Thermal spraying process of water pump shaft

       The thermal spraying process of the water pump shaft includes four parts: surface pretreatment, preheating, spraying and coating machining.



    1 Surface pretreatment

    (1) Remove oil stains. Use gasoline, acetone, etc. to remove oil from the surface to be repaired.

    (2) Flaw detection inspection. Inspect the journal surface for fatigue cracks or other defects by tinting.

    (3) Calibration. Adjust the water pump shaft on the lathe, check the size and wear of the two journals, and check the shape accuracy and position accuracy of each journal. Correct its shape and position errors.

    (4) Turning to remove the fatigue layer. When the thickness of the coating on one side is less than 0.6mm after the fatigue layer is removed by turning, the one-side continues to be turned to 0.6mm to ensure that the coating has sufficient strength.

    (5) Cover the parts that do not need to be painted with copper sheet.

    (6) Coarsening treatment. The roughening treatment adopts the method of turning the thread and adding nickel to further improve the bonding strength of the coating and the substrate. When turning the thread, the pitch is 0.8mm, the tooth depth is 0.3mm, the sharp angle of the turning tool is 90°, and the arc chord length of the sharp corner is 1mm. .

    2 warm up

    The purpose of preheating is to eliminate the moisture and moisture on the working surface, increase the interface temperature when the sprayed particles are in contact with the workpiece, reduce the coating stress caused by the thermal expansion of the workpiece, and avoid cracking of the coating, so as to improve the bonding between the coating and the substrate. strength. The spraying part of the water pump shaft adopts oxyacetylene medium flame preheating, and the preheating temperature is controlled at about 100 ℃.

    3 spray

    (1) Spray primer coating material (NiAl). The bottom layer material in the spray coating design is about 0.1mm thick, and the surface floating ash powder is removed with a wire brush.

    (2) Spray the working layer (2Cr13). When spraying the working layer, only about 0.3 mm can be sprayed each time. Before the next spraying, the surface floating ash powder must be removed with a wire brush. The size of the journal after spraying is the three of the journal's working size, machining allowance and thermal expansion. The sum is about 201.45mm. Selection of process parameters during spraying: oxygen pressure is 0.6MPa, acetylene pressure is 0.09MPa, compressed air pressure is 0.5MPa, workpiece linear speed is 0.417m/s, spray gun moving speed is 5~7mm/r, and spraying distance is about 120mm.

    (3) Coating cooling. After spraying, the coating can be air-cooled. During the air-cooling process, the water pump shaft needs to continue to rotate until the coating is cooled to room temperature to prevent the shaft from bending.

    4 Coating process

    Due to the high processing technology requirements of the bearing position of the water pump shaft, the original design dimensional accuracy is 200js6, and the surface roughness Ra is 0.40μm. In addition to the processing characteristics of the coating itself, grinding should be used. In the actual spray repair, in order to shorten the repair time, the water pump shaft and impeller are not disassembled. The diameter of the impeller itself is 1300mm, so the radius of gyration of the general grinder cannot meet the requirements, so grinding cannot be selected. Therefore, the coating processing adopts the method of rough turning → fine turning → fine turning → metallographic sanding. In order to meet the requirements of the original assembly properties, considering the existence of a certain porosity of the coating and the characteristics of the surface of the coating, the dimensional accuracy of the original design was changed from 200js6 to 200h6.

    The optimal cutting line speed during spray coating turning is about 0.667 m/s, the cutting amount f during roughing is 0.2～1.0mm/r, and f can be 0.05～0.2mm/r during finishing; the cutting depth during roughing is 0.1 ～0.2mm, 0.05～0.10mm for fine turning.