Microstructure and dynamic tensile characteristics of dissimilar fiber laser welded advanced high strength steels

Abstract

The desire for production of anti-crash weight-optimized modern vehicles has resulted in the development of tailor welded blanks (TWBs) of 3rd-generation advanced high strength steels (3G-AHSSs). Therefore, understanding crash response (dynamic loading: strain rate of 1–100 s−1) of TWBs is essential for deploying new classes of AHSSs. The present work investigates dissimilar fiber laser joints between press-hardened 22MnB5 steel - medium manganese transformation induced plasticity (MMn-TRIP), and TRIP1100 steels. Lack of mixing within the fusion zone has been evidenced in the MMn-TRIP and TRIP1100 sides of the joints through retention of considerable austenite within the structure and the non-uniform hardness distribution, respectively. It was found that the inter-critical heat-affected zone of the MMn-TRIP side of the joints with a large volume fraction of austenite is responsible for the initiation of Lüders bands (discontinuous yield). The detailed digital image correlation technique demonstrated that the rapid decomposition of martensite in the heat-affected zone of the 22MnB5, and TRIP1100 sides of the joints localizes the plastic flow. Although the strength of the dissimilar joints showed positive strain rate sensitivity, however, under the higher applied strain rates elongation to fracture shows a transition from positive to negative strain rate sensitivity as a consequence of the suppression of the TRIP effect.