Analyze the production process of high-speed train brake pads.

The mainstream materials for high-speed train brake pads include powder metallurgy copper-based/iron-based compounds, carbon-ceramic composites, and synthetic materials. The core manufacturing processes involve ingredient mixing, shaping, sintering, post-treatment, assembly, and quality inspection. Among these, the copper-based powder metallurgy process is the most mature and widely applied.

I. Material Selection and Ingredient Mixing

(A) Material System

Copper-based brake pads use copper powder (60%–80%) as the base material, with additives such as tin, lead, and nickel to enhance strength, graphite or molybdenum disulfide as lubricants, and alumina or silicon carbide to improve performance. Coupling agents are also added to ensure good interfacial bonding between the various components. Iron-based brake pads are relatively cheaper and mainly consist of elements like Fe, Cr, and Mo; they are suitable for heavy-duty applications and require strict control over oxidation resistance and material toughness during production. Carbon-ceramic brake pads are made by combining carbon fibers with silicon carbide or resin and possess excellent high-temperature stability, making them ideal for high-speed braking scenarios. Synthetic material brake pads use resin as the base, with added fibers and fillers, and are suitable for low-speed driving or auxiliary braking.

(B) Key Points of Ingredient Mixing

The weighing accuracy during the ingredient mixing process must be controlled within ±0.1%. Double-action mixers or V-type mixers are commonly used, with mixing times ranging from 2 to 3 hours and equipment speeds maintained at 15–30 rpm to ensure uniform mixing of all components. Pre-alloying treatments or the addition of additives such as zinc stearate can be used to enhance the strength and demolding properties of the pressed components.

II. Shaping Processes (Cold Pressing/Hot Pressing/Isostatic Pressing)

Shaping processes are mainly divided into cold pressing, hot pressing, and isostatic pressing. Cold pressing is carried out at room temperature, with pressures ranging from 15 to 500 MPa and holding times of 10 to 20 seconds. This process is suitable for mass production of brake pads with simple shapes and offers advantages such as high performance and low cost. Hot pressing requires temperatures between 150 and 200°C, pressures of 10 to 30 MPa, and holding times of 5 to 15 minutes; it is particularly suitable for resin-based or carbon-ceramic brake pads, as it improves the material's strength and density. Isostatic pressing uses a pressure-transmitting medium, with pressures ranging from 100 to 300 MPa, and is ideal for brake pads with complex shapes and high density requirements. The resulting products have uniform density, with densities that can be 5%–10% higher than those achieved by mold pressing.

The molds used for shaping are made of Cr12MoV or SKD11 materials, with hardness levels of HRC58–62 and surface roughness of Ra≤0.4μm, to ensure accurate shaping of the brake pads and extend the mold's service life.

III. Sintering/Curing (Core Density Improvement Steps)

(A) Powder Metallurgy Sintering

The sintering process must be carried out in a protective atmosphere, typically using a mixture of 75% H? and 25% N? or pure hydrogen, which effectively prevents material oxidation. The dew point of the atmosphere must be controlled below -40°C. The sintering temperature varies depending on the material; copper-based brake pads are sintered at 850–1000°C, while iron-based brake pads are sintered at 1100–1250°C, with holding times of 1–5 hours and heating rates controlled at 5–15°C/min. In some cases, pressurized sintering is used, with pressures of 2–4 MPa, to further improve the product's density and strength. Cooling is done either in the furnace or in stages, with cooling rates not exceeding 20°C/min to prevent cracking.

(B) Synthetic Material Curing

Resin-based synthetic material brake pads require heating at 160–180°C for 4–6 hours to complete resin cross-linking and curing. Carbon-ceramic brake pads need to be subjected to carbonization or silicization treatments at temperatures above 1000°C to ensure stable material properties.

Fourth, post-processing and finishing

Firstly, mechanical processing is carried out, and the surface roughness of the brake pad reaches Ra≤0.8μm and flatness ≤ 0.05 mm through grinding process; Drill or tap according to the installation requirements, and the tolerance should reach H7 level; After deburring, the chamfer size is controlled at R 0.5–1.0 mm..

In surface modification, 80-120 mesh ALO sandblasting can be used to improve the surface adhesion, or copper plating and nickel plating can be used to enhance the anti-corrosion performance (mainly based on actual reports), and MoS? anti-wear coating can also be applied to optimize the braking effect.

The heat treatment process is adjusted according to the material, and the copper-based brake pad is aged at 400-500℃/1-2h to eliminate the internal stress. Iron-based brake pads improve the strength of materials through quenching and tempering.

Five, assembly and quality inspection (to protect the installed machine)

(1) Assembly

There are three main connection modes between the friction block and the back plate: riveting, bolt connection and welding. 304 or 316 rivets are selected for riveting, and the shear strength is required to be ≥ 300 MPa; The torque of bolt connection should be controlled at 15-30 N m; Welding adopts vacuum electron beam or laser welding process, and the weld strength should reach more than 90% of the matrix strength. Dovetail groove or bolt hole design is mostly used for assembly interface, which should conform to TJ/CL 307 standard, and ensure interchangeability and installation accuracy ≤ 0.1 mm..

(II) Whole process quality inspection

In the raw material stage, components (detected by ICP), particle size (laser particle size analyzer) and loose density (according to GB/T 1479 standard) need to be detected. The detection indexes in the green pressing stage include density (drainage method), strength (three-point bending test) and size (three-coordinate measurement). After sintering, the metallographic structure (grain size ≤5), hardness (HB≥80) and bending strength (≥200MPa) should be tested. In the finished product stage, the core detection indexes are friction coefficient (kept at 0.35-0.45 in the temperature range of 20-600℃, with fluctuation ≤ 0.05), wear rate (≤ 1× 10 cm/(n m)), cold and hot cycle performance (no crack after 10 cycles from -50℃ to +350℃) and dynamic balance (≤

VI. Comparison of Typical Process Routes

The process route of copper-based powder metallurgy brake pads is batching → powder mixing → cold pressing → sintering → grinding → riveting → quality inspection. The production cycle is 3-5 days, and the cost is moderate. It is suitable for motor trains with a speed of ≤ 350 km/h. The process route of carbon ceramic brake pad is carbon fiber prefabrication → resin impregnation → curing → carbonization → silicification → processing → assembly. The production cycle is 7-10 days, and the cost is high, so it is suitable for high-speed scenes with speed ≥ 350 km/h. The process route of synthetic brake pads is batching → banburying → hot pressing → curing → processing → assembly. The production cycle is 2-3 days, and the cost is low, which is suitable for working conditions with speed ≤ 200 km/h.

VII. Direction of Process Optimization

In the aspect of intelligent control, closed-loop adjustment is realized by on-line monitoring key parameters such as sintering temperature, forming pressure and protective atmosphere, and the process capability index CPK is confirmed to be ≥ 1.33. In the application of new materials, graphene or carbon nanotubes are added to improve the thermal conductivity and friction stability of brake pads. Near-net molding technology is adopted in the optimization of molding process, and complex structures are manufactured through 3D printing, which reduces the machining allowance and improves the material utilization rate by 30%. Green manufacturing direction, promote the recycling of waste powder and the recycling of protective atmosphere, and reduce energy consumption by 20%.