In order to determine the optimal process parameters for cladding Stellite6-8%WC powder on the surface of 45 steel, the three process parameters of laser power, scanning speed and powder feeding rate were selected by orthogonal test method, and a three-factor three-level orthogonal test was carried out. The hardness of the cladding layer and the dilution rate were used as evaluation indicators, and the range analysis was carried out to optimize the process parameters of laser cladding Stellite6-8%WC. The results show that the order of influence on the hardness of the cladding layer is laser power jump powder feeding rate jump scanning speed, and the order of influence on the dilution rate is laser power jump scanning speed jump powder feeding rate. The optimal process parameter combination is optimized as laser power 1 600 W, scanning speed 5 mm/s, and powder feeding rate 20 g/min. The cladding layer was prepared using the optimal process parameter combination, and the average microhardness was 397HV0.2, which is about 1.6 times that of the 45 steel substrate.
Laser cladding technology is an advanced material surface modification technology. This technology uses a high-energy laser beam as a heat source to heat the cladding powder and the substrate surface, and forms a cladding layer on the substrate surface that is metallurgically bonded to it, so as to achieve the purpose of strengthening the metal surface performance. Laser cladding technology has the advantages of fast heating and cooling, small heat-affected zone, fine grains of cladding layer, dense organization and high material utilization rate, and is widely used.
The forming quality and morphology of the cladding layer are closely related to the process parameters. The process parameters that affect the surface performance and internal microstructure of the cladding layer are usually not single, but the result of the comprehensive effect of various process parameters. They influence and restrict each other. In practice, the process parameters should be comprehensively considered according to the requirements of the cladding layer. Appropriate combination of process parameters can effectively reduce the generation of internal defects in the cladding layer and improve the quality of the cladding layer. Therefore, it is very necessary to optimize the laser cladding process parameters. In this regard, some scholars have conducted a lot of research. Lei Jingfeng et al. clad Ni60-25%WC coating on the surface of U71Mn steel, used melt width, melt height and dilution rate as evaluation indicators to optimize parameters, and used the range table to analyze the primary and secondary effects of process parameters on indicators. The results show that the primary and secondary effects of process parameters on quality indicators are different. The optimized parameters can prepare a tightly bonded cladding layer, and the surface hardness is significantly improved. Jiang Jibin et al. clad WC-reinforced Ni-based alloy composite coating on the surface of 45 steel substrate. The results show that the scanning speed has a significant effect on the generation of cladding layer defects. From the substrate to the top of the cladding layer, the microhardness shows a gradual increase trend, and the maximum hardness is 70HRC.
In this paper, 45 steel is selected as the base material, and Stellite6-8%WC is used as the cladding material. By designing orthogonal experiments, the hardness and dilution rate of the cladding layer are used as evaluation indicators, and the three process parameters of laser power, scanning speed and powder feeding rate are optimized respectively to obtain the optimal process parameters.
1 Experimental design
1.1 Experimental materials and equipment
45 steel is selected as the base material, and 45 steel is cut into 60 mmx60 mmx10 mm rectangular blocks by using an electric spark wire cutting machine. Before the cladding test, the grinder is used to grind to ensure the flatness of the cladding surface. The sample block is cleaned with an ultrasonic cleaner to avoid the influence of impurities on the cladding quality, and then the sample block is placed in a vacuum drying oven at 200°C for preheating to relieve the stress caused by rapid cooling and cracking after cladding. The cladding material is Stellite6-8%WC cobalt-based composite powder. The selected Stellite6 alloy powder is spherical with a particle diameter of 48 μm~150 μm, and the WC powder particles are spherical with a diameter of about 150 μm. The powders are fully mixed using a ball mill and then placed in a vacuum drying oven at 200 ℃ for drying.
The laser cladding test is a laser cladding system composed of a TSR-2000-01 fiber laser, a water cooling system, a control system, an ABB robot and a coaxial powder feeder. A single cladding layer is prepared on the surface of 45 steel. During the cladding process, argon with a purity of 99% is used as a protective gas and a powder feeding gas to avoid oxidation of the molten pool during the cladding process.
1.2 Orthogonal Experimental Design
The orthogonal experimental method is used to optimize the process parameters in the cladding process, which can avoid excessive experiments and is an economical and effective method. In the laser cladding process, laser power, scanning speed and powder feeding rate have an important influence on the morphology and quality of the cladding layer. Therefore, in order to analyze the influence of process parameters on the dilution rate and hardness of the cladding layer, this paper selects the main process parameters such as laser power (A), scanning speed (B) and powder feeding rate (C) to design a three-factor three-level orthogonal test, and the arrangement is shown in Table 1.
2 Experimental results and analysis
2.1 Effect of process parameters on the dilution rate and hardness of the cladding layer
The dilution rate refers to the phenomenon that the substrate and the cladding material diffuse with each other during the cladding process, thereby forming a metallurgical bond with high bonding strength. The dilution rate is calculated as the penetration depth (/ penetration height + penetration depth). If the dilution rate is too low, the bonding strength between the cladding layer and the substrate will be poor. However, if the dilution rate is too high, too many matrix elements will enter the cladding layer, causing “pollution” to the alloy elements in the cladding layer, resulting in reduced coating performance. A suitable dilution rate should be selected to ensure the bonding strength between the cladding layer and the substrate without affecting the performance of the cladding layer. Generally speaking, hardness is proportional to wear resistance. Therefore, when this paper uses orthogonal test to optimize the process parameters of single-pass laser cladding, dilution rate and hardness are used as evaluation indicators. The results are shown in Table 2.
In order to determine the optimal process parameters for cladding, the measured dilution rate and hardness results are analyzed by orthogonal test range analysis, as shown in Table 3. It can be seen from Table 3 that the influence of various process parameters on the evaluation indicators during laser cladding is different. The influence on the hardness of the cladding layer is the laser power jump powder feeding rate jump scanning speed; the influence on the dilution rate is the laser power jump scanning speed jump powder feeding rate. It was also found that laser power is a key factor affecting the dilution rate and hardness of the cladding layer. As a key factor in determining the energy input during the cladding process, when the laser power is too low, the heat energy input in the cladding layer is insufficient, resulting in insufficient melting of the cladding powder during the cladding process, causing defects. At the same time, the bonding strength between WC powder and the substrate is low, and it is easy to fall off during the friction and wear process. It exists between the coating and the substrate, which will greatly aggravate the surface wear. Only when the heat is sufficient and the powder is fully melted can it be beneficial to improve the hardness of the cladding layer. When the laser power is too high, the convection in the molten pool intensifies, the diffusion of elements between the substrate and the cladding layer intensifies, and the dilution rate increases, which is not conducive to improving the performance of the cladding layer. At the same time, it may cause partial ablation of the surface material of the cladding layer and reduce the hardness of the top [8]. When the dilution rate of the cladding layer is used as an evaluation index, the dilution rate should be kept below 10%. Therefore, the laser power is optimal when it is at level A2, and the scanning speed and powder feeding rate are optimal when they are at a larger value, which can not only ensure the bonding strength between the cladding layer and the substrate, but also avoid the dilution rate affecting the performance of the cladding layer[9]. Therefore, the optimal process parameter combination is A2B2C2, that is, laser power 1600 W, scanning speed 5 mm/s, and powder feeding rate 20 g/min. When the hardness of the cladding layer is used as an evaluation index, in order to make the cladding layer have a larger microhardness, the optimal process parameter combination is A3B2C2, that is, laser power 1800 W, scanning speed 5 mm/s, and powder feeding rate 20 g/min. When the laser power is 1800 W, the dilution rate is large, which is not conducive to ensuring the excellent performance of the cladding layer. Therefore, the laser power is selected to be 1600 W, and the dilution rate is moderate at this time. In summary, in order to make the cobalt-based composite coating have higher hardness and ensure a moderate dilution rate, the final process parameter optimization scheme is laser power 1 600 W, scanning speed 5 mm/s, and powder feeding rate 20 g/min.
2.2 Microhardness
The microhardness distribution curve of the cladding layer prepared under the optimal process parameters is shown in Figure 1. As shown in Figure 1, the hardness distribution from the cladding layer to the substrate is mainly divided into three parts, namely the cladding area, the heat-affected zone and the substrate area. The average microhardness of the cladding layer area is 397HV0.2, which is about 1.6 times that of the 45 steel substrate. At the bottom of the cladding layer, the hardness reaches a maximum of 455.7HV0.2. This is mainly because during the laser cladding process, the WC particles have a high melting point and will not be completely melted. At the same time, the WC particles have a high density, resulting in the sinking of WC particles. Under the action of the laser beam, the WC particles absorb heat, decompose W atoms and C atoms around them, and form carbide hard phases with the alloy elements in Stellite6, so that the hardness of the bottom area of the cladding layer reaches the maximum.
3 Conclusion
In this paper, Stellite6-8%WC composite coating was clad on the surface of 45 steel by orthogonal test, and the influence of process parameters on the dilution rate and hardness of the cladding layer was analyzed. The optimal process parameters were optimized in combination with the dilution rate and hardness of the cladding layer. The conclusions are as follows:
(1) The influence of process parameters on the hardness of the cladding layer is the laser power jump powder feeding rate jump scanning speed, and the influence on the dilution rate is the laser power jump scanning speed jump powder feeding rate.
(2) The process parameters were optimized in combination with the hardness and dilution rate of the cladding layer. The final optimized process parameter combination is: laser power 1 600W, scanning speed 5 mm/s, powder feeding rate 20 g/min. The cobalt-based alloy composite coating was prepared using the optimized parameters. The average microhardness of the prepared cladding layer is 397HV0.2, which is about 1.6 times the hardness of 45 steel.
