Short-Time Heat Treatment for Ti-6Al-4V Alloy Produced by Selective Laser Melting

In this study, we systematically investigated the effects of short-time heat treatment on the mechanical properties and fatigue strength of Ti-6Al-4V alloy produced by selective laser melting (hereafter SLM material). The short-time heat treatment was composed of the following two heat treatments: the first treatment was short-time solution treatment (ST treatment) in which the SLM material was heated at 1173-1298 K for 60 s and water-quenched; the second treatment was short-time aging treatment (AG treatment) in which the ST-treated materials were reheated at 823 K for 40s and air-cooled. Before the heat treatments, the microstructure of the SLM material was composed of the acicular alpha' martensite phase and the metastable beta phase. When the ST treatment was conducted at 1173 K and 1223 K lower than the beta transformation temperature of Ti-6Al-4V alloy (1271 K), the alpha' phase was transformed to the stable alpha phase during heating and the new fine alpha' phase was generated in the metastable beta phase by water quenching. These microstructural changes reduced the static strength but increased the ductility. When the ST treatment was carried out at 1298 K higher than the beta transformation temperature, the microstructure was almost the same as that of the SLM material and the mechanical properties were not greatly altered. AG treatment of the ST-treated materials induced the precipitation of the small alpha phase and reduced the volume fraction of the metastable beta phase. This treatment improved the static strength but reduced the ductility. The fatigue strength of the SLM material was much lower than that of the wrought material (63%) because the molding defects formed inside caused stress concentration and accelerated the initiation of fatigue cracks. However, the ST treatment at 1298 K and its combination with AG treatment effectively suppressed the initiation of fatigue cracks and markedly improved the fatigue strength to the same level as that of the wrought material. To improve the fatigue strength, the ST treatment at 1298 K was more effective than its combination with AG treatment because the volume fraction of the metastable beta phase was higher and the compressive residual stress near the surface was higher.