I. Overview of Aluminum Alloy Heat Treatment

Heat treatment of aluminum alloys is of crucial importance. It can significantly improve the various properties of aluminum alloys and enable them to play a greater role in numerous fields. The common heat treatment processes mainly include annealing, quenching, and aging.

Annealing treatment can eliminate the casting stress of castings and the internal stress caused by machining, and stabilize the shape and size of the processed parts. For example, after the product is heated to a certain temperature and kept at that temperature for a certain period of time, and then cooled to room temperature at a certain cooling rate, through the diffusion and migration of atoms, the microstructure can be made more uniform and stable, the internal stress can be eliminated, the plasticity of the material can be greatly improved, but the strength will be reduced. For homogenization annealing of ingots, keeping them at a high temperature for a long time and then cooling them at a certain speed can homogenize the chemical composition, microstructure and properties of the ingots, increase the plasticity of the material by about 20%, reduce the extrusion pressure by about 20%, increase the extrusion speed by about 15%, and at the same time improve the quality of the material's surface treatment.

Quenching involves heating aluminum alloy castings to a relatively high temperature and maintaining that temperature for more than 2 hours so that the soluble phases within the alloy can dissolve fully. Then, the castings are rapidly quenched into water to cool them down quickly, enabling the strengthening components in the alloy to dissolve to the greatest extent and remain fixed until room temperature. This process is also known as solution treatment or cold treatment. For example, for some alloy materials with low quenching sensitivity, solution treatment can be carried out by taking advantage of the high temperature during extrusion, and then quenching can be done by air cooling (T5) or water mist cooling (T6) to obtain certain microstructures and properties.

Aging treatment is applied to materials that have undergone solution quenching. When these materials are kept at room temperature or a relatively high temperature for a period of time, the unstable supersaturated solid solution will decompose, and the second-phase particles will precipitate from the supersaturated solid solution and distribute around the α(Al) aluminum grains, thus generating a strengthening effect. For natural aging, alloys such as 2024 can produce precipitation strengthening effects at room temperature. For artificial aging, alloys like 7075 do not show obvious precipitation strengthening effects at room temperature, but the precipitation strengthening effect is significant at relatively high temperatures. Aging treatment can be divided into under-aging, over-aging and multi-stage aging, etc.

Heat treatment of aluminum alloys has a positive impact on both mechanical properties and corrosion resistance.

In terms of mechanical properties, heat treatment can refine the grain structure, increase the strength and hardness of the material, and at the same time improve its plasticity and toughness. For example, solution treatment can evenly distribute solid solution elements such as Cu and Mg in the alloy within grain boundaries and inside the grains, forming a supersaturated solid solution, thereby enhancing the strength and hardness of the alloy.

Regarding corrosion resistance, heat treatment can eliminate microscopic defects and surface oxide layers, improve the surface quality of the alloy, and enhance its corrosion resistance. For instance, solution treatment can adjust the distribution of elements in the alloy and the purity of grain boundaries, which is conducive to the formation of a uniform and dense oxide film, thus improving the corrosion resistance of the alloy.

II. Main Heat Treatment Methods

(I) Annealing Treatment

Annealing treatment plays an important role in the heat treatment of aluminum alloys. It mainly eliminates the casting stress of aluminum alloy castings and the internal stress caused by machining by heating the aluminum alloy castings to a specific temperature and keeping them at that temperature for a period of time, and then cooling them to room temperature at an appropriate cooling rate. This treatment can stabilize the shape and size of the processed parts and make the microstructure of the aluminum alloy more uniform and stable.

For example, for Al-Si series alloys, the annealing treatment can also make part of the Si crystallize and spheroidize, thus significantly improving the plasticity of the alloy. According to research data, the plasticity of the aluminum alloy after annealing treatment can be increased by about 20%. The specific process is to heat the aluminum alloy castings to 280 - 300 °C, keep them at that temperature for 2 - 3 hours, and then cool them to room temperature along with the furnace, so that the solid solution decomposes slowly, the precipitated second-phase particles aggregate, and thus the internal stress of the castings is eliminated, achieving the purposes of stabilizing the size, improving the plasticity and reducing the deformation.

(II) Quenching

Quenching is one of the key steps in the heat treatment of aluminum alloys. Usually, aluminum alloy castings are heated to a relatively high temperature, generally close to the melting point of the eutectic, mostly above 500 °C, and kept at this temperature for more than 2 hours so that the soluble phases within the alloy can dissolve fully. Then, the castings are rapidly quenched into water with a temperature of 60 - 100 °C to cool them down quickly. Such an operation enables the strengthening elements in the alloy to dissolve to the greatest extent and remain fixed until room temperature.

For example, for some alloy materials with low quenching sensitivity, solution treatment can be carried out by taking advantage of the high temperature during extrusion, and then quenching can be done by air cooling (T5) or water mist cooling (T6) to obtain certain microstructures and properties. During the quenching process, it is hoped that the alloy has characteristics such as a wide temperature range between the solubility line and the solidus line, low extrusion deformation force at the solution temperature, and low quenching sensitivity.

(III) Aging Treatment

Aging treatment is a technological process in which the quenched aluminum alloy castings are heated to a certain temperature, kept at that temperature for a certain period of time, taken out of the furnace and air-cooled until reaching room temperature, so as to decompose the supersaturated solid solution and stabilize the microstructure of the alloy matrix.

During the aging treatment process of the alloy, with the increase in temperature and the extension of time, it will go through several stages, including the disappearance of the lattice region of the supersaturated solid solution, the segregation of second-phase atoms according to certain rules and the formation of G-PII regions, the subsequent formation of metastable second phases (transition phases), the combination of a large number of G-PII and a small number of metastable phases, and the transformation of metastable phases into stable phases and the aggregation of second-phase particles. Aging treatment can be divided into two major categories: natural aging and artificial aging. Natural aging refers to the aging in which the aging strengthening is carried out at room temperature. Artificial aging is further divided into incomplete artificial aging, complete artificial aging and over-aging.

(IV) Cycling Treatment Cycling treatment is a rather special heat treatment method for aluminum alloys. The aluminum alloy castings are cooled to a certain sub-zero temperature (such as -50 °C, -70 °C, -195 °C) and kept at that temperature for a certain period of time. Then, the castings are heated to below 350 °C, causing the lattice of the medium solid solution in the alloy to contract and expand repeatedly and making the grains of each phase shift slightly. The purpose of doing this is to make the atomic segregation regions within the crystal lattice of these solid solutions and the particles of intermetallic compounds in a more stable state, so as to achieve the goal of making the dimensions and volumes of product parts more stable. This heat treatment process of repeated heating and cooling is suitable for parts that require high precision and stable dimensions during use. Generally, ordinary castings usually do not undergo this treatment. III. Key Elements of Heat Treatment #(I) Requirements for Heat Treatment Equipment Heat treatment equipment plays a crucial role in the heat treatment process of aluminum alloys. Firstly, since the temperature difference range between the quenching and aging temperatures of aluminum alloys is not large, the temperature difference inside the furnace should be controlled within ±5 °C. This is because the quenching temperature of aluminum alloys is close to the melting point of the low-melting-point eutectic components within the alloy. If the temperature difference is too large, it may lead to an uneven microstructure of the aluminum alloy, thus affecting its properties. For example, in actual production, if the temperature difference inside the furnace exceeds ±5 °C, the degree of solid solution of aluminum alloy castings at different parts may vary, which will then affect mechanical properties such as their strength and hardness. Secondly, it is required that the temperature measuring and temperature controlling instruments be sensitive and accurate to ensure that the temperature is within the above error range. The accuracy of the temperature measuring and temperature controlling instruments should not be lower than grade 0.5. In this way, the temperature inside the furnace can be precisely controlled to ensure the stability and reliability of the heat treatment process. For example, some advanced heat treatment equipment is equipped with high-precision temperature measuring and temperature controlling instruments that can monitor the temperature inside the furnace in real time and make automatic adjustments according to the preset temperature curve to ensure that aluminum alloy castings are always in a suitable temperature environment during the heat treatment process. In addition, the temperature in each zone inside the furnace should be uniform, with a difference within the range of 1 - 2 °C. To achieve this goal, heat treatment equipment is usually equipped with a circulation device to ensure that the hot air inside the furnace can flow evenly, so that aluminum alloy castings can be heated and cooled evenly at all parts. For example, some large aluminum alloy heat treatment furnaces adopt a forced circulation ventilation system. High-power fans evenly blow hot air towards aluminum alloy castings, keeping the temperature in each zone inside the furnace within a small range. The quenching tank also has heating and circulation devices to ensure the heating of water and the uniformity of its temperature. Quenching is one of the key steps in the heat treatment of aluminum alloys, and the temperature uniformity of the quenching medium directly affects the cooling effect and mechanical properties of the castings. For example, during the quenching process, if the temperature of the quenching medium is not uniform, it may lead to different cooling speeds of aluminum alloy castings at different parts, resulting in problems such as internal stress and uneven microstructure. Therefore, the heating and circulation devices of the quenching tank can ensure that the temperature of the quenching medium is always kept within a suitable range, improving the quenching effect and the quality of the castings. Finally, the contaminated cooling water should be checked and replaced regularly. During the quenching process, the cooling water may be contaminated by impurities and oxides on the surface of aluminum alloy castings, thus affecting its cooling effect and the quality of the castings. Therefore, regularly checking and replacing the contaminated cooling water is one of the important measures to ensure the quality of heat treatment. For example, some enterprises have formulated strict regulations on cooling water management, regularly test and replace the cooling water to ensure the smooth progress of the quenching process.

(II) Quenching Media Quenching media are important factors to ensure the achievement of various purposes or effects of heat treatment. The higher the cooling rate of the quenching medium is, the more intense (faster) the cooling of the casting will be, and the higher the degree of supersaturation of the α solid solution in the metal microstructure will be, resulting in better mechanical properties of the casting. This is because a large number of strengthening phases such as intermetallic compounds are dissolved into the α solid solution of Al. For example, studies have shown that using PAG quenching liquids with different concentrations has different impacts on the mechanical properties, polarization curve characteristics and intergranular corrosion properties of 7249 aluminum alloy. The alloy quenched with 30% PAG quenching liquid has the best strength and plasticity, with a small corrosion current and a low corrosion rate during the polarization process. It has good intergranular corrosion resistance while ensuring relatively high strength and plasticity, and has the best overall performance. Another example is that for 2519A aluminum alloy sheets, the strength of the alloy that has been quenched in different media and pre-deformed is higher than that of the alloy without deformation. Under the T8 state, the alloy has the highest tensile strength when quenched in water at 20 °C; and it has the lowest tensile strength when quenched in air. Meanwhile, the intergranular corrosion resistance and exfoliation corrosion resistance of the alloy that has been quenched in different media and pre-deformed are better than those of the alloy without deformation. The alloy quenched in water at 20 °C has the best intergranular corrosion and exfoliation corrosion resistance, while the alloy quenched in air has the worst intergranular corrosion and exfoliation corrosion resistance. In addition, the temperature of the quenching water also has an impact on the mechanical properties of cast aluminum alloys. Studies have shown that the strength and hardness of alloy specimens are related to the temperature of the quenching water, and quenching at 80 °C can obtain alloy materials with the best overall performance. At this quenching water temperature, the fracture mode of the alloy specimens is mainly ductile fracture accompanied by local cleavage, and the alloy exhibits relatively good mechanical properties.

(III) Temperature Measuring and Temperature Controlling Instruments and Materials

The accuracy of the temperature measuring and temperature controlling instruments should not be lower than grade 0.5. The heat treatment heating furnace should be equipped with devices and instruments that can automatically measure and control the temperature, with functions such as automatic recording, automatic alarm, automatic power cut-off, and power restoration, so as to ensure that the temperature display and control in the furnace are accurate and the temperature is uniform.

High-precision temperature measuring and temperature controlling instruments can monitor the temperature inside the furnace accurately in real time and ensure that the temperature is always within a suitable range during the heat treatment process. For example, when the temperature inside the furnace exceeds the preset range, the automatic alarm device will give an alarm in a timely manner to remind the operators to make adjustments. The automatic power cut-off and power restoration devices can cut off the power supply in time when the temperature is abnormal or other failures occur, protecting the safety of the equipment and workpieces. After the problems are solved, the power supply will be automatically restored to ensure the continuity of the heat treatment process.

The automatic recording device can record the temperature changes during the heat treatment process, providing data support for subsequent quality analysis and process optimization. For example, by analyzing the temperature recording curve, we can understand the temperature change trends at different stages, find out possible problems, and make targeted adjustments and improvements.

The functions of these automatic temperature control devices lie in improving the precision and stability of heat treatment, reducing the interference of human factors, ensuring that aluminum alloy castings can obtain a uniform microstructure and properties during the heat treatment process, and improving the quality and reliability of products.

IV. Strengthening Methods for Aluminum Alloys

(I) Work Strengthening

Work strengthening is a method by which alloys obtain high strength through plastic deformation. The essence of work strengthening of alloys lies in increasing the dislocation density during plastic deformation. After intense deformation of metals, the dislocation density can increase from 10⁶ per cm² to more than 10¹² per cm². The greater the dislocation density in the alloy is, the more opportunities there will be for dislocations to intersect with each other during the slip process when continuing to deform, and the greater the resistance between them will be. As a result, the deformation resistance will also increase, and the alloy will thus be strengthened.

The degree of work strengthening varies depending on the deformation rate, deformation temperature and the nature of the alloy itself. For the same alloy material undergoing cold deformation at the same temperature, the higher the deformation rate is, the higher the strength will be, but the plasticity will decrease with the increase in the deformation rate. When the deformation temperature is relatively low, the mobility of dislocations is poor. After deformation, most of the dislocations are distributed in a disordered and irregular manner, forming dislocation tangles. At this time, the strengthening effect of the alloy is good, but the plasticity is also significantly reduced. When the deformation temperature is relatively high, the mobility of dislocations is greater, and cross-slip occurs. Dislocations can gather and tangle locally, forming dislocation clusters, and substructures and their strengthening appear. At this time, the strengthening effect is not as good as that of cold deformation, but the loss of plasticity is relatively small.

(II) Solution Strengthening

Alloying elements are added to pure aluminum to form aluminum-based solid solutions, which cause lattice distortion and hinder the movement of dislocations, thus playing a role in solution strengthening and increasing its strength. Binary alloys such as Al-Cu, Al-Mg, Al-Si, Al-Zn, and Al-Mn can generally form limited solid solutions and all have relatively large limiting solubilities, so they have significant solution strengthening effects.

For example, in scandium-containing ultra-high-strength aluminum alloys, the Sc element, as a common additive, can improve the strength and toughness of aluminum alloys by forming Sc-Al solid solutions. Meanwhile, appropriate amounts of elements like Ti and Zr can also effectively promote the strengthening solution process. Through their interaction with the Sc element, a multi-level and multi-phase composite strengthening system can be formed.

(III) Heterophase Strengthening

The heterophases in aluminum alloys are usually hard and brittle intermetallic compounds. They impede the movement of dislocations in the alloy and strengthen the alloy. For example, in ultra - high - strength aluminum alloys containing Sc, appropriate strengthening solution treatment can also improve the corrosion - resistance and high - temperature performance of the aluminum alloy. However, if the number of heterophases is too large, both the strength and plasticity will be reduced. The more complex the composition and structure of the heterophase and the higher its melting point, the better its high - temperature thermal stability.

(IV) Dispersion Strengthening

The smaller the size of the dispersion - phase particles and the more uniform their distribution, the better the strengthening effect. For example, adding fine dispersion - phase particles to aluminum alloys can impede the movement of dislocations and improve the strength and hardness of the alloys.

(V) Precipitation Strengthening

When the alloy is annealed at the solution treatment temperature, the alloying elements dissolve in the matrix, forming a supersaturated solid solution. Subsequently, quenching is carried out to rapidly cool the supersaturated solid solution and prevent the diffusion and precipitation of the alloying elements. During the aging process, the alloying elements precipitate from the supersaturated solid solution to form fine and dispersed second - phase particles. These particles are usually alloy - element - rich compounds or intermetallic phases, and their size, shape and distribution have a significant impact on the strength and hardness of the alloy.

(VI) Grain Boundary Strengthening

During the aging process, the precipitated phase tends to precipitate at the grain boundaries, forming a grain - boundary precipitation zone. The grain - boundary precipitation zone hinders grain - boundary sliding and improves the shear - resistance of the grain boundaries, thereby increasing the overall strength of the alloy. At the same time, the precipitated phase can also precipitate at the sub - grain boundaries, strengthening the sub - grain boundaries and further improving the overall mechanical properties of the alloy.

(VII) Combined Strengthening

In practical applications, several strengthening methods usually work simultaneously. For example, in ultra - high - strength aluminum alloys containing Sc, by optimizing parameters such as the types of additives, heating temperature and time, strengthening the solution can significantly improve the strength and toughness of the aluminum alloy. At the same time, strengthening the solution can also improve the corrosion - resistance and high - temperature performance of the aluminum alloy. In the future, the microstructure and properties of ultra - high - strength aluminum alloys containing Sc strengthened by multiple additives in a coordinated manner, as well as the mechanical and corrosion - resistance properties of ultra - high - strength aluminum alloys containing Sc in complex service environments can be further studied.

V. Influence of Heat Treatment on the Properties of Specific Alloys

(I) Influence on the Microstructure and Properties of 2024 Alloy

The 2024 alloy belongs to the high - strength and - hardness Al - Cu - Mg - series aluminum alloy and is widely used in the aerospace industry. Solution treatment and aging treatment have an important impact on the plasticity, strength and corrosion - resistance of the 2024 alloy.

During solution treatment, the first group of samples were solution - heat - treated and held at different temperatures. The results showed that after the alloy was solution - treated at a specific temperature (such as 500 °C) for 50 minutes, the soluble phases in the alloy could be fully dissolved, laying a foundation for subsequent aging treatment. Solution treatment can adjust the distribution of elements in the alloy, evenly distributing solid - solution elements such as Cu and Mg within the grain boundaries and inside the grains, forming a supersaturated solid solution, thereby increasing the strength and hardness of the alloy. At the same time, solution treatment can also improve the plasticity of the alloy. Research data shows that the plasticity of the 2024 alloy after solution treatment can be improved to a certain extent.

The impact of aging treatment on the properties of the 2024 alloy is also very significant. The third group of samples were first solution - treated and then subjected to long - term aging heat - treatment at different temperatures. The experiment found that the alloy could obtain the best microstructure and properties after artificial aging at 180 °C for 10 hours, and the hardness could reach 81.3 HRB. During the aging treatment process, the unstable supersaturated solid solution will decompose, and the second - phase particles will precipitate from the supersaturated solid solution and distribute around the α(Al) aluminum grains, thus producing a strengthening effect. Natural aging of alloys such as 2024 can produce precipitation - strengthening effect at room temperature, increasing the strength of the alloy. At the same time, aging treatment can also improve the corrosion - resistance of the alloy. By eliminating microscopic defects and surface oxide layers, the surface quality of the alloy is improved, which is conducive to the formation of a uniform and dense oxide film, thereby improving the corrosion - resistance of the alloy.

(II) Influence on the Microstructure and Properties of 7075 Alloy Single - stage aging has an important impact on the fiber structure, the formation of second - phase particles, micro - hardness and peak strength of 7075 alloy. By measuring the hardness, yield strength, tensile strength, elongation and reduction of area of the samples with different aging times under the 120 °C single - stage aging regime, it was found that the 7075 aluminum alloy could obtain the best combination of strength and plasticity when aged at 120 °C for 24 hours. Through the orthogonal experiment of double - stage aging, it was known that for the double - stage aging treatment of 7075 aluminum alloy, the pre - aging temperature was 140 °C and the holding time was 4 hours, and the second - stage aging temperature was 140 °C - 160 °C and the holding time was 10 hours. This treatment process could obtain products with better comprehensive properties. During the single - stage aging process, the fiber structure of 7075 alloy will change. As the aging time is extended, the fiber structure gradually becomes denser, which is beneficial to improving the strength of the alloy. At the same time, second - phase particles will also be gradually formed. These second - phase particles impede the movement of dislocations in the alloy and strengthen the alloy. For example, MgZn2 and Al2Mg3Zn3 have high solubility in aluminum and an obvious temperature - related relationship, and have a strong age - hardening effect. Single - stage aging can also significantly improve the micro - hardness and peak strength of 7075 alloy. As the aging time increases, the micro - hardness continuously increases, and after a certain time, the hardness tends to be stable. The peak strength also gradually increases during the aging process. This is because the aging treatment enables the strengthening components in the alloy to dissolve to the greatest extent and remain fixed until room temperature, thus increasing the strength of the alloy.

VI. Post - Weld Heat Treatment (1) Influence of Post - Weld Heat Treatment on Strength and Toughness For heat - treatable - strengthened aluminum alloys, after welding, heat treatment can be carried out again to restore the strength of the base metal's heat - affected zone to a level close to the original strength. Generally, the joint failure usually occurs in the fusion zone of the weld. After re - performing post - weld heat treatment, the strength obtained by the weld metal mainly depends on the filler metal used. When the composition of the filler metal is different from that of the base metal, the strength will depend on the dilution of the base metal by the filler metal. The best strength is compatible with the heat treatment used for the welding metal. Although post - weld heat treatment increases strength, there may be some loss in the toughness of the weld. Due to the precipitation near the weld and the melting of the grain boundaries in the fusion zone, the toughness of some weldments of heat - treatment - strengthened aluminum alloys is very poor. If the situation is not too serious, post - weld heat treatment can make the soluble components redissolve, obtaining a more uniform structure, slightly improving the toughness, and greatly increasing the strength. For example, for 6061 aluminum alloy welded in the T4 (solution treatment + natural aging) heat - treatment state and then treated with T6 (solution treatment + artificial treatment) after welding, the weld strength can reach 280 MPa. The T6 treatment involves heating the aluminum alloy to 535 ± 5 °C, holding it for 6 hours, and then quenching it in water at 80 ± 10 °C, with the quenching time not less than 5 minutes. Then it is aged in a low - temperature furnace at 165 ± 5 °C for 4 ± 0.5 hours. The hardness after treatment generally reaches HB80 - 90, the elongation is greater than 8%, and the tensile strength reaches 250 - 290 MPa. For the 6082 - T6 aluminum alloy welded joint, two heat - treatment methods, namely solution + aging and aging, were carried out. The tensile strength of the untreated 6082 - T6 welded joint was 225 MPa, the fracture location was in the heat - affected zone, and the lowest hardness value of the joint was in the heat - affected zone. After the aging treatment, the distribution of the strengthening phase at the 6082 - T6 welded joint was more uniform, there was no obvious change in the microstructure of the weld zone, and the microstructure of the fusion zone and the heat - affected zone was slightly refined. The tensile strength was 264 MPa, the fracture location was still in the heat - affected zone, and the lowest hardness value of the joint was still in the heat - affected zone. After the solution + aging treatment, fine strengthening phases were re - precipitated at the 6082 - T6 welded joint, the microstructure of the fusion zone and the heat - affected zone was significantly refined, the tensile strength was increased to 302 MPa, the fracture occurred in the weld zone, and the hardness value was significantly higher than that of the untreated 6082 - T6 welded joint, and the lowest hardness value was located in the weld zone.

(II) Key Technical Points 1. Heat Preservation Issue: The key technology lies in the heat preservation issue. It is essential to follow the process. The transfer from the high - temperature furnace (solution furnace) to water quenching should be as quick as possible; otherwise, the solution effect will be affected, and ultimately, the heat - treatment effect will be influenced. 2. Removal of Residues: After the welding of the welded parts, if gas welding or coated electrode welding is used, the remaining flux and slag on and on both sides of the weld need to be removed in a timely manner before the visual inspection and non - destructive testing of the weld. This is to prevent the slag and the remaining flux from corroding the weld and its surface and to avoid adverse consequences. The commonly - used post - welding cleaning methods include scrubbing in hot water at 60 °C - 80 °C; immersing in potassium dichromate (K2Cr2O7) or chromic anhydride (CrO3) with a mass fraction of 2% - 3%; then washing in hot water at 60 °C - 80 °C; and drying in a drying oven or air - drying. To test the effect of the removal of the remaining flux, distilled water can be dropped onto the weld of the welded part, and then the distilled water is collected and dropped into a small test tube containing a 5% nitric acid solution. If there is a white precipitate, it indicates that the remaining flux has not been completely removed. 3. Surface Treatment of Welded Parts: Through appropriate welding processes and correct operation techniques, the surface of the welded seam of aluminum and aluminum alloys after welding has a uniform and smooth wavy appearance. Anodizing treatment, mechanical polishing, etc. can be carried out to improve the surface quality of the aluminum workpieces. However, aluminum and aluminum alloys are soft metals with a relatively high coefficient of friction. If overheating occurs during the grinding process, it may cause the welded parts to deform or even fracture from the grain boundaries. This requires sufficient lubrication during the polishing process, and the pressure on the metal surface should be minimized. VII. New Heat - Treatment Methods and Approaches to Performance Improvement (1) Interface Replacement and Dispersion Strategy The team led by He Chunnian from the School of Materials of Tianjin University innovatively proposed an "interface replacement" dispersion strategy and successfully achieved the single - particle - level uniform distribution of oxide particles of about 5 nanometers in aluminum alloys. The oxide dispersion - strengthened aluminum alloy prepared by this strategy still exhibits unprecedented tensile strength (about 200 MPa) and high - temperature creep resistance at a temperature as high as 500 °C. For the temperature range of 300 °C - 500 °C, which is of the greatest concern in current fields such as aerospace, the mechanical properties of traditional aluminum alloys decline sharply, while the aluminum alloy prepared by this strategy far exceeds the best level of aluminum - based materials reported internationally. They first used the self - assembly effect during the decomposition of the metal - salt precursor to prepare ultra - fine oxide particles coated with few - layer graphite. The stronger chemical - bond combination between the nanoparticles was replaced with the weaker van der Waals force combination between the graphite - coating layers, thereby reducing the adhesion between the nanoparticles by 2 - 3 orders of magnitude. On this basis, through a simple mechanical ball - milling - powder - metallurgy process, the uniform dispersion of single - particle - level ultra - fine oxide particles with a high volume fraction (volume fraction of 8%) in the aluminum matrix was achieved, and the aluminum alloy exhibited extremely outstanding high - temperature mechanical properties and high - temperature creep resistance. The tensile strengths of this material at 300 °C and 500 °C are 420 MPa and 200 MPa, respectively; under the creep condition of 500 °C and 80 MPa, the steady - state creep rate is 10⁻⁷ s⁻¹. This research reveals the extraordinary heat - resistance mechanism of ultra - fine nanoparticles in enhancing lightweight metals and provides new ideas for the development of lightweight, high - strength, and heat - resistant metal materials and their applications in aerospace, transportation and other fields. (2) Electric Pulse Treatment Technology In 2015, Xu Xiaofeng from Jilin University proposed the electric pulse treatment technology (EPT) for aluminum alloys. This technology relies on instantaneous high - energy input to shorten the solution time of 7075 aluminum alloy to 220 ms. The electric pulse treatment technology can significantly accelerate the solution process in 7075 aluminum alloy. Although, compared with the traditional solution method, the supersaturation of the samples treated with pulse current is slightly lower, the combined effect of grain refinement and precipitate - phase refinement caused by the pulse - current treatment is better, and the strength after artificial aging is higher than that of the samples treated with the conventional T6 treatment. In addition, the process time of the pulse - current treatment is less than 1 s, which can avoid the deformation and oxidation of the material during the heat - treatment process. After SST (solution treatment) and EPT, it can be seen from the optical microstructure diagram that recrystallization has occurred in the alloy. The grain size of the samples treated with pulse current is only 15 μm, while the grain size of the traditional solution samples is 53 μm. The tensile strength and elongation of the alloy after solution treatment and pulse - current treatment have been improved. After artificial aging, the alloy treated with pulse current has higher strength and a small loss of elongation. It can be considered that the fine - grained structure after pulse - current treatment makes an additional contribution to the strength of the alloy.

(III) Cryogenic Treatment Cryogenic treatment can not only eliminate the residual stress of aluminum alloys, but also improve the dimensional stability of the alloys and reduce machining deformation. Cryogenic treatment can enhance the mechanical properties of aluminum alloys such as strength, plasticity and impact toughness. For example, after the aluminum alloy is subjected to hot - cold cycle high - low temperature treatment using the DJL (Dejieli) integrated cryogenic tempering furnace, the value of the residual stress of the aluminum alloy is significantly reduced, and the residual stress of the aluminum alloy can be reduced by more than 50% at most. The cryogenic treatment of aluminum alloys using the DJL integrated cryogenic tempering furnace can effectively reduce the machining deformation of aluminum alloy products and improve the product processing yield. Cryogenic treatment can effectively reduce the residual stress inside the aluminum alloy components, improve the dimensional stability and precision of the aluminum alloy, and prevent deformation during later use. The effect of multiple hot - cold cycle treatments is better than a single treatment, and the generally recommended number of times is 2 - 3 times.