Cracking and Microstructure of LMD-formed AlMo_0.5NbTa_0.5TiZr Refractory High-entropy Alloy Under Room-temperature and Inductive Heating Conditions

Refractory high-entropy alloys (RHEAs) are potential alternative materials for high-temperature structural components in the aerospace field. Among them, AlMo_0.5NbTa_0.5TiZr, as a low-density RHEA, is prone to significant internal stresses during the laser metal deposition (LMD) process due to rapid thermal cycling, which further leads to crack defects and becomes a key bottleneck restricting the improvement of forming quality. In this study, LMD processing experiments of AlMo_0.5NbTa_0.5TiZr RHEAs were conducted under room temperature and induction heating conditions at 200 °C, 300 °C, and 400 °C. TC4 titanium alloy was used as the substrate, and samples were fabricated using process parameters such as a laser power of 1000 W and a scanning speed of 600 mm/min. Meanwhile, a multi-scale temperature field–flow field simulation model was established using ANSYS software. Combined with Fourier’s law of heat conduction and the Navier–Stokes momentum equations, the heat and mass transfer as well as solidification behavior of the molten pool were revealed. Optical microscopy, backscattered scanning electron microscopy (BSE), and energy-dispersive X-ray spectroscopy (EDS) were used to characterize crack features, microstructure, and elemental distribution of the samples. The regulation mechanism of induction heating on the thermodynamic behavior, microstructure, and crack formation during the LMD process was systematically analyzed. The results show that under room temperature conditions, the molten pool of AlMo_0.5NbTa_0.5TiZr RHEAs exhibits relatively low temperature, and a severe temperature difference exists between the molten pool and the cold substrate, leading to large temperature fluctuations and a high vertical temperature gradient, which induces Marangoni convection dominated by vertical flow. At the same time, the flow field distribution is not sufficiently uniform, with a higher flow velocity at the surface but a significantly lower velocity at the bottom, resulting in elemental segregation and stress concentration. Consequently, continuous Al-Zr brittle phase bands and through-thickness cracks are formed. In addition, problems such as inhomogeneous microstructure, low interlayer bonding strength, and relatively high porosity are also observed. With the application of induction heating, the overall temperature field becomes smoother. The temperature gradient vector tends to shift toward the horizontal direction, and Marangoni convection is dominated by horizontal flow. The velocity field distribution becomes more uniform, and the flow is more stable. Elements are fully mixed and uniformly distributed. Induction heating significantly improves the LMD forming quality, eliminating cracks and greatly reducing elemental segregation. As the induction heating temperature increases, the fraction of the Al–Zr phase decreases while the dendritic regions increase. When induction-assisted LMD is performed at 400 °C, the internal microstructure of AlMo_0.5NbTa_0.5TiZr RHEAs becomes dense and uniform, achieving the optimal forming condition. This study provides a theoretical basis and a direction for process optimization for the high-quality LMD fabrication of RHEAs samples.