Surface Repairing Technology of Cast Rolling Sleeve by High Speed Laser Cladding

In aluminum sheet casting and rolling production, the casting roll sleeve is the main working part. When in service, the surface of the roll sleeve is subjected to the interaction of high temperature and load, which often affects the service life and the quality of aluminum plate due to the early appearance of wear pits, cracks and wear. The price of roller sleeve can reach hundreds of thousands of yuan per set. If it fails too early, it will certainly increase production costs and is not conducive to improving economic benefits of production enterprises. Surfacing welding is the traditional repair method of roll sleeve, but its large heat transfer leads to deformation of the matrix and affects the repair effect. In order to solve this problem, many experts and scholars put forward the method of laser cladding to repair the roller sleeve. As a high-tech surface modification technology, laser cladding has the characteristics of high processing precision, small thermal deformation degree, and less follow-up processing amount. Cobalt-based alloys are widely used for laser cladding surface strengthening of steel materials because of their high temperature strength, hardness and thermal fatigue resistance.

The high speed laser cladding technology has the characteristics of high cladding efficiency, high powder utilization rate and ultra-low dilution rate, and can quickly prepare laser alloying layers on the surface of large rotating workpieces in equal phase of casting roll sleeves. The surface roughness of the cladding layer is low, which is conducive to subsequent processing, saving cladding materials, and overcoming the problems of low efficiency and high cost of conventional laser cladding. The process parameters have a great influence on the performance of the cladding layer. It is the premise and key to improve the performance of the casting roll to prepare the high speed laser cladding layer with excellent quality by optimizing the process parameters.

After the surface repair cladding is completed, the surface roughness of the cast roll sleeve often cannot meet the requirements of actual working conditions, and needs to be reprocessed by grinding machine and other processes. Therefore, the thickness and surface roughness of the cladding layer become the key indexes to evaluate its quality. In this paper, 32Cr3Mo1V cast rolling roll sleeve steel was used as the matrix. Firstly, the influence of different process parameters on the thickness and surface roughness of cobalt-based coating by ultra-high speed laser cladding was discussed. After determining the optimal range, the optimal process parameters were optimized by design orthogonal test. The microstructure and microhardness of the cladding prepared by using the optimal parameters were analyzed, in order to provide some experimental and theoretical basis for the exploration of a new and more efficient technology for the surface repair and strengthening of the cast roll sleeve.

The base material of the test was 32Cr3Mo1V aluminum casting and rolling mill casting roll sleeve, and the cladding material was Co-06 alloy powder with particle size of 15-53μm. The SEM morphology of the powder was shown in Figure 1, and the composition was shown in Table 1. Before coating, the powder was dried in a drying oven at 120℃ for 120 min. The turning streaks and oxide layers on the specimen surface were removed by mechanical grinding, and the oil was cleaned with alcohol.

                                                                                                          Fig.1SEMimageofCo-06powder

When the laser power is 2500 W and the cladding rate is 15 m/min, the thickness of the ultra-high speed laser cladding layer increases with the increase of the powder feeding rate within a certain parameter range. Because the powder utilization rate can exceed 90% in the ultra-high speed laser cladding process, most of the cladding powder can be fully melted above the workpiece and injected into the molten pool in the liquid state. With the increase of powder feeding rate, more cladding powder is injected into the melt pool in unit time, and the thickness of cladding layer increases. But the test also found that when the powder feeding rate is increased to the laser power and other parameters do not match, it is not enough to make most of the cladding powder can be fully melted on the top of the workpiece, and the unmelted or semi-melted cladding powder will seriously affect the forming quality of the cladding layer.

When the laser power is 2800 W and the powder feeding rate is 30 g/min, the thickness of the cladding layer decreases with the increase of the cladding rate within a certain parameter range. The reason is that the ultra-high speed laser cladding coating is composed of multiple thin layers stacked in the thickness direction. When the overlap rate is constant, the thickness of the thin layer directly determines the thickness of the cladding layer. When the cladding rate increases, the amount injected into the molten pool in the form of droplets per unit time will decrease, thus reducing the thickness of the single thin coating, and ultimately the thickness of the cladding layer.

When the powder feeding rate is 15 g/min and the cladding rate is 15 m/min, there is no obvious correlation between the thickness of the cladding layer and the laser power within a certain parameter range. Because ultra-high speed laser cladding only produces ultra-thin and minimal molten pool on the surface of the substrate, the dilution rate is extremely low, the laser power rate has little effect on the substrate, and the utilization rate of ultra-high speed laser cladding powder is very high, the amount of powder that can be melted by increasing the laser power is limited, so the laser power has no significant effect on the thickness of ultra-high speed laser cladding layer.

Through the observation of the macroscopic surface topography of the cladding layer, it is found that within the above parameter range, the surface topography of the cladding layer changes significantly only when the cladding rate changes. It is found that when the cladding rate reaches 75 m/min, the surface shape of the cladding layer is poor and the roughness is high, while when the cladding rate is 15 m/min or 25 m/min, the surface shape of the cladding layer is better, and the roughness is low at the same time.

1. In a certain range of parameters, the thickness of the cladding layer increases with the increase of the powder feeding rate, and decreases with the increase of the cladding rate, and the thickness of the cladding layer has no obvious correlation with the laser power.

2. The optimized process parameters are laser power 2500 W, scanning rate 15 m/min and powder feeding rate 30 g/min. Under these process parameters, the cladding layer can obtain higher thickness and microhardness.

3. The microstructure of ultra-high speed laser cladding layer is uniform and fine, and there is obvious arch delamination phenomenon. The bottom and middle regions of the cladding layer are mostly columnar crystals growing perpendicular to the fusion line of the cladding layer/matrix interface or the lap fusion line, and the growth direction is highly consistent. There is slight grain coarsening in the fusion line region. The growth direction of dendrites in the top region of cladding layer is relatively disordered.

4. The microhardness of the ultra-high speed laser cladding layer is significantly higher than that of the substrate, which is conducive to improving the wear resistance of the cast roll sleeve.