Tailoring Microstructure and Mechanical Properties of Additively-Manufactured Ti6Al4V Using Post Processing

Additively-manufactured Ti-6Al-4V (Ti64) exhibits high strength but in some cases inferior elongation to those of conventionally manufactured materials. Post-processing of additively manufactured Ti64 components is investigated to modify the mechanical properties for specific applications while still utilizing the benefits of the additive manufacturing process. The mechanical properties and fatigue resistance of Ti64 samples made by electron beam melting were tested in the as-built state. Several heat treatments (up to 1000 degrees C) were performed to study their effect on the microstructure and mechanical properties. Phase content during heating was tested with high reliability by neutron diffraction at Los Alamos National Laboratory. Two different hot isostatic pressings (HIP) cycles were tested, one at low temperature (780 degrees C), the other is at the standard temperature (920 degrees C). The results show that lowering the HIP holding temperature retains the fine microstructure (similar to 1% beta phase) and the 0.2% proof stress of the as-built samples (1038 MPa), but gives rise to higher elongation (similar to 14%) and better fatigue life. The material subjected to a higher HIP temperature had a coarser microstructure, more residual beta phase (similar to 2% difference), displayed slightly lower Vickers hardness (similar to 15 HV10N), 0.2% proof stress (similar to 60 MPa) and ultimate stresses (similar to 40 MPa) than the material HIP'ed at 780 degrees C, but had superior elongation (similar to 6%) and fatigue resistance. Heat treatment at 1000 degrees C entirely altered the microstructure (similar to 7% beta phase), yield elongation of 13.7% but decrease the 0.2% proof-stress to 927 MPa. The results of the HIP at 780 degrees C imply it would be beneficial to lower the standard ASTM HIP temperature for Ti6Al4V additively manufactured by electron beam melting.