Production and Process Control of Cracks in Die Castings of Alloy Automobiles
[China Aluminum Network] Hot chamber die casting machine is an ideal equipment for the production of small and medium size magnesium alloys, because it has less heat loss. Because of its good ductility, material AM60B is often used as a material for the production of automotive steering components. The superior ductility of AM60B also derives from its unique microstructure. The properties of AM60B in the hot cell are determined by its non-dendritic matrix, which is essentially separated by the beta eutectic (A117Mgl2).
Because when the metal fluid is in a rapid solidification process, the β eutectic cannot form a rough sheetlike structure sufficient to reduce the metal ductility and creep resistance, but exists as a separate body. The matrix structure of magnesium is between dendritic and spherical, and the spherical structure is usually seen in the semi-solidification casting process. This solution of magnesium alloy in the injection process, through the inlet position of the runner, in the "goose neck" part is compressed forward, and the channel surface heat exchange, the formation of forced heat convection. This process is one of the main reasons for producing non-dendritic structures.
Because the aluminum content of AM60B is lower than that of AM9D, the metal fluid of AM60 is inferior to that of AM91D in the die casting process. Also because of the fast solidification of AM60B metal fluid (much faster than AZ91D), the surface of AM60B castings solidifies faster than other parts of their castings. In addition, because AM60B has a long solidification zone, it takes a long time to achieve complete solidification. This defect unique to magnesium alloy castings is its internal layering, or Defect Band. The main difference is in the surface and internal structure of the casting. The generation of such defects is also affected by its lack of type and solidification process. Buying has proven that these defects can be avoided by choosing a better location for the runner inlet and optimizing the geometry of the casting.
AM 60M automotive steering castings
This casting is used to secure the steering column housing. Castings are required to provide higher elongation and creep resistance at a sacrifice of strength.
Hot cracking and fracture
Thermal cracking usually occurs in the T-zone. Defective zones created in the center of the casting are evidence, and more studies have shown that this defect is the main cause of thermal cracking in the casting.
Fluid flow pattern in die casting process
When the metal fluid is pressed into the eight-cavity at a high speed, the fluid receives a greater resistance at the boundary due to the viscosity of the fluid itself, and the fluid center receives a smaller resistance measurement. As a result, the velocity of the fluid at the boundary is close to zero, while the fluid at its center is progressing quickly. Figure 4 shows the flow velocity field distribution of the fluid. The surface of the fluid will actually be filled with backflow because the surface layer of the fluid is relatively thermally conductive, resulting in a temperature at the surface of the casting that is lower than the temperature at the center of the casting. This results in two different temperatures at the interface between the alkali and the pressure inside the casting. . This interface will directly create a defect circle inside the casting. Studies have shown that this kind of defect circle begins at the early stage of die casting and is further strengthened during the solidification of the casting. It can be concluded that the different solidification rates at the surface and center of the casting will strengthen the generation and strengthening of this defect circle, and it has been proved practically and theoretically that the fluid with high Reynolds number (high speed) has a smaller velocity gradient distribution. Therefore, high-speed die casting will be more feasible in magnesium alloy die casting.
Microstructure
The above conclusion of the casting defect circle due to the internal interface is also supported by the micro-structure photograph. Figure 5 shows the microstructure of this defect. An internal fracture zone can be clearly observed. The upper part is the surface area of ​​the casting and the lower part is the central area of ​​the casting. All regions show that the non-dendritic α-magnesium primary crystalline phase (white) is surrounded by the separated β-eutectic phase (black), which proves that the surface region has a finer crystal particle shape, the crystals of the internal region The particles appear coarser.
We also believe that another reason for the origin of this non-dendritic crystal structure is due to the forced convection of the heat generated by the metal fluid as it passes through the hot runner inlet. EDS (X-ray Energy Dispersion Probe) is used to test whether the internal interface of the casting has an important alloy separation zone. EDS can perform this kind of chemical testing in a small area and can detect unchecked chemical elements from atoms. The test results from EDS show that the surface layer of the casting has smaller crystal grains than the central portion, but there is no obvious alloy segregation in the region between the surface layer and the interior. This conclusion will help improve the design, that is, change the fluid model. Castings without defects are manufactured.
How non-dendritic crystals are produced
In Figs. 5 and 6, the morphology of the microstructure shows that this non-dendritic crystal structure is very different from those formed in other processes. The non-dendritic crystal structure source actually comes from it. The rheological characteristics. This principle is currently used in the development of semi-solid die casting processes. Several different conditions are usually required to produce this non-dendritic crystal structure, first with rapid cooling followed by mechanical or other agitation, both of which will result in smaller crystal particles and can eliminate this Dendritic crystals. The hot chamber gooseneck-shaped sprue inlet passage under certain conditions exactly matches the above two conditions. Figure 7 shows that the molten metal fluid must pass through the heating chamber before being cast into the cavity.
Gooseneck-shaped sprue inlet, this “Zâ€-shaped sprue inlet allows metal fluid to exchange heat with the pipe wall earlier through its interface layer. Because the alloy AM60 has a high solidification temperature, some of the primary crystalline phase of magnesium will be generated first. Under the dual action of forced convection and the “z†shaped metal flow, the metal fluid in the inlet pipe of the runner will be cooled, thus destroying it. The dendritic crystals in the metal fluid produce approximately spherical crystals. Afterwards, these metal fluids containing partially solidified bodies are injected into the mold cavity for cooling, and the rapid cooling also disperses around the α crystal phase. An isolated eutectic, this form enhances the ductility and creep resistance of the metal. It is worth mentioning that this non-dendritic microcrystalline structure is not a true semi-solidified body and the temperature range it generates is not In the semi-solidified Wencheng, the convective mode is not laminar at the same time.
Because of the early solidification, the metal fluids of magnesium alloys in die casting are not Newtonian fluids, but rather belong to non-Newtonian fluid mechanics. The speed of the metal fluid therefore depends on the microstructure of the material. This non-dendritic crystal structure has low fluidity. This characteristic makes the metal flow into the well with rolling phenomenon during metal flow. This characteristic is very important for non-dendritic crystal structure. In addition, the gooseneck shape of the hot chamber also supports this strength convection, thereby contributing to the formation of such non-dendritic crystal structures in die casting of magnesium alloys.
discuss
The hot cracking of the casting surface is due to the different solidification temperatures and shrinkage rates of the casting during cooling. Heat shrinkage accumulates in the "T" zone of the metal that has not yet completely solidified, and thermal cracking usually occurs during the first filling of the mold because the use of the mold at that time has not yet entered a steady state.
Increasing the radius does not necessarily reduce the surface hot cracking of the casting. And to achieve better design changes. It is very important for metal flow analysis and solidification temperature analysis in defect-prone areas.
For alloys with large solidification intervals and the presence of some small eutectic, such as AM60B, internal and surface thermal cracking defects are more likely to occur.
Internal thermal cracking occurs at the interface of the “layers†(microstructured crystals of different structures) that occur during the solidification process or at locations that are not easy to be filled away from the runner inlet due to lack of filling. Again, or they have already solidified before the solidification process. Due to the different solidification temperatures of the alloy and the different shrinkage strengths of the mold, the thermal cracking of the surface is delayed. At the same time, the thermal cracking of the casting surface will expand outward with some tiny internal thermal cracks.
in conclusion
The defect circle is generated during die-casting filling and cooling and solidification. The improvement method is to modify the design parameters. First, to achieve a low velocity ladder flow pattern, metal fluids are required to have high velocity and good fluidity. This will improve the shape of the inner runner inlet and the position of the cast.
Second, by redesigning the shape of the casting, such as increasing or decreasing the volume and shape of some castings. The purpose of this is to increase the speed of cooling and solidification, thereby reducing the accumulation of heat in certain parts of the casting.
Some other techniques such as changing the casting radius and adding side ribs or grooves. Large fillets and the use of local temperature cooling rods can reduce the accumulation of heat in the brittle position of the casting: Simultaneously make necessary simulation tests. In order to make the design more perfect, such engineering improvements can further reduce the defects of castings.
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