Effect of heat treatment on the ratcheting behaviour of additively manufactured and thermo-mechanically treated Ti–6Al–4V alloy

Abstract

The ratcheting behaviour of the additively manufactured and thermo-mechanically treated Ti–6Al–4V alloy has been investigated. Post heat treatment microstructures of comparable mechanical strength properties have been developed from both the material. Additively manufactured Ti–6Al–4V has a heterogeneous microstructure with defects and residual stresses as compared to the homogeneous microstructure of thermo-mechanically treated Ti–6Al–4V. It has been found that the fatigue life of the as-built additively manufactured sample is less than 50% of the thermo-mechanically treated sample and the fatigue life decreases further after heat treatment. The macroscopic cyclic stress-strain behaviour could not be predicted well using the combined hardening model in the Finite Element Method for the additively manufactured alloy. This has been attributed to the heterogeneous evolution of microscopic strain investigated using the Digital image correlation (DIC) technique. The detrimental effect of heterogeneity of the microstructure on the ratcheting behaviour of the heat-treated additively manufactured alloy has been investigated using the Electron backscatter diffraction (EBSD) technique. It reveals that the presence of multi-variant needle-shaped martensite is effective in increasing the crack propagation resistance and increasing the fatigue life of the as-built additively manufactured alloy as compared to its heat-treated counterparts in which both lamellar and acicular morphology of alpha is prevalent.