Selective laser melting of hybrid ex-situ/in-situ reinforced titanium matrix composites: Laser/powder interaction, reinforcement formation mechanism, and non-equilibrium microstructural evolutions

Highlights: • SLM of 5wt.% B₄C/Ti-6Al-4V composite powder was studied and compared with the monolithic unreinforced Ti-6Al-4V. • TiB and TiC reinforcements were formed through the dissolution of guest B₄C particles rather than their melting. • Non-equilibrium solidification sequence showed deviation from equilibrium conditions. • The fabricated composites showed 30–80% enhancement in microhardness depending on the laser energy density. -- Abstract: Hybrid ex-situ/in-situ reinforced titanium matrix composites (TMCs) were fabricated by selective laser melting (SLM). The optimized pre-processed 5 wt% B₄C/Ti-6Al-4V composite powder feedstock and the un-reinforced Ti-6Al-4V powder were consolidated using energy densities in the range of 50–75 J/mm³. Despite the full melting of the powder particles in the monolithic Ti-6Al-4V system, complete melting of the host Ti-6Al-4V constituent in the composite case took place by energy densities exceeding 62.5 J/mm³. Presence of the guest B₄C particles surrounding the un-melted/partially melted host particles gave evidence of the non-efficient guest-to-host heat transfer. In-situ formation of (TiB + TiC) reinforcements was discussed based on a mechanism proposing dissolution rather than melting of the guest particles. The degree of dissolution was a significant function of the energy density and the guest particle size. Microstructural evolutions during SLM of 5 wt% B₄C/Ti64 composite were studied, and the non-equilibrium solidification sequence was suggested based on the microstructural observations and the equilibrium solidification path. High cooling rates during SLM inhibited some of the liquid and solid-state transformations in the TMCs. This was confirmed by microstructural observations of the arc-melted parts fabricated by the same composite feedstock. The SLM processed TMCs showed 30–80% enhancement in microhardness compared to the unreinforced Ti64.