【Objective】Laser cladding can improve the working performance of the pick head and increase its working life. Whether the cladding parameters are appropriate has a direct impact on the performance of the pick. Therefore, it is necessary to conduct numerical simulation research on the laser cladding of the pick. 【Method】The temperature field during the laser cladding process of the pick was studied, and the influence of laser power and scanning rate on the temperature field and stress field was studied. In the Ansys workbench platform, APDL programming and unit birth and death technology were used to obtain the temperature field and stress field of the pick at different scanning rates and laser powers. 【Result】The higher the laser power, the higher the peak temperature and the lower the cooling rate. Increasing the laser power will increase the residual thermal stress during cladding and after cooling. The higher the scanning speed, the lower the peak temperature. Increasing the scanning speed will reduce the residual stress during cladding and after cooling. At the same time, increasing the scanning rate and laser power can improve the cladding efficiency, and the residual stress during cladding and after cooling will be reduced overall. 【Conclusion】While improving the cladding efficiency, the impact on performance should be considered. It is necessary to comprehensively change the overall process parameters. At the same time, appropriate heat treatment is required after cladding to further reduce the residual stress.
The pick is an important part of the cutting part of the coal mining machine. Due to the complex coal seam environment, the parts of the cutting part of the coal mining machine must have sufficient strength and service life to meet the working and life requirements of the parts [1]. Untreated picks can no longer meet the current industry requirements. Picks must be surface strengthened before they can be put into use. The commonly used surface treatment method for picks is surfacing. Although surfacing can improve the strength of the pick to a certain extent, with the development of the industry, the main properties of the picks after surfacing treatment, such as hardness, wear resistance and corrosion resistance, can no longer meet the requirements. At present, most companies choose laser cladding to treat the surface of the pick. This study simulates the temperature field and stress field of different laser cladding powers and scanning speeds to obtain appropriate parameters.
Leunda et al. [2] conducted a laser cladding NiCr-WC powder test on the inner wall of a double-cylindrical hole component. The research results show that spherical WC particles have a higher powder utilization rate for irregular shapes, and it is easy to produce a cladding layer that meets the requirements. Emamian et al. [3] studied the influence of laser cladding process parameters on the macroscopic morphology of TiC cladding layer. The results showed that the process parameters have a direct influence on the morphological formation of TiC and play a major role. Xu Mingsan et al. [4] used the shear method to test the bonding strength between the cladding layer and the substrate, and studied the influence of multiple influencing factors on the bonding strength. The results showed that the bonding strength of the laser cladding layer can meet the working requirements. Deng Dewei et al. [5] used laser cladding to improve the surface properties of water-lubricated bearings, and studied the influence of various process parameters on the Ni40 cladding layer through experiments. Liu [6] used a three-dimensional model to analyze the effect of laser remelting on the residual stress of NiCrBSi coating. The results showed that the remelted area showed tensile residual stress. Luo et al. [7] used laser shock technology to eliminate the large residual stress inside the coating and optimize the microstructure and mechanical properties of the coating. Das et al. [8] found that laser remelting can produce residual compressive stress on the coating surface, which inhibits the generation and expansion of cracks, thereby significantly reducing the crack density of the coating. 1. Model establishment and numerical simulation
1.1 Part model and mesh division
U47-22 pick-type pick was selected in this study. The laser cladding of the pick is to clad a 2 mm thick cladding layer in the main stress area. The finite element model of the pick is shown in Figure 1. Meshing is very important in finite element analysis. The quality of the mesh directly affects the calculation accuracy. The main calculation area of the pick is at the cladding layer position, so the mesh at this position is encrypted, and the mesh size of other positions can be appropriately increased to reduce the calculation time.
In order to simplify the numerical simulation calculation model of the temperature field, the following assumptions are made for the simulation process [9]: the material is isotropic; the influence of the flow of the molten pool fluid on the temperature field is ignored; the latent heat of phase change during the cladding process is ignored; the heat loss caused by thermal radiation is ignored; and the vaporization of the material is ignored.
1.2 Heat source model
At present, the commonly used heat sources for laser cladding simulation include Gaussian surface heat source, Gaussian body heat source, ellipsoid heat source, double ellipsoid heat source, etc. For the research object and focus, Gaussian surface heat source is more suitable.
1.3 Boundary conditions and birth and death units
The boundary conditions of transient thermal analysis include convection heat transfer and thermal radiation. To simplify the calculation, only the heat convection between the parts and the environment is considered. The heat convection coefficient is related to the temperature, as shown in Figure 2. The birth and death unit technology kills the unit before the heat source is loaded, so that when the heat source has not been loaded to the position, the material model at the position does not have any properties. After the heat source is generated, the corresponding unit is activated, and the activated unit begins to apply material properties. The application of birth and death units makes the simulation closer to the real situation [10].
2 Simulation results and analysis
2.1 Temperature field results
The temperature field distribution at a certain moment when the power is 3.5 kW and the scanning speed is 0.4 π/s is shown in Figure 3. As shown in Figure 3, the unit scanned by the laser is activated, with a maximum temperature of 2 082.8 ℃ and a minimum temperature of 25 ℃. The temperature diffuses outward in the form of heat transfer inside the pick. The closer to the laser spot area, the higher the temperature. Since the application of the heat source is an instantaneous process, the cladding area will experience a process of rapid heating and cooling, which will cause a large temperature gradient near the cladding area and generate a large thermal stress instantly. However, as the cooling time increases, the thermal stress will slowly decrease but still be large [4]. When the pick is working, the alloy head and the cladding area are mainly stressed. Excessive stress will greatly reduce the working life of the pick. In order to increase the life of the pick, post-processing must be performed to reduce the internal stress.
The temperature-time curves of different reference points are shown in Figure 4. As shown in Figure 4, during the cladding process, the temperature of each reference point will have multiple peaks. The first peak is when the laser heat source is applied. At this time, the temperature is the highest and the material is clad onto the substrate at this moment. The subsequent peaks are generated by heat transfer when the laser heat source is applied to the corresponding positions of the following cladding layers. The temperature at this time will affect the organizational evolution of the cladding layer and the substrate material, and will directly affect the hardness and wear resistance of the material surface [5]. During the cladding process of the cladding layer at different positions of the pick, the peak temperature will be different due to the influence of heat conduction.
The temperature-time curves at different scanning rates are shown in Figure 5. As shown in Figure 5, the greater the scanning rate, the shorter the laser spot stays on the surface of the part, the less energy the part absorbs, and the lower the maximum temperature of the part. When the scanning speed exceeds a certain extreme value, the energy generated by the laser can no longer melt the material.
The temperature-time curves at different powers are shown in Figure 6. As shown in Figure 6, the higher the power, the higher the maximum temperature of the part, and the lower the cooling rate. The cooling rate will affect the material properties, especially the residual stress and material hardness [11]. It can be inferred that increasing the cladding speed will inevitably lead to a decrease in the energy absorbed by the material. Increasing the laser power can increase the energy absorption of the material, thereby increasing the peak temperature. When the scanning rate is increased from 0.1 π/s to 0.4 π/s and the laser power is increased to 3.5 kW, the time to reach the peak temperature is the same as that of increasing the rate alone, but the peak temperature is increased by about 400 ℃. Compared with increasing the laser power alone, the time to reach the peak temperature is 10 s earlier and the peak temperature is reduced by about 1 000 ℃.
2.2 Stress field results
The stress field cloud diagram at the end of cladding with a power of 3.5 kW and a scanning speed of 0.4 π/s and 1 h of natural cooling is shown in Figure 7. As shown in Figure 7, since the laser cladding process is a process of rapid heating and melting, followed by rapid cooling and solidification, great thermal stress will be generated on the cladding layer. The thermal stress is mainly concentrated on the cladding layer, and the maximum stress is at the boundary of the molten pool [12]. As shown in Figure 7 (a), the maximum stress at the end of cladding is 2.294 7×108 MPa. Because different temperatures have different cooling rates under the same convection environment, the maximum stress is reduced to 7.693 4×107 MPa after cooling for one hour.
The residual stresses at different laser powers and different scanning speeds are shown in Tables 1 and 2. The laser scanning rate remains unchanged, and the laser power is increased to 0.5 kW and 1.5 kW respectively. The stress at the end of cladding increases by 0.28×108 MPa and 1.13×108 MPa respectively. After cooling for 1 h, the stress increases by 0.16×107 MPa and 0.57×107 MPa respectively. When the laser power remains unchanged, the scanning speed is increased by 0.1 π/s and 0.3 π/s respectively, and the stress at the end of cladding is reduced by 0.04×108MPa and 0.07×108MPa respectively. After cooling for 1h, the stress is reduced by 0.46×107MPa and 0.81×107MPa respectively. When the laser power is increased to 3.5kW and the laser scanning speed is increased to 0.4π/s at the same time, the residual stress at the end of cladding is 2.29×108MPa, which is 0.88×108MPa lower than that of only increasing the power, and 0.32×108MPa higher than that of only increasing the scanning speed. After cooling for 1h, the residual stress is 7.69×107MPa, which is 1.23×107MPa lower than that of only increasing the power, and 0.15×107MPa higher than that of only increasing the scanning speed. In summary, increasing the laser power alone will increase the residual thermal stress during cladding and after cooling, while increasing the scanning speed alone will reduce the residual stress during cladding and after cooling. Compared with increasing only a single parameter, increasing the power and scanning rate at the same time, the residual stress after the end of cladding and cooling for 1 h is generally on a downward trend.
3. Conclusion
This study uses the Workbench platform to simulate the temperature field during the laser cladding process of the pick, and analyzes the effects of laser power and scanning rate on the temperature field of laser cladding, and draws the following conclusions.
① Laser power affects the peak temperature and residual stress during cladding. The higher the laser power, the greater the peak temperature, and as the power increases, the cooling rate decreases. Increasing the laser power will increase the residual thermal stress during cladding and after cooling.
② The greater the scanning speed, the lower the peak temperature. When the speed is too high, the energy absorbed by the part is not enough to melt the material. Increasing the scanning speed will reduce the residual stress during cladding and after cooling.
③ Increasing the scanning rate and laser power at the same time can improve the cladding efficiency, and the temperature can also reach the required temperature. The overall residual stress during cladding and after cooling will be reduced.
