Aluminum alloys and their composite materials have the characteristics of low density, high specific strength, corrosion resistance, and easy surface treatment, and are therefore widely used in industries such as construction, packaging, transportation, electrical and electronic, mechanical manufacturing, aerospace, and petrochemicals, as well as in people's daily lives.

The commonly used methods for preparing aluminum alloys and composite materials include melting casting (IM) and powder metallurgy (PM). Since the 1970s, researchers have found that the effectiveness of developing new aluminum based materials based on IM processes, such as increasing purity, adjusting composition, and changing heat treatment specifications, has been decreasing. The use of PM technology can not only avoid material composition segregation, but also improve solid solubility, obtain some aluminum alloys that cannot be produced by IM technology, and refine the structure, improve its morphology and distribution characteristics. Compared with aluminum alloys prepared by IM with similar composition, aluminum alloys prepared by PM have superior physical, chemical, and mechanical properties. Therefore, powder metallurgy has become one of the main methods for preparing high-performance aluminum alloys and composite materials. The preparation process of powder metallurgy aluminum alloy and composite materials can be roughly divided into three steps: powder preparation, forming and solidification, and subsequent processing.

Due to the inevitable coating of a dense and irreducible aluminum oxide film on the surface of aluminum based powders, metallurgical bonding between particles is hindered during the forming and sintering processes. Conventional powder metallurgy processes, such as compression sintering, often fail to obtain high-density and clean interface aluminum alloys and composite materials, resulting in poor final material properties. In order to eliminate the adverse effects of aluminum oxide film, improve material density, and obtain high-performance powder metallurgy aluminum alloys and composite materials, it is essential to carry out subsequent treatments (rolling, extrusion, forging, etc.) after sintering process. In addition, machining is usually required to obtain the desired shape of the final product, significantly increasing the preparation cost of powder metallurgy aluminum alloys and composite materials, and limiting their application scope. This article focuses on the preparation process of powder metallurgy aluminum alloys and composite materials, introducing their research status and exploring their development trends around the three stages of powder preparation, forming and solidification, and subsequent treatment.

1 Preparation process

1.1 Powder preparation process

There are various methods for preparing aluminum and its alloy powders, among which gas atomization is currently the main method for industrial production of aluminum powder, and mechanical ball milling is mainly used for preparing nanoscale aluminum powder.

1.1.1 Gas atomization method

In the process of preparing aluminum alloy powder by gas atomization, the cooling rate of droplets can reach 1× 103～１× 105 K/s, therefore this method is also known as rapid solidification method. A high cooling rate can increase the solubility of alloy elements, refine grains, reduce component segregation and phase segregation. This means that using gas atomized aluminum powder as raw material to prepare aluminum alloys has great flexibility in controlling chemical composition and microstructure, which helps to achieve breakthroughs in material properties.

During the development and production of Al Zn Mg Cu alloys used in aerospace, it has been found that as the total content of main alloying elements (Zn, Mg, and Cu) increases and the degree of alloying improves, the properties of the material are improved to a certain extent. However, when the total content of main alloying elements exceeds a certain limit (mass fraction of 12% -13%), due to the limitation of solidification cooling rate (generally not exceeding 10 K/s), a large number of coarse primary precipitates will form in the alloy, which are difficult to dissolve back into the matrix through subsequent solid solution treatment, seriously deteriorating the various properties of the material and causing the ultimate tensile strength (σ b) of this type of alloy to hover for a long time. 500~600 MPa.

After adopting the rapid solidification process, due to the increased solubility of alloy elements during the milling process, even if the composition design of the new alloy breaks through the limit of 12% to 13% of the total content of the main alloy elements, there will not be a large number of coarse primary precipitates, and the structure will be significantly refined, which is conducive to the formation of higher volume fractions of aging strengthening phases and fine grain structure in the final alloy, greatly improving the final properties of the material. The ultimate tensile strength can be increased from 600 MPa to over 800 MPa. The main disadvantage of preparing aluminum powder by gas atomization method is the wide range of powder particle size distribution and low yield of fine powder. The particle size distribution of aluminum powder obtained usually ranges from 1 to 200 μ m; Between m, with most of the powder particles ranging in size from 45 to 100 μ m; Between m, with a particle size of 10μ Fine powders below m account for only about 1% of the total production. This fine powder is not only difficult to separate and collect, but also difficult to adjust the yield according to demand. Gas atomized aluminum powder usually requires surface protection treatment.

According to different purposes, there are two common methods: oil immersion and formation of alumina film. Mixing oxygen with a volume content of 0.1% to 2% into inert atomizing gas media (such as N2 and He) can form a thin aluminum oxide film on the surface of particles. However, the water vapor in the atomizing medium and container can cause the following problems:

(1) The formation of chemically adsorbed water on the surface of particles requires additional vacuum thermal removal treatment for subsequent use, which increases costs;

(2) Impurities such as Na, S, Cl, K, Ca, Fe, Cu, and Zn in the aluminum liquid tend to agglomerate on the surface of the particles, reducing the protective effect of the oxide film;

(3) Increasing the thickness of the oxide film makes it difficult to eliminate the adverse effects of the oxide film during the solidification process. In order to eliminate the influence of water vapor, I.E. Anderson et al. used ultra-high purity nitrogen gas (purity of 99.998%, dew point of -67 ℃) as the atomization medium, pumped the pressure inside the atomization chamber to 6.67 Pa, and recharged ultra-high purity nitrogen gas before atomization to prevent outdoor air from entering. Finally, they obtained aluminum powder with a clean surface and an oxide film thickness of 2-5nm. This aluminum powder not only has good stability (heating at 300 ℃ for 100 hours in atmospheric environment only increases the thickness of the oxide film by 80%), but also has extremely high sintering activity (sintering at 300 ℃ can form sintering necks between particles; sintering at 550 ℃ and above can quickly eliminate the oxide film on the surface of particles). This type of atomized aluminum powder exhibits excellent sintering activity when combined with hard particles such as SiC.

1.1.2 Mechanical ball milling method

Compared with gas atomization method, mechanical ball milling method can prepare nano-sized aluminum powder. For low melting point nano metal powders represented by aluminum, maintaining low temperature during ball milling is crucial. In the presence of a large number of defects within the grains, the recovery and recrystallization temperatures of these powders are extremely low. When the particle size of Al-7.6Mg alloy is reduced to around 25 nm, its recovery temperature is only 100-230 ℃, and its recrystallization temperature is 370 ℃. 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 degrees or lower, and the use of process control agents can be reduced or avoided.

1.2 Forming consolidation

1.2.1 Compression molding

The required compression pressure for aluminum alloy powder and mixed powder is often relatively low. Liu Gaihua et al. used compression molding to form Al Si alloy powder, and when the compression pressure was 270MPa, they could obtain a compact with 80% density. However, due to the moisture absorption or irregular shape of aluminum powder, its flowability is poor, and its loose density and green strength are low, making it difficult to form complex parts with thin cross-sections. In addition, aluminum powder particles are prone to cold welding with molds, which can damage the molds. Therefore, it is best to apply a wear-resistant layer on the surface of the mold or use mold wall lubrication to improve the fitting accuracy between the mold punch and the negative mold as much as possible. At the same time, a certain amount of lubricant, usually 1.2% to 1.8% amide wax, needs to be added to the powder.

1.2.2 Spray forming

The basic process of preparing aluminum alloy by spray forming is to use high-pressure inert gas to atomize and break the aluminum liquid into small droplets, and make them fly at high speed along the nozzle axis direction. Before these droplets are completely solidified, they are deposited onto a receiving substrate with a certain shape and specific motion and shaped. This technology is equivalent to combining the powder making, storage, transportation, screening, pressing, and firing required for rapid solidification process into one step, avoiding material pollution caused by surface oxidation of aluminum powder and the introduction of impurities in various processes, greatly improving the plasticity and toughness of products, and to some extent retaining the characteristics of powder metallurgy final forming, thereby greatly shortening the production cycle and reducing costs. Spray formed blanks require subsequent densification treatment, such as hot isostatic pressing or forging. Spray forming technology is mainly used for producing large-sized semi-finished products, and has developed from single nozzle to double nozzle, which can prepare larger sized products. The co spray deposition technology, developed on the basis of spray deposition, is used to prepare aluminum based composite materials by simultaneously spraying alloy melt and particles onto a collector to obtain the desired composite material. This method not only has the inherent advantages of jet deposition, but also avoids interface reactions due to the short contact time between the enhanced particles and metal droplets, thus obtaining high-performance composite materials.

1.2.3 High speed compression

High speed pressing (HVC) technology is a new technology jointly proposed by Hoganas and Hydropulsor companies in Sweden in 2001 for the efficient and low-cost preparation of high-performance powder metallurgy parts. The process of producing parts using this technology is the same as traditional molding processes, and the mold design is also similar. The difference is that HVC achieves instant powder densification through strong shock waves generated by a hydraulic controlled heavy hammer (speed 2-10 m/s). This technology combines the characteristics of compression molding and powder forging, with features such as dynamic impact, near net forming, continuous stability, and low cost. From the obtained compaction performance, HVC compaction has the characteristics of high density and uniform distribution, low elastic aftereffect, high precision, and high green strength. High speed pressing has the characteristics of powder forging to a certain extent, which helps to form metallurgical bonds between particles during the forming process, thus exhibiting higher sintering activity in the billet. Chen Jin et al. used HVC method to form pure aluminum powder and obtained a 100% density compact.

1.2.4 Sintering

During solid-state sintering, the dense alumina film on the surface of aluminum alloy powder can seriously hinder the migration of substances. In fact, the density of aluminum alloy billets usually decreases after solid-state sintering. This is because the residual internal stress during the sintering process is eliminated, and the mass transfer process cannot be fully carried out, resulting in a relative decrease in the particle contact surface and causing volume expansion. For this purpose, researchers systematically studied the liquid-phase sintering of aluminum alloys. These studies mainly added some sintering aids to the matrix alloy, and used the sintering aids to react with alumina to destroy the oxide film and improve the wettability between the liquid-phase and alumina phases.

When selecting sintering aids, the following principles should be followed: the melting point of the sintering aid 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 aids in alloys should be low to facilitate the formation of sufficient liquid phase; Alloys should have a certain solubility in sintering aids to facilitate rapid sintering.

According to these principles, commonly used sintering aids in sintering aluminum alloys include sintering aids containing elements such as Mg, PB, Sn, and Bi. Due to the large grain size and sometimes the inclusion of coarse eutectic phases in aluminum alloys obtained by liquid-phase sintering, it can have some adverse effects on material properties. Adding elements such as Zr, Sc, Cr, and Mn can to some extent suppress grain growth during liquid-phase sintering of aluminum alloys.

In addition, liquid-phase sintering poses difficulties in controlling the dimensional accuracy and surface smoothness of the product. When using liquid-phase method to sinter aluminum based composite materials, there is a prolonged contact between the reinforcing phase particles and the molten metal, and a reaction occurs between them. If SiC is unstable in aluminum liquid, it will generate Al4C3 at the solid-liquid interface; Al2O3 is prone to generate Al2MgO4 in aluminum alloy liquid containing Mg. The interface reaction between the enhanced particles and the matrix alloy often significantly reduces the performance of the material. Therefore, it is necessary to select parameters such as sintering temperature and time based on the composition of the matrix alloy and the type of reinforcing phase.

1.3 Subsequent processing

At present, although a lot of efforts have been made in powder preparation and sintering processes, due to the obstruction of alumina film, it is difficult to obtain fully dense aluminum based composite materials even through hot pressing sintering. To obtain high-performance powder metallurgy aluminum alloys and their composite materials, they often rely more on cold/hot processing after forming and solidification, such as forging, rolling, extrusion, etc. By using these methods, not only can the particle interface bonding be improved, but also the density and microstructure of the material can be further increased, ultimately improving the strength and plasticity of the material. For subsequent deformation processing, it is generally necessary to choose a relatively large compression ratio, such as 20:1 or greater. Only in this way can the oxide film on the surface of metal particles be effectively destroyed, enabling metallurgical bonding between metal particles. A high extrusion ratio can further improve the uniformity of the dispersion of reinforcing phase particles in the alloy base, but a larger extrusion ratio is not necessarily better. Excessive extrusion ratio and temperature can lead to the rupture of reinforcing phase particles or degradation of the properties of the matrix alloy.

In recent years, some new strong plastic deformation processes have been developed on the basis of conventional forging, rolling, and extrusion, such as high-pressure twisting and equal channel angular pressing (ECAP). From the perspective of the severity of shear deformation, the complexity of the process, and the applicability of the material, ECAP method is the most advantageous process for achieving grain refinement. It has shown great industrial application value and has gradually received widespread attention. Traditional refinement processes, such as adding grain refining agents, can successfully refine grains to 10 μ m; By using the ECAP method, a grain size of 1 μ m can be obtained; Aluminum alloy around m. For composite materials, the use of ECAP method can better arrange the reinforcing phases in an orderly manner along a certain orientation, thereby enabling the material to have ultra-high strength in a certain direction. At present, elemental metals and alloys such as Al, Mg, Cu, and Ti have been successfully prepared using the ECAP method. For example, B. Martin et al. chose an oxygen content of 1.6% and d50=1.3μ The aluminum powder of m is used as the raw material, and the aluminum material prepared by ECAP technology has a tensile strength of 316 MPa at room temperature. The tensile strength can still reach 188 MPa at 300 ℃. After 20 hours of annealing treatment at 350 ℃, the structure and mechanical properties of the material did not significantly decrease. In the past 20 years of development, ECAP method has been able to prepare block materials with no residual pores and clean interfaces, and the sample size has exceeded 20mm× 20 ｍｍ× 100mm, the operation process gradually achieved continuity.

2 Development Trends

From the perspective of the preparation process, every technical step in the preparation of powder metallurgy aluminum alloys and their composite materials has shown significant progress. The ultra-high purity nitrogen atomization technology significantly improves the quality of aluminum powder; The spray deposition method effectively solves many problems in powder production and forming, and can obtain aluminum alloys with excellent comprehensive properties (especially good toughness), which is currently the main method for preparing high-performance aluminum alloys. The products produced by this method are mainly large-sized semi-finished products, and theoretically, this method is also suitable for preparing aluminum based composite materials. However, there are many difficulties in practical operation, and there are currently no substantial reports on its application in this area.

The rapid development of new technologies represented by ECAP technology indicates that high-strength super plastic aluminum alloys and composite materials with micro nano structures have become the main development direction of aluminum alloys, and the performance of materials is expected to reach new heights. However, similar to the spray deposition method, the process flow of these new technologies is relatively long and basically loses the near net shape characteristics of powder metallurgy technology. The preparation cost is relatively expensive and greatly limited in practical applications. Based on powder metallurgy technology, the development of pressure firing processes similar to iron-based products, and the short process, near net shape preparation of high-performance aluminum alloys, composite materials, and products have been the long-term direction of efforts. At present, the development of technologies such as gas atomization and low-temperature high-energy ball milling has basically solved the problem of preparing high-quality, micro/nano aluminum based powders. Further effective destruction of the oxide film during the forming and solidification process has become the key to low-cost preparation of high-performance powder metallurgy aluminum alloys and composite materials. Traditional methods such as adding sintering aids or powder forging can damage the oxide film, but they have problems such as low performance or high cost. Overall, existing research mainly focuses on the preparation and subsequent processing of powders, with relatively less work done in the consolidation stage due to practical effects. The emergence of new forming technologies represented by high-speed pressing is expected to bring new breakthroughs to the forming and sintering processes of aluminum powder metallurgy.

3 Conclusion

Powder metallurgy has become one of the important means to improve the performance of aluminum based materials, and its high cost is the main reason limiting its widespread application. Researchers from various countries have done a lot of work around simplifying their process flow, reducing preparation costs, and further improving performance. From the three stages of powder preparation, forming and solidification, and subsequent processing, it is unlikely that there will be significant breakthroughs in the research of powder preparation in the short term. The future development direction should focus on seeking new forming and solidification technologies that can leverage the near net shape characteristics of powder metallurgy, in order to reduce or avoid dependence on subsequent processing steps, shorten the process flow, improve the cost-effectiveness of powder metallurgy aluminum alloys and their composites, and enable them to be widely applied.

The above is all the content shared by Yongji Industry for aluminum alloy powder coating.