In order to meet the upcoming regulations on greenhouse gas emissions, aluminum use in the automotive industry is increasing especially in car body structure and closure applications. To address the shift from steel to aluminum, there needs to be a corresponding shift of joining technologies which consequently drives the development of an effective single sided joining process for aluminum alloys. In this paper, a variant of gas metal arc welding, i.e. cold metal transfer, CMT, which is unique in its approach to reduce the overall heat input, has been introduced to address that concern. Three welding arc modes: Standard, Pulsed and CMT are investigated to identify the advantages of CMT mode and the CMT mode was further investigated by four CMT spot welding modes: direct welding (DW) mode, plug welding (PW) mode, direct welding on a chill block (DWB) mode and plug welding on a chill block (PWB) mode. The resultant welds were investigated by macro and micro metallographic examination, microhardness testing and mechanical property testing. The results showed that the CMT arc mode could obtain the strongest welds understandably with the fewest welding defects. Under CMT arc mode, the direct welding with chill block mode (DWB) produced the largest weld faying interface. These welds also exhibited fewest partial tearing defects which led to the highest strength and ductility among the four welding modes for single sided spot welding of 1 mm + 1 mm AA6061-T6 sheet combination.

With the broader utilization of adhesive bonding in the automotive industry for structural lightweight applications, hybrid joining methods such as weld or rivet-bonding are being employed to complement the strength of adhesive-only joints. In this paper, a novel method to significantly improve the energy absorption of adhesive bonds by the addition of solder balls was developed and experimentally verified. Numerical analysis predicted a maximum increase of cross-tension strength and energy absorption by 25% and 80%, respectively. Our experimental study exhibited the same trend and achieved a maximum increase of strength and energy absorption of 17.5% and 40%, respectively. Microscopy indicated the presence of a thin adhesive layer between the solder balls and substrate after bonding which is believed to limit the full theoretical potential of the solder-adhesive bond strength. Effects of volume fraction of solder balls, and pre-tightening of the solder-adhesive joint prior to curing on the mechanical performance of solder-adhesive bonds were investigated. Additional work is ongoing to explore avenues to achieve the full potential of solder adhesives.

The hybrid use of aluminum alloy and advanced high strength steel (AHSS) has become an inevitable trend for fabricating a lightweight auto-body. Self-piercing riveting (SPR) as a preferred cold-forming fastening method is facing problem like weak interlocking when joining dissimilar combinations with considerably unequal thickness. In this study, a hybrid joining method, named rivet-welding (RW) was proposed to improve the robustness and strength of the SPR joint, by applying an electric current to it. For better evaluating the new process, the effects of heating time and electrode design on the microstructure, micro-hardness distribution, and mechanical performance of the RW joints were studied and compared systematically with the traditional SPR ones. The results showed that the electric current could improve the microstructure of the steel rivet and bottom DP590, and under long heating time, the inter-metallic compounds (IMC) could be formed at the interface of trapped AA6061-T6 and bottom DP590. Meanwhile, the electric current could increase the micro-hardness of the rivet and bottom DP590, and soften the AA6061-T6 around the rivet leg. In addition, the RW process using lower annular electrode A (LAE_A) could obtain 12.1% higher tensile-shear strength compared with the traditional SPR process.

A multi-physics hybrid numerical model, which couples electric, magnetic, thermal and flow fields, was used to investigate electromagnetic stirring behavior in a resistance spot weld nugget. The differences of two kinds of different excitation inputs, i.e., a sinusoidal current and its root-mean-square (RMS) value, were studied to examine if they could produce equivalent electromagnetic stirring effects in the weld nugget. Research showed that the two types of current inputs could produce almost identical fluid flow and heat transfer patterns and consistent evolution of flow and thermal fields in the nugget. At the end of the welding cycles, the maximum flow velocity and temperature between the two inputs differed by 11.6% and 0.3%, respectively. Therefore, the RMS current can be assumed to produce an approximately equivalent electromagnetic stirring effect with the sinusoidal current, and can be used in the future research to greatly improve the solution efficiency of the electromagnetic stirring behavior in the resistance spot weld nugget.

Magnetohydrodynamic behaviors in a resistance spot weld nugget under different welding currents are investigated based on a multiphysics coupled numerical model, which incorporates phase change and variable electrical contact resistances at faying surface and electrode-workpiece contact surface. The patterns of the flow field and thermal field at the end of the welding phase under different welding currents are obtained. The evolutions of fluid flow and heat transfer during the whole welding process are also revealed systematically. The analysis results are also compared with a traditional electrothermal coupled model to obtain the quantitative effects of the magnetohydrodynamic behaviors on the resistance spot weld nugget formation.