Aluminum die-casting has the advantages of high production efficiency, low processing cost, easy mechanical automation in the production process, high dimensional accuracy of castings, good surface quality and good overall mechanical properties. However, it is easy to produce pores, flow marks and rubs during the casting process. Defects such as injuries, dents, cracks, under-casting, etc., which cause the appearance quality and mechanical properties of the die-casting parts to decrease. In order to avoid the above problems in the die-casting process, the structural designer needs to evaluate the plan in advance in the structural design of the die-casting part, and arrange it rationally in the structure design of the part, and narrow the defect to the minimum by optimizing the structure.
1 Aluminum alloy die casting molding principle
Aluminum alloy die-casting parts must be formed by molding, combined with die-casting machine and aluminum alloy. The principle of the die-casting process is to use high pressure to flow the molten metal into a precision metal mold cavity at a high speed, and the molten metal is cooled and solidified into a casting under the action of pressure. Cold and hot chamber die casting is the two basic methods of die casting process. In the cold chamber die casting, the molten metal is poured into the pressure chamber by a manual or automatic pouring device, and then the injection punch advances to hydraulically press the metal into the cavity. In the hot chamber die casting process, the pressure chamber is perpendicular to the crucible, and the molten metal automatically flows into the pressure chamber through the feed port on the pressure chamber. The injection punch moves downward, pushing the molten metal into the cavity through the gooseneck. After the molten metal is solidified, the die-casting mold is opened, the casting is taken out, and the entire die-casting forming process is completed.
2 Aluminum alloy die casting design points
The rationality of die casting design is related to the whole die casting process. When designing die castings, the structural characteristics of die castings and the process requirements of die casting should be fully considered to minimize the defects of the designed die castings during the die casting process. Occurs to maximize the quality of die castings with an optimal design.
2.1 Reasonable design of wall thickness of die castings
The wall thickness problem should be fully considered in the design of aluminum alloy die-casting parts. The wall thickness is a special factor in the die-casting process. The wall thickness has a close relationship with the whole process specification, such as the calculation of filling time and the selection of the ingate speed. Calculation of solidification time, analysis of mold temperature gradient, effect of pressure (final specific pressure), length of mold retention time, height of casting ejection temperature and operation efficiency; shrinkage, blisters, pores appear when the design wall thickness is too thick External surface defects such as internal grain coarseness, which cause the mechanical properties to decrease, and the increase in the quality of the parts leads to an increase in cost; if the design wall thickness is too thin, the aluminum liquid is poorly filled, the molding is difficult, the aluminum alloy is not well dissolved, and the surface filling of the casting is difficult to occur. Defects such as lack of material, and bring difficulties to the die-casting process; as the die-casting parts increase with the increase of the air holes, the defects such as internal air holes and shrinkage holes increase, so the casting wall should be minimized under the premise of ensuring sufficient strength and rigidity of the casting. Thick and keep the thickness of the section uniform.
2.2 Reasonable design of die casting reinforcement
For large flat or thin wall die castings, the strength and rigidity are poor and easy to deform. At this time, the reinforcing ribs can effectively prevent the shrinkage and fracture of the die casting, eliminate the deformation, and enhance the strength and rigidity of the die casting, and the column is too high. The structure of the table can be used to improve the stress distribution and prevent the root from breaking. At the same time, the reinforcing rib can assist the flow of the molten metal and improve the filling performance of the casting. The thickness of the root of the rib is not greater than the thickness of the wall, and the thickness is generally 0.8-2.0 mm; the draft of the rib is generally designed to be 1° to 3°, and the height is higher, the smaller the draft is designed; The root of the rib should be rounded to avoid sharp changes in the section of the part, and at the same time assist the flow of molten metal, reduce the stress concentration of the part, and increase the strength of the part. The rounded corner is generally close to the wall thickness; the height of the rib generally does not exceed its thickness. Double, the thickness of the rib is generally required to be uniform. If the design is too thin, the rib itself is easy to break. If it is too thick, it is easy to cause defects such as depressions and pores. Table 1 shows the relationship between the thickness of the rib and the wall thickness of the die casting.
2.3 Reasonable design of the die-casting slope of die-casting parts
The effect of the die-casting of the die-casting part is to reduce the friction between the casting and the mold cavity, and it is easy to take out the casting; to ensure that the die-casting surface is not scratched, and the life of the die can be extended. The draft angle is related to the height of the die casting. The higher the height, the smaller the draft angle. Under normal circumstances, the outer surface of the die-casting part has a draft of about 1/2 of the inclination of the inner cavity, but in the actual design, the inner and outer surfaces of the die-casting part can be designed to be uniform to maintain the wall thickness. Uniform, simplified structural design.
Table 2 shows the minimum draft angle of various alloy die-casting parts. Table 3 shows the relationship between the slope and depth of the cavity of each die-casting part.
2.4 Reasonable design of machining allowance
When designing die-casting parts, machining should be avoided as much as possible. Machining will damage the dense layer on the surface of the part and affect the mechanical properties of the parts. It will expose the internal pores of the die-casting parts, affecting the surface quality and increasing the cost of parts. When the die-casting parts cannot avoid machining, the design with large cutting amount should be avoided as much as possible. The structural design is as convenient as possible to increase or decrease the machine-added area and reduce the machine-adding cost.
The dimensional accuracy of the upper part of the die-casting part is high, or the surface roughness of some planes is high, and the die-casting process is difficult to meet the requirements. At this time, subsequent processing is required. For this part of the structure, the machining allowance should be reserved as much as possible. The strength of the surface of the die-casting part is higher than that of the interior. When machining, care should be taken to preserve the density of the surface. Therefore, the machining allowance cannot be excessive. If the machine is excessively added, it may cause pores and external surface defects. Table 4 shows the reserved reference for the machine plus margin.
2.5 Aluminum alloy die casting spray design
The surface spray coating design of die castings generally adopts the powder spraying process. The principle is electrostatic powder spraying: the coating is polarized mainly by electrodes, and the objects to be sprayed are charged with opposite charges. The powder is evenly attached to the surface of the object under the action of electric field force. . Characteristics of powder spraying process: powder electrostatic spraying will not cause air pollution, powder can be recycled to reduce the cost of materials consumption, and the film properties are good in acid resistance, alkali resistance and corrosion resistance.
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