Shrinkage cavity is a common casting defect, which is essentially a volume defect. According to the causes, it can be divided into the following types: hot spot shrinkage, riser neck shrinkage, runner shrinkage, axis dispersion shrinkage and concave corner shrinkage.
Hot spot shrinkage cavity, also known as internal shrinkage cavity, is the node with the thickest volume and the most heat release value after solidification in the casting geometry structure. Generally, the diameter of the inscribed circle is used to measure the size of the hot spot. The larger the diameter is, the more volume accumulation of metal is, the longer the solidification time will be. As a result, the volume deficit caused by volume shrinkage during the solidification period of hot spot will be larger. If there is no compensation measure, the metal compensation will form internal shrinkage cavity. Hot spot shrinkage cavity accounts for the largest proportion of metal shrinkage defects. In this paper, hot spot shrinkage cavity is produced in the axle housing.
The shrinkage cavity of riser neck is the secondary shrinkage cavity formed when the shrinkage cavity passes through the neck of riser and invades into the casting. Especially for steel castings, the shrinkage cavity is also the final solidification position of molten iron, resulting in the segregation of material composition, in which the content of sulfur, phosphorus and carbon exceeds the standard mainly. The main reason is the process hot spot and unreasonable riser setting.
Because the ingate is the channel through which molten metal is introduced into the mold cavity, the solidification is very slow after the pouring, and the shrinkage of molten iron and the casting in the later solidification stage can not be compensated, and then the shrinkage cavity occurs. The higher the temperature, the larger the shrinkage cavity.
Axis shrinkage cavity is a group of small shrinkage cavities distributed intermittently on the center line and axis of the casting. Different from shrinkage porosity, it can be seen by naked eyes, and its characteristics are not concentrated and dispersed.
The direction of the concave angle and shrinkage cavity on the surface of the casting is changed from vertical to horizontal, that is, the sand sharp angle. Because the solid shell formed here is thinner than that of the plane type, and the external gas invades, it is also called air shrinkage cavity.
According to the above theoretical calculation and simulation results of liquid metal temperature field, the location and area of shrinkage cavity of axle housing can be accurately predicted by using the thermal analysis shrinkage defect module of ProCAST simulation software, which provides a reliable guarantee for the process optimization in the future. The defect analysis of the three casting schemes is shown in the figure.
Through the comparison of the above flow field and temperature field, it can be seen from the filling process of the three schemes that the filling process of the three semi closed gating system is the most stable, and the molten metal has no interruption phenomenon. The molten metal is always full of the runner, and there is no air entrainment, so as to reduce the oxidation of the contact area between the metal flow surface and the air, thus reducing the generation of oxide inclusions; at the same time, the density of molding sand is less than that of metal The results show that the density of liquid will float on the surface of liquid flow, and the blocking effect of sand collecting pit and runner is good; the closed gating system takes the second place, and the mold filling is better. At the beginning of pouring, a little dispersed metal liquid is in the mold cavity. At the initial filling stage, the molten metal is not filled with the runner, which may produce oxygenated inclusions. The sand retaining effect of the runner is poor. In the middle and late stage, the molten metal fills the whole mold cavity and reduces the oxidation In scheme 2, there are many isolated molten metal in the open gating system, the cooling speed is fast, and the temperature drops below the liquidus rapidly, which is easy to produce fatal casting defects such as cold shut and cold beans. At the same time, the molten metal has not filled the runner during the whole pouring process, so the probability of oxide inclusion is greatly increased, and the sand retaining effect of the runner is poor.
From the analysis of temperature field and shrinkage defects of the three schemes, it can be seen that there are isolated liquid regions in the later stage of solidification. It can be seen from the figure that when the solidification fraction is the same, the isolated liquid region of the open gating system in scheme II is the largest, and the shrinkage cavity area is the largest. In order to further optimize the design, the open gating system of scheme II is abandoned. According to this reason, the first closed gating system and the third semi closed gating system are selected to optimize the feeding system for subsequent treatment.