Optimization of processing parameters and establishment of a relationship between microstructure and mechanical properties of SLM titanium alloy

The processing parameters of a selective laser-melted (SLM) Ti6Al4V alloy were systematically optimized for optimal surface roughness and relative density. The relationship between microstructure and mechanical properties were established in the optimal parameter range. The results showed that the optimal laser power and scan speed was 200-250 W and 850-1150 mm/s, respectively. The laser power and scan speed played contradicting roles in the resultant roughness and porosity. An appropriate increase in laser power and decrease in scan speed could reduce the surface roughness and simultaneously improve the density and dimensional accuracy. When the laser power was 200 W, the microstructure varied from equiaxed grains to mixed equiaxed-columnar grains, and then it varied to columnar grains as the scan speed changed from 250 mm/s to 850-1750 mm/s. The acicular structure inside the grains transformed from an alpha (alpha') + beta phase to an alpha' (alpha) phase with gradual grain refinement. Some nano-beta phase precipitated near the acicular phase. Micro-hardness measurements revealed different values at the top, middle and bottom of the samples with increasing laser power and showed a reduced hardness gap with increasing scan speed. However, the elongation continuously increased with increasing scan speed and reached a maximum value of 7.8% at a scanning speed of 1150 mm/s. Decrease in elongation was observed when the laser power increased to 275 W. These variations in the hardness and mechanical properties were attributed to the combined effects of nano-beta phase and grain refinement of acicular alpha (alpha') martensites.