Investigation on microstructure characteristics and mechanical properties of twin wire-directed energy deposition-arc fabricated TiAl alloy regulated by the line energy

Abstract

The microstructure of TiAl alloy is extremely sensitive to the thermal history. During additive manufacturing, multiple thermal cycles and partial remelting of the TiAl deposition layers take place. Therefore, the heat input would exert significant influences on microstructural and mechanical characteristics of additively manufactured TiAl alloys. In this study, TiAl alloys were prepared successfully by twin-wire directed energy deposition-arc (TW-DED-arc). Systematic studies were conducted to regulate the microstructure characteristic and the mechanical property of the as-deposited TiAl alloys via the line energy. The results indicate strong effects of the line energy on the microstructure and performance of the TiAl alloys, which show alternative distribution of layer-band like microstructure with the dendritic grains and the fully lamellar colonies along the deposition direction. With the enhancement in the line energy, the layer-band like feature gradually weakens, and the fully lamellar colonies increase. In addition, the discontinuous coarsen phenomenon and the discontinuous dynamic recrystallization occurred, and the anneal twin was also formed. On the other hand, tensile fractures were observed in the dendritic grains region, and the maximum tensile strength was achieved with the medium line energy. However, the elongation and microhardness are proportional to the line energy. With the elevation in the line energy, the elongation and microhardness gradually increases and decreases, respectively. The microstructure formation mechanism of as-fabricated TiAl alloy is analyzed systematically, and the fracture mechanics was also discussed in detail. The findings of this research contribute not only to the methodology for fabrication of TiAl alloys with the novel TW-DED-arc process, but also to the deep understanding on the regulation of the microstructural and mechanical characteristics of the additively manufactured TiAl alloys.