Fatigue Performance of Additively Manufactured Ti-6Al-4V: Surface Condition vs. Internal Defects

Abstract

Grade 23 Ti-6Al-4V additively manufactured by selective electron beam melting (SEBM) has found important clinical applications as bone implants since 2007. In general, an as-built rough surface is desirable for bone ingrowth, but at the expense of fatigue performance. This study assesses the relative influence of the surface condition and internal defects on the fatigue performance of SEBM Ti-6Al-4V. Chemical etching, standard machining, and precision machining are used to improve the as-built surface condition, while a significant two-step hot isostatic pressing (HIP) treatment is employed to heal internal defects. Detailed assessment of the fatigue performance of these samples with different surface and internal conditions leads to a range of informative observations. The fatigue results are superimposed on a well-established fatigue diagram for Ti-6Al-4V and further presented in a fatigue-processing condition diagram. It is shown that HIP is necessary only when the surface finish is sufficient and when there are no surface defects. Improving the surface condition is far more important than applying post-SEBM HIP. For example, as-built samples with machined surfaces of Ra = 0.05 µm and Rz = 0.45 µm without HIP exhibited much better fatigue performance than as-built samples with machined surfaces of Ra = 0.13 µm and Rz = 0.95 µm plus HIP. This study provides a quantitative basis for the design and application of SEBM Ti-6Al-4V as bone implants in terms of fatigue performance, as well as for other applications.