Abstract
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.
Highlights
Due to strict energy-efficiency regulations aimed at reducing exhaust emissions, researchers are making an effort to reduce vehicle weights to enhance vehicle fuel efficiency
Considerable efforts have focused on high manganese twinning-induced plasticity (TWIP) steels for car body manufacturing, which are composed of a fully austenitic microstructure with a high amount of manganese and a significant percentage of carbon
The weld zone consists of a fully austenitic microstructure, which has different morphologies at different zones of the joint depending on the peak temperature of the relevant regions during the Resistance spot welding (RSW) process
Summary
Due to strict energy-efficiency regulations aimed at reducing exhaust emissions, researchers are making an effort to reduce vehicle weights to enhance vehicle fuel efficiency. Innovative high-strength steels are frequently used to both reduce the vehicle weight and to improve passenger safety. The predominant deformation mechanism of TWIP steels is twinning, which is determined by the stacking fault energy (SFE) value, depending on the Mn, Al, Si and C content as well as on temperature [1,2,3,4]. Medium SFE values (between 20 and 35 mJ/m2 ) provide mechanical twinning inside the grains [4,5,6]
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