Effective post processing of SLM fabricated Ti-6Al-4 V alloy: Machining vs thermal treatment

Abstract

Anisotropy in the mechanical properties with respect to the build direction of Selective Laser Melting (SLM) fabricated titanium parts is a known, challenging problem, which can be mitigated using a post processing method such as heat treatment. This paper investigates the potential sources of anisotropy associated with SLM fabricated Ti64 parts and studies the influence of heat treatment on the anisotropy in mechanical properties' behaviour in two directions: vertical and horizontal. SLM fabricated samples were machined and stress-relieved at 670 °C. In addition, annealing at 850 °C was performed on the machined samples to investigate the effect of heat treatment at a higher temperature on the anisotropy behaviour of the machined parts. The tensile test results showed that the difference in elongation at fracture between the vertical and horizontal samples (elongation anisotropy) will reduce from 125% in non-heat treated samples to 36% when the samples are annealed. Microstructural analyses revealed an anomaly, which is named the fish scale feature in this work. This newly-found type of defect was significantly identifiable in non-heat treated samples; however, after annealing it completely disappeared from the microstructure. It is believed that the fish scale defect plays a role in the failure of vertical samples. The same stress-relieving and annealing processes were then investigated on as-built vertical samples. The outcome revealed that the annealing process provides a greater improvement than machining. The results demonstrate that each post process, either machining or annealing, can eliminate the premature failure problem of as-built vertical samples, which occurs at a fracture strain of 0.5% under 482 MPa stress. Machining improves the fracture strength of the non-machined vertical samples to 1361 MPa at a strain of 2.8%; whereas annealing can enhance the fracture stress (premature) from 482 MPa to 1069 MPa, with a significant increase in fracture strain from 0.5% to 4%. The observations were found to be correlated to the phase change in the microstructure, mitigating the influence of defects such as the fish scale feature and full removal of the residual stress during annealing.