Microstructural Evolution of a High-Strength Zr-Ti-Modified 2139 Aluminum Alloy for Laser Powder Bed Fusion

The demand for high-performance aluminum components drives research into the design of novel alloys that can be processed by laser-based additive manufacturing. In recent years, the addition of grain refiners proved to be an effective strategy to reduce the hot-cracking of high-strength Al alloys. In this study, the solidification and aging behavior of an Al2139 alloy doped with additions of Zr and Ti for L-PBF was investigated. These elements favored the formation of a fine-grained structure free of cracks. The formation of Al-3(Zr,Ti) inoculants was predicted by Scheil simulations and observed as cuboidal particles in the center of ff-Al grains. The microstructure of the as-built material featured fine and fully equiaxed grains, which appeared comparatively finer at the edge (300-600 nm) and coarser (0.8-2.0 mu m) at the center of the molten pools. In both cases, there was evidence of Cu and Mg micro-segregations at the grain boundaries. The microhardness of 109.7 HV0.5 in the as-built state was increased to 186.1 HV0.5 after optimized T4 heat treatment, responsible for the precipitation of many rod-shaped Zr- and Ti-based second phases and quasi-spherical Cu-, Mn-, and Fe-rich particles. Prolonged exposure carried out to simulate high-temperature service caused a drop in microhardness and marked modification of the microstructure, evidenced by the rearrangement and subsequent spheroidization of Cu- and Mg-rich particles at the grain boundaries.