Abstract: During the service and use of axles, the damage to the axles mainly includes the frictional micro-motion damage to the inner wall during the assembly process, the continuous damage and peeling after erosion and corrosion by sediment, fatigue damage caused by the rotating bending load during operation, and surface damage to the axle during the inspection and unloading process, which will cause the axle to be scrapped due to surface failure. In response to these problems, the CRRC-SP-13 alloy steel powder developed by CRRC Research Institute was selected to perform laser cladding treatment on the damaged EA1N axle, and a detailed laser cladding process plan was formulated to strictly control the axle repair process from three aspects: before cladding, during cladding, and after cladding. The results show that the axle has a good effect after laser cladding treatment, and no new defects such as pores and slag inclusions are generated. The successful completion of the laser cladding process research on the EA1N axle has great practical significance for extending the service life of the axle and reducing the scrap rate of the axle.
Keywords: EA1N axle; remanufacturing; laser cladding; alloy steel powder

1 Preface

At present, with the vigorous development of the domestic rail transit industry, rail transit equipment has basically achieved independent and large-scale production. However, how the rail transit industry can achieve green and low-carbon development is a new proposition put forward by the society to all rail transit equipment industry workers. Green and low-carbon development is an inevitable requirement for the country to build a green manufacturing system, follow the development path of ecological civilization, and achieve “carbon peak and carbon neutrality”, and “remanufacturing and repair technology” is one of the most effective ways to achieve resource recycling. “Remanufacturing and repair technology” can extend the full life cycle chain of products (manufacturing, use, scrapping, remanufacturing, reuse, and scrapping), extend the service life of products, improve product technical performance and added value, and provide information for product design, modification and maintenance. Finally, the full life cycle of products can be completed at the lowest cost and with the least resource consumption, and the potential value of products can be maximized, which has extremely high economic and social benefits. Therefore, it is urgent to carry out relevant research work in the rail transit industry and realize the in-depth application of “remanufacturing and repair technology” in the rail transit equipment manufacturing industry as soon as possible.

As one of the key components of rail transit vehicles, axles require a lot of manpower and financial resources in their production process. Since the performance of axles has a significant impact on the safety of the entire vehicle, there are extremely strict requirements for the various parameters of axles. However, during the installation and removal of axles, it is inevitable that bumps, strains and other problems will occur, which makes the number of axle scrapped high all year round, causing great economic losses. Figure 1 is a physical picture of axle withdrawal and strain. At this time, if “remanufacturing repair technology” is applied to the repair of axles, the scrap rate of axles can be greatly reduced. Among them, laser cladding technology is a typical application of “remanufacturing repair technology”.

Foreign research on axle laser cladding repair technology started early. In 2013, SOODI et al. used commercial 420 stainless steel and 17CrMoV5 powder to repair the notched axle specimens. The rotary bending fatigue test confirmed that the CRMoVe powder had better repair effect. However, the new defects such as pores generated during the repair process caused the dispersion of the experimental results to increase.

In the field of domestic rail transit industry, many subsidiaries of CRRC, such as CRRC Sifang Co., Ltd., CRRC Qiji Co., Ltd., and CRRC Shijiazhuang Co., Ltd., have carried out research on the application of remanufacturing technology. Qi Xiansheng et al. [7] proposed the feasibility of laser cladding repair of EA4T axles. In 2020, Hou Youzhong et al. [8] of Qingdao Sifang Co., Ltd. used CRH380A/AL EMU EA4T axle steel as the matrix and NiCrMo alloy as the additive material, and used the selective laser cladding process method for repair and process evaluation. The results showed that the laser cladding effect was good.

In view of the fact that the research on the laser cladding process of EA1N axles at home and abroad is not perfect enough, and the annual scrap volume of EA1N axles is huge, this paper conducts research on the laser cladding process of EA1N axles, aiming to extend the service life of EA1N axles and reduce their scrap rate.

2 Characteristics and advantages of laser additive repair technology

Jiang Jibin et al. explained the powder feeding working principle of laser cladding. According to the integration of the powder injection head and the laser working head, the powder feeding method is divided into coaxial powder feeder method and lateral powder feeding method. The laser additive repair of the axle should adopt the coaxial powder feeding method. Figure 2 and Figure 3 are the laser additive system composition diagram and laser additive repair forming schematic diagram respectively.

The current mainstream repair technologies are laser additive repair technology, welding repair technology and thermal spray repair technology. Compared with welding repair technology and thermal spray repair technology, laser additive repair technology has the following advantages.

(1) High energy density The laser beam has the characteristics of high energy density, low heat input, and small heat-affected zone, that is, the thermal impact on the substrate is small. Due to the high energy density and short heating time, the residual stress between the repair layer and the substrate is small.

(2) High strength of the repair layer The repair layer has high density, and there is metallurgical bonding between the repair layer and the substrate. The dilution rate is low and the bonding strength is high. It can be used to repair parts under heavy load conditions.

(3) High repair flexibility It can achieve large-thickness and large-area laser cladding on the surface of the workpiece, meet the surface repair requirements of shaft parts of different sizes and shapes, and realize “near-net-shape” remanufacturing of selected areas of defective parts, with small subsequent processing allowance.

3 Process plan

This paper takes the technology-improved HXD2 electric locomotive axle as an example for verification. The specific chemical composition and main performance requirements of the EA1N axle alloy steel are shown in Table 1 and Table 2 respectively.

In order to solve the laser cladding problem of EA1N axle, CRRC-SP-13 alloy steel powder developed by CRRC Research Institute was selected for laser cladding of EA1N axle. The three aspects of pre-cladding, cladding process and post-cladding treatment were controlled. The specific axle laser cladding process is shown in Figure 4.

3.1 Preparation before cladding

(1) Powder treatment Based on the previous experimental results, according to the composition and performance requirements of EA1N alloy steel, alloy steel powder with a particle size of 53-150μm was selected for laser cladding. Before cladding, the powder needs to be sieved with an 80-mesh powder sieve (aperture of about 178μm) to ensure that there are no impurities and agglomerated powder in the powder, and then dried in a vacuum drying oven at 80℃ for 30 minutes.

(2) Machining to remove defects In order to remove the surface defects of the axle wheel seat, the position needs to be machined. Machining operations must meet the following principles:
1) Based on previous surveys and statistical results, the total machining cutting depth should be greater than or equal to 95% of the defect depth.
2) For a single part, the wheel seat of the axle after machining is guaranteed to be defect-free.
3) Although the defect morphology of each axle is different, all axles of a single model should share a set of machining procedures. Based on the above principles, the entire wheel seat surface of the axle is uniformly turned and the diameter is reduced by 2mm. No specific requirements are made for machining roughness.
(3) Axle surface treatment Since there are pollutants such as cutting fluid and anti-rust oil on the surface of the axle after machining, the axle needs to be rusted and degreased to ensure the quality of laser cladding surface repair. First, visually inspect the axle to be repaired, and use an angle grinder to grind and remove the rusted parts. At the same time, ensure that the grinding process does not cause grinding marks that are too deep or have steep edges on the surface, and do not damage the unrepaired parts. After grinding, measure the diameter of the part to be repaired, and use alcohol to clean the entire surface to be repaired to completely remove residual oil stains. After cleaning, avoid contacting the surface to be repaired again to avoid secondary contamination.

3.2 Laser cladding repair

(1) Laser cladding equipment and process parameters Based on the previously accumulated data and the characteristics of the axle material, the laser cladding process parameters that have been verified are used to perform laser cladding repair of axle surface defects. It should be noted that, considering the differences between different hardware, the process parameters are limited to the use of the laser additive repair dedicated experimental platform. The specific equipment and process parameters are shown in Table 3.

(2) Laser cladding path planning Considering that the axle to be repaired is a simple cylindrical surface, as shown in Figure 5 (red part), the laser cladding path is planned using manual programming. The specific sequence is as follows:

1) Start cladding from the edge of the axle repair area, using the conventional cladding strategy of a single spiral line.

2) After repairing to the inner edge of the red area, perform an additional cladding of 1 to 2 circles to ensure that no area is missed.

3) After the red part is clad, perform edge cladding on the outermost edge of the shaft and the edges on both sides of the keyway. At this time, the cladding head can be appropriately tilted to ensure that there is no missing meat during machining.
4) After the single-layer cladding is completed, the entire cladding layer is polished with an angle grinder to evenly remove the uneven surface parts and restore the metal luster of the surface. The grinding depth is about 0.3mm.
5) After the polishing is completed, the previous 4 steps are repeated until the cladding of the second layer is completed. At this time, it should be ensured that the diameter after cladding is at least 2mm larger than the original diameter. When cladding the second layer, it should be ensured that the upper and lower cladding tracks are staggered.

3.3 Post-cladding treatment

(1) Local heat treatment After the laser additive repair is completed, the axle is locally heat treated. Considering the application scenario requirements of on-site maintenance, under the premise of ensuring that the heat input causes the least thermal damage to the substrate, a customized heating sleeve is selected to perform heat treatment in an air environment. The heating temperature is 500~550℃, and the insulation time is 2~3h. During the heating and cooling process, the heating part is wrapped with heat insulation cotton to increase the heating speed and reduce the cooling speed. During the heating process, a thermal imager is used to monitor the temperature to ensure that the heating temperature is accurate.
(2) Nondestructive testing of the repaired parts of the cladding layer To ensure the quality of the repair of the cladding layer, in accordance with the standard GB/T 18851.1-2012 “Nondestructive testing Penetrant testing Part 1: General Principles”, fluorescent penetrants and black light lamps are used to perform nondestructive testing of the repaired parts. After the flaw detection, the penetrant is washed off with clean water, and the parts are quickly dried to prevent rust.
(3) Hardness test of the repaired parts of the cladding layer The surface of the cladding layer is lightly polished with an angle grinder to form a bright plane of at least 1 cm2. The hardness of the cladding structure is measured and filed using an ultrasonic hardness tester.
(4) Packaging for rust and corrosion prevention The repaired axle is cleaned and marked before packaging, and anti-rust and anti-corrosion treatment is performed. It is then packaged and sealed in a strong, impermeable neutral plastic bag.

4 Experimental verification

Take the HXD2 electric locomotive axle for technical improvement as an example, by preparing a set of finished test axles, the axle laser cladding preparation work was carried out according to the above axle laser cladding process flow, and the powder treatment, wheel seat machining to remove defects, axle rust removal and oil removal were completed; then the equipment and process parameters were determined, and the laser cladding path was planned. Figure 6 is a physical picture of the axle laser cladding. After the axle laser cladding was completed, the axle was subjected to local heat treatment, non-destructive testing of the cladding repaired parts, and hardness testing of the cladding repaired parts. The results showed that the laser cladding effect was good, and no new defects such as pores and slag inclusions were generated after the repair.

At present, laser cladding axle processing, residual stress detection, non-destructive testing and wheelset press test have been carried out, all of which meet the corresponding standard requirements. Figure 7 and Figure 8 are the actual picture of the axle after laser cladding and the wheelset press test picture, respectively. It has a certain effect on verifying the performance of laser cladding remanufactured axles, and can effectively avoid major safety accidents caused by the repaired axles after they are put into use.

5 Conclusion

Aiming at the problems of imperfect axle laser cladding process methods and lack of research on EA1N axle laser cladding process, this paper selects CRRC-SP-13 alloy steel powder developed by CRRC Research Institute for EA1N axle laser cladding, and controls the three aspects of pre-cladding, cladding process and post-cladding treatment. The results show that the axle laser cladding effect is very good, and no new defects such as pores and slag inclusions are generated after repair. The research on EA1N axle laser cladding process has been successfully completed. The engineering application verification of axle repair is currently underway, and the results of this test also provide a theoretical basis for the development of subsequent axle repair processes.