Creep behaviour of additively manufactured Ti-6Al-4V alloy using small specimen testing techniques

The Ti-6Al-4V alloy is one of the most widely used alloys in additive manufacturing. In this study, it was processed using Powder Bed Fusion – Laser Beam with Metals (PBF-LB/M). Its creep behaviour was investigated using small specimen testing techniques, namely Small Punch Creep Tests (SPCT) and uniaxial creep tests on miniature specimens. The present work focuses on the study of the PBF-LB/M Ti-6Al-4V in the as-built state in a moderate temperature range (400-500 °C) corresponding to the upper limit of the alloy's potential operating temperature. The present study aims to complement existing literature data and addresses the possibility of microstructure evolution during mechanical testing. Additionally, the obtained SPCT results were compared with uniaxial creep test results. The test results also contributed to the improvement of a finite element model dedicated to simulating SPCT on as-built PBF-LB/M Ti-6Al-4V. This allowed the evaluation of various parameters, including the friction coefficient related to the punch/specimen contact. To better understand the operating creep mechanism, microstructural characterisation was performed on a cross-section on an interrupted SPCT test at 400 C – 450 MPa. A heterogeneous microstructure was identified and related to the local mechanical state obtained through finite element simulations. The accumulation of local strain (plastic + creep) was identified as the driving force for the formation of nanograins. These nanograins are formed by the fragmentation and rotation of α' martensite lamellae’s fractions, allowing deformation by grain boundary sliding. High strain levels can thus be achieved, and an advantageous thermomechanical domain for using the as-built PBF-LB/M martensitic microstructure is identified.