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Cast aluminum alloys are promising materials that can simplify the manufacturing process of automobile body structures. However, the low ductility of cast aluminum poses significant challenges to existing riveting technologies. In the present work, dissimilar AA6061-T6 aluminum alloy and Al–Si7Mg cast aluminum were joined by self-piercing riveting (SPR) and friction self-piercing riveting (F-SPR) processes to reveal the effect of friction heat on rivetability of low-ductility cast aluminum alloys. The joint macro-morphology, microstructure, peak tooling force, microhardness distribution, tensile-shear, and cross-tension performance of the two processes were comparatively studied. Results indicated that the in-situ softening effect of friction heat in the F-SPR process could effectively improve the ductility of cast aluminum, avoid cracking, and reduce the tooling force by 53%, compared to the SPR process. The severe plastic deformation and friction heat induced by rivet rotation results in refined equiaxed grains of aluminum near the rivets and solid-state bonding between aluminum sheets in the rivet cavity. The F-SPR joints are superior to SPR joints in both tensile-shear and cross-tension performance due to the avoidance of cracking, increase of mechanical interlocking, and solid-state bonding of interfaces. Significantly, when Al–Si7Mg is placed on the lower layer, the peak tensile-shear and cross-tension loads of the F-SPR joints are 7.2% and 45.5% higher than the corresponding SPR joints, respectively.

This paper describes an experimental study in which a traditional resistance spot weld process with Multi-Ring Domed (MRD) welding electrodes was used to join AA5754-O to low carbon steel sheets in a coach peel configuration. A linear relationship was identified for the resistance spot weld C-gun stationary arm deflection with force applied by the movable gun arm. Due to the relative rotation of the electrodes during deflection, the point of electrode/coupon surface contact moved away from the original centerline to a point inboard towards the C-gun arm. This led to asymmetric features in aluminum/steel resistance spot weld morphology, intermetallic compound (IMC) characteristics, weld defect distribution, and aluminum hardness. The aluminum weld nugget solidified closer to the weld gun with respect to the centerline of the MRD electrode imprint and exhibited a softer thermo-mechanically affected zone (TMAZ) and expulsion which produced a thinner IMC layer and fewer oxide film defects. This in turn resulted in strong button pull-out strength and the welds were able to absorb 36% more energy under an applied directional load in comparison to aluminum weld nuggets that solidified further away from the weld gun with respect to the electrode imprint centerline. The mechanism responsible for these weld asymmetries was further revealed through analysis of the variation in contact condition between electrodes and workpieces due to weld gun deflection.

Coaxial one-side resistance spot welding (COS-RSW) is an advanced technique for joining CFRP/metal dissimilar materials. In the present work, a three-dimensional model was developed based upon an in-house finite element code JWRIAN-SPOT to simulate the multi-physical process of the COS-RSW. The thermal cycle and joining strength of COS-RSW joints with different welding parameters were measured experimentally. The sensitivity of material properties of electrode couple and component of accumulated heat generation were discussed in detail. The predicted thermal cycles presented a reasonable agreement with measurements, which also explained the influence of welding parameters on temperature field. The joining strength of COS-RSW joints can be evaluated qualitatively via the area of effective molten zone, which reached about 20 MPa with a 4800 A welding current. The column and cylinder electrodes determined the peak value of interface temperature and molten zone area, respectively. The cylinder electrode was suggested to be fabricated by a high resistivity material to generate sufficient Joule heat. Moreover, the SUS304 electrode and its contact zone provided more than 70% of the entire heat generation. The materials of electrode couple should be determined carefully when employing COS-RSW for joining CFRP and other metals.

In this paper, an external magnetic field was introduced to resistance spot welding (RSW) to improve the weldability of A7N01 aluminum alloy. Comparative studies of the macro- and micro-structures, microhardness, and the static and dynamic mechanical properties between welds produced by traditional RSW and magnetically assisted RSW (MA-RSW) were completed. The results show that the external magnetic field could enable larger nugget diameter, the softened columnar grain zone (CGZ-I), the hardened columnar grain zone (CGZ-II) and the finer equiaxed grain zone (EGZ). As a result, compared to traditional RSW joints, the MA-RSW joints exhibited better strength, improved toughness, and greater energy absorption capacity. This resulted in a transition of the failure mode from an interfacial fracture (IF) to a partial thickness fracture (PTF) in lap-shear tests and a BP fracture in cross-tension tests, respectively. In fatigue tests, the larger nugget diameter, hardened CGZ-II and finer EGZ in the MA-RSW welds can inhibit cracks from propagating from EGZ, but from the softened CGZ to the base metal. This change of fracture mode greatly improved the fatigue life of MA-RSW welds under both high and low load conditions.

Single point incremental forming (SPIF) is a die-less manufacturing technology using a numerical controlling system for complex and customized parts. However, single-step SPIF cannot achieve high wall angle shapes due to the formability limit of the material. As wall angle gets larger, local thinning becomes severe, and cracking occurs easily. Generally, a multi-step strategy is used to get a hard-to-form deep drawing shape. However, inappropriate multi-step strategies can cause a stepped feature (corrugation). In this study, a novel parameterized multi-step strategy is proposed to solve the local thinning and stepped feature issues of forming high wall angle shape. Both experimental and simulation results showed this strategy not only improves the formability of a high wall angle, by avoiding over-thinning in the local region and redistributing the thickness of the material, but also minimizes geometric deviation by stepped feature. The multi-step strategy made the local thickness increase from 0.47 mm to 0.68 mm, preventing both cracking observed in a single-step strategy and the occurrence of the stepped feature. With the good agreement between experimental and simulation results, simulations with different parameters in a parameterized multi-step strategy were carried out to compare thickness distribution at each step, the stepped feature at the bottom and the ductile failure tendency. The comparison reveals that the locally large plastic strain, which was caused by the mismatch between the tool radius and vertical displacement, is the main reason for uneven thickness distribution, obvious stepped feature, and high failure tendency. With appropriate parameters, a multi-step strategy results in a more uniform thickness distribution, higher geometric accuracy, and lower failure tendency.

In the present research, a digital twin of coaxial one-side resistance spot welding (COS-RSW) was established for the real-time prediction of transient temperature field. A three-dimensional (3D) model of COS-RSW joint was developed based on the in-house finite element (FE) code JWRIAN-SPOT. The experimental verified FE model was employed to generate the big data of temperature of COS-RSW process. Multiple dimension interpolation was applied to process database and output prediction. The FE model can predict the thermal cycle on COS-RSW joints under different parameter couples. The interpolation effect of individual welding parameters was discussed, and a power weight judgement for welding time was essential to ensure accuracy. With the support of big data, the digital twin can provide visualization prediction of COS-RSW within 10 s, whereas numerical modeling needs at least 1 h. The proposed application of digital twin has potential to improve the efficiency of process optimization in engineering.

Coaxial one-side resistance spot welding (COS-RSW) is a newly developed process for joining metals and composites. In the present study, Al5052 and carbon-fiber-reinforced plastic (CFRP) lap joints were fabricated via COS-RSW. The welding process was modeled numerically using an in-house finite element code called JWRIAN. Single-lap shear tests were performed to evaluate the joining strength. The molten zone diameter was defined and measured experimentally to verify the numerical model. An artificial neural network (ANN) was established based on multitask learning, and its training data set was prepared via finite element analysis (FEA). The well-trained ANN was employed to generate a process window for the COS-RSW. Results demonstrated that the FEA could accurately reproduce the COS-RSW process, which served as an efficient tool for generating a process data set without performing experiments. The ANN performed multitask learning well and predicted the welding output effectively. Furthermore, Tmavg, an index representing the average value of the maximum temperature in the molten interface of CFRP, was adopted to evaluate the contribution of the integral interface temperature field to the bonding strength qualitatively. An optimal Tmavg value, which was close to the CFRP decomposition temperature of 340 °C, was obtained, and it exhibited an excellent correlation with higher bonding strengths. The process window provided welding parameters directly to yield the desired results.

Friction self-piercing riveting (F-SPR) process has shown advantages over traditional self-piercing riveting (SPR) process in joining magnesium alloys and aluminum alloys. Frictional heat produced by the rotation of the rivet plays important roles to the inhibition of cracking in riveting low ductility materials and the formation of solid-state bonding. However, the current F-SPR process needs a matching die to facilitate rivet deformation and formation of mechanical interlock between lower sheet and the rivet, which poses a big challenge in riveting large thin-walled structures, where a matching die is hard to apply. In this research, single-sided F-SPR process was proposed to join 2.2 mm-thick aluminum alloy AA6061-T6 to 3.0 mm-thick aluminum alloy AA6061-T6, and effects of process parameters, e.g., rotation speed and feed rate, on the morphology and mechanical properties of the joints were studied experimentally. It was found that higher rotation speed and lower feed rate which signified higher heat input could effectively reduce defects but lead to insufficient undercut. As such, a two-stage process was proposed to reduce frictional heat generation in lower sheet and thus increase the deformation resistance acted at the rivet tip. A preliminary study showed that the two-stage F-SPR process increased the tensile-shear and cross-tension strengths by around 25% and 81% respectively, compared to the one-stage process.

The combination of adhesive and other joining processes has been an increasing interest in the transportation industry. In this paper, a hybrid joining process combining friction self-piercing riveting (F-SPR) and adhesive bonding was developed to join AA7075-T6 aluminum alloy sheets. The formation process, macro morphology, microstructure, microhardness, and mechanical performance of F-SPR bonded joints were investigated comparing with the F-SPR joints. It was concluded that the adhesive played a role of lubrication for reducing the contact stiffness between the aluminum sheets, which further reduced the interlocking amount but did not affect the solid-state welding between the aluminum sheets. After the F-SPR bonding process, the initial 0.3-mm-thick adhesive layer was squeezed to less than 0.06 mm under a maximum riveting force of about 20 kN. The subsequent baking treatment for the adhesive curing (30min@180℃) re-precipitated the η strengthening phase in the aluminum heat-affected zone, which further improved the aluminum hardness as well as the mechanical performance of the joint. The F-SPR bonded joint exhibited a combination of adhesive failure and rivet pull-out failure under quasi-static loading, which improved the tensile-shear strength by 128.7 % than the baked F-SPR joint but had no apparent effect on cross-tension performance. The F-SPR bonded joint also superposed the fatigue failure modes of the F-SPR joint and adhesive bonding joint, exhibiting the longest fatigue lives under the same cyclic load amplitude. The process provides a new method for aluminum alloy sheet joining in body-in-white production.

Mixed usage of dissimilar materials, such as aluminum alloys and steels, has become one of the main means for vehicle body lightweighting. Due to the formation of hard and brittle intermetallic compounds in fusion welded or brazed aluminum and steel welds, resistance spot welding is hardly used. Self-piercing riveting is a preferred cold forming fastening technology and has been widely used in joining aluminum sheets and aluminum to steel sheets of multi-material vehicle bodies. However, the vast variety of stack-ups with different gauges and material properties in a body-in-white brings problems to the selection of rivets and dies. How to make the most of the rivetable range of a specific rivet and die combination is essential for not only the simplification of production line layout but also cost reduction. In this paper, seven sets of rivet and die combinations with different die-to-rivet volume ratios were selected to investigate the effects of four key factors, i.e., rivet hardness, rivet length, die width and die pip height, on the rivetability and mechanical performance of the aluminum alloy AA6061-T6 and mild steel CR4. The lap-shear strength was found linearly correlated with the undercut for a fixed top sheet thickness and an equation was fitted to predict the lap-shear strength in terms of undercut and top sheet thickness. Rivetability diagrams were used to illustrate the rivetable ranges and joint quality under each set of rivet and die combinations. It was found that the softer rivets and larger dies could improve the rivetability range, meanwhile decrease the joint strength. The longer rivets and smaller dies could narrow down the rivetability range, but increase the joint strength. A die-to-rivet volume ratio of greater than 1.0 was necessary to avoid the quality issues of bottom cracking and raised rivet head for the selection of rivets and dies. These results would provide valuable reference for the determination of the riveting factors in engineering.

The increasing demand for lightweighting in the automobile industry drives the mix-use of high specific strength materials, such as aluminum alloy and advanced high strength steel (AHSS). However, the differences in physical and metallurgical properties of aluminum alloy and steel, and the increase of steel strength have brought great challenges to the traditional spot joining technologies. This paper proposed a novel friction stir riveting (FSR) process to join aluminum alloy and AHSS sheets. In the FSR process, a high-speed rotating semi-hollow rivet penetrates through the aluminum sheet and is then friction welded with the steel sheet to form an annular solid phase welding zone, which mechanically locks the aluminum sheet between the rivet cap and the steel sheet, creating a solid phase-mechanical hybrid joint. The joint macro morphology formation, the solid phase welding mechanism, the thermo-mechanical coupled effects on microstructures and microhardness, and the tensile-shear performance of the joints were investigated. The results indicate that the FSR process achieved successful joining of AA6061-T6 and DP600. However, the joint performance was limited by a thin layer of inclusion in the solid phase welding zone, which was produced after the rivet cavity being fully filled by the trapped aluminum alloy. Further optimization of the rivet cavity and corresponding process parameters is necessary to obtain an inclusion-free FSR joint. The FSR process is expected to provide a new industrial solution for the joining of light alloy to steel and offers new insight into the solid phase joining of tube-sheet structures.

AA7xxx series aluminum alloys have great potentials in mass saving of vehicle bodies due to pretty high specific strength. However, the use of these high strength materials poses significant challenges to traditional self-piercing riveting (SPR) process. To address this issue, friction self-piercing riveting (F-SPR) was applied to join aluminum alloy AA7075-T6 sheets. F-SPR is realized by feeding a high speed rotating steel rivet to aluminum alloy sheets to form a dissimilar material joint. The effects of spindle speed and rivet feed rate on F-SPR joint cross-section geometry evolution, riveting force and energy input were investigated systematically. It was found that the rivet shank deformation, especially the buckling of the shank tip before penetrating through the top sheet has significant influence on geometry and lap-shear failure mode of the final joint. A medium rivet feed rate combined with a high spindle speed was prone to produce a defect free joint with sound mechanical interlocking. F-SPR joints with the failure mode of rivet shear fracture was observed to have superior lap-shear peak load and energy absorption over the joints with mechanical interlock failure. The optimized F-SPR joint in this study exhibited 67.6% and 13.9% greater lap-shear peak load compared to, respectively, SPR and refill friction stir spot welding joints of the same sheets. This research provides a valuable reference for further understanding the F-SPR process.

The options for joining aluminum (Al) alloy to ultra-high strength steels are extremely limited due to the great differences in their thermophysical properties. In this work, a novel process, e.g., resistance rivet welding (RRW), was proposed to join Al alloys to press hardened steels (PHS) through combining traditional self-piercing riveting and resistance spot welding. The materials plastic flow, element diffusion and nugget growth characteristics during RRW were studied in depth to reveal the nugget formation mechanism. It was found that in RRW process, the intercepted Al alloy, internal wall of rivet shank and PHS near the faying surface were melted together and produced a unique Al/steel mixed nugget. The microstructure of the mixed nugget was equiaxed ferrite with a large number of B2-structured intermetallic compounds (IMCs) FeAl dispersed within it. The competition between the nugget and Al sheet determined the joint mechanical performance. The nugget dominated joint strength when the heat input is comparatively low while the Al sheet played a significant role with the increase of heat input.

In the present work, a recently developed solid state joining technique, Friction self-piercing riveting (F-SPR), has been applied for joining high strength aluminum alloy AA7075-T6 to magnesium alloy AZ31B. The process was performed on a specially designed machine where the spindle can achieve the motion of sudden stop. Effects of rivet rotating rate and punch speed on axial plunge force, torque, joint microstructure and quality have been analyzed systematically. During F-SPR, higher rotating rate and slower punch speed can reduce axial force and torque, which correspondingly results in a slightly smaller interlock between rivet leg and joined materials. Improved local flowability of both aluminum and magnesium alloys under a higher rotating speed results in a thicker aluminum layer surrounding the rivet leg, where formation of Al-Mg intermetallics was observed. Equivalent joint strength obtained in this study are higher than the yield strength of the AZ31 Mg alloy. One of the tensile failure modes is the rivet fracture, which is due to local softening of rivet leg from frictional heat. Other two failure modes include rivet pullout and shear through of bottom sheet.

Friction self-piercing riveting (F-SPR), as a combination of traditional self-piercing riveting and friction stir spot welding processes, has been proposed to solve the cracking issues in riveting low ductility materials. The F-SPR process creates mechanical and solid-state hybrid joints, which are quite different from existing spot welding/joining methods. However, the roles that mechanical interlocking and solid-state bonding play upon the mechanical behavior of the joints are unknown. To fill in this gap, the current body of work investigated the tensile-shear behavior and fracture mechanism of the F-SPRed AA7075-T6 aluminum alloy sheets. The results reveal that the ring-shaped solid-state bonding between aluminum sheets enhances the overall stiffness of the joints but shows a minor contribution to the tensile-shear performance. The solid-state bonding formed between the captured aluminum in the rivet shank and the lower sheet serves as an “anchor” to hinder the rotation of the rivet, which delays the action of rivet pull-out from the lower sheet and thus strengthens the joints significantly. The combination of a hard rivet, large rivet flaring and solid-state bonding inside the rivet shank is necessary to achieve the preferred rivet pull-out fracture mode, which shows a higher peak load and larger energy absorption compared to other fracture modes.

Friction self-piercing riveting (F-SPR) process has shown advantages over fusion welding, solid state welding, and traditional mechanical joining processes in joining dissimilar materials. Because of the thermo-mechanical nature of F-SPR process, formation of the joint is determined by both riveting force and softening degree of materials to be joined. However, it is still not clear that how exactly the riveting force and generated frictional heat jointly influence mechanical interlocking formation and crack inhibition during F-SPR process. To address these issues, F-SPR process was applied to join 2.2 mm-thick aluminum alloy AA6061-T6 to 2.0 mm-thick magnesium alloy AZ31B. The correlation of riveting force, torque responses, and energy input with joint quality was investigated systematically under a wide range of process parameter combinations. It was found that a relatively greater final peak force and higher energy input were favorable to produce sound joints. Based on that, a two-stage F-SPR method was proposed to better control the energy input and riveting force for improved joint quality. The joints produced by the two-stage method exhibited significantly improved lap-shear strength, i.e., 70% higher than traditional self-piercing riveting (SPR) joints and 30% higher than previous one-stage F-SPR joints. This research provides a valuable reference for further understanding the F-SPR joint formation mechanism and conducting process optimization.

Aluminum alloys have been increasingly adopted in the fabrication of automotive body structures as an integral component of mass savings strategy. However, mixed use of dissimilar aluminum alloys, such as sheet metals, castings, and extrusions, poses significant challenges to the existing joining technologies, especially in regard to single-sided joint access. To address this issue, the current study applied the friction stir blind riveting (FSBR) process to join 1.2 mm-thick AA6022-T4 aluminum alloy to 3 mm-thick Aural-2 cast aluminum. A newly developed, robot mounted, servo-driven, FSBR equipment and the procedure using it to make FSBR joints were introduced systematically. The effect of rivet feed rate and spindle speed on joint formation and cross section geometry was investigated, and it was found that a high spindle speed and a low rivet feed rate, i.e., high heat input, are prone to produce good joints, and that low heat input can cause severe problems related to insufficient softening of the sheets. The rivet deformation, especially the notch location on the mandrel relative to the shank has significant influence on lap-shear strength and fracture mode of the final joints. A rivet pull-out fracture mode was observed at higher rivet feed rates and lower spindle speeds and exhibited significantly improved energy absorption capability, i.e., 62% higher compared to traditional blind riveted (BR) joints.

In recent years, higher requirements on vehicle performance and emission have been posing great challenges to lightweighting of vehicle bodies. Mixed use of lightweight materials, e.g., aluminum alloys and magnesium alloys, is one of the essential methods for weight reduction. However, the joining of dissimilar materials brings about new challenges. Self-piercing riveting (SPR) is a feasible process to mechanically join dissimilar materials, however, when magnesium alloy sheet is put on the bottom layer, cracks occur inevitably due to the low ductility of the magnesium alloy. Friction self-piercing riveting (F-SPR) process is a newly proposed technology, which combines the SPR with friction stir spot welding (FSSW) and has been validated being capable of eliminating cracks and improving joint performance. However, in the F-SPR process, the generation of the transient friction heat and its effect on interaction between the rivet and the two sheets are still unclear. In this paper, a three-dimensional thermomechanical-coupled finite-element (FE) model of F-SPR process was developed using an ls-dyna code. Temperature-dependent material parameters were utilized to calculate the material yield and flow in the joint formation. Preset crack failure method was used to model the material failure of the top sheet. The calculated joint geometry exhibited a good agreement with the experimental measurement. Based on the validated model, the transient formation of F-SPR mechanical joint, stress distribution, and temperature evolution were further investigated.

Conventional magnesium alloys, due to their low ductility, have a poor self-piercing rivetability. Cracks always occur when the magnesium sheet is placed at the bottom layer, which brings great challenge to the use of the magnesium alloys. In this paper, friction self-piercing riveting (F-SPR) process was adopted to join 1 mm thick aluminum alloy AA6061-T6 to 2.2 mm thick magnesium alloy AZ31B, and the effect of rivet hardness and key geometrical features on joint formation were studied systematically. The experimental results showed that using rivets with a hardness of 190 HV, the top aluminum sheet could be well pierced and a larger rivet shank flaring value would be formed between rivet shank and the bottom magnesium. The effect of the rivet’s geometrical features, including ribs under shoulder and inclination angle under shoulder, were examined using two evaluation criteria, i.e., rivet shank flaring value and remaining thickness, and found that the rivet with no ribs and 10 deg inclination angle under shoulder is suitable for joining 1 mm AA6061-T6 to 2.2 mm AZ31B in F-SPR process. [DOI: 10.1115/1.4029822]