In order to study the effect of laser power on the microstructure and properties of tin-based babbitt alloy cladding layer, tin-based babbitt alloy cladding layer was prepared on the surface of 20 steel using laser powers of 800 W, 1000 W, and 1200 W. The microstructure, bonding zone morphology, and tribological properties of the cladding layer were studied using metallographic microscope (OW), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffractometer (XRD), and friction and wear tester. The results show that with the increase of laser power, the temperature of the molten pool increases, the cooling rate decreases, and the particle size of the hard point particles SnSb gradually increases with the increase of laser power. When the laser power is low, the SnSb particles are small and evenly distributed. With the increase of laser power, the size of the SnSb phase increases and the number decreases, which reduces the hardness and wear resistance of the cladding layer. When the laser power is 800 W, the microhardness of the cladding layer is the largest, which is 35.7HV, and the average friction coefficient is 0.257. The wear mechanism is abrasive wear and surface fatigue wear.

Tin-based Babbitt alloy, also known as white alloy, has good wear resistance, inlayability, corrosion resistance, etc., and is the main raw material for preparing automotive bearings. However, during the service of tin-based alloy bearings, the oil film between the tin-based alloy bearings and the bearings will be temporarily destroyed due to the start and stop of the automotive sliding bearings, resulting in the tin-based alloy bearings and the main shaft grinding, which reduces the service life of the tin-based alloy bearings. In severe cases, it will cause the bearings to be scrapped. Under the premise of ensuring other excellent properties of tin-based alloy bearings, how to improve their wear resistance is an urgent problem to be solved to increase the service life of the bearings.

The tribological properties of tin-based babbitt alloy can be effectively improved by improving the preparation process of tin-based babbitt alloy. Zhuang et al. proposed to prepare polyurethane coating on the surface of tin-based babbitt alloy, and studied the tribological mechanism of the coating under dry and wet conditions, and obtained a coating with good tribological properties; Zhuang et al. prepared tin-based babbitt alloy layer by laser ablation technology. The multi-layer structure produced on the surface can reduce the friction factor of babbitt alloy and improve the wear resistance and self-lubricating properties of tin-based babbitt alloy. Alcover et al. prepared tin-based babbitt alloy by flame spraying and arc spraying respectively. Through friction and wear tests, it was verified that the tribological properties of thermal spraying process were significantly better than those of tin-based babbitt alloy manufactured by traditional process.

Laser cladding technology can also improve the mechanical properties of alloys, and the tribological properties of cladding layer can be improved by regulating parameters such as laser power and scanning speed. Hao Yunbo et al. prepared a tin-based babbitt alloy cladding layer on the surface of 20 steel by laser cladding technology. The study showed that the cladding layer prepared by laser cladding has fine grains, good metallurgical bonding with the substrate and good wear resistance. Weng et al. prepared a Ti-Al-V composite coating on a Co-based surface. By comprehensively regulating the laser power and scanning rate, a dense cladding layer with a compact structure and no defects such as pores and cracks was prepared. In order to improve the wear resistance of gray cast iron, Zhao et al. prepared a Cu-Ti-Ni composite coating on its surface. By regulating the process parameters, the microhardness and tribological properties of the cladding layer were improved. Aghili et al. prepared Cr3C2-NiCr wear-resistant coating on the surface of Al-Ti substrate, studied the influence of laser power, scanning speed, etc. on the mechanical properties of cladding layer, and improved the microhardness of cladding layer by regulating process parameters.

This paper intends to prepare tin-based babbitt alloy cladding layer on the surface of 20 steel, and study the influence of laser power on the microstructure, heat-affected zone and tribological properties of tin-based babbitt alloy. By regulating process parameters, a tin-based babbitt alloy cladding layer with good tribological properties is prepared, which provides a theoretical basis for the preparation of tin-based babbitt alloy cladding layer by laser cladding technology.

1 Experimental materials and methods

The experimental substrate material is 20 steel with surface oxide scale removed, and the substrate size is 100mm×50mm×10mm. The cladding powder uses SnSb11-Cu6 standard grade tin-based babbitt alloy powder, the powder particle size is 50-100μm, and the powder composition is shown in Table 1. The test was conducted using the LDF6000-60 fiber laser produced by Siemens, Germany. The test platform is shown in Figure 1. The test process parameters are as follows: the laser power is 800W, 1000W, and 1200W respectively, the scanning rate is 1500mm/min, and the spot diameter is 3mm. The prefabricated powder method was used for the test, the thickness of the cladding layer was 1mm, and the overlap rate was 50%. In order to prevent oxidation during the cladding process, argon gas with a purity of 99.9% was used for protection during the test.

After laser cladding, the metallographic specimen was prepared using a standard mechanical polishing process, and the cross section of the cladding layer was corroded using a 4% (volume fraction) nitric acid alcohol solution for 15s. The Zeiss metallographic microscope was used to observe the microstructure of the cladding layer, and the Brucker D8ADVANCE X-ray diffractometer was used to analyze the phase composition of the cladding layer. The friction and wear test used an HT-1000 high-temperature friction and wear tester. The wear sample size was a Ø25mm×5mm cylindrical sample. The sample was ground and polished before grinding. The grinding ball was GCr15, the loading load was 300g, the wear time was 20min, and the wear scar radius was 3mm. After the wear test, the sample was ultrasonically cleaned, and then the wear scar morphology and wear mechanism were analyzed by a SU4800 scanning electron microscope, and the element distribution on the wear scar surface was tested by an energy spectrometer.

2 Analysis and discussion

2.1 Macromorphology of cladding layer

Figure 2 shows the macromorphology of tin-based babbitt alloy cladding layer prepared at three laser powers of 800 W, 1000 W, and 1200 W. For the convenience of description, they are recorded as samples 1, 2, and 3 respectively. The surface of the cladding layer prepared by the three laser powers is flat and the overlap is obvious. When the laser power is low, the surface of the cladding layer is bright white. When the laser power increases to 1200 W, the color of the cladding layer becomes darker. The analysis shows that the increase in laser power reduces the cooling rate of the cladding layer, resulting in oxidation of the cladding layer, which makes the color of the cladding layer darker. With the increase of laser power, the thermal stress in the cladding layer also increases, resulting in cracks on the surface of the cladding layer, and the number and width of the cracks also increase with the increase of laser power.

2.2 Phase composition of cladding layer

The XRD spectra of the three cladding layers are shown in Figure 3. The phase compositions of the three cladding layers are all SnSb phase, Cu6Sn5 phase, and α-Sn phase. By comparing the diffraction peaks of the cladding layers prepared under three laser powers, it can be seen that although the laser power changes the heat input of the cladding layer, it has no effect on the phase composition of the cladding layer, and only changes the intensity of the phase peak.

The microstructure morphology of the tin-based cladding layer prepared by different laser powers is shown in Figure 4 (a) to (c). The three cladding layers are all composed of bright white square SnSb phase, skeleton-like Cu6Sn5 phase, and black matrix α-Sn phase. The change of laser power does not change the phase composition of the cladding layer. However, the change of laser power changes the temperature of the molten pool during laser cladding. It can be seen from formula (1) that the temperature of the molten pool is related to the laser power P, the laser beam spot radius R, and the absorption coefficient α of the base material. When other parameters remain unchanged, as the laser power increases, the temperature of the molten pool also increases, and the solidification time of the molten pool also increases, so that the grains in the molten pool have sufficient time to grow, resulting in the aggregation of SnSb grains inside the molten pool due to the flow of the melt, resulting in the increase of the size of the SnSb particles.

Where: T0 is the ambient temperature; α is the power absorption coefficient of the base material 20 steel; k is the thermal conductivity; r is the spot diameter; ρ is the density; c is the specific heat capacity of the material.

In Figure 4 (a), the grain size of SnSb is small. This is because the laser power is small, the molten pool temperature is low, the cooling rate is fast, the melt has a large undercooling, and more SnSb nuclei are generated in the molten pool. The large undercooling makes the SnSb grains unable to grow completely and evenly distributed in the cladding layer. As the laser power increases to 1000-1200 W, the grain size of the SnSb phase increases rapidly, as shown in Figure 4 (b) and (c). The analysis shows that the liquidus point of Babbitt alloy is low, only 370 ℃. The higher laser power increases the temperature of the melt and substrate and reduces the cooling rate of the melt. After processing, the temperature of the substrate is high. When the cladding layer, which should be cooled rapidly, is cooled to the same temperature as the substrate, the cooling rate is delayed, so that the SnSb phase in the cladding layer has sufficient time to diffuse and aggregate, resulting in an increase in the size of the SnSb phase.

Figure 5 (a), (b), (c) are SEM morphologies of the bonding area of ​​tin-based Babbitt alloy/20 steel substrate prepared with different laser powers. The morphologies of the three bonding areas all show good metallurgical bonding. When the laser power is 800 W, the bonding area between the steel substrate and the Babbitt alloy is relatively flat, and a small amount of Fe element diffuses into the cladding layer, forming Fe-Sn intermetallic compounds in the bonding area. This is because the atomic distance between Sn and Sb is greater than that of Fe, causing the Fe element to float and diffuse at the interface. As the laser power increases, the molten pool temperature increases, causing the substrate to partially melt, and it is difficult for the Sn element to diffuse into the 20 steel substrate, resulting in almost no diffusion of the Sn element on one side of the substrate, and a small amount of Fe element diffused on the Babbitt alloy side. The diffusion depth of the Fe element in the substrate can be determined by EDS line scanning of the transition area and the corresponding element intensity curve, as shown in Figure 6. The diffusion distance of the Fe element is the position where the Sn element rises rapidly and the end point where the Fe element approaches 0%. As the laser power increases, the diffusion distance of the Fe element in the substrate also increases.

2.3 Microhardness of cladding layer

Figure 7 shows the average microhardness of the surface of the Babbitt alloy cladding layer prepared at different laser powers. The average microhardness of samples 1, 2, and 3 (test force 0.98N) are 35.7 HV, 29.8 HV, and 28.7 HV, respectively. Because the solidification rate of sample 1 is the fastest at a lower laser power, the cladding layer undergoes microstructural refinement, making the hard point particles

SnSb uniformly distributed in the cladding layer, thereby improving the microhardness of the cladding layer. The strengthening mechanism of the cladding layer can be attributed to microstructural refinement and second phase strengthening. However, due to the higher laser power, samples 2 and 3 reduce the cooling rate of the cladding layer, resulting in segregation and aggregation of hard point particles in the cladding layer, thereby reducing the microhardness of the cladding layer.

2.4 Analysis of tribological properties of cladding layer

The friction factor curves of the tin-based Babbitt alloy cladding layer prepared at different laser powers are shown in Figure 8. The average friction coefficients of samples 1, 2, and 3 are 0.257, 0.526, and 0.592, respectively. Sample 1 has a lower average friction coefficient of 0.257. This is because in the early stage of wear, the hard particles hinder the wear of the grinding ball, resulting in a large fluctuation in the friction coefficient. From the metallographic morphology of Figure 4, it can be seen that the smaller hard particles in sample 1 are evenly distributed in various areas of the cladding layer, making the ability of each area of ​​the cladding layer to resist deformation roughly the same, and the ability to resist deformation is stronger when worn. The surface of the cladding layer is not easily damaged, thereby reducing the friction coefficient of the cladding layer. The average friction coefficients of samples 2 and 3 are similar, 0.526 and 0.592, respectively. In the early stage of wear, due to the contact between the grinding ball and the cladding layer, tangential friction is generated under the action of the vertical load, and the friction coefficient rises rapidly. Since the SnSb phase in the two cladding layers is large in size and unevenly distributed, the surface hardness of the cladding layer is too different, and the ability to resist deformation is also different, which increases the friction factor of the cladding layer. When the wear reaches stability, the friction factor also tends to be stable.

The wear scar morphology of the tin-based babbitt alloy cladding layer prepared by three laser powers is shown in Figure 9, and the morphology of the tin-based babbitt alloy cladding layer prepared by 800 W laser power after wear is shown in Figure 9 (a) and (d). Obvious pitting and plowing can be seen under the low-magnification morphology of the wear surface. Due to the rapid cooling and heating, a large number of fine SnSb particles are generated in the cladding layer; as the wear progresses, the fine hard point particles fall off, forming fine pitting pits on the surface of the cladding layer. The fallen hard point debris joins the grinding ball and the wear surface, causing three-body wear, resulting in plowing grooves on the surface of the cladding layer, and also intensifies the formation of pitting, forming large-area pitting pits in the later stage of wear. The wear mark morphology is further magnified to 100 times, and relatively obvious plastic deformation is also found in the inner edge area of ​​the wear mark. Due to the action of the vertical load, the two sides of the wear mark are concave toward the center of the wear mark, which leads to the most serious wear in the center area of ​​the wear mark. The wear mechanism of the cladding layer is abrasive wear and surface fatigue wear, accompanied by pitting, plowing grooves and plastic deformation.

The morphology of the tin-based babbitt alloy cladding layer prepared by 1000 W laser power after wear is shown in Figure 9 (b) and (e). There are obvious signs of shedding in the inner area of ​​the wear mark, and there is obvious plastic deformation. Under the continuous action of the load, the hard point particles on the surface of the cladding layer cannot

bear the continuous load and the shear stress generated by the grinding of the grinding ball and the cladding surface, resulting in the shedding of the hard point particles and pitting. The wear scar morphology is further enlarged, as shown in Figure 8 (e). There are relatively fine cracks on the wear scar surface. This is because the shear stress generated by the vertical load and friction continues to act on the cladding layer, causing the cladding layer to continue to resist the external force, resulting in cracks on the cladding layer surface. There are light-colored areas and dark-colored areas on the wear scar surface. This is because the hard point particles are unevenly distributed during wear, causing the matrix phase α-Sn to contact the grinding ball, causing α-Sn to oxidize. EDS point scanning is performed on points 1 and 2 in Figure 9 (d). The element percentage is shown in Table 2. The oxygen content in the point 1 area is relatively high. Analysis shows that an oxidation reaction occurred in the dark area, generating Sn oxide. The wear mechanism of the wear scar is mainly oxidative wear and surface fatigue wear, accompanied by pitting and plastic deformation on the inside of the wear scar.

The morphology of the tin-based babbitt alloy cladding layer prepared by 1200 W laser power after wear is shown in Figure 9 (c) and (f). The wear inside and outside the wear mark is more serious, and both have undergone severe plastic deformation. There are two furrows with a width of about 50μm in the middle of the wear mark, and there are traces of shedding. The wear mechanism is similar to the wear mechanism of Figure 9 (a). Pitting first occurs on the inside of the wear mark, and the detached debris enters the grinding area, resulting in furrows on the wear surface; as the wear progresses, the wear surface can no longer withstand the shear stress caused by friction, resulting in plastic deformation on both sides of the cladding layer. The wear mechanism is mainly abrasive wear, accompanied by furrows and plastic deformation.

Conclusion

The laser cladding technology was used to prepare the tin-based babbitt alloy cladding layer on the surface of 20 steel. By changing the laser power, the effect of laser power on the tin-based babbitt alloy cladding layer was explored, and the following conclusions were drawn:

(1) The change of laser power did not change the phase composition of the cladding layer. The three cladding layers were composed of square SnSb phase, rod-shaped Cu6Sn5 phase, and black soft matrix α-Sn phase. When the laser power was low, the grain size of the SnSb phase in the cladding layer was small and the distribution was relatively uniform; with the increase of laser power, the size of the SnSb phase and Cu6Sn5 phase increased and agglomerated.

(2) When the laser power was low, as the laser power increased, the amount of SnSb phase decreased, resulting in a decrease in the hardness of the cladding layer. When the laser power is 800 W, the microhardness of the cladding layer is the highest, which is 35.7 HV. The improvement of the microhardness of the cladding layer can be attributed to the refinement of the structure and the strengthening of the second phase.

(3) In the bonding area between the substrate and the babbitt alloy, the Fe element tends to diffuse into the cladding layer and generates Fe-Sn intermetallic compounds. With the increase of laser power, the diffusion trend of the Fe element becomes more obvious. When the laser power reaches 1200 W, the diffusion distance is the largest.

(4) The friction coefficient of the tin-based babbitt alloy increases with the increase of laser power. The friction coefficients of 800 W, 1000 W, and 1200 W babbitt alloys are 0.257, 0.526, and 0.592, respectively. At low laser power, the wear mechanism of the cladding layer is abrasive wear and surface fatigue wear; when the laser power is 1000 W, the wear mechanism is oxidation wear and surface fatigue wear; when the laser power increases to 1200 W, the wear mechanism is abrasive wear.