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

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.