Surface roughness of Ti-6Al-4V parts obtained by SLM and EBM: Effect on the High Cycle Fatigue life

Abstract

Selective Laser Melting (SLM) and Electron Beam Melting (EBM) are powder bed fusion processing which allows to build-up parts by successive addition of layers using 3D-CAD models. Among the advantages, are the high degree of freedom for part design and the small loss of material, which explain the increase of Ti-6Al-4V parts obtained by these processes. However, Ti-6Al-4V parts produced by SLM and EBM contain defects (surface roughness, porosity, tensile residual stresses) which decrease significantly the High Cycle Fatigue (HCF) life. In order to minimize the porosity and tensile residual stresses, post-processing treatments like Hot Isostatic Pressing (HIP) and Stress Relieving are often conducted. But the modification of the surface roughness by machining is very costly and not always possible, especially for parts with complex design. The aim of this work is to evaluate the effect of the surface roughness and microstructure of Ti-6Al-4V parts produced by SLM and EBM on the HCF life. Five sets of specimens were tested in tension-compression (R=-1; f=120Hz): Hot-Rolled (reference); SLM HIP machined; SLM HIP As-Built; EBM HIP machined; EBM HIP As-Built. For each condition, microstructure characterization, observation of the fracture surface of broken specimens and surface analysis were carried out respectively by Optical Microscope (OM), Scanning Electron Microscope (SEM) and 3D optical profilometer. Results of fatigue testing show a significant decrease of the HCF life mainly due to the surface roughness. Along with experimental testing, numerical simulations using FEM were conducted using the surface scans obtained by profilometry. Based on extreme values statistics of the crossland equivalent stress averaged on a critical distance, a methodology is proposed to take into account the effect of the surface roughness on the HCF life.