Microstructural analysis of weld nugget properties on resistance spot-welded advance high strength dual phase (α+α /) steel joints

Abstract

This research work focuses on production of sound weld by Resistance Spot Welding (RSW) process and correlate the mechanical properties of Dual Phase (DP) steel joints with its microstructural features. The main application of resistance spot welding was used to build white body assembly in automotive industry. The welded joint recorded maximum value of 21.3 kN in TSFL and 18.3 kN in CTSFL. The Scanning Electron Microscope (SEM) and Optical Microscope (OM) of the Nugget Zone (NZ) revealed lath/needle like microstructures with effect of severe cooling rate. The TEM microstructure exposed the formation of martensite (α /) in the Nugget Zone (NZ) and tempered martensite (Tα /) in Heat Affected Zone (HAZ), which accounts for superior joint strength of DP lap shear joints. Furthermore, crystallography studies reported the phase composition and crystallinity in weldments as cementite and magnesium carbide in [101] and [200] indices that accounts for high hardness of 584 Hv in the weldments.