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Resistance rivet welding (RRW) process was applied to join three-sheet of Al/steel/steel. The formation mechanism, microstructure and mechanical performance of three-sheet dissimilar materials joints were studied in detail. The Al/steel/steel mixed nugget is produced by the fusion of Al/steel mixed and steel/steel nuggets. The microstructure of Al/steel/steel mixed nugget varies from mixed martensite and ferrite to full martensite as the increasing heat input. The ferrite contains numerous B2 structure nano-precipitates. The introduced Al element by the rivet cavity plays a significant influence on the microstructure, and the competition between ferrite and austenite stabilized elements in mixed nugget is responsible for the microstructure evolution. The phases performance of ferrite and martensite is determined. The average nano-hardness of ferrite is 2.66 GPa (271.3 HV), while that of martensite is 4.68 GPa (477.4 HV), and the yield strength for ferrite and martensite is calculated to be 475.0 MPa and 771.1 MPa, respectively. The average microhardness of the mixed nugget is 369.1 HV, 454.1 HV and 481.7 HV for welding parameters 7 kA-200 m s, 8 kA-200 m s and 9 kA-200 m s, presenting a trend of increasing with the heat input. Considering the multilayer joining containing both the Al/steel heterogeneous and steel/steel homogeneous interfaces, the comprehensive tensile-shear performance is optimal at welding parameters 8 kA-200 m s, 5334 N for Al/steel interface and 19,531 N for steel/steel interface.

The aging characteristics of Al/UHSS resistance rivet welding (RRW) joints are investigated for the first time in this paper. Microstructure studies have shown that the aging treatment does not change the phase composition of the Al/steel mixed nugget still remaining the ferrite structure, while the precipitation, growth and dissolution occur for the microstructure of Al alloy varying with the distance from the rivet. Mechanical tests revealed that for interfacial fracture (IF) and button pullout fracture (BPF) joints, the maximum peak load increased by 13% and 26% with the prolonged aging time, respectively. The improvement in the joint strength after aging can be attributed to the relaxation of the residual stress for IF joint and the age hardening of Al sheet for BPF joint. In addition, interestingly, the accelerated artificial aging is discovered firstly in Al/steel RRW joint. The phenomenon can be attributed to the higher solid solubility of Mg and Si in softening zone of Al sheet, resulting in faster and more precipitation during the aging treatment. The precipitation kinetics indicated that the average Avrami exponent was 1.25, proving the one-dimensional linear growth of fresh nuclei of the hardening phase β''. The block shear model analysis suggested that the Canadian Standards Association, giving the calculating/test values ratio ranged from 0.88 to 1.08, could provide a more reasonable calculation, which showed that the shear failure was mainly determined by the yield of the gross shear plane, and the tension plane failed because of tensile rupture during the lap-shear test.

The mixed use of aluminum (Al) alloy and high strength steel (HSS) is an effective way to reduce the weight of vehicles, which meanwhile brings great challenges to the traditional spot joining technologies. Three-stage friction stir riveting (FSR) process using a precisely designed semi-hollow rivet was developed for Al-steel joining. By coordination control the rotation and feed movement of the rivet, a “tube-sheet” annular solid-phase welding zone is formed between the semi-hollow rivet and the lower steel sheet, realizing the reliable joining of Al-steel dissimilar sheets combination. The rivet structure was designed to accommodate the trapped Al alloy in the rivet cavity during the friction welding stage, obtaining an inclusion-free friction welding interface which was not only mechanically mixed but also fully interatomic binding. The optimal FSR joint is composed of ferrite softening zone (FSZ), carbide precipitation zone (CPZ), ultrafine martensite zone (UMZ), and fine dual-phase zone (FDPZ). Moreover, due to the circumferential flow of materials during friction welding stage, the dynamic recrystallization (DRX) microstructures of the rivet and steel sheet both have shear texture consistent with {112} and {110} cube texture, which brings the uniformity of microstructure and performance. With the combined contribution of inclusion-free joining interface and the microstructure with uniform cube texture, the optimal FSR joints have the maximum average tensile-sheer strength of 281.1 MPa (90.7%·σ6061-T6) and peak load of 5.29 kN, presenting pull-out fracture mode with good fracture energy absorption.

Friction stir welding (FSW) is a new solid-state joining process for joining light metals, such as magnesium (Mg) alloys. However, the texture-induced softening mechanism (in stir zone, SZ) tends to cause the worsening of mechanical properties in FSW Mg alloys. In this work, multi-pass hot-rolling and subsequent annealing is used to ameliorate the FSW AZ31 alloy plate. The results show that multi-pass hot-rolling leads to the c-axes of most grains in the SZ approximately perpendicular to the normal direction (ND) transforming into these almost parallel to the ND, accompanied with multiple twin types. After subsequent annealing, twins, as nucleation sites, promote the formation of new grains with off-basal texture, resulting in the weak basal texture. Compared with the FSW plate and the initial plate, the rolled-annealed FSW plate exhibits a relatively uniform hardness distribution and a “strength-ductility” synergy, which are mainly attributed to an appropriate texture feature and a homogeneous grain structure. This work provides a new idea for the development of new high-performance FSW Mg alloys.

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.

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.

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.

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.

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