## Fabricating High-Quality 3D-Printed Alloys for Dental Applications
Moreover, the dominant factor affecting the fatigue strength of SLM builds still remains unidentified [12]. Although the mechanical anisotropy of SLM structures have been extensively researched, most of those studies were performed using \"tensile\" tests [11; 15; 17]. Tests for evaluating the mechanical and fatigue behaviors of SLM products for dental applications should be accompanied with \"bending\" tests, which simulate a long dental bridge under occlusal load. Future research should also include studies on the effects of post-processing heat treatment of as-built SLM products for relieving the residual stress and for improving the mechanical properties and fatigue resistance [13; 17]. At present, when an SLM part is applied as a dental prosthesis, it should be tested to find whether the alloy satisfies the requirements specified by ISO 22674 in terms of mechanical properties [19]. Figure 2: Processing strategies for the selective laser melting substructure of dental metal-ceramic restoration (**left**) and schematic representation of different types of molten pool boundaries (MPBs) (**right**): (a) single MPB; (b) “layer-layer” MPB; and (c) “track-track” MPB. It should also be noted that most dental metallic restorations produced by SLM need to be veneered with ceramic (porcelain) to match the color of the tooth for improved esthetic results [1]. The fracture of ceramic veneers is a common clinical problem that causes the premature failure of fixed partial dentures [8]. Therefore, a high bond strength between ceramic and metal in a ceramic-metal restoration is directly associated with the long-term clinical success of the restoration [1]. [PERSON] et al. [20] and [PERSON] et al. [21] reported a favorable porcelain adherence of SLM-produced alloys in comparison with cast alloys. Recently, [PERSON] et al. [22] demonstrated that Co-Cr dental alloy fabricated via SLM had significantly higher bond strength than cast alloy. In metal-ceramic restorations, another frequent reason for porcelain fracture is the lack of rigidity and the distortion of the metal substructure [23]. Therefore, mechanical anisotropy should also be taken into consideration when a metal substructure for a metal-ceramic restoration is built. As stated previously, a possible change in the marginal fit of metal-ceramic restorations during the application of veneering ceramic onto an SLM substructure needs to be researched further by carefully examining the dimension and microstructure during the firing procedures [8]. At present, the SLM alloys most used for dental applications are Co-Cr and titanium (Ti) alloys [2]. Dental alloys used in the oral environment should have a lower release of metallic ions, which may lead to adverse tissue reactions and/or hypersensitivity, to reduce risks to patient health [1,24]. For SLM alloys, therefore, a biocompatibility issue can be raised due to their potential release of toxic metal ions. Titanium-6 aluminum-4 vanadium (Ti-6 AI-4V) alloy fabricated by SLM is suitable for implant applications owing to its favorable biocompatibility [2,25]. [PERSON] et al. [24] reported that Co-Cr alloy fabricated with SLM showed a lower release of toxic Co ions than cast alloy. In their study [24], the SLM consisted of a homogeneous and compact structure, unlike the cast Co-Cr alloy that had a heterogeneous structure. Similarly, SLM Ni-Cr alloy may exhibit enhanced biocompatibility in comparison with cast Ni-Cr alloy, possibly by improving the microstructure and, as a result, reducing the release of toxic and allergic Ni ions [2]. This requires in vitro and in vivo investigations in future. As described above, SLM is a complicated metallurgical process, and many factors during the process are related to the final quality of the SLM products. SLM technology has great potential to replace existing dental manufacturing techniques such as casting and milling, for fabricating metallic structures of fixed and removable partial dentures [1].