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

Abstract

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 °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 °C), the other is at the standard temperature (920 °C). The results show that lowering the HIP holding temperature retains the fine microstructure (~1% β phase) and the 0.2% proof stress of the as-built samples (1038 MPa), but gives rise to higher elongation (~14%) and better fatigue life. The material subjected to a higher HIP temperature had a coarser microstructure, more residual β phase (~2% difference), displayed slightly lower Vickers hardness (~15 HV10N), 0.2% proof stress (~60 MPa) and ultimate stresses (~40 MPa) than the material HIP’ed at 780 °C, but had superior elongation (~6%) and fatigue resistance. Heat treatment at 1000 °C entirely altered the microstructure (~7% β phase), yield elongation of 13.7% but decrease the 0.2% proof-stress to 927 MPa. The results of the HIP at 780 °C imply it would be beneficial to lower the standard ASTM HIP temperature for Ti6Al4V additively manufactured by electron beam melting.

Directionally-Dependent Mechanical Properties of Ti6Al4V Manufactured by Electron Beam Melting (EBM) and Selective Laser Melting (SLM)

“…It is noticeable that the particle size of SLM was smaller than those of EBM, i.e., 10-40 µm and 20-70 µm, respectively. The size of SLM particle was comparable to those used by other researchers, e.g., 5-50 µm [20] and 25-45 µm [28][29][30][31][32][33]. Other works with EBM indicated the particle size of around 82 µm [15], 77 µm [28], 60-70 µm [34,35].…”

“…Note that porosity is defined as the volume fraction of pores. It is expressed as p = 1 − ρ/ρ T , where ρ is the sample density and ρ T is the theoretical density of the substance [29,30]. The theoretical density of Ti-6Al-4V was 4432 kg.m −3 or 4.432 g/cm 3 as indicated above.…”

Martensite Formation and Decomposition during Traditional and AM Processing of Two-Phase Titanium Alloys—An Overview

“…High isostatic pressing (HIP) was also proposed as a post-EBM treatment [77]. According to the authors, HIP of the as-built parts, having α martensitic microstructure, caused coarsening of martensite decomposition products.…”

Influence of Powder Bed Temperature on the Microstructure and Mechanical Properties of Ti-6Al-4V Alloy Fabricated via Laser Powder Bed Fusion

“…The martensitic α′ phase is bad for ductility, whereas the ductility of the lath-shaped α phase and the intergranular β phase, decomposed from the martensitic α′ phase, is improved by heat treatment [ 13 , 14 ]. To improve the ductility of the Ti-6Al-4V products manufactured by LPBF, the post-LPBF heat treatment is often necessary by the martensitic α′ phase decomposing into the α + β phase [ 1 , 7 , 15 , 16 ]. However, the post-LPBF heat treatment method has the disadvantage of decreasing the production efficiency.…”