Microstructure evolution and crack propagation mechanism during laser lap welding of Ti6Al4V and DP780 steel with CoCrNi powder

Joining steel and Ti alloys is important for aerospace applications. However, the brittle intermetallic compounds (IMCs) formed during welding of steel and Ti alloys are prone to cracks, which limit the preparation of high-strength steel/Ti joints. In this work, CoCrNi powder was used as an intermediate alloy to reduce the tendency of these brittle phases to produce cracks. The addition of this CoCrNi powder ensured that good quality joints were obtained and no cracks were observed. Three elements of CoCrNi are capable of infinite solid solution with Fe, so the alloy powder does not form a large amount of brittle IMCs with Fe by laser melting. A region of Fe-Ti IMCs formed at the steel/Ti interface due to the mixing of Fe and Ti. Moreover, the diffusion of CoCrNi from the melt pool into the Fe-Ti IMC interface layer reduced the interface layer brittleness, decreased the interface layer cracking tendency and improved joint performance. The plastic toughness of the interface layer of Fe-Ti compounds significantly improved after the addition of Co, Cr, and Ni alloying elements, which fully demonstrated the effectiveness of CoCrNi powder incorporation.