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ENGINEERING RESEARCH PROJECT

RESEARCH PROJECT - 2016

ANALYSIS OF TIG WELDED SP-700

TITANIUM ALLOY

Student: Nisarg D. Parekh

Student ID: 15892846

Supervisor: Timotius Pasang

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ACKNOWLEDGMENTS

1. Associate professor Timotius Pasang for his great knowledge in material

science and inculcation on my project every week which helped to keep my

work going along the right track throughout the course.

2. Mark Masterton and Jim Crossen for their ultimate patience and kind

suggestions in repeated technical services and support.

3. Stainless Design, Hamilton for their help with laser cutting the specimen and

explanations on material machining process.

4. Patrick Conor for his wisdom about micro world and manipulation with scanning

electron microscopy work.

I am truly grateful to all the mentioned faculties of Auckland University of Technology

for their rightful guidance, selfless helps, wise advices, great patience and plentiful

time spent to help me complete my project in half year course. I am also thankful to

the fabrication and designing company Stainless Designs to help me with the laser

cutting of samples. My greatest appreciation goes to school of engineering at AUT for

sharing of all experimental facilities throughout the journey.

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DECLARATION OF ORIGINALITY

I Nisarg Dipakbhai Parekh hereby declare that the work presented in this report, is

entirely my own, expect as acknowledged in the text or references. I also declare that

this work has not been submitted previously, either in whole or in part, for a degree at

this or any other institution.

Signed: _______________________

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ABSTRACT

This study investigates the microstructure and strength of TIG welded Titanium alloy

SP700. Microstructure of the weld were examined under scanning electron

microscopy and optical microscopy. To evaluate hardness of the joint the material

were tested under micro-hardness tests and to observe the tensile strength of the

joints the specimens were passed through the tensile test. To study the material

behavior under different heat treatment process the samples were heat treated at

590°c, 840°c and 950°c. Some samples were air cooled while others passed through

the aging process at 550°c. To attain the excellent TIG weld properties some pilot tests

were done before welding the test samples.

Keywords: Titanium, SP-700, microstructure, hardness, ultimate tensile strength, TIG

welding process, Heat treatment of Titanium alloy.

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TABLE OF CONTENTS

1. Introduction……………………………………………….………………………………. 6

1.1 Purpose of selection of Titanium alloy ………..………….………………….. 6

1.2 Basics of TIG Welding ………………………………………………………… 8

2. Literature Critique ……………………………………………………………………… 10

3. Experimental Setup ……………………………………………………………………..23

3.1 Post Weld Heat Treatment ………...………………………………………… 24

3.2 Fabrication and Mounting of Specimen ……………………………….…… 25

3.3 Micro-Hardness Test ………………………………………………………… 27

3.4 Ultimate Tensile Strength Test ……………………………………………… 28

4. Results and Discussion ……………………………………………………………….. 29

4.1 Ultimate Tensile Test Result ……………………………………………..…. 29

4.2 Hardness Test Results ……………………………………………….……… 31

4.3 Scanning Electron Microscopy ……………………………………………… 33

4.4 Optical Microscopy of Mounted Samples ………..…………………………. 38

5. Conclusion …………………………………………………..…………………………. 43

6. References …………………………………….……………………………………….. 45

Appendix …………………...……………………………………………………………….48

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1. INRODUCTION

1.1 Purpose of selection of Titanium alloy

In today’s highly progressive human civilization there is a desperate demand of

materials that could fit the strength and weight reduction purposes. In the areas such

as aerospace engineering, medical, sports, heavy machineries and even automobile

engineering where human race is heading towards excellence and precision it is very

important to select a perfect material which fulfill the overall needs of the project. Over

200 years ago such a material was discovered from the earth’s crust which was

enriched with the characteristics which are desired in the current market.

The two most desirable properties which a metal should contain are highest strength

to weight ratio and corrosion resistance. Titanium is as strong as steel and 45% less

heavy in its unalloyed condition. The properties of Titanium is physically and

chemically similar to Zirconium. Metallic Titanium was first isolated in impure form in

1887 and with higher purity in 1910. But it came into commercial use as simulating in

aircraft applications in 1950.

Although the aviation industry still provides the main market of the alloys, titanium have

widespread use in other industries because of their many desirable properties.

Titanium is a single material as strong as steel with less than 60% of its density, but

with a high corrosion resistance. Traditional applications are in aerospace and

chemicals. More recently, especially since the cost of titanium dropped significantly,

finds alloys increased use in other industries, such as the offshore sector.

We are using Titanium SP-700 which possesses structure of Ti-4.5A1-3V-2Mo-2Fe.

SP-700, is designed to improve hot workability and mechanical properties over most

commonly used Titanium alloy Ti-6A1-4V. The chemical composition enhances the

properties such as fatigues strength, hot and cold workability and superplastic forming.

The purpose of using SP-700 because it possesses a number of excellent properties

superior to those of the most widely used alloy, Ti-6Al-4V. For high strength and

excellent formability, examine the superior properties if SP-700 in the table 1.1.

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Characteristics of SP-700

Tensile Strength 10-15% higher than Ti-6Al-4V, for less weight, reduces thickness and ability

to produce larger parts.

Fatigue Strength 10-15% higher than Ti-6Al-4V, for increased durability and reliability.

Hot Working Cracking sensitivity is low. Permits hot working at lower temperatures,

which minimizes costs and makes open air forging possible.

Superplastic Forming

Lower superplastic forming temperature and higher elongation than

Ti6Al4V decreases fabrication costs significantly and makes the fabrication

of complicated parts possible.

Cold Working Cold rolling, blanking and bending workability are improved greatly. Unlike

Ti6Al4V, precision blanking is possible.

Heat Treatment

Response

Improved hardenability compared to Ti6Al4V makes heat treatment of

larger parts possible and suppresses distortion during heat treatment.

Table 1.1: Characteristics of SP-700 {1}

SP-700 Ti-6Al-4V

Figure 1.1: grain structure of SP-700 and Ti-6Al-4V {1}

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1.2 Basics of TIG Welding

Tungsten Inert Gas (TIG) welding was developed in 1930s when welding of

magnesium became apparent. Nowadays TIG welding is known by the name of GAS-

TUNGSTEN ARC WELDING (GTAW). In TIG welding metals are fused together by

heating them by an electric arc stablished between a non-consumable tungsten

electrode and the workpiece. The welding zone, tungsten electrode and molten metal

are protected from the atmospheric gases by a stream of inert gas through the welding

torch and sometimes from the passage below the welding zone in controlled

conditions. In result the same chemical integrity of the weld zone is achieved as the

base metal.

The arc can be controlled according to the melting temperature of the base metal. By

changing the current and power of the arc passing through the tungsten electrode the

penetration can be adjusted. The temperature can sometimes approach to 2500 °C.

The basic TIG principal can be seen the figure 1.2.

Figure 1.2: TIG Principal

Note: A filler rod may not be used depending on the design of the joint.

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Argon and Helium are widely used as shielding gases. Helium can be used alone or

in combination with Argon. Argon is 10 times heavier than air, thus it provides better

shielding than Helium. It can also be operated at lower flow rates because it has less

tendency to flow away from the welding area. Argon is a non-reacting gas, so there

are no chance of any chemical reactions between metal and shielding. The use of

Argon results in high weld quality and good appearance.

Apart from other welding techniques TIG welding is capable of achieving the highest

quality of welds. It is also most versatile in terms of what and be welded and in which

position. The welding quality obtained by TIG welding are so high that sometimes the

welds are not needed to be polished or no further machining process.

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2. LITERATURE CRITIQUE

Titanium alloys are light weight and high in strength and that is the reason of Titanium

being used widely in aerospace engineering. The applications of Titanium has also

expanded in other fields as sports, leisure goods, automobiles and motorcycles {1}.

Hot and cold formability and fatigue strength of Titanium SP-700 ae some of the most

desirable properties. Such properties are achieved by ultra-fine microstructure of the

material. β-rich, α+β type SP-700 possesses α grain size of 2-3 µm {2}. Looking at the

chemical properties of Titanium is its excellent resistance to corrosion. Titanium is

almost as resistant as platinum. It can withstand the attacks by acids, salt solutions

and moist chlorine gas.

In the current scenario Titanium alloys are widely used in many fields such as

aerospace, automobiles, sports etc. the efforts made to form the material into desired

tool the material has to go through various processes. To verify the material’s

processing ability and the sandwich structural formations lot of efforts have been made

on the Superplastic Forming (SPF). It is important to determine the SPF characteristics

of Titanium alloys to assess their potential for application to sandwich structure. Low

flow stress at forming, practical SPF temperature, high strain capability and

temperatures are the key attributes for a material. In this category α+β shows the

lowest flow stress and SP-700 shows the lowest processing temperature as well {3}.

This properties of SP-700 advances its usage in aircraft component manufacturing.

Recently Titanium SP-700 is also used in production of golf club heads and other

leisure goods. In automobile industries Titanium is used in manufacturing the engines

components, spoilers, mufflers for the exhaust system and turbo chargers etc.

Titanium SP-700 has the compositions of Ti-4.5A1-3V-2Mo-2Fe. This alloy has weight

percentage of Aluminum: 4.5; Vanadium: 3.0; Molybdenum: 2.0; Iron: 2.0; Oxygen:

0.15; Carbon: 0.08; Nitrogen: 0.05; Hydrogen: 0.01; Yttrium: 0.005 and other element

each: 0.1 which can be seen in the table 2.1 below. Aluminum is the only α stabilizer

while Molybdenum, Vanadium and Iron are β stabilizers. So, in chemical composition

of the alloy α stabilizer weight about 4.5% and β stabilizers contributes 5% of weight

in the alloy and this is the reason for SP-700 being β rich alloy.

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Element Weight %

Aluminum 4.0-5.0

Vanadium 2.5-3.5

Molybdenum 1.8-2.2

Iron 1.7-2.3

Oxygen 0.15

Carbon 0.08

Nitrogen 0.05

Hydrogen 0.01

Yttrium 0.005

Other Each 0.10

Table 2.1: Elements of SP-700 by % weight

Titanium SP-700 products shows superior properties over Ti-6Al-4V. Such qualities as

high sinterability, excellent mechanical properties, good cold workability and fine

microstructure. The existence of elements like iron and Molybdenum enhances the

alloy’s sintering and refines the microstructure {4}. No significant increase in flow

stress over 800°c can be noticed because of SP-700 alloy’s super plastic formability.

Extremely fine microstructure and low-transit temperature contributes the major factor

in achievement of this attribute. It is also possible to accomplish diffusion bonding at

this temperature. Better performance in isothermal and hot forging processes can be

obtained in SP700 than ordinary alloy Ti-6Al-4V.

While comparing SP-700 with Ti-6Al-4V on cold workability, Sp700 is found to have

superior cold forming ability. Hot worked SP700 has an ultra-fine microstructure which

contributes in possessing excellent mechanical property such as fatigue strength.

Table 2.2 shows the mechanical properties of the alloy.

According to the International Journal of Science and Research (IJSR) on “Titanium

and its Alloys” which depicts the characteristics and behavior of different alloys with

different stabilizers. SP-700 is described in the journal as “Heat treatable, high strength

Titanium alloy with superior strength and exceptional hot and superplastic formability

compared to Ti-6Al-4V, combined with good ductility and fatigue resistance.”{5}

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Properties Metric

Density 4.54 g/cc

Hardness 315

Tensile strength >=960 MPa

Tensile strength Yield >=900 MPa

Elongation at break >=10 %

Tensile modulus 110 GPa

Poisons ratio 0.33

Shear modulus 41.4 GPa

Electrical resistivity 0.000164 ohm-cm

Magnetic permeability 1.0020

Magnetic susceptibility 0.00000351

Specific heat resistivity 0.660 J/g-‘C

@ temp 800 ‘C

Thermal conductivity

19.0 W/m-K

@ temp 800 ‘C

Melting point 1593 ‘C

Beta Transus 900 ‘C

Table 2.2: mechanical properties of SP-700 {6}

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Titanium can also be crystallized at a wide range of temperatures. When a crystal

transforms completely into other structure, it is called as allotropic transformation and

the temperature at which it transforms is called as “transition temperature”. Hexagonal

Closed Packed (HCP) crystal structure, which is known as α-Titanium can be achieved

at lower temperatures. To achieve β-Titanium transformation occurs at higher

temperature which results into Body Centred Cubic (BCC) crystal structure. BCC is

characteristically stable in Titanium alloys. The β-transition temperature for Titanium

is 882±2. Figure 2.1 illustrates the HCP and BCC structure of α-Titanium and β-

Titanium respectively. Highlighted area represents most dense plane of the crystal{5}.

Fig. 2.1: Crystal structure for α-Titanium and β-Titanium

The classification of Titanium alloys are based on the number of α and β stored in its

structure at ambient temperature. In its pure state the Titanium possesses α

microstructure. After solution heat treatment process at higher than β-transus

temperature and air cooling β alloys possesses all β microstructures. α-β alloys carries

a mixture of both α and β microstructures at ambient temperature. The general

classification of alloying elements can be seen in fig 2.2 below.

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Fig.2.2: classification of alloying elements

α –STABILIZERS

Pure Titanium structure is found in hexagonal close-packed crystal (HCP) crystal

formation at room temperatures. Some of α-stabilizers are Aluminium, Gallium, Ge, O,

N, and C. Al has exceptional property of solubility which enables Al to dissolve in both

α and β phases which is the reason of using Al most extensively in Titanium alloys.

On the other hand Al also reduces alloys density. Apart from Al and O other stabilizers

have very low solubility in Titanium. Despite of being strong α stabilizers O, N and C

cannot be used much because these elements lowers the ductility of the alloy {5}.

Fig. 2.3: Schematic phase diagram with α stabilizing element

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β –STABILIZERS

The complete transformation from α phase to β phase takes place at 885°C, which is

known as β transition temperature. This temperature can be adjusted by alternating

the type or amount of impurities or the alloying elements. V, Nb and Mo are β-

isomorphous stabilizers and it is possible to stabilized β phase by adding sufficient

concentration of these elements. Some β-eutectoid elements such as Cr, Fe and Si

are also used for alloying Titanium. Addition of Cr to more than 5% weight forms the

intermetallic composition Ti-Cr2 which is not a desired outcome. Si addition in Titanium

alloys improves high temperature application and also increases creep resistance.

NEUTRAL ELEMENTS

Neutral elements reduces transformation temperature slightly but at higher

concentration transformation temperature riser at even higher rate. Such neutral

elements are Sn, Zr and Hf. The type and amount of these elements classifies alloys

into four categories such as α, near α, α+β and β alloys. Fig 2.4 shows the schematic

of a psuedobinary phase diagram of Titanium alloys. {5}

Fig. 2.4: the schematic of a psuedobinary phase diagram of Titanium alloys.

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α- ALLOYS

α stabilizers are the responsible elements to elevate β transition temperature, which

eventually stabilizes the α phase. Al, O, N, and C are commonly used α stabilizers.

Among these Aluminium plays a major role in enhancement of material’s tensile

strength and creep strength.

NEAR- α ALLOYS

If an alloys consisted of 1 to 2 of weight% β stabilizers, it can be categorized under

near α alloy. These alloys generally end up stabilizing at 5-10% of phase structure at

room temperature. Because of its best creep resistance ability near α alloys are widely

used into manufacturing of high operational temperature compressor sections.

α + β ALLOYS

α + β alloys can be fortified by heat treatment and thermo-mechanical processing.

These alloys possesses great combination of strength and ductility as compared to

near α alloys. α+β alloys are generally used in military and commercial aircraft,

manufacturing of fan and compressor blades in gas turbine engines.

Fig. 2.5: The effect of heat treatment process on the amount of α and β.

β ALLOYS

According to Fig. 2.4, β alloys can be categorised into two groups, such as β

isomorphous and β eutectoid. After metastable β composition the isomorphous α

phase is obtained. Candidates of isomorphous group are; Molybdenum, Vanadium,

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Tantalum and Niobium. While on the other group eutectoid group consistes of includes

Manganese, Iron, Chromium, Cobalt, Nickel, Copper, and Silicon.

PRINCIPLE OF TIG WELDING

High magnitude of current is transferred to the base metal with the help of a non-

consumable tungsten electrode under the flow of non-reacting gases such as Argon,

Helium or sometimes the mixture of both the gases. Use of filler metal is optional which

depends upon the material properties and thickness. Filler metal can be fed by both

manual and automatic manner.

Fig. 2.6: principle of TIG welding process

TIG WELDING TOOLS

TIG welding requires the source of welding current which produces and controls the

flow of current and voltage. The torch unit is a tool to produce an arc between metal

and the tungsten rod. It also allows the shielding gas to prevent the contamination and

corrosion in the weld. Shielding gas, purge gas and trailing gas are provided through

a gas cylinder with pressure reducing valve and flow meter. Nowadays the welding

machines are capable enough to produce both of the direct current (D.C.) and

alternating current (A.C.). Fig. 2.7 depicts the welding tools required for TIG welding

process.

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Fig. 2.7: TIG welding Tools

TIG WELDING TORCH WITH WATER COOLING

To reduce the heating of torch and current cable caused by high magnitude current

and thermal stressing two types of cooling systems are used in general. Which are air

cooling and water cooling techniques. In air cooling system inert gases cools down the

temperature of gas nozzle and arc burning. In water cooling system water circulates

through pockets of hand grip and reaches to the electrode. Fig. 2.8 shows the water-

cooled TIG welding torch.

Fig. 2.8: Water cooled TIG welding torch

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FLOW METER FOR TORCHES:

Flow meter are easily attachable device which shows and regulated the flow of

shielding gas in l/min. Proper adjustment of shielding and purge gas flow can be

measures and controlled with the help of flow meters.

Fig 2.9: Gas flow meter

Fig. 2.10: Effect of Current and Inert Gas

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WELD POOL INFLUENCED BY CURRENT TYPE

Using different types of electrodes and current types can affect the weld pool thickness

and depth. Sometimes the change in polarity of electrode and base metal can also

help to achieve desired size and shape of the weld pool.

Fig.2.11: Influence of Current Type on Weld Pool

FUNCTION OF FILTER CONDENSER

In A.C. current the amplitude cycles around positive and negative half cycles, which is

known as rectifying effect. Filter condenser is used to balance this effect and help to

achieve clean work surface.

Fig. 2.12: Function of Filter Condenser

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TIG WELDING WITH PULSATING A.C.

Because of the advancement in technologies it is now possible to weld with different

types of current. Such as direct current and Pulsating current. This enables us to

choose between pulse duration and pauses independently.

Fig. 2.13: Pulsating Square-Wave Alternating Current

TIG A.C. WELDING PARAMETERS

In TIG welding A.C. current can maximum work at 400°c while D.C. can be operated

at 600°c. Fig. 2.14 shows the different welding parameters for different material

thickness.

Fig. 2.14: A.C. Welding Parameters

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CURRENT LOADING OF TUNGSTEN ELECTRODE

While welding it is necessary that the arc surround the tip equally. The angle between

electrode and weld plate should be around 20° – 30°. If the current is too high for the

welding it can melt a metal and draw a hole in it. While using alternating current at

proper strength it produces a hemispheric molten bead which can be considered as

ideal weld pool bead.

Fig. 2.15: Current Loading of Electrode Fig. 2.16: Manual and Machanised Welding

MANUAL AND AUTOMATIC TIG WELDING

Manual welding

In this process torch and filler metal are fed manually by hand. It requires high amount

of accuracy and steadiness.

Fully automatic welding

Mechanical system is provided to feed filler metal and torch. Sometimes the whole

process is programed on the computer.

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3. EXPERIMENTAL SETUP

To perform the welding operation on 2.5 mm thick Titanium plate it needed to do some

pilot welding tests on the smaller plate size to achieve the optimum result. After

brushing the surface of 120mm x 30mm and giving a nice and flat edge by filing tool

the plates were clamped together on the welding jig. Initial welding current was set on

90 A and 9 V power supply. Welding speed was set at 172 mm/min. After approaching

the midway the current was increased to 100 A. By examining the weld joint the weld

pool did not penetrate through the plate in 90 A section, while the section which was

welded on 100 A showed a nice and clear penetration. From the results derived from

the pilot test it was decided to weld the 120mm x 50mm x 2.5mm Titanium plate on

100 A and at the speed of 172 mm/min.

Fig. 3.1: Dog bone shape sample dimensions

The as-received condition of material was mill annealed SP-700 alloy. Two plates of

Titanium were cleaned by brushing with stainless steel brush and the edges were filed

to remove any discontinuity or surface impurity. Welding parameters used for the

process are as shown in table 3.1 below.

100

30 30

32

25

10 6 R6

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welding current 100 A

Voltage 8 V

Welding speed 172 mm/min

Shielding gas 15 l/min

Purge gas 10 l/min

Time taken to complete the weld 52 s

Table 3.1: TIG welding parameters

Fig. 3.2: Weld jig and clamped plates Fig. 3.3: Ampere reading from the weld machine

3.1 Post Weld Heat Treatment

From the pseudo-binary phase diagram (fig 2.4) it has been decided to choose the

temperature from each stage. To observe the behavior of material in all the phases,

the temperature for PWHT were chosen which are above Tβ, between Tβ and Ms and

below Ms. After examining the past work and discussing with supervisor, the PWHT

temperature for this project were decided to be 950°c, 840°c and 590°c. Tβ for Titanium

SP-700 is around 900°c and Ms is at around 800°c. To observe the aging effect on

material, one of the 950°c sample and 840°c sample were aged at 550°c for 1 hour.

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All four samples were heat treated in the furnace for 1 hour regarding the sample’s

size.

Table 3.2 provides PWHT information. W-590, W-840/550, W-950 and W-950/550

represents the dog bone shaped samples heat treated at 590°c, 840°c, 950°c and

950°c respectively. One small piece (the part between each two dog bone) was also

processed with each samples for the purpose of performing optical microscopy and

hardness testing. Each dog bone sample was used for Ultimate Tensile Strength and

Scanning Electron Microscopy to analyze the fractured surface.

PWHT

temp. Duration

Aging

temp. Duration

Cooling

method

W-590 590’C 1 hour - - Air cooled

W-840/550 840’C 1 hour 550 ‘C 1 hour Air cooled

W-950 950’C 1 hour - - Air cooled

W-950/550 950’C 1 hour 550’C 1hour Air cooled

A.W. - - - - -

Table 3.2: PWHT temperatures

3.2 Fabrication and Mounting of Specimen

Mounting of the specimen was necessary to observe the microstructure of cross

section of weld piece. Each piece shows weld pool, heat affected zone and base metal.

To examine the microstructure of the specimen it is necessary to grind, polish and etch

the surface carefully. Mounting of the specimen on polymer material facilitate easier

observation. 1.75 spoonful of PolyFast (resin) was used for each sample. PolyFast is

a Phenolic hot mounting resin with carbon filler, it helps in edge retention while micro-

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scoping. The specimen is first set in the mount chamber and PolyFast is filled in the

chamber. Struers LaboPress 3 is a hot press machine which applies 20kN of high

pressure at 180°c temperature. The process takes about 8 minutes to complete. At

the bottom of it, specimen’s description is carved for easier recognition.

The mounted specimen are then grinded on the METASERY Rotary grinder.

Specimens are grinded with 180, 500, 1200 and 2400 grade abrasive sanding paper

respectively. The specimen is then coated with ethanol to remove any kind of moisture

and then dried rapidly with help of hot air blow drier.

Polishing of the specimen was done by both automatic and manual way. In automatic

polishing machine (Struers LabPol-2), 6 specimen holder disc was used to locate

specimens in it. A pneumatic arm holds the specimen in place and applies equal

amount of pressure. These specimens were then polished on a disc shaped layer of

cloth with fine grit and polishing media was provided to the process. Struers DP-Paste

M was used as a polishing media which contains a very fine paste of diamonds. There

were two types of paste available in the lab, with 6μm being coarser and 1μm being

finer. 6μm paste was applied first and then process was continued with 1μm diamond

paste. After each polishing stage specimens and cloth disc were cleaned with ethanol.

Manual polishing also follows the same procedure but the only difference is that the

person have to hold the specimen manually on rotating wheel. In manual polishing the

specimen must be rotated at 90° after certain period to avoid scratch marks. The

process must be repeated until a fine mirror finish surface appears. Once achieved

the specimen are then washed with ethanol and dried with hot air blow drier to avoid

any contamination. Caution for both grinding and polishing process is not to encounter

any physical contact with the metal surface which may lead to contamination and/or

corrosion.

Etching process reveals the grain and microstructure of the material. The agent used

for etching process was Kroll’s agent which is one of the two most suitable etching

agents for Titanium alloys. The acidic etching agent reveals the surface morphology

on the weld profile which were then observed using optical microscopy.

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Fig. 3.2.1: fabricated specimen for optical microscopy and microhardness

3.3 Micro-hardness Test

Measured load of 300 g was transferred to specimen with the help of rhombic diamond

for 10 seconds of time duration. This load creates indent on the specimen. The indent

on the specimen is then measured diagonally. The data taken from the indent

measurement are used to calculate hardness. This test was carried out on LECO

LM800AT hardness tester.

Fig. 3.3.1: hardness testing machine LECO LM800AT

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3.4 Ultimate Tensile Strength Test

Ultimate tensile test was performed on the dog bone shape specimens. Test was

carried out on HOUNSFIELD TENSILE TESTING RIG. Dog bone samples were

clamped at both sides by mechanical jaws and then introduced to the unidirectional

pulling force. The pulling force is increased constantly until the failure. Extension

speed for the test was 3 mm/min. fractured parts were stored carefully and further

used to carry out SEM.

Fig. 3.4.1: Housnefield Tensile Testing Machine

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4. RESULTS AND DISCUSSION

4.1 Ultimate Tensile Tests Result

Ultimate tensile strength test was carried out on all dog bone shaped sample including

un-welded sheet and as-welded samples. To find out the tensile strength of the

material two parameters are required which are cross sectional area and applied force.

Cross-sectional area for all the dog bone shaped sample was 14.75 mm2. Results of

all the tests were studied and compared. Following is the table showing the

comparison of UTS for all the samples.

Fig. 4.1.1: UTS comparison of all specimen

It can be seen from Fig. 4.1 that ultimate tensile strength of the specimen is increasing

with increase in heat treatment temperature. β transus temperature for SP700 is

900°C. It can be seen from the graph that the samples which were heat treated above

this temperature shows higher UTS than the ones heat treated below it. Dog bones

were cut as per the standards provided by ASTME to concentrate the force into

perticular area of observation. Failure on as-welded and W590 sample appeared on

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the base metal away from the weld zone. W840/550 sample was broke at the weld

zone. While performing test on W950 and W950/550, samples experienced

continuous slipping from the gripper jaws. To avoid this slipping holes were drilled in

gripping section and a heat treated steel pin was used to clamp samples. Drilling holes

in the sample braught a disadvantage of higher stress concerntration outside the hole

boundary. Because of this higher stress concentration W950 and W950/550 fractured

from the hole which lead to failure in measuring UTS at weld zone.

Fig. 4.1.2: fracture occurrence on each sample

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4.2 Hardness Test Results

Fig. 4.3 show the comparison of hardness test carried out on all the mounted samples.

Numbers in the figures indicates average Vickers Hardness (HV) of all samples.

Fig. 4.2.1: comparison between hardness tested samples

From the comparison above it can be seen that W840/550 and W590 possesses

almost equal hardness of 390 HV. Aging the sample increases its hardness by 8.5%.

This can be seen in W950 sample which was aged at 550°C for an hour. To examine

aging effect on A.W., we can consider at W590 sample. As 590°C can be considered

as aging temperature. From this observation it can be found that aging the samples

can increase hardness by 35 HV.

To observe the effect of heat treatment on each area of the sample such as base metal

(BM), heat affected zone (HAZ) and fusion zone (FZ) hardness test was carried out at

every 0.2 mm distance. The result was then noted and transferred into graphical

manner as shown below.

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Fig. 4.2.2: hardness profile of each sample

In W950/550 sample it can be seen that the hardness of heat affected zone (H.A.Z.)

is higher than the fusion zone (F.Z.) and base metal (B.M.). While looking at the W950

sample it seems that hardness drop occurs in heat affected zone. W590 and

W840/550 shows uniformity in hardness in all the zones.

4.3 Scanning Electron Microscopy

SEM test was carried on the samples broke from the weld zone and outside of weld

zone. W840/550, W590, as-welded and un-welded samples were observed under the

scanning electron microscopy test. Failure behavior and source of failure was carried

out from the results obtained from the test. Fracture properties such as brittleness or

ductility can be observed from the images taken under the electron microscopy.

Fig. 4.3.1 shows the SEM images of un-welded (base metal) sample failure. The

necking of the dog bone size sample can be seen. As the core of the sample is split

from the center, the fracture seems to be started due to the excessive tension between

peripheral atoms and the ones from the core. Small micro-voids on the fractured

surface indicates the ductile behavior of the un-welded Titanium SP-700 alloy. This

micro-voids approves extensive plastic deformation before cracking.

34

Fig. 4.3.1: SEM images for un-welded samples

35

Fig. 4.3.2: SEM images of as-welded sample (A.W.)

Fracture clearly shows the ductile behavior of the sample with no signs of visible

defects like gas pores or contamination. In some areas deep fracture has occurred

which is because of some irregularities in the material itself. Dimple presentation on

the fracture surface shows the significant amount of plastic deformation before rupture.

36

Fig. 4.3.3: SEM images of W840/550

Fig. 4.3.3 shows the images taken from the W840/550 fractured surface. Looking in

the lower magnification images the big planes on the fractured surface indicates the

material properties to be brittle in behavior. But magnifying the image reveals the tiny

dimples on the big plane like surfaces. This big plane paces are the crystal boundaries.

The fracture appeared from failure of the bond between two crystals. This crystals are

the big columnar structure in the weld zone. It has smaller grain size on the peripheral

Columnar structure of crystals

Crystal failure

37

faces while the crystals are made of coarse grain structure from inside. It can be seen

in some areas where the crystal failure has occurred. The dimples are shallower than

the ones found in un-welded sample. Which relates to shorter elongation under tensile

test.

Fig. 4.3.4: SEM images of W590 sample

38

4.4 Optical Microscopy of Mounted Samples

Etching reveals the microstructure of the polished samples which enables the sample

to examine under optical microscopy. Etching time cannot be fixed as the material

reacts at different level with etching agent, the etching duration can be determined by

the visual observation of metal surface. After removing from the etching agent the

sample were then quickly dried and stored with extra care to prevent any scratching

or surface contamination.

While inspecting the as-welded specimen under optical microscopy it can be seen that

the grains were settled in coarse columnar structure in fusion zone. Energy absorbed

from the welding arc the grains in heat affected zone grew larger in size, while the

base metal grain structure shows a small denser arrangements. Weld pool can be

easily distinguished under the optical microscopy test. Fig 4.4.1 shows the optical

microscopy images of as-welded sample.

Fig. 4.4.1: optical microscopy images for aw-welded sample (x50 magnification)

1

1

2

2

3

3

39

Fig. 4.4.2: optical microscopy images for W590 sample (x50 magnification)

For W590 sample it can be seen clearly that the microstructure in the weld zone is fine

columnar grains. Image A shows the clear partition between heat affected zone and

fusion zone. From image A, it can be seen that the existing grains has growth at fusion

line so, no nucleation of new gains are necessary. This can be understood by the

following image.

Fig. 4.4.3: growth of existing grains at fusion line

A

A

B

B

40

B.M. H.A.Z. F.Z. H.A.Z. B.M.

Fig. 4.4.4: optical microscopy images of W840/550 (x50 magnification)

Heat treated sample at 840°c and then aged at 550°c shows no clear fusion line. The

sample shows formation of both α and β grains distributed throughout the profile. In

fusion zone α grain structure is found bigger and coarser. Also the number of α crystals

is higher than β structure. While it can be seen that β grains are arranged around the

fusion line in dense manner. According to fig 2.4, 840°c heat treatment temperature

results under α+β structure with near to β phase alloy. Brighter grains indicates α

phase and darker grains indicates β phase gains. This can be clearly seen in Fig.

4.4.4.

H.A.Z. F.Z.

B.M.

α β

41

Fig. 4.4.5: optical microscopy images of W950

Heat treating a sample above β-transus temperature (900°c) transforms alloy into β

phase. Fig 4.4.5 shows the optical microscopy images of W950 sample. Coarse grains

are found in the fusion zone and base metal, but the grains around the welding pool

seems to be in small size. It can also be seen from the image that α grains are only

found in this fusion zone periphery area. In x200 and x500 images the grain boundary

between α and β grains can be seen clearly.

β β

α

α

x200

X500

Fusion zone Fusion zone boundary

42

Fig. 4.4.6: optical microscopy images of W950/550

Fig. 4.4.6 shows W950/550 under optical microscopy. After aging W950 sample its

can be easily derived that the small grains around fusion zone periphery are dissolved

and transferred into big grains. This uniformity relates to higher hardness and UTS of

the material. x500 images shoes the grain boundary situation.

X500 X500

X50 X100

43

5. CONCLUSION

Welding of 2.5 mm thick Titanium SP-700 sheet with using TIG welding process

is proven successful with used welding parameters. No porosity, contamination

or corrosion were found in weldment. Smooth surface finish and homogeneous

weldment is achieved with 6 mm wide weld pool.

Coarse columnar grain structure was found in weld zone. Grains in heat

affected zone were larger in size compared to base metal.

Gradual increase in UTS can be achieved with respective increase in post weld

heat treatment temperature.

Fracture occurrence in base metal indicates the higher ultimate tensile strength

of weld pool than base metal in case of A.W. and W590.

W840/550 heat treated sample shows homogeneous crystal construction

throughout the sample which caused it to break from weld zone under tensile

test.

Peak hardness is associated with the sample heat treated at highest

temperature (W950/550).

Aging of material at 550°C increases hardness by 8.5% compared to non-aged

sample.

Sample heat treated at highest temperature (950°C) shows 21% increment of

hardness compared to as-welded sample.

W840/550 and W590 shares almost equal hardness of 392 HV but shows

significant difference (100MPa) under UTS test.

44

Formation of micro-voids indicates ductile behavior of material.

Adjustments in post weld heat treatment can adjust the properties of whole

material and can also minimize differences between weld zone and base metal.

45

6. REFERENCES

1. JFE Technical Report - No. 5 (March 2005) - “Advantages of high formability

SP-700 Titanium Alloy and its Applications”

2. Ishikawa, M. et al. Titanium ’92 Science and Technology. Vol. 1, 1993, p.141.

3. William D. Brewer, R. Keith Bird, and Terryl A. Wallace. NASA Langley

Research Center – “TITANIUM ALLOYS AND PROCESSING FOR HIGH

SPEED AIRCRAFT”

4. Book, Materials Science and Engineering: A, vol. 213, Takahiro Fujita, Atsushi,

Chiaki Ouchi, Hidenori Tajima.

5. International Journal of Science and Research (IJSR) – Titanium and its Alloys

– Yassin Mustafa Ahmed, Khairul Salleh Mohamed Sahari, Mahadzir. Ishak,

Basim Ali khidhir.

6. Website - MatWeb – Material Property Data – SP-700

7. Titanium A Technical Guide: Matthew J. Doanchie, Jr. second edition

8. Fatigue properties of SCS-6/SP700 Titanium Matrix Composite: Akira

Fukushima, Chikara Fujiwara, Yukio Kawachi and Kouichi Yasuhira

9. Effect of post weld heat treatment on the fatigue properties of dissimilar

Titanium alloy joints: S.Q. Wang, J.H. Liu, D.L. Chen.

10. Interfacial microstructure evolution and shear strength of Titanium sandwich

structures fabricated by vrazing: Wentao Wang, Minyu Fan, Jinlong Li and Jie

Tao.

46

11. Notched tensile strength of SP-700 laser welds: L.W. Tsay, Y.S. Ding, W.C.

Chung, C. Chen.

12. Standard test for tension testing of metallic materials: ASTM international ,

Designation: E8/E8M – 09

13. The effects of hydrogen on fatigue crack growth behavior of Ti-6Al-4V and Ti-

4.5Al-3V-2Mo-2Fe alloys: Y.S. Ding, L.W. Tsay, C. Chen.

14. Infrared brazing Ti-6Al-4V and SP-700 alloys using the Ti-20Zr-20Cu-20Ni

braze alloy. : C.T. Chang, Z.Y. Wu, R.K. Shiue, C.S. Chang

15. Adsorption of vanillin and syngaldehyde onto a macroporous polymeric resin :

Maria Ines F. Mota, Paula C. Rodrigues Pinto, Jose Miguel Loureiro, Alirio E.

Rodrigues.

16. Notched tensile fracture of Ti-4.5Al-3V-2Mo-2Fe welds as elevated

temperatures. L.W. Tsay, C.L. Hsu, C.Chen

17. Microstructure and notched tensile fracture of Ti-6Al-4V to Ti-4.5Al-3V-2Mo-

2Fe dissimilar welds. : C.Y. Chu, C.T. hsieh, L.W. Tsay

18. Microtwin formation in the α phase of duplex titanium alloys affected by strain

rate. : Yi-Hsiang Lin, Shu-Ming Wu, Fang-Hsin Kao, Shing-Hoa Wang, Jer-Ren

Yang, Chia-Chih Yang, Chuan-Sheng Chiou

19. Low-frequency internal friction of α+β Titanium alloy SP-700. : X.S. Guan, H.

Numakura, M. Koiwa, K. Hasegawa, C. Ouchi

20. Microstructureal characteristics and unique properties obtained by solution

heat treating or aging in β-rich α+β titanium alloy : Chiaki Ouchi, Hideaki Fukai,

Kohei Hasegawa.

47

21. On the significance of matrix microstructure in fatigue strength of a

unidirectional SCS-6/SP700 titanium alloy matrix composite : Hiroshi Nakatani,

Masakazu Okazaki, Akihiro Mikami

22. Solid phase extraction and graphite furnace atomic absorption spectrometry

for determination of silver and palladium with diaion SP700 as sorbent. :

Chunhua Yang, Zhangjie Huang, Yunsong Ge, Jing Chen

23. L, G. and J. C. Williams, titanium, berlin springer. 23 – 30. (2003)

24. Welding science, A new look at a fundamental technology: J. Elmer, J. Wong.

25. Mechanical and fatigue properties of titanium alloy SP700 after simulated

superplastic forming conditions : F. Pitt, M. Ramulu, Material Science Forum,

Vol 735, pp. 372-382.

26. Aerospace part production using SP700 : Material Science forum Vols. – 357-

359, P.N. Comley

48

APPENDIX

Fig. A.1: pilot testing of TIG welding on SP700 sheet

Image A.1 shows the welding penetration occurred on Titanium SP700 sheet. When welding was done on 90 A current it did not penetrate thoroughly to the other side, it can be seen in the yellow circle. When the welding current was increased to 100 A it showed pretty good penetration and smooth surface finish rather than rough. Red circle shows the results obtained by increased current.

Fig A.2: left-out sheet of SP700 after laser cutting

.

Dog bone shaped samples were laser cut from the welded sheet of SP700. Images A.2 shows the remaining sheet of SP700. Small center pieces were used in process of mounting, grinding and polishing for optical microscopy and micro-hardness.

49

Fig A.3: wrapped samples in stainless steel foil

To imitate the effect of vacuumed chamber dog bone shaped samples were wrapped tightly in the SS foil sheet. Small center piece from the left-over sheet was also wrapped and heat treated with the sample to carry out micro-hardness and optical microscopy.

Fig. A.4: automatic polishing machine – Struers Tegra Force5

50

Fig. A.5: UTS result for un-welded sample

51

Fig. A.6: UTS result for as-welded sample

52

Fig. A.7: UTS result for W590 sample

53

Fig. A.8: UTS result for W840/550 sample

54

Fig. A.9: UTS result for W950 sample

55

Fig. A.10: UTS result for W950/550 sample

56

Fig. A.11: NTS of various preheated specimen{16}

Fig. A.12: The effects of solution heat treatment on the hardness upon aging{17}

57

Fig. A.13: the notched tensile strength of specimens upon aging{11}