Key points of aluminum alloy die casting design
Abstract: An excellent die casting designer should be familiar with the die casting process and the production process of die casting, so that the design of die casting conform to the most simple production and the most convenient production operation requirements. Die casting design is the most fundamental link to ensure the quality of die casting, the structural design and the rationality of the process directly determine the mold production, product size accuracy, die casting process parameters, production and installation efficiency and other high and low degree of ease. In this paper, the main points of attention in the design process of aluminum alloy die casting are summarized to improve the quality of die casting design, and provide a reference for more structural designers.
Aluminum die castings have the advantages of high production efficiency, low processing costs, easy mechanical automation in the production process, high dimensional accuracy of the castings, good surface quality, and good overall mechanical properties; but it is easy to produce pores, flow marks, and scratches during the casting process. Defects such as flaws, dents, cracks, under-casting, etc., which make the appearance quality and mechanical properties of die-castings deteriorate. In order to avoid the above problems in the die-casting molding process, the structural designer needs to evaluate the plan in advance in the structural design of the die-casting part, and make a reasonable layout in the structural design of the parts, and reduce the defects to a minimum by optimizing the structure.
1 The forming principle of aluminum alloy die casting
Aluminum alloy die-casting parts must be formed by molds, combined with die-casting machines and aluminum alloys for comprehensive use. The principle of die-casting process is to use high pressure to flow molten metal into a precision metal mold cavity at high speed, and the 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 the cold chamber die-casting, the molten metal is poured into the press chamber by 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 flows into the pressure chamber automatically through the feed port on the pressure chamber. The injection punch moves downward to push the molten metal into the cavity through the gooseneck. After the molten metal is solidified, the die-casting mold is opened and the casting is taken out to complete the entire die-casting forming process.
2 Design points of aluminum alloy die castings
The rationality of die-casting design is related to the entire die-casting molding process. When designing die-casting parts, the structural characteristics of die-casting parts and the process requirements of die-casting should be fully considered, and the defects of the designed die-casting parts during the die-casting molding process should be minimized. Occurred, and the best design plan is used to improve the quality of die castings to the greatest extent.
2.1 Reasonable design of the wall thickness of die castings
Wall thickness should be fully considered when designing aluminum alloy die casting structure. Wall thickness is a factor of special significance in the die casting process. Wall thickness has a close relationship with the entire process specification, such as the calculation of filling time and the selection of internal gate speed. , Calculation of solidification time, analysis of mold temperature gradient, the effect of pressure (final specific pressure), the length of mold retention time, the level of casting ejection temperature and operating efficiency; if the design wall thickness is too thick, shrinkage holes, sand holes, and pores will appear , External surface defects such as coarse internal grains, which reduce the mechanical properties, increase the quality of parts and increase the cost; too thin design wall thickness will cause poor aluminum liquid filling, difficult molding, poor aluminum alloy dissolution, and difficulty in filling the surface of castings. , Lack of material and other defects, and bring difficulties to the die casting process; with the increase of pores, the internal pores, shrinkage and other defects of die castings increase. Therefore, under the premise of ensuring sufficient strength and rigidity of the casting, the wall of the casting should be minimized Thick and keep the thickness of the section uniform.
2.2 Reasonable design of reinforcement ribs for die castings
For large-plane or thin-walled die-casting parts, their strength and rigidity are poor, and they are easy to deform. At this time, the use of stiffeners can effectively prevent the die-casting from shrinking and breaking, eliminating deformation, and enhancing the strength and rigidity of the die-casting. For structures such as, platform, reinforcement ribs can be used to improve the stress distribution and prevent root fracture. At the same time, the reinforcement ribs can assist the flow of molten metal and improve the filling performance of the casting. The thickness of the root of the stiffener is not greater than the thickness of the wall here, and the general thickness is designed to be 0.8～2.0mm; the draft angle of the stiffener is generally designed to be 1°～3°, the higher the height, the smaller the draft angle; A fillet should be added to the root of the rib 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 increase the strength of the part. The fillet is generally close to the wall thickness here; the height of the rib generally does not exceed 5 of its thickness. Generally, the thickness of the stiffener is 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 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 angle of die casting
The role of the die casting slope is to reduce the friction between the casting and the mold cavity, making it easy to take out the casting; to ensure that the die casting surface is not strained, and at the same time, it can extend the life of the mold. The inclination of the die is related to the height of the die casting. The greater the height, the smaller the inclination of the die. Under normal circumstances, the exit angle of the outer surface of the die-casting part is about 1/2 of the inner cavity exit angle, but in actual design, the inner and outer surface of the die-casting part can be designed to be the same in order to maintain the wall thickness. Uniformity, simplifying the structural design. For example, Table 2 is the reference value of the minimum die-casting angle of various alloy die-casting parts, and Table 3 shows the relationship between the die-casting angle and the 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 destroy the dense layer on the surface of the part and affect the mechanical properties of the part; it will expose the internal pores of the die-casting part, affect the surface quality, and increase the cost of the part. When machining of die-casting parts cannot be avoided, the design with larger cutting volume should be avoided as much as possible, and the structure design should be as easy as possible for machining or reduce the machining area and reduce the machining cost.
The upper part of the die-casting part requires high dimensional accuracy, or some flat surface roughness requirements are 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 when designing. The surface strength and hardness of die-casting parts are higher than that of the inside. Pay attention to keeping the surface density during machining, so the machining allowance should not be excessive. Excessive machining may cause pores and external surface defects. Table 4 is reserved for reference for machine plus margin.
2.5 Spraying design of aluminum alloy die castings
The surface spraying design of die-casting parts generally adopts the powder spraying process, the principle of which is electrostatic spraying: the paint is mainly polarized by the electrode, and then the object to be sprayed is charged with the opposite charge, and the powder is uniformly attached to the surface of the object under the action of the electric field force. . Characteristics of powder spraying process: powder electrostatic spraying will not cause air pollution, powder can be recycled to reduce material consumption costs, and the coating film has good acid, alkali and corrosion resistance.