Objective Large-scale integral titanium alloy structural parts have been used as an indicator to measure the technological advancement of defense equipment. Laser additive manufacturing technology, with its unique advantages, has gradually become one of the processing methods for large-scale integral titanium alloys. In laser additive manufacturing, formed parts are circularly heated by a focused and high-energy laser beam. A large temperature gradient will be established in the formed parts, resulting in complex stress and strain evolution during the forming process. After forming, enormous residual stress is generated inside the formed elements, causing the substrate to warp significantly. This problem has been one of the factors impeding the development of this technology. Many scholars have found that the method of subfield scanning can effectively control the internal stress during additive manufacturing of large-scale components. However, the layer configuration of the large frame structure is relatively complicated. It is necessary to consider the appropriate means for planning the scanning trajectory and jump sequence according to the structural characteristics of large-scale frame structures.
Methods To mitigate the deformation and cracking of formed parts in laser additive manufacturing due to uneven temperature distribution, a subfield scanning strategy based on characteristic regions was proposed. This study focused on two aspects: scanning starting point positions and jump strategy between characteristic regions. First, ANSYS was used to simulate two kinds of characteristic regions' deposition manufacturing processes, including the L-shaped and T-shaped regions. The influences of the ipsilateral side starting point scanning and the opposite side starting point scanning on the temperature field distribution of characteristic regions and the thermal cycle of substrates' nodes were analyzed. The best scanning starting points for the two characteristic regions were identified. After that, the frame structures' deposition manufacturing processes were simulated using three jump scanning strategies: continuous jump, interval jump, and maximum span jump. The temperature distribution nephograms of the frame structures under the three jump scanning strategies were obtained. Second, temperature field simulation results were loaded on stress analysis models to analyze the stress evolution process. Finally, the thermocouple's temperature variations of substrates' nodes during the deposition processes were monitored, and substrate deformation after deposition was measured. The experimental and simulation results were analyzed.
Results and Discussions The above research shows that changing the scanning start points will affect the L-shaped and T-shaped characteristic regions' temperature distribution. When using opposite side starting points to deposit characteristic regions, the influence range of temperature can be reduced; the cumulative heat effect can be reduced (Fig. 3), thereby decreasing thermal behavior's influence during the deposition process on the substrate's mechanical properties. In addition, when using opposite side starting points to deposit characteristic regions, the substrate's nodal thermal cycle curve rises slowly, and the node's peak temperature is also low (Fig. 4). When the frame structure's deposition is only completed, the maximum temperature of the molten pool with the maximum span jump scanning is lower than the interval jump scanning and continuous jump scanning (Fig. 7). During the deposition process, the three jump strategies' stresses are distributed in the characteristic region, where the scan starts, and the characteristic regions' junctions. The phenomenon of stress concentration becomes more visible as the deposition layer's height increases (Fig. 8). The deposited layer's stress distribution range does not change much after cooling; the substrate's high-stress area gathers to the constrained position, concentrated on both sides of the constrained end. The stress at maximum span jump scanning is lower than that at interval jump scanning and continuous jump scanning (Fig. 9).
Conclusions Following the above analysis, this article proposed a subfield scanning strategy based on characteristic regions for the large frame structure, classified according to its organizational characteristics. It is divided into two characteristic regions: T-shaped and L-shaped regions. The effects of scanning start point positions and character regions' jump scanning strategy on thermal behavior and stress evolution during the scanning process were investigated using theoretical analysis and numerical simulation. The simulation results were tested and verified through thermocouple temperature measurement and substrate warping deformation experiments. The results of the investigation agree with the simulation results. Studies have shown that the maximum span jump scanning strategy based on the characteristic regions can make the substrate's temperature distribution more uniform during the deposition manufacturing process, resulting in less stress and deformation of the formed parts. Discrete control of thermal stress is realized to reduce the macroscopic deformation and cracking of the formed parts. This study serves as a theoretical basis and method guidance for improving the forming quality of large-scale integral titanium alloy structural parts.
