Refill friction stir spot welding (RFSSW): a review of processing, similar/dissimilar materials joining, mechanical properties and fracture mechanism

Refill friction stir spot welding(RFSSW)provides a novel method to join similar and/or dissimilar metallic materials without a key-hole in the center of the joint.Having the key-hole free characterization,the similar/dissimilar RFSSW joint exhibits remarkable and endurable characteristics,including high shear strength,long fatigue life,and strong corrosion resistance.In the meanwhile,as the key-hole free joint has different microstructures compared with conventional friction stir spot welding,thus the RFSSW joint shall possess different shear and fatigue fracture mechanisms,which needs further investigation.To explore the underlying failure mechanism,the similar/dissimilar metallic material joining parameters and pre-treatment,mechanical properties,as well as fracture mechanisms under this novel technology will be discussed.In details,the welding tool design,welding parameters setting,and the influence of processing on the lap shear and fatigue properties,as well as the corrosion resistance will be mainly discussed.Moreover,the roadmap of RFFSW is also discussed.

Joining Technology of Dissimilar Materials for Automotive Components(Ⅱ)

Joining techniques of dissimilar materials for lightweight multi-material automotive body structure were discussed. The joining of 1 .4 mm thickness steel and 2 mm thickness of Al were performed by the new method that is hybrid laser welding system. After aluminum and steel were welded by laser hybrid welding process, the micro-structure investment and the micro-hardness test were carried out. Hybrid laser welding promises a bright future in joining technology of dissimilar materials for automotive components.

Electron-Beam Welding Joint Strength of Dissimilar Materials

This paper provides an in-depth discussion of the joint strength of electron beam welding of dissimilar materials.The effect of welding parameters and material properties on the joint strength was analyzed,and an argument for the optimal parameter combination is presented.Electron-beam welding technology offers several advantages,including high energy density and the ability to create fine weld seams.However,it also presents certain challenges,such as the complexity of welding parameters and the potential generation of brittle phases.The analysis conducted in this paper holds significant importance in enhancing the quality and efficiency of dissimilar material welding processes.

Microstructure and mechanical properties of butt joints of QCr0.8/1Cr21Ni5Ti equal-thickness dissimilar materials by electron beam welding

Butt joints of QCr0.8/1Cr21Ni5Ti equal-thickness dissimilar materials were obtained by electron beam welding with fixed accelerating voltage 60 kV and focus current ~1.99 A , changed electron beam current and welding velocity. Microstructure and composition of the EBW joint were investigated by means of optical micrography and EDX analysis, mechanical properties of the joint were also tested. The results show that joint’s macrostructure was divided into three zones: top weld zone near QCr0.8 and bottom weld zone consisting of Cu(ss.Fe) with a certain amount of dispersedly distributed (α+ε) mixed microstructure, middle weld zone consisting of (α+ε) microstructure with a small amount of Cu(ss.Fe) particles. Morphological inhomogeneous macrostructure and uneven chemical compostion of QCr0.8/1Cr21Ni5Ti joint by EBW are the most important factor to result in decreasing joining strength.

Development of processing windows for friction stir spot welding of aluminium Al5052/copper C27200 dissimilar materials

Friction stir spot welding technique was used to join dissimilar combinations of aluminium alloy(Al5052)with copperalloy(C27200)and friction stir spot welding windows such as tool rotational speed–dwell time and tool rotational speed?plungedepth diagrams for effective joining of these materials were developed.Using a central composite design model,empirical relationswere developed to predict the changes in tensile shear failure load values and interface hardness of the joints with three processparameters such as tool rotational speed,plunge depth and dwell time.The adequacy of the developed model was verified usingANOVA analysis at95%confidence level.Response surface methodology was used to optimize the developed model to maximizetensile strength and minimize interface hardness.A high tensile shear failure load value of3850N and low interface hardness valueof HV81was observed for joints made under optimum conditions,and validation experiments confirmed the high predictability ofthe developed model with error less than2%.The operating windows developed shall act as reference maps for future designengineers in choosing appropriate friction stir spot welding process parameter values to obtain good joints.

Joining and surfacing of advanced materials

The application of advanced materials, i.e. advanced ceramics, glasses, intermetallic phases and various type of composites, not only depends on their manufacture processes including a great input of know-how, but also on their abilities for processing, among which the joining processes play an important role. The uses of advanced materials are changing rapidly, with a major emphasis on technical applications, especially the components of machines, apparatus and technical devices expected to withstand very heavy exploitation conditions. Furthermore,these materials are becoming more complex, in terms of being strengthened and toughened by transformation processes as well as by the addition of other ceramic or metallic materials including nanomaterials. The successful use of advanced materials requires the development of equally advanced joining materials, processes and technology. Some selected examples of results of joining advanced materials with the use of various procedures as well as surface modification of structural components with the use of advanced materials obtained in the Welding Engineering Department of Warsaw University of Technology, Poland, are presented.

SEMI-WEIGHT FUNCTION METHOD ON COMPUTATION OF STRESS INTENSITY FACTORS IN DISSIMILAR MATERIALS

Semi_weight function method is developed to solve the plane problem of two bonded dissimilar materials containing a crack along the bond. From equilibrium equation, stress and strain relationship, conditions of continuity across interface and free crack surface, the stress and displacement fields were obtained. The eigenvalue of these fields is lambda. Semi_weight functions were obtained as virtual displacement and stress fields with eigenvalue?_lambda. Integral expression of fracture parameters, K Ⅰ and K Ⅱ, were obtained from reciprocal work theorem with semi_weight functions and approximate displacement and stress values on any integral path around crack tip. The calculation results of applications show that the semi_weight function method is a simple, convenient and high precision calculation method.

A CRACK EMANATING FROM THE TIP OF BONDED DISSIMILAR MATERIALS

A crack is assumed to emanate from the tip of bonded dissimilar materials with the crack on the bisector of one of the bonded wedges. The problem is firstly divided into symmetric and anti-symmetric modes according to the characteristics of the local geometry. By eigenexpansion method, the eigenequations for the two modes are derived, respectively, and the corresponding eigenvalues are obtained with different ratios of dissimilar material constants and angles of the wedges. The singularity of the crack is then analyzed by the eigenvalues that are less than one. The fields of displacement and stress in the vicinity of the tip of the crack are finally derived in an explicit form.

Challenge to Welding and Joining Technology for Applying Multi-material in Electric Vehicle Production

Advanced high strength steel,aluminum alloy and plastic materials are used in the right places for the purpose of reducing the weight of EV(electric vehicle)bodies and in-vehicle parts,and multi-material structures are advancing.Therefore,it is difficult to handle the welding and joining processes of automobile structures by the conventional arc welding and resistance spot welding,which have been applied to steel joining,and various joining processes are being applied depending on the material.Under above mentioned background,the authors have developed some unique joining processes for multi-materials that are used in the right place.This paper introduces the dissimilar metal joining between the galvanized steel and aluminum alloy by laser arc hybrid process,the metal/thermoplastic dissimilar material joining using laser process and the solid-state resistance spot joining process of advanced high strength steel for EV body structural parts.Moreover,the authors describe the high-speed plasma jet GTA(Gas Tungusten Arc)welding process of copper applied to electrical components such as motors.

Joining Performance and Microstructure of the 2024/7075 Aluminium Alloys Welded Joints by Vaporizing Foil Actuator Welding

Vaporizing foil actuator welding(VFAW) was used for joining 2024-T3 and 7075-T6 aluminum alloy sheets, and the resulting joint microstructure was analyzed. 2024/7075 aluminum alloy pairs with suitable processing parameters can be prepared by using VFAW. Dynamic preform addresses the poor formability problem of target material and advantage of VFAW on dissimilar materials in some conditions. But with standoff sheet inserting in the flyer and target, 2024/7075 welded pairs gets the better weld strength, compared with flyer preformed method. The microstructure of the circular weld area of the welded joint showed a wave interface, in which a thin melt layer formed at the center and edge parts. The crystal grains near the bonding interface were remarkably elongated and refined. Therefore, the joining of the 2024/7075 pairs was facilitated through plastic forming and melting.

Joining of AZ31 and AZ91 Mg alloys by friction stir welding

Two dissimilar magnesium(Mg)alloy sheets,one with low aluminium(AZ31)and another with high aluminium(AZ91)content,were successfully joined by friction stir welding(FSW).The effect of process parameters on the formation of hot cracks was investigated.A sound metallurgical joint was obtained at optimized process parameters(1400 rpm with 25 mm/min feed)which contained fine grains and distributed β(Mg_(17)Al_(12))phase within the nugget zone.An increasing trend in the hardness measurements has also confirmed more amount of dissolution of aluminium within the nugget zone.A sharp interface between nugget zone and thermo mechanical affected zone(TMAZ)was clearly noticed at the AZ31 Mg alloy side(advancing)but not on the AZ91 Mg alloy side(retreating).From the results it can be concluded that FSW can be effectively used to join dissimilar metals,particularly difficult to process metals such as Mg alloys,and hot cracking can be completely eliminated by choosing appropriate process parameters to achieve sound joint.

Microstructure and properties of an Al–Ti–Cu–Si brazing alloy for SiC–metal joining

An Al–Ti–Cu–Si solid–liquid dual-phase alloy that exhibits good wettability and appropriate interfacial reaction with SiC at 500–600°C was designed for SiC–metal joining. The microstructure, phases, differential thermal curves, and high-temperature wetting behavior of the alloy were analyzed using scanning electron microscopy, X-ray diffraction analysis, differential scanning calorimetry, and the sessile drop method. The experimental results show that the 76.5Al–8.5Ti–5Cu–10Si alloy is mainly composed of Al–Al2Cu and Al–Si hypoeutectic low-melting-point microstructures (493–586°C) and the high-melting-point intermetallic compound AlTiSi (840°C). The contact angle, determined by high-temperature wetting experiments, is approximately 54°. Furthermore, the wetting interface is smooth and contains no obvious defects. Metallurgical bonding at the interface is attributable to the reaction between Al and Si in the alloy and ceramic, respectively. The formation of the brittle Al4C3phase at the interface is suppressed by the addition of 10wt% Si to the alloy. © 2017, University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg.

The effect of zinc coating during joining aluminium to zinc-coated steel by welding-brazing process

An optical sensing system was constructed to study the coating when joining ahtminium alloy to zinc-coated steel by MIG welding-brazing process. The results showed that the coating fused to form a liquid zinc film on surface during process. The Al-Si filler metal reacted with the liquid zinc film to improve the spreadability of filler metal and thus a new method was established to join aluminium alloy and uncoated steel by depositing a mixed layer on steel surface.

Joining of dissimilar metals between magnesium AZ31B and aluminum A6061-T6 using galvanized steel as a transition joining layer

A galvanized steel is used to join Mg AZ31B alloy and Al A6061-T6 alloy as a joining transition layer by cold metal transfer(CMT)method.Firstly,to optimize the process variables,extensive welding tests were performed by a design of experiment method.Then,microstructures,joining mechanisms and tensile properties were characterized and analyzed.Results indicated that Mg and Al alloys can be joined by using galvanized steel as a joining transition layer and cold metal transfer welding method.The formed joint is called as a Mg–steel–Al CMT joint.By using galvanized steel transition joining layer,Mg–Al brittle intermetallics Al12Mg17 and Al3Mg2 were inhibited.The properties of Mg–steel–Al CMT joints have been improved after optimizing the welding variables.The strength of Mg–steel–Al CMT joint is comparable to those of Al–Al joint and Mg–Mg joint.The strength of Mg–steel–Al CMT joint can reach 100%that of Al–Al joint and Al–steel joint,and reach 90%that of Mg–Mg joint and Mg–steel joint.The optimized Mg–steel–Al CMT welded joint with galvanized steel transition layer is fractured at the heat affected zone of Al base metal rather than at the weld-brazed interface,due to softening of Al base metal.

WEIGHT FUNCTIONS FOR INTERFACE CRACKS IN DISSIMILAR ANISOTROPIC MATERIALS

Bueckner's work conjugate integral customarily adopted for linear elastic materials is established for an interface crack in dissimilar anisotropic materials.The difficulties in separating Stroh's six complex arguments involved in the integral for the dissimilar materials are overcome and thert the explicit function representations of the integral are given and studied in detail.It is found that the pseudo-orthogonal properties of the eigenfunction expansion form(EEF)for a crack presented previously in isotropic elastic cases,in isotopic bimaterial cases,and in orthotropic cases are also valid in the present dissimilar arbitrary anisotropic cases.The relation between Bueckner's work conjugate integral and the J-integral in these cases is obtained by introducing a complementary stress- displacement state.Finally,some useful path-independent integrals and weight functions are proposed for calculating the crack tip parameters such as the stress intensity factors.

Joining dissimilar metal of Ti and CoCrMo using directed energy deposition

We report laser cladding of pure titanium on a CoCrMo alloy using directed energy deposition.Using electron microscopy,the microstructural evolution upon varying the process parameters,especially laser power and powder feed rate,was investigated in relation to crack formation.Cladding layers showing dilution rates of more than 5%contained cracks due to the formation of the brittle Co_(2)Ti intermetallic phase.The observed cracks could be ascribed to a mismatch in thermal expansion and a resulting stress of more than 440 MPa acting on the Co_(2)Ti phase,as determined by density functional theory and nanoindentation.Furthermore,an excess laser energy caused chemical inhomogeneity and unmelted Ti powder particles,while a deficient laser energy resulted in a lack of fusion.Neither cracks nor partially melted powders were observed for a powder feed rate of 3 g/min and a laser power of 225–300 W,for which the dilution rate was minimized to less than 5%.For such samples,the cladding layers comprised pureα-Ti and a uniform CoT i interface with Co_(2)Ti islands.

A Novel Friction Stir Spot Riveting of Al/Cu Dissimilar Materials

In order to eliminate the disadvantages of the keyhole in conventional friction stir spot welding joint and attain the highstrength lap joint of Al/Cu dissimilar metals,a novel welding technique,named as friction stir spot riveting(FSSR),was proposed.A pinless tool and an extra filling stud were employed.The Al/Cu spot joints without keyhole defect were achieved by the FSSR.A Cu anchor-like structure was formed,which greatly increased the mechanical interlocking between the upper Al sheet and lower Cu sheet.The thin intermetallic compounds containing CuAl2 and CuAl at the Al/Cu interface strengthened the joining interface between the Al sheet and the Cu stud.Increasing rotating velocity increased frictional heat and plastic deformation and then eliminated the interfacial joining defects.The FSSR joint with the maximum tensile shear load of 3.50 kN was achieved at a rotating velocity of 1800 rpm and a dwell time of 20 s,whose fracture path passed through the softened region of upper Al sheet.In summary,the novel FSSR technique has the advantages of strong mechanical interlocking and metallurgical bonding between dissimilar materials,thereby attaining the high-strength spot joint.

Review on friction stir welding of dissimilar magnesium and aluminum alloys: Scientometric analysis and strategies for achieving high-quality joints

Magnesium and aluminum alloys continually attract interest as lightweight structural materials for transport applications. However, joining these dissimilar alloys is very challenging. The main obstacle that hinders progress in dissimilar Mg-Al joining is the formation of brittle intermetallic compounds(IMCs). As a solid-state joining technique, FSW is an excellent candidate to attenuate the deleterious IMC effects in dissimilar Al-Mg joining due to the inherent low heat inputs involved in the process. However, the IMCs, namely Al_(3)Mg_(2) and Al_(12)Mg_(17) phases, have also been reported to form during Al-Mg dissimilar FSW;their amount and thickness depend on the heat input involved;thus,the weld parameters used. Since the heat dissipated in the material during the welding process significantly affects the amount of IMCs,the heat input during FSW should be kept as low as possible to control and reduce the amount of IMCs. This review aims to critically discuss and evaluate the studies conducted in the dissimilar Al/Mg FSW through a scientometric analysis and also with a focus on the strategies recently applied to enhance joint quality. The scientometric analysis showed that the main research directions in Mg/Al FSW are the technological weldability of aluminum and magnesium during FSW, structural morphology, and mechanical properties of dissimilar welded joints. Considering the scope of application of the aforementioned joints, the low share of articles dealing with environmental degradation and operational cracking is surprising. This might be attributed to the need for well-developed strategies for obtaining high-quality and sustainable joints for applications. Thus, the second part of this review is conventional, focusing mainly on the new strategies for obtaining high-quality Mg/Al joints. It can be concluded that in addition to the necessity to optimum welding parameters to suppress the excessive heat to limit the amount and thickness of IMC formed and improve the overall joint quality, strategies such as using Zn interlayer, electric current assisted FSW(EAFSW), ultrasonic vibration FSW(UVa FSW), are considered effective in the elimination, reduction, and fragmentation of the brittle IMCs.

电子封装超声互连研究新进展

超声振动摩擦和局部高温高压可实现同材或异材的低温、快速、可靠的互连,超声技术在电子封装中显示出无与伦比的技术优势。综述了超声引线键合、块体母材直接超声键合、母材/焊料/母材三体超声固相键合等超声固相键合技术,超声钎焊、超声瞬态液相扩散焊等超声液相键合技术,超声纳米烧结、混合(复合)焊料超声互连等超声固液混合键合技术。无论是高频低功率的高速超声键合,还是中频高功率超声常速键合,超声的作用规律和效果相似,其互连质量和互连机理与接触材料及其状态有着最为密切的关系;考虑由于超声互连过程快、时间短、互连界面窄、接头不透明,可以采用焊接实时高速摄像技术和同步辐射X射线成像技术相结合,实时观察液态焊料润湿流动、焊料颗粒运动轨迹、母材的剥离过程及界面形成规律;考虑由于微焊点尺寸放大,可以借助温度传感器和压力传感器进行超声的温度效应和瞬态压强实时测量;基于双超/多超声诸多影响因素,多超声的数值模拟分析将成为微焊点超声互连机理研究的有效手段。

Direct joining of oxygen-free copper and carbon-fiber-reinforced plastic by friction lap joining

Oxygen-flee copper （Cu） was successfully joined to carbon-fiber-reinforced thermoplastic （CFRTP, polyamide 6 with 20wt% carbon fiber addition） by friction lap joining （FLJ） at joining speeds of 200-1600 mm/min with a constant rotation rate of 1500 rpm and a nominal plunge depth of 0.9 ram. It is the first time to report the joining of CFRTP to Cu by FLJ. As the joining speed increased, the tensile shear force （TSF） of joints increased first, and decreased thereafter. The maximum TSF could reach 2.3 kN （ 15 mm in width）. Hydrogen bonding formed between the amide group of CFRTP and the thin Cu20 layer on the Cu surface, which mainly contributed to the joint bonding. The influence factors of the TSF of the joints at different joining speeds were discussed. The TSF was mainly affected by the joining area, the degradation of the plastic matrix and the number and the size of bubbles. As the joining speed increased, the influence factors varied as follows： the joining area increased first and then decreased; the degra- dation of the plastic matrix and the number and the size of bubbles decreased. The maximum TSF was the comprehensive result of the relatively large joining area, small degradation of the plastic matrix and small number and sizes of bubbles.