Microstructures and failure analyses of DP980 laser welded blanks in formability context

Abstract

In the present study, Erichsen cup height and limiting drawing ratio of fiber laser welded DP980 blanks were compared with respect to that of parent metal to understand the role of deformation mode in formability context. The strain localization was observed consistently at the outer heat affected zone (HAZ) during both the tests in laser welded blanks with failure propagated in radial direction and parallel to the weld. The microstructure development in the outer HAZ was analyzed by SEM, TEM and instrumented nano-indentation. The SEM images of the necked region revealed void initiation at the interface of ferrite and martensite due to heterogeneity in properties between the phases. The TEM bright field images with SAD analysis confirmed tempering of martensite and carbide precipitation accompanied with reduction in hardness in the outer HAZ, referred as soft zone (SZ). FE simulation results considering local weld properties revealed strain localization and subsequent failure under plane strain deformation mode due to the presence of the SZ in both Erichsen cup and deep drawing tests. It was found that the reduction in formability due to the presence of SZ was not severe in deep drawing process as compared to the stretch forming test. This was due to the difference in thickness distribution and thinning development in the blanks deformed under different stress and strain states inherited from the tool geometry.