Generation and process control of crack defects in alloy automotive die castings
The hot chamber die casting machine is an ideal device for the production of small and medium size magnesium alloys, because it has less heat loss. The material has good ductility due to AM60B, so it is often used as the ductility of the production of automotive steering parts and materials of this Gen from the micro structure on its own. The characteristics of AM60B in the heat chamber are determined by its non dendritic matrix, which is essentially separated by beta eutectic (A117Mgl2).
Because when the metal fluid is rapidly solidified, the beta eutectic cannot form a rough sheet of tissue sufficient to reduce the ductility and creep resistance of the metal, but exists in the form of a separate body. The matrix structure of magnesium is between branches and spheres, and sphere tissues are usually found in semi freezing casting process. The solution of magnesium alloy in the injection process, the runner entrance position, in the "gooseneck" section is compressed on heat exchange with the surface of the channel, the formation of forced convection. This process is one of the main reasons for non dendrite structure.
The casting is used to secure the steering column housing. Castings are required at the expense of certain strength in exchange for higher ductility and creep resistance.
Thermal cracking and fracture
Hot cracking is usually generated in the T type region, and the defect zone in the center of the casting is evidence, and more studies show that the defect is the main factor leading to thermal cracking of the casting.
Fluid flow model in die casting process
When the metal fluid is pressed at a high speed of eight cavities, the resistance of the fluid at the boundary is greater due to the viscosity of the fluid itself, while the resistance of the fluid center is less measured. Therefore, the velocity of the most boundary fluid is close to zero, and the fluid in its central position is very fast. Figure 4 shows the flow velocity field distribution of the fluid. The fluid surface is actually will be back filled, because the fluid surface relative thermal conductivity is very strong, leading to the casting surface temperature is lower than the temperature of the casting center, which makes the final castings produce two different temperature pressure base interface. This interface will directly result in defects within the casting. Studies show that the defect ring begins at the initial stage of the die casting process and is reinforced by the solidification of the casting. It can be concluded that the produce and strengthen the casting surface and the center of the different solidification rate will strengthen this defect circle, the theory and practice proved that the high Reynolds number (high speed) distribution of fluid velocity gradient has smaller. Therefore, in high speed casting of magnesium alloy die casting will be more feasible.
The conclusion of the casting defect circle due to the internal interface is also supported by the microstructure photographs. Figure 5 shows the microstructure of this defect. An internal split zone can be clearly observed, with the upper part being the surface area of the casting, and the lower part the central region of the casting. All areas showed non dendritic primary magnesium alpha crystalline phase (white) isolated beta eutectic phase (black) surrounded by this proved that the surface area of crystal particle shape thin, crystal particles within the region is relatively large.
We also believe that another reason for this non dendritic crystal structure is the forced convection of metal fluid through the heat generated through the sprue inlet of the hot chamber. The EDS (X light energy dispersive probe) is used to test whether there is an important alloy separator in the interior of the casting. EDS can carry out this chemical test in small areas and can detect no chemical elements from atoms. The results of EDS show that the surface of the casting has smaller crystal particles than the central part, but there is no obvious alloy segregation between the surface and the interior. This conclusion will help to improve the design, that is, to change the model of the fluid to produce castings without defects.
Because of the early solidification, the metal fluid in the die casting is not a linear flow of Newton fluid, but belongs to the category of non Newton fluid mechanics. Therefore, the speed of metal fluid depends on the microstructure of the material. This non dendritic crystal structure have low mobility, which makes the metal flow in process with a small amount of metal in wells rolling phenomenon, this property is very important for non dendritic crystal structure. In addition, the heat chamber shape gooseneck also supports this contributed to the strength of convection, the non dendritic crystal structure of magnesium alloy in die casting.
The defect ring is produced during the process of die casting, filling and cooling solidification, and the modified method is to modify the design parameters. First of all, to obtain low velocity ladder fluid shape, metal fluids need to have high velocity and good flow. This will improve the sprue inlet shape and casting position.