In order to improve the self-lubricating property and wear resistance of tin-based babbitt alloy, copper-coated graphite (Cu-Gr) powder was mixed with tin-based babbitt alloy powder, and a tin-based babbitt alloy self-lubricating composite cladding layer was prepared on the surface of 20 steel substrate by laser cladding method. The results show that after laser cladding, the graphite wrapped by copper still exists in the composite cladding layer in the form of a single substance; the Cu-Gr/Babbitt alloy composite cladding layer is uniform and dense, and there are no defects such as cracks and pores on the surface. The cladding layer is mainly composed of α-Sn solid solution phase, hard particle phase SnSb and Cu6Sn5 phase. Due to the addition of copper-coated graphite, the structure of the composite cladding layer is significantly refined. The microhardness of the composite cladding layer is about 43.19HV, which is 27% higher than that of the babbitt alloy cladding layer. Due to the increase in microhardness and the self-lubricating properties of graphite, the friction coefficient and wear rate of the composite cladding layer have decreased, which are 0.359 and 1.36×10-6 mm/(N-1·m-1), respectively. The addition of Cu-Gr can effectively improve the self-lubricating properties and wear resistance of Babbitt alloy.

Tin-based Babbitt alloy has good friction reduction, embedding, compliance and other advantages, and is widely used in key components such as machine tool guide rails. Machine tool guide rails manufactured by traditional processes such as centrifugal casting are prone to severe wear and failure when facing harsh working conditions such as heavy loads and high wear. In order to improve the performance of tin-based Babbitt alloy, the use of surface modification technology to prepare tin-based Babbitt alloy cladding layer is a simple and effective method.

Laser cladding is an advanced surface modification technology with the advantages of low heat input and low dilution rate. The coating can achieve a strong metallurgical bond with the substrate, and has realized a variety of effective solutions in the preparation of multifunctional coatings. In recent years, the preparation of babbitt alloy coatings on the surface of steel substrates by laser cladding has attracted wide attention due to its high bonding strength and good wear resistance. Hao Yunbo et al. prepared babbitt alloy cladding layers on the surface of 20 steel by laser cladding technology and studied the microstructure and bonding strength of babbitt alloys. The results showed that compared with the traditional process, laser cladding of babbitt alloys improved the bonding strength between the substrate and the coating. Xu Tongzhou et al. prepared babbitt alloy cladding layers with nickel transition on the surface of 45 steel substrates by laser cladding technology. The results showed that the diffusion of nickel refined the grains in the cladding layer and improved the wear resistance of babbitt alloys. However, no related research on laser cladding of tin-based babbitt alloy self-lubricating wear-resistant and friction-reducing coatings has been found in the open literature.

The preparation of self-lubricating wear-resistant coatings by laser cladding technology is an emerging technology that introduces self-lubricating materials such as graphite, WS2, MoS2, Ag, etc. into the cladding layer to improve the wear resistance and friction reduction of the cladding layer. However, these self-lubricating materials are prone to react with other elements or decompose after being irradiated by laser, and the self-lubricating effect is poor. Therefore, the doping ratio and process parameters of the self-lubricating materials must be strictly controlled. Graphite has excellent self-lubricating properties, a high melting point, and a low thermal expansion coefficient, and is an excellent self-lubricating material. By modifying the graphite surface and depositing an electroplating layer on the graphite surface, the graphite can be effectively prevented from carbonizing during the laser cladding process. Liu et al. used laser cladding technology to prepare a nickel/graphite self-lubricating composite coating on the surface of 42CrMo, and improved the coating performance by adjusting the scanning rate. The results showed that when the scanning rate was below 300mm/min, graphite improved the hardness and wear resistance of the coating. Raghuram et al. prepared Fe-Ni/Gr composite coating on the surface of low carbon steel by prefabricated powder method. The study showed that the addition of graphite can significantly reduce the wear of the coating. With the increase of graphite content, the hardness and wear resistance of the coating are improved.

However, the report of preparing laser cladding babbitt alloy cladding layer with graphite as doping material is extremely rare. The primary reason is that the wettability of graphite to babbitt alloy is poor and it cannot form a good metallurgical bond with babbitt alloy. Secondly, the density difference between the two is large, which easily leads to the aggregation of graphite and reduces the toughness of babbitt alloy. The above problems can be effectively avoided by preparing copper plating on the graphite surface. In this paper, copper-plated graphite is used as a self-lubricating material. The wettability of graphite and babbitt alloy is improved by the transition of copper. The laser cladding babbitt alloy/copper-plated graphite self-lubricating cladding layer is prepared on the surface of 20 steel substrate. The effect of graphite on the microstructure, graphite distribution and morphology, microhardness, wear behavior and wear mechanism of the cladding layer is systematically studied. The research results can provide theoretical basis and technical support for the industrial production of laser cladding babbitt alloy self-lubricating cladding layer.

1 Experiment and method

1.1 Experimental materials

20 steel is used as the base material, and the base material size is 100mm ×100mm ×10mm. The base material surface is polished with sandpaper to remove the oxide scale on the base material surface. The polished base material is ultrasonically cleaned in ethanol for 15min. SnSbCu11-6 babbitt alloy powder with a powder size of 100 mesh is selected as the base powder of the cladding layer, and the chemical composition is shown in Table 1. Copper-plated graphite powder is used as the self-lubricating reinforcement phase, and the copper-plated graphite particle size range is 100 mesh (mass fraction: Cu60%, C40%). 3% (mass fraction) copper-coated graphite powder and 97% (mass fraction) babbitt alloy powder were uniformly mixed in a ball mill at a speed of 100 r/min for 100 min, so that the copper-coated graphite was evenly dispersed in the babbitt alloy powder. Subsequently, the composite powder was kept at 50°C in a vacuum drying oven for 3 h to remove the moisture in the composite powder.

1.2 Experimental equipment

The laser cladding test was carried out on a Trudiode 3006 fiber laser. The maximum output power of the laser is 3000 W. In order to prevent oxidation of the molten pool during the cladding process, argon gas was used to protect the molten pool. The DPSF-2 powder feeding system and coaxial powder feeding laser head were used to transport the powder into the molten pool through high-purity argon gas. The laser cladding process parameters are: laser power 800 W, spot diameter 3 mm, scanning rate 1000 mm/min, powder feeding rate 28 g/min, argon flow rate 15 L/min, overlap rate 50%. These process parameters are obtained from the preliminary cladding test to ensure that the copper-plated graphite does not carbonize during the cladding process and is evenly distributed in the cladding layer.

Metallographic specimens were cut from the cladding samples using a wire cutting machine. The specimen size was 10 mm×10 mm×10 mm. The metallographic specimens were prepared by standard grinding and polishing procedures, and the specimens were corroded by 4% (volume fraction) nitric acid-alcohol solution for 15 seconds. The phase composition of the cladding layer was analyzed by X-ray diffractometer (XRD, Rigaqu, D/max2500Pc, To-kyo, JAPAN), using Cu-Ka radiation, scanning speed of 4°/min, scanning angle of 10°~80°, tube voltage of 40kV, and tube current of 15mA. The microstructure of the cladding layer was characterized by S3400 scanning electron microscope (SEM, S-3400, Hitachi, Gemini300Zeiss) equipped with energy dispersive spectrometer (EDS). The acceleration voltage was 20kV and the beam spot was 1μm.

The microhardness of the cladding layer surface was tested by using a HVS-1000 Vickers microhardness tester with a test load of 1N and a loading time of 15s. The test spacing was 500μm. Each sample was measured 10 times and the average value was taken to reduce the error generated during the test. The T-1000 high temperature friction and wear tester was used to carry out room temperature dry sliding friction and wear tests on the surface of the Babbitt alloy cladding layer and the Cu-Gr/Babbitt alloy cladding layer. The surface of the test sample was polished before the wear test to eliminate the influence of the sample surface roughness on the wear performance. The GCr15 steel ball with a hardness of 60 HRCC was used as the friction pair for the test. The load was 3 N, the wear scar radius was 3 mm, the rotation speed was 200 r/min, and the wear time was 20 min. After the friction and wear test, the wear morphology was observed by scanning electron microscopy, and the wear rate of the two cladding layers was calculated by formula (1). W = V/S×L (1)

Where: W is the wear rate; V is the wear volume; S is the wear length; L is the applied load.

2 Analysis and discussion

2.1 Macromorphology of cladding layer

Figure 1 shows the macromorphology of Babbitt alloy cladding layer and Cu-Gr/Babbitt alloy cladding layer respectively. As can be seen from Figure 1, the two cladding layers are uniform and dense, without defects such as pores and cracks. In addition, in Figure 1 (b), a small amount of black particles are evenly distributed in the cladding layer, indicating that the graphite particles are not completely burned during the laser cladding process.

2.2 Phase composition of cladding layer

Figure 2 shows the X-ray diffraction spectrum of the two babbitt alloy cladding layers. In the Cu-Gr/Babbitt alloy composite cladding layer, due to the low content of added Cu-Gr, no diffraction peak corresponding to graphite is found. The phase composition of the two cladding layers is α-Sn, SnSb, and Cu6Sn5 phases. According to the Sn-Sb-Cu ternary alloy phase diagram, during the solidification process, the Cu6Sn5 phase first precipitates from the liquid phase, followed by the intermetallic compound SnSb phase, and finally the α-Sn phase. Part of Sb and Cu dissolve in Sn to form a tin-based solid solution, and the other part of Sb, Cu and Sn form intermetallic compounds SnSb and Cu6Sn5.

Figure 3 shows the microstructure of the Babbitt alloy cladding layer and the Cu-Gr/Babbitt alloy cladding layer under an optical microscope. In Figure 3 (a), there are rhombic precipitates and fine granular precipitates in the microstructure of the Babbitt alloy cladding layer, and these precipitates are evenly dispersed in the cladding layer. The rhombic precipitates are SnSb phase, the fine granular precipitates are Cu6Sn5 phase, and the black matrix is ​​α-Sn solid solution. Among them, SnSb phase and Cu6Sn5 phase are hard particle phases, and α-Sn phase is soft matrix phase. Figure 3 (b) shows the microstructure of Cu-Gr/Babbitt alloy cladding layer. Combined with the X-ray diffraction results, the phase composition of the two cladding layers has not changed significantly. The white rhombic precipitates are SnSb phase, the fine particles are Cu6Sn5 phase, and the black matrix is ​​α-Sn solid solution. Compared with the Babbitt alloy cladding layer, the microstructure of the composite cladding layer is significantly refined, which is closely related to the addition of Cu-Gr. However, since the color of α-Sn solid solution is similar to that of graphite after corrosion, the presence of graphite was not observed under the metallographic microscope.

The SEM image and EDS element distribution diagram of the Cu-Gr/Babbitt alloy cladding layer are shown in Figure 4. Among them, the area where only the Sn element is enriched is the solid solution α-Sn phase, the area where the Sn and Sb elements are enriched together is the SnSb phase, and the area where the Cu and Sn elements are enriched together is the Cu6Sn5 phase. In addition, it can be clearly seen in Figure 4 that graphite is evenly distributed in the cladding layer, such as the black particles shown in Figures 4 (a) and (b). It can be seen from the EDS spectrum that the carbon element is mainly distributed in the dark blue area, from which it can be inferred that the dark blue area is graphite. Since the melting point of Cu (1083 ℃) is much higher than that of Babbitt alloy (340 ℃), copper-coated graphite absorbs a lot of heat during the melting process, which can effectively reduce the burning and carbonization of graphite particles during laser cladding. In addition, the rod-like structure is mainly located around the graphite particles. This is because the graphite plays a non-uniform nucleation role during the solidification process, and the precipitated phase with a higher melting point preferentially nucleates near the graphite. In addition, due to the non-uniform nucleation effect of graphite, the grain size of the cladding layer is significantly refined.

The morphology of the bonding zone of the Babbitt alloy cladding layer and the Cu-Gr/Babbitt alloy cladding layer is shown in Figure 5. Both cladding layers form a relatively flat metallurgical bonding line near the bonding zone, and a good metallurgical bonding is formed between the cladding layer and the substrate.

2.3 Microhardness of cladding layer

Figure 6 shows the surface microhardness of Babbitt alloy cladding layer and Cu-Gr/Babbitt alloy cladding layer. The results show that the microhardness of Babbitt alloy cladding layer and Cu-Gr/Babbitt alloy cladding layer are 32.4HV and 41.3HV respectively, and the microhardness of composite cladding layer is significantly improved by about 27%. Combining the microstructures of the two cladding layers, it is believed that the improvement of the microhardness of Cu-Gr/Babbitt alloy cladding layer can be attributed to the synergistic effect of dispersion strengthening and fine grain strengthening. The synergistic strengthening effect of the microhardness of the composite cladding layer can be expressed as: Formula (2), where: H0 is the average hardness of the Babbitt alloy cladding layer; HO is the hardness increment caused by the dispersion strengthening of SnSb and Cu6Sn5 phases uniformly distributed in the cladding layer; Hg is the hardness increment caused by fine grain strengthening.

Combined with Figure 3, it can be seen that the addition of Cu-Gr significantly refines the grains in the Babbitt alloy cladding layer. Grain refinement can increase the number of grain boundaries, thereby causing grain boundary strengthening. Grain refinement strengthening can be described by the classic Hall-Pech relationship: Formula (3), where: H0, k are constants; d is the average grain size. The hardness of the material is inversely proportional to the grain size, as shown in Formula (3). The reason for the increase in hardness due to grain refinement is the increase in grain boundaries, which play a hindering role in the movement of dislocations.

Another strengthening mechanism of the composite cladding layer is that the addition of Cu-Gr leads to an increase in the number of second phase particles in the composite cladding layer. The dispersion strengthening mechanism of the composite cladding layer can be analyzed according to the classical Orowan theory: Formula (4), where: G is the matrix shear modulus; is the average diameter of the increased particles; is the volume fraction of the increased particles; b is a constant. It can be seen from Formula (4) that the fine particle radius and the increased volume fraction help to improve the hardness of the material. From the microstructure of the coating shown in Figure 3, it can be seen that the precipitated phase in the composite cladding layer is finer, the volume fraction of the Cu6Sn5 phase in the cladding layer is significantly increased, and the dispersion strengthening effect of the composite coating is more significant. Therefore, the improvement of the microhardness of the composite cladding layer can be attributed to the synergistic effect of the grain boundary strengthening effect caused by grain refinement and the dispersion strengthening effect caused by the second particle increase.

2.4 Analysis of tribological properties of cladding layer

The time-varying curves of the dry sliding friction coefficient of the two cladding layers are shown in Figure 7. The average friction coefficients of the Babbitt alloy cladding layer and the Cu-Gr/Babbitt alloy cladding layer are 0.525 and 0.359, respectively. In the early stage of wear, the normal load causes severe plastic deformation on the surface of the cladding layer, which increases the surface roughness of the coating and causes the friction coefficient of the cladding layer to increase rapidly. As the wear intensifies, the friction coefficient of the composite cladding layer decreases and remains relatively stable. This is because the second strengthening and fine grain strengthening improve the deformation resistance of the composite cladding layer. In addition, the interlayer slip characteristics of graphite make the composite cladding layer self-lubricating and can also reduce the friction coefficient of the cladding layer. It is worth noting that the friction coefficient of the Babbitt alloy cladding layer is higher and the fluctuation is larger during the wear process. This is because the Babbitt alloy cladding layer lacks self-lubricating properties. In addition, the Babbitt alloy cladding layer has low hardness and poor ability to resist deformation, which increases the roughness of the cladding surface and leads to an increase in the friction coefficient.

The wear volume and wear rate of the Babbitt alloy cladding layer and the Cu-Gr/Babbitt alloy composite cladding layer were calculated by formula (1), and the results are shown in Figure 8. The wear volume and wear rate of the Babbitt alloy cladding layer and the Cu-Gr/Babbitt alloy composite cladding layer are 8.32×10-6 mm³, 2.61×10-6 mm³/(N-1·m-1) and 4.54×10-6 mm³, 1.36×10-6 mm³/(N-1·m-1), respectively. The wear volume and wear rate of the composite cladding layer are about 50% of those of the Babbitt alloy cladding layer, which directly indicates that the composite cladding layer has better wear resistance than the Babbitt alloy cladding layer.

In order to reveal the wear mechanism of the two cladding layers, the wear surface of the cladding layer was characterized by scanning electron microscopy. Figure 9 (a) and (b) show the wear scar morphology of the Babbitt alloy cladding layer. It can be clearly seen in Figure 9 that due to the low hardness of the Babbitt alloy cladding layer, deep furrows and severe plastic deformation appear on the wear scar surface, accompanied by pitting. When the cladding layer is subjected to normal load, the friction pair and the cladding layer generate tangential friction, causing the soft matrix α-Sn to deform and sink downward, and the hard point particles SnSb and Cu6Sn5 to protrude, playing a role in resisting deformation. As the wear intensifies, the protruding hard point particles fall off, and the fallen hard point particles will re-enter the wear area, resulting in three-body wear, thereby intensifying the wear, deformation and shedding of the cladding layer. In addition, due to the continuous action of shear stress, the soft matrix α-Sn and the friction pair undergo cold welding, causing the soft matrix to fall off and adhere to the surface of the friction pair. As the wear intensifies, the adhered soft matrix gradually accumulates on both sides of the wear scar to form a transfer layer. The wear mechanism of the Babbitt alloy cladding layer is abrasive wear and adhesive wear.

The wear scar morphology of Cu-Gr/Babbitt alloy composite cladding layer is shown in Figure 9 (d) and (f). The grooves of the composite coating are shallow and sparse, and the plastic deformation is weakened. The wear mechanism of the composite coating is mainly abrasive wear. The improvement of the wear resistance of the composite coating is due to the improvement of the hardness and deformation resistance of the composite coating, which is consistent with the Arcchard law that the wear resistance of the material is positively correlated with the hardness. Compared with the Babbitt alloy cladding layer, the composite cladding layer has an increased volume fraction of hard particles in the composite cladding layer due to the grain refinement and the second precipitation strengthening, thereby improving the deformation resistance of the composite cladding layer. In addition, with the intensification of wear, the graphite particles dispersed in the cladding layer fall off and enter the wear zone, forming three-body wear. Due to the interlayer slip characteristics of graphite, graphite particles form a solid lubricating film during the wear process, adhere to the surface of the cladding layer, and reduce the friction coefficient of the cladding layer. EDS point test was performed on the point 1 area of ​​Figure 9 (d), and the element test results are shown in Figure 9 (f). In addition to Sn, Sb, and Cu elements, there are also a lot of C elements. Therefore, it can be proved that graphite particles form a lubricating film on the surface of the cladding layer during the wear process, which reduces the friction coefficient between the friction pair and the cladding layer and improves the self-lubricating properties and wear resistance of the cladding layer. In addition, the lubricating film formed by graphite particles hinders the cold welding between the soft matrix α-Sn phase and the friction pair, thereby avoiding the occurrence of adhesive wear. Therefore, the addition of Cu-Gr can make the composite cladding layer have higher microhardness and self-lubricating properties, thereby improving the wear resistance of the composite cladding layer.

Conclusion

Babbitt alloy cladding layer and Cu-Gr/Babbitt alloy composite cladding layer were prepared on the surface of 20 steel substrate by laser cladding technology. The phase composition, microstructure, microhardness and wear mechanism of Cu-Gr cladding layer were studied in depth, and the following conclusions were drawn:

(1) The Babbitt alloy cladding layer and Cu-Gr/Babbitt alloy composite cladding layer have uniform and dense structure without defects such as pores and cracks. Graphite exists in the composite cladding layer in the form of a single substance. Due to the heterogeneous nucleation characteristics, the grains of the composite cladding layer are significantly refined.

(2) The two coatings are mainly composed of α-Sn solid solution phase, Cu6Sn5 and SnSb hard point phase. Among them, the Cu6Sn5 phase in the composite cladding layer increases and the α-Sn phase decreases.

(3) The microhardness of the babbitt alloy cladding layer and the Cu-Gr/Babbitt alloy composite cladding layer are 32.4 HV and 41.3 HV, respectively. The microhardness of the composite cladding layer is significantly increased by about 27%, which can be attributed to the fine grain strengthening and second phase strengthening caused by the addition of graphite.

• The self-lubricating property of graphite significantly reduces the friction coefficient (0.359) and wear rate of the composite cladding layer [1.36×10-6mm3/(N-1m-1)], which is significantly lower than the friction coefficient (0.525) and wear rate [4.54×10-6mm3/(N-1·m-1)] of the babbitt alloy cladding layer. In addition, the presence of graphite lubricating film effectively avoids the occurrence of adhesive wear, and the wear mechanism of the composite coating is mainly abrasive wear.