Cast aluminum parts are a commonly used material in industrial production, widely used in fields such as aviation, automotive, and construction. However, cast aluminum parts often require heat treatment during the production process. So, why do aluminum castings need to undergo heat treatment?
1, The importance of heat treatment for cast aluminum parts
The metallographic structure of cast aluminum alloy is coarser than that of deformed aluminum alloy, so it also differs during heat treatment. The former has a longer insulation time, usually more than 2 hours, while the latter has a shorter insulation time, some only need a few tens of minutes. Because castings produced by metal mold casting, low-pressure casting, and differential pressure casting solidify under relatively high cooling rates and pressures Its crystal structure is much finer than that of castings made by gypsum casting and sand casting, so the insulation time of its heat treatment is also much shorter. Another difference between cast aluminum alloy and deformed aluminum alloy is the uneven wall thickness, complex structural shapes such as irregular cross-sections or internal channels. In order to ensure that there is no deformation or cracking during heat treatment, special fixtures are sometimes designed for protection, and the temperature of the quenching medium is also higher than that of deformed aluminum alloy. Therefore, artificial aging is generally used to shorten the heat treatment cycle and improve the performance of castings.
The purpose of heat treatment for casting aluminum alloys is to improve mechanical properties and corrosion resistance, stabilize dimensions, and enhance process performance such as machinability and weldability. Because the mechanical properties of many as cast aluminum alloys cannot meet the requirements for use, except for Al Si based ZL102, Al Mg based ZL302, and Al Zn based ZL401 alloys, all other cast aluminum alloys need to undergo heat treatment to further improve the mechanical and other performance of the castings. Its specific functions include the following aspects:
(1) Eliminate the internal stress caused by uneven cooling rate during crystallization and solidification of castings due to reasons such as uneven wall thickness and thick joints;
(2) Enhance the strength and hardness of the alloy, improve the metallographic structure, and ensure that the alloy has certain plasticity, cutting and welding properties;
(3) Stabilize the structure and size of castings, prevent and eliminate volume changes caused by high-temperature phase transformation; 4) Eliminate intergranular and compositional segregation to homogenize the structure.
2, Key points of heat treatment methods and operation techniques for casting aluminum alloys
The heat treatment methods for casting aluminum alloys currently include annealing, quenching [solution treatment], aging, and cyclic treatment, which are described as follows:
(1) Annealing. The function of annealing is to eliminate casting stress and internal stress caused by mechanical processing in castings, stabilize the shape and size of machined parts, and spheroidize some Si crystals in Al Si alloys, improving the plasticity of the alloy. The process involves heating the aluminum alloy casting to 280-300 ℃, holding it for 2-3 hours, and cooling it in the furnace to room temperature. This allows the solid solution to slowly decompose and the second particles to aggregate, thereby eliminating the internal stress of the casting, achieving stable size, improving plasticity, and reducing deformation. The heat treatment status code is T2.
(2) Quenching. Quenching is also known as solution treatment or rapid cooling treatment. The process is to heat the aluminum alloy casting to a relatively high temperature (usually close to the melting point of eutectic, mostly above 500 ℃), hold for more than 2 hours, and fully dissolve the soluble phases in the alloy. Then, the casting is rapidly quenched in water at 60-100 ℃, and due to the rapid cooling, it is fixed with the strengthening phase that dissolves to the maximum extent in the alloy and stored at room temperature.
(3) Timeliness. The process involves heating quenched aluminum alloy castings to a certain temperature, holding them for a certain period of time, and then air cooling them to room temperature to decompose the supersaturated solid solution and stabilize the alloy matrix structure.
Time processing can be divided into two categories: natural time processing and artificial time processing. Natural aging is a process of strengthening aging at room temperature. Artificial time efficiency can be divided into three types: incomplete artificial time efficiency, complete artificial time efficiency, and over time efficiency.
① Incomplete artificial time efficiency. Heat the casting to 150-170 ℃ (lower temperature) and hold for 3-5 hours to obtain better tensile strength, good plasticity, and toughness, but with reduced corrosion resistance.
② Completely artificial time efficiency. Heat the casting to 175-185 ℃ (at higher temperatures) and hold for 5-24 hours to obtain sufficient tensile strength, i.e. the highest hardness, but the elongation rate decreases.
③ Expired time limit. Also known as stabilization tempering. The process involves heating the casting to 190~-230 ℃ and holding it for 4~9 hours to reduce its strength and improve its plasticity, in order to achieve better stress corrosion resistance.