Relationship between the surface quality of micro-milling processing materials of laser cladding WC/ Ni alloy powder coating. Die steel Cr12MoV was selected as the substrate, WC/ Ni alloy powder was used as the cladding material, and fiber laser was used to carry out cladding processing experiments. Test samples were prepared, and micro-milling experiments were carried out on the prepared samples using a tungsten steel carbide milling cutter with a diameter of 1 mm. Single factor experiments and orthogonal experiments were designed respectively, and the surface roughness Ra was used as the evaluation standard to explore the influence of micro-milling processing parameters on the milling surface quality of WC/ Ni alloy powder cladding coating, and the milling mechanism was explained. The results show that the spindle speed has the most significant effect on the surface roughness, followed by the feed rate, and the milling depth has the least effect; when the spindle speed is 9000 r/min, the feed rate is 6 mm/min, and the milling depth is 0.04 mm, the minimum roughness is measured to be 0.014 μm; when micro-milling WC/Ni alloy powder cladding coating, smaller processing parameters can be selected to obtain better surface quality.
Laser cladding is a new type of surface strengthening method. By rapidly heating and melting the alloy powder and the substrate surface, a metallurgical bonding filler cladding layer is formed on the surface of the base layer, thereby significantly improving the surface properties of the substrate [1]. Cr12MoV is often used to manufacture various molds due to its high hardness and good wear resistance. Since the mold surface is often squeezed and rubbed, it is easy to fail [2]. Ni-WC alloy has good oxidation resistance, corrosion resistance and low magnetism, which can make up for the defects of mold steel Cr12MoV [3-4]. GAO et al. [5] used laser cladding to clad a layer of WC-reinforced Ni60 alloy on the surface of Cr12MoV to improve the mechanical properties of Cr12MoV. The test results showed that the hardness of the coating was as high as 745HV, which was 3.5 times the hardness of the substrate. HU et al. [6] studied the effect of WC content in laser cladding coating on coating performance. The results showed that as the WC content increased, the coating obtained higher thermodynamic stability and microhardness, but the surface of the workpiece after laser cladding often needs to be processed before it can meet the use requirements. Xu Shaofeng et al. [7] numerically simulated the milling process of nickel-based alloy welds and optimized the milling cutter. The results showed that a smaller cutting speed, a smaller feed rate and increased air cooling can obtain a smaller cutting force and cutting temperature. Zhou Jun and Shu Linsen [8] performed high-speed milling on nickel-based alloys after laser cladding. The results showed that the surface roughness of the clad nickel-based alloy increased with the increase of cutting depth and feed rate. ZHENG et al. [9] explored the milling mechanism of nickel-based high-temperature alloys, established the milling force model of nickel-based high-temperature alloy 718, optimized it, and gave the optimal processing parameters for milling nickel-based high-temperature alloy 718. ZHANG et al. [10] explored the influence of processing parameters on the surface of nickel-based single crystal high-temperature alloy DD5. The results showed that high cutting speed, low tool feed speed and low cutting depth can improve the surface integrity of DD5 after milling. An Yiwei et al. [11] studied the milling surface quality of 316L stainless steel laser cladding parts and found that the feed rate had the most significant effect on surface roughness, followed by the side cutting amount, and the spindle speed had the least effect. Zhang Lifeng et al. [12] studied the relationship between cutting force and processing parameters in laser cladding Ti-6Al-4V high-speed milling, and pointed out that the feed rate and milling depth had the most significant effect on the size of the cutting force.
Recently, the research on laser cladding has focused on its performance and mechanical properties, and there is little research on the micro-machining of materials. In this paper, the authors laser clad a layer of WC/Ni alloy powder on the mold steel Cr12MoV and performed micro-milling on it, analyzed the changes in the roughness of the machined surface, and explored the influence of spindle speed, feed rate and milling depth on the surface quality, providing an experimental reference for laser cladding WC/Ni alloy micro-milling.
1 Experimental design
1.1 Preparation of laser cladding test specimens
The laser cladding test uses die steel Cr12MoV as the cladding substrate, and its chemical composition is shown in Table 1. The size of the test substrate is 110 mm×90 mm×40 mm. Before cladding, the metallographic structure of the test substrate and the cold stamping die are first subjected to the same carbonitriding heat treatment to ensure the accuracy of the test. The cladding powder uses WC/Ni alloy powder, in which the mass fraction of WC is 25% and the particle size is 25-55 μm. The chemical composition of WC/Ni alloy powder is shown in Table 2. The powder is fully dried before cladding.
The equipment used for the laser cladding test is YLR-3000 fiber laser, as shown in Figure 1. The rated output power of the laser does not exceed 3000 W, the core diameter of the optical fiber is 1000 μm, the length is 10 m, and the input wavelength is 1070-1080 nm. N2 is used as the protective gas when preparing the test samples. The laser cladding process parameters are as follows: the laser power is 1200 W, the scanning speed is 4 mm/s, the powder feeding voltage is 8 V, and 3 test samples are prepared. The clad test samples are cut into 24 mm×16 mm×10 mm blocks by wire cutting to facilitate subsequent milling processing.
1.2 Micro-milling test
The equipment used for the milling test is CNC engraving machine Carver400G, the spindle speed is 2000~30000 r/min, the maximum cutting speed is 6 m/min, and the milling system is shown in Figure 2.
Before the milling test, the test sample divided by wire cutting is firstly rough-milled with a carbide milling cutter with a working diameter of 6 mm to make the surface of the test sample smooth and flat, and then a tungsten steel carbide milling cutter with a working diameter of 1 mm is used for micro-milling test. The processed workpiece is placed in an ultrasonic cleaner to remove impurities such as residual debris on the surface, and the surface roughness and three-dimensional morphology of the material processing surface are measured with a laser confocal microscope. The influence of processing parameters on the processing surface quality is studied by combining single factor test and orthogonal test.
1.3 Experimental plan
Based on milling experience and relevant literature, the milling test parameters were selected and determined. A total of 24 groups of experiments were designed, including 9 groups of three-factor three-level orthogonal experiments and 15 groups of single-factor experiments. The experimental parameters are shown in Tables 3 and 4.
1.4 Experimental result analysis
The roughness of the milled test specimens was measured at three points randomly selected by laser confocal microscopy, and the average value was taken for data analysis. The results are shown in Tables 5 and 6. The range analysis of each factor of the orthogonal experiment is shown in Table 7. In the table, Ki represents the sum of the roughness under the corresponding level factor, Ki is the corresponding Ki average value, and R represents the range. It can be seen from Table 7 that the extreme differences of spindle speed, feed rate and milling depth are 0.053, 0.046 and 0.037 respectively, indicating that when the laser cladding WC/Ni alloy coating is micro-milled, the order of influence of processing parameters on the processing surface quality is: spindle speed is the most significant, feed rate is the second, and milling depth is the smallest.
2 Analysis of measurement results
The laser confocal microscope is used to detect the processing surface of the material, and a square of 75 μm×75 μm is selected as the detection area to analyze the influence of different speeds, feed rates and milling depths on the processing surface quality of the material.
2.1 Effect of spindle speed on material surface defects
When the milling depth is kept at 0.04 mm and the feed speed is kept at 6 mm/min, the effect of the spindle speed on the machining surface quality is shown in Figure 3. It can be seen that: as the spindle speed increases, the machining surface roughness increases. When the spindle speed is 9000-13000 r/min, the surface roughness of the workpiece is 0.014-0.022 μm; when the spindle speed reaches 15000 r/min or even higher, the surface roughness of the workpiece has a significant increasing trend. Therefore, it can be inferred that in order to obtain better surface quality during micro-milling of laser cladding WC/Ni alloy coating, the spindle speed can be controlled at 9000-13000 r/min.
Theoretically, the more times the milling cutter speed increases, the more times the abrasive material cuts per unit time, and the smaller the surface roughness. However, the experimental results in this paper are contrary to the theory. This is because the experiment uses micro-milling, and the milling cutter processing diameter is 1 mm. As the spindle speed increases, the vibration generated by the processing increases, making the milling cutter’s cutting process discontinuous during milling, and producing ripples on the processed surface, resulting in reduced surface quality. Moreover, as the spindle speed increases, the cutting temperature rises, the Co element in the tungsten carbide tool diffuses[13], and the chip adhesion occurs between the tool and the workpiece, which increases the cutting force and the vibration generated during cutting, resulting in unstable cutting and increased surface roughness.
Figure 4 shows the three-dimensional morphology of the machined surface when the feed rate f = 6 mm/min and the milling depth ap = 0.04 mm are respectively 9000 and 17000 r/min. As shown in Figure 4 (a), when the spindle speed is 9000 r/min, the machined surface quality is relatively good. Although there are a few convex parts, the maximum height of the convex parts is only 0.21 μm. At the same time, there are a few ripples on the machined surface, but the ripple amplitude is not high. As shown in Figure 4 (b), when the spindle speed is 17000 r/min, there are more convex parts on the machined surface, the height is higher, and the highest part is 0.36 μm. In addition, ripples and pits are generated on the machined surface. This is because during micro-milling, the vibration amplitude generated during the machining process increases due to the increase in spindle speed, resulting in discontinuous and unstable cutting during machining, resulting in pits and bulges on the machined surface.
2.2 Effect of feed rate on material surface defects
Maintaining the spindle speed of 15,000 r/min and the milling depth of 0.06 mm unchanged, the feed rate is 4, 6, 8, 10, and 12 mm/min respectively. The effect on the machining surface quality is shown in Figure 5. It can be seen that: as the feed rate increases, the machining surface roughness gradually increases. When the feed rate is 4-6 mm/min, the curve rises relatively steadily, the surface roughness is 0.053-0.055 μm, and the surface quality is relatively good. Therefore, it can be inferred that in high-speed micro-milling of WC/Ni alloy, in order to obtain better surface quality, the feed rate can be selected to be 4-6 mm/min.
The reason for the above phenomenon is that as the feed rate increases, the height of the residual area produced by each tooth of the tool cutting the material per unit time increases. When the axial milling depth is constant, this phenomenon will cause the cutting area of the milling cutter teeth to increase, resulting in an increase in the load during cutting, causing the workpiece to produce greater deformation and vibration during cutting, and ultimately leading to an increase in the roughness of the machined surface. The increase in cutting area means that the contact area between the tool and the material increases, providing a favorable environment for the growth of the chip, increasing the friction factor during cutting, and the surface roughness also increases accordingly [14].
Figure 6 shows the three-dimensional morphology of the machined surface when the spindle speed n = 15,000 r/min and the milling depth ap = 0.06 mm, and the feed rate is 4 and 12 mm/min respectively. As shown in Figure 6 (a), when the feed rate is 4 mm/min, the machined surface quality is relatively good, with a surface roughness of Ra = 0.053 μm. Although there are convex parts, the maximum convex height is only 0.57 μm, and there is no obvious ripple on the machined surface. As shown in Figure 6 (b), when the feed rate is 12 mm/min, the machined surface produces obvious ripples, the ripple amplitude is large, and pits are generated. This is because during micro-milling, constant speed and increased feed rate will increase the cutting area per unit time, increase the load during cutting, and cause the workpiece to produce large deformation and vibration during cutting, making the cutting process unstable and producing ripples and pits.
2.3 Effect of milling depth on material surface defects
Maintaining the spindle speed of 17,000 r/min and the feed rate of 4 mm/min, the milling depth of 0.02, 0.04, 0.06, 0.08, and 0.1 mm respectively, the effect on the machining surface quality is shown in Figure 7. It can be seen that: as the milling depth increases, the machining surface roughness also increases linearly. When the milling depth is 0.02-0.06 mm, the surface roughness is 0.036-0.05 μm, and the surface quality is relatively good; when the milling depth increases from 0.06 mm to 0.1 mm, the workpiece surface roughness increases from 0.05 μm to 0.097 μm. It can be inferred that when micro-milling WC/Ni alloy, in order to obtain better surface quality, the milling depth can be controlled at 0.02-0.06 mm.
The main reason for the above phenomenon is that as the milling depth increases, the actual material involved in the processing also increases, which increases the cutting force generated during cutting. Under high-speed cutting, the increase in cutting force increases the vibration generated during the cutting process, resulting in more intense collisions between the tool and the material, and a decrease in the stability of the cutting process, which ultimately leads to poor surface flatness, increased roughness, and poor surface quality. At the same time, as the milling depth increases, the amount of material removed during cutting also increases, and the extrusion deformation between the tool and the material intensifies, resulting in more cutting residues on the material processing surface during cutting, which increases the roughness. As shown in Figure 8, the spindle speed n = 17,000 r/min and the feed rate f = 4 mm/min are kept unchanged, and the three-dimensional morphology of the processing surface with a milling depth of 0.02 mm and 0.1 mm is analyzed.
As shown in Figure 8 (a), when the milling depth is 0.02 mm, a small amount of burrs are generated on the machined surface, and there are ripples. This is because the spindle speed of the test is 17,000 r/min, which is relatively high, resulting in vibration during processing, causing ripples on the machined surface. At the same time, due to the small milling depth, the number of burrs is small. As shown in Figure 8 (b), when the milling depth is 0.1 mm, a large number of burrs are generated on the machined surface, and the burr height is relatively high, reaching up to 1.3 μm, and pits are generated on the machined surface, which makes the machined surface worse. This is because the increase in milling depth increases the amount of material involved in processing, the cutting force increases, and the vibration generated by cutting increases. In addition, the spindle speed of the test is 17,000 r/min, which also produces large vibrations. The combination of these two phenomena increases the surface roughness of the workpiece and the quality of the processed surface is poor.
3 Conclusions
(1) A test platform for the post-milling of mold steel Cr12MoV laser cladding WC/Ni alloy powder was built. Nine groups of three-factor three-level orthogonal experiments and 15 groups of single-factor experiments were designed. The test data were analyzed to explore the influence of milling parameters on the surface quality of material processing. The results show that among the milling parameters, the spindle speed has the most significant effect on the surface roughness, followed by the feed rate, and the milling depth has the least effect on the surface roughness. Therefore, in the process of micro-milling laser cladding WC/Ni alloy coating, in order to ensure the surface quality, a small range of processing parameters should be selected as much as possible while ensuring the processing efficiency.
(2) From the single factor test, it can be seen that the surface roughness of the material increases with the increase of the processing parameters. Therefore, in the process of micro-milling laser cladding WC/Ni alloy coating, in order to obtain better surface quality, a smaller milling parameter should be selected while ensuring the processing efficiency.
(3) By observing the three-dimensional microscopic morphology of the processed surface, it can be seen that when micro-milling laser cladding WC/Ni alloy, the main defects on the processed surface are burrs, pits, ripples and other structures. These defects are the main reasons for the poor surface quality of the material.
