Improve the surface wear resistance of 60Si2Mn steel. Laser cladding of two iron-based powders, X1 and X2, was carried out on the surface of 60Si2Mn steel by synchronous powder feeding. The microstructure, chemical element distribution and phase composition of the cladding layer were observed and analyzed by metallographic microscope, field emission scanning electron microscope and X-ray diffractometer. The hardness and wear resistance were tested by microhardness tester and multifunctional friction and wear tester. Both cladding layers had no defects such as cracks and pores. There were a large number of dendrites, equiaxed crystals and a small number of planar crystals growing along the surface of the substrate inside the coating. The number of equiaxed crystals in the top area of ​​the X1 cladding layer was larger, and the structure was finer and more uniform. Both cladding layers were composed of the same phase (α-Fe) solid solution, and no obvious diffraction peaks of other phases appeared. The average hardness of the substrate 60Si2Mn steel was about 300HV, and the hardness of the X1 cladding layer was 950~1 000HV, with an average hardness of 975HV. The hardness of the X2 cladding layer is 784~821HV, and the average hardness is 803HV. After the ball-disc wear test, the volume wear rates of the X1, X2 cladding layers and the substrate are 1.32×10’‒4, 1.94×10’‒4, and 3.29×10’‒4mm’3/(N·m), respectively. The hardness and wear resistance of the two cladding layers are better than those of the substrate. Among them, the average hardness of the X1 cladding layer is about 21% higher than that of the X2 cladding layer. Its volume wear rate is the smallest and its wear resistance is better.

The soil-contacting parts of agricultural machinery are often in direct contact with hard particles such as gravel and sand in the soil without lubrication. Their working environment is complex and prone to wear failure. The service life of the soil-contacting parts is not long and they need to be replaced frequently, which affects the farming efficiency and causes a large amount of metal material waste [1-4]. Therefore, the use of surface strengthening technology to reduce the scrap rate of agricultural machinery soil-contacting parts, increase the service life of materials, and enhance the wear resistance of parts has attracted widespread attention.

Domestic and foreign scholars often use plasma surfacing, flame spraying and thermal spraying technologies to prepare coatings on the surface of agricultural machinery soil-contacting parts. The purpose is to improve the hardness and wear resistance of the material surface, thereby improving its operating efficiency and reducing replacement costs. Hao Jianjun et al. [5] used plasma surfacing technology to prepare Fe-Cr-C-V coating on the surface of 65Mn steel. The Vickers hardness of the coating increased by 75% compared with quenched steel, and the wear loss was reduced by 67%. Benegra et al. [6] used plasma surfacing and supersonic flame spraying technology to prepare two coatings on the surface of stainless steel, and found that the wear resistance of plasma surfacing nickel-aluminum coating was better than that of supersonic flame spraying coating. Niranatlumpong et al. [7-8] prepared WC-Co coating and Al2O3/Ti2O3 coating on the rotary blade substrate by supersonic flame spraying and plasma spraying, respectively. The results showed that in the sand wear test, the WC-Co coating had better wear resistance, and the wear rate of the tool after thermal spraying WC-Co coating was only 38% of that of quenched steel. The above surface modification technologies can improve the wear resistance of agricultural machinery soil-contacting parts, thereby extending their service life. However, the above methods have the disadvantages of complex process flow, high dilution rate of cladding layer, and large deformation of substrate. Laser cladding, as an advanced surface strengthening and repair technology, can prepare a coating that is metallurgically bonded to the substrate. It has the advantages of high surface quality, low dilution rate, small substrate heat-affected zone, and low material loss. It is widely used in agricultural machinery, high-speed trains, railways, mining machinery and other fields [9-13]. This paper attempts to use laser cladding technology to clad two kinds of iron-based powders on the surface of 60Si2Mn steel, a common material for rotary tillers, and analyzes the microstructure, phase structure, bonding zone hardness and wear resistance of the two cladding layers.

1 Experiment

1.1 Materials

The base material used is 60Si2Mn steel plate with a size of 100 mm×100 mm×6 mm. The cladding materials are two kinds of iron-based powders, X1 and X2, with a powder particle size of 100~270 mesh, and their chemical compositions are shown in Table 1. Before cladding, the surface of the substrate is first polished to remove the surface oxide layer, and then flame preheated at 250 ℃ to eliminate residual stress and reduce temperature difference to prevent cracking caused by excessive cladding temperature.

1.2 Methods

The laser cladding equipment consists of an LDF6000-100 semiconductor laser, an MH50II-20 8-axis robot, a PF2/2 dual-hopper powder feeder, a platform base, a gas protection system, a water cooling system, an online monitoring system, and a complete set of automated software control systems [14]. The synchronous powder feeding method was used to clad the powder on the substrate surface. The specific process parameters are shown in Table 2.

The laser clad specimens were cut into small test blocks of 10 mm × 10 mm × 6 mm by wire cutting, and then ground and polished with sandpaper and a polishing machine. The cross-section of the test block was then etched with aqua regia solution. The microstructure of the junction area between the cladding layer and the substrate after etching was observed using a Zeiss Axio Vert A1 metallographic microscope. The distribution of chemical elements in the junction area between the substrate and the cladding layer was analyzed using a JSM-7800F field emission scanning electron microscope. The D8 Advanced X-ray diffractometer was used to analyze the surface of the cladding layer, and the diffraction parameters are shown in Table 3. The HVS-1000A digital microhardness tester was used to test the microhardness of the cladding layer to the substrate. The distance between the test points was 0.2 mm, the test load was 50 g, and the loading time was 15 s. The UMT multifunctional friction and wear tester was used to perform ball-disc friction and wear tests on the two clad samples and the unclad substrate sample. The grinding material was a silicon nitride ball with a diameter of 6.35 mm. The test parameters are shown in Table 4. The surface morphology of the worn sample was observed using a Nano focus laser confocal microscope, the wear scar area was obtained, and the volume wear rate was calculated.

2 Results and analysis

2.1 Macromorphology analysis of cladding layer

The macromorphology of the two coatings after laser cladding is shown in Figure 1. The grooves and protrusions on the surface of the cladding layer are arranged alternately, the surface flatness is poor, there are pits, but there are no defects such as macro cracks and pores. Comparing Figure 1a and Figure 1b, it can be seen that the surface of the X2 cladding layer is smoother, and there are a large number of molten alloy droplets on the surface of the X1 cladding layer. The reason for this phenomenon may be that the wetting performance of the X1 powder is poor or the powder is not fully decomposed. The degree of powder decomposition is related to the powder particle size. When the mass of the cladding powder is the same, the smaller the powder particle size, the less energy is required for melting, the higher the degree of decomposition, and the macroscopic morphology of the cladding layer prepared is also better [15-16].

2.2 Analysis of microstructure of the bonding zone

Figure 2 shows the cross-sectional morphology of the two cladding layers X1 and X2. Figure 2b and Figure 2e are partial enlargements of the interface of Figure 2a and Figure 2d, respectively. It can be seen from the figure that there is an obvious dividing line between the substrate and the cladding layer, that is, the fusion line. When the metal in the molten pool near the fusion line begins to crystallize, it grows into the cladding layer with the matrix grains as nucleation points. Laser cladding is a rapid cooling and heating process, and the grain growth area is small. Under this growth mechanism, the cladding layer and the matrix material undergo element diffusion and fusion, resulting in a transition zone, which has no defects such as cracks and pores. Under the same corrosion conditions, the colors of the various regions are different because the corrosion resistance of the various regions is different. From the depth of the color, it can be seen that the corrosion resistance of the cladding layer is better than that of the matrix. Figures 2c and 2f are partial enlargements of the top areas of the two cladding layers, from which it can be seen that the X1 cladding layer has a more uniform structure and finer grains. The reason is that the X1 powder contains a high content of the strong carbide-forming element V, which is easy to form nano-sized carbides, which seriously hinders the growth of grains at the grain boundaries and plays a role in refining grains [17]. The microstructure of the cladding layer is related to the composition supercooling during the solidification process, and is mainly affected by the ratio of temperature gradient to solidification rate (G/R). During the cladding process, the junction between the cladding layer and the substrate solidifies first, with the largest temperature gradient and the smallest solidification rate, so the G/R is the largest, there is no composition supercooling, and all the heat released during the solidification process is transferred to the substrate interface. The nucleation rate at the interface is lower than the grain growth rate, and the solid-liquid interface slowly advances forward to form a planar crystal structure; as the solidification process continues, the solid-liquid interface continues to advance toward the surface of the cladding layer, the temperature gradient continues to decrease, the solidification rate continues to increase, and the G/R gradually decreases, resulting in a large composition supercooling, interface instability, and the structure transforms into columnar crystals or dendrites; when the solid-liquid interface advances to the surface of the cladding layer, it contacts the outside air, and the heat dissipation rate is fast. At this time, the G/R is very small, and the grain nucleation rate is greater than the growth rate, causing the structure to transform into fine equiaxed crystals [18-22].

2.3 Analysis of the physical structure of the cladding layer

Figure 3 shows the microstructure and element distribution curves of the bonding area between the cladding layer and the substrate, X1 and X2. From the element distribution curve, it can be seen that the content of Si, Mn, and C in the two samples has no obvious change in the substrate and the cladding layer; the content of Fe element is relatively high in the substrate, and the content gradually decreases from the substrate to the cladding layer, and the element distribution changes suddenly at the interface; the content of Cr element is relatively high in the cladding layer, and the element content shows a downward trend from the surface of the cladding layer to the substrate, and the element content changes significantly at the interface. There is V element in the X1 cladding layer, and the local signal intensity of V element in the figure is higher than that of Cr element. The reason is that the ability of V element to form carbides is higher than that of Cr element, and a small amount of carbides are formed locally, making the composition distribution uneven. Compared with the line scanning results of the X1 cladding layer, the Cr element content in the X2 cladding layer is relatively higher, and the change of element content at the interface is more obvious. The Cr content not only affects the hardness of the cladding layer, but also affects the corrosion resistance of the cladding layer. As the Cr content increases, the hardness of the cladding layer decreases and the corrosion resistance improves[23].

Figure 4 shows the X-ray diffraction patterns of the two cladding layers. As can be seen from Figure 4, the two cladding layers show obvious diffraction peaks at 44.7°, 65.0°, and 82.3°, indicating that the two coatings are composed of the same phase (α-Fe) solid solution. The two coatings have the same structure and similar properties. Since no diffraction peaks of other phases appear, it indicates that a small amount of alloying elements such as Cr, Ni, V, Mn, Mo, and Si are dissolved in (α-Fe). The Cr content in the powder is relatively high, and it can replace the Fe atoms in (α-Fe) to form an infinite substitution solid solution at room temperature, so the (α-Fe) solid solution is mainly composed of Cr-containing solid solution[24].

2.4 Analysis of the hardness of the bonding zone

The cross-sectional hardness curves of the two cladding layers are shown in Figure 5. The hardness of the X1 cladding layer is 950~1 000HV, with an average hardness of 975HV. The hardness of the X2 cladding layer is 784~821HV, with an average hardness of 803HV. The average hardness of the X1 cladding layer is about 21% higher than that of the X2 cladding layer, because the top of the X1 cladding layer contains a large amount of fine equiaxed crystal structure. The outer surface hardness of the two cladding layers is the highest, which is related to the deformation resistance. During the solidification process, when the solid-liquid interface advances to the surface of the cladding layer, the G/R is very small, the crystallization speed is very fast, the structure changes to fine equiaxed crystals, and the grain boundary area is relatively large. Due to the different crystal orientations on both sides of the grain boundary, the high resistance of the grain boundary makes it difficult for plastic deformation to pass through the grain boundary from one grain to another [25-26]. In the figure, HAZ is the heat affected zone, and its hardness is between that of the cladding layer and the substrate. The reason is that the substrate absorbs part of the energy in this area and dissolves slightly, which makes a small amount of grains in the heat affected zone coarsened. Another reason is the dilution effect caused by the mixing of the cladding material and the substrate material [27-28].

2.5 Analysis of wear resistance of cladding layer

Figure 6 shows the micromorphology of two cladding layers and 60Si2Mn steel substrate after ball-disc wear test. By observing the width of the wear scar, it can be clearly found that the wear degree of the X1 cladding layer is the lightest (Figure 6a), the wear degree of the 60Si2Mn steel substrate is the most serious (Figure 6c), and the wear degree of the X2 cladding layer is between the two. It can be seen that the wear resistance of the X1 cladding layer is the best.

In addition, the volume wear rate of the two cladding layers and the substrate can be calculated by formula (1) and formula (2), and their wear resistance can also be determined by comparing the numerical values ​​of the volume wear rate.
Where: Wγ is the volume wear rate, mm’3/(N·m); ΔV is the volume wear amount, mm’3; F is the normal load, N; v is the wear velocity, mm/s; t is the wear time, s; S is the wear area, mm2 (can be obtained through Origin software data processing); L is the amplitude, i.e. the wear distance, mm.

The friction and wear test parameters in Table 4 are sorted out and substituted into the volume wear rate formula, and the volume wear rates of the X1, X2 cladding layers and the substrate are obtained as shown in Figure 7. As shown in Figure 7, the volume wear rates of the three samples are 1.32×10‘‒4, 1.94×10‘‒4, and 3.29×10‘‒4
mm’3/(N·m), respectively. Among them, the volume wear rate of the X1 cladding layer is the smallest, so its wear resistance is the best.

3 Conclusions

1) Both cladding layers have no defects such as cracks and pores. They are composed of a large number of dendrites, equiaxed crystals and a small number of planar crystals growing along the surface of the substrate. The surface of the X1 cladding layer has a larger number of equiaxed crystals and a more uniform and fine structure. The reason is that the X1 powder has a high content of V, a strong carbon compound forming element, which is easy to form nano-sized carbides, which hinder the growth of grains at the grain boundaries and play a role in fine grain strengthening. Both cladding layers are composed of the same phase (α-Fe) solid solution, and no obvious diffraction peaks of other phases appear.

2) The hardness and wear resistance of the two cladding layers are better than those of the substrate. The average hardness of the substrate 60Si2Mn steel is about 300HV, and the average hardness of the X1 and X2 cladding layers is 975HV and 803HV, respectively. The hardness of the X1 cladding layer is about 21% higher than that of the X2 cladding layer. The volume wear rates of X1 cladding layer, X2 cladding layer and substrate material are 1.32× 10‘‒4, 1.94×10‘‒4 and 3.29×10‘‒4mm’3/(N·m), respectively. Among them, the volume wear rate of X1 cladding layer is the smallest and the wear resistance is the best.