Objective Manufacturing requirements for semi-enclosed or fully-enclosed hollow parts are common in the development of advanced aircraft manufacturing, nuclear power, chemical industry production, and national defense equipment. Scholars have also attempted to develop closed hollow parts using additive manufacturing in recent years, but the process technology is still in its early stages. For example, when using selective laser melting (SLM) and electron beam selective melting (EBSM) technology to form hollow spheres, internal support is usually required to aid in the forming process. The support structure and metal powder that remain in the hollow sphere after forming are difficult to remove, limiting the application and promotion of powder-bed metal additive manufacturing hollow spheres. The use of powder-feeding metal additive manufacturing [such as laser metal deposition (LMD) technology] can effectively avoid this problem; however, there are issues such as limit inclination angle limitation, step effect, forming collapse, and difficulty in closing the sphere at the same time. Several scholars have studied these problems in LMD forming technology. Wang Xuyue et al. used the variable z-axis lift method to form nonclosed 136-layer semicircular inclined thin-walled parts. Paul et al. conducted a study on the deposition inclination angle and the closure of structural parts. Shi Jianjun et al. optimized the process of upside deposition. Wang Cong et al. realized the formation of a closed cavity rotary thin-walled part. However, due to the limitation of the forming process, effective additive manufacturing forming of hollow spheres has not been realized yet. Therefore, in this study, we conduct hollow sphere LMD forming experiments to improve LMD technology’s forming ability for complex structural parts and closed parts, as well as to provide support for the expansion of its application fields.
Methods A continuous multiposture LMD forming method is proposed. The original hollow sphere 3D model was segmented using the annular beam LMD experimental platform, and the continuous multiposture deposition path planning method was used for normal layering (Fig. 2). The force mathematical model analyses of the deposition process are conducted. And based on modeling analysis, the parameters of the forming experiment are optimized, multiple experiments are performed to achieve the best forming effect, and the hollow sphere formed by LMD technology are obtained. After deposition, the sample is cut into two hemispheres along the sphere’s centerline using wire electrical discharge machining and the size of the formed part is measured. Then, eight points are selected along the deposition angle α for cutting and sampling, and the sample is ground, polished, and corroded to observe its microstructure under an optical microscope. In addition, the Vickers microhardness tester is used to determine the microhardness of each point, a laser scanning confocal microscope (Keyence VK-X1000) is used to characterize the typical surface morphology, and the Mitutoyo SJ-210 portable roughness measuring instrument is used to determine the surface roughness (Ra) of the hollow sphere.
Results and Discussions The deposition head’s posture transition was stable during the hollow sphere deposition process, as were the molten pools; there was little splashing, and there was no collapse during the deposition and forming process, and the sphere’s surface was uniform and flat. Forming process and the appearance of the parts met the expected requirements. The size difference between the original model and formed parts was slight (Fig. 8), indicating that with the increase of the deposition angle α, the grains were refined first and then coarsened; the microhardness of the equiaxed crystal structure shows a trend of first increasing and then decreasing (Fig. 11). The Ra value fluctuates at 3 μm from α=20° to α=100°. When α reached 100° and above, the roughness value gradually increased, finally achieving a higher value of 15.8 μm(Fig. 12). The average value was as low as Ra of 6.55 μm (Fig. 13).
Conclusions This paper proposed a method for forming a continuous multiposture LMD. Stable hollow sphere LMD forming can be achieved through mathematical modeling analysis and process optimization, and the outer diameter and wall thickness of the sphere were generally uniform with the change of the deposition angle, whereas the sealing part fluctuated slightly. The microstructure of the formed part was dense and the grains were fine. With the increase in the deposition angle, the grain size decreased first and then increased, whereas the microhardness first increased and then decreased. The step effect on the surface of the hollow sphere is insignificant. The average roughness of the spherical surface of each deposition angle was as low as Ra of 6.55 μm, and even as low as Ra of 1.1 μm after remelting. Finally, the continuous multiposture LMD forming method significantly improves the forming ability of LMD technology for complex structural parts and closed parts, paving the way for its wider application.
