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A Review on Laser Powder Bed Fusion of Inconel 625 Nickel-Based Alloy  
Zhihua [PERSON] 1,2,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,1  
Inconel 625 (IN625) is a nickel-based superalloy. It mainly draws support from the solution strengthening of some elements in the Ni-Cr matrix, such as Mo and Nb, to obtain high-temperature strength, high creep resistance. It also exhibits excellent corrosion resistance in various environments and good weldability [20; 21; 22; 23; 24]. However, due to its high hardness, low thermal conductivity, and high work hardening rate, IN625 is considered as an alloy difficult to machining or subtractive processing [25]. Machining tools are worn too fast and it is difficult to control the properties of IN625 in casting or forging. Therefore, new technologies are in great need for manufacturing complex-shaped IN625 parts, which are often demanded in aerospace industries. Laser powder bed fusion (LPBF) is an emerging and powerful additive manufacturing process which is capable of producing metal parts with high geometrical complexity and freedom. As the aviation industry continuously strives to improve performance and efficiency in extreme temperatures and environments, LPBF additive manufacturing becomes a very attractive alternative solution for producing IN625 components in complex geometry [26; 27; 28]. LPBF of nickel-based alloys, including IN625 and IN718 [29; 30; 31; 32; 33; 34; 35; 36; 37], has been widely studied due to their high importance in aerospace industries. A detailed LPBF process schematic diagram is shown in Figure 1. LPBF is a cyclic super rapid heating and cooling process. For example, the cooling rate of the LPBF process can reach \(10^{6}\) K/s [38; 39], in which the microstructure is in a non-equilibrium state and the grain growth and segregation of alloy elements are restrained accordingly [40; 41; 42; 43; 44]. Because of the large temperature gradient and complex heat transfer caused by laser beam cyclic processing, the microstructure and properties of the alloy tend to be anisotropic, and new metastable or even amorphous phases may be generated [45; 46; 47; 48]. There are many variables associated with the LPBF process, such as laser power, scan speed, hatch spacing, and layer thickness. All of these variables need to be properly manipulated to successfully and reliably generate the final component, otherwise, it will result in balling, distortion, porosity, cracks, and low-density defects that seriously affect the performance of the parts [49; 50; 51; 52; 53]. For example, when using low laser power or high scanning speed, the molten pool is not wide enough or heated enough to combine the present track with the surrounding materials, resulting in the lack of fusion of parts [53]. In addition, the surface quality and dimensional accuracy of as-built parts are usually insufficient to meet the requirements of industrial production, so post-treatment processes such as surface processing and heat treatment are required [50; 54]. Due to the key roles of Inconel 625 as a critical material used for manufacturing high-temperature bearing parts of aeroengines, the poor workability of IN625 alloy, and the difficulties to achieve high geometrical complexity using conventional manufacturing processes, it is necessary to review the current research of LPBF of Inconel 625, which provides new possibilities for manufacturing  
Figure 1: Schematic diagram of laser powder bed fusion (LPBF) technology [30]. Inconel 625 parts with high geometrical complexity and good properties. This paper reviews the studies that have been done on LPBF of IN625 focusing on the microstructure, mechanical properties, the development of residual stresses, and the mechanism of defects formation. The effects of some additional treatments (e.g., preheating, post-processing heat treatments) to relieve or eliminate residual stress inside IN625 parts undergoing LPBF and improve the quality and properties of LPBF IN625 parts were reviewed.