The research is based on the laser cladding processing needs of coal mine hydraulic support manufacturing and remanufacturing. The functions, structures and key technologies of internal and external wall laser cladding equipment are studied, and the development direction of the equipment is prospected, in order to provide reference for related industries.
Laser cladding technology uses high-energy laser beams as heat sources to melt metal powder materials on the surface of parts to prepare high-performance protective coatings, improve the hardness of the surface of parts and components, and resist wear, corrosion, oxidation, and high temperature, thereby extending the service life of parts.
Hydraulic support is the main supporting equipment in the comprehensive mining process of coal mines. Hydraulic support equipment runs for a long time in the mine and bears a large load. Due to its complex and harsh working environment, it is easy to corrode and wear the equipment, resulting in sealing failure and shortening the service life of the equipment. Traditional hydraulic support manufacturing and remanufacturing mainly use cold arc welding and electroplating processes. Cold arc welding has problems such as low production efficiency and high dilution rate, while electroplating is a high-pollution process and will gradually be replaced. Laser cladding hydraulic support manufacturing and remanufacturing is an advanced, green and environmentally friendly new process technology, which can effectively improve the production efficiency and wear resistance and corrosion resistance of hydraulic supports, extend its service life, and replace traditional cold arc welding and electroplating processes. Therefore, the market prospects for the manufacture and remanufacturing of hydraulic supports using laser cladding technology are broad.
1 Development status of laser cladding equipment for inner and outer walls of coal mine hydraulic supports
In recent years, with the rapid development of fiber laser technology, laser cladding technology has been widely used in the manufacture and remanufacturing of the outer cylinder and the outer wall of the piston of coal mine hydraulic supports. At present, the demand for laser cladding processing for the manufacture and remanufacturing of the outer cylinder and the inner wall of the middle cylinder of hydraulic supports is increasing, and it is urgent to design intelligent equipment that meets the laser cladding of the inner and outer walls of hydraulic supports.
Laser cladding equipment for inner and outer walls involves many aspects of composite technology. Because the technology involved is interdisciplinary, cross-industry and difficult, it has developed relatively slowly. At present, the development and application of laser cladding equipment for inner and outer walls of coal mine hydraulic supports in China is still in its infancy. Most of the products that have entered the market have a power below 6kW, and the efficiency of inner wall laser cladding is lower than that of outer wall cladding. Therefore, there is an urgent need to obtain higher power and higher efficiency 10,000-watt inner and outer wall laser cladding equipment. The laser cladding head has a small space during inner wall cladding processing, and works in a high temperature and high concentration smoke environment for a long time, which makes it difficult for the laser cladding head to work continuously in deep holes and blind holes. The use and maintenance costs are high, the cladding head has poor stability, low cladding efficiency, and low powder utilization. These technical problems hinder the industrial promotion of inner and outer wall laser cladding equipment and technology in the coal hydraulic support industry.
2 Composition of laser cladding equipment for inner and outer walls of hydraulic supports in coal mines
According to the specifications and types of hydraulic supports in the domestic coal mine industry, the inner and outer wall laser cladding equipment suitable for more than 90% of the laser cladding processing needs of hydraulic supports in coal mines on the market should meet the requirements of minimum processing aperture of not less than 160mm, depth of not less than 3000mm, and machine tool load of not less than 2500kg. It also needs to meet the laser cladding processing requirements of the inner wall of the outer cylinder, the inner wall and outer wall of the middle cylinder, and the outer wall of the plunger of the hydraulic support. At the same time, it is suitable for the cladding process of various metal materials such as iron-based powder, copper alloy powder, and stainless steel powder. Combined with the user’s cladding efficiency requirements, the laser power should be not less than 6kW. The equipment suitable for laser cladding of the inner and outer walls of hydraulic supports in coal mines mainly consists of five parts: laser cladding machine tool, laser cladding head, laser, water cooler, and powder feeder, as shown in Figure 1.
2.1 Laser cladding machine tool
The laser cladding machine tool is a key component of the inner and outer wall laser cladding equipment, which can enable the workpiece and the laser cladding head to realize automatic laser cladding processing movement. The hydraulic support inner and outer wall laser cladding machine tool includes a bed, a head box, a control system, a support mechanism, a tailstock, a three-axis motion mechanism, and a laser head mounting rod, as shown in Figure 2. Among them: the bed is the supporting component for the installation of various motion modules and components; the head box is generally fixed at one end of the bed to drive the workpiece to rotate; the control system is used to control the automatic movement of the machine tool; when processing the inner wall workpiece, the support mechanism is used to support the outer wall of the workpiece, and has the function of adjustable support diameter range, which is convenient for meeting the processing requirements of workpieces of different diameters; when processing the outer wall workpiece, the tailstock is used to support the workpiece, and has cross sliding and fast positioning functions, which is convenient for switching between inner and outer wall workpieces and moving the tailstock when switching workpieces of different lengths; the three-axis motion mechanism can drive the laser cladding head to achieve axial movement in three directions: up and down, front and back, and left and right; in order to reduce the length of the laser head, the inner hole laser cladding head is installed on the laser head mounting rod to increase the strength of the laser head, thereby improving the stability of the internal optical path of the laser head, so that the laser head can achieve the processing of the inner wall of a deep hole of more than 3000mm.
At present, the commonly used laser cladding lasers on the market generally use fiber lasers, which belong to the fourth category of laser products. This type of laser direct beam and mirror-reflected beam can seriously damage the eyes and skin of the operator, so the inner and outer wall laser cladding machine should be equipped with necessary laser protective covers. A large amount of smoke and dust will be generated during the laser cladding process. The smoke contains harmful substances, which will cause environmental pollution and affect the health of the workers. Therefore, the laser cladding machine should also have effective smoking and dust removal devices.
As the core key component of laser cladding processing, the performance of the laser cladding head directly affects the cladding quality, cladding efficiency and powder utilization rate. The design of the inner and outer wall laser cladding head suitable for inner wall laser cladding processing is a key technical problem in the development of inner and outer wall laser cladding equipment for coal mine hydraulic support. Unlike the outer wall laser cladding head, the inner and outer wall laser cladding head needs to work in a small space, so it is necessary to highly integrate the design of laser beams, powders, cooling water and protective gas in a limited space. During the inner wall processing, the inner wall laser cladding head continues to work in a harsh environment of high temperature and high concentration of smoke and dust, which is prone to burnout of protective lenses and optical lenses, resulting in cladding quality defects. Since such defects are not easy to be found, the inner wall laser cladding head needs to have efficient water cooling and necessary abnormal monitoring and alarm functions. At present, the monitoring technical means commonly used by domestic related product research and development and manufacturing companies are water flow, air flow, air pressure and temperature abnormality monitoring and alarm.
The inner and outer wall laser cladding heads generally adopt two methods: pre-set powder feeding broadband laser cladding and synchronous powder feeding high-speed laser cladding. The pre-set powder feeding broadband laser cladding head has the advantages of high laser power, high cladding efficiency and high powder utilization. The synchronous powder feeding high-speed laser cladding head has a small heat output, which has little thermal impact on the workpiece substrate, and the surface flatness of the cladding layer is high and the post-cladding processing volume is small. When processing the inner wall of the outer cylinder of the hydraulic support, it is necessary to reach the position near the bottom of the cylinder. Therefore, the inner and outer wall laser cladding heads should have blind hole processing requirements, and the laser cladding head optical path needs to adopt a forward-inclined optical path design. The forward-inclined optical path of the preset powder feeding broadband laser cladding head generally uses a collimating lens to collimate the laser beam, and then refracts and shapes the laser beam through an integrating mirror with a refraction angle greater than 90° to obtain the required spot shape at the working position as shown in Figure 3. There are generally two forms of the forward-inclined optical path of the synchronous powder feeding high-speed laser cladding head. One is to use a collimating lens to collimate the laser beam and then refract and converge the laser beam through an off-axis parabolic mirror with a refraction angle greater than 90″, as shown in Figure 4; the other is to use a collimating lens to collimate the laser beam, and then converge the collimated laser beam through a focusing lens, and finally refract the laser beam through a plane mirror with a refraction angle greater than 90° as shown in Figure 5.
2.3 Powder feeder
In order to shorten the conveying distance of the powder feeding pipe and improve the powder feeding accuracy, the powder feeder of the inner and outer wall laser cladding equipment of the coal mine hydraulic support is generally fixed on the machine tool and moves with the three-axis motion mechanism of the machine tool. Because the metal powder needs to be fed to the laser head in the deep hole workpiece, the inner and outer wall laser cladding equipment generally uses a carrier gas powder feeder. When using the preset powder feeding broadband laser cladding processing technology, the powder needs to enter the front of the powder feeding nozzle and the air pressure carrying the powder is removed to prevent the gas carrying the powder from affecting the powder spreading quality of the powder feeding nozzle on the workpiece.
3 Development trend of laser cladding equipment for inner and outer walls of coal mine hydraulic supports
To extend the service life of the laser cladding head, it is necessary to optimize the cooling water circuit and protective gas circuit structure of the laser cladding head, optimize the process parameters, and develop metal materials suitable for inner wall cladding processing to reduce the smoke and splashes generated during the cladding process. During the cladding process on the inner wall of the workpiece, the working state of the laser cladding head and the surface quality of the cladding layer are not easy to observe. It should develop in the direction of functional diversification, display the powder flow and molten pool information of the cladding process in real time, and automatically judge the stability of the cladding process through the control system. At the same time, the detection device automatically collects and displays the uniformity parameters of the cladding layer, and automatically judges whether the workpiece is qualified according to the parameters set by the user.
Production efficiency is related to corporate profits. By studying laser cladding heads, metal materials and process parameters suitable for higher power, the processing efficiency of laser cladding can be improved. With the rapid development of automation technology, the shortage of labor and people’s yearning for a good working environment, laser cladding machine tools in the future should develop in the direction of automation such as automatic loading and unmanned production lines, reducing labor while improving product quality and production efficiency.
4 Conclusion
At present, the internal and external wall laser cladding equipment has been promoted and applied in the coal mine hydraulic support manufacturing and remanufacturing industry. Its basic functions initially meet the requirements of industrial application, and the gap with foreign technology in terms of functional diversity and performance stability is getting smaller and smaller. With the rapid development and integration of my country’s intelligent equipment technology, laser technology, detection technology, control system, metal materials, etc., the internal and external wall laser cladding equipment of coal mine hydraulic support will be more perfect in terms of function and stability in the future.
