Role of changes in heat input on additively manufactured Ti–6Al–4V fabricated by directed energy deposition

Changes in linear heat input commonly used to manipulate deposit geometries during laser-based directed energy deposition of Ti–6Al–4V also impact microstructural and mechanical properties. With increases in linear heat input from 141 J/mm to 283 J/mm, α lath widths increased from 0.7 μm to 2.3 μm, but prior β grain sizes were mostly unchanged. At the same time, V and Fe preferentially segregated to the β phase and led to an increase in the β phase volume fraction. Along with these microstructural changes, increases in linear heat input also produced a decrease in yield strength from 1027 to 900 MPa. After hot isostatic pressing, the α lath widths increased in size and further lowered the strength by approximately 70 MPa at lower heat inputs but by only 13 MPa for the highest heat inputs. Since it appeared that factors other than α lath width were impacting the strength, an empirical strength model was used to quantify the contributions from intrinsic, Hall-Petch, Taylor, and solid solution strengthening mechanisms. Although the reduction in strength corresponded with an increase in the α lath widths, suggesting a Hall-Petch strengthening relationship, this strengthening contribution was relatively small compared to solid solution strengthening and Taylor hardening. By comparing the strengthening components, the decrease in strength was instead attributed to decreases in Taylor hardening caused by a drop in dislocation density.