Causes of crack formation in aluminum alloy wheel during low pressure casting

Low pressure casting can realize high mechanization and automation, not only improve the productivity (10-15 type / h), but also reduce many human factors that are not conducive to the production process, improve the yield, and greatly reduce the labor intensity of workers. However, the quality of low-pressure casting parts is affected by such factors as process plan, process parameters, mold structure and manual operation, as well as the interaction between them. Unreasonable design or improper operation of any link may lead to defects of low-pressure casting parts. Among them, the generation of aluminum alloy wheel crack is an important factor affecting the production cost and production efficiency. Therefore, it is very important to study the cause of the wheel crack in low pressure casting aluminum alloy.

1 definition of low pressure casting

The dry compressed air is introduced into the sealed container filled with metal liquid, which acts on the metal liquid surface to maintain a certain temperature, so that the metal liquid can enter the mold cavity from bottom to top along the riser through the sprue. After the metal liquid fills the mold cavity, the air pressure is increased, and the metal liquid in the mold cavity solidifies and forms under a certain pressure, and then the gas pressure on the liquid surface is relieved, so that there is no gas pressure in the riser The solidified liquid metal falls back to the increasing pot, and then opens the mold to take out the parts.

2 crack in aluminum alloy wheel of low pressure casting

It is mainly produced in the parts with stress concentration, or the cracks caused by uneven stress when the wheel is demoulded, or the liquid solidifies at the liquid lifting pipe. Cracks are generally divided into cold cracks and hot cracks.

Cold crack refers to the crack formed when the temperature of alloy is lower than its solid phase line. Generally speaking, cold crack is produced when the casting is cooled to low temperature and the casting stress acting on the casting exceeds the allowable strength or plasticity of the casting itself. Cold cracks often appear on the surface of castings, and there is slight oxidation on the surface of cracks; hot cracks are generally considered to be produced in the process of alloy solidification, because of the heat transfer effect of mold wall, castings always start to solidify from the surface. When a large number of dendrites appear on the surface of the casting and form a complete framework, the casting will show solid shrinkage (usually expressed as linear shrinkage). However, there is still a layer of liquid metal film (liquid film) between the dendrites. If the shrinkage of the casting is not hindered, then the dendrite layer can contract freely without stress. When the shrinkage of dendrite layer is blocked, it can’t shrink freely or be affected by tensile force, then tensile stress will appear, and the liquid film between dendrites will be deformed by tensile force. When the tensile stress exceeds the strength limit of the liquid film, the dendrite will be pulled apart. However, there are still some liquid metals around the cracked part. If the liquid film is pulled out slowly and there is enough liquid around and flows into the cracked part in time, the cracked part will be filled and “healed” and the casting will not have hot cracks. If the pull crack cannot be “healed” again, hot cracks will appear in the casting. The surface of the hot fracture surface is strongly oxidized, showing a dark or black color without metallic luster.