# Superior Mechanical Behavior and Fretting Wear Resistance of 3D-Printed Inconel 625 Superalloy
## 1 Introduction  
Since the first 3D printing process appeared in the Japanese edition of the IEICE Transactions on Electronics in 1981, explorations on different metal components from metallic powders have rapidly increased worldwide for investigating high-performance metallic materials or complex structures in the fundamental scientific research. It is noteworthy that the additive manufacturing (AM) is becoming the focus of intensive research owing to its advantages of completely melting the powders and producing fully dense microstructures during the fabrication without pretreatment, thus making them a superior technique in AM processes. In addition to the ability of AM to create complex structures with a high density, the capability to prepare materials with an ultra-high tensile strength (TS) and a superior yield strength (YS) has also been demonstrated. The excellent TS reported for AM-processed steel, titanium and nickel-based alloy are 1470 [1], 1510 [2] and 1184 MPa [3], respectively, with corresponding TS of 1100, 1440 and 933 MPa. As one of the most important nickel-based alloys, Inconel 625 has shown exceptional properties such as strong resistance to corrosion, high tensile strength and yield strength, as well as high resistance to chloride-ion stress-corrosion cracking [4; 5] and thus has been widely applied in advanced areas including sea-water applications. It has been used as propeller blades for motor patrol gunboats, submarine auxiliary propulsion motors and submarine quick-disconnect fittings [4]. Potential applications are springs, seals, fasteners and oceanographic instrument components. Conventionally, Inconel 625 alloys are hard-to-cut materials during machining, which normally require high cutting temperatures due to the low thermal conductivity and volume-specific heat. Compared with traditional casting and forging parts, Inconel 625 obtained by the AM process shows much higher tensile strength [6; 7] and has hardness as high as 343 HV with primary dendrite arm space of \(\sim\)0.5 \(\upmu\)m due to the rapid cooling speed (106 K/s) [6]. These unique properties make AM processes veryattractive for designing the high-strength and durable Inconel 625 applied in aerospace, automotive and robotics. However, Inconel 625 superalloy applied in moving assemblies normally subjects to various critical conditions such as cyclic fatigue loading, corrosion and thermal stresses and thus suffers from serious mechanical energy dissipation caused by friction and wear. Accordingly, a fine control of friction and wear for Inconel 625 superalloy is desirable for improving the reliability of moving mechanical systems and is also an attractive fundamental scientific research. [PERSON] [8] first investigated fretting properties of Inconel 625 under different pressures at elevated temperature and high vacuum. The friction coefficient with a high value of 1.7 at pressures below 0.1 Pa was observed and it decreased to a low value of 0.55 at 105 Pa. In order to enhance the wear resistance, the reinforced Inconel 625 with considerably size-reduced TiC particles was successfully prepared by [PERSON] et al. [9] using the laser metal deposition (LMD). During the LMD process, the in-situ interfacial reaction could be tailored between the TiC reinforcement and the Ni-based metal matrix to improve the interfacial wettability and bonding coherence and thus greatly enhancing the wear resistance. Recently, an accurate 3D finite element model was developed by [PERSON] et al. [10] to predict the wear of Inconel 625 superalloy, acting as tools with complex geometries, with PVD-TiAlN coated carbide and ceramic inserts. It was found that the low-cutting speed, low-depth cut and the middle-level feed rate would be appropriate for Inconel 625 with PVD-TiAlN coated carbide insert, while ceramic insert demonstrated better performance under the condition of a high-cutting speed. Recently, increasing attentions on AM metallic materials have been received for designing components with a high complexity.