Key Points of Design for Aluminum Alloy Die Castings

2021-07-08 20:22

The rationality of die casting design is related to the whole process of die casting. When designing die castings, the structural characteristics and process requirements of die casting should be fully considered, so as to minimize the occurrence of defects in the process of die casting, and to maximize the quality of die castings with the best design scheme.
1. Reasonably design the wall thickness of die castings
When designing the structure of aluminum alloy die castings, the problem of wall thickness should be fully considered. Wall thickness is a factor of special significance in the die casting process. Wall thickness is closely related to the entire process specification, such as the calculation of filling time, the selection of ingate speed, the calculation of solidification time, the analysis of mold temperature gradient, the role of pressure (final specific pressure), the length of mold retention time, the height of casting ejection temperature and the operating efficiency; If the design wall thickness is too thick, external surface defects such as shrinkage cavity, sand hole, air hole and coarse internal grain will appear, which will reduce the mechanical performance and increase the quality of parts, leading to higher costs; Too thin design wall thickness will lead to poor filling of aluminum liquid, forming difficulties, poor dissolution of aluminum alloy, easy to appear defects such as filling difficulties and material shortage on the casting surface, and bring difficulties to the die casting process; With the increase of air holes, the defects such as air holes and shrinkage cavities in die castings increase. Therefore, on the premise of ensuring sufficient strength and rigidity of castings, the wall thickness of castings should be reduced as much as possible and the thickness of sections should be uniform.
2. Reasonably design the stiffeners of die castings
For die castings with large planes or thin walls, their strength and rigidity are poor and easy to deform. At this time, the use of stiffeners can effectively prevent the shrinkage and fracture of die castings, eliminate deformation, and enhance the strength and rigidity of die castings. For structures such as high columns and platforms, the use of stiffeners can improve the stress distribution to prevent root fracture. At the same time, the stiffeners can assist the flow of molten metal and improve the filling performance of castings. The thickness of the root of the stiffener shall not be greater than the thickness of the wall here, and the thickness is generally designed as 0.8~2.0mm; The demoulding angle of the stiffener is generally designed as 1 °~3 °, and the higher the height is, the smaller the demoulding angle is; Fillet shall be added at the root of the stiffener to avoid sharp changes in the section of the part, and at the same time, to assist the flow of molten metal, reduce the stress concentration of the part, and improve the strength of the part. The fillet is generally close to the wall thickness here; The height of the stiffener is generally not more than 5 times its thickness, and the thickness of the stiffener is generally required to be uniform. If the design is too thin, the stiffener itself is easy to break, and if it is too thick, it is easy to produce defects such as dents and pores.
3. Reasonably design the die casting out angle
The function of the die casting casting exit angle is to reduce the friction between the casting and the mold cavity, so that the casting can be easily taken out; It can ensure that the die casting surface is not strained and prolong the service life of the die. The mold exit angle is related to the height of the die casting. The higher the height is, the smaller the mold exit angle is. In general, the mold exit angle of the outer surface of the die casting is about 1/2 of that of the inner cavity. However, in the actual design, the mold exit angle of the inner and outer surfaces of the die casting can be designed to be consistent to maintain uniform wall thickness and simplify the structural design.
4. Reasonably design the machining allowance
During the design of die castings, machining shall be avoided as far as possible. Machining will damage the dense layer on the surface of the parts and affect the mechanical properties of the parts; It will expose the air hole inside the die casting, affect the surface quality, and increase the part cost. When machining cannot be avoided for die castings, the design with large cutting amount shall be avoided as far as possible. The structural design shall be convenient for machining or reduce the machining area as far as possible to reduce the machining cost.
The dimension accuracy of the upper part of the die-casting parts is required to be high, or the surface roughness of some planes is required to be high, so the die-casting process is difficult to meet the requirements. At this time, the subsequent processing is required. For this part of the structure, the processing allowance should be reserved as much as possible during the design. The strength and hardness of the die casting surface are higher than those of the internal part. During machining, attention should be paid to retaining the surface density, so the machining allowance should not be excessive. Excessive machining may cause porosity and external surface defects.
5. Spraying design of aluminum alloy die castings
The powder spraying process is generally adopted for the surface spraying design of die castings. Its principle is electrostatic powder spraying: the coating is polarized by the electrode, and then the object to be sprayed is charged with opposite charges. Under the action of electric field force, the powder is uniformly attached to the surface of the object. Features of powder spraying process: electrostatic powder spraying will not cause air pollution, the powder can be recycled to reduce the cost of material consumption, and the film has good acid resistance, alkali resistance and corrosion resistance.