Use of miniature tensile specimens for measuring mechanical properties in the steady-state and transient zones of Ti–6Al–4V wire-fed electron beam deposits

Abstract

Recent developments in additive manufacturing (AM) have shown the importance of small-scale mechanical testing due to material availability issues, the microstructural variance within the complex build geometries, or thin-type features such as lattices. The current approach in AM is to test standard-sized coupons and postulate that a component with complex geometry has similar mechanical properties even though their thermal histories are completely different in many cases. In this research, small-scale tensile testing methodologies of AM parts made by electron beam wire-fed (EB-WF) deposited Ti–6Al–4V were investigated. Specifically, specimens extracted from well-established Ti–6Al–4V microstructural zones of transient and steady-state regions were compared to understand the effect of local microstructure on mechanical properties, which were evaluated through microhardness and miniature tensile testing. The average microhardness in the transient and steady-state zones are 321 Hv and 306 Hv, respectively. Although distinct microstructural features were observed in these zones, their tensile properties were similar. Both deposited zones had tensile yield and ultimate strength values ranging between 825-832 MPa and 880–907 MPa, respectively. The measured properties of the miniature specimens were comparable with E8 standard-sized specimens, higher than the ASTM F1108 cast and lower than the AMS 4911P wrought Ti–6Al–4V standards. Deformation and fractography analysis were used to correlate microstructure and property relationships for different α lath morphologies. Basket-weave morphology in the transient zone specimen showed profound grain splitting by basal slip which led to microstructural refinement and hindered softening during plastic flow.