Process-structure relationships in laser directed energy deposition of molybdenum powder within a Ti–6Al–4V matrix

Laser directed energy deposition (L-DED) of multi-materials is capable of additively fabricating parts with enhanced properties for biomedical and aerospace applications. This study focuses on the multi-material printing of Ti–6Al–4V (Ti64) and molybdenum (Mo) with L-DED. Alloying Mo with Ti64 improves the high-temperature mechanical properties of Ti64. However, the dynamic and melting behavior of the Mo powder inside the Ti64 matrix and their impact on the L-DED process and the microstructure are still unclear. This study utilizes in situ monitoring techniques, namely high-speed X-ray imaging and infrared imaging, with post-process material characterization techniques to relate the L-DED process of depositing Mo powder into a Ti64 matrix to the final microstructure. Results showed that the motion of Mo powder particles inside the melt pool was governed by the convective fluid flow. The convective fluid flow achieved an overall homogeneous macro-scale chemical composition by influencing the melting behavior of Mo powder particles and contributing to the liquid mixing. The increase in Mo content in Ti64 did not impact the melt pool temperature. Lastly, Mo segregated at the micro-scale near the top surface, and Mo-rich regions were located near unmmelted powder particles in the final build. This work can help improve multi-material applications and verify simulation models for L-DED. • Marangoni forces played a key role in inducing convective flow within the melt pool. • The convective flow impacted the motion and melting of molybdenum powder particles. • The convective flow achieved an overall homogeneous macro-scale chemical composition. • Alpha formation caused micro-segregation of titanium and molybdenum.