## Fabricating High-Quality 3D-Printed Alloys for Dental Applications
However, it is still questionable whether this technique can be applied to SLM-processed dental prostheses in general, because it may alter the dimension of the products. SLM-fabricated fixed partial dentures have good dimensional accuracy and, in particular, a good marginal fit to minimize plaque accumulation and to reduce the chance of recurrent caries and periodontal disease (Figure 1) [1,7]. However, the dimensional accuracy of the final 3D product can be significantly jeopardized by many factors, including thermal distortion due to continuous melting and resolidification during the SLM process [6]. [PERSON] et al. [7] reported that SLM-fabricated Co-Cr alloy copings showed significantly larger marginal discrepancies than cast ones. In addition, [PERSON] et al. [8] showed a significantly increased marginal discrepancy after the application of veneering ceramic in the Co-Cr alloy copings produced by SLM (Figure 1). Therefore, at present, it cannot be ensured that dental SLM restorations have a good marginal fit both before and after ceramic veneering. Meanwhile, [PERSON] et al. [9] proposed a method of reducing the layer thickness or increasing the sloping angle during the SLM process to improve the accuracy of the final SLM framework. In addition, [PERSON] et al. [10] suggested the use of a compound scanning method with different spot compensations to fabricate a more accurate SLM structure. Thus, the strategies for guaranteeing an accurate fit of SLM objects should be researched further. For dental applications, SLM-fabricated restorations should retain sufficient mechanical properties and fatigue resistance, which are directly affected by the alloy microstructures [1,11,12]. Even a little residual porosity in the SLM parts may be problematic for some applications where high strength and fatigue resistance are necessary [5]. During the SLM process, therefore, various factors such as applied process parameters and cooling cycles should be optimized carefully, depending on the alloy powder materials used (e.g., type and size distribution) [2,6,13]. In particular, the anisotropy of the mechanical properties is a unique characteristic of SLM builds (Figure 2) [11,12,14,15,16,17]. The tensile  
Figure 1: Schematic diagram of dental metal-ceramic crown showing (**a**) over-extended and (**b**) under-extended margins (MD: marginal discrepancy; AMD: absolute marginal discrepancy). properties of SLM parts fabricated in different building directions depend on the metallurgical bonding between two neighbor melt tracks or layers. When an SLM structure is being built, \"layer-layer\" and \"track-track\" molten pool boundaries (MPBs) are generated by multi-layer and multi-track melt pool overlapping, respectively (Figure 2) [16; 18]. When the structure is built vertically (90\({}^{\circ}\)) or horizontally (0\({}^{\circ}\)), the loading direction caused by an occlusal force would act on the track-track MPB surface or on the layer-layer MPB surface, respectively. When the part is built diagonally (45\({}^{\circ}\)), the oriented MPBs are discontinuous not only on the x-y plane but also along the z axis, producing complex loads acting simultaneously along both the layer-layer and track-track MPB surfaces [15; 18]. Thus, the differences in MPB formation can eventually influence the mechanical properties of the processed SLM structure. [PERSON] et al. [12] reported that SLM-produced structures exhibit significant anisotropy in their fatigue strength as well. Therefore, when a metallic dental structure is fabricated via SLM, the building direction is a primary factor to be considered during the designing process. In addition, it should be noted that the microstructural anisotropy of the SLM part is also caused by the local heat transfer condition, which can be determined by the mean of the scanning strategy [16]. At present, however, there is no definite standard or consensus on how dental SLM prostheses should be designed when considering the anisotropy issue, which mainly depends on the building and scanning strategies.