Microstructural Evolution and Failure Behavior of Resistance Spot-Welded Dual Phase Steel

Abstract

In present scenario the automobile manufacturers are thriving to reduce the mass of passenger cars, by improving fuel efficacy and minimizing the CO2 emissions. Dual-phase steels are one of the most commonly used advanced high strength steel grades in the automotive industries. These steels have higher strength, good formability, and adequate weldability. The steel finds several applications in the fabrication of components such as chassis, suspensions, and body frame. Resistance spot welding (RSW) is one of the most widely used processes to join thin metal sheets. The current study aims to examine the microstructural and failure behaviour of spot-welded dual phase (DP) steel joints of 1.4 mm thickness. The findings show that the microstructure of fusion zone (FZ) of the joint mostly consisting of lath martensite, while the microstructure of heat affected zone (HAZ) manifests distinct characteristics viz. heat affected zone coarse grain (HAZ-CG) with coarser martensite, heat affected zone fine grain (HAZ-FG), and heat affected zone intercritical (HAZ-IC) with some ferrite and martensite packets at grain boundaries. The base metal (BM) predominately shows ferrite with martensite islands at grain boundaries. Under tensile-shear loading, crack initiated from the base metal. While the failure of coach-peel specimen takes place from HAZ-CG. The work concludes that the structural integrity of the joint is significantly controlled by microstructural variations, joint configuration, and location of the failure.