Abstract: Enhancing the performance of engineering components through surface modification or additive repair can effectively increase the service life of parts, in support of green and sustainable development strategy. In this study, Inconel 718 cladding layers were prepared on the surface of 316L stainless steel using the hot-wire laser metal deposition technology, with a line energy of 81.4 kJ/m and a wire deposition rate up to 3.1 kg/h. The microstructure, phase composition,
microhardness, and wear resistance of the Inconel 718 cladding layer were investigated in detail. The results revealed that the microstructure of the Inconel 718 coatings was predominantly comprised of numerous columnar dendrites. The columnar dendrites grew vertically from the substrate surface or the boundaries between adjacent deposited tracks towards to the center of the coating. The γ-Ni phase was identified as the primary phase in the Inconel 718 cladding layer, with a “chain-like” and “island-like” distribution of Laves phase detected in the inter-dendritic regions. The average microhardness of the
Inconel 718 coating was measured at 268.89 HV1, which was 37% higher than that of the 316L substrate. Moreover, the average friction coefficient of the Inconel 718 cladding layer was determined as 0.53, with a friction mass loss of 23.9% compared to the substrate of 316L stainless steel. The worn surface of the Inconel 718 coating was characterized by fine scratches and minor peeling, indicating an adhesive wear mechanism.

Highlights:(1) Inconel 718 coatings were prepared using laser metal deposition with hot wire, achieving a wire deposition rate of 3.1 kg/h.
(2) The microhardness and wear resistance of the 316L substrate were significantly enhanced.
Keywords: laser deposition with hot wire; Inconel 718; deposition rate; microstructure; wear resistance

316L austenitic stainless steel, as a low-carbon alloy steel, is widely used as engineering components. When such parts are in service for a long time under alternating loads and harsh environments, they are prone to failure due to surface wear and fatigue damage. Usually, damaged components are directly scrapped and replaced, resulting in a huge waste of materials. Inconel 718 alloy has good room temperature/high temperature strength, excellent oxidation resistance, fatigue resistance, wear resistance and corrosion resistance. By preparing Inconel 718 on the surface of parts Coatings can be used to strengthen or repair traditional engineering components, thereby extending the service life of components and reducing material and energy consumption. They have important academic value and economic significance.

As an advanced surface processing technology, laser cladding technology can obtain a cladding layer with a small heat-affected zone, high metallurgical bonding strength, and good physical and chemical properties such as wear resistance and corrosion resistance. According to the type of cladding material, laser cladding technology can be divided into laser powder cladding technology with pre-powder laying or synchronous powder feeding and laser melt deposition technology with synchronous wire feeding. In recent years, many scholars have conducted extensive research on laser powder cladding technology and achieved a series of research results. Zhang Jie et al. used laser cladding technology to prepare Inconel 718 coating on the surface of rolled Inconel 718 alloy, and compared the wear resistance of the coating at room temperature and high temperature. The study showed that the microhardness and wear rate of Inconel 718 coating at 600°C were 72% and 25% of those at room temperature, respectively. Wang Tao et al. prepared Inconel 718 on the surface of A3 steel. The influence of laser scanning speed on the wear resistance of the coating was studied. It was pointed out that when the scanning speed was 14 mm/s, the wear mass of the coating was only 0.02114 mg/m, and the wear resistance was the best. Li Dong et al. increased the hardness of the Inconel718 cladding layer from 287 HV0.2 to 306 HV0.2 by changing the powder feeding gas during laser cladding. Jia Xiaohui et al. compared and analyzed the microstructure and wear resistance of laser cladding Inconel 718 coating and WC/Inconel 718 composite coating. It was found that adding WC particles to the Inconel 718 coating can refine the microstructure of the coating and improve the microhardness of the coating. The wear rate of the WC/Inconel 718 composite coating is 65.3% of that of the Inconel 718 coating. Wu Jun et al. added TiC particles to the Inconel 718 coating, which increased the microhardness of the coating from 297 HV0.2 to 408 HV0.2, the friction coefficient decreased from 0.3402 to 0.2628, and the wear rate decreased from 35.15 ×
10-4g/(N·m) to 5.96 × 10-4g/(N·m).

However, the preparation process of metal powder is complicated, the material utilization rate is low, and the recycling of waste powder is difficult. The production cost of laser melting powder coating is often high. Compared with powder cladding, the material utilization rate of laser cladding technology based on metal wire is close to 100%, cleaner working environment, lower porosity and crack rate of cladding layer, is considered to be a more cost-effective surface modification process. However, due to the high reflectivity of welding wire to laser and low melting rate of welding wire, traditional laser fuse deposition technology cannot meet the application requirements of surface modification and additive repair of large-sized components. Laser hot wire deposition technology adds an additional power supply between the wire guide and the substrate to preheat the welding wire, which can reduce the dependence of welding wire melting on laser heat, increase welding wire melting rate and laser scanning speed, and has broad application prospects in the field of surface modification and repair of large components.

In order to explore the process feasibility of laser hot wire deposition technology in the field of surface modification, this paper uses laser hot wire deposition technology to prepare Inconel 718 cladding layer on the surface of 316L stainless steel, and systematically studies the microstructure, phase composition and wear resistance of the cladding layer. The research results of this paper will provide theoretical guidance and engineering experience for the application and promotion of laser hot wire deposition technology in actual industrial fields.

1 Test method

1.1 Test material
The substrate used in the test is 316L stainless steel, of which the diameter of the tube is 60 mm and the length is 1000 mm, and the size of the plate is 300 mm (length) × 200 mm (width) × 10 mm (thickness). The cladding consumables are ERNiFeCr-2 welding wire with a diameter of 1.2 mm. The chemical composition of the substrate and the welding wire is shown in Table 1.

1.2 Laser hot wire deposition system
The laser hot wire deposition test system is shown in Figure 1. The device consists of five basic parts: heat source supply system, welding wire preheating and feeding system, four-axis linkage motion mechanism, gas protection system and high-speed camera acquisition system.
The deposition heat source is provided by the IPG fiber laser produced by Laserline, Germany. The fiber core diameter is 600 μm, the wavelength is 976 nm, the maximum output power is 4 kW, and the minimum spot diameter is 3 mm. The JRS-400 laser beam laser, which is independently designed and developed, is used. The wire feeding mechanism feeds the welding wire into the molten pool, and the YC-400TX4 power supply produced by Panasonic is used to preheat the welding wire. The preheating voltage is kept constant at 5 V, and the preheating current is 0-300 A and is stable and adjustable. The four-axis linkage walking mechanism can realize the high-speed movement of the laser head in the three directions of x, y, and z, and can also rotate the rod-shaped substrate mounted on the flange. In order to prevent the cladding layer from being oxidized and forming defects such as pores, two argon gases are introduced from the laser head and the wire feeding nozzle to protect the molten pool and high-temperature welding wire. In order to observe the preheating, melting and transmission process of the welding wire, the Phantom VEO710 high-speed camera is used to collect the hot wire deposition process in real time.

1.3 Preparation of Inconel 718 cladding layer
The laser hot wire deposition process test was carried out after sandpaper and acetone were used to remove the oxide layer and oil stains on the surface of the 316L stainless steel substrate. The optimized process parameters are shown in Table 2.
The welding wire deposition rate φ during the test (unit: kg·h’-1) is expressed by formula (1): Where: ρ is the wire density (8.26 × 103kg·m‘-3), D is the wire diameter (1.2 × 10’-3 m), Vf
is the wire feeding speed (unit: m·min’-1).
The line energy QL (unit: kJ·m-1) consumed during the deposition process is expressed by formula (2): Where: PL is the laser power (unit: W); U is the preheating power supply voltage (unit: V); I is the wire preheating current (unit: A); VS is the scanning speed (unit: m·min‘-1).

1.4 Microstructure and performance testing
After the deposition test was completed, the cladding layer was cut into 10 mm × 10 mm × 5 mm metallographic specimens by wire cutting. The test surface of the specimen was ground and polished with sandpaper and diamond polishing agent, and then electrolytically corroded in aqua regia (DC power supply, 6 V, 5 s). The microstructure of the cladding layer was observed by optical microscope (OM) and scanning electron microscope (SEM); the phase composition and element distribution of the cladding layer were analyzed by X-ray diffraction (XRD) and energy dispersive spectrometer (EDS). The microhardness of the cladding layer was tested by HXD-1000 digital microhardness tester. The indenter load during the test was 1000 g, the loading time was 15 s, the distance between adjacent measuring points was 0.2 mm, and each sample was measured three times along the same path to take the average value. The coating and substrate were cut into blocks of 20 mm × 20 mm × 5 mm. After polishing, the wear resistance of the coating and substrate was tested by HT-1000 ball-disc friction tester at room temperature and dry sliding conditions. The friction pair was Si3N4 with a diameter of 6 mm. Ceramic ball, wear scar diameter is 6 mm, static load in friction test is 10 N, test lasts for 30 minutes; after the test, high-precision electronic balance is used to weigh the mass loss of wear sample, scanning electron microscope is used to observe the surface morphology of wear sample, and wear mechanism is analyzed.

2 Experimental results and discussion

2.1 Deposition process and forming characteristics
Figure 2 shows the preparation process and surface morphology of Inconel 718 cladding layer. As shown in Figure 2a, when preparing Inconel 718 coating on the regular curved surface of 316L stainless steel, the post-wire feeding method is adopted. The laser beam, welding wire and deposition layer are located in the same plane. The welding wire is located behind the laser beam along the deposition direction. As shown in Figure 2c, when preparing Inconel 718 coating on the 316L stainless steel plane, the welding wire is located behind the laser beam. When the Inconel 718 coating was prepared, the side wire feeding method was adopted. The welding wire was not in the plane formed by the laser beam and the deposition layer, and the feeding direction of the welding wire was perpendicular to the deposition direction. Whether the deposition test was carried out on a plane or on a curved surface, the end of the welding wire was always in contact with the molten pool, forming a closed loop of the preheating current, ensuring that the resistance heat fully heated the welding wire. At the same time, the welding wire was completely melted under the action of resistance preheating, laser heat and heat conduction of the molten pool, and a stable liquid bridge transmission process was formed after the liquid metal contacted the molten pool. As shown in Figures 2b and 2d, the surface of the Inconel 718 cladding layer prepared by laser hot wire deposition technology is flat and continuous, without defects such as unfusion and cracking. The cross-sectional morphology of the Inconel 718 cladding layer is shown in Figure 3. Figure 3a is the cross-sectional morphology. It can be seen from the figure that the boundary between adjacent layers in the Inconel 718 coating is clearly visible, and the width of a single deposition layer is about 1.3 mm; the height difference of the deposition layer surface is about 100 μm, indicating that the surface of the laser hot wire deposited Inconel 718 cladding layer only needs a small amount of cutting or grinding to meet the use requirements. Figure 3b is the longitudinal section morphology, from which it can be seen that the thickness of the Inconel 718 cladding layer is about 1 mm. The metallurgical bonding between the cladding layer and the substrate and between adjacent deposition layers is good, and there are no microscopic defects such as pores and cracks.

2.2 Wire deposition rate and line energy
Substituting the optimized deposition process parameters into equations (1) and (2), it can be calculated that the wire deposition rate during the laser hot wire deposition of Inconel 718 is 3.1 kg/h, and the line energy during the deposition process is 81.4 kJ/m. Figure 4 summarizes the material deposition rate and line energy consumed by other surface modification processes in the preparation of Inconel 718 alloy. As can be seen from the figure, based on relatively low heat input, traditional laser cladding technologies such as laser engineered net shaping (LENS), Laser
direct energy deposition (LDED) and Laser metaldeposition The material deposition rate of (LMD) in the preparation of Inconel 718 alloy is usually less than 2 kg/h. In contrast, the energy of the arc is much higher than that of the laser, and the melting speed of the welding wire is faster, so the traditional wire arc additive manufacturing (WAAM) [20-23] has a higher material deposition rate. During the laser hot wire deposition process, the absorption rate of the welding wire to resistance heat is much higher than that to laser, so preheating the welding wire can significantly increase the deposition rate of the welding wire. In addition, preheating the welding wire reduces the dependence of the welding wire melting on laser heat and molten pool heat conduction. During the deposition process, only a micro-molten pool needs to be formed on the surface of the substrate, which reduces the laser power and the irradiation time of the laser on the substrate surface, increases the laser scanning speed, and thus reduces the line energy during the deposition process. In summary, when the Inconel 718 coating is prepared by laser hot wire deposition technology, the welding wire deposition rate can basically reach the level of arc surfacing process, but the energy consumption remains at a relatively low level.

2.3 Microstructure and phase composition analysis
Figure 5 shows the microstructure of Inconel 718 Microstructure of the cladding layer under a light microscope. Due to the low linear energy during the laser hot wire deposition process and the fast cooling speed of the molten pool, the microstructure of the Inconel 718 cladding layer has typical rapid solidification characteristics and is composed of a large number of columnar dendrites. The growth direction of the columnar dendrites is determined by the heat dissipation direction or the direction of temperature gradient reduction. During the multi-pass deposition process, most of the heat in the molten pool is transferred vertically downward through the substrate, and a small amount of heat is conducted outward along the molten pool boundary and the solidified cladding layer. Therefore, as shown in Figures 5a and 5c, in the bottom area of ​​the deposition layer, the columnar dendrites grow vertically upward perpendicular to the substrate surface; at the junction of the layers, due to the intergrowth crystallization mechanism, the columnar dendrites extend perpendicular to the layer boundary to the center of the molten pool. In addition, during the laser hot wire deposition process, the laser scanning speed is fast, the molten pool is elongated, and in the upper surface area of ​​the molten pool, the direction of the temperature gradient changes, and most of the heat is directed to the solidified cladding layer at the rear of the molten pool. Therefore, as shown in Figure 5b As shown in Figure 5, the transition from vertical columnar crystals to horizontal columnar crystals was observed in the upper area of ​​the cross section of the cladding layer; in the middle and upper area of ​​the corresponding longitudinal section of the cladding layer, the columnar crystals were observed to be equiaxed, as shown in Figure 5d.
Figure 6 shows the SEM images of the transverse and longitudinal sections of the Inconel 718 cladding layer. Consistent with the microstructure observed under the optical microscope, the microstructure of the Inconel 718 cladding layer under the scanning electron microscope consists of a very narrow cellular crystal area on the upper side of the fusion line (see Figure 6a and Figure 6c), and a large area of ​​columnar dendrites in the middle and upper part of the cladding layer (see Figure 6b and Figure 6d). It is worth noting that the columnar dendrites appear as a dark gray continuous matrix under the scanning electron microscope, but a large amount of bright white precipitation is observed in the inter-columnar crystal area. In the middle and bottom areas of the deposited layer, the precipitates are small in size and are dispersed in an “island-like” and “granular” shape (see Figure 6a and Figure 6c). 6c); in the upper area of ​​the deposited layer, the precipitates are larger in size and are distributed in a “chain-like” manner (see Figure 6b and Figure 6d), indicating that secondary phases precipitate during the solidification process of the Inconel 718 molten pool deposited by laser hot wire.

In order to determine the phase composition of the Inconel 718 cladding layer deposited by laser hot wire, the Inconel 718 cladding layer sample was subjected to X-ray diffraction analysis, and the analysis results are shown in Figure 7. It can be seen from the figure that the spectrum peaks in the Inconel 718 cladding layer appear at 2θ = 43.5°, 50.7°, 74.6°, 90.5° and 95.8°, indicating that the basic phase in the Inconel 718 cladding layer deposited by laser hot wire is γ-Ni phase. Due to the small size of the precipitates, the diffraction peaks corresponding to the precipitates may overlap with the main peak of γ-Ni, and the existence of secondary phases cannot be detected by XRD.

For Inconel The elemental composition of the intergranular precipitates of the 718 cladding layer was further analyzed by energy spectrum analysis. Figure 8 shows the EDS surface scan results. As can be seen from the figure, compared with the gray matrix (γ-Ni), the bright white precipitates between the grains are rich in Mo and Nb elements, while the content of Ni and Cr elements is relatively poor, indicating that the bright white precipitates in the intergranular region of the Inconel 718 cladding layer are Laves phases.

Yang Hao et al. pointed out that the solidification of Inconel 718 alloy begins with the transformation of L➡ L+γ. At the same time, due to the low equilibrium distribution coefficient of alloying elements such as Mo and Nb, these alloying elements will diffuse into the liquid phase during the solidification of the molten pool, forming an element-enriched layer rich in Nb and Mo at the front of the solid-liquid interface. As the temperature of the molten pool decreases, the L➡ γ+MC+Laves eutectic reaction occurs, generating a large amount of γ/Laves and a small amount of γ/MC eutectic compounds. Therefore, the Laves phase is usually prepared by laser cladding/additive manufacturing. Secondary phases in the intergranular region of Inconel 718 alloy.

2.4 Microhardness
The microhardness of the Inconel 718 cladding layer and the 316L substrate was measured along the “AB” and “CD” paths in Figure 3, and the results are shown in Figure 9. As can be seen from the figure, the microhardness of the Inconel 718 cladding layer from top to bottom in the horizontal and vertical sections varies uniformly between 258.7 HV1 and 278.4 HV1, with an average hardness of 268.89 HV1. The microhardness of the 316L stainless steel substrate is between 190.7 HV1 and 209.2 HV1, with an average hardness of 196.28 HV1. Due to the high temperature laser quenching treatment of part of the substrate below the fusion line, the alloy elements such as Mo in the cladding layer diffuse downward, resulting in a significant increase in the hardness value of this area. The above study shows that the microhardness of the Inconel 718 cladding layer is 37% higher than that of the substrate. Qin Mingjun et al. [30] prepared Inconel 718 on the surface of 304 stainless steel. 625 cladding layer, similar research results were obtained. The laser hot wire deposition process reduces the line energy during the deposition process, accelerates the cooling rate of the molten pool, refines the microstructure in the alloy, and enhances the fine grain strengthening effect. On the other hand, the accelerated solidification rate of the molten pool weakens the segregation of alloying elements such as Nb and Mo to the intergranular region, and more strengthening elements are trapped by the matrix phase, which enhances the solid solution strengthening effect of the matrix phase. In addition, the hard and brittle Laves phase in the intergranular region of the columnar dendrites in the laser hot wire deposited Inconel 718 deposition layer is small in size and can move with the matrix during the deformation of the alloy. Based on the above comprehensive reasons, the laser hot wire deposited Inconel 718 cladding layer has a higher hardness. Meng et al. found that the microhardness of the Inconel 718 alloy prepared by laser additive manufacturing is even higher than that of the forged Inconel 718 alloy.

2.5 Wear resistance
The friction coefficient curve of the Inconel 718 cladding layer and the 316L stainless steel substrate is shown in Figure 10 As shown in the figure, the Inconel 718 cladding layer did not suffer from obvious wear failure during the friction and wear test, and the friction coefficient was stable at about 0.53. After a short running-in period, the friction coefficient of the 316L stainless steel substrate changed around 0.67, but the fluctuation was obvious. After the friction and wear test, the wear mass loss of different samples was weighed using a high-precision electronic balance. The mass loss of the Inconel 718 cladding layer was 0.517 × 10-3 g, accounting for 23.9% of the mass loss of the 316L substrate (2.164 × 10-3g). Generally speaking, the smaller the wear mass loss, the more conducive it is to inhibit the formation of microcracks and micropores on the alloy surface. Therefore, the wear resistance of the Inconel 718 cladding layer is significantly better than that of the 316L stainless steel substrate.

The surface morphology of the friction sample can reflect the wear failure mechanism of the alloy. Figure 11 shows the Inconel SEM images of the friction and wear surfaces of the 718 cladding layer and 316L stainless steel. As shown in Figures 11a and 11c, the wear surface of the Inconel 718 coating sample is relatively smooth, while the 316L stainless steel surface has a delamination phenomenon. Figures 11b and 11d are high-magnification SEM images of the local areas of the wear surface of the Inconel 718 cladding layer and 316L stainless steel, respectively. As shown in Figure 11b, Only slight scratches and a small amount of spalling and wear debris were observed on the wear surface of the 718 cladding layer, and the wear rate was low. As shown in Figure 11d, the wear surface of the 316L stainless steel substrate is mainly composed of deep grooves and large spalling, and severe plastic deformation is observed, indicating that fatigue wear failure occurred on the surface of the 316L stainless steel substrate.

The static load applied during the friction and wear test and the tangential movement of the Si3N4 ball will first form stress concentration on the surface of the specimen wear track. Due to the low microhardness of the 316L stainless steel substrate, it is difficult to resist the crack initiation tendency caused by stress concentration during the friction process. As the friction and wear test proceeds, the furrow depth increases, the stress concentration intensifies, and the alloy surface undergoes plastic deformation. At the same time, the cracks extend to the edge of the groove, causing part of the alloy to detach from the matrix and spall. The formation of spalling causes local stress unloading, resulting in fluctuations in the friction coefficient; on the other hand, large spalling and fine wear debris scratch the friction pair during the friction process, resulting in an increase in the friction coefficient. Therefore, the wear mechanism of the 316L stainless steel substrate is fatigue wear, adhesive wear and abrasive wear. Compared with 316L stainless steel, the Inconel 718 cladding layer has higher microhardness, stronger load-bearing capacity, lower friction coefficient in friction and wear test, and better wear resistance. Therefore, only shallow plow-shaped grooves were observed on the worn surface, which is a typical adhesive wear.

3 Conclusion
The Inconel 718 coating with a thickness of 1 mm was efficiently prepared on the surface of 316L stainless steel by laser hot wire deposition technology. The line energy during the deposition process was 81.4 kJ/m and the welding wire deposition rate was 3.1 kg/h. The Inconel 718 cladding layer has good metallurgical bonding with the substrate and has no structural defects such as pores and cracks. The microstructure, phase composition, microhardness and wear resistance of the Inconel 718 cladding layer were studied, and the following main conclusions were drawn:
(1) Inconel The microstructure of the 718 cladding layer has typical rapid solidification characteristics and is composed of a large number of columnar dendrites. The growth direction of the columnar crystals is related to the heat dissipation direction of the molten pool. At the bottom of the cladding layer, the columnar crystals grow vertically. At the junction of the layers, the columnar crystals extend perpendicular to the layer boundary to the center of the cladding layer. In the upper area of ​​the cladding layer, the vertical columnar crystals transform into horizontal columnar crystals. The γ-Ni phase is the basic phase in the Inconel 718 cladding layer. The Laves phase with “chain” and “island” distribution is observed in the inter-columnar crystal region.
(2) The average microhardness of the Inconel 718 cladding layer is 268.89 HV1, the average friction coefficient is 0.53, the wear mass loss is about 0.517 × 10-3 g, the wear surface is composed of fine scratches and a small amount of peeling and wear debris, and the wear mechanism is adhesive wear.
(3) The microhardness of the Inconel 718 cladding layer is higher than that of 316L The substrate is 37% higher, and the mass loss in the wear test is only 23.9% of that of the 316L substrate. Laser hot wire deposition technology provides an efficient solution for surface strengthening modification and cladding repair of large-size components.