Microstructure and Mechanical Behavior of Resistance Spot Welded Joints in Advanced High-Strength Q&P 980 Steel

Abstract

Abstract The third generation of advanced high-strength steel has garnered significant interest from both industries and researchers due to the remarkable combination of high strength and ductility. In this study, the relationships between welding parameters, microstructure evolutions, mechanical properties, and failure modes of resistance spot-welded (RSW) joints of Q&P 980 steel sheets were investigated. The microstructure of the alloy, composed of a ferritic matrix with embedded martensite and retained austenite, is analyzed using optical and field emission scanning electron microscopes. The welding process for Q&P steel sheets was examined under different welding currents and durations. Geometric measurements indicate that higher welding current and duration lead to larger weld nuggets, wider HAZ, and greater indentation depth. Tensile-shear tests were conducted to evaluate the mechanical integrity of the welded joints. These tests elucidate different failure modes, predominantly full interfacial failure, but also partial interface failure and peripheral failure in medium welding currents. The results emphasize the importance of nugget size in determining joint strength, with larger nuggets correlating with higher ultimate shear loads. The FESEM analysis of fracture surfaces highlighted the fracture modes, such as shear-dominated plastic deformation and ductile fracture in joints welded at high welding current. The pre-existing central defects and interfacial notches contribute to interfacial failure. Fracture surface analysis of welding samples confirms ductile fracture in peripheral and interfacial regions.