Welding Metallurgy of Aluminium Alloys E-Book

Reza Beygi

• Herausgeber: Wiley-VCH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 381

Veröffentlichungsjahr: 2025

Beschreibung

A must-have guide to welding aluminium alloys for optimized performance and precision

Aluminium alloys, widely used across industries due to their high specific strength and corrosion resistance, require precise joining methods to maintain their structural integrity. Welding Metallurgy of Aluminium Alloys: Design, Processes, and Simulations provides a detailed exploration of aluminium alloy welding, addressing the science, technology, and applications behind this essential process. Integrating essential theory and real-world practice, this comprehensive volume equips readers with the knowledge to analyze, optimize, and execute welding processes while preventing common defects that compromise weld quality.

Presenting insights rooted in over 450 research articles, an experienced team of researchers and practitioners examines microstructural evolution, thermal dynamics, defect mechanisms, and more. Detailed chapters cover various welding methods, including arc, beam, resistance, and solid-state welding, while providing clear guidance on topics ranging from mitigating hot cracking and porosity to simulating temperature distributions and residual stress.

Providing the knowledge to tackle an array of welding challenges, Welding Metallurgy of Aluminium Alloys:

• Features in-depth case studies and simulation methodologies that enable practical application and innovation in diverse industries
• Explores mechanisms behind welding defects and presents strategies for prevention and mitigation
• Discusses simulations for temperature distribution, thermal history, residual stress, and material flow during welding processes
• Highlights microstructural changes and metallurgical transformations unique to each welding process covered
• Includes extensive reference material, such as equations for heat conductivity analysis and defect prediction tailored to aluminium welding

An indispensable resource for designing robust, high-performance welds, Welding Metallurgy of Aluminium Alloys: Design, Processes, and Simulations is ideal for undergraduate and graduate courses in materials science, mechanical engineering, and welding technology, as well as a must-have reference for welding specialists, materials scientists, and construction engineers. It is also a vital addition to any technical library supporting professional development and academic programs focused on advanced joining processes.

Leseprobe

Table of Contents

Cover

Table of Contents

Title Page

Copyright

Preface

List of Abbreviations

1 Aluminum Alloys: Fundamentals of Welding Metallurgy

1.1 Introduction

1.2 Weldability

1.3 Thermal Cycle and Its Effect on the Weld Zone

1.4 Welding of Aluminum to Other Metals

References

2 Arc Welding Processes

2.1 Introduction

2.2 Processes

2.3 Modifications to the Processes

2.4 Hybrid Welding

2.5 Protective Gases

2.6 Filler Material

2.7 Grain Refinement in the Weld

2.8 Grain Structure

2.9 Defects During Arc Welding

2.10 Welding of Dissimilar Metals

2.11 Simulation of the Arc Welding Processes

References

3 Beam Welding Processes

3.1 Introduction

3.2 Laser Systems

3.3 Defects Formation During Laser Welding of Al Alloys

3.4 Shielding Gases in Laser Welding

3.5 Modifications to Laser Welding of Al

3.6 Special Cases of Al Welding

3.7 Simulation of Laser Welding

References

4 Resistance Welding Process

4.1 Introduction

4.2 Weld Zones

4.3 Defects During RSW of Al Alloys

4.4 RSW of Dissimilar Aluminum Alloys

4.5 RSW of Al to Dissimilar Metals

4.6 Modifications to RSW for Welding Al Alloys

4.7 Simulation of the Process

4.8 Evaluation of the Joints

References

5 Solid-State Welding Process

5.1 Introduction

5.2 FSW of Al Alloys

5.3 FSW of Al-Matrix Composites

5.4 FSW of Al to Dissimilar Materials

5.5 Joint Design

5.6 Simulation of the Process

References

Index

End User License Agreement

List of Tables

Chapter 1

Table 1.1 The applications of some Al alloys.

Chapter 2

Table 2.1 Filler materials used to join various Al alloys by arc w...

Table 2.2 Values of GRF for three Al alloys.

Chapter 5

Table 5.1 Process parameters for FSW of thin Al sheets.

List of Illustrations

Chapter 1

Figure 1.1 Designation system for wrought aluminum alloys accordin...

Figure 1.2 Joining of a stringer–skin aluminum part used in the fu...

Figure 1.3 The forming, solution treatment, welding, and age harde...

Figure 1.4 Schematic representation of mechanical properties of Al...

Figure 1.5 Structure of the Al oxide layer in wet air.

Figure 1.6 Solubility of hydrogen in aluminum at various temperatu...

Figure 1.7 Relation between the portion of the oxide layer and the...

Figure 1.8 Pores caused by oxide layer (inside the red squares) du...

Figure 1.9 SEM image of (a) technical or keyhole-induced pore and ...

Figure 1.10 (a) The contribution of eutectic compounds at the gra...

Figure 1.11 Schematic diagram of the semisolid molten pool.

Figure 1.12 Schematic model of hot cracking and its influencing v...

Figure 1.13 Fracture surface of an SC developed during welding of...

Figure 1.14 Effect of (a and b) solidification interval (Δ

T

) on (...

Figure 1.15 Length of the solidification crack with respect to th...

Figure 1.16 Effect of weld pool shape on the segregation and soli...

Figure 1.17 Effect of the welding speed on the solidification cra...

Figure 1.18 Dependency of critical strain rates of the weld with ...

Figure 1.19 Schematic curve of the effect of grain size on SC sen...

Figure 1.20 Dependency of the lateral growth of the grains with t...

Figure 1.21 (a) Al/Si binary phase diagram. (b) Plot used to pred...

Figure 1.22 Secondary dendrite arm spacing of some Al alloys as a...

Figure 1.23 (a and b)

T

–

f

S

curves based on the Scheil equation ta...

Figure 1.24 Temperature versus the square root of the solid fract...

Figure 1.25 (a) Top view of the weld metal. (b) PMZ susceptible t...

Figure 1.26 Schematic of hardness distribution in the HAZ of weld...

Figure 1.27 Schematic of microhardness profile of a HTA welded by...

Figure 1.28 (a) FZ of an AA6061 alloy after aging. (b) The FZ aft...

Figure 1.29 Schematic of hardness profile after FSW of O-temper a...

Figure 1.30 Schematic showing the effect of the temperature gradi...

Figure 1.31 Metallographic images of the failed tensile specimens...

Figure 1.32 Dependency of the joint strength to the IMC thickness...

Chapter 2

Figure 2.1 Position tolerance and weld-gap tolerance for various w...

Figure 2.2 Schematic of plasma arc welding.

Figure 2.3 A comparison of arcs of TIG and PAW.

Figure 2.4 Schematic of variable polarity.

Figure 2.5 Behavior of arc during MIG welding of Al.

Figure 2.6 Schematic of K-TIG.

Figure 2.7 Schematic waveform of DP-VPTIG.

Figure 2.8 Double-wire MIG welding of Al.

Figure 2.9 Schematic of the forces acting on the droplet during do...

Figure 2.10 Schematic of the pulsed currents in two electrodes du...

Figure 2.11 The schematic of direct and indirect electric arc MIG...

Figure 2.12 Current waveform during VP-CMT process.

Figure 2.13 Torches for (a) straight polarity, (b) reverse polari...

Figure 2.14 Cross sections of the weldments of Al—Mg plates welde...

Figure 2.15 Schematic of hybrid plasma-MIG welding when the two p...

Figure 2.16 (a) Arc shape of TIG. (b) Arc shape of a hybrid TIG-M...

Figure 2.17 Swing TIG-MIG welding for narrow-gap welding.

Figure 2.18 Schematic of hybrid TIG-MIG weld-brazing used to join...

Figure 2.19 (a) Effect of Ti content on the (a) average of grain ...

Figure 2.20 Schematic of the constitutional undercooling resultin...

Figure 2.21 Microstructure of a pure Al TIG-welded under periodic...

Figure 2.22 Wave current of DP VP-TIG.

Figure 2.23 Evolution of the microstructure during double-pass VP...

Figure 2.24 (a) Macrograph of the weld of a 6061 alloy. (b) Grain...

Figure 2.25 Schematic diagram of (a) conventional epitaxial growt...

Figure 2.26 Nanoindentation test of the (a) matrix and (b) eutect...

Figure 2.27 SEM images of the weld zone of an AA7075 welded with ...

Figure 2.28 Measured and calculated temperature distribution usin...

Figure 2.29 (a) Schematic of weld-brazing of Al to steel where th...

Figure 2.30 Schematic of weld-brazing of Al/Cu with Al placed on ...

Figure 2.31 (a) Schematic of plasma arc weld-brazing of Al to Ti ...

Figure 2.32 (a) Edge preparation for weld-brazing in butt configu...

Figure 2.33 (a) Schematic of friction surfacing on stainless stee...

Figure 2.34 Schematic illustration of the contour method to calcu...

Figure 2.35 Contours of longitudinal residual stresses during wel...

Figure 2.36 (a) A point at the bottom of a V-groove of a thick Al...

Figure 2.37 Schematic of the restraint applied on two Al sheets c...

Figure 2.38 Shell model of a train body.

Figure 2.39 Flowchart of inherent strain method.

Chapter 3

Figure 3.1 Titanium alloy and aluminum joined by laser welding in ...

Figure 3.2 (a) Hull structure of the airplane. (b) Skin-stringer p...

Figure 3.3 Laser beam welding of a Tee-joint.

Figure 3.4 (a) Heat conduction mode laser welding and (b) keyhole ...

Figure 3.5 (a) Shape of the catenoid and the parameters that defin...

Figure 3.6 (a) Schematic of laser beam parameters and (b) a compar...

Figure 3.7 Profile of energy intensity distribution of (a) Gaussia...

Figure 3.8 Schematic of pressures acting on the keyhole.

Figure 3.9 (a–d) Mechanism of keyhole-induced porosity.

Figure 3.10 (a) High height-to-width ratio leads to higher number...

Figure 3.11 Mechanism of pore formation during laser welding of a...

Figure 3.12 (a) Schematic presentation of the keyhole during lase...

Figure 3.13 (a) Schematic of the weld pool solidification during ...

Figure 3.14 (a) Macrograph of a laser spot weld of AA5083 taken b...

Figure 3.15 Schematic of SC during laser welding of Al alloys joi...

Figure 3.16 Effect of titanium content on the fraction (a) and si...

Figure 3.17 (a) Schematic of the laser welding of two aluminum al...

Figure 3.18 Schematic of the superimposed diode laser on pulsed N...

Figure 3.19 Schematic of the wobble technique showing various fre...

Figure 3.20 Schematic of the laser pulse shape.

Figure 3.21 Various shapes of ramp-down stage in one pulse of las...

Figure 3.22 SC susceptibility (measure by crack radius) with resp...

Figure 3.23 Shape of double-pulse laser welding.

Figure 3.24 Effect of the power of the imposed laser beam as well...

Figure 3.25 Schematic of LAHW with a leading arc.

Figure 3.26 Schematic of the influence of the melt flow on the mi...

Figure 3.27 (a) Pores in the absence of oscillation. (b) Reductio...

Figure 3.28 Effect of ultrasound vibration on the bubble behavior...

Figure 3.29 SEM images of cross section of weld-brazed joints bet...

Figure 3.30 SEM images of cross section of Al/copper joints made ...

Figure 3.31 Schematic of distortion made during laser welding of ...

Figure 3.32 Schematic of a cylindrical plus a Gaussian heat sourc...

Figure 3.33 (a) Hot cracking observed during laser beam welding o...

Figure 3.34 (a) Initiation and propagation of hot cracking in las...

Figure 3.35 (a and b) Phase distribution maps (liquid and solid) ...

Figure 3.36 (a) Schematic of the multiple reflection as well as t...

Figure 3.37 (a) Momentum and temperature distribution around the ...

Chapter 4

Figure 4.1 (a) Schematic of resistance spot welding. (b) Cycles of...

Figure 4.2 (a) Schematic of welding lobe determination for RSW of ...

Figure 4.3 Comparison of evolution of dynamic resistance during re...

Figure 4.4 Schematic of electrode life used for resistance welding...

Figure 4.5 Schedule of applying preheating (current

I

1

at interval...

Figure 4.6 Mechanism of electrode pitting during resistance spot w...

Figure 4.7 Mechanism of nugget formation during resistance spot we...

Figure 4.8 Schematic of the shunting effect during RSW.

Figure 4.9 Various microstructures observed in the different zones...

Figure 4.10 Schematic of microhardness profile in a 5xxx Al alloy...

Figure 4.11 EBSD image of the weld zone of an Al—Zn—Mg alloy weld...

Figure 4.12 Inverse pole figures showing the crystal orientation ...

Figure 4.13 (a) Cross section of the welded sample of AA611, show...

Figure 4.14 TEM image of regions (a) TMAZ-II and (b) the base mat...

Figure 4.15 Schematic of an electrode with an outer ring and an i...

Figure 4.16 Schematic of hot cracks seen in the cross sections in...

Figure 4.17 Schematic of (a) temperature–time history, (b) hot cr...

Figure 4.18 (a) C-gun arms of a RSW machine. (b) Asymmetry of the...

Figure 4.19 Diameter of the electrodes for joining two dissimilar...

Figure 4.20 Asymmetric weld nugget during RSW of stainless steel ...

Figure 4.21 Macro-image of spot weld between two dissimilar Al al...

Figure 4.22 (a) Cross section of the nugget in a joint between AA...

Figure 4.23 Effect of welding time on the nugget diameter and IMC...

Figure 4.24 Interface between Al and steel jointed by resistance ...

Figure 4.25 Application of a shim to increase the angle of the no...

Figure 4.26 Schematic of cross section of resistance spot welding...

Figure 4.27 Schematic of joint showing the bulge of steel into Al...

Figure 4.28 Usage of a composite electrode with a tungsten rod em...

Figure 4.29 (a) Electrodes used for resistance spot clinching. (b...

Figure 4.30 Schematic of a resistance spot clinching joint betwee...

Figure 4.31 Welding cycles for (a) conventional welding according...

Figure 4.32 (a) Schematic of external magnetic field and the curr...

Figure 4.33 Using a Cu-foil as a shield for electrode protection ...

Figure 4.34 Temperature distribution during (a) conventional RSW ...

Figure 4.35 (a) Schematic of resistance plug welding of 7xxx Al a...

Figure 4.36 (a) Solidification crack close to Al–steel interface ...

Figure 4.37 Cross section of the nugget and the predicted nugget ...

Figure 4.38 Maximum temperature map and temperature–time history ...

Figure 4.39 Tensile shear test for evaluation of the strength of ...

Figure 4.40 (a) Weld cross section. Three modes of failure of lap...

Figure 4.41 Schematic of cross-tension test.

Figure 4.42 (a) Coach peel tensile test preparation. (b) Coach pe...

Figure 4.43 (a–c) Typical load-displacement curve obtained during...

Chapter 5

Figure 5.1 Macro-image of an FSW weld made on an AA2519 (a) after ...

Figure 5.2 Schematic of hardness profile of a heat-treatable Al af...

Figure 5.3 Precipitation morphologies in (a) air-cooled and (b) wa...

Figure 5.4 SEM images of the bottom of the NZ of AA7xxx alloy FSWe...

Figure 5.5 Schematic of hardness profile of the weld of a non-heat...

Figure 5.6 Schematic of hardness profile of a weld of a non-heat-t...

Figure 5.7 Schematic of hardness profiles of the weld of a non-hea...

Figure 5.8 Schematic of hardness distribution in the weld of a thi...

Figure 5.9 (a) Fracture in an FSW weld with lack of penetration. (...

Figure 5.10 Schematic of cold lap and hook formed during FSW in l...

Figure 5.11 (a) Schematic of hardness profile in a cast Al alloy ...

Figure 5.12 Schematic of microstructure evolution during FSW of A...

Figure 5.13 Process of upward FSP used to make Al-matrix composit...

Figure 5.14 Schematic of the tool position during FSW of steel....

Figure 5.15 (a) Schematic of microstructure evolution during FSW ...

Figure 5.16 Fracture of the interface of Al/teel interface welded...

Figure 5.17 SEM images of the interface of Al/steel join made by ...

Figure 5.18 Schematic of fracture propagation path at the interfa...

Figure 5.19 (a) Schematic of buttering stainless steel on the edg...

Figure 5.20 Schematic of FSW for T-joining of Al sheets along wit...

Figure 5.21 Schematic of Tee-joint design produced by FSW in lap ...

Figure 5.22 Common defects that form in Tee-joints, when welded i...

Figure 5.23 Tee-joints of Al alloys by FSW in butt configuration....

Figure 5.24 Schematic showing the geometry of the pin and the too...

Figure 5.25 (a and b) Schematic of Al/steel Tee-joint design. (c)...

Figure 5.26 Equivalent plastic strain map obtained during FSW of ...

Guide

Cover

Title Page

Copyright

Preface

List of Abbreviations

Table of Contents

Begin Reading

Index

End User License Agreement

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Welding Metallurgy of Aluminium Alloys

Design, Processes, and Simulations

Reza Beygi, Eduardo André de Sousa Marques, Ricardo João Camilo Carbas, and Lucas Filipe Martins da Silva

Authors

Prof. Reza BeygiArak UniversityArak, Iran

Prof. Eduardo André de Sousa MarquesUniversity of PortoRua Dr. Roberto Frias4200-465 PortoPortugal

Dr. Ricardo João Camilo CarbasINEGI Institute of Science & InnovationRua Dr. Roberto Frias4200-465 PortoPortugal

Prof. Lucas Filipe Martins da SilvaUniversity of PortoRua Dr. Roberto Frias4200-465 PortoPortugal

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Print ISBN: 978-3-527-35334-7ePDF ISBN: 978-3-527-84535-4ePub ISBN: 978-3-527-84536-1oBook ISBN: 978-3-527-84537-8

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Preface

The field of welding metallurgy for aluminum alloys continues to evolve, addressing new challenges and meeting industry demands for advanced materials and methods. Welding Metallurgy of Aluminium Alloys: Design, Processes, and Simulations serves as a comprehensive guide for students, engineers, and professionals who seek a deep understanding of aluminum welding processes and the underlying principles. This book covers critical topics, from microstructure evolution during welding to defect mechanisms and prevention, offering both theoretical insights and practical applications.

Drawing on over 450 research articles, this book bridges the gap between fundamental science and industrial practice. Each chapter explores key aspects of welding metallurgy, providing detailed explanations of microstructural changes across various processes and an in-depth analysis of common defects – both technical and metallurgical. Particular attention is given to defects such as hot cracking, porosity, and softening, as well as process-specific issues such as lack of penetration and kissing bonds in friction stir welding. Mechanisms of defect formation and strategies for mitigation are discussed in depth, enabling improvements in weld quality and performance.

In addition to defect analysis, this book covers simulation approaches for each welding process, examining temperature distribution, thermal history, residual stress, and distortion. Special focus is placed on material flow in friction stir welding, while weld design principles are discussed for all processes, supporting researchers and practitioners in their work.

It is hoped that this book will serve as a valuable resource for those studying or working with aluminum welding, whether in academic settings or industrial applications. By presenting a detailed account of welding metallurgy for aluminum alloys, this book aims to contribute to a deeper understanding and advancement of this important field.

List of Abbreviations

AM

additive manufacturing

AC

alternative current

ALE

arbitrary Lagrangian–Eulerian

BTR

brittle temperature range

CGZ

coarse grain zone

CNT

carbon nanotube

CMT

cold metal transfer

CEL

coupled Eulerian–Lagrangian

DC

direct current

EBSD

electron back-scattered diffraction

EBW

electron beam welding

FE

finite element

FFT

Fast Fourier Transform

FSW

friction stir welding

FSP

friction stir processing

FZ

fusion zone

GMAW

gas metal arc welding

GTAW

gas tungsten arc welding

HAZ

heat-affected zone

HTA

heat-treatable aluminum

IMC

intermetallic compound

LAHW

laser arc hybrid welding

LBW

laser beam welding

LTW

laser transmission welding

LPBF

laser powder bed fusion

MFDC

medium frequency direct current

MIG

metal inert gas

NHTA

non-heat-treatable aluminum

NZ

nugget zone

PMZ

partially melted zone

PSN

particle stimulated nucleation

PLBW

pulsed laser beam welding

RSC

resistance spot clinching

RSW

resistance spot welding

RW

resistance welding

NZ

nugget zone

PAW

plasma arc welding

PFZ

precipitate free zone

PWHT

post-weld heat treatment

SEM

scanning electron microscopy

SC

solidification cracking

SCS

solidification cracking susceptibility

SMAW

shielded metal arc welding

TEM

transmission electron microscopy

TMAZ

thermo-mechanically affected zone

TIG

tungsten inert gas

UTS

ultimate tensile strength

VP

variable polarity

YS

yield strength

1Aluminum Alloys: Fundamentals of Welding Metallurgy

1.1 Introduction

Aluminum (Al) alloys have been used traditionally for structural parts of aircrafts. At the beginning of the twenty-first century, the leading aircraft companies started to use composite materials due to their advantages such as low weight. Nonetheless, the usage of Al in structural parts still continues due to better recyclability of these materials and satisfying the weight reduction of the structure [1]. Al is a light alloy with a density of 2.7 g/cm3, is nonmagnetic, has an FCC crystal structure, has high formability and low-temperature toughness, and has a relatively high corrosion resistance depending on the alloying elements [2]. Al alloys are also easily processible with casting, machining, and extrusion. Some Al alloys capable of precipitation hardening have strengths comparable to steels [2]. In contrast to carbon and low-alloy steels, Al does not possess any polymorph; therefore, only the solidification behavior determines the properties of the weld zone in the fusion welding [3]. Cooling of the solidified weld zone does not have a prominent effect on the microstructure of the weld due to absence of any polymorph transformation in Al alloys.

Al alloys are classified based on their mechanical properties, manufacturing process, and alloying elements. Wrought Al alloys are determined by the degree of cold work and chemical composition. Cast Al alloys are determined by the casting process and chemical composition. Other mechanisms such as precipitation hardening and solid-solution hardening may also contribute to the strength of Al alloys [2]. The alloying elements that promote precipitation hardening in Al are Cu, Cu—Mg, Cu—Li, Cu—Si, Zn, Zn—Mg, Zn—Mg—Cu, and Li—Cu—Mg [2]. The alloying elements that promote solid-solution hardening are Mn, Si, Zn, Mg, and Cu [2].

The designation system for wrought Al alloys in the European standard system is a four-digit numeric. Figure 1.1 shows the designation system according to the European standard. There is also a supplementary designation system whose basis is a chemical symbol. The applications of some Al alloys are provided in Table 1.1.

Along with austenitic stainless steel and Ni alloys, Al alloys are good candidates for the construction of LNG cargo containment, as Al possesses a high toughness due to its FCC structure [11]. The usage of Al alloys as structural materials requires their joining in various joint designs. Mechanical joining methods such as riveting have been used in lap configuration, which adds to the weight of the structure. An example is the stringer–skin connection used in the fuselage of the aircraft (Figure 1.2a), where additional adhesive is used to seal the joint. Welding technologies help to reduce the weight by eliminating the overlap region and the sealant and while maintaining the same performance (Figure 1.2b) [12]. In Tee-joining, the high heat conductivity of Al causes a high thermal gradient to form between the weld pool and base material. Lap joining also adds to the weight of the structure and hence is used for thin sheets less than 1 mm, where welding processes such as resistance welding are to be used. Regarding the weld design, fillet welds are usually avoided in the structural parts under fatigue due to crack initiation from the root of the weld [13]. The joint design also affects the residual stress during welding. For instance, the Tee-joints cause a higher residual stress during welding than lap joints due to higher heat sink effect in the Tee-joints, which causes a higher cooling rate [14].

Figure 1.1 Designation system for wrought aluminum alloys according to the European standard.

Table 1.1 The applications of some Al alloys.

Alloy

Application

AA2xxx

Propellant cabin of launch vehicles

[4]

AA5xxx

Shipbuilding

[5]

AA6xxx

Car body

[6]

, battery tray

[7]

AA7xxx

Bicycle frame

[8]

, panel structure of launcher

[9]

, underframe in high-speed trains

[10]

The Al alloys that are heat-treatable (HTA) need to undergo specific sequences during forming and assembling to obtain desired mechanical properties. For example, in the car body, the Al sheets are welded in a solution-treated or hot-formed state. The final aging treatment, which is connected to the paint bake cycle, is performed after welding [15]. Figure 1.3 shows the schematic workflow of Al alloy used in the car body. Hot forming of Al alloy is performed in the solid-solution state, which is quenched subsequently to preserve the solid-solution state. The joining methods are performed after this stage, and then a paint bake cycle is carried out to regain the strength through aging treatment.

Figure 1.2 Joining of a stringer–skin aluminum part used in the fuselage of the aircraft made by (a) riveting and (b) welding.

Source: Ref. [12]/with permission of Elsevier.

Figure 1.3 The forming, solution treatment, welding, and age hardening while paint baking of Al alloys used in the car body.

There are some challenges during welding of Al alloys, and understanding their mechanisms helps to avoid them and hence obtain the optimum performance of the welded structure. Some issues that exist during welding steel are not present during welding Al due to its physical properties. For instance, Al is a nonmagnetic material and therefore the problems of arc blowing will not occur during arc welding processes. Instead, other issues are pronounced during welding Al. The thermal conductivity of Al is six times that of steel and therefore the welding processes with high power density need to be used for Al. Though the melting point of Al is low, due to high specific heat of Al the heat sources need to have high intensity. Due to the high heat conductivity of Al, the use of welding processes with low heat intensity for wrought alloys and precipitation-hardened alloys produces a wide heat-affected zone (HAZ) with lower strength (due to softening) and distortion. In precipitation-hardened alloys, it is also probable that the precipitates in the HAZ dissolve and reprecipitate, which leads to brittleness [2]. The thermal expansion coefficient of Al is twice that of steel. This along with its low Young's modulus causes a high distortion, especially in thin structures, making it difficult to maintain the tolerances [16]. To keep the distortion within the limits of tolerance, the components need to be tightly clamped and often tack-welded. The welding sequence also needs to be planned carefully [2].

A significant difference between steel and Al arises from metallurgical aspects. Structural steels encounter phase transformation during cooling; the most important one is the austenite to ferrite transformation. While the strength of structural steels increases in the HAZ during welding, Al alloys lose their strength in the HAZ. A higher number of issues exist while welding Al alloys that are heat-treatable, which means their strength is increased by precipitation. Other Al alloys get their strength by other mechanisms such as solution hardening and work hardening. Three main series of heat-treatable Al alloys are introduced in the following.

1.1.1 Aluminum–Copper Alloys

Al—Cu alloys known as 2xxx family are mainly used in the structure of the airframes and possess high fatigue strength. The precipitation hardening is the main mechanism of strengthening in these alloys. From the supersaturated state, a series of phases form by the increase in temperature:

GP phases are coherent phases and differ in the size and the lattice strain they induce. θ′ and θ are semi-coherent and incoherent, respectively, the latter being the equilibrium phase, whose contribution to the strength is the lowest compared to the other phases.

1.1.2 Aluminum–Zinc–Magnesium Alloys

MgZn2 is the main precipitate in this group of Al alloy. These Al alloys are less sensitive to quenching, and thus a supersaturated condition can be obtained even after air cooling. Natural aging can occur in these alloys after a couple of months to recover their initial hardness, making these alloys to have a self-hardening effect [17]. So, these alloys do not need a post-weld heat treatment (PWHT) to recover their hardness. However, these alloys are susceptible to solidification cracking (SC) in the fusion zone (FZ) and liquidation cracking in HAZ. Evaporation of Mg and Zn due to their low boiling point can cause pore formation during welding [18].

1.1.3 Aluminum–Magnesium–Silicium

The strength in the HAZ of these alloys is only partially restored by subsequent aging, as explained in Figure 1.28. The degree of strength lost in the HAZ depends on the heat and the time it receives this heat. It means that the welding process and its parameters determine the degree of strength lost in the HAZ. A lower heat input favors a narrower HAZ region that is less softened.

Figure 1.4 Schematic representation of mechanical properties of Al—Li—Cu alloys in relation to Cu and Li content.

Source: Adapted from [19].

1.1.4 Aluminum–Lithium Alloys

These alloys, as special Al alloys, are used in the aircraft industry. Lithium has a low density and a high solubility in Al and hence offers a great potential to be used in the structure of airplanes. 1% lithium decreases the density of Al by 3% and increases the elastic modulus by 6% [19]. Some Al–Li alloys belong to Al—Cu—Li alloys such as AA2196 (found as extruded parts) and AA2198 (found as sheets and plates) [19]. The mechanical properties of these alloys primarily depend on the content of copper and lithium, as shown in Figure 1.4[20]. The red line shows the solubility limit at 500 °C. The increase in Cu leads to an increase in Al2CuLi precipitation hardening phases, which increase the strength. An increase in Li promotes the formation of Al3Li precipitates and lowers the density. In addition to precipitates, the dispersoids may also exist. Dispersoids, in contrast to precipitates, are formed above the solubility limit temperature, and hence they cannot be altered by heat treatment of the solid alloy. Manganese is an element that forms incoherent dispersoids and increases the toughness and fatigue strength [19]. In Al—Mg—Li alloys, the main precipitate is Al3Li. Al—Li alloys are prone to pore formation during welding, which downgrades their weldability [21].

1.2 Weldability

Welding of Al affects the mechanical properties due to the change in the microstructure during solidification as well as the heat effects in the solid state. The surface tension and the viscosity of the Al melt are low [22]. This increases the risk of melt burn through and spatter during welding Al. The physical, chemical, and metallurgical properties of Al alloys influence their behavior during the welding processes. Despite having a low melting temperature (up to 650 °C), a high thermal conductivity of Al (λ = 120 − 240 W/m K) necessitates a high heat intensity for welding. The thermal conductivity of Al decreases with increasing temperature. Some Al alloys, despite having excellent properties, exhibit low joint efficiency by welding and hence their usage is less favored than other Al alloys that have lower mechanical properties but exhibit better joint efficiency. Some features of Al make it different from steels in terms of weldability. The first feature is the oxide of Al alloy. The special characteristics of Al oxide necessitate some considerations to obtain weldability. The presence of hydrogen is another factor that causes weldability challenges. This is very different from steel in which hydrogen causes completely different problems in welding. Hot cracking is another problem that occurs in the FZ during the final stage of solidification. A similar phenomenon named liquation cracking occurs adjacent to the FZ in partially melted zone (PMZ), which degrades the mechanical properties. The mechanisms of all these phenomena are explained in detail for Al alloys, whose understanding helps to avoid them during welding and improve the weldability.

Figure 1.5 Structure of the Al oxide layer in wet air.

Source: Adapted from [19].

1.2.1 Aluminum Oxide

The oxide of aluminum (Al2O3) is very dense, tough, and thermally stable and has a high melting temperature (Tm = 2050 °C). It is also nonconductive to the electricity. The oxide layer consists of a narrow barrier layer and a thick layer, which have a certain porosity. The presence of alloying elements promotes the formation of complex oxides as well as the change in the thickness of the oxide layer. This oxide layer can trap hydrogen, which changes the characteristics of the oxide layer [19]. The scheme of the oxide layer on the surface of Al in the wet air is shown in Figure 1.5.

The oxide layer can cause an incomplete bonding by inhibiting a metallurgical bonding during welding if it is not removed away by proper methods. In arc welding processes this is done by cleaning effect of the arc in reverse polarity, during which the workpiece is in the positive pole. The oxide layer also influences the plasma spectra and its electronic temperature during arc or laser welding processes [23]. In the absence of the oxide film, the elements are more easily heated to the ionization temperature. Also, the electronic temperature of the plasma is higher, which is due to the absorption of the heat by the oxide film. This causes the depth and the width of the weld to be lower in the presence of the oxide film [23].

Due to its higher melting temperature, the oxide is not melted during welding Al and covers the Al surface with a firm shell, which interferes with the formation of the weld pool [24]. Due to its detrimental effect, the oxide layer has to be removed before welding.

1.2.2 Hydrogen and Pore Formation

Hydrogen is the main gas that forms pores during welding Al alloys. The elements of nitrogen and oxygen do not cause pore formation, as they have a high affinity to make stable nitrides and oxides and therefore are not available in the form of gas. Hydrogen is soluble to a high extent in molten Al, whose solubility decreases abruptly after solidification (Figure 1.6). After solidification, the excess amount of hydrogen, which is not soluble in the solid state, will exit and stay in the melt as pores. This problem is exacerbated when the cooling rate is rapid. By increasing the welding speed, the gas escape is hindered and the pores will become higher in quantity. The position of the welding is also determinant in pore formation. The gas can leave the melt pool easily when welding is performed in the flat position. In the overhead position, there is no escape path for the gas bubbles, and therefore, this position of welding should be avoided for welding Al. One source of hydrogen is humidity, by which the water reacts with Al in the arc space and consequently hydrogen is released according to:

(1.1)

The sources of hydrogen are the humidity in the protecting gas, the humidity entered from the atmosphere, and the residues on the surface of the workpiece or the filler, such as paint, oil, and hydroxides.

Another source of hydrogen is the oxide layer of the Al. The pores that result from the oxide layer of the base material appear as uniform coarse pores over the weld length. The pores that result from the hydrogen of the oxide layer of the filler material appear as uniform fine pores [19]. The thicker the oxide layer, the higher the amount of hydrogen. The thickness of the oxide layer, as mentioned before, depends on the alloying elements or, in other words, the type of Al alloy. For example, AA2198 possesses a thinner oxide layer than AA2196. Another factor that determines the amount of hydrogen from the oxide layer is the thickness of the Al sheet [19]. By increasing the thickness of the Al sheet, the ratio of the oxide/total mass of Al decreases, which in return lowers the amount of hydrogen introduced. The portion of the oxide layer decreases exponentially with thickness, as shown schematically in Figure 1.7. The annealing temperature and time can also increase the thickness of the oxide layer.

Figure 1.6 Solubility of hydrogen in aluminum at various temperatures.

Figure 1.7 Relation between the portion of the oxide layer and the thickness of the sheet for two Al alloys.

Source: Adapted from [19].

The oxide layer can also produce pores in another way. During laser welding of Al, the high temperature inside the keyhole vaporizes the oxide layer at the interface, which appears as pores (red squares in Figure 1.8) [25]. These pores are distinguished by the pores caused by the keyhole instability, which form at the bottom of the weld (blue squares in Figure 1.8). The mechanism of pore formation by keyhole is explained in detail in Chapter 3. Also, the mechanism that controls these kinds of pores will be discussed. As an example, introducing a gap between the plates provides a vent to relieve the pressure, causing a more stable keyhole [25]. This leads to less pore formation and a stronger weld.

Figure 1.8 Pores caused by oxide layer (inside the red squares) during laser welding of a 6061 alloy. (a) Cross section view and (b) longitudinal view.

Source: Ref. [25]/with permission of Elsevier.

Figure 1.9 SEM image of (a) technical or keyhole-induced pore and (b) metallurgical pore.

Source: Ref. [27]/Taylor & Francis.

Based on these, the surface of Al needs to be cleaned before welding either by mechanical or chemical methods [19]. The degree of removal of the surface oxide depends on the thickness of the oxide layer. Considering that the thickness of the oxide is not uniform (see Figure 1.5), the removal of the surface should make sure that the oxide layer is removed completely. The mechanical methods of removing the oxide layer, especially the milling processes, are the most proper ones. Though in these methods the surface becomes wavy, it is free of adhered deposits, which appear after chemical methods [2]. One example of the chemical cleaning method is soaking in 20% sodium hydroxide solution and subsequent soaking in dilute nitric acid to neutralize the residual alkali before welding [26]. Due to the rapid formation of the oxide layer, even in the vacuum, the welding process should be carried out right after the surface treatment.

In fusion welding processes wherein a keyhole exists, such as laser beam welding, another kind of pores can form during welding. The source of these pores is not hydrogen, but the metallic vapor and the shielding gas entrapped by the keyhole collapse. When the melt pool solidifies rapidly, these entrapped vapors and gases do not have enough time to escape from the melt pool, forming a keyhole-induced pore [26]. A scanning electron microscopy (SEM) image of this hole is provided in Figure 1.9a. These pores have irregular shapes with step-like inner walls and traces of solidification contraction [26]. The pores that are caused by hydrogen or by metal vapor produced by the burning of the metal elements are referred to as metallurgical pores [27]. The metallurgical pores have regular shapes, and the inner interface is smooth due to free crystallization cell boundary on the wall of the pore. The internal pressure of accumulated hydrogen inside the pore in the solidification stage initiates after nucleation of the pore and then the continuous diffusion of hydrogen into it [27]. The pores formed by keyhole collapse in the unstable welding pool are also referred to as the technical pores, which are irregular and large [27]. These pores can be reduced by controlling the process parameters. The pores caused by hydrogen are smaller in size than the ones induced by instability of the keyhole [28]. The details of the formation of the technical pores are addressed in Chapter 3.

1.2.2.1 Effects of Porosity on the Mechanical Behavior

The porosity in the weld metal of Al reduces the ductility, tensile strength, and fatigue stress [29]. Deformation concentrates around the porosities, and a higher strain rate exists in the regions with these defects, which subsequently reduces the ductility [30]. If the porosity percent is lower than 4% volume, it does not influence the static mechanical behavior [31]. The pores exhibit on the fracture surface, showing their contribution to the initiation of the fracture [32].

The fatigue strength will not be influenced if the diameter of the pores is less than 0.2 mm, as these pores are present in globular form. By increasing the diameter of the pores, the dynamic behavior will be influenced. The dispersion of the pores is also important in the determination of the mechanical behavior. A solitary scattered pore or pore nest is less detrimental than line pores with very small sizes. In the welded specimens without any pores, the fatigue crack initiates at the surface. While in the presence of porosity, the fatigue cracks initiate near the edge of the porosities, and by increasing the porosity, the fatigue stress reduces [29, 33].

The pores are not the only cause of degradation of the mechanical properties. The cracks made by metallurgical phenomena such as hot cracking that occurs during solidification also have an impact. The existence of various defects such as pores and cracks in the FZ is reported in the literature [34]. Understanding the mechanism of formation of these cracks is necessary to avoid them during welding.

1.2.3 Hot Cracking

SC is a very typical failure in Al alloys. In contrast to pure metal, alloys solidify within a temperature range where both solid and liquid phases are present. A simple explanation of this crack is provided in Figure 1.10. The occurrence of SC is accompanied by local stress or strain as well as eutectic compounds with low melting temperatures present in the grain boundaries between the growing dendrites. Usually, Al alloys are more susceptible to this cracking than pure Al [35]. At the final stage of the solidification process, while the grains are solidified the eutectic phase is present in the form of liquid between the grains. The shrinkage of solidification causes the just solidified grains to move with respect to each other whose boundaries are filled with this liquid. This causes a separation of the grains, which, by further cooling and shrinkage, leads to the formation of cracks lined up together (Figure 1.10a). When enough liquid as well as a path for this liquid to fill these cracks are available, a healing process occurs that eliminates SC, as shown in Figure 1.10b.

Various theories have been proposed to address the details of hot cracking. During solidification when cooling down the melt, there is a slurry stage where both solid and liquid phases are present and the dendrite arms of the solid phase are not coalescent. The temperature is between the temperature of coherency and the liquidus (Tcoh < T < Tl). By further cooling, when the temperature is below the coherency temperature and above the solidus temperature, the dendrites become coalescent and form a coherent solid network named mushy zone (Ts < T < Tcoh) [36]. The schematics of these regions (slurry and mushy zones) are shown in Figure 1.11.

Figure 1.10 (a) The contribution of eutectic compounds at the grain boundaries to the formation of solidification cracking. (b) Filling of the crack by available eutectic compound.

Figure 1.11 Schematic diagram of the semisolid molten pool.

Source: Ref. [37]/Springer Nature.