In this study the effect of the weld current on the microstructure and mechanical properties of a resistance spot-welded twinning-induced plasticity (TWIP) steel sheet was investigated using optical microscopy, scanning electron microscopy–electron back-scattered diffraction (SEM–EBSD), microhardness measurements, a tensile shear test and fractography. Higher weld currents promoted the formation of a macro expulsion cavity in the fusion zone. Additionally, higher weld currents led to a higher indentation depth, a wider heat-affected zone (HAZ), coarser grain structure and thicker annealing twins in the HAZ, and a relatively equiaxed dendritic structure in the centre of the fusion zone. The hardness values in the weld zone were lower than that of the base metal. The lowest hardness values were observed in the HAZ. No strong relationship was observed between the hardness values in the weld zone and the weld current. A higher joint strength, tensile deformation and failure energy absorption capacity were obtained with a weld current of 12 kA, a welding time of 300 ms and an electrode force of 3 kN. A complex fracture surface with both brittle and limited ductile manner was observed in the joints, while the base metal exhibited a ductile fracture. Joints with a higher tensile shear load (TSL) commonly exhibited more brittle fracture characteristics.

AA6082-T6 alloy was joined by friction stir spot welding using five different pin profiles, namely cylindrical, conical, triangular, hexagonal, and cylindrical with two grooves, at different dwell time. The joints welded by cylindrical pins had larger effective weld width. However, grooves on the cylindrical pin decreased the effective weld width. In the weld made with the hexagonal pin, the hook was bent downward from the interface of the sheets and had the smallest effective weld width. When the conical pin was used, the effective weld width increased with increasing dwell time. In the case of using tools with cylindrical and conical pins, HAZ hardness was relatively lower. With increasing dwell time, HAZ hardness of the joints made with the conical pin decreased. Effective weld width determined the weld strength under the tensile shear loading condition: larger effective weld width resulted in higher weld strength. Weld strength of the joints made with cylindrical pins was higher. The joints fabricated with hexagonal pins had the lowest weld strength. In general, higher dwell time led to higher weld strength. The welds with the higher strength experienced both brittle and ductile fractures, while the joints with the lower strength exhibited completely brittle fracture.

A study was conducted to investigate the microstructure and the mechanical properties of dissimilar resistance spot welds between steel sheets of DP600 and DP1000 using different welding currents for automotive applications. Microstructural characterization, microhardness tests and tensile shear tests of the joints were conducted. The dissimilar joints had three main microstructural zones: the base metals; the heat-affected zones of both steels; the fusion zone. Additionally, the DP1000 side of the joints also had a transition zone between the heat-affected zone and the base metal, which exhibited significant softening. Such a soft transition zone was not observed on the DP600 side. The peak hardness values were observed in the centre heat-affected zones of both sides of the joints. The welding current significantly influenced the microstructure and the mechanical properties of the joints. With increasing welding current, the nugget size increased almost linearly and the widths of the heat-affected zones on both sides became smaller. The use of higher welding currents reduced the microhardness values in the outer heat-affected zones of both sides and the fusion zone of the joints; the hardness values in these zones decreased almost linearly with increasing welding current. For a welding current of up to 10 kA, the tensile shear loads of the joints increased linearly with increasing welding current. The peak values of the tensile shear load and tensile deformation were obtained for the joints welded with a current of 10 kA. All joints exhibited the full button pull-out failure mode under the tensile shear loading test. The failure location of all joints was in the centre heat-affected zone. The joints welded with relatively lower welding currents (8–9.5 kA) failed on the DP1000 side, while the failure of the other joints fabricated with relatively higher welding currents (10–11 kA) was located on the DP600 side of the weld.