Effect of Spot Weld Parameter on HAZ of Advanced High Strength Steel Joint

To meet different target of light-weighting, lower fuel economy, crash safety and emission requirement, advanced high strength steel (AHSS) is commonly used in automotive vehicles and has become popular now a days. AHSS material up-to 1500 MPa is commonly used for structural components and major reinforcement of automotive BIW. Manufacturing of AHSS material requires precise control of chemical composition, and subsequent rolling and heat treatment to get optimum combination of required phases In most of the AHSS material microstructure, martensite is present along with ferrite or other phases. Hot stamp steel with strength level 1500 MPa strength also have martensite phase in microstructure after press hardening. However during heating and cooling cycle in resistance spot welding, martensite phase tempering affects hardness at Heat Affected Zone (HAZ). Softness at HAZ lower downs shear and tensile strength of joint which impacts fatigue properties of the joint and energy absorption during vehicle crash. Base material and spot weld joint’s fatigue strength and energy absorption at high strain rate are two important properties and performance criteria. These are generally considered while designing automotive parts and their joining. This paper discusses HAZ softening while spot welding of AHSS, impact of weld parameters (welding current, weld time, electrode force) on microstructure, micro-hardness and derivation of optimum spot weld parameter for spot welding with AHSS material.