Influence of PWHT on Microstructure and Mechanical Properties of Similar TRIP-TRIP and Dissimilar DP-TRIP Laser Welds for Automobile Applications

Abstract

The need for lower fuel consumption along with improved safety of passengers has encouraged the auto industry to use Advanced High Strength Steels (AHSS). Fabrication of such applications involves welding, which leads to the alteration of microstructure and mechanical properties. The aim of the present work is to study and understand the effect of laser welding on similar Transformation Induced Plasticity (TRIP-TRIP) and dissimilar Dual Phase (DP)-Transformation Induced Plasticity (TRIP) welds. This weld fabrication using low heat input techniques results in fully martensitic structure in the weld and Heat Affected Zone (HAZ) rendering brittleness of the weldment. The effect of isothermal post-weld heat treatment (PWHT) on the properties of weld joints was also studied. The optical microscopy has revealed the martensite in the Fusion Zone (FZ), while the HAZ consisted of a mixture of ferrite and martensite. The variation in microstructure in the transverse direction to the weld has led to variation in microhardness. The results of the tensile test indicated the weld to be stronger and the failure location was far from the weld centre. The Ultimate Tensile Strength (UTS) and ductility of the tensile samples were less compared to the base metals. Post weld heat treatment resulted in a small decrease in fusion zone hardness. Because of the lower ductility of the weldments, cracks were observed in the fusion zone during the Erichsen cup test.