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Friction self-piercing riveting (F-SPR) process based on a pip die has been invented to solve the cracking problems in riveting high-strength and low-ductility light metals, such as magnesium alloys, cast aluminum, and 7 series aluminum alloys. In this paper, in order to solve quality issues caused by the misalignment between rivet and pip-die in F-SPR, a flat-die based F-SPR process was proposed and employed to join 1.27 mm-thick AA6061-T6 to 3 mm-thick AZ31B. The results indicate that a 1.0 mm die distance is effective to avoid rivet upset and insufficient flaring. As the feed rate increases, the heat input in the whole process decreases, resulting in a larger riveting force, which in turn increases both the bottom thickness and interlock amount. Besides, solid-state bonding, including Al-Mg intermetallic compounds (IMCs), Al-Mg mechanical mixture, and Al-Fe atom interdiffusion was observed at the joint interfaces. The upper Al layer was softened, but the lower Mg layer was hardened, and both sheets exhibited a narrowed affected region with the increase of feed rate, while the rivet hardness shows no obvious change. Three fracture modes appeared accompanying the variations in lap-shear strength and energy absorption as the feed rate increased from 2 mm/s to 8 mm/s. Finally, the F-SPR process using a flat die was compared to those using a pip die and a flat bottom die to show the advantage of flat die on coping with the misalignment problem.

A recently developed friction self-piercing riveting (F-SPR) technique based on the combination of friction stir processing and riveting has been reported to possess both solid-state bonding and mechanical fastening characteristics. However, there is still a lack of quantitative understanding of the hybrid enhancement mechanism, hindering its engineering application. To fill in this gap, the current research investigated the microstructure evolution, microhardness distribution, and miniature-tensile performance of the aluminum alloy AA7075-T6 F-SPR joints by experiments. An accurate numerical simulation model was established to quantitatively evaluate the individual contributions of microstructure, local bonding strength, and macro interlocking to the performance of the joint, which could well explain the experimental results. It was found that due to the friction stirring of the rivet, solid-state bonding driven by dynamic recrystallization is realized between the trapped aluminum in the rivet cavity and the bottom aluminum sheet. The solid-state bonding zone has 75% yield strength, 81% ultimate tensile strength, and 106% elongation compared to the base material. This solid-state bonding enables the internal interlocking between the trapped aluminum and the rivet to withstand the additional load, which forms a novel dual-interlock fastening mechanism and increases the peak cross-tension force by 14.3% compared to the single-interlock joint.

The Al alloy and carbon fiber reinforced polymer (CFRP) hybrid structures, incorporating the performance advantages of the two materials, have been attracting more attention in high-end manufacturing fields. In the current investigation, the flat friction spot joining (FSJ) was employed in joining the AA6061-T6 alloy and CFRP sheets. The significance of temperature distribution in influencing joint quality was highlighted through analyzing interface microstructural features, weld defect formation as well as fractography. To understand the role of thermal energy generation and conduction in the process comprehensively, a 3D thermal-mechanical coupling finite element model was established. The interfacial temperature was characterized by an uneven distribution behavior due to the inhomogeneous heat distribution. The peak temperatures on the top surface and Al alloy to CFRP interface at 1500 rpm rotational speed with 0.1 mm/s plunging speed were 498 °C and 489 °C, respectively. The peak interface temperature was reduced to 286 °C at 250 rpm, which produced an extremely small melted area. Compared with the plunging speed, rotational speed was found to be the predominant parameter for determining the joint property, which could be optimized to simultaneously realize the avoidance of thermal decomposition of CFRP, the sufficient melting duration time, and the wide enough melted area. Simulated thermal histories and melted area profiles were in agreement with experimental ones. The findings could be utilized to provide some feasible guidance for process optimization of dissimilar FSJ of metals and composites.

In this paper, self-piercing riveting (SPR) and friction self-piercing riveting (F-SPR) processes were employed to join aluminum alloy AA5182-O sheets. Parallel studies were carried out to compare the two processes in terms of joint macrogeometry, tooling force, microhardness, quasi-static mechanical performance, and fatigue behavior. The results indicate that the F-SPR process formed both rivet-sheet interlocking and sheet-sheet solid-state bonding, whereas the SPR process only contained rivet-sheet interlocking. For the same rivet flaring, the F-SPR process required 63% less tooling force than the SPR process because of the softening effect of frictional heat and the lower rivet hardness of F-SPR. The decrease in the switch depth of the F-SPR resulted in more hardening of the aluminum alloy surrounding the rivet. The higher hardness of aluminum and formation of solid-state bonding enhanced the F-SPR joint stiffness under lap-shear loading, which contributed to the higher quasi-static lap-shear strength and longer fatigue life compared to those of the SPR joints.

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.

Friction self-piercing riveting (F-SPR) is an emerging technique for low ductility materials joining, which creates a mechanical and solid-state hybrid joint with a semi-hollow rivet. The severe plastic deformation of work materials and localized elevated temperatures during the F-SPR process yield complex and heterogeneous microstructures. The cut-off action of the work materials by the rivet further complicates the material flow during joint formation. This study employed the F-SPR process to join AA7075-T6 aluminum alloy sheets and systematically investigated the microstructural evolutions using electron backscatter diffraction (EBSD) techniques. The results suggested that as the base material approached the rivet, grains were deformed and recrystallized, forming two distinct fine grain zones (FGZs) surrounding the rivet and in the rivet cavity, respectively. Solid-state bonding of aluminum sheets occurred in the FGZs. The formation of FGZ outside the rivet is due to dynamic recrystallization (DRX) triggered by the sliding-to-sticking transition at the rivet/sheet interface. The FGZ in the rivet cavity was caused by the rotation of the trapped aluminum, which created a sticking affected zone at the trapped aluminum/lower sheet interface and led to DRX. Strain rate gradient in the trapped aluminum drove the further expansion of the sticking affected zone and resulted in grain refinement in a larger span.

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.

A three-dimensional finite element thermal-mechanical coupling model was developed to simulate the friction spot joining with a flat shoulder tool for AA6061-T6 Al alloy and carbon fiber reinforced polymer (CFRP) at different welding conditions. When joining at 1500 rpm rotation speed and 0.1 mm/s plunge speed, the peak temperature at the Al alloy-CFRP interface reached up to 575 °C as the plunge depth was increased to 0.6 mm. The interfacial temperature was reduced as rotation speed or plunge depth was decreased. Good correspondences between predicted temperature field distribution and experimental bonded area have been achieved. Thermal history and temperature distribution at the top surface of Al alloy were well validated by experimental measurements. The temperature zone within 220–340 °C made the greatest contribution to improving joint integral resistance to tensile-shear testing, which was recommended to enlarge during the process for a given welding parameter. On the basis, the design of tool structure enabling to reduce peak temperature and enlarging melted area was proved numerically as a feasible way to enlarge processing windows to increase the overall strength of joint. The findings could hopefully help optimize friction spot welded joints of dissimilar metals and composites.

Self-piercing riveting (SPR) is a mature method to join dissimilar materials in vehicle body assembling. Friction self-piercing riveting (F-SPR) is a newly developed technology for joining low-ductility materials by combining SPR and friction stir spot welding processes. In this paper, the SPR and F-SPR were employed to join AA6061-T6 aluminum alloy and AZ31B magnesium alloy. The two processes were studied in parallel to investigate the effects of stack orientation on riveting force, macro-geometrical features, hardness distributions, and mechanical performance of the joints. The results indicate that both processes exhibit a better overall joint quality by riveting from AZ31B to AA6061-T6. Major cracking in the Mg sheet is produced when riveting from AA6061-T6 to AZ31B in the case of SPR, and the cracking is inhibited with the thermal softening effect by friction heat in the case of F-SPR. The F-SPR process requires approximately one-third of the riveting forces of the SPR process but exhibits a maximum of 45.4% and 59.1% higher tensile–shear strength for the stack orientation with AZ31B on top of AA6061-T6 and the opposite direction, respectively, than those of the SPR joints. The stack orientation of riveting from AZ31B to AA6061-T6 renders better cross-section quality and higher tensile–shear strength and is recommended for both processes.

In this paper, a traditional resistance spot welding (RSW) process in combination with a GM-patented Multi-Ring Domed (MRD) electrode was used to join two types of aluminum alloys, AA5754-O and AA6022-T4, to interstitial-free low carbon steel (LCS). Parallel studies were carried out for AA5754-LCS and AA6022-LCS resistance spot welds to investigate the effects of aluminum contact resistance on the weld profile, interfacial microstructure, defect distribution, and coach peel performance. The results indicated AA5754-O develops a higher contact resistance when ex-posed to atmospheric conditions. This resulted in a degradation of the RSW process due to increased internal expulsion of the molten aluminum nugget and concurrent reduction in aluminum nugget size that contributed to a loss in joint mechanical performance. By contrast, AA6022-T4 exhibited a lower contact resistance, which minimized internal expulsion and promoted the retention of larger aluminum nuggets. The larger AA6022 weld nuggets exhibited improved mechanical performance in comparison to the smaller 5754 nuggets.

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.

Safety evaluation of resistance spot welds necessitates the accurate measurement of local constitutive properties. This study employed miniature mechanical tests to investigate the deformation and failure behaviors of nugget, heat affected zone (HAZ), and corona bond of resistance spot welded JSC980YL steel. A novel mini-peel test was developed to enable local fracture in HAZ for numerical inverse calibration of constitutive parameters. The fracture constants of weld zones calibrated using Cockcroft-Latham ductile failure criterion were incorporated in finite element models to predict the failure modes of spot welds in tensile-shear and cross-tension coupon tests. The result indicates that the ultimate tensile strengths of the nugget and the corona bond were 37.6% higher and 5.8% lower, respectively, than that of the base material. The nugget and HAZ exhibited ductile fracture, whereas the corona bond was brittle fracture with only 1.2% elongation. In the coupon tests, the increase of nugget diameter slowed down the damage accumulation rate in the nugget and accelerated that in the HAZ, resulting in the transition of failure mode from interfacial to pullout. The failure load of corona bond in coupon tests increased with the increase of nugget diameter while its contribution to the peak load decreased.

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.

Friction self-piercing riveting (F-SPR) process has been proposed to achieve crack-free joining of low-ductility materials by combining SPR process with the concept of friction stir processing. The inhibition of cracking in an F-SPR joint is related to the in-process temperature as well as plastic deformation of materials, which are controlled by the process parameters, i.e., spindle speed and feed rate. However, the relationship between F-SPR process parameters and the temperature characteristics within the joint has not been established. In the current study, a coupled thermal-mechanical model based on solid mechanics was setup to study the F-SPR process of aluminum alloy and magnesium alloy. Temperature and strain rate-dependent material models and preset crack surface method were integrated in the model and geometry comparisons were conducted for model validation. Based on this model, the evolutions of temperature and plastic deformation in the rivet and the sheets of an F-SPR joint were obtained to reveal the formation mechanism of the joint. The temperature distribution and evolution of the sheet materials were correlated with F-SPR process parameters, and a critical spinning speed of 2000 rpm at a feed rate of 1.35 mm/s was determined capable of inhibiting cracking in the magnesium sheet.

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.

Zinc-coated ultra-high-strength steel (UHSS) sheets are highly susceptible to liquid metal embrittlement (LME) during resistance spot welding. However, systematic understanding on LME cracks in the pullout fracture of UHSS welds is still lacking. This study employs artificially created pre-cracks on bare DP980 steel welds to simulate the LME cracks, allowing accurate control of the crack shape and location. To investigate the fracture initiation and propagation behavior of welds with various pre-crack characteristics, a combined experimental and computational approach, which incorporated tensile tests on newly designed miniature specimens for detailed mechanical property measurement and fracture parameter calibration of different weld zones, was proposed. Results indicate that external pre-cracks in a highly strained region accelerated damage accumulation at the fracture onset region and deflected the fracture propagation path to pass through the pre-crack, causing a 12.1% loss of tensile-shear peak load. Conversely, internal pre-cracks vary the fracture initiation site and exhibit a more detrimental influence, with 24.1% loss of tensile-shear strength. The effects of the pre-crack length, orientation, and depth on the tensile-shear performance were quantified. Results show that the negative effect of the LME crack on the weld performance can be mitigated or avoided by controlling the crack location and geometry.

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.

Mixed use of high strength materials and lightweight materials is becoming one of the essential methods for vehicle body lightweighting. However, multi-material vehicle bodies need multi joining processes for assembling, which significantly increases the assembly cost compared with traditional steel bodies. In this paper, an improved friction self-piecing riveting (F-SPR) process was proposed, aiming at joining different multi-material stack-up using the same equipment with just changing rivet and dies. The effectiveness of this process was validated via joining three typical stack-ups, i.e., F-SPRing of aluminum alloy to low ductility magnesium alloy, single-sided F-SPRing of aluminum alloys and F-SPRing of aluminum alloy to high strength steel. The effects of process parameters on the plastic deformation of rivet and materials inside the joint and the ductility improvement of materials were investigated.

To improve the joint quality by reducing the deformation resistance of dual-phase (DP) steels, electric-aided self-piercing riveting (EA-SPR) was conducted in this paper. The effect of current density on the joint forming process, geometry characteristics and mechanical properties of EA-SPR for both combinations of aluminum to steel and steel to aluminum were investigated and compared with the traditional SPR joints. It was found that the riveting force decreased for all the DP steels with different strength grades. The increase amplitude of undercut increased with the elevated strength grades of DP steels. When riveting from aluminum to steel, the tensile-shear and cross-tension strengths of EA-SPR joints could improve up to 13.3% and 15.7% respectively, compared with the traditional SPR joints. For joints riveted from steel to aluminum, the current showed less effect on the tensile-shear strength due to the rivet shank fracture. Nevertheless, the cross-tension strength could still improve by 13.04%.

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.

A new material model of magnesium alloys, combining both Hill&rsquo, 48 yield function and Cazacu&rsquo, 06 yield function, was developed and programmed into LS-DYNA using user subroutine, in which both slip dominant and twinning/untwinning dominant hardening phenomena were included. First, a cyclic load test was performed, and its finite element analysis was carried out to verify the new material model. Then, the deformation behaviors of the magnesium crash box subjected to the compressive impact loading were investigated using the developed material model. Compared with the experimental results, the new material model accurately predicted the deformation characteristics of magnesium alloy parts. Additionally, the effect of the thickness distribution, initial deflection and contact friction coefficient in simulation models on deformation behaviors were investigated using this validated material model.

A traditional resistance spot weld process in combination with Multi-Ring Domed welding electrodes was applied to join AA5754-O to low carbon steel. The coach peel performance of welds was compared between the as-welded (unbaked) and post-paint bake condition (baked). The unbaked joints exhibited button pullout fracture mode, whereas the baked joints were severely weakened and demonstrated an interfacial fracture mode which is attributed to the baking process inducing thermal stress at the dissimilar material interface which cracked the semi-continuous oxide film inclusions at the edge of the solidified aluminum nugget to form a more continuous defect layer across the diameter of the aluminum nugget. These defects act as low energy crack paths under applied loads and reduce joint strength by promoting undesirable interfacial fracture in baked welds.

Welding of magnesium (Mg)/steel faces huge challenges due to the extremely low mutual solid solubility. In this paper, the resistance rivet welding (RRW) was used to join Mg alloy to steel for the first time, and a metallurgical-mechanical hybrid joint was formed among the semi-tubular rivet, Mg alloy and sheet steel. A steel hollow sphere weld was formed by fusion welding between steel rivet and lower steel sheet, with re-solidified Mg alloy maintained inside it. The microstructure of the hollow sphere weld consisted of lath-like martensite. The welds present both interfacial fracture (IF) mode and button pullout fracture (BPF) mode in tensile shear tests and the strength reaches the maximum of 4381 kN at 7 kA-200 ms with BPF mode. The failure mechanism of the hollow sphere weld for IF shows the ductile characteristic, and the fracture characteristics of Mg sheet for BPF show the mixture of ductility and brittleness. This study provided an alternative technology for welding Mg/steel immiscible materials.

Resistance element welding (REW) is a recently developed hybrid joining process for aluminum (Al)/steel dissimilar materials. In this study, the microstructure and mechanical performance of the Al/press hardened steel (PHS) joints generated using an integrated REW process were investigated. The microstructure of fusion zone (FZ) between rivet and PHS was lath martensite, which was transformed from the fast cooling of austenite. The Al could be divided into four zones according to the microstructure and microhardness distribution: re-solidified zone (RZ), softening zone (SZ), transition zone (TZ) and base metal (BM). The dissolution and coarsening of the precipitates are responsible for the hardness reduction of Al sheet. The variation of mechanical performance was explained in light of the increasing FZ size and the softening Al sheet as heat input rising. Moreover, four block shear models were introduced to predict the peak load of REW joints using the average hardness of SZ in Al sheet, among which the models from Architectural Institute of Japan (AIJ) provided a relatively good prediction. Considering both the average hardness of SZ and sheet thickness, an analytical model was established to predict the variation of critical nugget sizes and explain the failure mode transition.

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.

Carbon fibre reinforced plastics (CFRP) and Al5052 sheets were joined by coaxial one-side resistance spot welding (COS-RSW). The cross-section features and mechanical behaviour of the joints in single lap shear (SLS) test were investigated. The underlying joining mechanism was found to be the N C O covalent bond between silane coupling agent and CFRP via the X-ray photoelectron spectroscopy (XPS) analysis. The joints of low welding current exhibited interfacial fracture with low strength and small displacement during the SLS test, while the joints of high welding current presented a staged failure process with tearing of the CFRP, which gave higher strength and larger displacement. The bonding zone was divided into three subzones, which showed specific surface morphologies and distinct contributions to the load-bearing capacity of the joint. The joining strength showed a positive correlation with welding current until the occurrence of decomposition in CFRP, which reduced the area of effective bonding zone and played a negative effect on strength.

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.

Carbon fiber reinforced plastic (CFRP) is a prospective lightweight material in automobile industry. However, joining metal and CFRP is a great challenge. In the present study, an innovative process called coaxial one-side resistance spot welding (COS-RSW) is proposed to fabricate Al5052 (Al) and CFRP joints. Based upon our newly developed finite element code JWRIAN, the electric-thermal-mechanical coupled process of COS-RSW is modeled and validated with experiments. The influences of welding current, welding time and electrode force on CFRP molten zone are investigated in detail. The results show that the depth of molten zone has a strong correlation with both welding current and welding time. A relative low welding current or short welding time that results in the Al/CFRP interface temperature below the melting temperature of the resin matrix is insufficient to form a sound connection between the two sheets. While excessive current or too long welding time may lead to overheating of CFRP and decomposition of the resin matrix, which increases the risk of weak joining. Furthermore, it is found that the depth of molten zone increases even at the cooling stage, which indicates accurate simulation for both the heating and the cooling stages of the COS-RSW process is indispensable. Keywords: Coaxial one-side resistance spot welding, Carbon fiber reinforced plastic, Al5052, Dissimilar materials, Multiphysics coupling simulation

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.

Friction self-piercing riveting (F-SPR) process has been proposed to join low ductility lightweight materials, and has shown advantages over fusion welding, solid state welding and traditional mechanical joining processes in joining dissimilar as well as low ductility materials. Because of the thermo-mechanical nature of F-SPR process, the formation of the joint is determined by riveting force and softening degree of the materials. However, it is still not clear that how exactly the riveting force and generated frictional heat jointly influence the mechanical interlocking formation and inhibit cracks during F-SPR process. To address these issues, in current study, F-SPR process was applied to join 2.2 mm-thick AA6061-T6 aluminum alloy to 2.0 mm-thick AZ31B magnesium alloy. The correlation of riveting force, torque responses as well as energy input with joint quality were 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 method was proposed to better control the energy input and riveting force. It was found that the joints produced by the two-stage method exhibited significantly improved lap-shear strength, i.e., 70% higher than traditional SPR joints and 30% higher than one-stage F-SPR joints. This research provides a valuable reference for further understanding the F-SPR joint formation and 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.