In order to improve the decomposition and uneven distribution of WC reinforced metal matrix composite coatings during laser cladding, WC/316L composite coatings with 50% WC content were prepared by laser cladding technology. The microstructure, hardness and friction and wear properties of the cladding layer were characterized by scanning electron microscopy (SEM), micro Vickers hardness tester and friction and wear tester. The results show that low heat input can prevent WC particles from sinking and improve the uniformity of WC particle distribution in the thickness direction of the cladding layer; however, the plane distribution uniformity of WC is poor, and low heat input leads to weak bonding of WC particles, which is easy to fall off; during the cladding process, 15% of WC in the powder decomposes. Studies have shown that by reducing the laser heat input, a composite coating with relatively uniform WC thickness direction can be prepared, and the wear resistance of the cladding layer can be improved, but the uniformity of WC plane distribution and the bonding performance of WC particles need to be optimized.
With the development of industries such as petrochemicals, electric power, and aerospace, the service environment has higher and higher requirements for the wear resistance of structural parts. For example, roller drill bits are important components of oil and mining drilling equipment. Dissolved salts and solid particles in the well will cause roller drill bits to wear and break, and the cost of roller drill bits accounts for about 40% of the drilling cost. Therefore, using surface technology to prepare high-hardness and wear-resistant coatings on the surface of components is of great significance to improving drilling efficiency and component life and reducing costs.
WC-reinforced metal-based composite coatings are widely used as wear-resistant parts coatings due to their high hardness and high wear resistance, and good wettability with Fe-based, Ni-based, and Co-based coatings. The main preparation methods of WC-reinforced metal-based composite coatings include thermal spraying and laser cladding. Laser cladding has the advantages of low dilution rate and metallurgical bonding between the substrate and the coating. It is widely used in petrochemical, electric power and other industries. During the laser cladding process, secondary compounds such as W2C and Fe3W3C can serve as nucleation centers, increase the nucleation rate, promote the formation of secondary dendrites, and improve the hardness of the cladding layer. W, C and Ni, Cr and other elements decomposed by WC at high temperature are easy to form new phases with high hardness, which play a role of dispersion strengthening. However, WC is easy to form CO and CO2 when it decomposes at high temperature, forming pores in the cladding layer. In addition, due to the difference in linear expansion coefficient between WC and metal-based parent phase, when the WC content increases to a certain amount, defects such as longitudinal cracks will appear in the cladding layer. When the WC mass fraction is 20%, no cracks appear in the cladding layer, and the friction performance is good. When the WC mass fraction is 20%, the hardness of the cladding layer is low, and there is still room for improvement. However, when the WC content continues to increase, the number of WC particles sinking increases, and the hard particles come into contact, affecting the continuity of the cladding layer structure [15-16]. This will not only deteriorate the uniformity of WC particle distribution, but also cause microcracks inside the cladding layer, increase the number of weakly bonded WC particles, and reduce the wear resistance of the cladding layer.
Studies have shown that adding auxiliary fields, such as induction heating, magnetic fields, mechanical vibrations, and ultrasonic vibrations, can improve the uniformity of WC particle distribution. Although induction heating can improve uniformity to a certain extent, the force can easily cause cracks in the coating. The external electromagnetic field can not only reduce the amount of WC decomposition but also improve the uniformity of WC distribution, but it will increase the temperature of the molten pool and reduce the nucleation rate, resulting in coarse grains in the coating and affecting the mechanical properties of the coating. Under the action of mechanical vibration, the uniformity of WC is improved, and defects such as pores and cracks in the coating are significantly reduced, but mechanical vibration can easily cause poor organization and looseness. The use of ultrasonic auxiliary field can make the organization structure uniform and improve the uniformity of WC, but the ultrasonic thermal effect will increase the amount of WC decomposition.
In summary, external field assistance can improve the performance of the coating and the uniformity of WC distribution to a certain extent, but there are still problems that have not been solved. Studies have shown that low heat input can increase the viscosity of the molten pool and reduce WC decomposition. The increase in viscosity leads to an increase in the buoyancy of the molten pool and the fast solidification speed of auxiliary laser cladding, which can effectively alleviate the problem of WC sinking and reduce the probability of cracks and pores.
Based on this, this study prepared WC/316L composite cladding layer by controlling the heat input, studied the dissolution and distribution characteristics of WC at low heat input, and the influence of high WC content on cracks and pores, and explored the hardness and friction and wear properties of the cladding layer.
1 Experimental method
The substrate was Q235 steel with a size of 150 mm×150 mm×13 mm, and sandblasting was used to remove rust before cladding. The cladding layer material was a mixed powder of 316L and WC in a ratio of 1:1. The two powders were mixed for 10 min by mechanical mixing until they were evenly mixed, and then the mixed powder was fed into the powder feeder. Figure 1 shows the microscopic morphology of 316L powder. 316L is a solid powder with an irregular
morphology and a size of 45 ~75 μm. WC is a solid powder with a spherical shape and a size of 50 ~100 μm.
The mixed powder was dried at 120 ℃ for more than 1 h before cladding. The laser cladding parameters are shown in Table 1. After cladding, the cladding layer was cut into metallographic specimens of 20 mm × 20 mm. The specimens were polished using different types of sandpaper (180#, 400#, 800#, 1 000#, 1 500#, 2 000#), and then polished using diamond polishing liquid with a particle size of 1.5 μm. The structure, composition, and friction and wear surface morphology and composition of the cladding layer were analyzed using a JSM-6390A scanning electron microscope and an energy spectrum analyzer. The hardness of the cladding layer was tested using a cross-section with a load of 300 g and a loading time of 10 s. The surface of the cladding layer was taken as the initial position, and the hardness was tested every 200 μm until the hardness of the three test points on the substrate side was the same. The friction and wear properties of the cladding layer were tested by a pin-disk friction pair. The friction pair was GCr15. The size of the cladding layer wear specimen was Φ4.8 mm × 12.7 mm. The loading force was 50 N. The test time was 5 min and the rotation speed was 100 r/min.
2 Results and analysis
2.1 Morphology and composition of the cladding layer
Figure 2 shows the macroscopic morphology of the laser cladding layer. The surface of the cladding layer has a metallic luster. Literature studies have shown that when WC coatings are prepared by laser cladding, cracks and pores are prone to appear on the coating [9,13,23]. Based on the macroscopic morphology of Figure 2, it can be seen that no obvious cracks and pores appeared on the surface of the cladding layer in this study. It can be preliminarily judged that low heat input can reduce the number of pores and cracks to a certain extent and improve the density of the cladding layer.
Figure 3 shows the cross-sectional morphology of the cladding layer. The white contrast in the figure is WC particles, and the gray contrast is 316L. When the mass fraction of WC is 50%, there are no obvious pores and cracks in the cladding layer. Compared with the obvious sinking of WC with high heat input, the WC particles in the cladding layer prepared in this study did not sink significantly. The local black circular defects in Figure 3 are pits left by the shedding of WC particles during the preparation of the metallographic sample, not pores during the preparation process. The cladding parameters in this study determine that the heat input is low, and the bonding between some WC particles and 316L is weak. During the metallographic preparation process, the weakly bonded WC particles peel off, forming the pits shown in Figure 3. The peeling of weakly bonded hard particles can easily cause abrasive wear, aggravating the wear of the cladding layer and the counter-abrasive material. In addition, weakly bonded WC particles will cause stress concentration, which may lead to fatigue wear during friction and wear. Therefore, when the heat input is low, no problems such as microcracks occur, but weak bonding defects will still occur in the cladding layer.
During the laser cladding process, WC usually decomposes. Under the condition of low heat input in this study, the WC particles also decomposed, but a large number of WC hard particles remained in the coating. The mass fraction of large WC particles in the cladding layer is about 35%. Therefore, about 15% of WC decomposes during the preparation of the cladding layer. Since WC and 316L particles are mechanically mixed, the mixed powder is not evenly fed under the influence of factors such as density, gravity, buoyancy and drag force, resulting in uneven distribution of large WC particles in the cladding layer. However, the metal solution with higher viscosity improves the buoyancy of WC to a certain extent, which is conducive to alleviating the sinking problem. Therefore, the large WC particles in the cladding layer of this study did not sink significantly, and were distributed more evenly along the thickness direction of the coating, but the uniformity of powder feeding caused the uneven distribution of WC along the plane.
After entering the molten pool, WC will metallurgically combine with liquid Fe, Cr, etc., and new products will be generated in the cladding layer. The reaction products will affect the organizational structure of the cladding layer. Figure 4 shows the morphology of the cladding layer and the interface characteristics of the WC and 316L combination area. The energy spectrum analysis results at different positions of the cladding layer are shown in Figure 5 and Table 2. Based on Figures 4 and 5, most of WC and 316L form a metallurgical bond, and new phases are generated in the cladding layer in addition to 316L and WC. From the results in Table 2 and Figure 5, it can be seen that white contrast 2 is the original WC hard particles, and gray contrast 1 and black contrast 3 are new phases generated by 316L after WC decomposition. Based on the chemical composition, they are W2C and Fe3W3C, respectively. During the cladding process, undecomposed WC, W2C, and Fe3W3C can serve as nucleation centers to promote the formation of secondary dendrites and improve the hardness of the cladding layer. In addition, WC hard particles will produce fine grain strengthening, solid solution strengthening, and dispersion strengthening due to the expansion coefficient of the matrix, grain size, dislocation movement, and distribution of WC. WC can effectively improve the hardness and wear resistance of the composite material to a certain extent, but WC particles will cause a splitting effect on the matrix. In addition, as shown in Figure 3, the shedding of weakly bonded WC particles will greatly reduce the wear resistance of the cladding layer and cause abrasive wear. Therefore, low input can improve the uniformity of WC distribution, but it will affect the bonding strength of some WC particles.
2.2 Hardness of cladding layer
The cross-sectional microhardness distribution of the cladding layer is shown in Figure 6. Due to the low laser heat input, the cladding layer has no obvious transition zone. The hardness of the cladding layer near the surface area is about 580HV0.3, and the hardness near the substrate is about 640HV0.3. From the surface to the interface, the hardness of the cladding layer gradually increases, which is mainly caused by the sinking of WC during the cladding process. Therefore, when the heat input is low, the cross-sectional morphology shows that the WC is more evenly distributed in the depth direction, which greatly improves the distribution uniformity of large WC particles, but the problem of WC sinking still exists.
2.3 Friction and wear results
The friction and wear results show that when the composite coating is worn against GCr15, the friction coefficient is 0.49±0.04, and the wear rate of the cladding layer is about 0.35 mg/(N·min). Since the heat input is still high and the hard phase is melted, reducing the heat input is a feasible way to further improve the hardness and wear resistance of the cladding layer. Increasing the scanning rate can reduce the heat input, but this method will increase the defect density of the cladding layer. From the cross-sectional structure of the cladding layer, it can be seen that the metal phase can be completely melted at a laser power of 900 W and form a dense cladding layer, but WC still decomposes. Continuing to reduce the laser power can reduce the heat input and further reduce the decomposition of WC, but it is easy to cause the problem of weak bonding of WC particles. It is necessary to further adjust the relationship between the heat input and the powder feeding rate to improve the bonding strength of the weakly bonded WC particles.
Figure 7 shows the wear morphology of the cladding layer. As can be seen from Figure 7, the surface of the sample presents an obvious plowing morphology, which is a typical abrasive wear morphology. Under the cladding conditions of this study, due to the high WC content and low heat input of the laser, a certain number of weakly bonded WC particles exist in the cladding layer. During the friction and wear process, WC particles fall off to form a three-body wear of the cladding layer-hard particles-abrasive sample. The hard particles cut and squeeze the cladding layer, causing wear. Some literature shows that weakly bonded particles may cause fatigue wear, but no fatigue wear morphology is found in the figure. Although there is a certain amount of weak bonding in the cladding layer prepared under the conditions of this study, the WC particles have not fallen off to form microcracks. Therefore, the cladding layer mainly suffers from abrasive wear.
3 Conclusion
(1) Low heat input can improve the uniformity of WC particle distribution in the thickness direction of the cladding layer, but there are problems with the uniformity of WC particle plane distribution and weak bonding. During the cladding process, 15% of the WC in the powder decomposes.
(2) When the WC content of the 316L cladding layer is 50%, the hardness of the cladding layer is higher than 580 HV0.3, which is 2 times higher than that of the substrate.
(3) Under the friction and wear conditions of this study, the cladding layer is mainly abrasive wear, with a friction coefficient of about 0.49 and a wear rate of about 0.35 mg/(N·min).
(4) By reducing the heat input of laser cladding, a composite coating with a relatively uniform WC thickness direction can be prepared, which can improve the wear resistance of the cladding layer, but it is necessary to optimize the uniformity of WC plane distribution and weak bonding problems.
