Nugget formation, microstructural features and strength of resistance spot welded cold-rolled dual-phase steel lap joints for automotive applications

Abstract

Abstract The main objective of this research paper is to investigate the nugget formation, microstructure and strength performance of DP 800 steel joints developed using resistance spot welding (RSW) process for automotive application. The influence of RSW parameters on nugget formation was studied using one variable at a time approach. The RSW joint showing optimum nugget area was further characterized for microstructural features and strength performance. The microstructural features of nugget were studied using optical and scanning electron microscopy (SEM). The lap tensile shear fractured specimens were analyzed using SEM for optimized condition. The microstructural features were correlated to the tensile shear strength and hardness of RSW joints. Results showed that welding current significantly influences the nugget formation than electrode force and welding time. The DP 800 RSW joints made using a welding current of 5.5 kA, an electrode pressure of 4.0 MPa and a welding time of 2.0 s disclosed defect free weld nugget of 7.28 mm diameter. It showed a higher tensile shear strength of 20 kN and microhardness of 564 HV thereby satisfying the requirement of automotive applications.