The initial gap (IG) is frequently occurring in the process of resistance spot welding (RSW) for automotive body-in-white structures. It is an inevitable challenge that the RSW with IG can negatively impact the welding quality, subsequently reducing the structural integrity and safety of the vehicle. This research aims to study the influence of the IG on RSW mechanical behaviors based on the refined finite element model (FEM) of RSW with different IGs under tensile shear load. The influence of six types of IGs on the peak load and fracture modes of RSW of plates with similar thicknesses of 1.0 mm and 1.5 mm is investigated through FEM and experiments. To quantify the influence of the IG on the RSW’s deformational behavior, a prediction model is introduced to predict the peak load of RSW with different IGs. The prediction model is locally optimized through the Limited memory Broyden Fletcher Goldfarb Shanno (L-BFGS-B) optimization algorithm. Based on the prediction model, the relationship among the peak load values of the tensile shear specimens, the IG, and the mechanical behavior of the RSW is revealed. The results show that the IG has an obvious influence on the peak load values of RSW under tensile shear load, and the fracture modes are both the pull-out fracture (PF) mode. The peak load values of the RSW’s tensile shear specimens are decreased with the increment in the IG. Finally, the prediction model can accurately predict the peak load for various IGs, with errors of no more than 3%.

High-strength steel materials play a crucial role in improving automotive crash safety and reducing vehicle weight. However, the high hardness and strength characteristics of high-strength steel make it challenging to be joined by the conventional self-piercing riveting (C-SPR) process. In this study, the C-SPR and pre-holed self-piercing riveting (PH-SPR) processes are employed to join high-strength steel DP590 and aluminum alloy AlSi10MnMg-T7 sheets. The forming quality, microstructure, mechanical properties, and failure behavior of the PH-SPR joint are investigated and compared with the C-SPR joint. The effects of rivet strength, rivet length, sheet thickness, and sheet material on the forming quality of the two types of joints are compared systematically and numerically. The results show that the forming quality of the PH-SPR is sensitive to rivet parameters while not to the material of the top sheet. The rivet with lower strength and larger length can improve the forming quality of PH-SPR joints. the PH-SPR process reduces the riveting force and energy consumption by 12.1% and 46.3%, respectively, compared with the C-SPR process. The presence of the top steel sheet in C-SPR joint brings an increasing hardness of both the rivet leg and aluminum sheet during the riveting process. The mechanical responses of the PH-SPR joint are superior to those of the C-SPR joint. Specifically, the peak force of the C-SPR joint is 4.8% lower than that of the PH-SPR joint, while the energy absorption of the PH-SPR joint is 3.18 times higher than that of the C-SPR joint.

Energy shortage is a significant challenge faced by humanity, and energy conservation and carbon reduction are a common choice for global sustainable development. Among them, improving the lightweight level of carrier tools is a key way to promote global energy conservation and carbon reduction. Low-ductility light alloys have been gradually applied in the lightweight design of carrier tools due to their characteristics of high strength and low density. However, due to the large differences in physical properties such as melting points between low-ductility light alloys and high-strength steels, it is difficult to achieve effective connection of multi-material vehicle bodies, which limits the further promotion and application of low-ductility light alloy materials. As a cold joining technology, the riveting process has become an important means to support the mass application of low-ductility light alloy materials. In traditional riveting processes, solid rivets improve the uniformity of deformation by optimizing geometric and process parameters; blind rivets enhance the practicality of single-sided operation by regulating mandrel tension and deformation rate to suppress brittle fracture. New processes are constantly innovated: for example, self-piercing riveting without pre-opening reduces damage to the base material; pre-holed self-piercing riveting improves the bearing capacity of multi-layer dissimilar materials; adhesive-riveted hybrid joining has the advantages of strong bearing capacity and reliable connection; friction self-piercing riveting realizes the dual strengthening of “mechanical interlocking - solid-state joining” by softening materials through frictional heat; electromagnetic riveting improves the uniform deformation of materials through high strain rate dynamic loading. However, due to the low elongation and high sensitivity of low-ductility light alloy materials, the joints are prone to process-induced damage such as macroscopic cracks, which affect the forming quality and mechanical properties of the joints. Thus, it is necessary to deepen mechanism research, promote process optimization, expand the path of performance improvement in various environments, and promote the high-quality development of lightweight carrier tools.

To further improve the joint strength and identify the potential failure behavior of self-piercing riveted (SPR) steel/aluminium hybrid joints under shear and cross-tension loading conditions, a simplified 2D-to-3D simulation process chain is proposed. The finite element models (FEM) for the failure process of SPR joints are established in LS-DYNA and verified against experimental results. The effects of rivet strength, sheet material and thickness, and riveting direction on the joint strength and failure behavior of SPR joints are investigated. The results show that when the rivet strength is less than 1.3 GPa, increasing the rivet strength can improve the joint strength of AS joints under shear load and SA joints under cross-tension load. The joint strength of two types of SPR joints increases gradually and then decreases with an increase in thickness ratio R _t . When the steel/aluminium sheet thickness ratio R _sa exceeds 1.47, the aluminium-to-steel direction should be chosen. Increasing material strength can improve the joint strength of two types of SPR joints, while their sensitivities to the material strength vary under different loading conditions. The sheet material, sheet thickness, and riveting direction can change the failure behavior of the joints, while the rivet strength does not alter the failure behavior. This work contributes to understanding the mechanism of joint strength improvement and enhances riveting quality in industrial practice.