[China Aluminum Network] The sprocket is a critical component in the bicycle crank assembly. The manufacturing process involves several key steps: material cutting, blanking, punching the center hole, forming, tooth cutting, and finally, punching 25 precise holes. After these operations, the part undergoes solution treatment to enhance its mechanical properties.
During the solution treatment, the temperature is maintained between 495°C and 503°C for 15 to 20 minutes, with water at 25°C to 35°C used as the cooling medium. While the flatness of the sprocket meets specifications after stamping, it often exceeds the allowable limits after solution treatment. To address this issue, the sprocket is mounted on a custom fixture using bolts and pressure plates before the treatment. However, even after removing the fixtures, the flatness still does not meet the required standards.
Water is commonly used as a quenching medium due to its low cost, fast cooling, and effectiveness in achieving the desired properties of aluminum alloys. However, from the perspective of minimizing deformation, water may not be the best choice. Its rapid cooling can create significant temperature differences between the surface and core of the part, especially in areas with varying wall thicknesses. This leads to internal stresses that can cause distortion or cracking, particularly when the alloy is soft at high temperatures.
In industrial practice, methods such as increasing the water temperature, oil quenching, isothermal quenching, or using polymer-based media are often employed to reduce deformation. Oil quenching, while effective, requires an extra degreasing step. Isothermal quenching, although more controlled, adds complexity to the process. Polymer quenchants offer flexibility, as their cooling rates can be adjusted by altering the concentration. Low-concentration solutions cool quickly, similar to cold water, while higher concentrations slow down the cooling, approaching the characteristics of oil or hot water. However, polymer solutions tend to be more expensive and require more careful management than water.
Based on the analysis, it was decided to use 100°C hot water as the quenching medium. Following the modified solution treatment process—temperature 495–503°C, holding time 15–20 minutes—the flatness of the sprocket was successfully reduced to less than 0.26 mm, meeting the product specifications. This adjustment significantly improved the quality and consistency of the final product.
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