In order to solve the problem of severe surface wear and difficulty in repair during the overhaul process, laser cladding technology was used to remanufacture the worn reducer shaft surface. The effects of laser cladding on the metallographic microstructure, hardness, wear resistance and corrosion resistance of the reducer shaft surface were studied. The results show that after the worn reducer shaft is remanufactured by laser cladding, the surface cladding layer is fine and dense, and the performance of the cladding layer is significantly improved. The wear resistance and corrosion resistance are about 2.5 times and 2 times of the reducer shaft substrate, respectively. Laser cladding technology has broad application prospects in the remanufacturing of reducer shafts.

The reducer is mainly composed of transmission parts (gear or worm), shaft, bearing, housing and its accessories [1]. Production practice shows that with the long-term operation of the reducer, the reducer shaft surface is prone to wear. When the reducer shaft is worn to a certain extent, it will cause the equipment to fail to work normally. Once this happens, it is necessary to replace the reducer shaft or repair and remanufacture its surface. Although replacing the reducer shaft can restore the performance of the reducer, this method has limitations. On the one hand, when the maintenance period is short, the reducer shaft for replacement may not arrive on time; on the other hand, the cost of replacing the reducer shaft is high. Therefore, in recent years, the repair and remanufacturing of the reducer shaft surface has been widely studied and applied in practice. At present, the commonly used repair and remanufacturing technologies for the reducer shaft surface include brush plating, repair welding, thermal spraying, etc. Although good application effects have been achieved, there are still some shortcomings. For example, the brush plating technology is used on the reducer shaft surface, and the obtained brush plating coating has a limited thickness and is easy to peel off [2]; the repair welding technology is used, and the temperature is high during the remanufacturing process, which generates thermal stress and a large heat-affected zone, which is easy to cause cracks or cracks on the shaft surface; the thermal spraying technology is used, and the obtained thermal spray coating is mechanically bonded to the reducer shaft substrate, and the bonding strength is not high [3], and the coating has pores and a high porosity. Laser cladding technology is a surface modification technology that has been increasingly applied in industrial applications in recent years. It has the advantages of small heat-affected zone, small deformation, low material consumption, low cost, clean and pollution-free [4-6]. For the reducer shaft with severe wear, it is feasible to use laser cladding technology to remanufacture its surface. The reducer shaft under repair in a certain enterprise was severely worn. It was difficult to restore its size using traditional technical methods. If the shaft was customized from the manufacturer, it would take about 2 months. In order to meet the maintenance period requirements, laser cladding technology was used to repair and remanufacture the worn reducer shaft surface to restore its performance and extend the service life of the reducer shaft.

1 Test materials and process
1.1 Test materials and equipment
The test materials are the reducer shaft of a scraper conveyor repaired by a certain enterprise and laser cladding powder. In view of the fact that the failure of the reducer shaft is mainly caused by surface wear, when selecting laser cladding powder, in order to ensure the repair and remanufacturing effect of the reducer shaft, first of all, the wear resistance requirements of the reducer shaft surface are considered, and secondly, the cost of laser cladding repair and remanufacturing is considered. In addition, the use conditions of the reducer shaft also need to be considered. Taking all the above factors into consideration, the laser cladding Fe-based powder is selected for the test, and its composition is shown in Table 1 below.
When selecting the test equipment, according to the laser cladding efficiency, laser cladding repair and remanufacturing quality, cost and other factors, the laser is selected as a semiconductor type with a rated power of 4 kW and an equipment model of IGJR-4.

1.2 Test process
The main test process of remanufacturing the reducer shaft surface using laser cladding technology is as follows.
(1) Surface pretreatment
The worn reducer shaft surface is cleaned, and the oil, coal slag and other debris on the surface are thoroughly removed. The fatigue layer on the outer surface of the reducer shaft is turned by a CNC lathe, and the fatigue layer is turned cleanly until there are no corrosion points or scratches. The general turning thickness is 0.5-1.0 mm.
(2) Dimension measurement and parameter formulation
The reducer shaft dimensions after turning are measured with a vernier caliper, and the thickness of the laser cladding layer is calculated according to the assembly dimension requirements of the reducer shaft, and the laser cladding test parameters are formulated, as shown in Table 2.
(3) Laser cladding
Before laser cladding, the reducer shaft surface is cleaned with anhydrous ethanol, and after drying, laser cladding is performed according to the established laser cladding parameters. Figure 1 shows the laser cladding process of the reducer shaft. Figure 2 shows the surface morphology of the reducer shaft after laser cladding.
(4) Machining
In order to meet the assembly requirements of the reducer shaft, the reducer shaft after laser cladding needs to be machined. The test first uses a CK61100 CNC lathe to turn it. After turning, there are no scratches, pits and other defects on the surface, and a processing allowance of 0.1mm is reserved; then the reducer shaft surface is polished by an external cylindrical polishing machine until the size and surface roughness requirements of the reducer shaft are met. Figure 3 shows the final surface morphology of the reducer shaft after machining.

(5) Quality inspection and performance test
The machined reducer shaft is quality inspected, where the size is inspected by a vernier caliper, the roughness is inspected by a roughness meter, and the defects such as pores and cracks are inspected by penetrant testing (PT). The laser cladding repair and remanufacturing effect of the reducer shaft is mainly judged by the microstructure analysis and performance test of the cladding layer.

Analysis of the repair and remanufacturing effect of the reducer shaft by laser cladding
2.1 Microstructure analysis
The microstructure of the laser cladding layer of the reducer shaft after laser cladding remanufacturing is shown in Figure 4.
As shown in Figure 4, the microstructure morphology of the laser cladding layer (bright white area) of the reducer shaft is obviously different from that of the substrate (dark area on the right). The laser cladding layer and the substrate are closely connected, and the interface is a plane crystal, indicating that the two are metallurgically bonded. Regardless of the upper, middle or lower part of the laser cladding layer, the grains are relatively small overall. The main reason is that laser cladding is a process of rapid heating and cooling, and the action time is relatively short, so the grains have no time to grow [7-8]. The cladding layer has a fine and dense structure, which helps to improve the comprehensive mechanical properties of the cladding layer. In addition, in Figure 4, there are certain morphological differences in the microstructure of different positions of the cladding layer. The middle part of the cladding layer shows obvious dendrite morphology. This is mainly because the temperature gradient in the middle part of the cladding layer is reduced relative to the lower part of the cladding layer, and the solidification rate increases, so that the fine plane crystals formed in the lower part of the cladding layer gradually grow into dendrites. However, according to the solidification theory, dendrites cannot grow all the time. The closer to the upper part of the cladding layer (the leftmost side in Figure 4), the greater the influence of air, and the dendrite morphology gradually disappears.

2.2 Hardness analysis
The Rockwell hardness test was conducted five times on the surface of the laser cladding layer of the reducer shaft using a HR-150A Rockwell hardness tester. The test reference standard GB/T230.1-2018 “Metallic Material Rockwell Hardness Test Part 1: Test Method”. Table 3 shows the hardness test results of the laser cladding layer of the reducer shaft.

As shown in Table 3, the hardness values ​​of the five test points of the laser cladding layer on the surface of the scraper conveyor reducer shaft are not much different, and the average hardness reaches 50.02HRC. High hardness is beneficial to the surface strength and wear resistance of the reducer shaft. According to the relationship between material properties, the laser cladding layer has high hardness and strong ability to resist deformation and anti-destruction. At the same time, hardness value is also one of the important indicators for measuring wear resistance [9-10]. After the reducer shaft is repaired and remanufactured by laser cladding technology, the surface laser cladding layer has high hardness, which helps to improve the surface performance of the reducer shaft and enhance the application effect of laser cladding remanufacturing.

2.3 Wear resistance analysis
The wear resistance test uses MLG-130 abrasive wear testing machine to conduct three groups of reducer shaft cladding layer and substrate comparison tests. The test refers to the standard JB/T7705-1995 “Loose abrasive wear test method rubber wheel method”. Figure 5 shows the test results of the wear resistance comparison between the laser cladding layer and the substrate of the reducer shaft.

As shown in Figure 5, the wear mass loss of the laser cladding layer of the reducer shaft and the substrate are different. The results of the wear resistance comparison tests of the cladding layer and the substrate of the three groups of scraper conveyor reducer shafts are consistent, and the wear loss of the substrate is greater than the wear loss of the cladding layer. This result not only shows that the wear loss of the substrate is greater under the same test conditions, and the laser cladding technology can improve the wear resistance of the reducer shaft surface, but also indirectly reflects that the different microstructures of the laser cladding layer and the substrate lead to differences in wear resistance. In the three groups of wear resistance comparison tests, the average wear loss of the reducer shaft substrate was about 0.65g, and the average wear loss of the cladding layer on the reducer shaft surface after laser cladding repair and remanufacturing was about 0.28g. Under the same wear test standards and wear test conditions, the wear loss of the laser cladding layer on the reducer shaft surface is about 2/5 of the reducer shaft substrate. It can be approximately considered that the wear resistance of the laser cladding layer on the reducer shaft surface is 2.5 times that of the reducer shaft substrate.

2.4 Corrosion resistance analysis
The corrosion resistance test used LRHS-412-RY salt spray corrosion test box to conduct five groups of neutral salt spray tests (NSS tests) on the reducer shaft substrate and cladding layer. The test reference standard GB/T10125-2012 “Artificial atmosphere corrosion test salt spray test”. The specific test conditions are: the mass concentration of sodium chloride solution is 50g/L, the pH of the spray solution is 6.5-7.2, the spray pressure is 120kPa, and the test time is 48h. After the test, the corrosion products are completely removed and washed with ethanol, and the mass loss is measured after thorough drying. Figure 6 shows the corrosion resistance comparison test results of the reducer shaft laser cladding layer and the substrate.

As can be seen from Figure 6, the corrosion mass loss of the laser cladding layer of the reducer shaft is different from that of the substrate. The results of the five groups of corrosion resistance comparison tests of the cladding layer and the substrate of the reducer shaft are consistent, and the corrosion mass loss of the substrate is greater than the corrosion mass loss of the cladding layer, indicating that under the same test conditions, the corrosion mass of the substrate is more, and the corrosion resistance of the laser cladding layer of the reducer shaft is better. The reason for the different corrosion mass loss is mainly because the microstructure and morphology of the laser cladding layer of the reducer shaft are different from that of the substrate, and the ability to resist corrosion is different. In the five groups of corrosion resistance comparison tests, the average corrosion mass loss of the reducer shaft substrate is about 0.44g. After laser cladding repair and remanufacturing, the average corrosion mass loss of the cladding layer on the surface of the reducer shaft is about 0.21g. Under the same corrosion test standards and test conditions, the corrosion mass loss of the laser cladding layer on the surface of the reducer shaft is about 1/2 of the reducer shaft substrate. It can be approximately considered that the corrosion resistance of the laser cladding layer on the surface of the reducer shaft is twice that of the reducer shaft substrate.

3 Conclusions
Laser cladding technology has achieved good application results in the repair and remanufacturing of the reducer shaft surface. After the scraper conveyor reducer shaft surface was repaired and remanufactured by laser cladding technology, the grains in the middle of the laser cladding layer were dendrites, the overall grains were small and the structure was dense, the surface hardness of the cladding layer reached 50.02HRC, the wear resistance of the cladding layer was about 2.5 times that of the reducer shaft base material, and the corrosion resistance was about 2 times that of the base material. It can be seen that laser cladding technology can effectively solve the problem of reducer shaft surface wear, not only can it restore the performance of the failed shaft and extend the service life of the reducer shaft, but also has low repair and remanufacturing costs, energy saving and emission reduction, and has broad application prospects.