Mechanical Strength and Fracture of Resistance Spot Welded Advanced High Strength Steels

Abstract

In recent years, quenching and partitioning (Q&P) steel sheets have been proposed for the fabrication of car body components. However, the use of this steel grade isconditional on its attitude to be assembled with other automotive steel grades by welding techniques. In this work, a Q&Psteel sheet and a transformation induced plasticity (TRIP) steel sheet were welded together byresistance spot welding at varying process parameters. Particularly, the effects of the welding current, clamping force, and welding time on the mechanical strength of the Q&P/TRIP joints have been assessed. An L-9(33) orthogonal array was used to design the welding tests, performing three replications for each welding condition. Optical microscopy and SEM fractographic examinations were conducted to evaluate the microstructure of the steels and the weld nugget size, and to detect the possible occurrence of welding defects, in each welding condition. The welded samples mainly failed by button pull, with interfacial fracture occurring in very few cases. Moreover, metal expulsion, which occurred in the samples welded at the highest welding current, limits the achievable maximum extent of the weld nugget and the shear tension strength.