In order to improve the microhardness and wear resistance of the 45# steel surface, Fe60 self-fluxing alloy powder was used as the raw material for the cladding layer, and the cladding coating was prepared through laser cladding and laser remelting processes. Use a scanning electron microscope (SEM) to observe the microstructure of the coating, and use Vickers hardness tester, wear testing machine, three-dimensional topography instrument and other instruments to test the microhardness, wear resistance and wear mechanism. The results show that the dendrite structure of the coating prepared by laser cladding is larger, and the dendrite grains of the coating after laser remelting are refined and denser. Compared with the substrate, the average hardness of the coating prepared by laser cladding is 670 HV, which is 3.38 times that of the substrate. The average hardness of the coating after laser remelting is 682 HV, which is 3.43 times higher than the substrate hardness. After laser remelting and laser cladding, the friction coefficient and wear amount of the coating are significantly reduced compared to the substrate. The wear amount is reduced by 8 times and 2.67 times respectively. The wear resistance is significantly improved. The friction and wear of the coating The forms are abrasive wear and adhesive wear.
As a high-quality carbon structural steel, 45# steel has good comprehensive mechanical properties and cutting processing advantages, so it is widely used in machinery manufacturing, aerospace, power engineering and other fields. However, the hardness of 45# steel is relatively low and the surface is not wear-resistant, so the surface of parts is prone to scratches and wear under heavy loads, causing parts to fail and seriously affecting the economic benefits and production efficiency of the company. Therefore, some scholars use surface modification technology to improve the performance of 45# steel parts, such as plasma spraying, arc welding, laser cladding and other technologies. Laser cladding technology is an advanced material surface modification technology that uses high-energy-density laser The beam fuses the base material and the powder together, forming a strong metallurgical bonding cladding layer with the surface of the base material. This technology has been widely used in aerospace, machinery, chemical industry, mold and other fields, and has become an important surface modification One of the technologies.
Laser remelting means that after the first laser cladding is completed, the laser beam is scanned again to clad the coating without powder coming out. Laser remelting technology can further refine the structure, reduce segregation, suppress coating defects, and improve mechanical properties. Zhang et al. performed laser surface remelting treatment on aluminum alloy plates by changing the scanning speed. After laser remelting, the columnar grains became significantly smaller and thicker than the nearly equiaxed grains in the matrix. As the scanning speed increased, the columnar grains became The width is reduced and the internal grains are refined, and the microhardness is improved. Wang et al. prepared iron-based amorphous structure coatings on the surface of H13 steel through two methods: laser cladding and laser remelting, and compared the structures and properties obtained by the two methods. The results show that after laser remelting, the amorphous structure content of the coating decreases, and defects such as cracks or pores are significantly reduced. However, in the remelting laser beam, there are transgranular cracks extending through the crystal region to the matrix. After remelting, the hardness of the crystallized region is about 200 HV higher than that of the amorphous region, and the overall corrosion resistance of the remelted layer is higher than that of the cladding layer.
The above research results all confirm that laser remelting technology can help reduce coating defects and improve part performance. Therefore, in order to improve the low hardness and non-wear resistance of 45# steel, this paper studied the effect of laser remelting power on the microstructure, microhardness and wear resistance of the cladding layer by changing different laser remelting powers, in order to Provide certain experimental and theoretical basis for improving the wear resistance of 45# steel.
1 Experimental process
1.1 Experimental materials
This test uses 45# steel as the base material, and its chemical composition is shown in Table 1. Fe60 self-fluxing alloy powder with a particle size of 100~325 mesh is used as the cladding material. Its chemical composition is shown in Table 2. Powder morphology The transitive phase characteristics are shown in Figure 1.
Before the experiment, the surface of the 45# steel was polished with an angle grinder and 1200-grit sandpaper, ultrasonically cleaned with absolute ethanol and then dried to remove surface dirt. At the same time, the Fe60 alloy powder was placed in a vacuum drying oven at a temperature of 120°C to keep it warm. After 3 hours, dry and dehumidify, and take it out for later use after the temperature drops to normal temperature.
1.2 Experimental methods
The coaxial powder feeding method is used to laser clad Fe60 alloy powder on the surface of 45# steel, and the ILAM series laser cladding device is selected. Its maximum output power is 4000W. The laser cladding sample preparation process is shown in Figure 2. In this experiment, based on laser cladding, the coating was remelted by changing the laser power. The specific process parameters are shown in Table 3.
1.3 Material characterization and performance testing
The laser-remelted sample was vertically cut into 10 mm × 10 mm × 6 mm samples along the coating cross section. After polishing, 6% nitric alcohol was used for etching, and an Axio Observer 3M Zeiss microscope was used to observe the structure of the coating. The Huayin HMV-10 microhardness tester was used to test the microhardness of the substrate and the coating surface. The load was 300g for 10s. Each sample was marked once at an interval of 0.1mm, and the average value was taken three times in parallel. The multifunctional friction and wear testing machine MFT-5000 was used to test the friction and wear performance of the samples. The friction and wear testing machine measured the wear resistance of the coating after cladding, and a high-precision electronic balance was used to measure the quality difference before and after wear.
2 Experimental results and analysis
2.1 Macromorphological analysis
Figure 3 shows the macroscopic morphology of laser cladding and laser remelting. Laser remelting significantly improves the smoothness of the coating. During the cladding process, the powder is ejected from the nozzle driven by the air flow. The cladding layer will have incompletely penetrated powder adhering to the surface of the coating, causing surface roughness. After remelting, the surface will not be fully penetrated. The powder melts completely and blends with the coating, making the surface of the coating smooth.
2.2 Microstructure Analysis
Laser cladding has the characteristics of rapid heating and cooling, and is a non-equilibrium crystallization process. Therefore, the microstructure of the coating mainly depends on the relationship between the temperature gradient (G) and the solidification rate (R). In the early stage of cladding, due to the low temperature of the substrate, the temperature gradient (G) of the interface between the substrate and the cladding layer is the largest, and the solidification rate (R) is the smallest, so the substrate and the bottom mainly grow in the form of plane crystals, indicating that the coating and the substrate show obvious metallurgical bonding. With the continuous increase of the temperature of the molten pool, the temperature gradient (G) gradually decreases, so that the plane crystals perpendicular to the bonding surface gradually transform into larger dendrites. In the middle of the cladding layer, due to the lack of obvious directionality in heat dissipation, the grains grow in no fixed direction, and the microstructure orientation is disordered, as shown in Figure 4b. On the surface of the cladding layer, due to the fast heat dissipation rate and the faster solidification rate (R), the grain size becomes smaller and grows in no fixed direction, as shown in Figure 4a. As shown in Figure 5, the microstructure of the coating under the remelting power of 1kW, the coating is stirred by the laser and the cooling and solidification process of remelting, the distance between the dendrites of the coating is reduced and the grains are refined, and the spacing between the organizations becomes more dense. Due to the increase of the temperature gradient (G), the solidification rate (R) decreases. Under the action of the stirring force of the molten pool, the powder that is not fully melted is promoted to melt again and the dendrites of the organization are refined and dense, and the liquid phase swallows the nearby particles and re-nucleates into “petal-like” particles.
2.3 Microhardness
As shown in Figure 6, the average Vickers hardness distribution diagram of the vertical section of the substrate and the coating, it can be seen that the hardness of the substrate is about 198 HV, and the hardness of the alloy coating is about 670 HV, which is 3.38 times higher than that of the substrate. The average hardness after remelting is about 682 HV, which is 3.43 times higher than that of the substrate. As can be seen from the figure, the hardness after laser remelting is slightly higher than that after laser cladding treatment. The reason is that the internal structure of the coating after remelting undergoes secondary melting, and the grains are refined and dense, which improves the hardness after remelting.
2.4 Wear resistance study
Wear loss and friction coefficient are key parameters to characterize wear resistance. Under the same experiment, the less wear loss and the smaller the friction coefficient, the better the wear resistance. As shown in Figure 7, the friction coefficient diagram and wear mass loss diagram of the coating after 30 minutes on the friction and wear test machine. The average friction coefficient of the substrate is 0.481, and the wear amount is 0.8mg. The average friction coefficient of the coating after laser cladding is 0.117, and the wear amount is 0.3mg. Its wear performance is 2.67 times higher than that of the substrate. The average friction coefficient of the coating after laser remelting is 0.087, and the wear amount is 0.1mg. Its wear performance is 8 times higher than that of the substrate. The figure shows that after laser remelting, the coating structure is finer, the hardness is higher, the friction coefficient is better, and the wear resistance is significantly improved.
3 Summary
Fe60 alloy powder coating was prepared by laser cladding, and the effects of laser cladding coating and laser remelting on microstructure, hardness, wear resistance and other properties were investigated. The following conclusions were drawn:
(1) After laser remelting, the distance between dendrites in the coating was reduced and the grains were refined, and the spacing between the tissues became denser. Under the action of the stirring force of the molten pool, the powder that was not fully melted was promoted to melt again and the dendrites of the tissue were refined. The liquid phase swallowed the nearby particles and re-nucleated them into “petal-like” particles.
(2) The coating after laser remelting had a larger heat input again, resulting in an increased dilution rate, and the hardness of the coating was slightly reduced compared with that of the laser remelted coating.
(3) The friction coefficient and wear loss of the coating after laser cladding and laser remelting were significantly improved. The wear loss was increased by 2.67 times and 8 times respectively compared with the substrate, and the wear resistance was significantly improved.
