Comparison of Microstructure, Fracture Mode and Formability of Ultra-High-Strength DP1180/DP590 Tailor Welded Blanks

This paper investigated phase transformation and fracture modes in formability test of dissimilar ultra high strength DP1180-DP590 tailor welded blanks. The welded joints with full penetration were obtained by increasing input power. The fusion zone was mainly consisted of lath martensite (LM), bainite (B) and retained austenite (gamma). With the increase of input power, microstructure evolution in fusion zone is: finer LM + B -> coarse LM + B + B-axial (B-axial represents bainite formed in axial zone). And with finner martensite and bainite dispersed, interfacial cohesion and plasticity was improved and phases were elongated and extensively deformed, which resulted in the transition of fracture modes in formability test. Furthermore, the formability of TWBs achieved superior cupping height, and increased by 20.4% compared with the specimen cracked in fusion zone.