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J Weld Join > Volume 41(1); 2023 > Article
Shim, Park, and Kim: An Overview of Resistance Element Welding with Focus on Mechanical and Microstructure Joint and Optimization in Automotive Metal Joints

Abstract

As lightweight automotive structures improve fuel efficiency and reduce carbon dioxide emission, they have garnered extensive attention. Vehicle mass reduction, which is a key problem for next generation eco-friendly vehicles, can significantly increase mileage. Hence, industries have committed to replace conventional materials with lightweight materials, such as advanced high strength steel. Additionally, automotive industries are hindered by challenges in the field of joining technology. A novel welding technology called resistance element welding (REW), which is an appropriate thermal-based joining method, was developed recently for joining hybrid materials with other structural steel grades. In this study, the state of the art joining and the process characteristics for dissimilar metal joints have been presented because related studies show limited investigation in this area. Following by the state of them, the principal and welding quality of REW, experimentally and numerically, are reported to give comprehensive information on the current practices and research interest related to technologies. Finally, extensive work was concentrated on portional joining optimization techniques to improve different materials.

1. Introduction

Recently, most of the automotive manufacturers for manufacturing lighter, safer, more environmentally friendly, more performant and cheaper products have been employed light metals to reduce the car body weight. Also, the gas emission standards required by many countries stimulate the active researches on joining of dissimilar material in the automotive industries1,2). The application of light metals such as aluminum has been expanding since the metals are advantageous in terms of weight reduction of a car body. However, the joining of a dissimilar metal pair such as aluminum and steel is problematic due to their vastly different solidus temperature and thermal expansion coefficient when conventional fusion-based welding technologies hae been attempted3,4). Arc welding, the universally accepted method of permanently joining all metals might generally be considered a mature industry but it is still a growing industries5). The development of new welding techniques for automotive applications has been carried out meeting the new material combinations for auto body parts focusing on lighter yet strong and fuel efficient vehicles employing light weight alternative materials6).
The typical technology employed for joining automotive metals is the welding process7). There are numerous researches8-17) in this area. The technologies included Resistance Spot Welding(RSW), Resistance Seam Weld- ing(RSW), Friction Welding(FW), Laser Beam Welding (LBW), Metal Inert Gas(MIG) welding, Tungsten Inert Gas(TIG) welding and Plasma Arc Welding(PAW) have been applied for the automotive manufacturing. Among them, the most commonly used welding method is the RSW due to being a cost-effective process easily with automation and high productivity9). Much spot-joining technologies10-12) such as fusion, solid-state, and mechanical joining have been developed to obtain an acceptable joint of dissimilar metals for car bodies because of significant differences in the material-physical properties(e.g. melting points, thermal conductivities, electrical resistances). One of the advanced technologies, SPR(Self-Piercing Riveting) which is a cold forming technique used to fasten together two or more sheets of materials with a rivet without the need to pre-drill a hole, has increasingly been become popular mainly due to the growing use of lightweight materials in transportation applications16,17). However, SPR technology for joining of these advanced light materials remains a challenge as these materials often lack a good combination of high strength and ductility to resist the large plastic deformation.
By combining the mechanical(riveting) and thermal (welding) joining principles, a new joining technology, called REW has recently been developed as a fusion joining technology18). The developed technique was reported to address the challenges of joining Al alloys to steels. As demonstrated by Meschut et al.19), a hole is pre-punched in the Al alloy, and an auxiliary element is inserted into the hole, followed by RSW on the rivet/steel. Also, the technological performance among a REW process should be indicated that REW and FEW offered high potential for profile-intensive constructions. Qiu et al.20) observed that the peak load of A6061 Al alloy/ Q235 steel joints produced by REW was 37% higher than that produced by RSW. Ling et al.21) compared the mechanical performance of 2mm 6061 Al alloy/1.8mm uncoated 22MnMoB boron steel joints produced by RSW and REW. Manladan et al.22) reported the mechanical performance of REW and RSW bonded Mg/steel joints, indicating that the REW bonded joints exhibited excellent lap-shear properties with high energy absorption. Ling et al.23) investigated that the REW nugget made from Q235 rivet and DP780 steel consisted of large lath martensite. For Al sheet, the evolution of the precipitates resulting from the high heat input will deteriorate the hardness, which influences the mechanical performance of joints24). The metallurgy mechanisms of the nugget formation and the Al sheet softening are still not clearly defined25).
Oh et al.26) employed a newly designed rivet for bonding between the A365 and GACC plates, and observed mechanical performance by minimizing the head that protrudes from the aluminum plate. It was difficult, however, to maintain mechanical properties for the misalignment between the rivet and electrodes with a 6mm electrode diameter for the rivet. Manladan et al.27) studied Mg alloy/ASSs REW joints in multi-sheet con- figurations. It could be revealed that the nugget size at the rivet/austenitic stainless-steel interface was lower than that at the austenitic stainless steel/austenitic stainless and therefore the latter mainly influenced the failure mode transition. AA6061-T6 Al alloy/HS1300T PHS joints with REW process and proposed an analytical model of critical nugget size to predict the failure mode were reported by Cetin and Thienel28).
Despite these obvious advantages, the optimization of the REW process is rarely reported in the literature. Ling et al.21) observed the joining of a A6061 sheet and high-strength boron steel by REW, and found that the lap-shear strength of the REW joint could be nearly seven times that of the RSW joint. As most of the mechanical properties came from the interlock created by the steel rivet, the method can also be applied to other material combinations that cannot be joined by fusion welding. REW and FEW technologies in combination with adhesive bonding to produce Al alloys/ultra-high strength steel joints with outstanding load-bearing capacity was investigated by Meschut et al.29). Holtschke and Jüttner30) reported that the REW technique could be used to join a thermally sensitive sandwich material such as a high-strength steel.
In recent study, REW process could be applied for significant improving the mechanical performance of Al alloy to coated and un-coated steels joints31). However, it could still be verified the potentials to become a dominant technique for joining difficult-to-join, metallurgically incompatible light alloys/steel combinations6). Literature survey on joint performance to understand the mechanisms of REW joints has been lacking. Until now, the related studies32-34) on REW process have been concentrated on comparing the mechanical properties of joints manufactured by SPR and RSW techniques, but those are not well organised and properly linked. In fact, it is not easy to have a complete understanding in technical mechanical and microstructure performance which is essential for improving the efficiency and effectiveness of REW process4).
The present paper critically reviews and scientifically links research works related to theory, optimization and welding quality of REW process. The review presented in this section is on different techniques proposed and investigated by researchers for improving the efficiency and effectiveness of REW process on different automotive metals resulting in mechanical and microstructure joint properties as welding quality. Scrutiny of the published research work emphasizes the requirement for such a review reporting all the available literature and the future direction for research. Finally, the present survey explores different methodologies and processes regarding the experimental and computational analyses for REW process.

2. Advanced REW Techniques

2.1 What is REW process

The REW process, a further development of the conventional RSW process, can be defined as a novel joining technology to join multi-material-compounds in the production lines with the same joining technology as with conventional steel parts35). With this joining technology, there were many problems for the related joining technology of dissimilar materials by a conventional RSW process. In principle, the REW process involves the use of an auxiliary joining element which is integrated into the aluminium carrier sheet in different ways36). In addition, REW could be understood as a thermal-mechanical joining technology which combines the advantages of mechanical(form-fit) and thermal (metallic bond) joining principles and enables a boundary stretch of joining dissimilar material combinations.
As shown in Fig. 1, the principle of REW process combines both thermal and mechanical joining principles, by creating a metal bond between an auxiliary joining element and the bottom plate in combination with a force- and form-locking connection of the auxiliary joining element with the top plate. In a first step, a hole is punched into the top(cover) sheet. Then the auxiliary element called weld rivet is inserted or positioned in the hole. One electrode is lowered onto the rivet and the other is positioned onto the bottom sheet. Pressure (F) and electric current(I) are applied simultaneously. The heat generated by the electrical resistance creates a weld nugget in the contact zone between the weld rivet and the base sheet, and forms the connection. In the final phase, an increase of the electrode force leads to a deformation of the weld rivet in the axial direction and therefore to a tight force connection(surface pressure) between the rivet head and the cover sheet. A frictional connection is obtained at the contact between the rivet shaft and the cover sheet as well as between the rivet head and the cover sheet(surface pressure). The individual process stages can be controlled by a variation of the welding parameters(weld time, electric current and force) generally employed in the state-of-the-art mid- frequency and servo-controlled spot welding equipment37).
Fig. 1
Principles of REW process37)
jwj-41-1-37gf1.jpg
Li et al.38) reported the advanced version of two-step REW, called Self-Penetrating Resistance Element Welding(SPREW) which can be divided into three subprocess steps to overcome the metallurgical incompatibility of dissimilar material compounds. The welding process and its characteristics as well as the chemical composition of the resulting dissimilar joint were described17). Also, It could be illustrated that the welding nugget between the welding rivet and the steel sheets enabled sufficient joint strengths since failure always occurred in the aluminium.
Jesweit et al.39) developed the new heating element for resistance welding of TPC with a PEI-based conductive nano-composite. Observations of the fracture surfaces revealed a cohesive failure mode within the nano-composite HE and non-uniform heating over the weld area. It could be shown that PEI/MWCNT HE present an alternative to electric current HE, although more work is needed to improve the temperature homogeneity over the weld area. The REW technology was also applied for experiment to verify the maximum shear strength of compounds of different aluminium sheets joined within the scope of further development of this joining technology. Maximum shear strength of about 4500N was achieved with a compound of Al5- Std carrier sheet with a thickness of 1.1mm40).
The approach of embedding metal inserts in TPC (Thermo-Plastic Composite) during compression moulding without fiber damage which based on the concept of moulding holes by a pin and simultaneously placing the weld insert in the moulded hole was studied by Troschitz et al.41). The process is schematically illustrated in Fig. 2. At first, a pre-consolidated TPC sheet is warmed up above melting temperature of the matrix polymer by an infrared heating device. The TPC sheet is then transferred into the open compression mould. A pin tool is shifted forward, forming a hole by displacing the reinforcing fibres and the still molten thermoplastic matrix after mould closing immediately. After cooling and solidification of the TPC specimen, the pin retainer is retracted and the tapered pin is separated. Finally, TPC specimen with the integrated weld insert is demoulded. Subsequently, the TPC sheet is transferred into the open compression mould.
Fig. 2
Schematic illustration of process-embedding of weld insert in TPC41)
jwj-41-1-37gf2.jpg

2.2 Optimization of Welding Parameters in REW Process

Most common problem that has faced the manufacturer, is to control the welding parameters to obtain a good welded joint with minimal detrimental residual stresses and distortion as weld quality42). To sole this problem, a time-consuming trial and error effort with welding parameters chosen by the skill of the engineer or machine operator is required. Then welds are examined to determine whether they meet the specification or not. Finally the welding parameters can be chosen to produce a welded joint that closely meets the joint requirements. Controlling the welding parameters which included electric current, force and welding time, plays an important role in the quality of the REW process43). In these cases, only a single electric current without any up- or down slopes is selected to guarantee a simple process control.
A self-pierce rivets, fastener element for a resistance based joining process which are inserted in the non-ferrous joint member in a pre-riveting-step were observed44). In this case, the well-known and stable processes of SPR and RSW were combined. Also, an embossing element was reported45). Both solutions should be improved the corrosion resistance by generating a media-impermeable barrier. Especially, a sufficient strength of the element for the second and third variant is crucial for a stable punching/riveting and final welding process. All variants are welded with typical copper-chrome-zirconium electrode caps46). A weld nugget is formed by applying electric current and force as welding parameter. A melting of the surrounding aluminum alloy and the formation of inter-metallic phases in the weld nugget should be avoided by choosing suitable welding parameters while the formation and consequences of heat-affected zones in REW should be considered.
Meinhardt et al.18) studied the significant influence factors of the forming process on joint strength of the considered REW process on the joint strength and its controllability. Shear strength of REW joints with cold forged auxiliary joining elements is on a comparably high level to the other REW processes, regarding the chosen sheet thickness as one of the thinnest in the preceding comparison. The research also revealed that the initial relative position of the carrier sheet and the auxiliary joining element is highlighted as the most significant factor on joint strength. Baek et al.47) observed the robust bonding of Al-high strength steel joints for automobiles by examining the microstructure-mechanical properties according to the welding conditions of the Al-steel joints welded with resistance elements while maintaining the RSW process. The test specimens bonded with 3.5kA and 4.5kA electric current were not sufficiently melted to achieve robust bonding, and gaps and micro-cracks at the bonding interface were the main causes for inducing the brittle failure of the bonding interfaces.
Tomohiro et al.48) studied ERW technology for high grade line-pipe by optimizing the chemical composition and rolling conditions of the steel coils used as starting material. It is considered that circumferential heat transfers from the edge decreases, and as a result, the heating width also decreases as the welding speed increases. Proper control of the welding parameters corresponding to the thickness and speed of the steel band in reducing oxide inclusions in the seam are also important. The optimal welding conditions for dissimilar joining of electro-galvanized DP780 steel to 6061- T6 aluminum alloy compared with traditional RSW was reported49). Not only the microstructure of the joints varied with the distance from the center of the nugget, but also the fatigue fracture modes of the REW and RSW joints were dependent on the load levels and these joints at high load levels. A REW with hybrid materials in combination with high-strength steel materials by giving a summary of the material properties, the boundary conditions and the welding process itself as well as the influences of the welding parameter on the nugget formation were investigated Schmal and Meschut50). It could be concluded from their study that only low maximum forces of up to 4 kN are required to punch in the weld rivet, but riveting guns for the use of existing equipment is sufficient. Mahieu et al.51) observed the macro- and microstructure characteristics of nugget and Al sheet outside the rivet shank for the REW joints of Al alloy/ UHSS to improve the joining efficiency. This results the lap shear strength reached the peak point at 7-200 when the strength decreased with the increase of heat input.
Recently, a 3D numerical model for high-strength Dual-Phase(DP) 600 steel and Q235 steel by considering contact resistances as functions of temperature and surface contacting area was developed52). The electric current flow and thus Joule heat generation at the faying interface between the element and workpiece as the welding parameters in REW were observed. In addition, the copper electrode adjacent to the thinner workpiece experiences an extremely high temperature(over 550℃) that would reduce the electrode life. The study also confirmed that nugget is only formed at the faying interface between the welding element and workpiece with a smaller electric current and a shorter welding time. Sun et al.53) studied the process window and microstructure evolution caused by external magnetic field and the enhancement of external magnetic field on weld nugget enlargment and grain refinement. The weld nugget was also compressed by the external magnetic field in the vertical direction and decompressed in the horizontal direction. It can be concluded that the diameters of weld nugget increased with increasing electric current from 9 to 11kA. Günter and Meschut42) reported the welding process and its characteristics for REW process. The study also showed that selected welding parameters generate sufficient joint strengths, and the ultimate shear load of the welded joints is significantly dependent on the head design of the embedded weld inserts. By analysing the quality and strength of the joints, high-quality joints could be achieved with this innovative technology.

3. Numerical Analyses

REW has been studied both experimentally and numerically to improve the quality of the produced welds. Experimental studies on welding quality provide valuable information about the influence of different welding parameters on the weld properties54). Despite providing practical information, experimental studies on REW process are very costly. However, numerical studies using FEA technologies could be helped reducing costs and providing an effective compliment to experimental work. Many researchers55-62) have carried out the numerical analyses for REW with considering different electrical, thermal and mechanical phenomena such as formation of temperature field and welding nugget growth. Generally, the REW process involved multiple inter-metallic contacts, leading to more complex dynamic electric current flow, heat generation, transfer and nugget growth. One of the pioneering works on REW process was conducted by Manladan et al.22) who simulated the thermal contacts with heat generation between two solids to study the thermal characteristics due to interfacial conditions based on the thermal-electrical analogy.
Troschitz et al.41) studied a two-dimensional, axisymmetric simulation model to simulate for REW process to determine suitable welding parameters of the TPC-steel-joints for creating between the isotropic metallic weld insert and a steel sheet as shown in Fig. 3. The simulated results were compared to the results of corresponding experimentally welded joints with respect to both the geometry(height and diameter) of the weld nugget and the shape of the heat-affected zone of the heat-affected zone. Furthermore, two surfaces in contact with each other which directly affects the electric current distribution and heat generation during the welding process were illustrated the same or similar contact stiffness. As the multiple interfacial thermal contact behaviors with heat generation of REW are unclear yet, it is essential for investigating the underlying mechanisms related to the nugget shifting and mitigation with REW for dissimilar steels.
Fig. 3
Two-dimensional(2D) axisymmetric model used for the simulation of REW41)
jwj-41-1-37gf3.jpg
The 2D axis symmetric forming simulation model for welding rivet design was developed by Günter and Meschut54). The developed model has been employed for analysing the its characteristics and the chemical composition of dissimilar joint for overcoming the metallurgical incompatibility of dissimilar material compounds using a Self-Penetrating Resistance Element Welding(SPREW). The simulated results showed that a numerically optimized welding rivet geometry might be guaranteed sufficient joint strength. Baek et al.56) studied a 3D Finite Element Method(FEM) modeling for mechanical simulation using the commercial finite element code ANSYS. It could be found through numerical simulation that the stress and strain distributions showed a significant difference to the same fatigue load according to the geometrical effect of the welding interface. This trend was in good agreement with the experimental results of the fracture mode and fatigue S-N curve.
Eshraghi et al.57) developed a 2D thermal-electrical- mechanical-metallurgical finite element model for simulating the RSW process of two DP600 sheets and quantifying the dependence of the weld properties on different welding parameters. The Design of Experiments (DOE) method was also employed for analyzing the main effects and interactions of the electric current intensity, welding time, sheet thickness, electrode face radius, and squeeze force over a realistic range of values. The simulated results were supplied to verify the main effects and interactions of the welding parameter and their significance on weld properties(nugget radius, nugget thickness, HAZ radius, and MZ volume). A finite elements analysis of REW for joining of aluminum alloy and advanced high strength steel sheets was researched by Chen et al.58). It could be found that the thermal contact resistance decreased when the interface temperature increases and the interface pressure decreases. Not only the developed models were simulated nugget forming and stress distribution, but also thermal distributions were compared with respect to the amount of electric current input and load. The bond strength was experimentally compared for various process conditions by simulating the lap-shear test59).
A finite element modeling to identify the welding parameters that lead to the formation of acceptable joints and check possibility of a new REW process of producing invisible lap joints between steel-polymer-steel composite laminates was reported by Calado et al.60). An electrical-thermal-mechanical coupled REW model for high-strength Dual-Phase(DP) steel and Q235 steel by considering contact resistances as functions of temperature and surface contacting area was developed by Tomohiro et al.48). Also, the REW of 2.0-mm thick 6061 aluminum alloy and 2.0-mm thick boron steel was simulated and compared with the published experiment results21) to validate the developed numerical model. As illustrated in Fig. 4, the nugget of REW joint is marked in the red dotted line, which is apparently formed by the welding element and the lower workpiece. It is clear from both the experimental and calculated results that the nugget are just located beneath the element and obviously shift downward with a higher penetration rate within the lower workpiece. The developed model will also be helpful to understand the fundamentals on the advantages of REW and provide a guidance for welding schedule development of the REW of dissimilar steels. Therefore, the calculated results are consistent with the experiment by comparing the calculated nugget dimensions and shifting phenomena62).
Fig. 4
The comparisons of 6061 aluminum alloy-boron steel joint wit REW21)
jwj-41-1-37gf4.jpg
Günter and Meschut54) developed the 2D axis symmetric forming simulation models for welding rivet design as well as 3D welding simulation models for determining the temperature fields and the dynamic resistances during REW process. In this case, only the welding process is simulated since hot penetration cannot be simulated within 3D model due to missing damage criteria in the computational model. It seems that three and four interfaces are included in the developed model for two-sheet application and in the model for three-sheet application respectively as shown in Fig. 5. The simulated results showed that the aluminium is fully displaced during hot penetration.
Fig. 5
The developed model for two-sheet combination (upper left) and three-sheet combination (upper right)54)
jwj-41-1-37gf5.jpg

4. REW Joint quality Monitoring

4.1 Mechanical Analyses

As it is unacceptable for vehicle structures directly related to safety to be mass-produced with defects, automotive industries must find better alternatives that can minimize defects. Furthermore, the critical issue is that the applicability of welding and bonding between dissimilar materials, including the investment cost and production time, however, needs to be examined by automobile manufacturers36). Meinhardt et al.18) reported a REW process with upset auxiliary joining elements to analysis the maximum shear strength for specimen with upset steel-elements in aluminum carrier sheets of different thickness. Maximum shear strength of about 4500N was achieved with a compound of Al5-Std carrier sheet with a thickness of 1.1mm and an auxiliary joining element made of CR240LA. Ling et al.21) studied bonding between the A6061 and 22MnMoB plates through the REW process and confirmed tensile strength approximately seven times higher than that of the conventional RSW process.
In3), a short-time(≤20ms) welding process is investigated, while the shear tensile-properties are analysed18). The join-ability of magnesium to stainless steel and the join-ability of aluminium to ultra-high-strength steel are reported in21,22), respectively. Oh et al.26) employed a newly designed rivet for bonding between the A365 and SGACC plates, and secured mechanical performance by minimizing the head that protrudes from the aluminum plate. It was difficult, however, to maintain mechanical properties for the misalignment between the rivet and electrodes with a 6mm electrode diameter for the rivet.
Troschitz et al.41) investigated a innovative technology which based on the concept of moulding holes by a pin and simultaneously placing the weld insert in the moulded hole for embedding metal weld inserts in TPC during compression moulding without fibre damage. It could be seen that the ultimate shear load of the welded joints using weld insert type A is 3.7kN on average with a maximum displacement of 13mm to15mm. Also, the average ultimate shear load of joints using type B weld inserts is significantly lower(3.0 kN) with a maximum displacement of approximately 5mm to 6mm. Baek et al.47) observed the tensile shear load-displacement curves which depend on the welding conditions for REW. It could be revealed the tensile-shear peak loads were gradually increased as the electric current increases as shown in Fig. 6. In the 3.5kA condition, the peak loads were distributed between 5000N and 7000N, while the peak loads in the 4.5kA condition were between 7000N and 8000N. In addition, the tensile-shear peak loads had a high strength approaching 9000N when the electric current was increased to 6.5kA.
Fig. 6
Tensile-shear load-displacement curve after tension-shear tests47)
jwj-41-1-37gf6.jpg
Park et al.63) reported the mechanical and corrosion performance of steel/aluminum dissimilar materials joints obtained by SPR and REW processes. The REW joints for shear tests indicated an 18.5% higher peak load than the SPR joints. The effect of weld mechanical properties and the failure mode on the distance of misalignment that occurs during REW process was investigated by Jun et al.64). It seems that differences exist in the tensile shear strength, cross tension strength, ductility ratio and failure mode based on the distance of misalignment. Also, the length of the nugget diameter is a major factor which effected the mechanical properties and failure mode in the REW. Duric et al.65) studied the possibility of joining Carbon Fiber-Reinforced Polymer(CFRP) and DP500 steel using REW process. The experimental results showed that DP steel and CFRP could be joined by REW with the maximum failure load of 2411.5N and the failure energy of 1.2J, but the joint failed through the weld in the interfacial(IF) mode. It could be indicated that formation of the asymmetrical nugget can be attributed to the differences in electrical resistivity and thermal conductivity.

4.2 Microstructure Analysis

The mechanical properties, fracture morphology, nugget formation process, dynamic resistance, microstructure and hardness distribution of joint of 6061 Al alloy and uncoated 22MnMoB boron steel using a REW technique were reported Ling et al.21). A nugget of REW formed first at the interface of the rivet and boron steel. Significant heat was also conducted from the rivet to the nearby Al base metal which leaded to a partial melt and metallurgical bonding between the rivet and aluminum. Manladan et al.11) observed the RSW and REW technologies for joining AZ31 Mg alloy and 316L Austenitic Stainless Steel(ASS) to verify the weldability of Mg alloy to ASS. It could be drawn that the RSW joints were produced through welding-brazing mode in which the Mg alloy melted and spread on the solid steel, forming a nugget only on the Mg side.
Niu et al.25) investigated the macro- and microstructure characteristics of nugget and Al sheet outside the rivet shank of Al alloy/UHSS. It could be indicated that the Al could be divided into four zones by microstructure and hardness distribution such as Re-solidified Zone(RZ), Softening Zone(SZ), Transition Zone(TZ) and Base Metal(BM). In addition, an analytical model was also established to predict the critical nugget size of REW using the average SZ hardness and sheet thickness, which well explained the failure mode transition of the existing REW joints. The microstructure of the REW joint which mainly consists of martensite, was reported by Oh et al.26). It seems that the microstructure of the REW joint is similar to that of RSW, revealing microstructure modifications caused by heating and cooling at a high rate. The industrial implementation of the process was also required to carefully consider the electrode misalignment with the rivet to prevent possible arcing through the material adjacent to the rivet.
REW technology was employed to join 1.5 mm-thick AZ31 Mg alloy to two 0.7mm-thick 316 L ASS sheets using different joint configurations27). 3D Digital Image Correlation(DIC) method was also employed to measure the out-of-plane deformation and strain distribution in the axial direction during the lap-shear test. As illustrated in Fig. 7, the nugget size at the ASS/ASS interface was larger than that at the rivet/ASS interface (asymmetrical nugget) at all electric current. An alternative technology for welding Mg/steel immiscible materials for realizing car body light weighting was studied by Cetin and Thienel28). It could be concluded from the study that the relationship between hollow sphere weld and re-solidified Mg alloy was similar to the flesh and core of a drupe. A method of assembling lap joints through a simple approach to AA5052 and SPFC980 steel using REW was studied56). It could be revealed that fatigue properties had the highest fatigue strength at 10.5kA. However, the fatigue cracks in the case of the AA5052/SPFC980 joints welded under the condition of 10.5-12kA initiated and propagated at the HAZ of AA5052.
Fig. 7
Nugget diameters as a function of electric current27)
jwj-41-1-37gf7.jpg
The possibility of RSW and REW techniques to produce Mg alloy/Austenitic Stainless Steel(ASS) joints with excellent lap-shear performance was explored by Manladan et al.61). Both joints consisted of two zones, namely, the adhesive zone and weld zone. It seems that the microstructure of the fusion zone consisted of austenite and intercrystalline delta ferrite as well as the nugget microstructure consisted of fine columnar dendritic structure. Wang et al.66) attempted to join Al alloy to Ti alloy by REW and investigated the mechanical properties, fracture behavior, microstructure and interface characteristics of 7075 Al/Ti6Al4Vjoints. It was observed that the maximum peak load and energy absorption of the REW joints improved with the increasing of rivet diameter.

5. Conclusions and Future Challenges

The paper had critically reviewed the motivation for the joining technology called REW process, the process characteristics, optimization, welding quality and simulation issues. The intention of the review is to give comprehensive information on the current practices and research interests related to improve mechanical and microstructure performance during REW process and to bridge the gap between the untouched areas in automobile industries. The review has been focused on the REW technologies of different thickness sheet and dissimilar steel included Advanced High Strength Steel (AHSS), AISI 1008 low carbon steel/DP600 steel, stainless steel/non-stainless steel, aluminium and magnesium so on in automotive manufacturing processes.
The summary of research works performed shows that conventional techniques are successfully employed in optimization of welding parameters on mechanical and microstructure performance in REW process. In addition, experimental design and optimization are presented to give the experimentalist useful tools in the real experimental situation, as well as the necessary theoretical background. The observation can be utilized as a guideline document for future research in carrying out optimization of REW process. Form the above presented review of optimization of REW process, it is evident from many researchers that electric current is the major factor to affect the weld quality and weld strength by increasing or decreasing other welding parameters, but there is a need to study the effects of all welding parameters on the weld quality in REW process.
Finally, it is apparent that joints made from different metals and thickness are one of major issues for automotive design due to the requirement for not only weight reduction of automotive, but also the increased safety and structural integrity of automotive. Future research based on the reviews should concentrated on the development of advanced materials such as composites, high strength steels, aluminium and magnesium alloy with a good combination of high strength and ductility for REW process.

Acknowledgements

This work was supported by the Technology Innovation Program (20014618, Development of car body assemblies based on ultra high strength steel above 1.0GPa using multimaterial joining technology) funded By the Ministry of Trade, Industry & Energy(MOTIE, Korea)

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