This paper studies the process of repairing the inner surface of the cylinder of the hydraulic support column by laser cladding technology. From the aspects of maintenance cost, hardness of cladding layer, bonding strength of cladding layer, etc., it comprehensively compares the advantages and disadvantages of copper alloy cladding technology and laser cladding technology on the inner surface of the cylinder, and analyzes the application prospects of laser cladding technology in the process of repairing the inner surface of the cylinder of the hydraulic support column. (Keywords: hydraulic support column cylinder; inner hole laser cladding; service life; economic benefits)

1  Introduction
In the comprehensive mechanized coal mining operation in coal mines, hydraulic support is the most commonly used mechanical equipment. Using hydraulic support to support the roof can improve the safety of coal mining operations. However, the working conditions in coal mines are poor, and the wear resistance and corrosion resistance of hydraulic supports face great challenges, especially in the humid environment underground, which contains corrosive media such as S2, SO4’2-, HS, and C1-, which cause corrosive damage to the outer surface of the support column cylinder and the active column, and then cause rust and plating peeling on the surface of the workpiece. The inner surface of the outer cylinder and the middle cylinder of the column is contaminated by impurities or the emulsion ratio concentration does not meet the standard, resulting in the inner surface of the column cylinder being scratched and rusted to form pits. The inner surface of the column cylinder is rusted and scratched, which affects the sealing and support of the hydraulic support to the top plate, bringing serious hidden dangers to the safe production of coal mines.

In the past, the traditional electroplating iron repair process was often used to repair the damaged column cylinder. With the rapid development of my country’s economy, high-energy consumption and high-pollution industries are strictly restricted and shut down by national environmental protection policies. The copper alloy cladding process has become the mainstream method for repairing the inner surface of the hydraulic support column cylinder.

2  The significance of the research on laser cladding on the inner surface of the cylinder of the hydraulic support column

The technology of cladding copper alloy on the inner surface of the cylinder is essentially to weld the high manganese aluminum bronze welding wire to the inner surface of the cylinder of the hydraulic support column with an electric welder. The heat input during welding is large and the dilution rate is high (generally above 20%), which can easily cause the cylinder to deform. During assembly, the guide sleeve and the cylinder body often do not match the coaxiality and cannot be installed. The hardness index of the copper alloy layer clad on the inner surface of the cylinder is about 180HB, and the bonding strength with the substrate is about 430MPa. The molten copper layer on the inner surface of the cylinder after repair is soft and has poor wear resistance. In the later use, the inner surface of the cylinder will still have scratches, pulls and dimensional deviations, which need to be repaired again, resulting in an increase in the maintenance cost of the hydraulic support cylinder.

my country is short of copper resources, and the price of copper welding wire has increased year by year. For example, the price of copper welding wire was 83.48/kg in October 2022, and the price has risen to 97.23/kg in May 2024. Moreover, the price of copper welding wire is still rising. Therefore, the price of using cladding copper alloy technology to repair the inner surface of the cylinder is relatively high.

Large mining height fully mechanized mining face is an important development direction of coal mining technology. The domestic large mining height fully mechanized mining support hydraulic system is developing towards high pressure, large flow, intelligence, energy saving and safety. The cylinder diameter of the hydraulic support column is getting larger and larger, and the cladding copper alloy process can no longer meet the needs of repairing the inner hole of the hydraulic support column cylinder. The laser cladding process of the inner surface of the hydraulic support column cylinder has the characteristics of high bonding strength of the cladding layer, good wear resistance, corrosion resistance and low maintenance cost. Therefore, the laser cladding technology of the inner surface of the hydraulic support column cylinder is expected to replace the cladding copper alloy technology and become the mainstream method of repairing the inner surface of the hydraulic support column cylinder in the future.

3  Laser cladding technology for repairing the inner surface of the cylinder of the hydraulic support column

The main equipment for laser cladding of the inner surface of the cylinder of the hydraulic support column is a system integration of a 10,000-watt fiber laser, a rotary machine tool, a four-axis motion mechanism, and a pneumatic (gravity) powder feeder. The 10,000-watt fiber laser is composed of a laser generator, a spot shaping system, a spot module, a divergent lens module, a stacking station, and a lens protection and drying system. The repair process is mainly divided into two parts: inner surface laser cladding and mechanical processing.

3.1  Inner surface laser cladding
(1) Inspection of the cylinder to be repaired. The technicians inspect the cylinder to be repaired. First, they inspect the inner surface of the cylinder for defects such as missing substrate, rust pits, and damage to the cylinder thread. They mark the damaged parts and propose a repair process route.
(2) Position tolerance mapping of the cylinder to be repaired. The technicians measure the cylinder to be repaired, draw the repair drawing, and compare it with the original assembly drawing to determine the correctness of the repair drawing.
(3) Preparation before laser cladding. Before the inner hole laser cladding, the internal failure layer of the cylinder to be repaired should be bored. When encountering a large hemp pit corrosion point, the corrosion point should be cleaned. Then, the hemp pit is welded flat and polished with electric welding to meet the technical requirements. Then, the inner wall surface of the workpiece is wiped clean with a degreasing agent. The treated cylinder to be repaired is transferred to the laser cladding machine and fixed to prepare for the inner hole laser cladding.
(4) Laser cladding. Enter the process parameters such as the laser rated power, actual output power, scanning speed, powder feeding parameters, spot area, defocus coefficient, wavelength, power density, overlap rate, and cladding layer thickness into the operation screen of the laser cladding machine. After checking and confirming the flatness of the workpiece, start the machine to start the inner hole laser cladding.
(5) Inspection and cooling. After the inner hole cladding is completed, the inner hole cladding quality is carefully checked. If there is a cladding defect, the defect needs to be repaired. After confirmation, it is transferred to the designated location to cool naturally to room temperature. After cooling, it is transferred to the mechanical processing process.

3.2  Machining
The cylinder after laser cladding needs to be rough and fine bored, and leave a reasonable honing size. The cylinder meets the dimensional tolerance requirements and roughness requirements required for assembly through honing, and undergoes a comprehensive quality inspection. After all indicators are qualified, it can be used for column assembly.

4  Performance indicators of the laser cladding layer on the inner surface of the cylinder
Performance indicator values ​​of the laser cladding layer on the inner surface:
Cladding layer thickness: ≥ 0.5mm;
Cladding layer hardness: 200 ~ 280HB;
Cladding layer roughness: ≤ 0.4μm;
Cladding layer and substrate bonding strength: above 480MPa;
Cladding layer wet sand wear test: wet sand rubber wheel wear test 4.5g/3000 turns;
Cladding layer neutral salt spray test: 3000h no rust.

5  Comparison between laser cladding process and copper melting process on inner surface of column cylinder

5.1  Comparison between consumables of laser cladding and copper melting on inner surface
For repairing the inner surface of hydraulic support column cylinder, the copper alloy cladding process actually consumes 23 to 24 kg of copper welding wire per square meter, while the laser cladding process consumes 13 to 14 kg of alloy powder per square meter. At the same time, the price of alloy powder for laser cladding on the market is lower than that of copper welding wire. From the perspective of consumables cost, the maintenance cost of laser cladding process is lower than that of copper alloy cladding process.

5.2  Comparison of technical performance of cladding layer
See ​​Table 1

6  Conclusion
The application of laser cladding technology to repair the inner hole surface of hydraulic support column cylinder can improve the wear resistance and corrosion resistance of hydraulic support column cylinder, greatly improve the overall performance of hydraulic support, extend the service life of hydraulic support column, and reduce the maintenance cost of hydraulic support.