The heat treatment process of cast aluminum is composed of three stages: heating, heat preservation and cooling. The whole process can be described by several parameters, such as heating speed, heating temperature, heat preservation time, cooling speed and heat treatment period. In each heat treatment process specification, temperature is very important. If the temperature measurement is not accurate, the heat treatment process specification can not be correctly implemented, which will cause the quality of aluminum castings to decline or even scrap. Temperature measurement and control is the key of heat treatment technology and the key factor affecting deformation. The influence of cooling process on deformation of aluminum castings after quenching is also an important reason. After the metal heat treatment, the thin part is always cold fast and the thick part is cold slowly. In order to reduce the distortion and cracking tendency of transition zone due to stress concentration, the difference between thickness and thickness of workpiece should be minimized when the actual production needs are met. After heat treatment, aluminum castings are processed. According to the deformation law of the castings, reverse deformation and pre expansion holes at the shrinkage end are applied to improve the qualified rate of deformation after quenching.
The datum plane must be determined for the selection of elements, the dimension inspection of the die cavity, the marking of the casting and the machining of the die. The outer circle, plane, inner hole and end face of the casting can be used as the reference plane. The appropriate datum plane is generally agreed by design, processing and casting. The following points should be considered when selecting datum:
- the non machined surface is generally selected as the reference plane of the investment casting. If the machining surface is selected, it is better to select the surface with less machining allowance.
- the datum shall be selected with the surface with precision requirements between the surface to be processed, and the design datum and processing process datum of the parts shall be coincident as far as possible.
- the number of datum plane shall be restricted by six degrees of freedom, so generally three base surfaces (two base surfaces are selected for rotating parts) are generally selected, and the line marking and processing shall be the same base plane.
2、Structural design of cast aluminum parts
Whether the structure of castings is reasonable has great influence on the quality of castings, the feasibility and simplicity of production technology and the production cost. The structure of the investment casting should conform to the production characteristics of the investment casting. The example of the reasonable structure of the part of the casting is shown.
In order to ensure the quality of the casting, process ribs and process holes are often set on the casting according to the needs. The example of application of process rib is shown. The reference dimensions for the process rib design are shown. The application example of process hole is shown.
The casting of the molten metal can be used to cast complex parts. In order to improve productivity and improve accuracy, the assembly and weldment of several parts produced by other methods can be directly cast into a molten casting after slightly structural improvement. The structural example of the multiple parts assembly and welding parts changed into the whole casting of the molten mold is shown.
3、Selection of casting structural elements and process parameters
Small wall thickness: because the inner surface of the mold shell is smooth and dry, and it is generally poured by hot shell, the wall thickness of the investment casting can be designed thinner. The recommended values for small wall thickness and the possible small values of the various alloy casting are shown. For local parts, thin wall thickness can be cast, which is 30% - 50% smaller than the small value in the table.
The application range of die casting is universal. Aluminum casting is one of the metal forming methods, which is the way to complete the process of less chip and no chip. It is widely used and developed rapidly. At present, die casting alloy is no longer limited to zinc, aluminum, magnesium and copper of non-ferrous metals, but also gradually expanded to cast iron and steel castings. The size and weight of die casting parts depend on the power of die casting machine. Because of the increasing power of die casting machine, the shape size of casting can be from several mm to 1 ~ 2m; The weight can range from a few grams to dozens of kilograms. Die casting is no longer limited to the automotive industry and instrument industry, and gradually extends to other industrial sectors, such as agricultural machinery, machine tool industry, electronic industry, national defense industry, computer, clock, camera and daily hardware industry. In the aspect of die casting technology, new technologies such as vacuum die casting, oxygen injection die casting, precision and density die casting and application of soluble core are presented.