Aluminum Powder Metallurgy - JH MIM

Powder metallurgy aluminum alloy materials have the characteristics of low density, high specific strength, corrosion resistance, and easy surface treatment, so it is widely used in construction, packaging, transportation, electrical and electronic, machinery manufacturing, aerospace, and petrochemical industries, and People’s Daily life.

The common methods for preparing aluminum are mainly molten casting (IM) and powder metallurgy (PM).

Since the 1970s, researchers have found that the results of developing new aluminum substrates based on molten casting processes, such as increasing purity, adjusting composition, and changing heat treatment norms, have been getting smaller and smaller. The Aluminum powder metallurgy PM process can not only avoid the segregation of the material composition, but also improve the solid solubility, obtain some aluminum alloys that can not be prepared with the IM process, and fine the structure, improve its shape and distribution characteristics. Compared with the aluminum alloy prepared by IM with similar composition, the aluminum alloy prepared by PM has better physical, chemical, and mechanical properties, so the powder metallurgy method has become one of the main methods for preparing high-performance aluminum products.

The powder metallurgy aluminum preparation process can be roughly divided into powder preparation, forming consolidation, and follow-up treatment of three links. Because the surface of aluminum powder is inevitably coated with a dense non-reducible alumina film, it will hinder the formation of metallurgical bonding between particles during the forming and sintering process, and it is often difficult to obtain aluminum alloy and composite materials with high-density and clean interface through conventional powder metallurgy process, that is, pressing and sintering process. Resulting in poor final performance of the material. To eliminate the adverse effects of aluminum oxide film, improve the material density, and obtain high-performance powder metallurgy aluminum alloy and composite materials, it is necessary to carry out after-treatment (rolling, extrusion, forging, etc.) after the sintering process. In addition, machining is usually required to obtain the final product of the desired shape, which significantly increases the preparation cost of powder metallurgy aluminum alloys and limits their application scope.

1. Preparation process

1.1 Powder preparation of aluminum powder preparation methods are various, the gas atomization method is currently the main method of industrial production of aluminum powder, mechanical ball milling rule is mainly used for the preparation of nano-scale aluminum powder.

1.1.1 Gas atomization In the process of preparing aluminum alloy powder by gas atomization, the cooling rate of the droplet can reach 1×10. ~ 1×10jK/s, so the method is also known as the rapid solidification method. The high cooling rate can improve the solid solubility of alloying elements, refine the grains, and reduce the composition and phase segregation. This means that the preparation of aluminum alloy with gas-atomized aluminum powder has great flexibility in the regulation of chemical composition and microstructure, which helps to achieve breakthroughs in material properties. For example, in aerospace Al-Zn-Mg-Cu alloy in the use of IM process development and production process, it has been found that with the increase of the total content of main alloying elements (Zn, Mg, and Cu) and the improvement of alloying degree, the performance of the material has been improved to a certain extent. However, when the total content of main alloy elements exceeds a certain limit (mass fraction is 12 ~ 13), due to the limitation of solidification cooling rate (generally not exceeding 10 K/s), a large number of coarse primary precipitates will be formed in the alloy, and these primary precipitates are difficult to dissolve back into the matrix through subsequent solid solution treatment. The severe weight deteriorates the properties of the material, resulting in the extreme tensile strength of this kind of alloy hovering around 500 600 MPa for a long time. After the rapid solidification process, due to the increase of solid solubility of alloying elements in the pulverization process, even if the limit of the total content of main alloying elements of 12 ~ 13 is broken through the composition design of the new alloy, there will not be a large number of coarse primary precipitated phase, and the structure is refined. It is beneficial to form a higher volume fraction of the aging strengthening phase and fine crystal structure in the final alloy, so that the final performance of the material is greatly improved, and the ultimate tensile strength can be increased from 600 MPa to more than 800 MPa. However, the main disadvantage of the gas atomization method is that the powder particle size distribution is wide, and the fine powder yield is relatively low. Usually obtained aluminum powder particle size distribution between 1 ~ 2O0 /,m, most of the powder particle sizes between 45 ~ 100 m, and particle sizes below 10 m accounted for only about 1 of the total production [6]. This fine powder is not only difficult to separate and collect but also difficult to adjust the yield according to demand.

1.1.2 Compared with the gas atomization method, the mechanical ball milling method can prepare nano-sized aluminum powder. For the low-melting nano-metal powder represented by aluminum, maintaining low temperatures in the ball milling process is very important. In the case of a large number of defects in the inner grain, the recovery and recrystallization temperatures of these powders are extremely low. For example, Al 7.6Mg alloy, when its particle size is reduced to about 25 nm, its recovery temperature is only 100 230℃, and the recrystallization temperature is 370℃ll. At present, low-temperature ball milling technology has been developed, using liquid nitrogen as the cooling medium, the temperature of the entire ball milling process can be controlled at zero or lower, and the use of process control agents can be reduced or avoided.

1.2 Forming and consolidation

1.2.1 Molding

The pressing pressure required for aluminum alloy powder and mixed powder is often compared to Low. When the Al-Si alloy powder is molded and the pressing pressure is 270 MPa, the compact with 80 density can be obtained. However, due to moisture absorption or the irregular shape of aluminum powder, it is difficult to form complex parts with thin sections because of poor fluidity, loose packing density, and low compact strength. In addition, aluminum powder particles are easy to cold weld with the mold, damaging the mold. Therefore, it is best to coat the surface of the mold with a wear-resistant layer or use the mold wall lubrication to improve the accuracy of the mold and the mold as much as possible, while the powder also needs to add a certain amount of lubricant, usually 1.2 ~ 1.8 amide wax.

1.2.2 Jet forming

The basic process of jet-forming aluminum alloy is to atomize the liquid aluminum into small droplets by high-pressure inert gas and make them fly at high speed along the axis of the nozzle. Before these droplets are fully solidified, they are deposited on the receiving base with a certain shape and a specific movement and are formed. The technology is equivalent to combining the pulverization, storage, transportation, screening, pressing, and burning required by the rapid solidification process in one step, avoiding the surface oxidation of aluminum powder and the material pollution caused by the introduction of impurities in various processes, and greatly improving the plasticity and toughness of the product. To a certain extent, the characteristics of powder metallurgy final forming are retained, thus greatly shortening the production cycle and reducing the cost. Jet-forming bodies often require subsequent densification, such as hot isostatic pressing or forging. Jet forming technology is mainly used for the production of semi-finished products with larger sizes and has been developed from single nozzle to double nozzle, which can prepare larger-size products.

Developed based on jet deposition for the preparation of aluminum base

The co-jet deposition technology of composite materials is to spray alloy melt and particles onto the sink collector at the same time to obtain the required composite materials. This method not only has the inherent advantages of jet deposition, but also can avoid interface reactions due to the short contact time between the reinforced particles and the metal droplet, and can obtain high-performance composite materials.

l_2. 3 High-speed pressing

High-speed pressing (HVC) technology is a Swedish company by Hoganas and Hydropulsor, 2001 jointly proposed a high-efficiency, low-cost preparation of high-performance powder metallurgy parts of the new technology. The process of producing parts with this technology is the same as the traditional molding process, and the mold design is similar, but the difference is that the HVC is a strong shock wave generated by a hydraulically controlled heavy hammer (speed 2 ~ 10 m/s) and the powder is densified in a short time. This technology has the characteristics of both die pressing and powder forging, and has the characteristics of dynamic impact, near net forming, continuous stability, and low cost. From the performance of the compact obtained, the HVC compact has the characteristics of high density and uniform distribution, low elastic aftereffect, high precision, and high strength of the green compact. High-speed pressing has the characteristics of powder forging to a certain extent, which helps to form a metallurgical bond between the particles during the forming process because the billet shows a higher sintering activity. The pure aluminum powder was formed by the HVC method, and the compact with 100 density was obtained.

1.2.4 Sintering

During solid phase sintering, the dense alumina film on the surface of aluminum powder will seriously hinder the material migration. In fact, the density of the aluminum alloy body usually decreases after solid-phase sintering. This is because the residual internal stress during pressing in the sintering process is eliminated, the mass transfer process cannot be fully developed, and the particle contact surface is relatively reduced, resulting in volume expansion. The researchers systematically studied the sintering of aluminum alloy by liquid phase method. These studies mainly added some sintering additives to the matrix alloy, and the sintering additives reacted with alumina to destroy the oxide film and improve the wettability between the liquid phase and alumina. The following principles should be followed when selecting sintering additives: the melting point of sintering additives should be lower than the melting point of the alloy, or it can form a low melting point eutectic with the alloy; The solubility of sintering additives in the alloy should be low to facilitate the formation of enough liquid phase; The alloy must have a certain solubility in the sintering additives to facilitate the rapid sintering. According to these principles, sintering AIDS commonly used in sintered aluminum alloys include MgEzo-zz3, pbE23-241, and SnE.  And BiE and other elements of sintering AIDS. Because the aluminum alloy obtained by liquid phase sintering is large, sometimes it will include some coarse eutectic phase, which will have some adverse effects on the material properties, and the addition of Zr, Sc, Cr Mn, and other elements E. The grain growth during liquid phase sintering of aluminum alloy can be inhibited to some extent. In addition, liquid phase sintering will make it difficult to control the dimensional precision and surface finish of the product. When aluminum matrix composites are sintered by the liquid phase method, there is a long time of contact between the reinforced phase particles and the molten metal, and there will be a reaction between them. For example, SiC is unstable in liquid aluminum and will generate A1C at the solid-liquid interface. ; And Al. O. A1 is easily formed in aluminum alloy liquid containing Mg. MgO. This interfacial reaction between the reinforced particles and the base alloy often severely degrades the properties of the material. Therefore, the parameters such as sintering temperature and time should be selected according to the composition of the matrix alloy and the type of reinforcement phase.

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