle formed by multi-pass SPIF

Results and discussions on percentage thinning of 60o and 70o wall angle square pyramids formed out of 50o wall angle pyramids by multi-pass SPIF

77

(c) Average percentage thinning Vs Component depth

FIGURE 4.9 Pyramid of 60o wall angle formed using multi-pass SPIF and average percentage

thinning

Thickness measurement for the pyramid of 60o wall angle obtained by multi-pass SPIF is

carried out using point micrometer as shown in Fig. 4.9 (b). The maximum amount of

average percentage thinning is 86.67 percent at 12 mm depth of measurement as shown in

Fig. 4.9 (c). No failure is observed during multi-pass SPIF to convert 50o wall angle pyramid

into 60o wall angle pyramid. The obtained results lead to conclusion that it is preferable to

apply multi-pass SPIF to form higher angle of pyramid out of difficult to form materials such

as AMS4902 in cold forming condition.

4.6.2 Percentage thinning of 70o wall angle pyramid formed by multi-pass

SPIF

Fig. 4.10 (a) shows the pyramid with wall angle of 70o deformed by multi-pass SPIF out of

50o wall angle pyramid with the same combination of operating parameters. The maximum

amount of average percentage thinning for the pyramid with 70o wall angle reaches up to

98.67 percent at 12 mm depth of measurement as shown in Fig. 4.10 (b). This higher amount

of average percentage thinning for higher angle pyramid walls lead to crack propagation due

to necking of sheet which ultimately results into failure of pyramid walls. The failure depth of

13 mm is observed for 70o wall angle pyramid which is double than 70o wall angle pyramid

formed by single-pass SPIF.


78

(a) 70o wall angle pyramid formed out of 50o wall angle pyramid using multi-pass SPIF

(b) Average percentage thinning Vs Component Depth

FIGURE 4.10 Pyramid of 70o wall angle formed using multi-pass SPIF and average percentage thinning

Based on results and discussion, the conclusion and future scope of presented work are

summarized in next chapter.

Conclusions

79

CHAPTER: 5

Conclusions and Future Scope

5.1 Conclusions

The outcome of this research encompasses effects of speed, feed, tool diameter and

incremental step depth on formability of AMS4902 sheet in terms of maximum formable

depth during single-pass Single Point Incremental Forming (SPIF). The effect of parametric

interactions between tool diameter and incremental step depth on average percentage thinning

and average surface roughness of pyramid walls formed out of AMS4902 sheet using single-

pass SPIF is an integral contribution of this presented research. The results of experimental

work also comprise of effect of tool diameter on geometrical accuracy of square pyramids,

formed using single-pass SPIF, in terms of wall angle. The research efforts have been

extended to determine maximum formable wall angle of pyramid walls during single-pass

and multi-pass SPIF performed using optimum values of process parameters derived from

previous set of experiments. Following conclusions are drawn based on results of

experimental work reported in previous chapter.

5.1.1 Effect of tool diameter, speed, feed and incremental step depth on

maximum formable depth of AMS4902 sheet during single-pass SPIF

At the combination of higher speed (3250 rpm) with higher feed (4000 mm/min) and

intermediate speed (2250 rpm) with intermediate feed (2600 mm/min) of selected range,

8 mm diameter tool tend to offer pinning effect on AMS4902 sheet which ultimately

resulted into failure of pyramid wall before targeted depth of forming.

Failure of pyramid was observed due to rapid removal of material from AMS4902 sheet

when smaller diameter hemispherical headed tool is operated at the combination of higher

speed and higher feed of selected range.

Single point incremental forming of AMS4902 sheet at much higher speed with lower

feed is not conducive as excess local friction and heat generation due to rubbing at a point


80

of tool-sheet interface leads to weld the tool with the sheet which does not allow tool to

carry forward the process of forming.

Wavy wall surface is observed for the pyramid of full depth formed with the combination

of intermediate speed (2250 rpm) and higher feed (4000 mm/min) out of selected range

even with medium size tool diameter (12 mm) and lower incremental depth (0.25 mm).

Interaction of tool rotational speed and feed plays vital role to satisfy the

thermomechanical demand to form AMS4902 sheet at reduced operating forming force

which ultimately leads to improved tool life, maintain dimensional accuracy and surface

quality of pyramid of AMS4902 formed by SPIF.

5.1.2 Effect of parametric interactions between tool diameter and

incremental step depth on average percentage thinning of pyramid walls

formed out of AMS4902 sheet using single-pass SPIF

Tool diameter is found as significant parameter influencing average percentage thinningof

pyramid walls formed out of AMS4902 sheet as its percentage contribution is higher

(58.96%) than percentage contribution of incremental step depth (38.38%).

Irrespective of any of the values of the incremental step depths of selected range, 12 mm

diameter tool offers comparatively uniform thinning than 16 mm diameter tool.

Interaction of 12 mm diameter tool with 0.25 mm incremental depth offers minimum

thinning compared to combination with 0.50 mm and 0.75 mm incremental step depths.

Irrespective of any of the values of tool diameters of selected range, combination with

0.75 mm incremental step depth offers maximum thinning.

5.1.3 Effect of parametric interactions between tool diameter and

incremental step depth on average surface roughness of pyramid walls

formed out of AMS4902 sheet using single-pass SPIF

Incremental step depth is found as significant parameter influencing average surface

roughness of pyramid walls formed out of AMS4902 sheet as its percentage contribution

is higher (51.53%) than percentage contribution of tool diameter (43.55%).

Conclusions

81

Irrespective of any of the values of the incremental step depths of selected range, average

surface roughness of pyramid wall is less for the pyramids formed using 16 mm diameter

tool compared to pyramids formed using 12 mm diameter tool.

Interaction of 16 mm diameter tool with 0.25 mm incremental depth offers minimum

roughness compared to combination with 0.50 mm and 0.75 mm incremental step depths.

Combination of 12 mm diameter tool with 0.50 mm incremental step depth and

combination of 16 mm diameter tool with 0.75 mm incremental step depth offers

maximum surface roughness.

5.1.4 Effect of tool diameter on geometrical accuracy of pyramid wall

angles formed out of AMS4902 sheet using single-pass SPIF

Geometrical accuracy in terms of pyramid wall angle is observed much closer to the

targeted geometry of 50o wall angle of pyramids formed using 16 mm diameter

hemispherical tip tool compared to the pyramid walls formed using 12 mm diameter tool.

More spring back is observed in pyramid walls formed using smaller diameter (12 mm)

tool compared to larger diameter (16 mm) tool of selected range.

5.1.5 Effect of optimum process parameters to form 60o and 70o wall angle

pyramids using single-pass SPIF

Failure depth of 8 mm is observed for 60o wall angle pyramid during single-pass SPIF of

AMS4902 sheet performed using optimum process parameters derived from previous set

of experiments (12 mm diameter tool, 1250 rpm speed, 4000 mm/min feed and 0.25 mm

incremental step depth).

Failure depth of 7 mm is observed for 70o wall angle pyramid during single-pass SPIF of

AMS4902 sheet performed using optimum process parameters derived from previous set

of experiments (12 mm diameter tool, 1250 rpm speed, 4000 mm/min feed and 0.25 mm

incremental step depth).


82

5.1.6 Effect of optimum process parameters to form 60o and 70o wall angle

pyramids out of already formed pyramids of 50o wall angle using multi-

pass SPIF

Pyramid with 60o wall angle is successfully formed up to initial 13 mm depth of 50o wall

angle square pyramid without failure during multi-pass SPIF using optimum process

parameters derived from previous set of experiments.

The maximum average percentage thinning does not exceed 87 percent for 60o wall angle

pyramid formed out of 50o wall angle pyramid using multi-pass SPIF which leads to

conclusion that it is preferable to apply multi-pass SPIF to form higher angle of pyramid

out of difficult to form materials such as AMS4902 in cold forming condition.

The failure depth of 13 mm is observed for 70o wall angle pyramid formed using multi-

pass SPIF of already formed pyramid of 50o wall angle which is almost double than 70o

wall angle pyramid formed by single-pass SPIF of AMS4902 sheet.

5.2 Future Scope

Effect of tool end geometry on average percentage thinning, geometrical accuracy and

surface roughness of component formed using SPIF at room temperature is the extended

scope of research in future.

Potential scope in future is also to include the study on effect of spiral and planer tool

path in order to obtain geometrical accuracy along with uniform thickness distribution of

component formed using SPIF at room temperature in comparison with hot incremental

forming.

Validation of results of average percentage thinning and average surface roughness of

present experimental work with the results of numerical simulation using the same

property parameters determined by UTM testing.

List of Publications

83

List of Publications

(a) Publications in International Journals

1. Shah HN, Trivedi SV, Gandhi AH (2018) Significance of parameters influencing

surface roughness during Incremental Sheet Forming of AISI202, International

Journal of Engineering, Technology, Science and Research (IJETSR), Vol. 5, Issue 4,

ISSN 2394-3386, pp. 700-708.

2. Trivedi SV, Gandhi AH (2019) Investigation of Grid Marking Techniques to assess

formability of AMS4902 Sheet formed using Single Point Incremental Forming,

Journal of Emerging Technologies and Innovative Research (JETIR), Vol. 6, Issue 3,

ISSN 2349-5162, pp. 105-110.

(b) Publications in National Conferences

1. Bhatt PR, Trivedi SV, Gandhi AH (2016) An Incremental Sheet Forming Process: Its

Need, Application and Characteristics, 7th National Conference on Emerging Vistas

of Technology (NCEVT) - Smart Innovations in Mechanical and Allied Engineering,

ISBN: 978-93-85777-49-3, pp. 180-184.

2. Gandhi PA, Trivedi SV, Gandhi AH (2017) Numerical investigation of effect of wall

geometry on formability of parts in single point incremental forming process, National

conference on Progress, Research and Innovation in Mechanical Engineering

(PRIME) SCET - Multidisciplinary Conference on Engineering and Technology,

ISBN: 978-81-933591-5-0, pp. 1-6.

Documents

PARAMETRIC INVESTIGATION ON SINGLE POINT INCREMENTAL ...... · x Thesis Approval Form The viva-voce of the PhD Thesis submitted by Shri Snehal Viranchibhai Trivedi (Enrollment No