Microstructure evolution in compositionally graded Ti(4–12 wt% Mo) prepared by laser directed energy deposition

Abstract

Compositionally graded Ti(4–12 wt% Mo) alloys were successfully prepared by laser directed energy deposition (L-DED) using two hoppers from Ti and Ti-15Mo master alloy powders. Detailed SEM, EDS and XRD analysis reveals the variation of the microstructure and consequently explains the evolution of the microhardness with the Mo concentration. High laser power is required to dissolve Ti-15Mo particles, to improve the homogeneity of the material at the scale of particle size, and to achieve a smooth linear gradient of the chemical composition. In the bottom part of the samples, the microstructure consists of elongated beta grains of the length of several mm containing big α-Ti laths. With increasing Mo concentration, the volume fraction of α phase decreases. Starting from the composition of about 9 wt% Mo the presence of ω phase was detected. Microhardness values span over a wide range of 270–550 HV and are affected by phase composition. The highest values of microhardness are achieved at around 10 wt% of Mo. However, phase composition and microhardness depend also on the utilized laser power and position in the sample determining the cooling rates. L-DED is capable of producing functionally graded materials (FGM) on the basis of metastable β-Ti alloys providing large variations of mechanical properties within a single sample/product.