A three-dimensional aluminum alloy part may be manufactured by a process in which a layer of aluminum alloy powder feed material is distributed over a substrate and scanned with a high-energy laser or electron beam in selective regions corresponding to a cross-section of the aluminum alloy part being formed. During the manufacturing process, the selective regions may melt and form a pool of molten aluminum alloy material. Thereafter, the pool of molten aluminum alloy material may cool and solidify into a solid layer of fused aluminum alloy material. During solidification of the pool of molten aluminum alloy material, solid phase particles may form within a solution of liquid phase aluminum prior to formation of solid phase aluminum dendrites. The resulting aluminum alloy part may exhibit a polycrystalline structure that predominantly includes a plurality of equiaxed grains, instead of columnar grains.