SiCp/Ti6Al4V functionally graded materials produced by laser melt injection

With a well-controlled laser melt injection (LMI) process, for the first time the feasibility is demonstrated to produce SiC particles (SiCp) reinforced Ti6Al4V functionally graded materials (FGMs). SiCp are injected just behind the laser beam into the extended part of the laser melt pool that is formed at relatively high beam scanning velocities. The process allows for the minimization of the decomposition reaction between SiCp and Ti6Al4V melt, and also leads to FGMs of SiCp/Ti6Al4V instead of a homogeneous composite layer on Ti6Al4V substrates. An injection model is designed based on the temperature/viscosity field of the laser pool for a deeper understanding of the mechanism of formation of the FGMs with LMI. The model is based on finite element calculations of the temperature field in the melt pool, physical considerations of the LMI process and it is supported by experimental observations. Three types of reaction layers are observed around SiCp, namely a thin monocrystalline TiC layer, a cellular polycrystalline TiC layer and a thick mixed layer of TiC with Ti5Si3. Among them, only the monocrystalline TiC layer exhibits particular orientation relationships (ORs) to the SiCp lattice, i.e. (111)TiC∥(0001)SiC and〈110〉TiC∥〈11̄00〉SiC or (111)TiC∥(101̄2)SiC and 〈11̄0〉TiC∥〈12̄10〉SiC. These two kinds of TiC reaction layers act as a barrier against the interfacial reaction and its swift formation during rapid cooling hinders the dissolution of SiCp in the Ti-melt.