Effects of Powder Feed Rate on Formation of Fully Equiaxed beta Grains in Titanium Alloys Fabricated by Directed Energy Deposition

A near beta titanium alloy, Ti5Al2Sn2Zr4Mo4Cr, was fabricated by directed energy deposition (DED) with different powder feed rates to investigate the formation of fully equiaxed beta grains. A two-dimensional numerical model was developed to investigate the thermal conditions of the molten pool. Experimental results showed that the formation of an epitaxial cellular structure at the bottom of the molten pool is almost unavoidable. An increase in the powder feed rate produces a moderate thermal condition and promotes the formation of equiaxed grains in a single cladding layer. However, it could not guarantee the formation of a fully equiaxed microstructure in a block sample. From a low to high powder feed rate, fully columnar, mixed equiaxed-columnar, and fully equiaxed microstructures were obtained. Grain morphology was also affected by the remolten process. Increasing the powder feed rate reduced the remolten depth and broke the continuity of the epitaxial cellular structure, leading to different grain morphologies.