When metal components are manufactured by selective laser melting, a suspension structure cannot be directly formed in the unmelted powder layer. A suitable support structure must be added in time to dissipate the heat generated during the metal powder melting process and restrain the deformation of the overhanging characteristics. Therefore, the manufacturability and formation quality of the overhang structure are related to the added support structure. Various types of support structures, such as block, plane, tree, and cone supports, have been designed for overhanging structures, and considerable research has been conducted on support structure optimization, support strength, and easy removal. However, the type of performance that differs between support structures is not clear. In practice, this depends primarily on the experience of selecting a certain support structure. In this study, the effects of cone, block, and combination supports on the thermodynamics and formation quality of overhanging specimens are studied, and the selection principle of the support type is formulated; this can provide a technical reference for the selection of supporting structures.

First, the thermal elastic-plastic finite element method is used to simulate the additive manufacturing process of the cone, block, and combination support specimens. By quantitatively analyzing the temperature data at the end of the build process along vertical and horizontal paths, which are extracted from the model, the influence of the heat conduction characteristics of the different support structures on the temperature field of the specimen is revealed. Three types of supporting specimens made of 316L powder are prepared using the selective laser melting (SLM) method. The surface morphologies of the specimens after removing the support are analyzed using the laser scanning confocal microscope and scanning electron microscope. The effect of the supporting structure on the surface morphology is studied. The effects of the supporting structure on the microstructure and elemental distribution are studied using an optical microscope and electron probe. Finally, the microhardness of the formed specimens is measured using a microhardness tester to characterize their mechanical properties.

The geometry of the support structure affects the distribution of the temperature field in the overhanging specimen. The block support reduces the temperature gradient in the edge region (Fig. 6), thus reducing the specimen warpage caused by thermal strain. The block support reduces the peak temperature by 6.7%, temperature gradient by 14.05%, and temperature oscillation amplitude by 41.07% of the specimen, indicating that the block support structure has excellent cooling performance, whereas a higher cooling rate helps the overhang structure to form fine grains with good microstructural properties. In addition, it is found that the combination support specimen has the smallest warpage (0.29 mm) and surface roughness (70.804 μm) among the three types of specimens (Fig. 12). The excellent support strength and heat dissipation performance of the combination support improve the formation accuracy of the specimen. The block and combination support specimens have refined grain and reticulated alloying element phases such as Cr and Ni in the sub-grains. The dense metal structure and the brittle and hard reticular phases increase the microhardness to 234 HV (Fig. 17). However, the microhardness fluctuation of the block support specimen is the largest owing to the existence of many porosity defects, and its value is 22 HV. Therefore, a combination support structure with better comprehensive performance is preferred to improve the mechanical properties of the overhanging structure.

The support structure changes the thermomechanical evolution process of the overhang plate. The block support can significantly reduce the peak temperature, temperature oscillation amplitude, and edge temperature gradient of the specimen, concluding that the block support structure exhibits excellent cooling performance. The combination support can not only restrain the specimen's warpage by providing sufficient support strength but also improve the surface topography quality by reducing the powder clusters on the lower surface of the overhanging specimen. Therefore, choosing a combination support is conducive to the preparation of overhang structures with higher dimensional accuracy. The support structure affects the microstructure of the overhanging area of the specimen. The metal structure of the cone support specimen is coarse, uniform, and has fewer porosity defects that can be removed by setting a small machining allowance. Owing to the intense competition of metal grain growth in the block support specimen, the refined grains and the brittle and hard reticular phases formed in the overhang area significantly improve the microhardness. The combination support has the advantages of the other two types of support. Increasing the proportion of block structures in combination-type supports is beneficial to improve the mechanical properties of specimens, and increasing the proportion of cone structures is conducive to reduce material consumption.