The hydraulic cylinder is the core component of the shield machine, and its remanufacturing is an important part of the shield machine remanufacturing. The main damage forms of the cylinder piston rod are nicks, scratches, and coating shedding. The traditional repair method is to strip the original piston rod coating and then re-electroplate the new coating. With the introduction of laser cladding technology, it provides a new solution for the repair of the cylinder piston rod. In this paper, combined with the remanufacturing of the shield cylinder, the laser cladding repair piston rod process is described and analyzed.
Shield remanufacturing is the focus of engineering machinery remanufacturing due to its high value, large volume, and complex structure. As the core component of the shield machine, the hydraulic cylinder is an important part of the shield machine remanufacturing. Remanufacturing is a process of professionally repairing or upgrading the remanufactured blank to make its quality characteristics not lower than the level of the prototype new product [1]. The current development of laser cladding technology provides a new solution for cylinder repair.
Laser cladding is an important method to enhance the surface properties of materials and repair the surface of materials. It uses a high-energy-density laser beam to rapidly melt alloys with various compositions and properties into a matrix, forming a high-performance alloy layer on the surface of ordinary metals, thereby improving the wear resistance, corrosion resistance, high-temperature resistance, and oxidation resistance of the parts surface, and extending the life of the workpiece. The thickness of the cladding layer of laser surface cladding is generally 0.7 to 1.5 mm. This repair technology can repair hydraulic cylinder rods with surface scratches and strains exceeding 1.5 mm in depth [2].
At present, a new generation of laser cladding technology (ultra-high-speed laser cladding) has gradually been applied to the repair of workpiece surfaces. Ultra-high-speed laser cladding is an iterative technology of conventional laser cladding on the surface additive technology, which mainly solves the problems of low processing efficiency, large heat-affected zone, large material waste, and many post-processing steps.
1 Introduction to the principle of ultra-high-speed laser cladding
Ultra-high-speed laser cladding adopts a synchronous powder feeding method. By adjusting the relative position of the powder focal plane and the laser focal plane, the cladding powder intersects with the laser beam above the substrate and melts, and then fuses with the smaller substrate molten pool, evenly coats the substrate surface, and forms functional coatings with various properties after rapid solidification, as shown in Figure 1. Compared with conventional electroplating, it is more environmentally friendly and simpler to operate; compared with ordinary laser cladding, it has the characteristics of low dilution rate, small thermal impact, high efficiency and low cost. Ultra-high-speed laser cladding can be used for external wall cladding, internal hole cladding, additive repair and other contents.
2 Advantages and application of laser cladding technology
Compared with conventional electroplating repair solutions, its ultra-high-speed laser cladding technology has the following advantages:
(1) It is more environmentally friendly and simpler to operate, and at the same time solves the problem of easy detachment of electroplating. It adopts metallurgical bonding and has stronger bonding force.
(2) Compared with thermal spraying, it has the characteristics of high environmental protection, high bonding strength, low cost, etc., and the coating density is higher, more materials can be selected, and the lower heat input can be used for the processing of thin-walled or slender parts.
(3) The surface of the coating prepared by ultra-high-speed laser cladding is fine and relatively smooth. In most cases, the subsequent processing process only needs grinding before it can be used. The material utilization rate is high and the subsequent processing volume is small. It is understood that at present, ultra-high-speed laser cladding technology has become the preferred coating treatment solution in the coal mining industry due to its advantages such as environmental protection, high efficiency and short maintenance cycle. The remanufactured parts that have been laser clad in coal mines mainly include hydraulic support cylinders, articulated pins, spline shafts, guide shoes, support shoes, etc. [3]. In addition, laser cladding is gradually replacing electroplating processes in the field of engineering machinery.
3 Plating and damage forms of shield cylinder piston rods
The main cylinders of shield machines include propulsion cylinders, articulated cylinders, etc., and there are many of them. Taking the thrust cylinder as an example, the number of each unit is 20 to 50. The piston rod of the thrust cylinder is required to have anti-corrosion and wear-resistant properties. The original surface treatment process of the piston rod is: double-layer chrome plating on the surface, milky white chrome plating on the bottom layer, thickness 0.03 to 0.04mm, hard chrome plating on the surface layer, thickness 0.03 to 0.05mm, total thickness of the chrome layer
0.06 to 0.08mm, and the surface hardness can reach HRC60 or above. The construction environment of the shield tunnel is relatively harsh. Most cylinders will be dented, rusted, scratched on a large area, and the coating will fall off during use (see Figure 2). On the other hand, the storage of the shield after use is often not standardized, which further aggravates the rust damage of the cylinder piston rod. Therefore, when remanufacturing the cylinder, the surface coating treatment of the piston rod is one of the most important links.
4 Traditional Cylinder Piston Rod Remanufacturing Methods
For the shield cylinders that are damaged, rusted, scratched and plating peeled off during use, there are two main traditional electroplating piston remanufacturing methods:
(1) When the damage and rust are not serious, the method of stripping, grinding down the rod (reducing the size) and re-plating is adopted. The reduced rod size is compensated by the original seal gap tolerance of the seal. This method has a high risk and is prone to internal and external leakage of the cylinder, and the cylinder performance stability is poor.
(2) When the damage and rust are serious, the method of stripping, reducing the overall size of the piston rod and re-chrome plating is adopted. After the piston rod is reduced, the special-shaped seal needs to be customized again, and the cylinder guide sleeve needs to be re-processed to restore the cylinder function. When this method is used for maintenance, the main risk is that the original design size of the cylinder changes, whether the structural strength is satisfied; whether the newly processed seal and guide sleeve meet the original design size and tolerance requirements of the cylinder.
At present, the oil cylinder maintenance manufacturers in the industry have a large technical gap in the design and process of the oil cylinder compared with the original manufacturers of the shield oil cylinder. The technical level is also uneven when changing the oil cylinder size and reselecting the oil cylinder seal. The methods adopted in the face of the knocks, scratches and large-scale shedding of the coating on the oil cylinder are basically the same, that is, deplating and re-plating. The electroplating process of the electroplating manufacturer selected by the oil cylinder maintenance manufacturer is also uneven, and the requirements of each manufacturer for the thickness of the electroplating coating are also different. There is a risk of thinning the coating, which is basically only 0.03-0.04mm, which has a greater risk.
5 Laser cladding repair process for shield oil cylinder piston rod
5.1 Selection of cladding alloy
The cladding materials that can be used for laser cladding at present are mainly iron-based alloys, nickel-based alloys, titanium-based alloys, copper-based alloys, composite materials, etc. When remanufacturing the shield oil cylinder, iron-based stainless steel materials and nickel-based materials can be selected mainly to restore the original hardness and corrosion resistance of its coating.
Compared with the two materials, nickel-based materials have better corrosion resistance, but the corresponding cost is higher; iron-based stainless steel materials have better corrosion resistance than traditional electroplating processes, with similar hardness and lower material cost. Therefore, the current surface treatment of the piston rod remanufacturing of shield propulsion cylinders mainly uses iron-based stainless steel materials.
5.2 Pre- and post-processes of laser cladding
Ultra-high-speed laser cladding requires that the base material of the oil cylinder should not have defects, such as pits, cracks, rust, etc. For the surface of the oil cylinder piston rod, the surface quality (such as flatness, smoothness, etc.) is required to be high, and the surface quality of the base material needs to be improved to obtain a high-quality cladding surface. For the oil cylinder piston rod remanufactured by the shield, the requirements of laser cladding can be met by turning.
For the ultra-high-speed laser cladding of oil cylinder maintenance, it is necessary to go through the previous turning process to ensure that the original electroplating coating, embroidery pits and other defects are removed, and then the rod surface is color-detected to avoid cracks caused by the original stress, and cladding molding is performed without defects. For the single-sided cladding thickness of the maintenance oil cylinder, considering that the single-sided cladding thickness is thick, it is necessary to ensure that the subsequent processing allowance is 0.15mm on one side.
The thickness of ultra-high-speed laser cladding ranges from 0.05 to 1.5mm, and the cladding thickness can be determined according to the actual working conditions of the oil cylinder. When laser cladding the current shield thrust cylinder, the thickness of one side is about 0.3mm. After cladding, grinding is performed, and the final effective coating thickness is retained at 0.2mm per side. This can reduce the use of materials and the amount of subsequent processing. After ultra-high-speed laser cladding, the surface flatness can be directly ground and polished to achieve the required size and finish.
5.3 Laser cladding of piston rods repaired by cold welding For the cylinder repaired by cold welding, in order to ensure that there are no defects after laser cladding, the following points need to be considered before laser cladding remanufacturing:
(1) Make a basic judgment on the piston rod or cylinder to be repaired, whether there are dense cold welding points on the surface, whether there are deep large-sized pits, etc. If there are no, cladding can be performed according to the above process.
(2) If there is a cold welding point, it is necessary to perform secondary flaw detection on the part after turning to prevent cracks and defects in the weld overlap.
(3) For cold welding intensive cracks, large pits, rust pits, fatigue cracks, etc., local turning is usually used to remove defects, other materials are used for filling, and then laser cladding is performed on the whole.
6 Conclusion
Using ultra-high-speed laser cladding technology, combined with machining turning and grinding processes, the shield propulsion cylinder piston rod can be remanufactured, and compared with traditional electroplating methods, it has the advantages of environmental protection, high efficiency, and simple subsequent processing.
Through the ultra-high-speed laser cladding process, the appropriate cladding material can be selected to achieve the corrosion and wear resistance requirements of the hydraulic cylinder piston rod, and meet the use requirements of the remanufactured shield.
