With the rapid development of high-tech industry, the function and performance requirements of products are increasing on a daily basis. It has been difficult for a single metal to meet the requirements of industrial field for comprehensive performance of materials. Therefore, multi-material parts with multiple metal properties have significant development prospects. Titanium alloys are widely used in aerospace, biomedicine, automobile manufacturing, and other fields because of their high specific strength, good corrosion resistance, good biocompatibility, and high thermal strength. Aluminum alloy has the advantages of lightweight, high conductivity, and good workability. Both titanium and aluminum alloy are lightweight and high-strength materials that are focused on development in the aerospace field. If their properties are integrated, the preparation of titanium aluminum heterogeneous materials is expected to significantly improve the lightweight and comprehensive performance of components. Because of the special requirements for the manufacturing of curved surface matrix additives such as rocket body in the aerospace field, it is crucial to establish a three-dimensional model that conforms to the actual evolution process of the molten pool in the curved surface additive manufacturing process by the laser direct energy deposition and improve the surface quality of the formed components.

The laser directional energy deposition experiment with coaxial powder feeding adopts 4000 W semiconductor pumped Nd∶YAG laser. TC4 powder is used for deposition processing on 6061 aluminum alloy cylinder substrate. The substrate is sanded to remove the oxide layer before use. The integrated powder feeding laser head remains stationary above the cylinder body, which rotates around the axis with a constant angular velocity. The laser beam with constant power is used to melt the TC4 powder and the base material is sent out, thus forming a molten pool, which is protected by passing argon. The initial and steady deposition processes of Al-Ti heterogeneous materials by laser direct energy deposition are numerically simulated by using finite element simulation method. The effects of laser power and scanning speed on molten pool morphology, width-to-depth ratio, and temperature field are studied by controlling variables, and the simulation accuracy is verified by experiments.

When the laser heat source interacts with the substrate, the maximum and average temperatures of the molten pool rise rapidly within 100 ms. Subsequently, the maximum and average temperatures inside the molten pool continue to increase, but the growth rate slows down and reaches the maximum value at 300 ms (Figs. 4 and 5). Comparing the molten pool cross-section morphology of the simulated cladding layer with the actual molten pool cross-section morphology, it is observed that the morphology and temperature distribution of the molten pool obtained by the experiment are basically the same as those obtained by the finite element simulation , and the simulated spot diameter is consistent with the test spot diameter (Fig. 12). The higher the laser power is, the higher the volume energy density is. The width-to-depth ratio of the molten pool is inversely proportional to the volume energy density of the laser (Tables 4 and 5). The difference of the width-to-depth ratio between the initial deposited layer and the stably deposited layer is smaller than that between Al-Ti heterogeneous material layer and the initial deposited layer, which is due to the preheating effect of the initial Al-Ti heterogeneous material layer cladding on the stably deposited Ti layer cladding. Compared with the Al-Ti heterogeneous material layer, the cladding matrix of the stably deposited Ti layer is transformed from aluminum alloy to titanium alloy, with low thermal conductivity and high density. Additionally, when the stably deposited Ti layer is cladded, the depth of the titanium alloy matrix is higher than that of the initial deposited Ti layer. Therefore, the temperature difference between the initial deposited and the stably deposited Ti layers is lower than that between the Al-Ti heterogeneous material layer and the initial deposited layer.

The results show that the thermal behavior and morphology of the molten pool of Al-Ti heterogeneous material layer formed by laser directed energy deposition change significantly with the laser parameters. When the scanning speed is 0.32 rad/s, with the laser power increasing from 1400 W to 2300 W, the maximum temperature of the molten pool increases from 1525.5 ℃ to 3289.8 ℃, and the volume of the molten pool increases from 1.16 mm³ to 7.73 mm³. The width-to-depth ratio of the molten pool is negatively related to the laser energy density. The width-to-depth ratio of the molten pool in the Al-Ti heterogeneous material layer is the highest, 1.84, when the laser power is 2000 W and the scanning speed is 0.32 rad/s, followed by the width-to-depth ratio in the initial deposited Ti layer, 1.42, and the width-to-depth ratio in the stably deposited Ti layer is the smallest, 1.22. The width of the molten pool obtained in the experiment is 0.61 mm, and the molten pool morphology is in good agreement with that obtained by finite element simulation.