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Application of powder metallurgy technology in automobile power system


Application of powder metallurgy technology in automobile power system

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The use of powder metallurgy (PM) technology for manufacturing parts of automotive power systems continues to grow. The parts made with the PM process have many important and unique advantages. The residual porous structure in these parts is beneficial to self lubrication and sound insulation. The use of PM technology can produce complex alloys which are difficult or impossible to manufacture by traditional casting technology. The amount of parts manufactured by such technology is usually few or even less, which makes them cheaper and less waste in materials. But unfortunately, behind the attractiveness of these features, PM parts are hard to be machined.
Although one of the original intentions of the PM industry is to eliminate all processing, this goal has not yet been reached. Most parts can only be "close to the final shape" and still need some kind of finishing. However, a small amount of material that needs to be removed from the PM parts is a typical wear-resistant material compared to the castings and forgings.
The porous structure is one of the characteristics that make PM parts widely used, but the life of the tool will also be damaged by the porous structure. The porous structure can be stored in oil and can be sound insulation, but it also leads to micro cutting. When moving from the hole to the solid particles, the tip of the knife continues to be impacted, which can lead to a small fatigue rupture and a small cutting edge along the cutting edge. What is worse, these particles are usually very hard. Even if the material between the measured hardness Rockwell 20~35 degrees, but the components of a particle's 60 degrees high of talok. These hard particles cause serious and rapid edge wear. A lot of PM parts are heat-treated, and the hardness and strength of the materials are higher after heat treatment. Finally, because of the sintering and heat treatment technology and the gas used, the surface of the material will contain hard and wear-resistant oxides and (or) carbides.
The performance of PM parts.
Most of the properties of PM parts, including machinability, are not only related to the chemical composition of the alloys, but also are related to the porosity of the porous structure. The porosity of many structural parts is up to 15% to 20%.  The porosity of a part used as a filter may be as high as 50%. At the other end of the series, or HIP (forging die casting parts with hole thermionic) rate is only 1% or less. This material is becoming particularly important in automotive and aircraft applications because they can get higher levels of strength.
The tensile strength, toughness and ductility of PM alloy will increase with the increase of density, and machinability can also be improved, because the porosity is harmful to the tip.
The increase of the hole rate can improve the performance of the sound insulation of the parts. The common damping oscillation in standard parts is significantly reduced in PM parts, which is important for machine tools, air conditioning blowers and pneumatic tools. The high porosity is also necessary for a self lubricating gear.
The difficulty of cutting.
Although one of the developing goals of PM industry is to eliminate machine processing, and the main attraction of PM technology is only a small amount of processing. However, many parts still need post-processing to get higher accuracy or better surface finish. Unfortunately, it is extremely difficult to process these parts. Most of the trouble encountered is caused by the porosity. The porosity leads to the micro fatigue of the edge. The cutting edge is cut and cut continuously. It causes small cracks on the cutting edge from the small impact between particles and holes, and these fatigue cracks grow until the cutting edge is slightly collapsed. This micro - Avalanche blade is usually very small and generally shows normal abrasive wear.
The porosity also reduces the thermal conductivity of PM parts. The result is that the temperature on the cutting edge is very high and causes the wear and deformation of the crescent crescent. The interconnected porous structure provides the path of the cutting fluid discharged from the cutting area. This causes hot cracks or deformation, which is especially important in drilling.
The increase of surface area caused by internal porous structure also leads to easy oxidation and (or) carbonization during heat treatment. As mentioned earlier, these oxides and carbides are hard and very wear-resistant.
It is extremely important that the porous structure also give the failure of the hardness reading of the parts. When it is intentional to measure the macro hardness of a PM part, it contains the factors of the hardness of the hole. The porous structure causes the collapse of the structure, and the wrong impression of relatively soft parts is obtained. The particle is a lot harder. As described above, the difference is dramatic.
The existence of inclusions in powder metallurgy parts is also unfavorable. In processing, these particles will be pulled from the surface, and when it is erasing from the front of the tool, the scratches or scratches will be formed on the surface of the parts. These inclusions are usually large, leaving a visible hole on the surface of the part.
The difference in carbon content leads to disagreement of machinability. For example, the carbon content of FC0208 alloy is between 0.6% and 0.9%. A lot of carbon containing 0.9% of the material is relatively hard, resulting in poor tool life. Another batch of carbon containing 0.6% of carbon has excellent tool life. All two kinds of alloys are within the allowable range.
The final processing problem is related to the type of cutting on the PM parts.  As the part is close to the final shape, it is usually shallower. This requires a free cutting edge. The scraps on the cutting edge often lead to micro avalanche.
process technology
In order to overcome these problems, several technologies are applied (which are unique to the industry). The porous structure of the surface is often closed by impregnation. Free cutting is usually required. Recently, an improved powder production technology designed to increase powder cleanliness and reduce oxide and carbides in heat treatment has been used.