Aluminum die castings have advantages of high production efficiency, low processing cost, easy mechanical automation in production process, high dimensional accuracy of castings, good surface quality, and good overall mechanical properties; however, defects such as pores, flow marks, scratches, depressions, cracks, and undercasting are prone to occur during casting process, these defects reduce appearance quality and mechanical properties of die-casting parts.   degrade appearance quality and mechanical properties of die-casting parts. In order to avoid above problems in die-casting process, structural designer needs to evaluate plan in advance in structural design of die-casting part, make a reasonable layout in structural design of part, and minimize defects by optimizing structure.

 

Aluminum alloy die-casting parts must have a process of mold forming, combined with die-casting machine and aluminum alloy for comprehensive use. Principle of die-casting process is to use high pressure to flow molten metal into a precision metal mold cavity at high speed, molten metal is cooled and solidified under pressure to form a casting. Cold and hot chamber die casting are two basic methods of die casting process. In cold chamber die casting, metal liquid is poured into pressure chamber by manual or automatic pouring device, then injection punch advances to hydraulically inject metal into cavity. In hot chamber die-casting process, pressure chamber is perpendicular to crucible, molten metal flows into pressure chamber automatically through feed port on pressure chamber. Injection punch moves downward to push molten metal into cavity through gooseneck. After molten metal solidifies, die-casting mold is opened and casting is taken out to complete the entire die-casting forming process.

Rationality of die-casting design is related to the entire die-casting process. When designing die-castings, structural characteristics of die-castings and process requirements of die-casting should be fully considered, occurrence of defects in designed die castings during die casting molding process should be minimized, and quality of die-casting parts should be improved to the greatest extent with optimal design scheme.

Structural design of aluminum alloy die castings should fully consider wall thickness. Wall thickness is a factor with special significance in die casting process. Wall thickness has a close relationship with the entire process specification, such as calculation of filling time, selection of ingate speed, calculation of solidification time, analysis of mold temperature gradient, effect of pressure (final specific pressure), length of mold retention time, level of casting ejection temperature and operating efficiency; if design wall thickness is too thick, surface defects such as shrinkage cavities, blisters, pores, and coarse internal grains will appear, which will reduce mechanical properties and increase quality of parts, resulting in an increase in cost; If designed wall thickness is too thin, it will lead to poor filling of molten aluminum, difficulty in forming, poor dissolution of aluminum alloy, defects such as difficulty in filling casting surface and lack of material, which will bring difficulties to die-casting process; With increase of pores in die castings, defects such as internal pores and shrinkage cavities increase. Therefore, under premise of ensuring sufficient strength and rigidity of castings, wall thickness of castings should be reduced as much as possible and thickness of section should be kept uniform.

For die-castings with large planes or thin walls, their strength and rigidity are poor, and they are easy to deform. At this time, use of reinforcing ribs can effectively prevent shrinkage and fracture of die-castings parts, eliminate deformation, enhance strength and rigidity of die-castings parts. For structures such as columns and platforms that are too high, reinforcing ribs can be used to improve stress distribution and prevent root fractures. At the same time, reinforcing ribs can assist flow of molten metal and improve filling performance of castings. Thickness of root of reinforcing rib is not greater than thickness of wall here, and general thickness is designed to be 0.8-2.0mm; demoulding slope of reinforcing rib is generally designed to be 1°-3°, the higher height, the smaller design demoulding slope; Fillets need to be added to the root of ribs to avoid sharp changes in cross-section of part, at the same time assist flow of molten metal, reduce stress concentration of part, and improve strength of part. Fillet is generally close to wall thickness here; height of rib is generally not more than 5 times its thickness, and thickness of rib is generally required to be uniform. If design is too thin, rib itself is easy to break, and if it is too thick, it is easy to produce defects such as depressions and pores. Table 1 shows relationship between rib thickness and die casting wall thickness.

 

Role of draft angle of die casting is to reduce friction between casting and mold cavity, making it easy to take out casting; ensuring that die casting surface is not strained, and at the same time prolonging life of mold. Draft angle is related to height of die casting, the larger height, the smaller draft angle. In general, draft slope of outer surface of die casting is about 1/2 of draft slope of inner cavity, but in actual design, draft slope of inner and outer surfaces of die casting can be designed to keep wall thickness uniform and simplify structural design. For example, Table 2 shows minimum draft slope reference values of various alloy die castings, and Table 3 shows relationship between cavity draft slope and depth of each die casting.

When designing die-casting parts, machining should be avoided as much as possible. Machining will destroy dense layer on the surface of part and affect mechanical properties of part; it will expose internal pores of die-casting part, affecting surface quality and increasing cost of part. When machining cannot be avoided for die-casting parts, designs with large cutting volume should be avoided as far as possible, structural design should be as easy as possible for machining or reduce machining area and reduce machining costs.
Die-casting parts have higher dimensional accuracy requirements, or some plane surface roughness requirements, and die-casting process is difficult to meet requirements. At this time, follow-up processing is required. For this part of structure, processing allowance should be reserved as much as possible during design. Strength and hardness of surface of die-casting parts are higher than those of interior. Care should be taken to preserve surface density during machining, so machining allowance should not be excessive. Too much machining may cause pores and outer surface defects. Table 4 is a reference for machining margin reservation.

Surface spraying design of die castings generally adopts powder spraying process, and its principle is electrostatic powder spraying: coating is mainly polarized through electrodes, then object to be sprayed is charged with opposite charge, and powder is evenly attached to surface of object under action of electric field force. Features of powder spraying process: powder electrostatic spraying will not cause air pollution, powder can be recycled to reduce material consumption costs, coating performance is good in acid resistance, alkali resistance and corrosion resistance.