Material Interactions in a Novel Refill Friction Stir Spot Welding Approach to Joining Al-Al and Al

297 Pages·2016·20.29 MB·English

Material Interactions in a Novel Refill Friction Stir
Spot Welding Approach to Joining Al-Al and Al-Mg
Automotive Sheets

A thesis submitted to the University of Manchester for the degree of Doctor
of Philosophy in the Faculty of Sciences and Engineering

2016

BY

Basem Mohysen Mohammed Al-Zubaidy

SCHOOL OF MATERIALS
Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets

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Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets
Contents
Contents ............................................................................................................................ 3
List of Tables..................................................................................................................... 7
List of Figures ................................................................................................................... 8
List of Abbreviations....................................................................................................... 15
Abstract ........................................................................................................................... 16
Associated Publications .................................................................................................. 17
Declaration ...................................................................................................................... 18
Copyright ........................................................................................................................ 19
Acknowledgements ......................................................................................................... 20
Chapter 1: Introduction ................................................................................................... 23
1.1 The Effects of Automobiles on Global Warming (Greenhouse Gas Emissions) .. 23
1.2 Lightweight Materials in Automobile Body Applications .................................... 25
Influence on Emissions .................................................................................. 25
Multi-Material Automotive Structures ........................................................... 30
1.3 Objectives of this Study ........................................................................................ 35
1.4 Thesis Overview.................................................................................................... 36
Chapter 2: Literature Review .......................................................................................... 37
2.1 Introduction ........................................................................................................... 37
2.2 Joining Overview .................................................................................................. 37
2.3 Fusion Spot Welding ............................................................................................. 39
Resistance Spot Welding (RSW) ................................................................... 39
Laser Welding ................................................................................................ 42
2.4 Mechanical Fastening............................................................................................ 45
Self-Piercing Riveting .................................................................................... 45
Clinching ........................................................................................................ 46
2.5 Solid State Spot Welding ...................................................................................... 47
Ultrasonic welding (USW) ............................................................................. 47
2.6 Friction Stir Spot Welding (FSSW) Techniques. .................................................. 50
Background: Friction Stir Welding (FSW) .................................................... 50
Friction Stir Spot Welding (FSSW) ............................................................... 54
Refill Friction Stir Spot Welding (RFSSW) .................................................. 58
2.7 Heat Generation in FSSW and RFSSW ................................................................ 61
2.8 Effects of Welding Parameters on the RFSSW process........................................ 68
2.9 Material Flow in FSSW and RFSSW ................................................................... 72
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Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets
2.10 Microstructure Formation and Weld Defects in Al-Al Similar Friction Stir Spot
Welds .......................................................................................................................... 78
Microstructure Formation in Al-Al Similar Welds ...................................... 78
Weld Defects in Al-Al Similar Welds ......................................................... 80
2.11 Microstructure Formation of Dissimilar Al-Mg Spot Welds .............................. 83
2.12 Al Alloys ............................................................................................................. 86
Overview and Classification of Al Alloys ................................................... 86
Precipitation Hardening of Al Alloys .......................................................... 90
Aluminium Alloy AA6111 ......................................................................... 94
The Microstructures of AA6111 FSSWs ..................................................... 97
2.13 Magnesium Alloys .............................................................................................. 99
Overview and Classification of Mg Alloys.................................................. 99
Magnesium Alloy AZ31 ............................................................................ 103
2.14 Aluminium and Magnesium Intermetallic Compounds .................................... 106
Al-Mg Binary Phase Diagram .................................................................... 106
Gamma Phase ............................................................................................. 107
Beta Phase .................................................................................................. 108
Other Phases ............................................................................................... 109
2.15 FE Modelling of the Mechanical Behaviour of RFSSW Joints ........................ 110
Chapter 3: Experimental Methods ................................................................................ 115
3.1 Introduction ......................................................................................................... 115
3.2 Materials.............................................................................................................. 115
Surface Preparation for Welding .................................................................. 116
3.3 Welding Method: Refill Friction Stir Spot Welding (RFSSW) and Modified
FSSW ........................................................................................................................ 116
3.4 Heat Treatments .................................................................................................. 121
3.5 Thermal Measurements ....................................................................................... 121
3.6 Hardness Test ...................................................................................................... 123
3.7 Mechanical Testing ............................................................................................. 124
3.8 Metallographic Examination ............................................................................... 125
Sample Preparation ...................................................................................... 125
Optical Microscopy ...................................................................................... 126
Scanning Electron Microscopy (SEM) ........................................................ 126
Energy Dispersive X-ray (EDX) .................................................................. 128
Electron Backscatter Diffraction (EBSD) .................................................... 128
Focused Ion Beam (FIB) Milling ................................................................. 129
Transmission Electron Microscopy (TEM) ................................................. 130
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Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets
3.9 Material Flow during RFSSW ............................................................................ 131
Sample Preparation for Dissimilar Aluminium Alloy Flow Experiments ... 131
Sample Preparation for X-Ray Tomography ............................................... 132
Description of the X-Ray Computational Tomography Technique (XCT) . 133
3.10 FE Modelling of the Lap Tensile-Shear Test of RFSSW Joints ....................... 135
Chapter 4: Refill Friction Stir Welding of Al-Al Similar Welding (Results and
Discussion) .................................................................................................................... 137
4.1 Introduction ......................................................................................................... 137
4.2 Welding Optimisation ......................................................................................... 137
4.3 Effect of the Process Parameters on Material Flow in Refill Friction Stir Spot
Welding and Weld Defects ....................................................................................... 139
4.4 Effect of Welding Variables on the Weld Thermal Field and Thermal History . 158
4.5 Weld Microstructure ........................................................................................... 163
4.6 Effect of Welding Variables on Joint Hardness Profiles in the as-Welded,
Naturally Aged, and Artificially Aged Conditions ................................................... 168
4.7 Effect of Welding Variables on Weld Strength .................................................. 176
4.8 Effect of Natural and Artificial Ageing on Weld Strength ................................. 182
4.9 Weld Fracture Surfaces and Cross-Sections in the As-Welded, Naturally Aged,
and Artificially Aged Conditions .............................................................................. 187
4.10 Summary and Conclusions ................................................................................ 190
Chapter 5: Refill Friction Stir Welding of Mg-Al Dissimilar and Mg-Mg Similar
Welding (Results and Discussion) ............................................................................... 193
5.1 Introduction ......................................................................................................... 193
5.2 Welding Optimisation and RFSSW Modification .............................................. 194
5.3 Effect of Welding Variables on the Weld Thermal Field and Thermal History of
Mg-Al Dissimilar Joints ............................................................................................ 196
5.4 Effect of Welding Variables on Joint Hardness Profile of the Mg-Al Dissimilar
Weld in the As-Welded Condition ............................................................................ 201
5.5 Effect of Welding Variables on the Interface Microstructure of the Mg-Al
Dissimilar Welds ....................................................................................................... 203
5.6 Effect of Welding Variables on the Weld Strength of Mg-Al RFSSWs and
Modified RFSSWs .................................................................................................... 212
5.7 Comparison of the Effect of the Tool Rotation Rate on the Strength of Mg-Al and
Mg-Mg Modified RFSSWs ....................................................................................... 220
5.8 Weld Failure Modes ............................................................................................ 222
5.9 Summary and Conclusions .................................................................................. 225
Chapter 6: FE Modelling of the Mechanical Behaviour of Refill Friction Stir Spot
Welds (Al-Al and Mg-Al) ............................................................................................. 229
6.1 Introduction ......................................................................................................... 229
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Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets
Model Characteristics ..................................................................................... 229
Geometry and Dimensions of the Similar Al-Al Lap-Shear Specimen and
Weld Regions (SZ, HAZ, BM) ............................................................................. 230
Geometry and Dimensions of Dissimilar Mg-Al Lap-Shear Specimen and
Weld Regions (SZ, HAZ, BM) ............................................................................. 233
Material Properties ......................................................................................... 235
Al-Al welds ............................................................................................. 235
Mg-Al welds ........................................................................................... 242
Boundary Loading Conditions in the Model .................................................. 244
Mesh building ................................................................................................. 245
Results and Discussion ................................................................................... 249
As-welded similar Al joints .................................................................... 249
Artificially-aged Similar Joints ............................................................... 258
Mg-Al Dissimilar Welds ......................................................................... 264
Summary and Conclusions ............................................................................. 270
Chapter 7: Conclusions and Recommendations for Further Work ............................... 273
7.1 General Conclusion ............................................................................................. 273
Al to Al similar welds .................................................................................. 273
Mg to Al dissimilar welds ............................................................................ 276
FE Model of the Similar and Dissimilar Joints ............................................ 277
7.2 Recommendations for Further Work .................................................................. 279
Chapter 8: References ................................................................................................... 281

(79,971 words)

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Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets
List of Tables
Table 1-1: Vehicle mass analysis by system and components ........................................ 27
Table 2-1: Properties of pure aluminium, magnesium and iron at their melting points . 41
Table 2-2: Aluminium Alloy Designation Systems ........................................................ 88
Table 2-3: Aluminium Association and European Union (EN) Aluminium Alloy
Temper Designation System ........................................................................................... 88
Table 2-4: Some of the common aluminium precipitation hardening systems ............... 93
Table 2-5: Composition of some of the 6xxx series aluminium alloys used for
automotive panel applications ......................................................................................... 95
Table 2-6: Literal symbols for alloying elements in magnesium alloys ....................... 102
Table 2-7: General divisions of the temper designations for magnesium alloys .......... 103
Table 2-8: Composition of magnesium wrought alloys used for sheets and plates ...... 104
Table 2-9: Mechanical properties of an AZ31 sheet at room temperature ................... 104
Table 3-1: Chemical compositions of the alloys used in this study. ............................. 116
Table 3-2: Range of welding parameters used in this study. ........................................ 119
Table 4-1: The range of welding parameters used in RFSSW of AA6111 similar joints.
....................................................................................................................................... 139
Table 5-5-1: The range of welding parameters used in the RFSSW and modified
RFSSW process for welding AZ31 to AA6111 dissimilar joints. ................................ 196
Table 6-1: True stress-strain data used for modelling the RFSSW joint in the as-welded
condition. ....................................................................................................................... 239
Table 6-2: True stress-strain data used for modelling the RFSSW joint in the paint-
baked condition. ............................................................................................................ 242
Table 6-3: True stress-strain data used for modelling the AZ31-H24 magnesium alloy in
the dissimilar RFSSW joint of Mg-Al. ......................................................................... 243

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Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets
List of Figures
Figure 1-1: Greenhouse gases emitted by transportation ................................................ 24
Figure 1-2: Grams of CO emitted by transportation according to the region and
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projected future targets .................................................................................................... 25
Figure 1-3: Typical energy consumption during the life cycle of a vehicle .................. 26
Figure 1-4: Mercedes-Benz AMG SL63 aluminium body structure .............................. 29
Figure 1-5: Vehma/Ford MMLV a) Mach I BIW b) Mach II BIW ................................ 31
Figure 1-6: Schematic illustration of the RFSSW processes .......................................... 34
Figure 2-1: Classes of methods used to join similar and dissimilar materials ................ 38
Figure 2-2: Schematic illustration of the resistance spot welding (RSW) process ......... 39
Figure 2-3: Schematic illustration of laser welding ........................................................ 43
Figure 2-4: Laser weld bonding process ......................................................................... 45
Figure 2-5: a) Schematic illustration of the self-piercing riveting process; b) Cross-
section of two sheets joined by a self-piercing rivet ....................................................... 46
Figure 2-6: Schematic illustration of the clinching process ........................................... 46
Figure 2-7: Schematic illustration of the ultrasonic welding (USW) process ................ 48
Figure 2-8: The effect of welding time on the cross-sectional appearance of USWs of
Al-Al ............................................................................................................................... 49
Figure 2-9: Schematic diagram of the friction stir welding (FSW) process ................... 50
Figure 2-10: Friction stir welding tool ............................................................................ 51
Figure 2-11: Some selected tools designed at TWI ........................................................ 52
Figure 2-12: Schematic illustration of the friction stir spot welding (FSSW) process ... 55
Figure 2-13: The typical macrostructure of a cross-section of a friction stir spot weld. 56
Figure 2-14: Failure loads of AA6111-T4 sheets joined using FSSW ........................... 57
Figure 2-15: Schematic illustration of the friction stir spot welding refilling process ... 57
Figure 2-16: Schematic illustration of the RFSSW tool system ..................................... 59
Figure 2-17: Schematic illustration of the two RFSSW process variants ....................... 60
Figure 2-18: Constitutive behaviour of an AA6056 during FSW ................................... 62
Figure 2-19: Temperature cycle during the complete welding process (a) in different
distances from the SZ and (b) at a distance of 2.66 mm (Shen, et al., 2015). ................ 64
Figure 2-20: (a) Results of the simulated temperature distribution in the RFSSW process
for a half weld at the welding time of 3 s and (b) thermal cycles in different tracked
positions .......................................................................................................................... 65
Figure 2-21: Results of the simulated strain rate distribution in an RFSSW process for a
welding time of 3 s .......................................................................................................... 66
Figure 2-22: A schematic illustration of the RFSSW process with the placement of the
thermocouples during the temperature measurements .................................................... 67
Figure 2-23: The temperature profile during RFSSW of an Al-Mg dissimilar joint ...... 67
Figure 2-24: The effect of key welding parameters on the tensile shear strength of
RFSSW joints .................................................................................................................. 69
Figure 2-25: The effect of plunge depth and welding time on the lap shear strength of
RFSSW joints .................................................................................................................. 70
Figure 2-26: Effect of the RFSSW process parameters on the lap shear strength of
AA5754 to AZ31 joints................................................................................................... 71
Figure 2-27: Interlamellar regions produced during dissimilar Al 5754-Al 6111 FSSW
using a threaded pin tool ................................................................................................. 73
Figure 2-28: Schematic illustration of a material flow model during FSSW ................. 73
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Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets
Figure 2-29: Material flow observed with a pinless tool from welds produced with the
upper and lower sheets split though the weld centre into dissimilar copper content alloys
......................................................................................................................................... 75
Figure 2-30: X-ray tomography results showing the dished nature developed by the
weld interface for welds produced with the pinless tool ................................................. 75
Figure 2-31: Cross-sectional views of a kinematic model after different numbers of tool
rotations, for three surface stick-slip conditions ............................................................. 76
Figure 2-32: Simulation results of material flow during RFSSW................................... 77
Figure 2-33: Material flow in RFSSW during sleeve-plunging ...................................... 78
Figure 2-34: Optical micrograph of the cross-section of an RFSSW joint, of AA6181-T4
......................................................................................................................................... 79
Figure 2-35: Microhardness profile measured in the middle of the upper sheet at the
cross-section of an RFSSW joint of AA6181-T4 ........................................................... 80
Figure 2-36: Macrographs of RFSSW joint cross-section showing detrimental weld
zone, geometric/metallurgical features and weld defects ............................................... 80
Figure 2-37: Different types of hook defect in RFSSW welds ....................................... 81
Figure 2-38: The effect of welding parameters on the hook height ................................ 82
Figure 2-39: Cross-section of an Al5754-AZ31 RFSSW joint: (a) as-welded sample, (b)
micrographs of the eutectic structure in the IMC layer................................................... 84
Figure 2-40: (a) Macrograph of an RFSSW joint between AA5754 Al and AZ31 Mg,
(b) enlarged microstructures from region 2, and (c) the chemical composition
distribution across the interface ...................................................................................... 84
Figure 2-41: Cross-section of an Al5754-AZ31 RFSSW joint ....................................... 85
Figure 2-42: Typical SEM images of (a) the interfacial layers in low- and high-
rotational-rate welds; (b) the corresponding interfacial layer in the fractured welds ..... 85
Figure 2-43: The increase in aluminium content in North American small vehicles over
the last three decades ....................................................................................................... 87
Figure 2-44: Typical aluminium alloy applications in an automobile ............................ 90
Figure 2-45: (a) The Al-rich side of an Al-Cu binary diagram showing GP zone, θ and
θ solvus lines (dotted lines), (b) θ unit cell, (c) GP zone, (d) θ unit cell and (e) θ unit
cell ................................................................................................................................... 91
Figure 2-46: Activation energy barriers to the nucleation of precipitate phases in Al-Cu
binary alloys .................................................................................................................... 93
Figure 2-47: The effect of copper percentage and ageing time on the hardness of Al-Cu
binary alloys on ageing at (a) 130C and (b) 190C ....................................................... 94
Figure 2-48: The pseudo-binary Al-Mg2Si system ......................................................... 96
Figure 2-49: SEM images showing coarse precipitates of the second phase present in
weld zones of the AA6111 FSSW joints ........................................................................ 98
Figure 2-50: TEM micrographs for: the AA6111 parent material (a) in the T4 condition
and (b) after the paint-bake treatment, and from the pinless FSSW joint centre of a 0.5
second dwell-time weld (c) in the as-welded condition, and (d) after the paint-bake
treatment .......................................................................................................................... 99
Figure 2-51: Comparison of the specific stiffness and specific strength of pure Mg with
Al and iron ................................................................................................................... 100
Figure 2-52: Magnesium production data in the period 1938–2008. ............................ 100
Figure 2-53: Timeline summarising magnesium automotive applications ................... 101
Figure 2-54: Schematic of magnesium’s hexagonal close-packed (HCP) crystal lattice
and major planes ........................................................................................................... 102
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Material interactions in a novel refill friction stir spot welding approach to joining Al-Al and Al-Mg automotive sheets
Figure 2-55: Typical microstructures for an AZ31 sheet .............................................. 104
Figure 2-56: The effects of Al and Zn additions on the mechanical properties of Mg in
annealed and hard rolled conditions.............................................................................. 105
Figure 2-57: Al-Mg equilibrium phase diagrams ......................................................... 106
Figure 2-58: The Al-Mg binary phase diagram drawn by Okamoto using calculated data
from Murray and experimental results from Su et al .................................................... 107
Figure 2-59: Crystal structure of the gamma phase (Al12Mg17) .................................... 108
Figure 2-60: Complete unit cell of β-Al3Mg2 phase ..................................................... 109
Figure 2-61: The middle section of the Mg-Al phase diagram proposed by Su et al ... 109
Figure 2-62: ε-Al30Mg23 phase crystal structure ........................................................... 110
Figure 2-63: The geometric features of the welded joint. ............................................. 111
Figure 2-64: The engineering stress-strain curves assumed for the BM, SZ, and TMAZ
....................................................................................................................................... 113
Figure 2-65: Stress distribution in the RFSSW joint under loading ............................. 113
Figure 2-66: Finite-element model of a FSSW joint with 5 different material property
regions ........................................................................................................................... 114
Figure 3-1: The refill friction stir spot welding machine. ............................................. 117
Figure 3-2: Typical monitoring chart for the RFSSW process. .................................... 117
Figure 3-3: Welding head for the RFSSW process. ...................................................... 118
Figure 3-4: The RFSSW tool parts. .............................................................................. 118
Figure 3-5: The RFSSW machine variable control table. ............................................. 119
Figure 3-6: Schematic illustration of the modified RFSSW process ............................ 120
Figure 3-7: Schematic illustration of the different thermocouple locations for AA6111
similar welding. ............................................................................................................. 122
Figure 3-8: Schematic illustration of the thermocouple location for the dissimilar AZ31-
H24 to AA6111-T4 welding. ........................................................................................ 122
Figure 3-9: Schematic illustration of the locations of the microhardness measurements
....................................................................................................................................... 123
Figure 3-10: Schematic of the Vickers Microhardness indentation test. ...................... 124
Figure 3-11: Schematic illustration of lap-shear test specimens ................................... 125
Figure 3-12: Effects produced by the electron beam–specimen interaction ................. 127
Figure 3-13: Schematic diagram illustrating the FIB procedure of TEM sample
preparation .................................................................................................................... 130
Figure 3-14: Schematic diagram illustrating the TEM optical system ......................... 131
Figure 3-15: Schematic illustration of one of the material flow sample alignments .... 132
Figure 3-16: Schematic illustration of the alignment for the X-ray Tomography sample
....................................................................................................................................... 133
Figure 3-17: (a) Principle of X-ray tomography; (b) synchrotron and laboratory
tomography ................................................................................................................... 134
Figure 4-1: The effect of plunge time and tool rotation rate on the weld surface
appearance and the completeness of the refilling process. ........................................... 139
Figure 4-2: The output of the kinematic model, showing the surface velocity profile by
assuming complete sticking .......................................................................................... 141
Figure 4-3: Effect of weld plunge depth and process interruption on the material flow
....................................................................................................................................... 143
Figure 4-4: Effect of sleeve plunge depth and process interruption on material flow in
the lower sheet .............................................................................................................. 144
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