In order to improve the quality and efficiency of laser cladding remanufacturing of failed hydraulic cylinder piston rods, a remanufacturing process method combining reverse data acquisition and laser cladding path planning was proposed. First, by analyzing the failure form of the piston rod, the failure area model was obtained by using three-dimensional scanning data acquisition, point cloud processing, and data reverse; then, by laser cladding path planning and process parameter setting; finally, the laser cladding remanufacturing process was obtained. The results show that this method can effectively remanufacture the piston rod, and it is feasible in terms of technology, economy, and environment. The process can meet or even exceed the technical requirements of the original product.
1 Research background
In recent years, many problems such as global resource consumption and environmental pollution have attracted widespread attention, and green, low-carbon and circular development has become a global consensus. The increasing number of various scrapped and retired products has led to secondary environmental pollution, and a large part of these resources can still be recycled. The United Nations has issued «Transforming our world: the 2030 Agenda for Sustainable Development», with a total of 17 sustainable development agendas. development goals, calling on all countries to take action to protect the earth, which proposed “adopting sustainable consumption and production patterns”. The State Council of my country issued “Made in China 2025”, proposing “building a green manufacturing system and taking the path of ecological civilization development. Comprehensively promote green manufacturing and accelerate the green transformation and upgrading of the manufacturing industry”. The “Outline of the 14th Five-Year Plan for National Economic and Social Development of the People’s Republic of China and the Long-Term Goals for 2035” released in March 2021 also clearly proposed “building an ecological civilization system and promoting the comprehensive green transformation of economic and social development”.
As an important actuator of fluid mechanical equipment, hydraulic cylinders are widely used in engineering machinery, machine tools, vehicles, ships, automatic control and other fields. The market share is huge, and there are relatively complete international and national standards. It is a standard part. The piston rod is the core component of the hydraulic cylinder. It is a slender shaft used to connect the internal piston of the hydraulic cylinder and the external mechanism to transmit hydraulic pressure to perform work. The piston rod material is generally medium carbon steel or alloy steel, which needs to be tempered and chrome-plated. The dimensional accuracy, roundness, and roundness are all good. The requirements for the shape and position accuracy of the cylinder and coaxiality are high, and the roughness of the outer surface is low, so its application value is relatively large. Its working environment is generally harsh, which leads to different failure forms. It needs regular maintenance and replacement. It is a wearing part, so a large number of retired damaged parts are piled up in the engineering machinery maintenance department. When the piston rod is worn, it is easy to cause the hydraulic oil inside the hydraulic cylinder to leak. When the piston rod is cracked, it is easy to cause the piston rod to break, thus causing a safety accident. Since the piston rod has the characteristics of high manufacturing technology content, difficult processing, and high price, if the new part is directly replaced, it is expensive, resulting in economic losses, and more scrapped parts also cause waste of resources. The solid waste generated will also cause environmental pollution, which is obviously not in line with the concept of sustainable green development; if the piston rod is repaired blindly, it will not only cause secondary waste of raw materials, but improper repair will even cause safety accidents. Therefore, studying the green repair technology of the piston rod, extending its service life, and improving its repeated recycling rate can not only save resources and energy, save costs, but also generate huge Great economic benefits.
Remanufacturing is an important technology for implementing green and sustainable manufacturing. Laser cladding technology is a typical remanufacturing technology and belongs to a type of laser repair technology. At present, it is widely used for its advantages of high timeliness, strong nodulation, low dilution rate, no pollution, good surface quality, low material consumption, strong controllability, easy to realize automation, flexibility, greenness, no influence on the organization and performance of the overall workpiece material, and can increase the high temperature resistance, corrosion resistance and wear resistance of the material. The definition of laser cladding in “GB ∕ T29795-2013 Laser Repair Technology Terms and Definitions” is “a laser processing technology that uses a high energy density laser beam to quickly heat and melt the cladding material, forming a molten pool on the surface of the substrate, and forming a metallurgical bonding layer on the surface of the substrate after cooling and solidification”.
2 Research status at home and abroad
The process flow of laser cladding remanufacturing is mainly: substrate surface pretreatment (degreasing and rust removal) → feasibility analysis (appearance evaluation, non-destructive testing, Evaluation) → Material and metallurgical analysis (sampling, determining raw material model, configuring cladding materials, testing, metallographic analysis, evaluation) → Data collection → Technical solution formulation → (presetting cladding materials) → Preheating → Laser cladding process (synchronous) → Post-processing (machining, surface flaw detection, testing, assembly, testing). This study mainly focuses on the data collection, technical solution formulation, and laser cladding process of hydraulic cylinder piston rod.
2.1 Research status of data collection and model processing of failed parts
After the parts fail in the form of friction, wear, corrosion, cracks, fractures, etc., the shape and structure are often irregular, and the accurate parameterization of the shape of the failed parts is extremely important in the process of part remanufacturing, so it is necessary to use high-precision measuring instruments for data collection. If conditions permit, new product models can be prepared.
Dongaonkar and Metkar proposed a damaged part reconstruction method based on reverse engineering and rapid prototyping technology. Taking mechanical gears as an example, the failed gears were 3D scanned, and the 3D printing was completed after the reconstruction model. Parts repair. Yu Zongyu et al. proposed a part modal evaluation method based on Geomagic Design X reverse engineering, taking the camshaft of an internal combustion engine as an example. After reverse modeling the camshaft, they studied the modal analysis and calculation of the model, and obtained the fixed frequency and vibration mode of the camshaft under normal conditions, proving the accuracy and effectiveness of the evaluation method. Zhou Yanan and Qiao Xun proposed a 3D laser scanning point cloud data filtering method. By constructing a non-uniform grid, streamlining the original point cloud data, using the KD-Tree neighborhood search algorithm, selecting the optimal filtering neighborhood of the point cloud data, calculating the local convergence point to make the noise point drift to the local mode point position, and iterating and updating multiple times to improve the accuracy of the point cloud data. Wei Yanbiao et al. took the cam parts of engineering machinery as an example, combined three-dimensional measurement and 3D scanning, reconstructed the model of the parts, and obtained high-precision 3D model data. Jie Hui et al., Zhou Xinan et al. took the damaged impeller of an aircraft as the research object, used 3D scanner point cloud acquisition, used Geomagic Wrap software to process and repair the model data, and imported it into the 3D laser scanning point cloud data filter. Geomagic Design X is used for modeling to improve model accuracy and modeling efficiency. Xiao Hongtao et al. collected point clouds using 3D laser scanning, processed data to obtain STL data, and used 3D printing to manufacture the model. Zhang Chuan et al. introduced the method of 3D scanning reverse modeling and used CATIA for reverse modeling. Nishikawa et al. used a Xe plasma focused ion beam scanning electron microscope (PFIB-SEM) system to analyze the crystallographic characteristics of small fatigue cracks in the microstructure of high-temperature alloys, and successfully observed the 3D fatigue crack propagation path using large-volume, high-resolution 3D images. Gu et al. proposed a calibration method suitable for large and medium-sized components of instruments based on the free-form surface reconstruction model, and considered the uncertainty components introduced in the coordinate registration and surface approximation process to estimate the combined uncertainty of the reconstructed surface. The scanning measurement device was calibrated using a reconstruction model composed of NURBS surface expressions and their uncertainty predictions, and finally the surface of a standard part with a diameter of 4 m was tested. da Silva Santos et al. proposed a new three-dimensional scanning detection method for industrial seals based on the fusion of two-dimensional laser beam sensor and robot arm motion mode data. This method provides the three-dimensional geometric shape and volume of the inspected parts as output, so as to perform automatic compliance inspection according to process requirements.
2.2 Current status of numerical simulation and process parameter research of laser cladding
Laser cladding can be divided into synchronous powder feeding method, synchronous wire feeding method and preset method according to the different supply methods of cladding materials. The main laser cladding process parameters are: powder ratio PR, laser power P, laser scanning speed v, defocus amount, i.e. spot diameter D, powder feeding speed, gas flow rate, and energy absorbed per unit area of cladding layer E = P/Dv.
Liu Jichang pointed out the reason why metal additive manufacturing is relatively less used, and proposed that in the future, process planning should be optimized to improve forming efficiency and quality, and metal materials suitable for additive manufacturing should be developed. Jiao et al. studied the laser cladding of a layer of T15M high-speed steel on the surface of Q235 substrate. The influence of laser scanning speed on the microstructure and wear performance of cladding layer was studied. The results showed that with the increase of laser scanning speed, the carbides precipitated at the original austenite grain boundary increased and the large local segregation decreased. The coating had the lowest hardness when the scanning speed was 100 mm/min. When the scanning speed was 200 mm/min, the wear resistance was the best. When the laser scanning speed reached 300 mm/min, the wear performance of the coating deteriorated and a large number of debris fell off. Huang Haibo et al. proposed a geometric model reconstruction method based on NURBS surface fitting and obtained a path planning method for obtaining a stable laser power density. Liu Lijun et al. studied the influence of H13 die steel wear on die service life and used finite element analysis software to simulate and analyze the process of laser cladding Ni-based coating on die steel. Shayanfar et al. deposited Inconel 625 powder on ASTM A592 steel substrate and established a regression statistical model of laser cladding parameters. A process parameter map was formed to select the optimal laser process parameter combination in order to quickly obtain the best cladding layer. Wang Lei et al. comprehensively considered the optimal inter-layer stop time and the laser scanning path layer by layer, formulated the cladding head empty travel path selection rules, and established a multi-region multi-layer laser cladding path planning model. Cui Lujun et al. used ANSYS finite element analysis software to establish a slender shaft laser cladding model based on spiral and raster scanning trajectories, and simulated the temperature field under different cladding trajectories. In order to overcome the technical defects of the spiral blades of the coal mining machine drum after traditional cutting, re-welding and surface surfacing, Yue Haitao carried out a study on the laser cladding remanufacturing of the blades. By constructing a multi-parameter alloy cladding layer geometry model and analyzing the influence of various laser process parameters on it, the optimal process parameters were obtained, and consistent measures for the defects of the alloy cladding layer were proposed. In order to improve the remanufacturing and repair quality of failed parts, Shu Linsen et al. proposed a remanufacturing process based on geometric reconstruction and path position planning. First, the model of the failed part was reconstructed, and then the laser remanufacturing was formed. The laser nozzle position is determined by the laser cladding path, and the effectiveness is finally proved by practice. Hu Yanfeng analyzed the failure mode of the centrifugal pump impeller, established a remanufacturing evaluation system with three indicators of technology, economy and re-service, and then analyzed the influence of laser cladding process parameters on remanufacturing quality, constructed a laser cladding model, found the optimal process parameters, and finally performed laser cladding remanufacturing on the impeller to verify the quality. The current research status of the above two aspects at home and abroad has basically solved the data acquisition and numerical simulation problems in the laser cladding process, and provided a certain theoretical and technical basis for this study to pursue high-quality and high-efficiency laser cladding remanufacturing. However, there are still few studies on the laser cladding remanufacturing process of hydraulic cylinder piston rod. Combined with engineering practice, this study selected the steering cylinder piston rod of Liugong 855 loader as the research object. The rated load of the loader is 5000kg, the operating weight of the whole machine is 16500kg, and the loader turns frequently during work, requiring the steering system to be light and flexible. The steering pump of the steering system is a gear pump 14MPa, flow rate 134.4L, steering angle 35°.
3 Analysis of failure forms of piston rod
As shown in Figure 1, the steering cylinder assembly of Liugong 855 loader is a double-acting, single piston rod, welded, HSG type engineering hydraulic cylinder. Its structure is mainly composed of sealing gland, front cover, guide sleeve, cylinder, rear cover, piston, piston rod, sealing ring, guide device and other parts. The cylinder diameter of the steering cylinder is Ф90mm, the rod diameter is Ф40mm, the speed ratio is 1.25, the thrust is 101790N, the tension is 76340N, the rated working pressure is 16MPa, it belongs to medium and high pressure, and the stroke is 377~611mm. The piston rod is a solid rod with a round earring on the outer end. It is made of 45 steel, and the quenching and high-frequency quenching depth is 0.5~1mm. It mainly bears thrust or tension loads. The rod outer diameter tolerance is f7~f9, and the roughness is Ra0.1~0.3μm, the surface is chrome plated 20~50μm (0.02~0.05mm) and ground and polished.
Loaders are widely used in sand fields, stone fields, coal fields, mixing stations, logistics transfer and other places. According to actual engineering experience, although the surface of the steering cylinder piston rod has been technically treated, the piston rod will still have the following major failure forms after long-term use:
(1) Surface friction and wear. Due to the lack of regular maintenance, hard objects such as stones and sand in the actual operation process are splashed onto the surface of the hydraulic cylinder. The piston rod is rubbed by foreign hard particles during the reciprocating telescopic movement. In addition, the piston rod itself is affected by many factors such as high pressure, large flow, hydraulic oil performance, corrosion, external impact vibration, collision, periodic tensile and compressive stress and strain. The vicious cycle leads to piston rod surface wear, chrome plating corrosion and shedding, and the gap between the piston rod and the seal increases, resulting in oil leakage. As shown in Figure 2, the wear depth of the piston rod surface is greater than 0.25mm.
(2) Surface scratches. After the hydraulic cylinder is used for a long time, a large amount of sediment is stuck in the seal. When the piston rod is running, these sediments cause Scratches are generated on the piston rod surface. As shown in Figure 3, there are as many as 20 to 25 scratches on the piston rod surface, with a scratch depth of 0.3 to 0.6 mm, a scratch width of 1 to 60 mm, and a scratch length of 100 to 300 mm. The chrome plating layer falls off, so that the burrs on the surface further damage the oil seal.
(3) Pits and cracks are generated on the piston rod surface. Due to the flying of hard objects at the construction site, the piston rod surface is directly damaged, resulting in pits and cracks. Long-term stress causes the cracks to increase, and even the piston rod breaks, as shown in Figure 4.
(4) Bending deformation. When the piston rod surface is rubbed and worn, and the piston inside the hydraulic cylinder is damaged, it is easy to cause the tensile and compressive loads on the piston rod to increase instantly, resulting in bending and deformation of the piston rod, or even breaking.
4 Reverse data acquisition of piston rod
In order to accurately parameterize the damaged parts, reverse data acquisition of the damaged parts is required.
4.1 Preparation
(1) Disassembly. Fix the steering cylinder on the disassembly device and disassemble the cylinder non-destructively to prevent secondary damage to the parts. When pulling out the piston rod, it is necessary to keep it in a vertical position, move slowly and evenly, and be careful not to let the internal hydraulic oil flow out.
(2) Cleaning. Select appropriate physical or chemical cleaning methods to clean the dirt, sludge, rust, etc. on the surface of the piston rod, such as solution immersion, high-pressure jet, ultrasonic cleaning, laser cleaning, etc.
(3) Inspection. Non-destructive inspection of the cleaned piston rod requires the use of tools and other inspection devices, and compares the dimensional accuracy, shape and position tolerance, surface quality, internal material conditions and other indicators at the time of leaving the factory, and inspects and evaluates them one by one. In addition, the wear surface and its internal material defects are also inspected to see if there are cracks that affect the structural strength. All inspection results should be recorded.
(4) Pre-processing. Determine the parts on the piston rod that need to be remanufactured, remove the chrome plating, and repair irregular shapes such as pits, cracks, and fractures. For parts with roundness and straightness errors, mechanical processing methods are used for pre-processing, such as turning, milling, grinding, polishing, etc. to make them regular in shape and eliminate errors. During the processing, the process reference of the part can be found based on the unworn surface.
4.2 Reverse process
(1) Data acquisition. The parameters of the failed parts are collected by 3D scanning, and the discrete point cloud data obtained from various angles are spliced to obtain relatively complete part surface information. As shown in Figure 5, the piston rod is sprayed with imaging agent, the scanning system is calibrated, and special marking points are attached to the 2D turntable and parts. This study uses SHINING 3D’s EinScan-Pro industrial-grade 3D scanner to scan the parts to obtain data.
(2) Point cloud data processing. The unfavorable influencing factors such as the collected data error, noise, and measurement accuracy are filtered out to make the point cloud fitting surface smoother and more in line with the actual parts. As shown in Figure 6, the redundant points are deleted. Cloud, splicing, packaging, smoothing, sharpening, simplifying, creating references, etc.
(3) Data reverse. Importing data into 3D software for 3D modeling, which occupies an important position in the entire remanufacturing process, must be carried out strictly in accordance with the original design and the current structure, size, and technical requirements of similar new products. This study uses Autodesk Inventor software from the American Autodesk Company, as shown in Figure 7, to compare the 3D models of the new piston rod and the failed piston rod, perform 3D Boolean operations, obtain the failed area model and save it as an STL format file, and perform facet segmentation.
5 Laser cladding of piston rod
5.1 Material analysis
The material of the oil separation cylinder piston rod is 45 steel, the material structure is ferrite and pearlite, and it belongs to medium carbon structural steel, with good cold and hot price performance and mechanical properties. The mechanical properties are uniform, and its chemical composition is shown in Table 1.
5.2 Laser cladding
5.2.1 Technical solution formulation
For scratches that only slightly damage the chrome plating layer, re-chrome plating and polishing can be restored to the technical requirements. The milky white chrome is used as the base layer with a thickness of 25 μm, the surface hard chrome plating thickness is 35 μm, and the total coating thickness is 60 μm. The chrome plating process parameters are shown in Table 2.
For piston rods with severe wear and scratches, pits, cracks and other failure forms, the laser cladding process flow is as follows: non-destructive testing of failed parts → surface cleaning → surface turning → surface grinding → removal of surface dirt → part surface cleaning → drying in resistance furnace → laser cladding.
The actual operation process is shown in Figure 8. The whole set of test equipment is mainly composed of a horizontal rotating device, a three-jaw chuck, a top, a heating device, a laser, a powder feeding system, a gas feeding system, a lifting mechanism, an automatic feeding mechanism, an automatic grinding and polishing device, etc. The spindle speed of the horizontal rotating device can be adjusted according to The spindle needs to be regulated in speed. The front end of the spindle is equipped with a three-jaw chuck or a top for clamping the workpiece, and the rear end is equipped with a tailstock top for supporting the workpiece. The heating device is used to heat the parts in the early, middle and late stages of laser cladding to ensure that a good cladding interface is formed between the cladding metals.
Cladding path planning: The piston rod continuously rotates counterclockwise and clockwise under the clamping of the three-jaw chuck and the tailstock top. The piston rod rotates one circle, and the laser clads a cladding layer. The feeding mechanism drives the laser and other devices to feed intermittently along the axis of the piston rod until the cladding of the failed surface is completed. There is a certain distance between each cladding layer, which is conducive to heat dissipation during the cladding process.
5.2.2 Laser cladding process parameters
The influence of preheating on the formation of cladding interface: Increasing the preheating temperature can reduce the temperature difference between metals, which is conducive to the diffusion of alloy elements to the interface.
The influence of cladding process parameters: After fine processing, the thickness of the laser cladding layer is generally 5 to 10 times that of the electroplating layer, and the cladding layer and the substrate can form a stronger metallurgical bond, the grain milling organization, and the workpiece deformation is small. The laser cladding process parameters are shown in Table 3. The cladding material is self-fluxing alloy powder, which has good properties and steel metal parts. The density of the alloy powder material is ρ = 7.85g/cm3, the cladding thickness is 2mm, and the semiconductor laser is selected. The laser spot size is 2mm.
5.3 Post-processing
The laser cladding surface is ground, re-chrome plated, and polished to meet the technical requirements of its original design and similar new products. The finished product of the remanufactured piston rod is shown in Figure 9. The shape and position tolerance detection and non-destructive testing are carried out on it. The mechanical properties of the material such as elasticity, plasticity, strength and toughness are comprehensively tested. The X-ray diffractometer can be used for phase analysis, the metallographic microscope can be used for microstructure morphology analysis, and the microhardness analysis can be performed with a micro Vickers hardness tester.
The remanufactured piston rod is assembled with other parts and installed into the hydraulic cylinder. It is tested one by one on the hydraulic cylinder test bench according to international standards, national standards, and factory indicators, including hydraulic cylinder trial operation, starting pressure characteristics, pressure resistance test, durability test, internal and external leakage test, load test, stroke inspection, etc.
６ Conclusion and discussion
This study proposes a remanufacturing process method combining reverse data acquisition with laser cladding path planning for the laser cladding remanufacturing process of the hydraulic cylinder piston rod. First, the common failure forms in engineering practice are analyzed, namely surface friction wear, surface scratches, piston rod surface pit cracks and bending deformation. It is proposed to perform reverse three-dimensional scanning data acquisition, point cloud processing, and data reverse for the failed parts, and obtain the failure The regional model is then developed by analyzing the piston rod material, formulating a technical solution, and putting forward requirements for the on-site test equipment. The laser cladding path is planned, the process parameters are set, and the laser cladding remanufacturing process is obtained. After post-processing, the piston rod remanufacturing can be completed.
Compared with the current actual status of recycled materials, the laser cladding remanufacturing process for hydraulic cylinder piston rods is not only technically, economically and environmentally feasible, but also has obvious advantages. It can extend the service life of the hydraulic cylinder and effectively improve the reuse rate. From a long-term perspective, this method is in line with the concept of sustainable green development, and can save resources and energy, generating huge economic benefits.
