The thermal treatment of aluminum alloy die-casting parts refers to the control of heating temperature, holding time and cooling speed according to a certain heat treatment specification, and the arrangement of the alloy is changed. The main purpose is to improve the mechanical function, enhance the corrosion resistance function, improve the processing function, and obtain Scale stability. The heat treatment process of aluminum alloy castings can be divided into the following four categories:
The aluminum alloy casting is heated to a relatively high temperature, generally about 300 ° C. After a certain time of heat preservation, the process of cooling to room temperature with the furnace is called annealing. During the annealing process, the solid solution decomposes and the second phase particles accumulate, which can eliminate the internal stress of the casting, stabilize the casting scale, reduce the deformation, and increase the plasticity of the casting.
2. Solution treatment
The casting is heated to the highest possible temperature, close to the melting point of the eutectic, at this temperature for a sufficiently long time, and then rapidly cooled to maximize the dissolution of the strengthening component, this high temperature state is fixed to At room temperature, this process is called solid solution disposal. Solution treatment can improve the strength and plasticity of the casting and improve the corrosion resistance of the alloy. The role of solution treatment depends mainly on the following three elements:
(1) Solution treatment temperature. The higher the temperature, the faster the strengthening of the strengthening elements,
The better the reinforcement. Generally, the upper limit of the heating temperature is lower than the initial firing temperature of the alloy, and the lower limit of the heating temperature is such that the strengthening component is dissolved as much as possible into the solid solution. In order to obtain the best solid solution strengthening effect, but it is not convenient for the alloy to be over-fired, sometimes the stepwise heating method is adopted, that is, the temperature is kept at the low melting point eutectic temperature, so that the low melting point eutectic does not exist after the component is diffused and dissolved, and then Raise to a higher temperature for holding and quenching. In the case of solution treatment, it should also be noted that the heating rate of heating should not be too fast, so as to avoid melting of the casting and partial melting of the low melting point arrangement to cause over-burning. The transfer time of the bonfire of the solution heat treatment should be as short as possible, generally not more than 15 s, so as to prevent the diffusion of the alloying elements and reduce the function of the alloy.
(2) Holding time. The holding time is determined by the dissolution rate of the strengthening element.
This depends on the type of alloy, the composition, the arrangement, the casting method and the shape and wall thickness of the casting. The holding moment of cast aluminum alloy is much longer than that of deformed aluminum alloy, which is generally determined by experiment.
General sand castings are 20%-25% longer than the same type of metal castings.
(3) Cooling rate. The cooling rate given to the casting during quenching is greater,
The higher the supersaturation of the solid solution from the high temperature state, the higher the mechanical function of the casting, but the greater the internal stress formed, the greater the possibility of deformation of the casting. The cooling rate can be changed by selecting a cooling medium having different heat capacity, thermal conductivity, latent heat of vaporization and viscosity. In order to obtain the minimum internal stress, the casting can be cooled in a heat medium (boiling water, hot oil or molten salt).