A Comparative Examination of the Resistance Spot Welding Behavior of Two Advanced High Strength Steels 2006-01-1214

Advanced high-strength steels (AHSS) are a class of steels that have a minimum tensile strength of 500 MPa. The advantages of AHSS include superior formability and better crash energy absorption compared with conventional low-strength steels having a minimum tensile strength of 270 MPa. Several steels with a minimum tensile strength of 590 MPa have already found use in current vehicles, and others with minimum tensile strength up to 980 MPa have been qualified for use in future vehicle models. Two 780 MPa steels of interest are 780 DP (Dual Phase) and 780 TRIP (TRansformation Induced Plasticity). In this study, an examination was undertaken to compare the resistance spot-welding behavior of commercially produced 1.6 mm-thick, hot-dipped galvannealed, 780 MPa DP and TRIP steel sheet. Included in the study were evaluations of the weld lobes, weld microhardness, and the shear- and cross-tension strengths of resistance spot welds for the two steels. The results showed that both steels exhibited good weldability and that the overall spot weldability behavior of the DP and TRIP steels was very similar to one another. Weld strength and hardness are explained in terms of the weld and base-metal microstructures. A general description of the methods of production, physical metallurgy and structure of these steels is presented to explain the weld microstructural evolution.