Ni60 alloy powder was laser clad on 42CrMo steel plate to prepare nickel-based composite coating, and the surface morphology, microstructure, phase, Vickers hardness, wear resistance and shear strength of the cladding layer were analyzed. At the same time, the comprehensive effect of laser power and scanning speed on the organization and friction and wear properties of the laser cladding layer was analyzed. The results show that the phase types of each Ni60 cladding layer are consistent under different specific energies. When the specific energy is too high or too low, the surface of the cladding layer will produce large splashes or ablation; its microhardness is about 1.4~1.96 times higher than that of the substrate, and its hardness gradually increases with the decrease of specific energy, while the friction coefficient and wear amount gradually decrease with the decrease of specific energy; the cladding layer has a high shear strength, which can reach 225~259 MPa; when the specific energy is 4.8 kJ/cm2, the surface of the cladding layer has small splashes, the fish scale pattern is closely arranged and the thickness is uniform, and the organization is dense, the mechanical properties are excellent, and the cladding layer and the substrate have achieved good metallurgical bonding.

42CrMo steel is a kind of steel with high strength and high toughness. It is often used in metallurgy, mining, aerospace and other fields, such as plunger pump cylinder material. The plunger pump is an indispensable part of the hydraulic system, in which the plunger is the core component of the plunger pump. The liquid can be transported only through the reciprocating movement of the plunger and the cylinder. Due to the nature of the movement between the plunger and the cylinder, this continuous high-intensity work greatly reduces the service life of the plunger pump, and it often fails due to surface wear. If the parts are directly replaced during maintenance, it will cost a lot, and the use of low-cost imitation parts will bring more hidden dangers to the use of the plunger pump. Laser cladding is an emerging technology that uses high-energy laser as a heat source to quickly and efficiently repair the surface of the workpiece. Cladding the surface of the workpiece with suitable alloy powder can significantly improve the surface wear resistance, corrosion resistance and other properties. Compared with surface treatment technologies such as chemical plating and vapor deposition, laser cladding has advantages that are difficult to replace with other technologies, such as high energy density, low dilution rate, fast cooling rate, and small heat-affected zone. Li et al. used laser cladding to clad a mixture of 30% SiC and 70% Ni-based alloy onto a steel plate, studied the microstructure and dry sliding wear behavior of the cladding, and found that its friction coefficient and wear rate were significantly reduced, and the surface of the steel substrate obtained good wear resistance. Ni60 alloy powder is a nickel-based self-fluxing alloy powder uniformly mixed with Ni, Cr, B, and Si. The alloy has a series of advantages such as good wear resistance, corrosion resistance, high temperature resistance, high hardness, and oxidation resistance, and is widely used in surface strengthening technology. The surface layer of 42CrMo steel can be repaired and strengthened by laser cladding Ni60, giving full play to the advantages of nickel-based alloys and improving the surface properties of steel. The process parameters in the cladding process will directly affect the microstructure and mechanical properties of the cladding layer, so it is extremely important to explore the process parameters. Huang et al. studied the effect of scanning speed on the microstructure and friction properties of Ni3Al-based composites containing graphene nanosheets (NGs). The results show that the NG samples synthesized at a scanning speed of 450 mm/s have a dense and fine microstructure, as well as a higher relative density (98.6%), a lower friction coefficient (0.23) and a wear rate (5.5×10-6 mm3/ (N·m). Optimizing the scanning speed can effectively control the surface hardness and relative density of NG as well as the friction performance. Kobryn et al. studied the effects of laser power and lateral speed on the microstructure, porosity and stacking height of laser deposited Ti-6Al-4V, and found that the width of the columnar crystals decreased with increasing cooling rate, and the unfused and porosity decreased with increasing lateral speed and power level. Qiu et al. studied the microstructure and properties of Invar36 samples selectively laser melted. The results showed that when the laser speed v was less than 53.33 mm/min, the porosity was low (less than 0.5%); v>53.33 mm/min When the porosity increases significantly. In addition to the parameters mentioned above, the effects of factors such as preset layer thickness, spot diameter, and pulse width on the cladding layer are also studied. In addition, the comprehensive effects of different parameters are also studied, such as laser specific energy (E), which is a comprehensive influencing factor regulated by laser power, scanning speed, and spot diameter, and is an important evaluation index of laser cladding process.

Ni60 coating was prepared on the surface of 42CrMo steel using Ni60 alloy powder. The specific energy was controlled by adjusting the laser power and scanning speed. The influence of specific energy on the forming quality, microstructure, microhardness, shear strength, and wear resistance of the cladding layer was analyzed in order to obtain a high-quality cladding layer.

1 Experimental materials and methods

1.1 Experimental materials
The experimental substrate material is 42CrMo alloy structural steel with a size of 150 mm×100 mm×10 mm. Before the test, the surface was polished smooth with metallographic sandpaper, and then acetone and alcohol were used to remove surface impurities. The cladding powder was spherical Ni60 with a mesh size of about 300. Its microstructure is shown in Figure 1, and its chemical composition is shown in Table 1.

1.2 Experimental method
This test adopted the method of pre-setting powder. The alloy powder and alcohol were evenly mixed in a mortar. When the mixture was viscous, it was spread on the surface of the substrate using a powder spreader and placed in a drying oven for preheating at 120 °C for 0.5 h. The LWS-1000 Nd:YAG laser was used for laser cladding in an argon atmosphere with a gas flow rate of 12 L/min. The schematic diagram of the cladding process is shown in Figure 2.

Specific energy E is an important indicator of laser cladding process, which can be calculated by the following formula: E = P/v * D (1).

Where: E——specific energy, kJ/c㎡;

P——laser power, W;

v——scanning speed, mm/s;

D——spot diameter, mm.

The samples under different specific energies were numbered L1~L6, and the specific laser processes are shown in Table 2.

After cladding, the samples were cut out in a direction perpendicular to the scanning direction by using electric spark wire cutting, and the samples were ground and polished, and the metallographic samples were etched out with aqua regia with a mass percentage of 33%, and the microstructure and mechanical properties were studied. The X´Pert PRO MPD X-ray diffractometer was used to analyze the phase of the cladding layer, the Regulus8230 scanning electron microscope was used to observe the microstructure of the cladding layer, the VTD401 digital micro-Vickers hardness tester was used to measure the hardness of the cladding layer to find out the distribution law, and the AGS-X electronic universal testing machine was used to perform shear tests on the samples. The friction and wear test was carried out using a MMW-1A universal friction and wear tester. The sample size was a Φ4.8mm×12mm cylinder. The grinding ring material was 45 steel after heat treatment. The test speed was 100 r/min at room temperature, the load was 20 N, and the test time was 30 min.

2 Experimental results and discussion

2.1 Macroscopic morphology of the coating
Figure 3 shows the surface morphology of laser cladding layers with different specific energies under a stereo microscope. It can be seen from L1 to L6 in Figure 3 that the coatings obtained under different specific energies all show a tightly arranged fish scale morphology under the action of the laser, and have an obvious metallic luster under the protection of argon. When the specific energy is small (L1, L2), the irradiation energy per unit area is low, and some powders fail to melt in time to form splashes. With the increase of laser specific energy (L3, L4), the irradiation energy is moderate, the splashes are reduced, and the connection between the fish scales becomes compact from loose, and the thickness is uniform. When the specific energy increases to 5.6 kJ/cm2 (L5), some burns and splashes appear on the surface, the fish scale pattern gradually becomes uneven, and the forming quality is reduced. This is because with the increase in specific energy, the radiation received per unit area becomes stronger, while the powder’s absorption rate of the laser remains approximately unchanged, which increases the force between the laser and the powder and easily causes splashes, affecting the surface morphology. In addition, when the specific energy is large, the unit area receives too much energy, which leads to ablation and oxidation of the powder.

2.2 Phase composition and microstructure
Figure 4 shows the X-ray diffraction patterns of the cladding layers with three laser specific energies (L1, L4, and L6). As shown in Figure 4, when the laser specific energy is 4.35 kJ/c㎡, 5.40 kJ/c㎡, and 5.80 kJ/c㎡, respectively, the main phases of the cladding layer are (Fe, Ni), Austenite, and FeNi3, and the intensity of the diffraction peak is high. As the specific energy increases, the irradiation energy per unit area increases, the crystallinity of the coating increases accordingly, and the diffraction peak intensity gradually increases. It can be seen that the peak intensity increases significantly from L1 to L4 (specific energy increases from 4.35 kJ/c㎡ to 5.40 kJ/c㎡). By comparing the X-ray diffraction peak areas of the two, it can be judged that L4 contains more (Fe, Ni) and other phases than L1, while the specific energy changes from L4 to L6 (specific energy increases from 5.40 kJ/c㎡ to 5.8 kJ/c㎡) and the corresponding diffraction peak intensity changes are relatively small.

Figure 5 shows the microstructure of the cladding layer under different specific energies. The entire cladding layer has no pores, no cracks, and a dense structure. According to the organizational morphology, it can generally be divided into three regions: upper, middle, and lower. The results show that the microstructure of the cladding layer under different laser specific energies is mainly composed of parenchyma, dendrites and columnar crystals (L1~L6). Due to the rapid solidification characteristics of laser cladding, secondary dendrites are difficult to grow, so most of the dendrites are short primary dendrites. In contrast, the higher the laser specific energy, the more complex its microstructure. In the L1 cladding, the specific energy is small, the energy absorbed per unit area is low, and the solidification speed of the molten pool is fast, so that the grains solidify before they have time to grow, forming fine parenchyma grains (L1) in the cladding. When the specific energy increases from 4.35 kJ/c㎡ (L1) to 4.64 kJ/c㎡ (L2) and 4.80 kJ/c㎡ (L3), the absorbed energy increases, and the partial cladding crystals in the middle area of ​​the L2 coating are elongated into short rods, tending to columnar grains, and the short rod-shaped grains in the lower area of ​​L3 are transformed into slender columnar grains, which grow preferentially along the vertical isotherm. According to Laxmanan’s alloy solidification theory, the formula at this time is GL/R＜ΔT0/2DL (2)
Where: GL——temperature gradient of the liquid phase at the interface front, ℃;
R——solidification rate, mm/h;
T0——solid-liquid interface temperature difference, ℃;
DL——solute diffusion coefficient in the liquid phase;
Generally, ΔT0 and DL of alloys are constants. If GL is considered unchanged, when R is greater than the critical solidification rate of the component supercooling (Rc≈1.9 μm/s) is close to 2 times, the parenchyma gradually transforms into dendrites, and some dendrites can be seen in the gaps between columnar crystals in the middle area of ​​L4. As the specific energy increases, the columnar crystals gradually enter the stage of stable growth, and a large number of columnar crystals can be seen in the middle. When the growth rate is greater than 20 times the critical solidification rate Rc, it breaks through the stable growth stage, and some short secondary arms (L6) gradually appear on both sides of the columnar crystals.

2.3 Hardness distribution and friction and wear performance
Figure 6 shows the hardness distribution of the cladding layer to the substrate under different specific energies. The hardness is measured point by point from the surface of the cladding layer to the scanning direction perpendicularly. Five points are measured horizontally at 50 μm intervals and the average value is taken as the average hardness at this depth. The results show that the average hardness at 50 μm from the surface of the cladding layer from L1 to L6 is 380.2HV0.1, 364.5HV0.1, 358.1HV0.1, 350.4HV0.1, 350.3HV0.1, and 348.1HV0.1, respectively. The surface hardness is highest when the specific energy is 4.35 kJ/cm2 (L1). Compared with the 42CrMo matrix (average hardness 210HV0.1), the Ni60 cladding layer exhibits higher microhardness, which is about 1.4~1.96 times the matrix hardness, and the microhardness distribution law is similar, that is, the hardness of the cladding layer area is high and the matrix hardness gradually decreases. The higher hardness of the cladding layer is mainly attributed to the combined effect of the solid solution strengthening of (Fe, Ni), the dispersion strengthening of the in-situ generated phase FeNi3 in the cladding layer, and the fine grain strengthening brought by the rapid heating and cooling of laser cladding. It can be seen from the curve in Figure 6 that with the increase of specific energy, the hardness of the cladding layer gradually decreases. At the same time, as discussed above, with the increase of specific energy, the cladding crystal structure gradually transforms into columnar crystals and dendrites, and the increase of columnar crystals reduces its microhardness. Among them, the dynamic behavior of L3 and L5 molten pools has certain differences. The hardness of the middle structural transformation zone is higher than that of the surface. This is due to the full thermal convection of the molten pool under this specific energy. The average microhardness of the coating of L1~L3 is higher than that of L4~L6. On the other hand, the increase of specific energy makes the irradiation energy per unit area higher, the dilution rate of the cladding layer increases, and more Fe elements in the parent material enter the molten pool, which reduces the overall hardness of the coating. Affected by the rapid solidification characteristics of laser cladding, the structure of the middle and lower regions of the cladding layer is the most complex. When the specific energy increases, it can be seen that the columnar crystals and dendrites increase and intersperse with the cladding crystals, resulting in complex changes in the hardness of the coating area, which corresponds to the fluctuation distribution of the coating hardness in Figure 6.

Figure 7 shows the friction coefficient-time curve of the Ni60 cladding layer under different specific energies. It can be seen that the friction coefficient increases rapidly in the initial stage of L1~L6 friction and reaches a stable friction stage at about 100 s. When the specific energy is 4.35 kJ/c㎡ (L1), the average friction coefficient of the cladding layer is about 0.29, and when the specific energy is 5.8 kJ/c㎡ (L6), the average friction coefficient is about 1.3. Comparing the cladding layers under different specific energies, it can be found that the average friction coefficient of the cladding decreases with the decrease of specific energy, and the overall change trend is basically consistent with the change trend of microhardness with specific energy. According to Archard’s law, the wear resistance of the cladding layer is positively correlated with its microhardness. With the increase of specific energy, the dilution rate of the cladding layer increases, a large amount of Fe elements flow into the cladding layer, and the content of the hard phase decreases, thereby reducing the hardness of the cladding layer. At the same time, the coarsening of the microstructure also has an adverse effect on the hardness. This makes the adhesion of the coating material relatively increased, and more plastic deformation and transfer of friction products occur on the surface during friction, which increases the friction resistance between the grinding ring and the friction coefficient. From the comparison of the wear amount of the Ni60 cladding layer (L1~L6) in Figure 8, it can be seen that the wear amount of the cladding layer is different for different specific energies. The wear amount is 0.8 mg when the specific energy is 4.35 kJ/c㎡ (L1), and the wear amount is 1.9 mg when the specific energy is 5.8 kJ/c㎡ (L6). From L1~L6, as the specific energy increases, the hardness of the coating decreases, the friction products generated at the friction interface increase, and the wear amount gradually increases. The friction and wear surface of the Ni60 coating was characterized by SEM. As shown in Figure 9, different damages appeared on the surface of the L1~L6 coating, including furrows, abrasive particles and partially peeled adhesives. In L1 (4.35 kJ/c㎡), shallow and narrow furrows can be seen on the wear surface of the cladding layer. At this time, the cladding layer has high hardness and the best wear resistance, which is consistent with the friction coefficient and wear amount of the cladding layer under different specific energies. At the same time, some scattered abrasive particles can be seen on the surface of the cladding layer. These are hard particles that fall off during the friction process, indicating that abrasive wear has occurred in the cladding layer. When the specific energy increases (L2~L4), the number of abrasive particles that fall off the surface gradually increases, and a small amount of peeling occurs (L3). As the surface hardness decreases, the friction surface is further worn, the friction products increase, and a certain thickness of transfer film is formed under the action of shear force and adheres to the surface. When the specific energy is greater than 5.6 kJ/cm2 (L5~L6), a clear transfer film can be seen attached to the wear surface under high magnification, and the wear amount reaches up to 1.9 mg. Based on the morphology of the friction interface under different specific energies, it can be seen that the main wear behavior of the cladding layer under dry friction wear conditions is a composite wear combining abrasive wear and adhesive wear.

2.4　Shear strength of cladding layer
Figure 10 shows the shear force-time curve of Ni60 cladding layer under different specific energies of laser cladding. It can be seen from the figure that the maximum load that the cladding layer and the substrate bonding interface can withstand with different specific energies is about
2500 N, and its shear strength can reach 225~259 MPa, indicating that the Ni60 cladding layer and the 42CrMo substrate have achieved good metallurgical bonding. When the specific energy is 4.35 kJ/cm2 (L1), the shear strength is 234 MPa, while when it is 4.46 kJ/cm2 (L2), it is 259 MPa. After that, with the increase of specific energy, the shear strength fluctuates slightly around 230 MPa. Due to the difference in irradiation energy per unit area under different specific energies, on the one hand, the cladding layer has different dilution rates, which affects the distribution of alloy elements near the fusion line, and on the other hand, it has a complex microstructure. The two have a comprehensive impact on the shear strength of the cladding layer and the substrate.

3 Conclusion

(1) The phase types of Ni60 cladding layer are consistent under different specific energies, which has a great influence on the surface forming quality and the distribution of the structure. When the specific energy is too high or too low, the surface will produce large spatter or burn. With the increase of specific energy, the structure of the cladding layer coarsens to a certain extent, and changes from fine parcel crystals to columnar crystals; when the specific energy is 4.8 kJ/c㎡, the surface of the cladding layer has small spatter, the fish scale pattern is closely arranged and the thickness is uniform, the structure is dense and defect-free, and good metallurgical bonding is achieved.
(2) After laser cladding Ni60 coating, the microhardness of the cladding layer is significantly improved compared with the substrate, which is about 1.4~1.96 times the hardness of the substrate. In the cladding layer, the combined effect of (Fe, Ni) solid solution strengthening, in-situ formation of hard phase FeNi3 dispersion strengthening and cladding layer fine grain strengthening makes the cladding layer have a higher microhardness. Its hardness value gradually increases with the decrease of specific energy. At the same time, affected by the specific energy and the distribution law of the cladding structure, its hardness gradually decreases from the cladding layer to the substrate.
(3) Consistent with the law of microhardness change of the cladding layer under different specific energies, the friction coefficient and corresponding wear amount of the cladding layer will decrease with the decrease of specific energy. When the specific energy decreases from 5.8 kJ/c㎡ to 4.35 kJ/c㎡, its friction coefficient decreases from 1.3 to 0.29, and the wear amount decreases from 1.9 mg to 0.8 mg; under dry friction wear conditions, the main wear behavior of the cladding layer is a composite wear form combining abrasive wear and adhesive wear.
(4) The laser cladding Ni60 coating has good shear resistance, which can reach 225~259 MPa. The cladding layer and the substrate achieve good metallurgical bonding, and the specific energy has little effect on the shear strength. Considering the effect of specific energy on the forming, microstructure and mechanical properties of the cladding layer, it can be seen that the cladding layer has better performance when the specific energy is 4.8 kJ/c㎡.