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 outer wall cladding, inner 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 0.04 mm, hard chrome plating on the surface layer, thickness 0.03 to 0.05 mm, total thickness of the chromium layer 0.06 to 0.08 mm, 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 dented, rusted, scratched over a large area, and have plating peeled off during use, there are two main traditional electroplated piston remanufacturing methods:
(1) When the dents 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 dents 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 in size, it is necessary to customize the special-shaped seal again, and at the same time re-process the cylinder guide sleeve 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, and 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 oil cylinder’s knocks, scratches and large-scale shedding of the coating are basically the same, that is, stripping 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 thinning of 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
Currently, the cladding materials that can be used for laser cladding are mainly iron-based alloys, nickel-based alloys, titanium-based alloys, copper-based alloys, composite materials, etc. When remanufacturing the shield oil cylinder, mainly in order to restore the original hardness and corrosion resistance of its coating, iron-based stainless steel materials and nickel-based materials can be selected.
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 of the shield propulsion cylinder mainly uses iron-based stainless steel materials.
5.2 Pre- and post-processes of laser cladding
Ultra-high-speed laser cladding requires that the cylinder base material must not have defects, such as pits, cracks, rust, etc. For the surface of the cylinder piston rod, it has high requirements for surface quality (such as flatness, smoothness, etc.), and it is necessary to improve the surface quality of the substrate to obtain a high-quality cladding surface. For the piston rod of the shield remanufactured cylinder, turning can meet the requirements of laser cladding.
For ultra-high-speed laser cladding of cylinder maintenance, it is necessary to go through pre-turning 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-side cladding thickness of the maintenance cylinder, considering that the single-side cladding thickness is relatively thick, it is necessary to ensure that the subsequent processing allowance is 0.15mm on one side.
The ultra-high-speed laser cladding thickness ranges from 0.05 to 1.5mm, and the cladding thickness can be determined according to the actual working conditions of the cylinder. When the laser cladding of the current shield propulsion cylinder is performed, the single-side thickness is about 0.3mm. After the cladding, grinding is performed, and the final effective coating thickness is retained at 0.2mm on one side, which 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 cylinders repaired by cold welding, in order to ensure that there are no defects after laser cladding, the following points should be considered before laser cladding remanufacturing:
(1) Make a basic judgment on the piston rod or cylinder to be repaired to see if there are dense cold welding points on the surface, deep large-sized pits, etc. If not, cladding can be performed according to the above process.
(2) If there are cold welding points, secondary flaw detection should be performed on the part after turning to prevent cracks and defects at the weld overlap.
(3) For cold welding dense point cracks, large pits, rust pits, fatigue cracks, etc., local turning is usually used to remove defects, fill with other materials, and then laser clad the entire part.
6 Conclusion
Using ultra-high-speed laser cladding technology, combined with machining turning and grinding processes, the remanufacturing of shield propulsion cylinder piston rods can be completed. 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 and the selection of appropriate cladding materials, the corrosion and wear resistance requirements of the hydraulic cylinder piston rod can be achieved, meeting the use requirements of remanufactured shield machines.
