Evaluation of fatigue crack propagation behaviour in Ti-6Al-4V manufactured by selective laser melting

Total fatigue life performance of high strength titanium alloy Ti-6Al-4V manufactured by Additive Manufacturing (AM) based methods such as Selective Laser Melting (SLM) is of significant interest and has gained much attention recently. Researchers often compare the total fatigue life of materials manufactured from SLM with conventional manufacture, and often the SLM lives are reduced because cracks initiate from "defects" such as porosity or Lack of Fusion (LOF). But for repair and alternative manufacture of complex and critical aerospace structures for example, the crack growth/propagation phase of the fatigue process is also very important and it has not been studied as extensively. The aim of the work presented here was to evaluate and better understand crack propagation under constant amplitude loading in Ti-6Al-4V samples manufactured using SLM with a variety of layer thicknesses and build directions (vertical or horizontal). The "as-manufactured" condition was studied, with no post heat treatment. Cracks typically initiated at LOF features which had a negative impact on the total fatigue life. The focus here was on the crack growth phase. Modelling was performed with a conventional Linear Elastic Fracture Mechanics approach using literature data obtained from testing on Compact Tension (C(T)) specimens which were also in the as-manufactured condition with a variety of build directions. The modelling provided a very useful correlation of the data and provided a way of assessing the LOF features in terms of an equivalent initial crack. The crack growth properties of the SLM cases were also compared against literature data for conventionally manufactured material. The work will lead to a better understanding of fatigue crack growth characteristics for components manufactured by AM methods such as SLM. That understanding is an essential requirement for full certification and acceptance into service for critical applications such as aerospace structures.