In-situ laser additive manufacturing of Ti6Al4V matrix composites by gas–liquid reaction in dilute nitrogen gas atmospheres

Abstract

Processing titanium (Ti)-based materials in high-concentration nitrogen gas (N2) atmosphere is credited with reducing ductility/plasticity arising from the excessive formation of brittle agglomerated TiN. However processing these materials in dilute N2 may reduce the risk. In this study, a novel method is developed for laser additive manufacturing of in-situ synthesized TiN and AlN co-reinforced Ti6Al4V matrix composites (NTMCs) by the gas–liquid reaction in low-concentration N2 atmospheres. The manufacturing process of the NTMCs involves melting Ti6Al4V powders followed by laser-induced pyrolysis of N2 near the melt pool. The process is facilitated by the reaction between the decomposed nitrogen and molten Ti6Al4V, dissolution and precipitation of nitrides, and formation of the composites layer-by-layer. The formation of the nitride precipitates was verified by XRD, SEM, EDS, and HR-TEM. Such in-situ synthesized nanoscale reinforcements exhibited good dispersion and strong interfacial bonding with the matrix alloy in the composites. The microhardness, 0.2% compressive yield strength, and ultimate compressive strength of the NTMCs significantly increased with increasing N2 concentration in additive manufacturing; their maxima were 511 HV, 1721 MPa, and 2010 MPa, respectively, increased by 36.3%, 67.9%, and 16.8% from those of the Ti6Al4V alloy. The formation and strengthening mechanisms of the NTMCs were elucidated.