Ultrasonic fatigue testing in as-built and polished Ti6Al4V alloy manufactured by SLM

Abstract

Additive Manufacturing (AM) has carved a significant leap in mechanical design history, offering an almost infinite geometric freedom. It enables complex geometries with lower weight and enhanced behaviour, like topology optimisation. AM material mechanical properties is under continuous research due to the ever-increasing available materials, methods and an overall higher number of influential variables, such as printing parameters and powder shape and size (associated with metal AM). A rough and low quality-built surface is obtained from most AM methods. It has been proven to have a far from neglectable influence on the final mechanical properties and fatigue strength. Different post-treatment methods have shown a positive impact on correcting the rough surface impact on fatigue strength. In this work, the Ti6Al4V titanium alloy additively manufactured by Selective Laser Melting (SLM) is placed under Ultrasonic Fatigue Testing (UFT) within the Very High Cycle Fatigue (VHCF) regime. Ti6Al4V is widely applied in the aerospace industry due to its high strength, low-density relation. This work aims to evaluate the influence of surface roughness, defect size and location on fatigue strength. Several non-heat treated Ti6Al4V specimens were manufactured in one printing session. The present defect type, size and density, were quantified, and their tension, ductility and fatigue life properties evaluated. Tensile UFT were conducted with R =-1 to as-built and polished mirror-like specimens. The resulting fatigue fracture showed surface crack initiation for as-built specimens and sub-surface in polished specimens. A significant fatigue strength increase accompanied the crack initiation change. Sub-surface crack initiation occurred on the most common AM present defects, gas pores and smooth facets. Their location in respect to the surface appeared to influence fatigue resulting life directly.