Effects of Defects, Surface Roughness and HIP on Fatigue Strength of Ti-6Al-4V manufactured by Additive Manufacturing

Abstract

The additive manufacturing (AM) is expected to be the promising manufacturing process for high strength or hard steels such as Ti-6Al-4V for the aerospace industry components having complex shapes. However, disadvantage or challenge of AM is presence of defects which are inevitably contained in the manufacturing process. This paper focuses on the effects of defects, surface roughness and Hot Isostatic Pressing (HIP) process on the fatigue strength of a Ti-6Al-4V manufactured by AM. The guide is presented for the fatigue design and development of high quality and high strength Ti-6Al-4V by AM processing based on the combination of the statistics of extremes on defects and the √area parameter model.Defects were mostly gas porosity and those made by lack of fusion. Many pores which were formed near surface were eliminated by HIP and eventually HIP improved fatigue strength drastically to the level of the ideal fatigue limit to be expected from the hardness. Surface roughness had strong detrimental influence on fatigue strength. The method for estimating the effective size √areaeffmax of irregularly shaped defects and interacting adjacent defects was proposed from the viewpoint of fracture mechanics.Although the statistics of extremes analysis is useful for the quality control of AM, the particular surface effect and interaction effect of adjacent defects must be carefully considered. The effective defect size for adjacent defects is much larger than a single defect. Since the orientations of defects in AM materials are random, a defect in contact with specimen surface has a higher influence (termed as the effective defect size √areaeff) than the real size of the defect from the viewpoint of fracture mechanics. Considering the volume and number of productions of components, the lower bound of the fatigue limit σwl based on √areaeffmax can be determined by the √area parameter model.