The coaxial powder feeding laser cladding technology was used to prepare Ni-Cu alloy cladding layer on the ductile iron node sleeve sample to repair the ductile iron node sleeve. On the basis of the same process parameters, three different scanning paths were designed according to the structural characteristics of the node sleeve, including half-partition overlap in the width direction of the sample A and B, axial overlap along the circumference of one-way cladding, and “bow” overlap in the axial reciprocating cladding. The macroscopic morphological characteristics of the laser cladding samples under three different scanning paths, the results of nondestructive testing of the service interface by penetrant flaw detection, and the metallographic structure and microhardness were compared and analyzed. It was found that the surface cladding layer of the sample obtained by the axial overlap of the circumference of one-way cladding was defect-free and of good quality.

When the traction motor base of the EMU was overhauled, it was found that the traction motor base was often scrapped unnecessarily due to problems such as excessive size, scratches, and bumps on the inner wall of the node sleeve. Conventional subtractive repair methods cannot restore the size and performance of components, but the application of laser cladding additive remanufacturing technology can effectively solve such problems. With the development of additive remanufacturing technology, its application in the field of advanced manufacturing technology is becoming increasingly widespread. In the laser additive remanufacturing process, it is first necessary to obtain the shape and size data of the damaged area of ​​the part through three-dimensional detection, and then repair it.

The laser cladding forming process is relatively complex. The process parameters, preheating temperature, laser scanning path, etc. are closely related to the quality of the cladding layer. At present, the research on factors affecting the formability of laser cladding is mostly focused on the research of cladding materials. Conventional laser cladding has two forms: single-layer multi-pass cladding and multi-layer multi-pass cladding. During the cladding process, the temperature field and stress field are superimposed on each other. The process of superposition of the temperature field and stress field under different cladding paths is different. Reasonable selection of cladding paths can obtain better cladding forming effects. Wang Yu et al. studied the path planning problem of laser cladding forming of arc-shaped curved surface structural parts by using the laser cladding method with internal powder feeding, and obtained the cladding layer with high dimensional accuracy and good surface quality; Zhang Deqiang et al. tested four different scanning paths respectively, and obtained their microhardness and Rockwell hardness after cladding forming; Tabernero et al. found that the mechanical properties of the cladding layers obtained by different scanning paths were different; Chen et al. studied the influence of parallel line and 45° inclined line scanning methods on the residual quality of SLM (selective la-ser melting) formed parts; Chen Dening et al. studied the influence of two scanning paths, serpentine scanning method and island scanning method, on the forming quality of SLM; Deng Shishi et al. studied the influence of S-shaped orthogonal scanning strategy and partition scanning strategy on the residual stress, deformation and cracks of SLM formed parts. Based on the structural characteristics of the node sleeve of the motor base of the EMU, this paper studies the influence of the cladding path on the forming quality of the cladding layer, and obtains the optimal cladding path for laser cladding forming of the node sleeve surface under specific materials and process parameters.

1 Experimental process and method

1.1 Laser cladding test powder material

The chemical composition and mass fraction of the Ni-Cu alloy powder used in the experiment are shown in Table 1, and the chemical composition and mass fraction of the node sleeve matrix material GJS-400-18LT ductile iron are shown in Table 2.

1.2 Experimental plan

The maximum damage depth of the node sleeve of the base is 0.5 mm. After optimizing the process parameters, the optimal single-layer cladding thickness obtained in the experiment is 0.6~0.7 mm. Therefore, the double-layer multi-pass overlap method is used to repair the node sleeve by laser cladding.

The laser cladding equipment used in the experiment is shown in Figure 1. The laser cladding powder feeding adopts parallel axis synchronous powder feeding, and the protective gas and powder feeding gas are both argon. In order to achieve the best cladding effect, the inner surface of each node sleeve is polished with sandpaper before cladding to remove the oxide layer, and industrial alcohol and dust-free paper are used for cleaning. At the same time, the metal powder is placed in a dryer for drying and left to stand at 120 °C for 2 h to ensure that the moisture in the powder is completely removed.

Two layers of Ni-Cu alloy cladding layers with a size of about 424 mm × 50 mm are clad on the inner surface of the node sleeve ring. The optimized laser cladding process parameters are shown in Table 3.

According to the best process parameter combination, three different scanning paths were used for the node sleeve structural characteristics, including half-partition overlap in the width direction of the sample A and B, axial overlap along the circumference of one-way cladding, and “bow” overlap in the axial reciprocating cladding, and three node sleeve cladding samples were obtained. The three scanning paths are shown in Figure 2.

Scanning path 1 is the half-partition cladding path in the width direction of the sample A and B. The size of the spot diameter and the defocus amount, scanning speed, and overlap rate are ensured by adjusting the turntable clamping workpiece angle, the cladding head angle and its vertical distance to the inner surface of the workpiece, the turntable rotation speed, and the stepping lateral distance of the cladding head. The entire inner surface of the node sleeve workpiece is divided into two areas, A and B, and the cladding is completed in two times.

Scanning path 2 is axial overlap along the circumference of one-way cladding, and its process parameters are set in the same way as path 1.

Scanning path 3 is an axial reciprocating cladding “bow” shaped cladding path. The cladding head moves along the axial direction of the workpiece for cladding. The chuck stepping is controlled by inching. The workpiece rotates through a fixed arc length to ensure the overlap rate. The setting method of scanning speed and spot size is consistent with path 1.

After the cladding test is completed, the sample is pre-processed according to the metallographic inspection steps of sample selection, inlaying, sample rough grinding, sample fine grinding, sample polishing, and sample corrosion. The corrosion is carried out by chemical corrosion for about 1 min with a liquid volume ratio of VFecl3: VHcl: VH2O = 1: 10: 20. After the sample is fully corroded, the geometric morphology, metallographic structure, and microhardness of the cross section of the cladding layer perpendicular to the scanning direction are observed using a GX50A inverted metallographic microscope and a scanning electron microscope.

2 Experimental results and analysis

2.1 Analysis of macroscopic morphology of cladding layer

The macroscopic morphology of laser cladding layer is very important in actual industrial production. The better the macroscopic morphology of cladding layer, the smoother the surface, the less post-processing machine processing, the higher the production efficiency and the lower the production cost. On the contrary, if the surface macroscopic morphology of laser cladding layer is poor and there are a lot of defects on the surface, there will be disadvantages such as low production efficiency and high production cost, and even worse, the laser cladding repair parts may be directly scrapped. There are many factors that affect the macroscopic morphology of cladding layer, such as laser cladding process parameters, external environment, cladding materials, etc. When the laser cladding process parameters, external environment and cladding materials are determined, the cladding path directly affects the macroscopic morphology of laser cladding layer.

The three scanning paths of the experimental scheme were used for laser cladding test, and the macroscopic morphology of the obtained samples is shown in Figures 3 to 5.

As can be seen from the figure, the cladding layer obtained by the half-partition cladding method of sample 1 shown in Figure 3 has better flatness than sample 3, and there is no obvious element burning phenomenon. However, due to the limitation of the processing stroke of the ordinary cladding head and the offset angle relative to the sample to be processed, the overlap between the A and B zones is inevitably insufficient, and there are slight hole defects on the surface, which affects the overall cladding quality. Sample 2 shown in Figure 4 adopts the axial overlap method of unidirectional circumferential cladding, so the surface quality of the cladding layer obtained is the best, the surface flatness is the best, the overlap effect is good, and there is no obvious unmelted powder adhesion on the surface. Sample 3 shown in Figure 5 uses a “bow” shaped cladding path, so the surface flatness of the cladding layer is the worst, and the single-pass cladding line along the axial direction is too short. During cladding, the laser has the strongest “tempering” effect on the previous cladding line, the heat accumulation is too large, and the heat dissipation is poor, resulting in a wide single-pass cladding line, a large thickness, and obvious element burnout. Therefore, from the macroscopic morphology, it can be seen that the scanning path has an important influence on the surface quality of the cladding layer of ductile iron-based cladding Ni-Cu alloy powder.

2. 2 Nondestructive detection analysis of cladding surface defects

After the node sleeve sample is repaired by laser cladding, it is allowed to cool naturally. Then, the cladding layer is machined according to the actual size of the product, and the surface coloring is subjected to penetration testing. The surface quality is analyzed and evaluated by nondestructive testing. The flaw detection results obtained by using different laser cladding scanning paths are shown in Figures 6 to 8.

The results of nondestructive testing show that the quality differences of the service surfaces of various samples after laser cladding are basically consistent with the macroscopic morphology. As shown in Figure 6, the product has a certain degree of pore defects when the half-partition scanning path is used for cladding. The defect situation is consistent with the macroscopic morphology, mainly located at the overlap of the two half areas A and B. This cladding method indirectly increases the temperature distribution of the two cladding areas A and B. When the powder is melted and formed, the temperature gradient of the two areas is large, which is easy to produce defects such as pores and cracks. As shown in Figure 7, the use of unidirectional cladding axial overlap method can effectively solve the problems of excessive heat accumulation and inconsistent temperature gradient of the cladding layer, so that the thermal stress is fully released during the cladding process, and the generation of cracks and pores is effectively avoided. As shown in Figure 8, the annular node sleeve sample adopts the “bow” scanning path for cladding, and there are circumferentially distributed crack defects and a large number of them. The cracks develop axially and are consistent with the cladding direction. The location is the overlap of the cladding line. Because the cladding path prepared by this scanning path is short, the relative cooling time of adjacent cladding paths is too short, the heat dissipation is insufficient, and the relative heat accumulation of the entire workpiece is too large, thus cracks are generated.

2.3 Microstructure Analysis

Samples were taken at the defects of each sample for metallographic structure observation, and the defect characteristics of the cladding layer were further analyzed.

As can be seen from Figure 9, there are obvious defects such as black slag inclusions and pores in the surrounding area of ​​the overlap line of the half-partition cladding layer. The maximum pore size is 302 μm, which is basically consistent with the non-destructive testing results. Analysis of the cause: The heat source was interrupted during the conversion of cladding in areas A and B. The cladding area A, which had been completed, cooled down during the process of adjusting the workpiece direction and changing the clamp. The uneven heat at the overlap of the two areas caused insufficient melting of some powders, resulting in poor forming quality in the overlap area.

As can be seen from Figure 10, the overall wavy shape of the cladding layer and the substrate bonding area is caused by the dilution effect of the substrate on the cladding layer. The carbon atom diffusion degree of the graphite ball is weak, which effectively inhibits the formation of the white cast structure. The cladding layer has a uniform structure and the interface area is well bonded, ensuring the bonding strength of the interface.

As can be seen from Figure 11, the “bow”-shaped cladding scanning path is used, and the heat dissipation effect is very poor during the processing, which is easy to cause heat concentration and pore defects. The maximum pore size reaches 350 μm, and the smallest pore is 140 μm. The dense pores are also an important cause of cracks. The cracks germinate or expand in the part with pores. When subjected to stress concentration, the connection between the two pores breaks and cracks are formed.

In summary, only unidirectional lap cladding can obtain a cladding layer with uniform metallographic structure, good metallurgical bonding and no defects. This is because the upper cladding line preheats the powder on the next cladding line during unidirectional lap, reducing the temperature difference of the adjacent cladding lines, so the temperature gradient is small when the powder is melted and formed.

2.4 Microhardness of cladding layer

The microhardness analysis of the axial lap cladding layer along the circumference of the unidirectional cladding is carried out. The microhardness distribution of each region of the cladding layer prepared by the unidirectional overlap scanning path is shown in Figure 12.

As can be seen from the figure, from the surface of the cladding layer to the middle of the cladding layer, then to the bonding interface, heat-affected zone and substrate, its hardness shows a gradual downward trend as a whole. The surface hardness of the cladding layer is the largest, reaching 288.2HＶ10; there is a small range of fluctuations at the bonding interface, and finally it tends to be consistent with the substrate hardness, which is 159.2HＶ10. The change of the hardness value of the cladding layer at different depths shows that the farther away from the surface of the cladding layer, the lower its hardness, but the change trend is slow, which is beneficial to the entire cladding structure, and can effectively reduce the residual stress inside the cladding structure and improve the metallurgical quality of the cladding layer and the substrate.

3 Conclusion

This paper focuses on the unique structure and material properties of the node sleeve of the motor base of domestic high-speed railway EMUs. A certain thickness of Ni-Cu alloy powder cladding layer is clad on the GJS-400-18LT low-temperature ductile iron node sleeve substrate. The quality difference between the cladding layers prepared by different scanning paths is studied with the same process parameters such as laser power, powder feeding speed, scanning speed, and overlap rate. The results show that the axial overlap scanning path of unidirectional cladding along the circumference can effectively alleviate the heat accumulation, the thermal stress is fully released, and the cladding layer of the prepared sample has uniform structure and reliable quality. The cladding layer obtained by the axial overlap method of unidirectional cladding along the circumference has the best macroscopic morphology and formed service surface quality, which meets the performance requirements of the product.