The influence of process parameters on the geometric characteristics and hardness of Stellite 157/WC/TiC composites by laser cladding on high-speed steel surface was studied by response surface Box-Behnken design experiment. A mathematical model between the variables laser power, scanning speed, powder feeding rate and the response target cladding layer width, aspect ratio, dilution rate and hardness was established. For blade cladding, the geometric characteristics and hardness of the cladding layer were optimized and controlled, and the optimal process parameter combination was obtained: laser power 820W, scanning speed 237mm/min, powder feeding rate 9g/min. The blade cladding test was carried out using the optimized process parameters. The hardness after cladding was 840HV, reaching the original hardness of the blade and meeting the processing allowance, which provides a reference for the optimization of laser cladding process parameters and the repair of high-speed steel broach blades. (Laser cladding; Process parameters; Response surface; Co-based alloy; Broach repair)

1 Introduction

Laser cladding technology is an advanced material forming technology with the characteristics of low dilution rate, high forming efficiency, good bonding of cladding layer, green, and flexible application scenarios [1]. There are many studies on laser cladding technology, including different process parameter optimization methods, different cladding material components and properties, and different application fields [2-4]. As an important tool in mechanical processing, broaches are widely used in precision processing fields such as automotive equipment, aerospace, and marine equipment [5]. However, due to problems in the design, manufacturing, and use process, broaches often suffer from excessive wear and chipping, which greatly reduces the life of broaches [6]. The wear morphology of broaches is shown in Figure 1. At present, the research on extending the life of broaches mainly focuses on coating, design, manufacturing, and testing [7-9]. There are few studies on using laser cladding technology to repair broach blades.

In this paper, the response surface method is used to mathematically model, optimize and test the cladding process parameters of the high-speed steel broach blade, and a better combination of process parameters is obtained. The cladding test is carried out on the high-speed steel blade, which provides an important basis for improving the life of the high-speed steel broach and repairing the damaged blade.

2 Experimental equipment and experimental plan

2.1 Experimental materials
W18Cr4V high-speed steel is selected as the matrix, and a 20mm×20mm×10mm sample is made by DK7732 type electric spark wire cutting machine. It is polished with 240#~1000# sandpaper, polished with alumina polishing powder with a particle size of 0.5μm on a polishing machine, and ultrasonically cleaned with anhydrous ethanol and dried for use. Table 1 shows the chemical composition of W18Cr4V high-speed steel.

The cladding materials are 25% homemade bulk Co-wrapped WC (200-300 mesh), 2% agglomerated small-scale TiC (0.5 μm) and 73% spherical Stellit 157 (140-240 mesh). Table 2 shows the composition of Stellit 157 cobalt-based alloy materials. The cladding materials are weighed in proportion and mixed in a QM-QX04 planetary ball mill. The ball-to-material ratio is 5:1, the mixing time is set to 12h, and the speed is 220r/min. After the mixing is completed, the materials are dried in an electric furnace and stored in a vacuum for standby use.

2.2 Test equipment

The laser cladding system is shown in Figure 2a, which includes three parts: HWL-RAW1000 semiconductor laser generator, wavelength 980nm, maximum power 1000kW; automatic powder feeding system uses argon as protective gas, and the powder feeding method is three coaxial fixed nozzles; four-axis CNC system to control the movement of the laser cladding head. The cladding sample was cut and polished to a mirror surface, and the cladding layer was corroded with aqua regia with HNO3: HCl = 1:3. The geometric characteristics and microstructure of the cladding layer were observed with the help of the MDS400 metallographic microscope shown in Figure 2b. The hardness was tested using the HMV-G21 Vickers microhardness tester shown in Figure 2c. The test force was set to 1.96N, the holding time was 15s, the interval between two points was 100μm, and the average value was taken for three measurements at the same horizontal position.

2.3 Experimental method
Through preliminary experiments, the optimal process parameter interval parameters are determined as follows: laser power 800-900W, scanning speed 200-300mm/min, powder feeding rate 4.825-8.825g/min, defocusing amount 11mm, carrier gas flow rate 3.5L/min, substrate preheating 200℃. On this basis, the response surface method is used to study the geometric characteristics and performance of the cladding layer in this interval. With the help of the Box-Behken module in the Design-Excel software, a three-factor test table is designed. The cladding layer width, aspect ratio, dilution rate and hardness are used as response targets. The variance analysis method is used to analyze the significance of the experimental measurement results, and a second-order regression model of process parameters and response targets is constructed to achieve the prediction and control of the geometric characteristics and hardness of the cladding layer. Table 3 is the response surface test design table, and Figure 3 is the surface macroscopic morphology of the cladding layer of the test group.

3 Experimental results and discussion

Table 4 shows the response surface test group and the geometric parameters and hardness measurement results. The response values ​​are the cladding layer width, aspect ratio, dilution rate and hardness. Since the rake angle of the broach blade in this experiment is 15° and the back angle is 2.5°, the area at the blade is small, and the powder is not easy to stay during cladding. In addition, in order to achieve blade repair, the size of the cladding layer must meet the tooth grinding allowance, so a smaller aspect ratio is used as the optimization target. Since the dilution rate [10] is closely related to hardness, the process parameters with a smaller dilution rate are selected while ensuring that the cladding layer and the substrate achieve metallurgical bonding [11]. Hardness is an important performance indicator of the broach blade. According to the technical standard of round broaches, the hardness of 63~66HRC is used as one of the optimization targets. The square model in Design-Excel software is used for fitting analysis.

3.1 Model analysis
It can be seen from Table 5 that the P values ​​of the cladding layer width, aspect ratio, dilution rate and microhardness are all less than 0.05, indicating that the model is significant. R2 is the model correlation coefficient, and the closer it is to 1, the better the correlation. The signal-to-noise ratio is greater than 4, which proves that the model is highly identifiable. The signal-to-noise ratios of the four models in the table are all greater than 4, so the regression equation can be used to replace the real point to analyze the test.

The mathematical models of the cladding layer width, aspect ratio, dilution rate and hardness are as follows: see formula (1)-(4) in the figure

3.2 Residual normal probability distribution and prediction analysis
Figure 4 is the residual normal probability distribution diagram of the optimized target width, aspect ratio, dilution rate and hardness of the cladding layer. The residual of the test value is distributed along the straight line and meets the normal distribution, indicating that the model has a good fit. Figure 5 is a comparison diagram of the actual value and predicted value of the optimized target width, aspect ratio, dilution rate and hardness of the cladding layer. The test data are distributed on both sides of the straight line, and there are no points with abnormal positions, which further proves that the model has good prediction accuracy.

3.3 Geometric feature analysis

3.3.1 Influence and analysis of process parameters on cladding layer width
Table 6 is the variance analysis table of cladding layer width. The P values ​​corresponding to laser power and scanning speed are 0.0033 and 0.0016, respectively, both less than 0.05, indicating that laser power and scanning speed have significant effects on cladding layer width, among which scanning speed has the greatest influence, followed by laser power, while the P value corresponding to powder feeding rate is 0.5602, greater than 0.05, indicating that the powder feeding rate has no significant effect on cladding layer width. By comparing the mean square value, the influence of process parameters on cladding layer width is B>A>C. In other studies, the influence of laser power on cladding layer width is greater than scanning speed [12], which is not completely consistent with this study, which may be related to the selection range of the optimization interval of each process parameter.

In order to further study the influence of process parameters on the width of the cladding layer, the perturbation diagram of the influence of process parameters on the width of the cladding layer is analyzed, as shown in Figure 6. As the laser power increases, the width of the cladding layer tends to increase. The laser energy density increases with the increase of laser power. As the power density increases, on the one hand, the substrate absorbs more energy, the melting range of the substrate increases, the molten pool becomes wider, the molten pool can capture more powder, and the cladding layer becomes wider; on the other hand, the increase of laser power increases the energy absorbed by the cladding powder, more powder melts, and under the action of gravity, the melted material spreads to both sides, increasing the width of the cladding layer [13]. This can be seen from Figure 7. When the laser power is 800W, the width is almost unaffected by the powder feeding rate. When the laser power is 900W, the width of the cladding layer increases to a certain extent with the powder feeding rate. As the scanning speed increases, the width of the cladding layer gradually decreases. Since the linear energy density is inversely related to the scanning speed (laser power/scanning speed) [14], the laser irradiation time of the substrate becomes shorter, the absorbed energy decreases, the melting range of the substrate becomes smaller, and the width of the cladding layer decreases [15]. As the scanning speed increases, the amount of powder reaching the unit area of ​​the substrate during the cladding process decreases, and the powder forming the cladding layer decreases, which also leads to a decrease in width.

3.3.2 Effect and analysis of process parameters on the aspect ratio of the cladding layer
Table 7 shows the variance analysis results of the aspect ratio of the cladding layer. Among them, the P value corresponding to the powder feeding rate is <0.0001, indicating that the effect on the aspect ratio of the cladding layer is extremely significant; the P value of the interaction term of the scanning speed, laser power and scanning speed is <0.05, indicating that the effect on the aspect ratio of the cladding layer is significant. The order of the influence on the aspect ratio of the cladding layer is C>B>AC.

It can be seen from Figure 8 that the powder feeding rate is negatively correlated with the aspect ratio of the cladding layer, and the scanning speed is positively correlated with the aspect ratio. The effect of laser power on the width is not significant. As the powder feeding rate increases, the width of the cladding layer remains almost unchanged, but the amount of cladding material that reaches the substrate and melts increases, resulting in an increase in height, so the aspect ratio decreases [16]. As the scanning speed increases, the width of the cladding layer decreases, and the height of the cladding layer also decreases due to the decrease in laser energy density and the amount of powder reaching the substrate. The effect of scanning speed on the height of the cladding layer is smaller than that on the width, so the aspect ratio tends to increase. Figure 9 shows the interaction between laser power and powder feeding rate on the aspect ratio. As the powder feeding rate increases, the aspect ratio decreases at all powers.

3.3.3 Effect and analysis of process parameters on the dilution rate of cladding layer
The dilution rate directly affects the metallurgical bonding performance and physical properties of the cladding layer. Table 8 shows the variance analysis of the dilution rate of the cladding layer. The powder feeding rate (P<0.05) and scanning speed (P<0.05) have significant effects on the dilution rate, while other items have no significant effects on the dilution rate. By comparing the size of the mean square value, the influencing factors are ranked as B>C>A.

Figure 10 shows the influence of various factors on the dilution rate of the cladding layer. The increase in the powder feeding rate will reduce the dilution rate, and the increase in the scanning speed will increase the dilution rate. The main reason why the powder feeding rate affects the dilution rate is that the increase in the amount of powder feeding makes more powder reach the substrate, and the powder absorbs more laser energy, so the dilution rate continues to decrease. Similarly, the increase in the scanning speed reduces the amount of powder reaching the unit area of ​​the substrate, more energy is allocated to the substrate, the depth of the molten pool increases, the height of the cladding layer decreases, and the dilution rate increases.

3.4 Hardness analysis
The hardness of the cladding layer is mainly affected by the size of the bonding phase grains and the hard phase particles. Table 9 is a variance analysis table of the hardness of the cladding layer. The laser power, scanning speed, and powder feeding rate all have significant effects on the hardness of the cladding layer, and the order of influence on hardness is C>A>B. The interaction of each factor has no significant effect on the microhardness.

Figure 11 is a perturbation diagram of the influence of process parameters on hardness. The hardness of the cladding layer decreases with the increase of laser power and scanning speed, and increases with the increase of powder feeding rate. When the laser power increases, the life of the molten pool is extended, and the growth time after grain nucleation increases accordingly, resulting in grain coarsening.

According to the Hall-Pech formula, the larger the grain size of the material, the lower the microhardness, so the increase of laser power reduces the hardness [17]. The scanning speed has a significant effect on the dilution rate of the cladding layer. The scanning speed is positively correlated with the dilution rate. The increase in the dilution rate causes more Fe in the matrix to enter the cladding layer, diluting the original material, increasing the soft phase in the cladding layer, and thus the hardness continues to decrease [18]. The powder feeding rate increases, the dilution rate decreases, and the hardness of the cladding layer increases.

3.5 Process parameter optimization and model verification
With the help of the parameter optimization module in Design-Excel software, the width of 1100μm, the minimum dilution rate, the minimum aspect ratio, and the hardness of 850HV are selected as the optimization targets, and the importance is set to 3, 3, 3, and 5, respectively, as shown in Table 10.

11 is a set of process parameters with the largest expected value after optimization, and the model verification test is carried out on this basis. The optimal process parameter group (laser power 820W, scanning speed 237mm/min, powder feeding amount 9g/min) was selected, three cladding tests were carried out and the average of the measurement results was taken. As shown in Table 12, the cladding layer width is 1091μm, the aspect ratio is 2.22, the dilution rate is 34%, and the hardness is 885HV. The corresponding errors are 0.82%, -9.49%, 8.11%, -4.12%, and the average error rate is 5.653%. The model has high accuracy through experimental verification. The optimized cladding layer has a good cross-sectional morphology, no macroscopic pores and cracks appear, and the hard phase particles are evenly distributed. The cross-sectional morphology of the cladding layer is shown in Figure 12.

4 Laser cladding test of broach blade

4.1 Process parameter optimization and model verification
The sample with the same angle as the blade was processed and the cladding test was carried out in the manner shown in Figure 13a. The number of layers was two. Figure 13b shows the cross-sectional morphology of the blade after cladding. There are no macro cracks and pore defects. The total thickness of the cladding layer is 799μm, and the size meets the machining allowance of the blade.

4.2 Micromorphology
Figure 14a shows the micromorphology of the back face after grinding. There are no pores and cracks in the cladding layer. The unmelted tungsten carbide particles are preserved, and the micron-sized titanium carbide is evenly distributed. Figure 14b shows the metallographic structure of the heat-affected zone, where the fusion line between the cladding layer and the substrate can be clearly seen. The martensite is decomposed and transformed into austenite under the action of rapid heating and cooling of laser cladding, and the carbides are distributed in the structure. Figure 14c shows the metallographic structure of the bottom of the cladding layer, which is plane crystal, columnar crystal and dendrite in sequence. Unmelted tungsten carbide is embedded in the structure, which plays a role in grain refinement and hard phase strengthening. Figure 14d shows the middle and upper part of the cladding layer, which is composed of dendrite and fine equiaxed crystal [19].

4.3 Hardness test
The average hardness of the cladding layer is 840HV, which is within the hardness requirement of 63-66HRC for high-speed steel broach blades. The hardness of the blade cladding layer is lower than that of the flat substrate cladding layer. Due to the thin blade and poor heat dissipation, the blade melts too much when the temperature rises, resulting in more Fe elements in the substrate entering the cladding layer, which dilutes the cladding layer and reduces the microhardness.

5 Conclusion
(1) The mathematical model between the laser cladding process parameters and the optimized target width, aspect ratio, dilution rate and hardness was established by response surface methodology, and the significance of the model was proved.
(2) The width increases with increasing laser power, and the width decreases with increasing scanning speed; the aspect ratio and dilution rate increase with increasing scanning speed, and the aspect ratio and dilution rate decrease with increasing powder feeding rate; the power and scanning speed are negatively correlated with hardness, and the powder feeding rate is positively correlated with hardness.
(3) The parameters of blade cladding are optimized, and the optimized process parameters are: laser power 820W, scanning speed 237mm/min, powder feeding amount 9g/min. Cladding test verification is carried out, and the measured width is 1091μm, the aspect ratio is 2.22, the dilution rate is 0.34, and the hardness is 885HV. The errors with the predicted values ​​are 0.82%, -9.49%, 8.11%, -4.12%, respectively, and the average error is 5.653%. The model has high accuracy and good predictability.
(4) The optimized process parameters were used to carry out blade cladding test. The cross-sectional size of the cladding layer met the grinding allowance. The microstructure of the cladding layer after grinding was good, with no macroscopic pores or cracks. The microhardness was 840 HV. The hardness of the blade after cladding was lower than that of the plane cladding.