Fatigue Failure from Inner Surfaces of Additive Manufactured Ti-6Al-4V Components

Abstract

Selective laser melting (SLM) is an additive manufacturing process for producing metallic components with complex geometries. A drawback of this process is the process-inherent poor surface finish, which is highly detrimental in materials submitted to fatigue loading situations. The goal of this work is to analyze the fatigue behavior of Ti-6Al-4V specimens with internal axial channels under the following different conditions: hole drilled, hole as manufactured, and hole threaded M4 × 0.7. All the cases studied showed a lower fatigue performance as compared with solid samples due to the surface roughness and geometry effect that produced a surface stress concentration leading to a reduction in fatigue strength. The fractography revealed that crack initiation occurred from the internal surface in all specimens with internal channel mostly from defects as unfused particles and lack of fusion zones, while for the solid specimens crack initiation was observed from the external surface due to insufficient fusion defect. The application of the Smith-Watson-Topper energy-based parameter was revealed to be a good tool for fatigue life prediction of the different series studied.

Metallurgical Modelling of Ti-6Al-4V for Welding Applications

“…They noted that the induced phase change caused by the electropulse treatment yielded a 30% improvement in fatigue life. Jesus et al [26] considered heating cycles from an additive manufacture process for the Ti-6aAl-4V alloy, noting that residual stress within the build, caused primarily by the induced thermal fields, caused significant deterioration in the fatigue life of the components.…”

Cubic Lattice Structures of Ti6Al4V under Compressive Loading: Towards Assessing the Performance for Hard Tissue Implants Alternative

“…It can be observed that when a strut failed, all the neighbouring struts existing in the same plane also failed. The mechanism of failure is usually dependent upon the cell geometry [ 54 ]. For instance, in bending-dominated PLS components made of diamond and dodecahedron unit cells, failure occurs along shear bands at 45 degrees to the PLS as the struts are at an angle to the loading direction hence experience shear failure [ 55 ].…”

A critical review on the effects of process-induced porosity on the mechanical properties of alloys fabricated by laser powder bed fusion

“…Material Stress Ratio Effect of porosity on fatigue life of LPBF Ti64 [191] Ti64 R = 0.1 Without surface machining, internal pore closure using HIP alone does not improve fatigue life With HIP and machining, LPBF Ti64 can outperform an annealed one in high-cycle fatigue despite a coarser microstructure [111] Ti64 R = -1 Effect of pore closure can be compared between heat-treated and HIPed samples where both were conducted at 920 °C/2 h but the latter with a pressure of 100 MPa which reduced porosity from * 0.01% to * 0.001% and the average pore diameter from * 30 lm to * 15 lm The heat treatment improved fatigue life from a fatigue limit of \ 300 MPa to 350-400 MPa, but HIP improved it to 450-500 MPa [117] Ti64 R = 0.1 Surface machining is crucial for improving fatigue life, though sub-surface pores exposed from the machining process can lead to fatigue failure [118] Ti64 R = -1 Polishing alone does not significantly improve fatigue life as exposed sub-surface pores still lead to fatigue failure HIP improved fatigue life but still resulted in considerable scatter while subsequent polishing reduced scatter and achieved 70% fatigue strength of a wrought equivalent was achieved [192] Ti64 R = -1 Pore morphology is less critical than pore location relative to the surface [193][194][195][196] Ti64 R = 0, -1, 0.1…”