Effect of Annealing Heat Treatment on Microstructure and Mechanical Properties of Ti-6242 Alloy Fabricated by Laser Powder Bed Fusion Method

Abstract

Introduction and Objectives: Ti-6242 alloy is highly regarded in additive manufacturing due to its excellent thermal resistance. Titanium laser-based additively manufactured components, however, often require post-heat treatments because of their high solidification rate and formation of undesirable phases. The aim of this study is to investigate the effects of annealing heat treatment on the microstructure and mechanical properties of Ti-6242 specimens fabricated by laser powder bed fusion method. Materials and Methods: Annealing heat treatments were conducted at three temperatures of 950, 1005, and 1050 ℃ for holding times of 1, 3, 5, and 7 hours. The resulting microstructure, phase composition, and mechanical properties were characterized using optical and scanning electron microscopy, X-ray diffraction, and shear punch testing. Results: The as-built microstructure consisted of columnar primary β grains with colonies of metastable martensitic α′. Heat treatment led to decomposition of α′ and formation of more stable α and β phases. At 1005 and 1050 °C, the microstructure transformed from columnar to equiaxed, whereas at 950 °C, the columnar structure was retained. Williamson–Hall plots obtained from XRD patterns revealed a reduction in the lattice microstrain of heat treated samples compared with the as-built one. The applied heat treatment, improved both ultimate shear strength and ductility of the samples. The highest ultimate shear strength (761 MPa) and normalized displacement (0.638) were achieved at 1005 °C. Conclusion: Appropriate heat treatment leads to a more stable microstructure, reduction in lattice microstrain, and transformation of the columnar structure to an equiaxed one. This process could simultaneously enhance the ultimate shear strength and ductility compared to the as-built sample.