Hasan, Ahmed Falh (2016) Modelling of tool wear and metal flow behaviour in friction stir welding

243 Pages·2017·10.55 MB·English

by  Christine Middleton

Modelling of Tool Wear and Metal Flow
Behaviour in Friction Stir Welding (FSW)

Ahmed Falh Hasan, BSc, MSc

This thesis is submitted to The University of Nottingham for the
degree of Doctor of Philosophy

May 2016
Abstract
Friction Stir Welding (FSW) is a solid-state joining process that was invented in 1991;
it is particularly useful for joints difficult to make using fusion techniques. Significant
advances in FSW have been achieved in terms of process modelling since its inception.
However, until now experimental work has remained the primary method of
investigating tool wear in FSW.
In this project, two main objectives were set; the first one was to produce a numerical
approach that can be used as a useful tool to understand the effect that worn tool
geometry has on the material flow and resultant weld quality. The second objective
was to provide a modelling methodology for calculating tool wear in FSW based on a
CFD model.
Initially, in this study, a validated model of the FSW process was generated using the
CFD software FLUENT, with this model then being used to assess in detail the
differences in flow behaviour, mechanically affected zone (MAZ) size and strain rate
distribution around the tool for both unworn and worn tool geometries. Later, a novel
methodology for calculating tool wear in FSW is developed. Here a CFD model is
used to predict the deformation of the highly viscous flow around the tool, with
additional analysis linking this deformation to tool wear. A validation process was
carried out in this study in order to obtain robust results when using this methodology.
Once satisfied with the tool wear methodology results, a parametric study considering
different tool designs, rotation speeds and traverse speeds was undertaken to predict
the wear depth. In this study, three workpiece materials were used which were
aluminium 6061, 7020 and AISI 304 stainless steel, while the materials used for the
tools used were of H13 steel and tungsten-rhenium carbide (WRe-HfC) with different
tool designs.
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The study shows that there are significant differences in the flow behaviour around
and under the tool when the tool is worn and it shows that the proposed approach is
able to predict tool wear associated with high viscous flow around the FSW tool.
With a simple dome shaped tool, the results shows that the tool was worn radially and
vertically and insignificant wear was predicted during welding near the pin tip.
However, in other regions the wear increased as the weld distance increased.
Additionally, from the parametric study that was undertaken for the two tool designs -
a dome and a conical shape- the study has found that for both tool designs, wear depth
increases with increasing tool rotation speed and traverse speed. It was also shown
that, generally, the wear depth was higher for the conical tool design than the dome
tool in the pin tip zone. The research concludes that a proposed methodology is able
to calculate tool wear associated with high viscous flow around the FSW tool, which
could be used as a method for calculating tool wear without the need for experimental
trials. The CFD model has provided a good tool for prediction and assessment of the
flow differences between un-worn and worn tools, which may be used to give an
indication of the weld quality and of tool lifetime. Furthermore, from the results, it can
be concluded that this approach is capable of predicting tool wear for different process
parameters and tool designs and it is possible to obtain a low wear case by controlling
the process parameters.

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Acknowledgements
I would like to thank my supervisors, Assistant Prof. Chris Bennett and Prof. Philip
Shipway for their guidance, support, expertise and their patience. It is my pleasure to
have an opportunity work with them. I would like to thank my internal assessor,
Associate Prof. Kathy Simmons for the feedback during my progression assessments.
I would like to acknowledge the support of the Higher Committee for Education
Development in Iraq (HCED) for the research scholarship. I also acknowledge the
service of High Performance Computing (HPC)- University of Nottingham, and the
support of TWI Technology Centre – Yorkshire. I would like also to thank my
colleagues in the Structural Integrity and Dynamics Group and UTC, in particular the
advice and discussion of Dr Simon Woolhead and Ossama Muhammad.
Indeed I have to sincerely thank my family and kids who love, support and encourage
me. Finally, I would like to say thank you to my incredible wife Asra for everything
throughout the last four years.

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Table of contents
1 CHAPTER 1 INTRODUCTION ...................................................................... 1
1.1 BACKGROUND ............................................................................................... 1
1.2 MOTIVATION AND GOALS .............................................................................. 3
1.3 LIST OF PUBLICATIONS .................................................................................. 5
1.3.1 Journal paper ............................................................................................ 5
1.3.2 Conference paper ..................................................................................... 5
1.4 THESIS STRUCTURE ....................................................................................... 6
2 CHAPTER 2 LITERATURE REVIEW .......................................................... 8
2.1 GENERAL FSW MODELLING .......................................................................... 8
2.2 MATERIAL FLOW IN FSW ........................................................................... 14
2.2.1 Experimental Material Flow Visualization ............................................ 18
2.2.2 Numerical Material Flow Visualization ................................................. 22
2.2.3 Final remarks on flow behaviour ........................................................... 34
2.3 TOOL WEAR ................................................................................................. 36
2.3.1 Wear ....................................................................................................... 36
2.3.2 Numerical simulation of component geometry changes as a result of
wear 41
2.3.3 Experimental investigation of tool wear in FSW ................................... 46
2.3.4 Final remarks on tool wear ..................................................................... 54
2.4 SUMMARY AND KNOWLEDGE GAPS ............................................................. 55
3 CHAPTER 3 NUMERICAL TECHNIQUES ............................................... 57
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3.1 GENERAL FLUID FLOW BACKGROUND ......................................................... 57
3.1.1 The Reynolds number ............................................................................ 58
3.1.2 Newton’s law of fluid dynamics ............................................................ 59
3.1.3 Fluid flow governing equations ............................................................. 60
3.2 COMPUTATIONAL FLUID DYNAMICS (CFD) ................................................. 61
3.2.1 Numerical method .................................................................................. 62
3.3 CONSTITUTIVE EQUATIONS ......................................................................... 65
3.4 USER DEFINED FUNCTION (UDF) ................................................................ 66
4 CHAPTER 4 MODELLING METHODOLOGY ......................................... 67
4.1 INTRODUCTION............................................................................................ 67
4.2 ASSUMPTIONS ............................................................................................. 67
4.3 GEOMETRY.................................................................................................. 68
4.4 BOUNDARY CONDITIONS ............................................................................. 69
4.4.1 Tool boundary conditions ...................................................................... 69
4.4.2 Thermal model ....................................................................................... 72
4.5 SOLVER ....................................................................................................... 74
4.6 CONVERGENCE CRITERIA ............................................................................ 75
4.7 MESH DEVELOPMENT .................................................................................. 75
4.7.1 Structural hexahedral mesh study .......................................................... 78
4.7.2 Hybrid mesh study ................................................................................. 84
4.8 FLOW BEHAVIOUR VALIDATION .................................................................. 93
4.8.1 Model details .......................................................................................... 93
4.8.2 Model validation .................................................................................... 94
4.8.3 The effect of using strain rate dependent viscosity on the flow behaviour
100
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4.9 DISCUSSION .............................................................................................. 105
4.10 CONCLUSIONS ........................................................................................... 107
5 CHAPTER 5 MECHANICALLY AFFECTED ZONE (MAZ)
VALIDATION AND FLOW COMPARISON .................................................... 108
5.1 INTRODUCTION.......................................................................................... 108
5.2 MODELS DESCRIPTION ............................................................................... 108
5.3 VALIDATION THE SIZE OF THE MAZ.......................................................... 110
5.3.1 Geometry of model 1 ........................................................................... 110
5.3.2 Validation procedure ............................................................................ 110
5.3.3 Results of model 1 ................................................................................ 113
5.4 COMPARISON OF THE FLOW BEHAVIOUR AROUND UNWORN AND WORN TOOLS
116
5.4.1 Geometry of the model 2...................................................................... 116
5.4.2 Mesh Study .......................................................................................... 118
5.4.3 Results of model 2 (Results for the unworn and worn tool) ................ 121
5.5 DISCUSSION .............................................................................................. 130
5.6 CONCLUSIONS ........................................................................................... 131
6 CHAPTER 6 MODELLING OF TOOL WEAR ........................................ 133
6.1 INTRODUCTION.......................................................................................... 133
6.2 EXPERIMENTAL TESTS ............................................................................... 133
6.3 TOOL WEAR MODELLING ........................................................................... 136
6.3.1 CFD modelling ..................................................................................... 136
6.3.2 Tool wear methodology ....................................................................... 139
6.4 RESULTS ................................................................................................... 145
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6.4.1 Computed temperature fields ............................................................... 145
6.4.2 Material flow behaviour (velocity distribution and Pressure).............. 147
6.4.3 Tool wear prediction ............................................................................ 151
6.5 DISCUSSION .............................................................................................. 156
6.6 CONCLUSIONS ........................................................................................... 158
7 CHAPTER 7 THE EFFECT OF WELD PARAMETERS ON THE
PREDICTION OF TOOL WEAR ........................................................................ 160
7.1 INTRODUCTION.......................................................................................... 160
7.2 MODEL DESCRIPTION ................................................................................ 160
7.3 RESULTS ................................................................................................... 164
7.3.1 The dome shaped tool .......................................................................... 164
7.3.2 The conical shaped tool ........................................................................ 175
7.3.3 Tool profile results ............................................................................... 184
7.3.4 Wear depth comparison........................................................................ 190
7.4 DISCUSSION .............................................................................................. 194
7.5 CONCLUSION ............................................................................................. 196
8 CHAPTER 8 CONCLUSIONS AND FUTURE WORK ............................ 198
8.1 CONCLUSIONS ........................................................................................... 198
8.2 FUTURE WORK .......................................................................................... 200
9 REFERENCES ............................................................................................... 202
10 APPENDICES ................................................................................................... A

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Table of figures
Figure 1-1 Friction stir welding process diagram ........................................................ 2
Figure 2-1 The elements and space in both the Lagrangian and Eulerian approaches
[18] ............................................................................................................................... 9
Figure 2-2 A schematic diagram explains the material flow in the FSW process ..... 15
Figure 2-3 Processing zones in FSW and material flow patterns [3] ......................... 16
Figure 2-4 Onion ring zone (Nugget zone) in the FSW Process [34] ........................ 16
Figure 2-5 FSW microstructural zones according to Attala for AA2095 Al-alloy[35]
.................................................................................................................................... 17
Figure 2-6 Material flow pattern on the retreating side of the weld [45] ................... 21
Figure 2-7 Tool advance and extrusion area per revolution.[45] ............................... 21
Figure 2-8 Experimental weld micro-section with simulation flow diagram at (a) 1600
rpm and (b) 400 rpm rotation speeds and 0.2 m/min welding speed for 2014 Al alloy.
.................................................................................................................................... 27
Figure 2-9 Experimental observation and numerical study: (a) shape of thermal zone
and simulation temperature profile, (b) observation of MAZ with simulated equivalent
strain Geuerdoux and Fourment [53] ......................................................................... 28
Figure 2-10 Iso-surface of the velocity at (a) 3mm s-1, (b) 5mm s-1, and (c) 20mm s-1
[54]. ............................................................................................................................ 29
Figure 2-11 Influence of the rotation intensity on the flow lines [56] ....................... 31
Figure 2-12 Material flow on the surface of the retreating side (RS): (a) t = 0s and (b)
t = 0.9s [58] ................................................................................................................ 33
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Figure 2-13 Two surfaces in intimate contact with each other during sliding, represents
adhesive wear [70] ..................................................................................................... 38
Figure 2-14 Schematic diagram representing the abrasive wear process [64]. .......... 39
Figure 2-15 Wear rate of copper, subjected to two–body abrasion by SiC abrasive
paper, as a function of abrasive particle size at two different sliding velocities [63]. 40
Figure 2-16 Simulation of fretting wear [73] ............................................................. 42
Figure 2-17 SEM photo of taper pin shows smeared material, suggesting abrasive
wear [67] .................................................................................................................... 48
Figure 2-18 SEM photo of the shoulder shows a rolling pattern, suggesting abrasive
wear [67] .................................................................................................................... 49
Figure 2-19 Tool geometries at different welding lengths. Note that the total length of
accumulative welds is different for the three different tools (length in inches) [87]. 51
Figure 2-20 Cross-section images of (a) CY16, (b) W–La, and (c) WC411 tools were
taken by optical microscopy graphs [87]. .................................................................. 52
Figure 2-21 Weld zone cross-section images after FSW with a tool rotation rate of
1000 rpm and traverse speed of 50mm per minute using (a) CY16, (b) W–La, (c) W–
La-L, (d) WC411 and (e) WC411-L tools [87]. ......................................................... 53
Figure 3-1 FLUENT solvers [102] ............................................................................. 64
Figure 4-1 Computational domain and boundary conditions ..................................... 68
Figure 4-2 Cell shapes [121] ...................................................................................... 76
Figure 4-3 FSW model domain blocking strategy ..................................................... 79
Figure 4-4 Hexahedral mesh shape at the tool surface .............................................. 79
Figure 4-5 Radial line at angle position of 45° used for data analysis ....................... 81
Figure 4-6 Velocity profile with the radial distance away from the tool surface at an
angle of 45o, for the hexahedral grids ......................................................................... 82
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