Vacuum Heat Treatment of Aluminum Alloys

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The metallographic structure of cast aluminum alloy is coarser than that of deformed aluminum alloy, so it is also different during vacuum heat treatment. The former has a long holding time, generally more than 2h, while the latter has a short holding time, only tens of minutes. Because the metallographic structure of metal mold castings, low-pressure castings, and cast aluminum alloys is coarser than that of deformed aluminum alloys, it is also different during vacuum heat treatment. The former has a long holding time, generally more than 2h, while the latter has a short holding time, only tens of minutes. Because metal mold castings, low pressure castings and differential pressure castings crystallize and solidify under relatively large cooling rates and pressures, their crystalline structures are much finer than those of gypsum and sand castings, so their heat preservation during vacuum heat treatment is also much shorter. Another difference between cast aluminum alloys and deformed aluminum alloys is that the wall thickness is uneven, and there are complex structures such as special-shaped surfaces or inner channels. In order to ensure no deformation or cracking during vacuum heat treatment, special fixtures are sometimes designed to protect, and vacuum The temperature of the quenching medium is also higher than that of the deformed aluminum alloy, so artificial aging is generally used to shorten the heat treatment cycle and improve the performance of the casting.

The purpose of vacuum heat treatment

The purpose of vacuum heat treatment of aluminum alloy castings is to improve mechanical properties and corrosion resistance, stabilize dimensions, and improve cutting processing.

Machining properties such as workmanship and welding. Because the mechanical properties of many as-cast aluminum alloys cannot meet the requirements for use, except for Al-Si series ZL102, Al-Mg series ZL302 and AI-Zn series ZL401 alloys, the rest of the cast aluminum alloys must be further processed by vacuum heat treatment. Improve the mechanical properties and other performance of castings, specifically in the following aspects: 1) Eliminate the uneven cooling rate of castings during crystallization and solidification due to the structure of castings (such as uneven wall thickness, large thickness of the transition) and other reasons. 2) Improve the mechanical strength and hardness of the alloy, improve the metallographic structure, and ensure that the alloy has a certain plasticity, machinability, and welding performance; 3) Stabilize the structure and size of the casting, prevent and eliminate high-temperature phase transformation. Change the volume; 4) Eliminate intergranular and component segregation, and homogenize the organization.

Vacuum heat treatment method

1.Vacuum annealing treatment

The function of vacuum annealing treatment is to eliminate the casting stress of the casting and the internal stress caused by machining, stabilize the shape and size of the workpiece, spheroidize part of the Si crystal of the Al-Si alloy, and improve the plasticity of the alloy. The process is: heating the aluminum alloy casting to 280-300C, holding the temperature for 2-3 hours, and cooling it to room temperature with the furnace, so that the solid solution slowly decomposes, and the precipitated second particles gather, thereby eliminating the internal stress of the casting and achieving stable size, The purpose of improving plasticity, reducing deformation and warping.

2.Vacuum quenching

Vacuum quenching is to heat the aluminum alloy casting to a higher temperature (generally close to the melting point of the eutectic, mostly above 500C), and keep it for more than 2 hours to fully dissolve the soluble phase in the alloy. Then, it is rapidly quenched into 60-100 ℃ of water, so that the casting is quenched, so that the strengthening components can be dissolved in the alloy to the maximum extent and kept at room temperature. This process is called vacuum quenching, also known as solution treatment or cold treatment.

3.Aging treatment

Aging treatment, also known as vacuum low temperature tempering, is a process of heating the vacuum quenched aluminum alloy casting to a certain temperature, holding it for a certain period of time and cooling it to room temperature, so as to decompose the supersaturated solid solution and stabilize the structure of the alloy matrix. During the aging process of the alloy, as the temperature rises and the time prolongs, the atoms in the supersaturated solid solution lattice are recombined, and the solute atom-enriched region (called the G-Pl region) and the G-Pl region disappear. , the atoms of the second phase segregate according to a certain rule and generate a G-PH region, and then a metastable second phase (transition phase) is generated, a large amount of G-PH region and a small amount of metastable phase are combined, and the metastable phase is transformed into stable There are several stages of particle aggregation of phase and second phase. Aging treatment is divided into two categories: natural aging and artificial aging. Natural aging refers to the aging in which aging is strengthened at room temperature. Artificial aging is divided into incomplete artificial aging and complete artificial aging. There are 3 types of aging and over-aging.

4.Cyclic processing

Cool the aluminum alloy casting to a certain temperature below zero (such as -50C, -70C, -195C) and keep it for a certain period of time, and then heat the casting to below 350 °C, so that the alloy medium solid solution lattice repeatedly shrinks and expands, and makes the A small amount of displacement occurs in the grains of each phase, so that the atomic segregation regions and the particles of the intermetallic compounds in the solid solution crystal lattice are in a more stable state, so as to achieve the purpose of increasing the size and volume of the product parts and making them more stable. This heat treatment process of repeated heating and cooling is called cyclic treatment. This treatment is suitable for parts that require very precise and stable dimensions in use (such as some parts on testing instruments). General castings are not do this treatment.

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