Joining dissimilar metal of Ti and CoCrMo using directed energy deposition

Abstract

Dissimilar metal joining has received great attention for combining the advantageous properties of two different material systems in a single part. However, such a process is challenging due to the formation of intermetallic compounds which leads to cracks. The present study focuses on laser cladding of pure titanium on a CoCrMo alloy using directed energy deposition with variation of process parameters (laser power: 150-300 W

powder feed rate: 0.5-3 g/min). Cladding layers showing dilution rates of more than 5% contained cracks due to the formation of brittle intermetallic phase Co$_2$Ti. The excess of laser energy caused chemical inhomogeneity and partially melted powders, while the deficiency laser energy resulted in lack of fusion. Neither crack nor partially melted powder was observed for a powder feed rate of 3 g/min and a laser power of 225-300 W, for which the dilution rate was minimized to less than 5%. For such samples the cladding layers comprised pure α-Ti and a uniform CoTi interface with Co$_2$Ti islands. Crack-free of joining Ti-CoCrMo can be explained by thermal and mechanical behaviors of the evolved phases during process.