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Vacuum Heat Treatment Furnace

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Why Vacuum Heat Treatment is Increasing in Use

Vacuum annealing and vacuum hardening are the basic heat treatment forms of aluminum alloys. Vacuum annealing is a softening process. Its purpose is to make the alloy uniform and stable in composition and structure, eliminate work hardening, and restore the plasticity of the alloy. Vacuum hardening is an enhanced heat treatment, the purpose of which is to improve the hardening of aluminum alloys, also known as solid solution treatment, that is, through high-temperature heating, as many alloying elements existing in the form of the second phase in the metal are dissolved into the solid solution as possible, and then rapidly cooled to Suppress the precipitation of the second phase, thereby obtaining a supersaturated aluminum-based α solid solution, which prepares the structure for the next step of aging treatment.

The prerequisite for obtaining a supersaturated α solid solution is that the solubility of the second phase in the alloy in aluminum should increase significantly with the increase of temperature. Otherwise, the purpose of solid solution treatment will not be achieved. Most alloying elements in aluminum can form a eutectic phase diagram with this characteristic. Taking Al-Cu alloy as an example, the eutectic temperature is 548°C, and the room temperature solubility of copper in aluminum is less than 0.1%. When heated to 548°C, its solubility increases to 5.6%. Therefore, Al containing copper below 5.6% -Cu alloy, after the heating temperature exceeds its solid solution line, enters the α single-phase region, that is, the second phase CuAl2 is completely dissolved into the matrix, and a single supersaturated α solid solution can be obtained after hardening.

Vacuum hardening process

Vacuum hardening is the most important and demanding heat treatment operation for aluminum alloys. The key is to select the appropriate hardening heating temperature and ensure a sufficient hardening cooling rate, and to strictly control the furnace temperature to reduce hardening deformation.

The principle of selecting the vacuum hardening temperature is to increase the hardening heating temperature as much as possible while ensuring that the aluminum alloy does not over-burn or the grains grow too much, so as to increase the supersaturation of the α solid solution and the strength after aging treatment. Generally, vacuum hardening furnaces require furnace temperature control accuracy within ±3°C, and at the same time, the air in the furnace is forced to circulate to ensure the uniformity of the furnace temperature.

Overburning of aluminum alloys is caused by the local melting of low melting point components within the metal, such as binary or multicomponent eutectics. Over-burning not only reduces the mechanical properties, but also has a serious impact on the corrosion resistance of the alloy. Therefore, once overburning occurs in aluminum alloys, it cannot be eliminated, and alloy products should be scrapped. The actual over-burning temperature of aluminum alloys mainly depends on the alloy composition and impurity content, and is also related to the processing state of the alloy. The over-burning temperature of products processed by plastic deformation is higher than that of castings. The greater the amount of deformation processing, the more non-equilibrium low melting point compositions. The easier it is to dissolve into the matrix when heated, so the actual overburning temperature increases.

The cooling rate during vacuum hardening of aluminum alloys has a significant impact on the aging strengthening ability and corrosion resistance of the alloy. During the hardening process of LY12 and LC4, it must be ensured that the α solid solution does not decompose, especially in the temperature sensitive area of 290~420℃, there must be enough Great cooling rate. It is usually stipulated that the cooling rate should be above 50℃/s, and for LC4 alloy, it should reach or exceed 170℃/s.

The most commonly used hardening medium for aluminum alloys is water. Production practice shows that the greater the cooling rate during hardening, the greater the residual stress and residual deformation of the hardened material or workpiece. Therefore, for small workpieces with simple shapes, the water temperature can be slightly lower, generally 10~30℃, and should not exceed 40℃. For workpieces with complex shapes and widely varying wall thicknesses, in order to reduce hardening deformation and cracking, the water temperature can sometimes be increased to 80°C. However, it must be pointed out that as the water temperature of the hardening tank increases, generally speaking, the strength and corrosion resistance of the material also decrease accordingly. High alloy strength, mainly used in aluminum alloys that can be strengthened by heat treatment.