Application of plastic Electroplating-Electroplating Technology in Modern Automobile Industry.
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Piston, piston ring and connecting rod are the three major moving parts of automobile engine, and their friction losses account for about 4050% of the total loss of the engine. Reducing the weight of piston, piston ring and connecting rod and improving their friction environment are of great significance to reduce engine vibration, noise and fuel consumption. In the past, the piston did not carry out any surface treatment or only chemical tin dipping (the initial running-in layer was formed); the piston ring was plated with chromium and phosphating; the connecting rod was oxidized only by connecting rod bolts; and the bearing on the connecting rod was treated with electroplated Pb-Sn binary alloy and electroplated Sn. These electroplated coatings can no longer meet the needs of high speed, high power, compact and EGR technology development of modern engines.
Piston.
The AC8A high silicon aluminum alloy piston recently developed in Japan, compared with the traditional piston, shortens the height between the top surface of the piston and the connecting rod pin, reduces the area and wall thickness of the apron, changes three ring grooves into two ring grooves, and reduces the width of the ring grooves, thus reducing the weight of the system and reducing the friction loss. However, it also brings some negative effects: the deterioration of the heat dissipation condition of the system is easy to cause burning and gluing between the piston ring and the ring groove, and thermal cracking occurs at the top of the piston; for the guiding piston apron, due to the reduction of the area, the friction pressure with the cylinder wall increases, the lubrication condition becomes worse, and it is easy to cause vibration, noise and friction loss. The traditional tin plating process plays an important role in the initial running-in, but the tin coating is quickly worn away and can not improve the friction environment for a long time. Countermeasures taken:
* hard oxidation treatment is carried out above the first ring groove. The hardness and thickness of the oxide film vary according to the piston exhaust volume, which is generally about 30 ~ 90 μ m and HV350.
* Piston aprons are mostly treated by MD and MPC processes. The effect of high pressure air spraying MoS2 powder (ø 11 μ) treatment (MPC) is the best. The reasons include: the piston surface is impacted by MoS2 particles at high speed, and the microscopic appearance of the surface changes, forming an oil film layer; the piston surface is impacted by high-speed particles, and the increase of temperature is conducive to the diffusion of MoS2 to the matrix, so it has a lasting lubrication effect; the treatment process also has the advantages of low cost, no pollution, no change in size and so on.
Surface treatment of piston ring.
According to the literature, if the friction force of the piston ring is reduced by 40%, the fuel cost at medium and low speed can be reduced by 7%. The first ring of modern engine is mostly made of high-strength stainless steel, the surface is nitrided and heat treated, and then coated with a thin layer of Ni-Co-P-Si3Ni4 composite coating. With the increase of Si3Ni4 content in the coating, the wear resistance of the ring and the wear of the cylinder liner are increased; the wear amount of the cylinder liner can be reduced by increasing the Co content in the coating; with the increase of P content in the coating, the hardness and brittleness of the coating increase, and the bonding of the cylinder liner can be reduced. In addition, the piston ring can also be coated with CrN or TiN by PVD, or ploughed on the chromium-plated surface and sprayed with ceramics in the groove.
Connecting rod.
According to the American technical standard, FAW No.2 Plant uses electroplating technology (repairing copper plating) to repair the connecting rod with over-poor machining size of the inner hole in production. This kind of copper plating must use cyanide plating solution, and the coating is limited to the inner hole, and the whole electroplating process must ensure that there is no hydrogen permeation.
Bearing bush.
In order to prevent the thermal diffusion of Sn in the coating to the substrate to form a brittle phase, a nickel gate needs to be plated on the bearing bush of the high-power engine, and the Pb-Sn-Cu ternary alloy coating is used to replace the Pb-Sn binary alloy coating.
Intake and exhaust valve.
The friction loss of intake and exhaust valves accounts for about 2% of the engine energy loss. Measures to reduce loss: reduce the structural size of intake and exhaust valves and use lightweight materials. For example, titanium alloy is used instead of heat-resistant steel. Working at 750 °C and the friction of scales in the exhaust gas of the engine, the titanium exhaust valve will undergo fierce oxidation and must be treated by surface treatment, which is usually treated by molybdenum injection and salt bath furnace nitriding. Recently, the remarkable treatment method is plasma Carburizing treatment, which has little impact on the environment. The TiC hardened layer formed on the surface is 50 μ m deeper than the nitrided layer treated by heat-resistant steel salt bath furnace, and its hardness reaches HV700. The oxidation resistance and wear resistance are the same as those of heat-resistant steel gas valve, and the treatment cost is higher than that of salt bath furnace treatment.
Exhaust cylinder, silencer.
The exhaust system installed in the automobile chassis is not only corroded by the impact of road mud and sand, but also eroded and oxidized by high temperature waste gas above 400 °C. the working environment is very bad. The materials of exhaust system are mostly stainless steel and hot-dip aluminized plate. In the process of working and using, the coating blackening and corrosion often occur on the aluminized plate. Grade 5 corrosion occurred in 40 cycles of vehicle corrosion resistance test, and the corrosion resistance was not as good as that of stainless steel. The addition of silicon element to hot-dip aluminizing can restrain the growth of Fe-Al alloy layer and improve the processability. The service life of hot-dip aluminized sheet is related to the thickness of coating, forming process and working temperature.
Shock absorber connecting rod.
In the past, the connecting rod of shock absorber was electroplated with ordinary hard chromium, and its corrosion resistance could not meet the requirements. At present, HEEF25 micro-crack chromium plating process is widely used, and the thickness of the coating is 20 ±5 μ. Different from 10 ±3 μ, the number of cracks is generally required to 400 μ 600 / cm, which plays a positive role in ensuring the corrosion resistance and lubricity of the shock absorber connecting rod. In this process, the current density is obviously related to the number of microcracks, the shape of cracks and the amount of hydrogen permeation. It is better to control it in 58A/dm2. The design of the tubular anode and the polishing after plating have an effect on the quality.
Automobile decoration parts.
Most of the decorative parts of modern cars are plastic parts. High-grade automobile plastic decorative electroplating parts require extremely high corrosion resistance, thermal vibration resistance and decoration. Volkswagen TL-VW528/B standard stipulates: cu ≥ 20 μ m, Ni ≥ 10 μ m, Crmc= 0.8 μ m or Crmp= 0.5 μ m; pass CASS test for 48h; temperature alternating test through 3 cycles; the whole surface must have a uniform and highly bright appearance, that is, a small defect is not allowed on such a large decorative electroplating surface. Decorative electroplating of automobile plastic parts is a systematic project, from the design and use of products.
