Urban transportation systems have benefited significantly from 3D printing over the past few years. Now, the railway system is benefiting too. The speed of high-speed railways can reach upwards of 350km/h. Although it is relatively easy to drive a train, it is less easy to stop it. The main difficulty is braking. Considering that each carriage can weigh more than 60 tons – the weight produces significant inertia which, in turn, generates proportional kinetic energy, and the heating up of the brake discs (when braking) due to the friction. Therefore, the brake pads and brake discs should not only withstand the high temperature (around 900 °C), but should also ensure that the parts perform well at the same time. The parts also need to perform in harsh environments such as rain, snow, sand, and dust.
In recent years, the R&D and production technology of high-speed railway brake pads and brake discs has made gradual breakthroughs. With the in-depth integration of metal 3D printing technology into the railway industry, metal 3D manufacturers started to explore the application of metal 3D printing technology for the performance improvements of brake pads and brake discs.
The safe and stable operation of a train is inseparable from the long-term maintenance and functionality of its parts. When these parts are damaged and need to be replaced, but the parts are not immediately available at the storage facilities, railway providers need to order the parts from the manufacturer – resulting in higher procurement and management costs. In addition to the unprofitable and, ultimately, unnecessary waiting time. It should, therefore, come as no surprise that 3D printing is now the chosen means of manufacturing for some of these parts – providing customized, high-quality parts on-demand.
Earlier this year, Eplus3D, a leading Chinese AM manufacturer, collaborated with a railway company – providing the company with a tailored metal AM solution. Eplus3D did this by printing several brake discs with its large-format metal AM machine, EP-M650 (quad-laser).
Before 24CrNiMo was used to produce brake discs, high-speed trains of the early days used composite brakes. Due to its low mechanical strength and poor impact toughness, the composite brake discs used in early high-speed trains were likely to develop microcracks during operation. The composite brake material is also sensitive to water, resulting in less friction during the rainy season, and when traveling through wet areas, resulting in poorer brake performance.
In the traditional manufacturing process, the brake pads and brake discs are created through powder metallurgy. In the sintering process, due to the different shrinkage coefficients of each component in the brake disks, it is likely to produce defects such as porosity and coarseness, resulting in low compactness of the brake disc, which affects its final mechanical and friction properties.
Brake discs are traditionally manufactured using reductive manufacturing, combined with casting and forging, which, compared to 3D printing, is a cumbersome process, with a low material utilization rate and a long production cycle.
After data analysis and a survey, 24CrNiMo was selected as the printing material for the brake discs. 24CrNiMo steel is a high-strength, low alloy, steel with excellent mechanical properties. It has high ultimate tensile strength, good fracture toughness and thermal stability, and is widely used in the manufacturing of high-speed railway brake discs. Parts printed with this material can effectively reduce the parts’ weight and improve the mechanical properties.
The EP-M650, quad-laser large format metal 3D printer with a build volume of 655 x 655 x 800mm³, was selected for this project. Together with the high beam quality and a detailed resolution, the forming accuracy and mechanical properties of the brake discs can be guaranteed.
Using 3D printing technology such as the Eplus3D printer, railway operators are able to avoid material waste, and shorten production time and cost. The brake disc’s surface hole, in particular, benefits from 3D printing, as it is possible to directly print it through the model design, and fully improve the heat dissipation of the brake disc.
The 3D printing technology directly prints the wear-resistant layer on the steel back surface. The high-energy laser beam used in the 3D printing process can fully react and highly densify the components in the brake disc, so that the obtained brake disc has improved mechanical and friction properties.
3D printing can also optimize spare parts at a lower cost and in a shorter time. In the past, one-time custom parts manufacturing was very expensive, because it was almost impossible to produce only one part at a time and manufacturers usually made a large number of parts at the same time – resulting in wasted materials and a production cycle of, generally, about two months.
With the trajectory of technological development in mind, it is clear to see that 3D printing will play a wider role in the railway industry in the relatively near future.