Effects of post heat treatment on microstructure and mechanical properties of Ti5553 parts made by laser powder bed fusion

Abstract

Rapid heating and solidification rates involved in additive manufacturing result in non-homogenous microstructural features, microsegregation and residual stresses, leading to deteriorated mechanical properties. Although the rather new Ti-5553 (Ti‐5Al‐5Mo‐5V‐3Cr) metastable β-Ti alloy is printable by laser powder-bed fusion (LPBF), it exhibits poor mechanical strength compared to conventionally forged components due to the lack of second phase strengthening in the as-printed state. Therefore, post-processing heat treatment is essential to enhance the mechanical properties of the printed parts. In this research work, LPBF-made Ti-5553 specimens were subjected to varying heat treatment cycles to modify their corresponding microstructure in terms of the morphology and distribution of α particles. Differential Scanning Calorimetry (DSC), X-Ray diffraction (XRD) patterns, and electron backscatter diffraction (EBSD) results suggest that solutionizing at the upper α + β region (800 °C) offers a good combination of finer grain size and satisfactory α dissolution. On aging in the range of 500 – 700 °C for 0.5 – 4 h, growth of α particles with different morphologies was observed. A combination of multiple α morphologies, including Widmanstätten, lenticular, cylindrical and lamellar was identified based on the heat treatment conditions. Consequently, a wide range of microhardness from 300 to 500 HV was observed due to phase transformations in the microstructure, which control the mechanical properties. The subsequent tensile tests validated the improved mechanical properties further as the tensile strength increased from 780 ± 10–1640 ± 6.29 MPa, compared to the as-printed specimens. Examination of the fracture surfaces revealed intergranular failure following aging at 500 °C due to segregation of α particles at grain boundaries. At 600 °C, the fracture was initially intergranular, which slowly transformed to a ductile fracture with time. At 700 °C, the fracture was mainly ductile irrespective of the aging duration.