Influence of heat treatment, microstructure evolution, and damage mechanism on high cycle fatigue behaviour of additively manufactured Ti-modified Al 2024 alloy

Abstract

The high cycle fatigue behaviour of titanium modified Al 2024 alloy produced by laser powder bed fusion (LPBF) is investigated to elucidate the effect of the microstructural features, residual stress, and defects (such as porosity and lack of fusion) on dynamic behaviour. The titanium acts as a grain refining agent; it can reduce the solidification cracking and produce the ultrafine-grained (UFG) microstructure in aluminium alloys. The 2% titanium addition resulted in the UFG microstructure with an average size of 0.52 μm in as-built samples. The microstructural features were examined through scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron back-scattered diffraction (EBSD) analysis. The influence of the microstructural features, such as grain size, dislocation density, precipitate-dislocation interaction, etc., on the fatigue life of as built and T6 heat treated samples was analysed in detail. The effect of varying stress amplitude at a stress ratio of 0.1 and a frequency of 30 Hz was investigated for the as-built and heat-treated sample. The T6 heat-treated sample showed 21.9% higher fatigue strength compared to the as-built samples (87.8 MPa in the heat-treated sample compared to 72 MPa in the as-built sample). The corresponding damage mechanisms are thoroughly explored through scanning electron fractography, TEM, and EBSD. The defects like porosity and lack of fusion defects in the surface or sub-surface level were observed to be critical regions as crack initiation sites.