Influence of laser welding on the formability of AHSS steels

Limiting dome height (LDH) tests were used to evaluate the formability of both base metal and laser butt welded blanks of AHSS (including High strength low alloy (HSLA), Dual phase (DP) steels of different grades). Mechanical properties of the base metal and welded blanks were assessed by uniaxial tensile and biaxial LDH tests, and related to measured microhardness distributions across the welds. The formability ratio of laser welded dual phase sheet steels generally decreases with increased base metal strength. A significant decrease of LDH was observed in the higher strength DP steel welded specimens due to the formation of a softened zone in the Heat Affected Zone (HAZ). Softened zone characteristics were correlated to the LDH. Larger softened zones led to a larger reduction in the LDH. HAZ softening has been shown to be a function of the base metal martensite content and the weld heat input. Formability also decreased with increased weld heat input. Both in experiment and numerical simulations strain is localized in the softened HAZ in the uniaxial tensile testing, indicating that strain localization decreases tensile strength and elongation of laser welds in DP980. Copyright © 2007 MS&T07®.