Dissimilar Friction Stir Welding between /67531/metadc955097/m2/1/high...DISSIMILAR FRICTION STIR WELDING BETWEEN MAGNESIUM ... Gregory A. Dissimilar Friction Stir Welding between

APPROVED: Aleksandra Fortier, Major Professor Kyle Horne, Committee Member Xiaohua Li, Committee Member Yong X. Tao, Chair of the Department of

Mechanical and Energy Engineering Costas Tsatsoulis, Dean of the College of

Engineering Victor Prybutok, Vice Provost of the

Toulouse Graduate School

DISSIMILAR FRICTION STIR WELDING BETWEEN MAGNESIUM

AND ALUMINUM ALLOYS

Gregory A. Reese

Thesis Prepared for the Degree of

MASTER OF SCIENCE

UNIVERSITY OF NORTH TEXAS

December 2016

Reese, Gregory A. Dissimilar Friction Stir Welding between Magnesium and Aluminum

Alloys. Master of Science (Mechanical and Energy Engineering), December 2016, 35 pp., 2

tables, 20 figures, 53 numbered references.

Joining two dissimilar metals, specifically Mg and Al alloys, using conventional welding

techniques is extraordinarily challenging. Even when these alloys are able to be joined, the weld

is littered with defects such as cracks, cavities, and wormholes. The focus of this project was to

use friction stir welding to create a defect-free joint between Al 2139 and Mg WE43. The stir

tool used in this project, made of H13 tool steel, is of fixed design. The design included an 11

mm scrolled and concave shoulder in addition to a 6 mm length pin comprised of two tapering,

threaded re-entrant flutes that promoted and amplified material flow. Upon completion of this

project an improved experimental setup process was created as well as successful welds between

the two alloys. These successful joints, albeit containing defects, lead to the conclusion that the

tool used in project was ill fit to join the Al and Mg alloy plates. This was primarily due to its

conical shaped pin instead of the more traditional cylindrical shaped pins. As a result of this

aggressive pin design, there was a lack of heat generation towards the bottom of the pin even at

higher (800-1000 rpm) rotation speeds. This lack of heat generation prohibited the material from

reaching plastic deformation thus preventing the needed material flow to form the defect free

joint.

ii

Copyright 2016

by

Gregory A. Reese

iii

ACKNOWLEDGEMENTS

First and foremost, I would like to thank my advisor Dr. Aleksandra Fortier. Dr. Fortier

was a fantastic mentor who introduced me to the world of friction stir welding. Dr. Fortier helped

arranged my research assistant position with the Material Science and Engineering department at

UNT and guided me each step of the way. She offered autonomy in the lab while always being

available, understanding, and quick to respond.

Thank you to Dr. Rajiv Mishra for allowing me to utilize his well-equipped FSW and

processing lab as well as his technical guidance throughout this project. Also thank you to the

other students on the FSW and processing team who provided training in and outside of the lab.

Thank you to the Army Research Lab, and in particular Mr. Kevin Doherty, who was the

primary party of interest in this project, as well as much of the funding. Mr. Doherty and his

team also provided all the samples needed for this project.

Thank you to the University of North Texas and, in particular, my committee members

Dr. Xiaohua Li and Dr. Kyle Horne for all the expert guidance, use of facilities and opportunities

made available during my academic years.

iv

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ........................................................................................................... iii LIST OF TABLES AND FIGURES.............................................................................................. vi LIST OF ABBREVIATIONS ...................................................................................................... viii DISSIMILAR FRICTION STIR WELDING BETWEEN MAGNESIUM AND ALUMINUM ALLOYS ..........................................................................................................................................1

Introduction ..........................................................................................................................1

Background ..............................................................................................................1

Welding Zones .........................................................................................................3

Welding Parameters .................................................................................................4

Tool Geometry .........................................................................................................6

Base Materials Used in this Project .........................................................................9

Intermetallic Compounds .......................................................................................10

Mechanical Properties ............................................................................................13

Problem Statement .................................................................................................15

Thesis Overview ....................................................................................................15

Experimental Setup ............................................................................................................16

Tilt, Plunge Depth and Speed ................................................................................19

Tool Offset .............................................................................................................20

Opportunistic Obstacle...........................................................................................21

Tool Rotation Speed ..............................................................................................22

Material Depletion .................................................................................................24

Results ................................................................................................................................24

Microscopy ............................................................................................................24

Summary ................................................................................................................27

Future Work .......................................................................................................................28

Tool Geometry .......................................................................................................28

Tool Offset .............................................................................................................29

Tool Rotation Speed ..............................................................................................29

Microstructure and Characterization ......................................................................30

v

Mechanical Testing ................................................................................................30 REFERENCES ..............................................................................................................................32

vi

LIST OF TABLES AND FIGURES

Page

Tables

1. Mg WE43 chemical composition.........................................................................................9

2. Al 2139 chemical composition ..........................................................................................10

Figures

1. Schematic of FSW process ..................................................................................................1

2. Advancing and retreating sides ............................................................................................2

3. (a) Schematic of microstructural zones in aluminum; (b) micrograph showing various microstructural zones [3] .....................................................................................................3

4. No plow vs full plow in regards to tool tilt and plunge depth .............................................4

5. Varying shoulder geometries ...............................................................................................6

6. (a) Shoulder radius; (b) pin's base radius; (c) pin's tip radius; (d) shoulder concavity; (e) pin height .............................................................................................................................7

7. Scrolled Shoulder schematic [23] ........................................................................................7

8. Varying basic pin geometries...............................................................................................8

9. (a) Whorl pin; (b) MX-Triflute pin ......................................................................................8

10. Optical micrographs of the cross-sections perpendicular to the tool traverse direction of the plates friction-stir-welded with tool rotation speeds of (a) 1000, (b) 1200, and (c) 1400 rpm [4] ......................................................................................................................11

11. XRD analysis of joined Mg and Al alloys .........................................................................13

12. (a) Tool length of 102.8 mm; (b) Pin length of 6 mm; (c) Fixed tool design; (d) Scrolled shoulder with two flutes ............