چکیده：The Stellite6 Co-based alloy layers were fabricated on 316L stainless steel substrate using laser wire feeding cladding. The formation, dilution rate, microstructure, chemical composition, microhardness and wear resistance of the cladding layers were analyzed. The results indicate that the two-layer laser cladding process can ensure the optimal formation of the cladding layer with a dilution rate below 5%, and the dilution rate is uniform. The microstructure of the cladding layer is a subeutectic structure composed of dendritic Co-rich solid solution and network eutectic carbides, and it has a higher concentration of network carbides at the top layer compared to the bottom layer. The two-layer cladding process results in a stepwise increase of hardness in the vertical direction of the cladding layer, which stabilizes after reaching a thickness of 1.8mm. The horizontal microhardness of the cladding layer near the surface is relatively uniform, ranging from 450 HV to 550HV. The two-layer cladding process demonstrates superior wear resistance, with a friction coefficient of 0.37 and a 53.9% reduction of mass loss compared to the base material. The wear mechanism of the two-layer cladding process changes from adhesive wear of the substrate to abrasive wear.

316L stainless steel is a stainless steel with Mo added to 18Cr-8Ni, which improves its pitting resistance in Cl- environment and has excellent mechanical properties. It is widely used in the energy, chemical and other fields to manufacture pipelines and structural parts in harsh environments such as high temperature, high pressure and corrosion [1-2]. In order to improve the wear resistance and corrosion resistance of 316L, it is usually necessary to prepare a functional layer with excellent performance on its surface [3].

A commonly used surface treatment method is chemical chromium plating, but the processing of this method is seriously polluting, and the quality of the coating is unstable. The coating and the substrate are non-metallurgically bonded, and there is a risk of the coating falling off when used at high temperatures for a long time [4]. Cobalt-based alloys (Stellite alloys) have high strength, hardness, wear resistance and corrosion resistance at both room temperature and high temperature, and are ideal wear-resistant and corrosion-resistant layer materials for 316L stainless steel in harsh environments [5]. Currently, argon arc welding, plasma arc and other cladding methods or spraying methods are commonly used to form a metallurgical bond between cobalt-based alloys and the substrate to obtain an alloy layer with excellent performance. Sankarapandian et al. [6] used argon arc welding to clad Stellite6 alloy on A36 substrate. The more carbides in the Stellite6 alloy layer can effectively improve the wear resistance of the substrate. However, cobalt-based alloys usually have large differences in performance from the substrate. The energy of arc cladding is large and dispersed, which can easily lead to a high coating dilution rate and a greater tendency to crack [7]. Oxyacetylene flame spraying is used in coating preparation because its flame center temperature is lower than that of electric arc, the dilution rate is easy to control, and the equipment accessibility is good. Sun Delin [8] used oxyacetylene flame powder spraying process to perform surfacing on the surface of hollow thin-walled rollers. Through strict process control and preheating, slow cooling and other measures, a coating with good forming and excellent performance was obtained. Costel-Relu et al. [9] used oxyacetylene flame spraying of NiCrBSi/WC-12Co composite coating on the surface of stainless steel to improve the cavitation corrosion resistance of the substrate. However, this method is manually operated, and there are problems such as strict temperature control during the spraying process, high labor intensity, and unstable coating quality due to human influence.

Laser cladding is a surface treatment method that uses laser to rapidly melt the cladding material on the substrate surface and then rapidly solidify it to obtain a high-performance functional layer that forms a metallurgical bond with the substrate [10]. Laser cladding has the advantages of concentrated energy, low dilution rate, good cladding layer performance, and easy automation. In addition, its spot diameter can reach less than 1 mm, and the control is more precise compared to other surface treatment methods [11]. Soltanipour et al. [12] studied the effect of laser cladding process parameters on the microstructure of Stellite6 powder cladding layer on X19CrMoNbVN11-1 stainless steel substrate. The increase in laser power led to an increase in cellular crystals at the interface of the cladding layer and equiaxed crystals on the surface. By optimizing the process parameters, a well-fused and defect-free cladding layer was obtained. Maximilian et al. [13] laser clad Stellite6 alloy powder on a gray cast iron surface and studied the effect of laser power on the dilution rate, geometry and hardness of the cladding layer. They found that lower power resulted in finer structure and more carbides, thus obtaining a coating with higher hardness. Wang Weidong et al. [14] performed multi-pass laser cladding of Stellite6 alloy powder on the surface of H13 hot working die steel and found that the multi-pass cladding layer had a higher hardness than the single-layer cladding layer, and its longitudinal hardness fluctuation was greater, but it did not affect its overall hardness change trend. Ultimately, laser cladding increased its microhardness and effectively improved the surface properties.

Leunda et al. [15] used laser cladding to prepare NiCr-WC cladding layers on the inner wall of double-cylindrical hole parts. By optimizing the process parameters, the substrate was preheated to 350°C and an intermediate buffer layer was added to reduce the crack tendency of the cladding layer. The utilization rate of the cladding material was improved by using irregular-shaped powders, and finally a well-formed and excellent cladding layer was prepared on the inner wall of a double-cylindrical hole with a length of 300 mm and an inner diameter of 110 mm. Zhu Mingdong et al. [16] performed Stellite6 alloy powder laser cladding on the plane of 304LN stainless steel and the inner wall of a small hole. By optimizing the process parameters of plane laser cladding, a low-dilution rate and defect-free cladding layer was obtained. The results of wear tests and corrosion tests showed that laser cladding improved the wear resistance and corrosion resistance of its surface and the inner wall of the small hole. At present, the research on laser cladding of cobalt-based alloys mainly focuses on powder materials. However, laser cladding of powder materials has the problems of low material utilization and high pollution. In addition, the discontinuity of powder materials easily causes the laser to act on the substrate, resulting in large thermal stress. Certain temperature control measures are required to prevent cracks, deformation and other problems. However, the coupling between wire and laser is better, the temperature control needs to be smaller, and the material utilization rate is close to 100%, so it has received widespread attention [17]. Therefore, this paper uses laser wire feeding cladding to prepare a 3mm thick Stellite6 cobalt-based alloy coating on a 316L substrate, studies the influence of laser cladding process on the dilution rate and forming quality of the cladding layer, analyzes the influence of the cladding process on the structure and performance, and provides an experimental basis for improving the wear resistance and service life of the 316L stainless steel surface.

1 Experimental materials and methods

• Laser cladding materials

The test selected a 30mm×30mm×10mm substrate, the substrate material was 316L stainless steel. The cladding material was a quasi-1.2mm Stellite6 solid welding wire. The chemical composition of the substrate material and the cladding material is shown in Table 1.

• Laser cladding method

The YLS-4000 fiber laser was used in the experiment, and the nominal diameter of the spot was 200μm. During the cladding process, a MOTOMAN NX100 six-axis industrial robot was used for motion control, and a welding wire clamping device was used to ensure the coaxial movement of the welding wire and the laser. The wire feeding method was lateral feeding. The substrate surface was polished before cladding and cleaned with anhydrous ethanol.Remove impurities such as surface oxide scale and oil. During cladding, a “bow-shaped” path is used to perform multiple cladding on the substrate surface. The cladding thickness is 3mm, and in order to achieve uniform thickness, an extra layer is clad at the edge of the substrate to prevent the molten pool from collapsing. The experiment selected several representative groups of laser wire feeding cladding process parameters for discussion, considering three factors: laser power, wire feeding speed, and number of cladding layers. The specific test plan is shown in Table 2. The remaining cladding process parameters are 50% overlap rate, 99.9% pure argon gas for coaxial shielding gas, and 20L/min gas flow rate.

• Detection Method

Two block samples of 5.5 mm × 4 mm × 10 mm were cut from the cladding layer near the middle and near the edge by wire cutting, and the elements of the cladding layer were quantitatively analyzed using a ThermoScientific Niton XL5 Plus handheld X-ray fluorescence spectrometer (XRF). Taking Fe as the object, the dilution rate was calculated according to formula (1).

Where: ω layer

(Fe) is the Fe content of the cladding layer; ω base

(Fe) is the Fe content of the base material 316L, which is considered as a constant of 71.329% after XRF detection; ω material

(Fe) is the Fe content of the cladding material.

Wire cutting was used to cut 12mm×10

mm×2mm slices from the cross section of the cladding layer. After the slices were inlaid, they were first polished step by step with 150#~3000# water sandpaper, and then mechanically polished to a mirror surface with 2.5

μm diamond paste. The corrosive solution prepared by 5g

CuSO4

+50mLHCl+50mLH2

O was used to abrade

10s, and then rinsed with alcohol and dried to make a metallographic sample. The macroscopic morphology of the cross section of the cladding layer was observed using a stereoscopic

AOSVI macroscopic magnifier  The ICX41 inverted metallographic microscope was used to observe the microstructure of each micro-area of the cladding layer, and the TM4000Plus desktop scanning electron microscope (SEM) was used to observe the microstructure at a higher magnification, and the EDS spectrum was used for line scanning and point scanning analysis. The HV-1000TPTA micro-Vickers hardness tester was used to test the microhardness of the cladding layer in the longitudinal direction and 0.5mm from the surface in the transverse direction to detect its hardness change law and hardness uniformity. The hardness test parameters were load 500g and holding time 10s. The MFT-5000 friction and wear tester was used to perform dry friction and wear tests on the substrate and the cladding layer at room temperature. The friction mode was ball-disc friction, the friction partner was a Si3N4 ceramic ball with a diameter of 6.35mm, and the friction test conditions were: load 35N, speed 300 r/min, wear time 30min, and wear radius 4.5mm. Before and after wear, a precision balance (accuracy 0.1 mg) was used to measure the weight gain and loss. The OLS51003D laser scanning microscope was used to observe the morphology of the sample surface after wear, and SEM and EDS were used to observe the microscopic wear morphology.

2 Experimental results and discussion

2.1 Macromorphology analysis of cladding layer

The cross-sectional macroscopic morphology and dilution rate test results of laser wire feeding cladding layers of different processes are shown in Figure 1. Comparing Scheme 1 and Scheme 2, the dilution rate of two cladding layers is significantly lower than that of one cladding layer. The difference in dilution rate between the edge and middle positions of the cladding layer of Scheme 2 is small, indicating that the two-layer cladding process is conducive to the uniformity of the dilution rate. The thickness of the second layer in the middle position of the cladding layer of Scheme 2 is smaller, while the thickness of the second layer at the edge position is larger. This is because when the laser cladding reaches the edge and the laser power is large, the temperature of the cladding layer accumulates too high when the wire feeding speed is slow, resulting in the collapse of the molten pool and excessive melting of the first layer. It can also be seen from the fusion lines of Scheme 1 and Scheme 2 that more base material is melted near the edge, which is also related to the accumulation of temperature.

Comparing Scheme 2 and Scheme 3, when the laser power is reduced from 3200W to 2600W and the wire feeding speed is increased from 40mm/s to 55mm/s, the dilution rate of the cladding layer of Scheme 3 is significantly reduced, and the difference in dilution rate between the edge and the middle is small and more uniform. The fusion line of Scheme 3 is straighter than that of Scheme 2, indicating that its temperature accumulation effect is smaller and the substrate is melted less at the edge. The thickness of the second layer of Scheme 3 is also more uniform at the edge and the middle position, which also indicates that its temperature accumulation effect is smaller. Considering the dilution rate and the formation of the cladding layer, Scheme 3 is determined to be the optimal process parameter for wire feeding laser cladding.

2.2 Microstructure of cladding layer

The microstructure of the cladding layer of each process scheme was observed, as shown in Figure 2. It can be seen that the microstructures of the wire feeding laser cladding layers of the three process schemes are similar, all of which are hypoeutectic structures composed of white dendritic structure and gray-black intergranular network structure. From the bottom of the cladding layer to the top of the cladding layer, the gray-black intergranular structure gradually increases, and the white dendritic structure gradually decreases. This phenomenon is more obvious in the structure of Scheme 3, because it uses a smaller laser power and a faster wire feeding speed, resulting in a lower dilution rate. From the bottom of the cladding layer to the top of the cladding layer, the grain morphology is roughly divided into four regions: planar crystal region, cellular crystal and columnar crystal region, dendrite region, and equiaxed crystal region.

For the bottom layer of the cladding layer, there is an obvious interface between the cladding layer and the substrate, namely the fusion line, indicating that the cladding layer and the substrate form a good metallurgical bond. At the fusion line, the molten pool is in direct contact with the substrate, the temperature gradient (G) of the solid-liquid interface is large, the solidification rate (R) is small, and the G/R is large, causing the cladding layer crystals to grow in the form of planar crystals. As the solid-liquid interface continues to move upward from the bottom, heat gradually accumulates, the temperature gradient decreases, and the solidification rate increases, that is, the G/R decreases, resulting in a very narrow composition supercooling zone at the front of the solid-liquid interface. The crystal grows along the direction with the fastest cooling rate, and the other growth directions are suppressed, resulting in bulges, forming cellular crystals and columnar crystals.

For the middle of the cladding layer, the solid-liquid interface further moves toward the center of the cladding layer, G/R further decreases, and the crystal gradually changes from coarse columnar crystals to columnar dendrites, which are mainly composed of coarse primary dendrites and fine secondary dendrites. The growth direction of the dendrites depends on the heat flow direction and crystallographic orientation. When the second layer is clad, its thermal cycle acts on the top of the first layer, causing it to partially melt and present the characteristics of co-crystallization with the second cladding layer, and causing its grains to grow. Comparing Figures 2 (a2), (b2) and (c2), the lower laser power and faster wire feeding speed of Scheme 3 make its interlayer gray-black intergranular structure more; and the difference in grain size between the second layer and the first layer is more obvious, with more dendrites. The higher temperature thermal cycle when cladding the second layer causes the grains of the first layer to grow, while the smaller laser power and faster wire feeding speed cause the grains of the second layer to be fine.

For the top of the cladding layer, it is in direct contact with the air, with a faster heat transfer rate, a faster solidification rate, a very small G/R, and the crystals are mainly small, non-directional dendrites and equiaxed crystals. Comparing Figure 2 (a3), (b3) and (c3), Scheme 3 has more gray-black intergranular structures and the structures are obviously finer because of its low laser power and fast wire feeding speed.

The element distribution of the laser wire cladding layer in the longitudinal direction of the optimal process of Scheme 3 is scanned, and the results are shown in Figure 3. It can be seen that from the substrate to the cladding layer, the Co element increases and the Fe element decreases. The element content changes in a step-by-step manner, and the mutation position is at the fusion line and between the first and second layers, indicating that the cladding layer is well fused with the substrate, and cladding two layers helps to reduce the dilution rate of the top. Under Scheme 3, the laser and the welding wire are well coupled, and the latter layer melts less of the previous layer when cladding.

The microstructure of the laser wire feeding cladding layer of the optimal process of Scheme 3 was observed by SEM, as shown in Figure 4. It can be seen that the microstructure of the Stellite6 laser cladding layer is a typical hypoeutectic structure, consisting of a dendritic pro-eutectic structure (A) and a reticular eutectic structure (B). The eutectic structure at the bottom of the cladding layer is less skeleton-like, while the eutectic structure at the top of the cladding layer is more reticular, which is consistent with the structural characteristics under the optical microscope. The typical structural areas (A and B) of the cladding layer were further scanned. The results are shown in Table 3. The dendritic pro-eutectic structure (A) has more Co elements and is a Co-rich solid solution. The reticular eutectic structure (B) has more C, Cr, and W elements. It is speculated that it is mainly eutectic carbides of Cr and W. Hu Xiyun [18] also reported that the structure of the Stellite6 laser cladding layer is a pro-eutectic solid solution and eutectic carbides, which is similar to the situation in this article. Compared with the bottom of the cladding layer, the top of the cladding layer has more eutectic carbide network distribution, indicating that the two-layer cladding process is beneficial to increase the top carbide by reducing the dilution rate.

2.3 Performance of cladding layer

The hardness of the laser wire feeding cladding layer of each scheme was tested in the longitudinal and transverse directions, and the results are shown in Figure 5. As can be seen from Figure 5 (a), the hardness of the substrate 316L is 180-230HV, which is significantly lower than that of the cladding layer. The hardness of the substrate near the fusion line increases slightly. From the substrate to the cladding layer, the hardness increases significantly. From the bottom to the top of the cladding layer, schemes 2 and 3 use a two-layer cladding process, and the hardness shows a step-by-step increase. That is, there is a large change in the hardness of the cladding layer at the position of 1.8-2mm thick, which is consistent with the trend of element changes shown by the EDS line scan in Figure 3, indicating that the two-layer cladding process can improve the hardness of the top of the cladding layer by reducing the dilution rate. Compared with Scheme 2, Scheme 3 has a low laser power and a fast wire feeding speed. The hardness of the second cladding layer is higher, at 450-550HV. The top of the cladding layer of Scheme 3 has more eutectic carbide structures distributed in a network and finer dendrites and equiaxed crystals, which are conducive to improving its hardness. Figure 5 (b) tests the uniformity of the hardness of the cladding layer in the transverse direction. The hardness trend of the cladding layer of each scheme is similar to the test results of the longitudinal hardness. The transverse hardness of the cladding layer of Scheme 3 is between 450 and 550HV, which is relatively uniform. The hardness of the center of the cladding layer is slightly higher than that of the position near the two edges. This is mainly because when cladding to the edge, the accumulation of heat causes the temperature to rise. When cladding at the edge, the melted substrate is slightly more, the dilution rate is slightly higher, and the grains are coarser.

Friction and wear tests were conducted on the 316L substrate and the Stellite6 laser cladding layer of Scheme 1 and Scheme 3. The friction and wear performance of the substrate, one-layer cladding, and two-layer cladding processes were compared and analyzed. The results are shown in Figure 6. Figure 6 (a) shows that the friction coefficients of the three samples all changed dramatically at the beginning, which was the running-in wear stage. This is because the existence of surface roughness and the unevenness of the contact surface at the microscopic level caused the friction coefficient to fluctuate greatly. Subsequently, the friction coefficient tended to stabilize because the microscopic protrusions on the contact surface were smoothed [19]. In the stable wear stage, the friction coefficient of the 316L substrate was 0.47 on average, which was higher than the friction coefficients of one-layer cladding (0.37) and two-layer cladding (0.37). Figure 6 (b) compares and analyzes the weight loss of the three samples before and after wear. The weight loss of the 316L substrate is 11.5 mg, while the weight loss of the Stellite6 wear-resistant layer after cladding one layer is 7.3 mg, which is 36.5% less than that of the substrate; the weight loss of the Stellite6 wear-resistant layer after cladding two layers is 5.3 mg, which is 53.9% less than that of the substrate. Therefore, the Stellite6 laser cladding layer can improve the wear resistance of the 316L substrate, and the wear resistance of the two-layer cladding process is better.

The wear scar morphology of the three samples after friction wear is shown in Figure 7. Figure 7 (a), (d), and (g) show that the wear scar of the 316L substrate is wide and deep; while the wear scar of the Stellite6 laser cladding layer is narrow and shallow, and there are a small number of groove marks distributed macroscopically; the wear scar of the two-layer cladding is narrower and shallower, and there are more groove marks. Figure 7 (c), (f), and (i) show the transverse morphological profiles of the wear scars of the three samples. The wear width of the 316L substrate is 1380 μm, and the maximum wear depth is 41.5 μm; while the wear width of the cladding layer is 867.5 μm, which is much smaller than the substrate, and the maximum wear depth is 43.8 μm, which is equivalent to the substrate; the wear width of the two-layer cladding is 750 μm, which is the smallest, and the maximum wear depth is 29.0 μm, which is also the smallest. Therefore, the Stellite6 laser cladding layer can reduce the wear width and depth of the 316L substrate, and the two-layer cladding process can reduce it even more.

SEM was further used to characterize the wear morphology of the three samples, as shown in Figure 8. Figure 8 (a), (c), and (e) show that the groove feature of the Stellite6 laser cladding layer is more obvious than that of the 316L substrate. The wear scar edge of the 316L substrate can be clearly observed to be extruded and accumulated due to plastic deformation during wear; the extrusion phenomenon at the wear scar edge of the cladding layer is more minor, and the wear scar edge of the cladding layer is relatively smooth. The EDS line scanning results show that there is a protrusion in the O content at the wear site of the 316L substrate, indicating that the heat generated by friction and wear has caused it to oxidize, while the O content of the wear surface of the cladding layer and the cladding layer has little change, indicating that the degree of oxidation during wear is small. Further enlarging the wear morphology, as shown in Figure 8 (b), (d), and (f), the plastic deformation, cracking, shedding of the material, and the adhesive wear phenomenon of the material forming wear debris can be observed from the 316L substrate. This is because the 316L matrix is relatively soft. When the shear force during friction and wear is greater than the yield strength of 316L, the matrix will undergo plastic deformation, forming tiny holes and microcracks. As the wear time increases, the microcracks expand, and the matrix falls off in layers. A small amount of plowing grooves can be observed in the wear morphology, indicating that the wear form of the 316L matrix is mainly adhesive wear and a small amount of abrasive wear. Wang et al. [20] also reported similar wear morphology characteristics of 316L stainless steel. The plastic deformation phenomenon of the wear morphology of the cladding layer is less, mainly a large amount of plowing groove morphology, indicating that its wear form is mainly abrasive wear and a small amount of adhesive wear. Stellite6 laser cladding layer contains solid solution of Cr, W and other elements, which improves the hardness and deformation resistance of the cladding layer. The network eutectic carbides existing between the grains act as hard points, further hindering the deformation of the matrix. During wear, the hard carbides partially fall off due to stress concentration to form pits. The fallen hard particles act as abrasives, causing the cladding layer to form a furrow morphology. The wear morphology of the two cladding layers is similar to that of the one cladding layer. The plastic deformation phenomenon is further reduced, and its wear form is consistent with that of the one cladding layer.

۳ نتیجه‌گیری

• Stellite6 alloy was clad on the surface of 316L stainless steel using a two-layer laser wire cladding process. By adjusting the process parameters, a cladding layer of about 3 mm thick with good formation and no defects such as inclusions and cracks can be obtained. The dilution rate at the center and edge of the cladding layer is relatively uniform and is less than 5%.
• The microstructure of the laser wire feeding cladding layer is a hypoeutectic structure, namely, a dendritic Co-rich pre-eutectic solid solution and a network of Cr and W eutectic carbides. The eutectic carbides are less at the bottom of the cladding layer and more at the top. Smaller laser power and faster wire feeding speed will lead to the formation of more network eutectic carbides at the top of the cladding layer. (3) From the substrate to the first cladding layer and then to the second cladding layer, the hardness of the Stellite6 wire feeding laser cladding layer increases in a step-like manner in the longitudinal direction and finally stabilizes after a thickness of 1.8 mm; the hardness of the cladding layer is relatively uniform in the transverse direction near the surface, ranging from 450 to 550 HV, and the hardness of the center is slightly higher than that of the edge, which is related to the heat accumulation during cladding. (4) The wear resistance of the laser wire feeding cladding layer was evaluated by friction and wear test. The results show that the wear mode of the cladding layer changes from the adhesive wear of the 316L substrate to the abrasive wear due to the solid solution of Cr, W and other elements and the presence of a large number of network hard carbides, which significantly improves the wear resistance of the substrate. The friction coefficient of the cladding surface with two layers of cladding is 0.37, and the weight loss is reduced by 53.9% compared with the substrate.