Tin bronze is a basic material for wear parts and is widely used in the industrial field. The metallographic structure and energy spectrum of CuSn12Ni2 tin bronze were analyzed, and the CuSn12Ni2 tin bronze powder was clad on the 42CrMo alloy steel substrate using a high-speed laser cladding process to conduct a bonding strength test. The research results show that metallurgical bonding is achieved between CuSn12Ni2 tin bronze and the 42CrMo alloy steel substrate.
1. Research background
Tin bronze is widely used in the industrial field as one of the basic materials for friction and wear parts. This material is particularly suitable for low-speed and heavy-load conditions. The main forms used in sliding bearings include single metal sleeves and thrust bearings, powder sintered bimetal sleeves and thrust bearings, centrifugally cast bimetal sleeves and thrust bearings, spinning single metal sleeves, powder metallurgy single metal sleeves, etc. Laser cladding is an efficient surface strengthening and remanufacturing repair technology with the advantages of good bonding with the substrate, low dilution rate, and small heat-affected zone. Laser cladding is a complex process of multi-parameter coupling. Parameters such as laser power, laser scanning speed, powder feeding speed, and spot diameter are very important for the quality of the cladding layer. Laser cladding additive manufacturing has been studied in many aspects at home and abroad. However, for conventional laser cladding, the powder absorbs 20% of the energy, the energy utilization rate is low, the dilution rate is 5%~15%, and the subsequent processing volume is large after the cladding is completed, and the processing cost is high. For high-speed laser cladding, the powder can absorb 80% of the energy, the energy utilization rate is high, the dilution rate can be less than 3%, and the subsequent processing volume is small after the cladding is completed, and the processing cost is low. High-speed or even ultra-high-speed laser cladding technology optimizes the melting form and energy absorption ratio of the powder, increases the material deposition rate, and obtains a high-efficiency, defect-free, high-bonding strength, and low-dilution rate cladding layer, which is more advantageous than traditional laser cladding. The high-speed laser cladding preparation process is used to prepare the tin-bronze alloy layer on the steel shaft substrate, which can solve the problem of running circles caused by creep caused by the long-term interference fit between the shaft sleeve and the steel substrate. And after the tin bronze alloy layer fails, it can be processed and removed and then re-clad to achieve remanufacturing. At present, there are relatively few studies on high-speed laser cladding of tin bronze powder on steel shaft substrates. The author applies high-speed laser cladding technology to clad CuSn12Ni2 tin bronze powder on 42CrMo alloy steel substrate to study the micro-composition and organization of the material and the macro-bonding strength of the double-layer metal material. The research results show that CuSn12Ni2 tin bronze and 42CrMo alloy steel substrate have achieved metallurgical bonding.
2 Sample preparation
In order to fully study the bonding strength of the material, the research samples are first prepared, including plane samples used to test material defects and chemical composition near the material bonding surface, and circular samples used to test the material bonding strength.
2.1 Powder preparation
The more concentrated the particle size, the better the spherical shape, and the more uniform the composition distribution of the powder used for high-speed laser cladding, the better the fluidity of the powder, and the fewer defects after cladding, especially for the bonding surface, there will be fewer defects. The CuSn12Ni2 tin bronze powder used by the author is obtained by gas atomization process. The principle is to use high-speed airflow to break the copper alloy liquid into tiny droplets, and then quickly cool it to form spherical metal particles. The particle size is mainly concentrated in 50~150μm, and the sphericity is good, as shown in Figure 1. The metallographic grains inside the tin bronze powder are fine. Figure 2 (a) shows most of the equiaxed crystals, and Figure 2 (b) shows a small part of the dendrites. In addition, the cross-sectional energy spectrum analysis of the tin bronze powder shows that the distribution of copper, tin, and nickel elements is relatively uniform, and no segregation occurs.
2.2 Sample preparation
The sample preparation adopts a high-speed laser cladding process, in which the light source of the laser cladding equipment is a fiber laser with a laser wavelength of about 1.06μm and a maximum power of 6kW. After the laser is emitted from the fiber connector, it is converted into parallel light through a collimating lens, and then focused through a focusing lens to concentrate the energy at one point, and the metal is melted at the focus to achieve laser cladding processing. The coaxial annular gas carrier is used to deliver powder evenly. The powder delivery gas is argon. At the same time, argon is used as a protective gas to reduce the oxidation of materials during laser cladding. In order to remove the excess heat generated by the laser in the process of converting electrical energy into light energy, and to remove part of the heat absorbed by the lens reflecting the laser beam in the external optical path, a water cooling system is provided for the laser.
The thickness of the cladding layer in the author’s study is 1.2mm, the cladding speed is 60~100mm/s, the spot diameter is 2mm, the powder feeding amount is 40~50g/min, and the laser power is 4500kW~4800kW.
The plane sample prepared by the high-speed laser cladding process is shown in Figure 3, which is used to characterize and analyze the material near the bonding surface of CuSn12Ni2 tin bronze and 42CrMo alloy steel substrate. In the specific operation, it is necessary to take samples from the plane sample, and then prepare the sample for metallographic structure analysis and energy spectrum analysis. The normal bonding strength test sample prepared by the high-speed laser cladding process is shown in Figure 4, which is used to determine the bonding strength between CuSn12Ni2 tin bronze and 42CrMo alloy steel substrate.
3 Characterization and analysis of high-speed laser cladding materials
3.1 Metallographic structure
The sample was subjected to metallographic analysis. The analysis equipment used an ultra-depth of field microscope. Figure 5 shows the microstructure morphology of the sample before corrosion, and Figure 6 shows the metallographic structure of the sample after corrosion. The solution used for the corrosion sample is composed of a mixture of three substances: 10gFeCl, 6H, 0, 2mL hydrochloric acid solution with a density of 1.16g/mL, and 98mL ethanol solution with a volume fraction of 95%. It can be seen from Figure 5 that the CuSn12Ni2 tin bronze prepared by the high-speed laser cladding process still has certain pores, and the largest pore diameter is 97.14μm. It can be seen from Figure 6 that the metallographic structure of the sample after corrosion is mainly dendrites near the bonding surface, and equiaxed grains are mainly formed closer to the surface of CuSn12Ni2 tin bronze. The main reason is that the closer to the surface, the greater the degree of supercooling, the easier it is to form equiaxed grains, and the closer to the bonding surface, the smaller the degree of supercooling, which is more conducive to the formation of dendrite grains.
3.2 Energy spectrum analysis
During the laser cladding process, a certain amount of elements in CuSn12Ni2 tin bronze will penetrate into the 42CrMo alloy steel matrix and form a metallurgical bond near the bonding surface. The purpose of energy spectrum analysis at the bonding surface is that the dilution rate of CuSn12Ni2 tin bronze is not high, so the process has little effect on the composition and mechanical properties of tin bronze. Although the dilution rate is not high, a small amount of elements enter the alloy steel matrix, indicating that metallurgical bonding occurs near the bonding surface.
4 Bonding strength test
After the CuSn12Ni2 tin bronze material is clad on the 42CrMo alloy steel matrix by high-speed laser cladding process, it needs to have a high bonding strength with the matrix when it is used as a friction-reducing and wear-resistant layer of a sliding bearing. This can be obtained by adjusting the high-speed laser cladding process parameters. The author prepared the specimens for the bonding strength test according to the national standard GB/T12948-1991 “Destructive Test Method for Bimetallic Bond Strength of Sliding Bearings” and conducted a bonding strength test. The yield strength of CuSn12Ni2 tin bronze material is 140MPa~150MPa, and the tensile strength is 260MPa~300MPa. When the bonding strength is less than the yield strength, fracture will occur at the bonding surface. When the bonding strength is between the yield strength and the tensile strength, fracture will still occur at the bonding surface, but the CuSn12 tin bronze body has already yielded. When the bonding strength is greater than the tensile strength, fracture will occur in the CuSn12Ni2 tin bronze material body. The normal bonding strength test is shown in Figure 8, and the test results are shown in Figure 9. As can be seen from Figure 9, the normal bonding strengths of the two samples after the test are 429.5MPa and 326.6MPa, respectively, which are greater than the tensile strength of the material, indicating that the bonding strength of the bonding surface exceeds the tensile strength of CuSn12Ni2 tin bronze. The fracture surface of the sample is known from the test to be the CuSn12Ni2 tin bronze body, as shown in Figure 10, which also confirms that the bonding strength of the bonding surface exceeds the tensile strength of CuSn12Ni2 tin bronze. The bonding strength test results also show that CuSn12Ni2 tin bronze and 42CrMo alloy steel matrix have metallurgical bonding.
5 Conclusion
The author studied the bonding performance of CuSn12Ni2 tin bronze and alloy steel matrix prepared by high-speed laser cladding process, and found that CuSn12Ni2 tin bronze and 42CrMo alloy steel matrix produced metallurgical bonding.
Near the bonding surface, CuSn12Ni2 tin bronze is mainly dendrites. Near the surface of CuSn12Ni2 tin bronze, equiaxed crystals are mainly present. This indicates that the undercooling near the bonding surface is small and the undercooling on the surface is large.
The dilution rate of CuSn12Ni2 tin bronze by high-speed laser cladding process is not very high, so the process has little effect on the composition and mechanical properties of tin bronze.
When the high-speed laser cladding process parameters are adjusted to appropriate parameters, the bonding strength of the bonding surface can exceed the tensile strength of CuSn12Ni2 tin bronze.
