The traditional electroplating repair technology of hydraulic supports has serious surface chromium desorption, short life after repair, and long electroplating time. The repair process and technology of hydraulic supports are studied, and a rapid repair process of hydraulic supports based on laser cladding technology is proposed. The key technologies of laser cladding are analyzed, and YAG laser is selected for on-site application and debugging, and the effect of laser cladding repair of hydraulic supports is analyzed. The results show that the repair quality of laser cladding technology is high, the surface hardness of the hydraulic support column is improved, the service life of the equipment is extended, the reuse of waste equipment is realized, the purchase cost of the hydraulic support is saved, and the degree of environmental pollution is reduced.
Coal mine hydraulic supports are important mechanical equipment for supporting the working face of coal mines, and play a very important role in ensuring the output of coal mines and the safety of equipment and personnel. The column is the most important supporting component of the hydraulic support in coal mines, which can effectively support the roof and coal wall of the hydraulic support. Affected by the harsh and complex working conditions in coal mines, the hydraulic support columns exposed to the outside for a long time often suffer different degrees of damage during use. If the surface wear is serious, it will directly affect the sealing of the hydraulic support column and cylinder, and also weaken the supporting force. To this end, it is necessary to improve the repair quality of the hydraulic support column surface. At present, the surface damage repair of domestic hydraulic supports generally adopts electroplating technology, which uses the principle of electrolysis to plate a layer of metal or alloy on the surface of the column to enhance the surface strength and wear resistance. The coating layer formed by the traditional electroplating process has a short lifespan, and the surface is very prone to chromium removal, which is not conducive to later repair. At the same time, the electroplating process technology will cause a certain degree of pollution to the environment. Therefore, it is urgent to study new process technologies to improve the coating quality of the surface of hydraulic support columns and other parts and extend the service life of hydraulic supports.
1 Process flow and key technologies of laser cladding technology
1.1 Process flow of laser cladding technology
The common repair process for key components of hydraulic supports is electroplating. The corrosion resistance of hydraulic support column parts repaired by electroplating is weak. In order to solve the problems of traditional electroplating process, laser cladding repair process is proposed. Laser cladding technology is a surface improvement repair technology proposed in recent years. It improves the wear resistance and strength of the surface by strengthening the surface of parts. Figure 1 is a process flow chart of laser cladding.
As shown in Figure 1, the laser cladding process needs to go through two steps: preliminary preparation and film forming. First, the preparation of the base material and cladding powder needs to be completed, then the powder size is constructed, and finally, a certain density of laser beam is used to act directly on the coating surface, a certain metal is directly melted into the material, and the surface metal of the cladding material and the part is melted and solidified and cooled, and finally a layer of metallurgical bonding cladding layer with a certain low dilution rate is formed on the metal surface, which improves the performance of the metal surface to a certain extent.
1.2 Key technologies and equipment for laser cladding
Laser cladding technology allows the wear-resistant coating to fuse with the substrate, refines the grains to form a metastable phase, eliminates certain harmful phases in the matrix, and thus significantly improves the performance of the surface. During the laser cladding process, the gas-protected film is formed to ensure that the metal surface can be fully fused when the powder is coated. Figure 2 shows the key technologies and equipment for laser cladding of hydraulic support column parts.
As shown in Figure 2, metal powder is the basic material, which is fed in four ways by the powder feeder. At the same time, the high-energy laser beam emitted by the laser is used as a heat source, and the beam is emitted by the beam forming and focusing combination mirror to directly integrate the metal alloy powder with higher strength, better wear resistance and excellent corrosion resistance into the surface of the hydraulic support column parts. Through the metallurgical reaction between the metals, the original metal surface has the same performance as the powder alloy. After laser cladding, the wear resistance and corrosion resistance of the surface of the hydraulic support column parts are significantly improved, thereby extending the service life of the hydraulic support. The entire laser cladding process is supported by CAD/CAM software. Computer technology is used to control the laser head and powder feeding nozzle to achieve automatic control and continuous spraying. No smoke will be generated during the actual processing process, and the impact on the environment is minimal.
At present, there are mainly two types of laser cladding equipment in my country: the first is CO2 laser, which can achieve continuous output. The power of the cladding machine is above 3kW, which is relatively expensive and has high maintenance costs in the later stage; the second is YAG laser, which is pulsed output with a power of 580W. Compared with the first type, it is cheaper and has better application effect.
2 Application test of laser cladding technology in hydraulic support repair
In order to verify the application effect of laser cladding technology in hydraulic support repair, the hydraulic support column to be repaired in a coal mine in Shanxi was selected as the test sample, and YAG laser was selected, a powder feeder with non-gas-carrying conveying method was selected (powder utilization rate reached 92%), and iron-based materials were selected to prepare Fe-Cr-Nb-Mo alloy powder on the surface of three kinds of steel by plasma cladding technology. The proportions of each element in the alloy powder are as follows: Fe is 28.94%, Cr is 19.13%, Nb is 5.84%, and Mo is 2.76%.
To ensure the cladding effect, the substrate surface is first treated with sandpaper to remove surface impurities and rust, and the surface is cleaned and pre-treated using an ultrasonic cleaning machine. Then the substrate is preheated, and the hydraulic support column is laser clad using a laser cladding machine, and the powder alloy is sprayed onto the substrate surface for cladding through metallurgical reaction. The parameters set for the cladding process are as follows: working pressure is 220V, gas flow is 0.84m²/h, nozzle height is 10mm, scanning speed is 130mm/min, spot diameter is 3mm, and laser cladding power is 8.6kW.
For the selection of various parameters for laser cladding, it is necessary to consider the expected effects and characteristics of laser cladding. Generally, the spot diameter is selected to be 2~4mm. Since the larger the laser power, the larger the amount of melted cladding metal will be, and pores are very likely to be generated during this process, in order to ensure the quality after cladding, the laser cladding power is selected as 8.6kW. After completing the parameter setting, the laser cladding machine is started to perform laser cladding on the surface of the hydraulic support column. The entire cladding process is intelligently controlled by computer CAM. The surface of the hydraulic support column after laser cladding is uneven and needs to be polished to have a certain gloss. Figure 3 shows the scene and effect of laser cladding of the hydraulic support.
In order to verify the advantages and effects of laser cladding, the friction and wear performance test of the surface of the hydraulic support column after laser cladding was carried out, and the powder layer was diffracted and analyzed using an XRD diffraction tester to obtain the diffraction spectrum shown in Figure 4. As shown in Figure 4, the entire phase contains martensite and metal carbide, and the martensite has a high strength and high wear resistance and corrosion resistance.
The hardness of the surface of the cladding layer is detected using an intelligent hardness tester, and the hardness values at different distances from the surface of the cladding layer are obtained as shown in Figure 5.
As shown in Figure 5, the hardness value of the surface of the hydraulic support column cladding layer is 1000HV. As the depth of the cladding layer continues to deepen, the hardness value shows an overall downward trend. When the thickness of the cladding layer is 1800μm, the hardness value is 964HV, and as the distance increases, the hardness value increases slightly. The hardness value at a distance of 3560~3980μm from the surface decreases sharply. At this time, the hardness mutation is caused by the sudden decrease in the number of phases formed by hard metal carbides such as (Nb, Mo) C and VC to the matrix on the surface. At 4000μm from the surface of the cladding layer, the Vickers hardness decreases slowly.
The friction performance of the surface after laser cladding was tested, and the friction performance test results shown in Figure 6 were obtained. As shown in Figure 6, the overall friction coefficient of the surface after laser cladding shows a downward trend. When the applied load is 30N, the friction coefficient is 0.338, when the load is applied to 50N, the friction coefficient is 0.309, with the application of the later load (from 50N to 60N), the friction coefficient continues to increase, at 60N, the friction coefficient is 0.316, but as the load continues to increase, the Cu element contained in the soft tissue of the laser cladding surface layer plays a certain lubricating role, affecting the friction performance of the surface, and the friction coefficient gradually decreases.
According to the field test, after the laser cladding of the surface of the hydraulic support column, its surface hardness and friction performance are improved to a certain extent. The surface is treated with laser cladding technology, and the hydraulic support column at this time will not produce toxic and harmful gases and smoke. The traditional electroplating process repair is performed twice a year on average. After the laser cladding process repair, the hydraulic support has been used on site for more than 2 years. The electroplating repair cost of each hydraulic support is about 9 million yuan. After the laser cladding technology is used for repair, its life span is extended, and the average annual cost savings are about 9 million yuan. The application effect is satisfactory, and it has great promotion value and market prospects.
3 Conclusion
In view of the serious pollution, short life span and high cost of the traditional electroplating repair process of coal mine hydraulic supports, this paper analyzes the laser cladding repair technology and process flow, studies and explores the basic principles and processes of laser cladding repair of metal surfaces, and takes the hydraulic support columns to be repaired in coal mines as the experimental research object, and performs laser cladding treatment on their surfaces. The hardness and friction performance tests of the surface after laser cladding are carried out. The results show that the hardness value of the surface of the hydraulic support column increases significantly. With the deepening of the depth, the hardness gradually decreases, but in the process of decreasing, the hardness value increases to a certain extent, because there are some impurities in the laser cladding. The friction performance of the surface after cladding treatment is improved. After field application, the life of the hydraulic support column repaired by laser cladding technology is extended, which effectively saves enterprise costs and reduces the impact on the environment.
