Formation mechanism of crack in low pressure casting of automobile parts

Due to the use of lightweight aluminum alloy materials, compared with the steel and cast iron used before, automatic production has made rapid progress. Aluminum alloy materials have been used in the production of piston head, engine cover, suspension, wheel and other parts [25]. [25]. As for the aluminum alloy wheel, the aluminum alloy material can make the appearance more beautiful and the design more flexible. In the wheel manufacturing industry, the style pays more attention to the machining of the mechanical surface, and the bright spraying, small spokes and large size are more popular.

The existence of aluminum alloy wheel crack has an important impact on the production cost and efficiency of the enterprise, and has a major hidden danger to the safety performance of the car.

The stress concentration is the main reason for the crack of low pressure casting aluminum alloy wheel. Cracking often occurs in the process of demoulding because of the uneven stress and the solidification process of the liquid alloy in the riser because of the uneven stress and the solidification of the liquid alloy in the riser. The cracks formed when the temperature of alloy is lower than its solid phase line temperature are cold cracks. When the casting is cooled to a lower temperature, the stress in the casting exceeds the ultimate strength or plasticity of the material itself, and cold cracks will form. Cold cracks are mostly formed on the surface of castings, and slight oxidation will occur on the surface of cracks. It is generally believed that hot cracks are produced in the solidification process of alloy, largely due to the heat transfer effect of mold wall. In the process of casting, the solidification of castings starts from the surface. When a large number of dendrites appear on the surface of castings and are lapped into a complete framework, the solid shrinkage will appear (usually represented by linear shrinkage). At this time, if the shrinkage of the casting is unimpeded in the process of solid shrinkage, it can shrink freely, and the dendrite layer will not have the existence of stress, and it will not lead to the rupture of the liquid metal film (liquid film) which has not yet solidified between dendrites and lead to the hot crack; but when the shrinkage of the dendrite layer is blocked, there is tensile stress and it can not shrink freely, and the tensile stress exceeds the intergranular yet coagulated When the strength of solid liquid metal film (liquid film) is limited, the dendrite will crack. There are two kinds of results according to different conditions. The first is that there is liquid metal liquid around the crack after the crack is pulled. If the speed of the crack is slow, the metal liquid can be added to the position of the crack in time, then the crack will be filled and healed, that is, there will be no hot crack; the second is that the crack speed is fast, the metal liquid cannot be filled in time, and the crack position will not be healed, which will lead to the appearance of hot crack. The hot crack surface will appear dark or black due to strong oxidation. The process parameters of casting, the structure of wheel and the temperature of die are all the factors that may cause the cracks.

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