Development of LTTW consumables for the enhancement of mechanical properties and residual stress management in thin steel welded joints

Abstract

For automotive industry, distortion introduced by welding of the thin sheet metals could be problematic in the assembly of auto-body. In addition, fatigue life of the lap-joint panels using advanced high strength steels (AHSS) is typically limited by the weldments. Low transformation temperature welding (LTTW) consumables are characterized by low martensite start temperature and large fraction of martensite forming in the weld. It can efficiently reduce the tensile residual stress because the volume expansion associated with the martensitic transformation compensates for the thermal contraction during cooling. In this work, a LTTW wire, EH200B, was designed for arc welding of DP980 AHSS thin plates. Its composition was tailored for electrode weldability optimization. Effects of the LTTW wire on weld microstructure, distortion, residual stresses distribution, and fatigue properties of the lap joint panels were studied in this research, and compared to the conventional ER70S-3 wire. In comparison to the weld microstructure of complete ferrite using conventional wire, the weldment using LTTW wire consists of 90 vol.% martensite. Distortion pattern of thin sheets with bead-on-plate (BOP) welds using LTTW wire was opposite from those using conventional wire. Residual stresses in the BOP welded sheets using the two types of wires were measured using neutron diffraction method. The mapping results along middle thickness of the plates showed that in longitudinal direction, which typically yields a higher stress level than other directions, LTTW wire welded sheet showed lower residual stresses in the weld and HAZ. Fatigue life of lap joint panels using LTTW wire was almost twice of that using conventional wire.