Influence of Laser Types on Microstructure and Performance of Laser Multilayer Deposition of Ti-35V-15Cr Alloy

The laser is the main component of the laser additive manufacturing system. The different absorption rates of the material when applying different types of lasers will have a significant impact on the microstructure and properties of the laser additive manufactured alloy. In this research, we investigated the microstructure, flame-retardant properties, and microhardness of Ti-35V-15Cr alloy deposited by laser multilayer deposition (LMD) technology under the conditions of two different types of lasers, semiconductor and CO₂, and explored the mechanism of the influence of different types of lasers on the microstructure and properties of the alloy. The microstructure of Ti-35V-15Cr flame-retardant titanium alloy formed by the semiconductor laser with a wavelength of 1.06 μm and the CO₂ laser with a wavelength of 10.6 μm is significantly different. The average width of equiaxed grains is 100 μm, and the grain size and shape are basically the same, but in the transition area between the deposition layer and the substrate, there is an equiaxed-like transition microstructure with an average width of 200 µm, length of 526 µm, and aspect ratio of 2.6. The existence of this microstructure transformation is due to the variation of the content of the alloy elements Al, V, and Cr near the interface and the epitaxial growth characteristics of the grains. The formation of such subgrain structure is attributed to the local dislocation movement and alloy element diffusion caused by the large thermal accumulation and internal stress during laser forming, which not only inhibits the formation of segregation but also promotes the local dislocation movement and finally forms dislocation cells; whereas the microstructure of the alloy formed by the CO₂ laser is mainly columnar grains with an average size of 120-200 µm, and the top of the deposited specimen is a fine equiaxed grain layer with an average diameter of 52 µm. Compared with the former, the layer band morphology of the deposited specimens with CO₂ laser is more obvious and the top equiaxed layer is thinner, but unlike the former, the dendrite segregation inside the grains can be observed by scanning electron microscopy, and the distance between the dendrites is about 5-10 µm. The heat input and heat accumulation are therefore insufficient to cause significant local dislocation motion and diffusion of alloy elements, which leads to dendrite segregation. The effect of titanium alloy on the absorption rate of different types of lasers was studied by laser energy absorption rate equation and Jmat-pro software calculation, and it was found that the absorption rate of Ti-35V-15Cr titanium alloy to semiconductor laser energy is much higher than that of CO₂ laser energy, and the former is nearly three times of the latter, which proves that the laser absorption rate has a great influence on the formation of different microstructures. By means of a multivariate alloy solidification columnar/equiaxed transition model, it was found that the higher heat input and accumulation led to a lower temperature gradient at the solid-liquid interface front when the alloy was formed by a semiconductor laser, making it easier to form equiaxed grains. The microhardness of the alloys formed by the two lasers was tested, and the average hardness of the alloy formed by the semiconductor laser was 375HV and that of the alloy formed by the CO₂ laser was 363HV, the former being slightly higher than that of the latter. Then, the flame-retardant properties of the alloys formed by semiconductor and CO₂ laser were evaluated by laser ignition method, which was found that the flame retardancy of Ti-35V-15Cr flame retardant titanium alloy formed by a semiconductor laser was slightly better than that of the alloy ford by a CO₂ laser in both cases.