Effects of laser power on microstructure and mechanical properties of TiAl-based alloys fabricated via laser melting deposition

TiAl-based alloys are regarded as the most promising new-generation lightweight and high-temperature resistant structural materials. However, TiAl-based alloys exhibit high brittleness at room and high temperatures and are difficult to process and form. Laser melting deposition (LMD) is a typical additive manufacturing technology, representing the most novel and cutting-edge technology for TiAl-based alloy forming. In this work, TiAl-based alloys were fabricated by LMD. The effects of laser powers (1200 W, 1400 W, and 1600 W) on the phase compositions, microstructural features, grain boundary misorientation, recrystallization behavior, hardness and tensile properties of TiAl alloys were systematically investigated. The results indicate that the LMD-formed TiAl alloys are composed predominantly of the γ phase, with a small amount of the α 2 phase. With rising laser power, the γ phase content gradually decreases, whereas the α 2 phase content correspondingly increases. The macrostructure of the alloys shows a columnar crystal morphology, characterized by distinct lamellar bands. As laser power increases, the lamellar bands become more pronounced, and the growth of columnar crystals is enhanced. The microstructure of the alloys consists of γ/α 2 lamellar colonies and a minor amount of blocky γ phase at the margins of lamellar colonies. In addition, with the elevation of laser power, both lamellar colony dimensions and lamellar spacing in the formed parts progressively increase. The TiAl alloys exhibit recrystallized grains and annealing twins, suggesting that recrystallization occurred due to internal residual stress and in-situ thermal cycling. Consequently, TiAl alloys are predominantly composed of high-angle grain boundaries (HAGB). With increasing laser power, the microhardness and tensile strength of the TiAl alloys initially increase and subsequently decrease. The optimal mechanical properties are observed at a laser power of 1400 W, with a microhardness of 350.5 HV and a tensile strength of 476 MPa. At laser powers of 1200 W and 1400 W, the fracture mode of the alloys is characterized by a mixed fracture, including cleavage fracture, inter-lamellar fracture, and trans-lamellar fracture. In contrast, at 1600 W, the fracture mode is predominantly characterized by cleavage fracture. These findings innovatively establish the process-structure-property correlation of LMD-fabricated TiAl alloys by systematically investigating laser power effects, providing key insights for additive manufacturing-based TiAl alloy regulation. • The microstructure of the LMD-formed TiAl alloys exhibits a near-lamellar structure, primarily consisting of γ/α 2 lamellar colonies and a minor amount of blocky γ phase at the margins of lamellar colonies. In addition, with increasing laser power, the size of the lamellar colonies and the lamellar spacing in the formed parts both progressively increase. • The LMD-formed TiAl alloys exhibit recrystallized grains and annealing twins, which suggests that recrystallization occurred due to internal residual stress and in-situ thermal cycling. TiAl alloys are predominantly composed of HAGB. As the laser power increases, the in-situ thermal cycling temperatures increase, facilitating more complete recrystallization and an increase in HAGB content. • With increasing laser power, the microhardness and tensile strength of the TiAl alloys initially increase and subsequently decrease. At a laser power of 1400 W, the alloy demonstrates the optimal mechanical properties, with a microhardness of 350.5 HV and a tensile strength of 476 MPa.