Effects of build direction on thermal exposure and creep performance of SLM Ti6Al4V titanium alloy

With the development of typical additive manufacturing SLM, SLM Ti6Al4V has the potential to be used in high temperature vehicle structure. Mechanical properties after thermal exposure and creep performance of SLM Ti6Al4V and its build direction effects need to be studied in detail. In this paper, tensile and creep samples with 0°, 45° and 90° three build directions were designed and manufactured by SLM. Tensile tests were performed after 500 ℃ and 600 ℃ thermal exposure; And creep tests were carried out at 450 ℃ for all samples. The tensile properties after thermal exposure and the creep behaviors were studied and compared. The same size forged Ti6Al4V samples were manufactured and tested. It was shown that the yield strength (YS) and the ultimate tensile strength (UTS) of forged samples increased 24.23% and 16.70% after 500 ℃ exposure; while YS and UTS decreased after 600 ℃ exposure. The build direction had a significantly effect on the tensile ductility of SLM Ti6Al4V after thermal exposure. After 500 ℃ exposure, the tensile property of 90° sample was the best. Although the strength of 90° sample was slightly lower than that of 0° sample and 45° sample, the elongation was about 36.7% higher than that of 0° sample and 45° sample. After 600 ℃ exposure, 90° sample had both the highest strength and the elongation. Compared with forged samples, the SLM samples had longer creep ruptured time but lower ductility. For the SLM samples, 45° samples had best creep properties, while the properties of 0° samples were the worst. Fracture surfaces, failure modes and effects of build directions were investigated. Based on the features of SLM microstructures, two improved constitutive models were proposed and can be used to predict the anisotropic behavior of SLM material.