Abstract： To overcome the issue of poor wear resistance on the surface of In718, composite cladding layer with high molybdenum content was fabricated by laser cladding using In718 alloy as the matrix and nitrogen as the protective gas. The effects of high Mo content on the microstructure and tribological performance of the cladding layer were investigated using instruments such as X-ray diffractometer (XRD), scanning electron .microscopy (SEM), microhardness tester, and wear testing machine. The experimental results show that

the main phases in the cladding layer are the matrix phase Ni-Cr-Co-Mo, (Fe, Ni) solid solution, reinforced phase Mo2N, and free element Mo. The microstructure of the cladding layer exhibits dendrites. The long dendrites in the cladding layer have a distinct orientation. The secondary phase particles in the interdendritic regions were dispersedly distributed. The highest hardness of the cladding layer is 645 HV0.5, which is 2.8 times that of the substrate (230 HV0.5). This significant increase in hardness can be attributed to the solid solution strengthening

effect of Mo and the formation of the hard phase Mo2N. MoO3 has a layered structure, which was formed during the wear process, providing excellent friction reduction effects. The coefficient of friction of the cladding layer is 0.37, and the wear mechanism is mainly abrasive wear accompanied by slight oxidative wear. This study provides technological support for further optimization of parameters in engineering.

0 Introduction

Nickel-based high-temperature alloy Inconel718 (In718) is often used to manufacture key contact parts for aerospace. During service, wear is the main failure mode due to the relative movement of the contact parts [1-6]. In order to improve the wear resistance of the In718 surface and to avoid friction damage to the wear parts caused by In718, preparing a cladding layer with a certain hardness and a low friction coefficient on its surface is an effective solution [7-10]. Molybdenum has the characteristics of high thermal conductivity and good wear resistance at high temperatures. Molybdenum can generate MoO3 with a low friction coefficient through tribochemical reaction at high temperatures. Therefore, the preparation of a composite cladding layer containing molybdenum on its surface has attracted the attention of some scholars [11-14]. The Mo-containing composite cladding layer currently prepared faces defects such as easy cracking of the cladding layer and unmelted Mo. LIU et al. [15] also observed that Mo exists in the form of unmelted particles in the Mo-containing cladding layer prepared. LIU et al. [16] used laser cladding technology to prepare cladding layers with different Mo contents and found that the composition of Mo content was sensitive to its performance. SUN et al. [17] prepared Ni/Mo composite cladding layers at different laser powers and found that Mo cladding layers were prone to through cracks at high laser power. JIN et al. [18] successfully used laser cladding technology to prepare FeCoCrNiMox (x = 0.2, 0.5) cladding layers on EA4T axle steel substrates. As the Mo content increased, the toughness of the cladding layers decreased. HAN et al. [19] found that inappropriate laser parameters may lead to the presence of unmelted particles in the Mo coating. In order to break through the dilemma of unfused and crack defect suppression in laser cladding of high-content Mo composite cladding layers and improve the wear resistance of In718 surfaces, this paper carried out research on the preparation of Mo and In718 composite cladding layers on In718 surfaces, and evaluated their performance, in order to provide support for further optimization of engineering parameters.

1 Experiment

The substrate material is In718 with a size of 100 mm×100 mm×10 mm; the cladding layer materials are In718 powder and Mo powder. The chemical composition of In718 is shown in Table 1. The particle size of In718 powder is 150~300 mesh, and the particle size of Mo powder is 100~270 mesh; the weight ratio of In718 powder and Mo powder is 55% and 45%, respectively; the two powders are mixed by a drum mill for 40 min. Subsequently, in order to improve the flow properties of the powder and remove its internal moisture, the cladding powder is placed in a vacuum drying oven at 120 °C for continuous heating for 2 h. Before starting the laser cladding experiment, the oxide layer on the surface of the substrate was removed with sandpaper, and then the oil and dust on the surface of the substrate were removed with anhydrous ethanol. The laser cladding layer was prepared using an RFL-C3000S continuous fiber laser (Rike, Wuhan, China). During the experiment, coaxial powder feeding was used for laser cladding. The process parameters were set as follows: laser power of 2 kW, laser scanning rate of 8 mm/s, laser overlap rate of 65%, powder feeding rate of 11 g/min, and spot diameter of 4.2 mm. Nitrogen was introduced as a protective gas to ensure environmental stability during laser cladding. Single-layer multi-pass laser cladding was used in the experiment, and the length direction of the substrate was used as the scanning direction of the laser. A composite cladding layer with a length of 80 mm and a width of 30 mm was welded on the substrate. After the laser cladding experiment, the surface of the cladding layer was penetrant tested using a fluorescent penetrant. After the flaw detection, the metallographic specimens, X-ray diffractometer (XRD) specimens, and friction and wear specimens were cut vertically along the laser scanning direction using a wire cutting device (Zhongxin, Taizhou, China), with sizes of 10 mm × 5 mm × 8 mm, 10 mm × 10 mm × 8 mm, and 15 mm ×15 mm × 8 mm, respectively. The samples were inlaid using an XQ-1 metallographic inlay machine produced in Ningbo, Shaoxing, China. The cross-section of the metallographic sample cladding layer, XRD sample and friction and wear sample were polished with sandpaper of 60#~2000# grit, and then polished with diamond paste of 7 μm, 1 μm and 0.5 μm grit by a fully automatic polishing machine (MP-2B) until they had a mirror effect. The metallographic sample was then etched with a well-proportioned solution (V(HCL): V(HF): V(HNO3) = 3:2:5). The microstructure of the cladding layer was observed with a Phenom XL g2 scanning electron microscope (FeiNa, Netherlands) equipped with energy dispersive spectroscopy (EDS), and its elemental composition was quantitatively analyzed. The hardness of the metallographic sample was measured using a hardness tester (HVS-1000Z) (LiDun, Shanghai, China). The hardness tester applied a load of 500 g in the direction perpendicular to the sample surface for 10 s. The cross-sectional hardness of the cladding layer was tested, and a test point was made every 100 μm. The phase of the cladding layer was analyzed using a D/max-2500 XRD produced by Rigaku. During the acquisition, the diffraction rate was 6 (°)/min and the diffraction range was 10°~110°. The wear performance was tested using an MPX-3G friction and wear tester (Hengxu, Jinan, China) under the conditions of an experimental load of 50 N, an experimental temperature of room temperature, an experimental duration of 40 min, and a rotation speed of 300 r/min. The wear test adopted a rotary test method with a wear track radius of 1.5 mm. The friction and wear grinding ball was a ceramic ball Si3N4 with a diameter of 6.35 mm. The real-time coefficient of friction (COF) of the deposited cladding layer is automatically recorded by computer software connected to the friction sensor. In order to quantitatively characterize the wear characteristics, a DC-700 surface topography measuring instrument is used to perform a 3D scan of the worn surface. The wear volume is calculated by analyzing the wear profile characteristics, and the data is converted according to the wear rate calculation formula [20]:

Where: ΔV is the wear volume (mm3); F is the test force (N), s is the sliding distance during the test (m); δ is the wear rate

(mm3 /(N· m)).

2 Results and Discussion

2.1 Macroscopic morphology of cladding layer

Figure 1 shows the front morphology of the cladding layer. Figures 1a and 1b show the front morphology of the composite cladding layer prepared by laser and the front morphology after flaw detection, respectively. It can be observed that the prepared cladding layer has no cracks, indicating that the selected laser parameters are appropriate and the thermal coupling between the laser and the material does not exceed the material damage limit [21]. The laser parameters and laser scanning rate of laser cladding affect the 3D transient temperature distribution of the laser cladding process and will affect the forming quality of the cladding layer [22]. The formed quality of the prepared coating is intact, indicating that no cracks are generated under this parameter.

2.2 Cladding layer phase

As can be seen from Figure 2, the main phases of the cladding layer are Ni-Cr-Co-Mo, (Fe, Ni) solid solution, Mo2N and Mo.

The laser cladding process is a non-equilibrium solidification process [23]. Due to the high content of Mo powder in the cladding layer powder and the high melting point of Mo, the Mo with a high melting point solidifies first during the solidification process of the molten pool formed by laser cladding. Combined with the rapid solidification of the laser, the elemental Mo is retained in the cladding layer. Nitrogen is introduced as a protective gas during the laser cladding process, and some Mo atoms in the molten pool react with nitrogen to generate Mo2N in situ. Since Fe atoms and Ni atoms have similar electronic structures and lattice structures, Fe atoms continuously replace Ni atoms in the solute during the solidification process, causing lattice distortion and forming a (Fe, Ni) solid solution [24].

2.3 Microstructure evolution of cladding layer

Figure 3 shows the microstructure of the upper, middle and lower regions of the cladding layer. Figures 3d, 3e and 3f are partial enlarged views of Figures 3a, 3b and 3c, respectively. It can be observed that the cladding area is a fine dendrite structure, the granular secondary phase between the structures is dispersed, and the structure of the upper region of the cladding layer and the crystal core structure of the bottom region of the cladding layer are higher than those of the middle region. This is consistent with the phenomenon observed by Jiang et al. [25]. From the energy spectrum test of the crystal core region, this region is rich in Ni, Fe, Cr and Mo (as shown by point P1).

The energy spectrum test of the granular secondary phase shows (the results are shown in Table 2) that this area is mainly enriched with N atoms and Mo atoms. Combined with the XRD results in Figure 2, it is speculated that the secondary phase particles are Mo2N hard phases. The main phases in the intergranular region of the cladding layer are Cr, Ni and Mo, which is consistent with the phenomenon observed by WANG et al. [11].

Round particles were observed in the cladding layer. Through the energy spectrum line scan (as shown in Figure 4), the round material was simple molybdenum, indicating that in the high-content Mo-containing cladding layer, the high content of Mo powder will cause the fluidity of the molten pool to deteriorate. Mo has high thermal conductivity. Combined with the characteristics of rapid solidification of laser cladding, part of Mo melted and then re-solidified into newly generated simple Mo, resulting in no Mo2N formed in this area. Columnar crystals were observed growing along the edges of Mo particles. This grain orientation is due to the difference in thermal conductivity between the newly generated Mo particles and other liquid metals, which affects the local temperature field distribution of the molten pool [26]. Mo has a high melting point and good thermal conductivity, which causes the newly generated Mo particles to destroy the normal heat exchange of the liquid metal alloy. Therefore, a circumferential heat flow is formed around the Mo particles, causing the columnar crystals to grow perpendicular to the edges of the Mo particles. In addition, micro-region orientation is formed around the Mo particles, which further refines the grain size [27].

2.4 Hardness distribution of cladding layer

Figure 5 shows that the hardness distribution of the high-molybdenum-content cladding layer is characterized by a bimodal feature. The hardness value measured in the surface area is 415 HV0.5, and the hardness of the subsurface area is significantly increased to 596 HV0.5. The hardness of the middle area decreases, while the bottom area is significantly enhanced, reaching a maximum of 645 HV0.5. Due to the rapid solidification during the laser cladding process, the grains of the cladding layer are refined, thereby achieving a significant increase in hardness. At the same time, the strengthening effect of molybdenum and its compound Mo2N also plays an important role. Due to the high density values of Mo (density of 9.06 g/cm3) and Mo2N (density of 10.2 g/cm3), they tend to sink in the molten pool, resulting in a hardness peak at the interface between the coating and the substrate. The relatively low hardness in the middle of the coating may be related to the distribution difference of strengthening phases such as Mo and Mo2N.

2.5 Wear resistance of cladding layer

Figure 6 shows the variation of friction coefficient of cladding layer with high Mo content. It can be seen that the friction coefficient fluctuates significantly in the initial stage of wear and gradually stabilizes over time. This is because the actual contact area between the friction pair and the cladding surface is small at the initial stage of wear. At this time, point contact sliding causes the friction process to enter a short non-steady-state stage [28]. The average friction coefficient of the cladding layer is about 0.37. Under the action of a large load of 50 N, the surface of the cladding layer is damaged and hard fragments are generated. These fragments aggravate the damage of the wear surface under the reciprocating motion of the grinding ball. As time goes by, the friction heat generated by the friction between the grinding ball and the wear surface causes the hard fragments on the surface to be oxidized, and the grinding ball is in direct contact with the oxidized debris, thereby reducing the wear degree of the cladding surface, and the friction coefficient decreases and stabilizes.

Figure 7 shows the wear surface of the cladding layer at room temperature. It can be seen that there are a lot of wear debris, slight delamination and a small amount of plowing on the cladding layer. Under the action of a large load of 50 N, due to the uneven surface profiles of the two friction pairs, the surface of the cladding layer is destroyed under the action of shear force, and then falls off to form wear debris. The wear debris produces different degrees of plowing along the sliding direction of the grinding ball.

In order to further analyze the wear mechanism of the coating, the EDS energy spectrum scanning and XRD detection were performed on the worn surface of the coating. According to the EDS data analysis in Table 3, the elemental composition of these wear debris is Mo, Cr, Fe, Ni and O, and according to the friction and wear XRD in Figure 8, MoO3, NiMoO4, Cr2O3 and Fe2O3 may exist on the friction surface. The formation of the oxide layer is the result of sliding friction and local temperature change [29]. Due to the instantaneous friction heat generated during the rapid friction between the grinding ball and the cladding layer, part of the Mo element dissolved in the cladding layer is oxidized to generate a small amount of MoO3 oxide [30]. MoO3 has a layered structure and is easy to shear during the friction process, so it can effectively reduce the friction coefficient of the cladding layer. During the friction and wear process, Cr in some areas is oxidized by the instantaneous friction heat to generate Cr2O3. Cr2O3 is a high-hardness oxide that can play a supporting role to protect the surface of the cladding layer from damage under high load. At room temperature, Cr is more easily oxidized than Fe. In the later stage of friction and wear, part of MoO3 reacts with Ni in the solid solution to form NiMoO4. NiMoO4 is a molybdate with a low friction coefficient (0.15~0.37) [31]. Under normal temperature, the wear behavior of the cladding layer is mainly characterized by abrasive wear, accompanied by a certain degree of oxidative wear.

3 Conclusion

(a) A crack-free cladding layer containing 45% Mo in the cladding powder was successfully prepared using a laser power of 2 kW, a laser scanning speed of 8 mm/s and a laser overlap rate of 65%. The main phases of the cladding layer are the matrix phase Ni-Cr-Co-Mo, (Fe,

Ni) solid solution, Mo2N and Mo. And Mo is a metallurgical bond in the cladding layer.

(b) The hardness of the cladding layer increases first, then decreases, and then increases again, showing a trend of high at both ends and low in the middle. The maximum hardness of the cladding layer is

645HV0.5, which is 2.8 times that of the matrix (230 HV0.5).

(c) Since low friction coefficient materials MoO3 and NiMoO4 are generated during the friction and wear process. MoO3 has a layered structure and is easy to shear, the cladding layer has a low friction coefficient. The average friction coefficient of the cladding layer is 0.37, and the wear mechanism is mainly abrasive wear accompanied by slight oxidation wear.