[ET-494: Friction Stir Welding (FSW)]

[Senior Design]

Advisor: Dr. Mitra

Fall of 2014

Finale Report

Student: Jesse McDavitt

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Jesse McDavitt

Senior Design Term 2 Finale paper

Fall 2014

Advisor: Dr. R. Mitra

I. Abstract:

The project will investigate friction stir welding (FSW) beginning with an analytical

model using COMSOL. The objective is to design a welding tool that will give the required weld

qualities as determined by the American Welding Institute standards that are available for FSW.

FSW is a relatively recent addition to solid state welding techniques and has been under constant

and rapid development by both academia and industry. Much of the developmental work is

proprietary in nature because of the enormous commercial interest in this joining technique.

Some of the principal challenges to overcome are the geometry of the tool, control of the heat

transfer mechanisms involved, and the overall modelling of FSW using finite element models

such as COMSOL.

II. Background

Friction stir welding was invented in 1991 in Cambridge, England by The Welding

Society. The basic mechanism involves a downward vertical force applied by a rotating tool that

is inserted into the gap of the material to be joined (for a butt weld configuration). The tool

generates friction, causing heat which softens the workpiece and facilitates the welding process.

The rotating tool extrudes material in the workpiece from one side and forges the displaced

material into the other side.

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Fig.1. A schematic showing a butt weld using friction stir welding, tool rotation, tool travel

direction, and the downward force.

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Fig. 2. ASME (American Society of Mechanical Engineers) Section IX 2013 edition, and shows

details of a typical tool geometry used in friction stir welding.

III. Benefits of FSW:

Friction Stir Welding is a relatively new and rapidly developing technology that offers

huge benefits, including fewer welding defects compared to traditional welding techniques.

Important welding parameters, such as welding speed, heat input, and human skill, are more

easily controlled in FSW than in traditional welding. For example, the human skill factor is

eliminated completely, as the entire process is mechanized. The travel speed in FSW can be

controlled either manually or set up with robotics, and therefore allows for more control. The

heat input can be controlled by the geometry of the tool, and the same heat generation can be

expected provided that the tool does not deform or fuse with the workpiece. The mechanical

characteristics of a FSW weld are much better, resulting in high strength and toughness in the

welded section owing to low heat input and a small heat affected zone. The welded material is

much less distorted and requires less follow up treatment. In FSW no filler metal is required,

resulting in less material cost and less chance of adding an excessive or inadequate amount of

metal. These are some of the many advantages of FSW compared to traditional fusion welding.

IV. Objective/Plan:

This project aims to extend the first attempt at Southeastern by Dr. Mitra and a couple of senior

students in 2011 to butt weld three different aluminum alloys using a special tool. The

experiments that were performed by the team did not give the desired results. It was concluded

that further work was necessary in order to produce good quality welds at Southeastern.

Therefore, the current project intends to develop the methodology further, beginning with

improved tool design and experimental procedure that will produce the desired results. The first

objective of this project will be to develop a simulation of the FSW process in COMSOL, which

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will be used to determine an improved design of the tool. Once a suitable design or set of designs

are analytically determined, the next phase of the project will consist of fabricating a tool or a set

of tools and conducting the experimental work. All of this will be based on the funds becoming

available. The initial material to be joined will be aluminum alloys.

The main challenge of the first phase will be to simulate the heat transfer and material

deformation involved, because the process generates large amounts of heat (owing to friction)

and consists of large plastic deformation (extrusion and forging). One of the most difficult areas

to control is the heat affected zone (HAZ) around the weld. The HAZ is largely controlled by the

geometry of the tool. A great deal of effort must go into designing the tool tip and shoulder. The

curvature and length of the tip have to be within certain bounds. The diameter of the shoulder vs

the diameter of the tip is a factor that must be accounted for in the design. Also, based on work

done in this field, it is expected that a curve on the shoulder face as well as groves around the

tool tip would improve the joining process. The vertical force exerted by the tool and the speed at

which the tool rotates will also contribute significantly to the success of producing acceptable

welds. This is where the COMSOL simulation will play the most dominant role. The analysis

will provide thermal profiles around the tool bit area and how the work-piece material deforms.

The challenge in COMSOL will be to understand how moving mesh works in the program.

COMSOL will also allow for the rotational speed and linear speed to be changed in simulations.

My advisor and I are very confident that this simulation will work; however, if it does not

provide the expected results, the tool can still designed based on a new set of criteria that will be

developed as part of this project. The confidence in successfully using COMSOL for designing a

working tool is based on existing research conducted in Denmark and Germany where it was

used to study FSW. The simulation will take a large portion of this semester, so that it will be the

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main focus of the work for Spring 2014. I would also like to use Dr. Mitra’s original FSW tool to

test the simulations and also use it as the control or base line for future experiments planned in

the next phase. Once the simulation is put together, different tool designs can be input into

COMSOL and tested. Inventor will be used to construct the tool itself. Once the best tool design

(or designs) is found, it can be fabricated for actual testing (Pending Funding), after which the

writing and testing of the procedure can begin. Once the welded specimens are finished, I will

follow standard inspection procedures using my ANST level II Non-Destructive Testing (NDT)

certification knowledge. I will use some of the different testing methods I am trained to use to

test weld quality. Once my initial tests are completed, further tests using destructive testing

procedures and other NDT techniques will be conducted, possibly at a different location. The

material to be used in the initial work will be aluminum alloys. The procedure will be governed

by ASME Section IX 2013 Ed. Dr. Mitra wishes to continue this research because of its potential

for development in southern Louisiana.

V. Deliverables:

• A simulation using finite element analysis (COMSOL) that will determine the best options for

designing a suitable tool geometry (or geometries) for specific heat transfer and deformation

conditions.

• Tool-Design

A. 3-D Construct of Tool

B. The Tool Geometry (Curved Shoulder/Grooved tip/Cylindrical Shape, etc.)

• A procedure for friction stir welding that will be governed by Welding Standard ASME

Section IX 2013 ed.

• Test specimens showing the experimental results.( Pending Funding)

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VI. Tool Material/Workpiece

Tool –

A multiphase nickel-cobalt based ultra-high-strength, high ductility, high

temperature alloy, commercially available as Alloy MP159 from Latrobe Steel,

Pennsylvania, was used as the friction stir tool material. This alloy is used in jet

engine components, marine and petroleum industry applications, fasteners, and

prosthetic devices. The as-received cold-worked material was age hardened at

663°C (1225°F) for 4 hours as specified by SAE (Society of Automotive

Engineers) specification AMS 5843. The final hardness achieved was 47 - 48

HRC (Rockwell C).

Workpiece –

Aluminum alloy was chosen in this investigation. It is commercially available as

6061-T6511 alloy. It was age hardened 6061 alloy and was obtained in 6.4 mm

thickness Plate. The T6511 temper consists of solution heat treatment followed by

stress relieving by stretching and artificially aging.

VII. Term 1 Spring 2014

The first item on our list was to discuss and try to understand why Dr. Mitra’s previous

attempt with Friction Stir Welding did not fulfill the results that he and the students desired. Dr.

Mitra theorized that the vertical force on the mill and the resulting heat input were insufficient to

provide sufficient deformation and fusion. I begin researching the means of controlling the heat

input for FSW. Based on my review of the literature, I have concluded that the geometry of the

tool and the tool travel speed/revolutions are the dominant factors controlling the heat generation

and ultimately the weld quality. I then came up with the idea that we need a means to test or

simulate the heat input, so that we could try different numbers and find an optimal temperature.

This is when I approached Dr. Ma and asked him about using COMSOL to simulate this process.

He then instructed me to look at some large deformation simulations that COMSOL offered for

extruding to see how COMSOL works within those parameters. As I began to look through these

different simulations I found a simulation for Friction Stir Welding that was developed by some

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German investigators. I then went and discussed the simulation with Dr. Ma, who was able to

give me access to the simulation. Once we had the simulation under way, I then discussed with

Dr. Mitra how he would test his FSW tool. I began to look through ASME Section IX for the

essential variables to govern the procedure. I was able to find that just this past year 2013 that

ASME came out with essential variables that are to be followed in FSW. I am still going through

ASME Section IX, but should have the majority of the information needed to write the written

procedure. Section IX tells the reader the minimum and maximum tolerances for variables in the

welding procedure. I have experience qualifying welding procedures through the company I

work for, which is Performance Contractors Inc. in Baton Rouge. I have an understanding on

how welding procedures should be composed. Once I understood some of the variables I began

to work on the simulation again. Dr. Mitra and I reviewed the equations that was used in

COMSOL and compared it to what values he had from his previous work. Table 1 shows the

parameters that are used in COMSOL. At this time, we do not have our own values for the

parameters shown in red, namely the coefficient of friction and the vertical force exerted on the

tool during welding.

Parameters

Name Expression Description

T0 300[K] Ambient temperature

T_melt 933[K] Workpiece melting temperature

h_upside 12.25[W/(m^2*K)] Heat transfer coefficient, upside

h_downside 6.25[W/(m^2*K)] Heat transfer coefficient, downside

epsilon 0.3[1] Surface emissivity

u_weld 102[mm/min] Welding speed

mu 0.45[1] Friction coefficient

n 3200[1/min] Rotation speed (RPM)

omega 2*pi[rad]*n Angular velocity (rad/s)

F_n 25[kN] Normal force

r_pin 2.39[mm] Pin radius

r_shoulder 11.12[mm] Shoulder radius

A_s pi*(r_shoulder^2 - r_pin^2) Shoulder surface area

Table 1

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In the absence of experimental data generated at Southeastern, the values of the two parameters

will have to be estimated based on other work. For example, the coefficient of friction will be

estimated from the data for similar workpiece and tool material combinations. The normal force

is difficult to calculate because we do not have the equipment at Southeastern to measure it in the

vertical milling machine located in Anzalone Hall. Once all of these parameters are found, we

can test the simulation against Dr. Mitra’s previous data and show that it works. So at present I

am running the simulation and understanding how it works and trying different values for the

unknowns and seeing how it affects the simulation. To recap Term 1 I have researched what

affects the heat transfer, found a simulation in COMSOL that will tell me the heat transfer,

researched the essential variables for FSW in Section IX of ASME and defined the parameters of

the COMSOL simulation and setup the values for them.

IX. Term 2 Fall 2014

This Semester I have written the Welding Procedure Specifications or WPS (See

Appendix part A) for Dr. Mitra’s previous experiment and the supporting Procedure

Qualification Record or a PQR that would. (See Appendix part B) I wrote Dr. Mitra’s variables

from his first attempt on to an ASME Section IX suggested WPS and PQR forms. I decided to

attempt this task first because looking at the WPS would help me to see what Dr. Mitra’s data

looked like in an ASME format. Once I had all of his information written down I was able to

pick out what variables would be the best to change. Per QW-202.1 is says “when it has been

determined that test failure was caused by an essential variable …, a new test coupon may be

welded with appropriate changes to the variables…”. So with that statement we can still use his

same data and make some changes and then try and test a new coupon. We are going to focus on

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the technique of the procedure QW-410. In particular we decided to look at his linear speed and

the RPMs at which the tool turns. Researching the WPS in Section XI is what took most of my

time the semester, which is why I started with the part of my design first. I learned that a larger

amount of effort was going to have to be applied to this section. The welding portion of section

IX is broken down into Five Articles: Article I – Welding General Requirements; Article II –

Welding Procedure Qualifications; Article III – Welding Performance Qualifications; Article IV

– Welding Data; Article V – Standard welding Procedure Specifications. To write the WPS and

PQR I had to use Articles I, II, and IV. Article I mainly pertains to the PQR while Article II & IV

pertain to the WPS with the end of Article IV. The PQR tells the reader about the test coupon

specifically and the results of the test performed on the coupon. Article one pertains to all of the

different types of test that can be done and how to perform the test. There is a Visual test,

Tension Test, Guided Bends test, Notch-Toughness Tests, Fillet Weld Test, Fracture test, Marco-

Examination Test, and Other NDE (Non-Destructive Examination) Test. So I had and am still

going through Dr. Mitra’s Data to see what test where done on his test coupon and reading how

these tests affect the WPS. Depending on the test that you perform on the coupon it has different

grading criteria. Articles II & IV have more to do with the WPS. Article II tells the reader how to

prepare the test coupon, meaning what the test coupon can be. You can run a test procedure on

plate, pipe or other product forms. Plate and pipe are the most common; however, if a person had

square tubing then they could use Section IX to apply to their WPS. The WPS is only applies to

square tubing, while plate and pipe can me changed out with limits. Article II also tells the reader

the essential and non-essential welding variables that apply to the process they plan to use. Just

because it is listed as an essential variable does not mean that it must be used in the welding

process. Article IV is where large majority of the information are for the WPS can be found. It is

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where the data is such as the specifics of the variables and as well as the tolerances that a person

can differ on those variables once the WPS is qualified. The travel speed could vary by a ± % of

the speed used during the WPS. It goes into the base metal grouping and how ASME classifies

the different materials. This tells you how to label you base metals and fill out QW-403 of the

WPS and PQR. It also goes into the details about the test specimen that was used and what it

allows the WPS to do after qualification. For instance Dr. Mita used a 6.4mm thick plate as his

coupon. If his procedure was qualified he could weld the thickness ranges of 1.5mm to 2T

(6.4x2=12.8) per QW- 451.1(See Appendix Part C). That table also tell a person what test he or

she needs to perform and how many. So there is a large amount of data to dig through to write

the WPS and PQR. Once I had these written and was able to look at the procedure, I then

modeled Dr. Mitra’s Original tool in Inventor (See Appendix Part D). Once I had it in a 3-D

format I was able to do my design changes as you will see in part D. I this had the part 3-D

printed. When this was done, Dr. Mitra and myself needed away to lower the temperature down

during the welding process. We modeled his processes as best as we could in the COMSOL

simulation that I had found from the spring semester. The earlier experiments conducted at

Southeastern could not record some of data that would be needed for the simulation do to the

lack of tools and measuring devices. We estimated a Normal force of 25 kN and due to some

research Dr. Mitra had done as well that it would remain constant if the same machine was used.

We did not know the µ (Friction coefficient) of Aluminum and our tool material MP-159, so we

had to pick different values. Since the two variables that we wanted to change was the speed and

RPMs those were to two that we change in the simulation to see how it would affect the

temperature. Dr. Mitra original speed was 102 mm/min and 3200 RPMs. We tried the

simulations at three different µ (.35, .45, or .55). We also tried the simulation at two other speeds

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and two other RPMs (50 mm/min and 150 mm/min) and (2200 RPMs and 4200 RPMs). We did

a grand total of 27 simulations with each µ as the focal point (see Appendix Part E). We took the

Isosurface temperature from the simulations and put it into graph form (See Appendix Part F).

The chart showing the isosurface map shows the heat distribution over the workpiece and tool’s

surface and subsurface. The area we are most interested in is where the tool meets the workpiece.

Due to the lack of information from Dr. Mitra’s previous work we took three different µ to

accommodate the lack of knowledge. The first group of µ would simulate what we believe Dr.

Mitra’s work would like. How the graph reads is that the combinations of the different µ (three

small columns as a whole). So what we are looking for is if they stay consistent and the

temperature drops below the first simulations. The best result we found was a RPM of 2200 @

102 mm/min. as can be seen in the graph. So the new WPS will read with a new rotational speed

of 2200 RPMs and a travel speed of 102 mm/min (See Appendix Part G). Dr. Mitra can now take

this information and run the new WPS and get a PQR. This will be the foundation for Dr. Mitra

to continue his research.

X. Steps to write a WPS/PQR

1) Understand scope of work meaning what material is to be joined together, how it

will be done (welding process), and the thickness needed.

2) Obtain a copy of ASME BPVC Section XI or any other standard for welding

procedures.

3) Locate the recommended WPS/ PQR forms (As seen in Appendix Parts A & B)

4) Locate WPS Process welding Variables in QW-250 (See Appendix Part H FSW)

5) Review the essential variables that are to be used and compare to the WPS/PQR

forms.

6) Look for the information needed on the WPS/PQR forms in QW-403 Base

metals.

Locate material in Nonmonetary Appendix D (See Appendix Part I) it

will tell the reader the P number and SA number.

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Then go to Table QW/QB-422 (See Appendix Part J) which will provide

the information needed to fill out QW-403

At the bottom portion of QW-403 the thickness range can be found in

QW-451.1 (See Appendix C). This will determine the test coupon’s

thickness based on what thickness needs to be welded.

7) Using the QW-250 table and the welding data in QW-400 (See Appendix Part K)

begin going through the WPS and designing the WPS based on the scope of

work. I.e. what kind of environment one is on will determine what variables are

used?

8) Once the WPS has been design, the test coupon will be welded. This is where the

PQR is filled out.

9) Fill out the PQR with the exact vales of the essential variables used during the

welding

10) Test the PQR per QW table 451.1(Will tell how many and what test to perform)

11) Testing criterial can be found in Article 1 Welding General Requirements QW-

150 – QW-190 (See Appendix Part L)

Either perform the test yourself or send off to a lab that specializes in this.

(Welding Testing X-Ray / Laboratory)

12) Rewrite the WPS, if the tests are complainant with code, for the ranges of the

variables based on the welding data.

13) Document WPS and PQR

Revisions can be made of WPS as long as the variations fall within the

tolerances set by the Welding data

PQR can be used to make other WPS and be combine other PQR to make

new WPS

** Only a Qualified Welder can weld a test coupon** Information can be found in Article III – Welding Performance Qualifications

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XI. Appendix

A. *A mockup of Dr. Mitra’s Welding Procedure on the recommended QW-482 form.

(See Attachment 1)

B. *A mockup of Dr. Mitra’s Welding Procedure’s PQR on recommended QW-483 form

(Not a good PQR due to lack of test and visible defect)

(See Attachment 2)

C. *Table QW-451.1 Shows the ranges of thickness that can be welded after WPS is

qualified and what test and how many needed to be performed per coupon’s thickness.

(See Attachment 3)

D. A screen shot of Dr. Mitra’s Original tool with modifications per QW-461.10 and the 3-D

print outs of tool with modifications.

(See Attachment 4)

E. A picture of the focal map that was used to make the 27 simulations.

(See Attachment 5)

F. A graph representing that data collected from the COMSOL simulations showing the

different temperatures and a map of the distributed heat (µ=.45).

(See Attachment 6)

G. *The new proposed WPS for Dr. Mitra to qualify with changes made to QW-410

Technique.

(See Attachment 7)

H. *A table QW-267 of the welding variables for Friction Stir Welding. These shows the

Essential, Supplementary Essential, and Nonessential variables.

(See Attachment 8)

I. *An excerpt from Non-mandatory Appendix D found in the back of Section IX with the

Grade of Aluminum Boxed that was used. Tells the Reader the P Number and SA/SB

Number.

(See Attachment 9)

J. *An excerpt from Table QW/QB-422 with the material boxed in and all of the

information for QW-403 on the WPS/PQR.

(See Attachment 10)

K. *Excerpts from QW-400 Welding Data with all of the paragraphs boxed in that pertain to

the FSW stated in Attachment 8. The Welding Data tells the Reader the tolerance of the

variables.

(See Attachment 11)

L. *Excerpt of the table of contents from Section IX to Show all the different section for the

PQR test. These Sections tell how to prepare the test and how to perform the test.

(See Attachment 12)

Note: All Letters with * had there attachments obtained from ASME (American Society of

Mechanical Engineers) Boiler and Pressure Vessel Code 2013 Ed.

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Typewritten Text

Attachment 5