Effects of Powder Reuse on The Mechanical Properties of Electron Beam Additively Manufactured Ti-6Al-4V

Abstract

Metal Additive Manufacturing (AM) shows incredible promise for small batch, highly customizable part production. Powder bed fusion systems build parts layer by layer from metal powder that is manufactured according to stringent requirements, including chemistry, sphericity and particle size distribution. To maximize process economy, there is interest in reusing powder from inside the build chamber that is heat cycled but is not integrated into the fused metal. This practice can reduce process waste and lower material costs. However, the powder undergoes thermal cycling during printing and mechanical damage as a result of powder recovery. There has been limited investigation concerning the quality of metal that results from AM with reused powder. In order for recovered powder to be used for stress- or mission-critical engineering applications, the effect of powder reuse on the mechanical properties must be understood. In this investigation, the quality of Ti-6Al-4V metal developed in AM by electron beam melting was evaluated as a function of powder recovery and reuse in subsequent build cycles. The printed metal quality was characterized in terms of profilometry, mechanical properties that result from tensile testing, and fractographic evaluation of the fractured tensile specimens. Results showed that there are many aspects of the metal quality that are influenced by powder reuse in EBM of Ti-6Al-4V. Regarding the external surface quality of the built components, the average and ten-point surface roughness (Ra and Rz, respectively) decreased linearly with powder reuse. The mechanical properties also showed substantial dependence on reuse, but the tends were dependent on the orientation of the metal with regards to the build orientation. Regardless of build orientation, the elastic modulus, yield strength, ultimate tensile strength, and strength coefficient of the metal all increased linearly. There was a minor decrease in the strain hardening exponent. However the % elongation decreased significantly with reductions in the % elongation of 61.9% and 37.1%, in the horizontal and vertical build orientations, respectively. Fractography revealed a transition from ductile to brittle features with powder reuse, and also provided insight into the defects present that contributed to changes in the ductility. These included small spherical voids, lack of fusion voids, and cracks. Based on these findings, the preferred orientation for stress-critical AM parts of Ti-6Al-4V is in the vertical direction. The metal anisotropy represented by the differences in mechanical properties of the horizontally and vertically oriented parts is likely due to the texture in the microstructure. Complimentary work on the microstructure of the Ti-6Al-4V shows that the metal has columnar prior β grains oriented parallel to the build direction with α/β basketweave structure between them. The prior β grain boundaries could serve in pinning dislocations when loading is perpendicular to their orientation, thereby leading to lower strain to fracture of the horizontal specimens. However, the largest contribution to the changes in mechanical properties with powder reuse are related to increasing oxidation content of the powder during recovery. Interstitial oxides serve to strengthen and embrittle the material. The results achieved herein should contribute to the development of guidelines on powder reuse in EBM additive manufacturing of Ti-6Al-4V.