Effects of Cu addition and heat treatment on the microstructure and hardness of pure Ti prepared by Selective laser melting (SLM)

Formation of strong texture, columnar grains, and chemical inhomogeneity are some of the serious challenges associated with SLM of metallic alloys. This paper deals with in-situ SLM manufacturing of Ti-5Cu (wt.%) samples from pure Ti and Cu powders at a constant volumetric energy density (VED) of 50.26 J/mm3. The heat treatment of samples was carried out by heating at 1050 °C (above the β-transus temperature) for 3hr followed by furnace cooling. Then the effects of Cu addition to pure Ti and heat treatment on β-columnar to equiaxed transition (CET) morphology and size of the α phase as well as microhardness of pure Ti and Ti-5Cu samples were investigated. The results showed that the β columnar grains with an average equivalent diameter (deq) of 80 μm in SLMed pure Ti were effectively converted to equiaxed grains with an average deq of ~15 μm in the SLMed Ti-5Cu samples. The Cu addition increased the average microhardness from ~290 HV for pure Ti to ~415 HV for Ti-5Cu samples. This was attributed to the formation of equiaxed grains, increased lattice micro-strain resulting from Cu addition and decreased size of the lath-like α phase. The applied heat treatment led to the formation of equiaxed β grains with an average diameter of ~125 μm, dissolution of unmelted titanium particles and the non-dissolution zones of Cu and Ti, and a rather homogeneous structure in Ti-5Cu samples. It also resulted in the decomposition of α´ martensitic structure and the formation of different morphologies of Ti2Cu precipitates. Reduction of the average microhardness of the Ti-5Cu samples to ~312 HV after heat treatment was also related to the increase in deq of the equiaxed β grains, in addition to the increase in the size of the α laths phase and decrease in micro-strain.