Taking the repair and remanufacturing of friction damage areas of mechanical equipment as the background, 17-4PH martensitic precipitation hardening stainless steel for turbine blades was selected as the research object. Laser cladding technology was used for surface innovative repair. Ni and WC alloy powders were clad on the surface of 17-4PH stainless steel with different powder feeding amounts. The microstructure and composition distribution of the cladding layer, the interface between the cladding layer and the substrate, and the substrate were observed and tested. The mechanical properties of the cladding layer were tested. The direct effect of the powder feeding amount on the coating quality during the cladding process was analyzed during the experiment. The experimental results show that under the same laser power (1 250 W) and powder feeding rate (10 mm/s), the microhardness of the cladding layer increases with the increase of powder feeding amount.

According to incomplete estimates, more than 38% of mechanical material loss in the world is caused by wear each year, and the scrap rate of mechanical equipment and parts caused by wear is nearly 80%. Therefore, how to reduce resource loss is a common concern in the world. Repairing and remanufacturing the friction damage area of ​​mechanical equipment is an important way to save resources and reduce energy consumption, especially for some equipment that needs to work in harsh environments, such as engines, turbine blades, coal machines, etc. [1-3]. This paper uses laser cladding technology to clad Ni and WC alloy powders on the surface of 17-4PH stainless steel with different powder feeding amounts, and observes the microstructure and phase characterization of the sample cladding layer, and then conducts microhardness test and wear resistance test on the coating.

1 Experimental content
The base material selected in this test is 17-4PH martensitic precipitation hardening stainless steel with a size of 20 mm×20 mm×8 mm. Self-made Ni powder and WC composite alloy powder are used as cladding materials, and the mass ratio of WC alloy powder to Ni powder is 1:4, that is, 80% Ni powder and 20% WC alloy powder.

The grouping with powder feeding amount as the variable was designed. The laser power was 1 250 W and the powder feeding rate was 10 mm/s. The influence of powder feeding amount on the microstructure and performance of the cladding layer was discussed. The process parameter design is shown in Table 1.

The cladding equipment used in this experiment is a laser cladding device—fiber-coupled semiconductor laser, with a maximum output power of 4 000 W, a focal length of 150 mm, and a spot diameter of 1.5 mm.
After the laser cladding was completed, the microstructure and composition distribution of the sample cladding layer, the interface between the cladding layer and the substrate, and the substrate were observed by scanning electron microscopy. Then, the composition analysis of the cladding layer and the interface between the cladding layer and the base material was performed, including quantitative and qualitative analysis.
The microhardness of the test sample was tested. The load was set to 0.981 N (100 gf) and the loading time was 10 s. The hardness of 4 points on the surface of the cladding layer was measured in an equidistant manner and the average value was taken.
Without adding lubricant and maintaining room temperature, friction and wear tests were carried out on the polished cladding specimen surface coating and the polished substrate material surface using an HRS-2M testing machine. The test load was 20 N, the test time was 15 min, the running speed was 200 r/min, and the friction distance was 5 mm. After the friction and wear test was completed, the friction coefficient curve was drawn.

2 Test results and analysis

2.1 Microstructure analysis
Figure 1 is a line scan photo of the junction of the cladding layer of the sample obtained with different powder feeding amounts. The dark part in the picture is the substrate, and the light part is the cladding layer. Through energy spectrum detection, it is known that the content of Fe and Cr elements in the substrate is significantly higher than that in the cladding layer, and the content of Ni and W elements in the cladding layer is significantly higher than that in the substrate. The content of other elements such as C, Co, Nb and Cu elements in the coating and substrate is not much different. This shows that the elements in the substrate and the cladding layer will penetrate each other, the Ni and W elements in the cladding powder will be diluted to the substrate surface, and some elements such as Fe and Cr in the substrate will also transition to the cladding layer.
However, the relative strengths of Fe and Cr in the cladding layer are different at different powder feeding rates. When the powder feeding rate is 18 mg/s, the relative strength of Fe in the cladding layer is stable at about 120, and the relative strength of Cr is stable at about 45; when the powder feeding rate is 22 mg/s, the relative strength of Fe in the cladding layer is stable at about 100, and the relative strength of Cr is stable at about 40; when the powder feeding rate is 25 mg/s, the relative strength of Fe in the cladding layer is stable at about 45, and the relative strength of Cr is stable at about 20, and the relative strength of Fe and Cr in the cladding layer gradually decreases. From the analysis, it can be seen that under the same laser power (1250 W) and the same powder feeding rate (10 mm/s), the greater the powder feeding amount, the smaller the relative content of Fe and Cr elements diluted into the cladding layer[6]. The W and Ni elements account for a large proportion in the light-colored part of the cladding layer, and the content of Ni, Fe, and C in the dark part is greater than that of these three elements in the light-colored part. The content of other elements is not much different.

From Figure 2-1, it can be seen that the phase composition of the first diffraction peak of the substrate is mainly Fe, the second peak is mainly Fe-Cr composite phase, and the third peak is mainly Cr. According to Figures 2-2, 2-3 and 2-4, the first diffraction peak is Cr-Ni-W composite phase, the second peak is Ni-W composite phase, and the third peak is a composite phase composed of multiple elements, mainly including C, Co, Cr, Fe, Mo, Ni, W, etc., and the diffraction peak positions of the sample cladding layer are basically the same, and the phases contained are almost the same [7]. Therefore, under the same laser power (1250 W) and the same powder feeding rate (10 mm/s), the increase in powder feeding amount has no significant effect on the phase of the coating.

2.2 Hardness analysis
The cross section of the sample was polished after the oxide layer was removed by grinding. The hardness of 4 points on the surface of the polished cladding layer was measured at equal distances and the average value was taken [8-9]. The test results are shown in Figure 3. The load used in the test was 0.981 N (100 gf), the loading time was 10 s, and the powder feeding amount of samples 1 to 3 was 18 mg/s, 22 mg/s, and 25 mg/s from low to high.
As shown in Figure 3, the average microhardness (HV) of the base material is 325. The average microhardness of the cladding layer of samples 1 to 3 is greater than that of the stainless steel substrate. The average microhardness (HV) of the samples is 339.9, 375.8 and 404.7, respectively. The hardness value gradually increases with the increase of the powder feeding amount. Combined with Figure 2, it can be seen that with the increase of powder feeding amount, the white structure rich in W and Ni elements is more dispersed, which plays a certain strengthening role, so that the microhardness of the coating increases with the increase of powder feeding amount.

2.3 Wear resistance analysis
Figure 4 is a curve of the friction coefficient of the substrate and samples No. 1-3 changing with time.
It can be seen from Figure 4 that the friction coefficient of the substrate is basically maintained at about 1.8; when the powder feeding amount is 18 mg/s, the friction coefficient of the cladding layer shows a gradual upward trend, and finally stabilizes at around 1.4; when the powder feeding amount is 22 mg/s, the friction coefficient of the coating also shows a gradual increasing trend, and finally stabilizes at around 1.2; when the powder feeding rate is 25 mg/s, the friction coefficient of the cladding layer gradually increases, and finally stabilizes at around 1.0. The friction coefficients of the cladding layers of the three samples are all smaller than that of the substrate. Under the same laser power and powder feeding rate (1 250 W, 10 mm/s), the friction coefficient gradually decreases with the increase of powder feeding amount, and the fluctuation amplitude of the friction coefficient decreases, the change tends to be stable, and the wear resistance of the coating is obviously superior.

3. Conclusion
1) Ni/WC coating was prepared on the surface of 17-4PH stainless steel using laser cladding technology, and the main phase compositions are Cr4Ni15W and Ni17W3.
2) When other process parameters are constant, the microhardness of the material increases with the increase of powder feeding amount, the friction coefficient gradually decreases with the increase of powder feeding amount, and the fluctuation amplitude of the friction coefficient decreases, and the change tends to be stable.