Steel-based brake disc laser cladding coating preparation method and brake performance study

Abstract

In recent years, as the operating speed of high-speed trains has continued to increase, the thermal load challenges faced by brake discs during emergency braking have become more severe. Applying surface strengthening techniques to create high-performance coatings on the friction surface of brake discs is crucial for enhancing their thermal fatigue resistance and extending their service life. This study focuses on a 1:1 scale model of a steel-based brake disc for high-speed trains, using Stellite 21 cobalt-based alloy powder as the coating material. High-performance coatings were successfully prepared on the surface of the brake disc using laser cladding technology. A high-speed railway brake dynamometer test, ranging from 50 to 400 km/h, was conducted to validate the method for preparing the laser cladded brake disc (LC-disc). The prepared LC-disc exhibited good forming quality, with no defects such as cracks, pores, or incomplete fusion detected through non-destructive testing, and a powder utilization rate of over 80 %. The microstructure of the coating primarily consisted of the γ-Co phase, with minor amounts of ε-Co and M₇C₃. After the braking dynamometer test, columnar grains in the coating underwent deformation or recovery recrystallization, leading to grain refinement. During braking, the LC-disc demonstrated a stable coefficient of friction (COF), ranging between 0.296 and 0.44. The COF decreased with increasing initial speed and bilateral pad thrust, and was lower under wet conditions and higher pad thrust, indicating the LC-disc’s suitability for higher speeds and more demanding operating environments. Additionally, the maximum temperature of the friction surface during braking increased with the initial speed, reaching approximately 728.5 °C at an initial speed of 400 km/h, suggesting that this brake disc can adapt to the higher speeds of high-speed trains. However, repeated thermal stress cycles led to the formation of thermal fatigue cracks at the bolt holes of the coating. The laser cladding coating preparation method used in this study is efficient and cost-effective, producing coatings with high braking performance. This method meets the operational requirements of higher-speed high-speed trains and can extend the service life of brake discs, significantly reducing economic costs by adopting a replace-before-repair approach. It also provides an engineering example for the preparation of coatings on other large parts.