Influence of heat treatment on the structure, phase composition, and properties of an orthorhombic titanium aluminum alloy obtained by selective laser melting

Abstract

The impact of heat treatment on the structural-phase characteristics, physical and mechanical properties of an alloy based on orthorhombic titanium aluminide (O-alloy) produced by selective laser melting was investigated using differential scanning calorimetry, scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction analysis, and microindentation. Modifying the selective laser melting process parameters, specifically increasing the relative energy density from 49 to 97 J/mm³, results in the evolution of the porosity character of the samples after selective laser fusion. This transition is observed from porosity associated with the lack-of-fusion to the gas porosity inherited from the pores in the powder and gas entrapment during melting. The metastable β‑solid solution that is fixed in the O‑alloy during selective laser melting undergoes decomposition in two temperature ranges, 300–600 °C and 650–800 °C, when heated at a rate of 50 °C per minute up to 900 °C. An increase in the heat treatment temperature from 750 to 900 °C of the as-built O‑alloy results in a reduction in the volume fraction and an expansion in the thickness of the O‑platelets precipitated during the decomposition of the β‑solid solution. This phenomenon leads to a decline in the impact of dispersion hardening (hardness growth) and enhancement in the elastic modulus. This study examines the impact of volumetric energy density during selective laser melting on the chemical composition of the O‑alloy, the stability of as-built alloy to decomposition of β‑solid solution and the formation of a combination of physical and mechanical properties during heat treatment.