Generation and process control of crack defects in

2022-08-24
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Generation and process control of crack defects in alloy automotive die castings

hot chamber die casting machine is an ideal equipment for producing small and medium-sized magnesium alloys, because it has less heat loss. Material AM60B is often used as a material for producing automotive steering parts because of its good ductility. The ductility of this Eugen also comes from its unique microstructure. The characteristics of AM60B in the hot chamber are determined by its non dendritic matrix, which is basically controlled by β Eutectic (a117mgl2)

because when the metal fluid is in the rapid solidification process, β Eutectic cannot form a rough lamellar structure that can reduce the ductility and creep resistance of metal, but exists in the form of a separator. The matrix structure of magnesium is between dendritic and globular, and the globular structure is usually found in the semi solidification casting process. This solution magnesium alloy is compressed and advanced at the "gooseneck" part when it passes through the gate inlet during the injection process, and forms forced heat convection with the channel surface. This process is one of the main reasons for the generation of non dendritic structure

because the aluminum content of AM60B is less than that of am9d, the fluidity of metal fluid of AM60 is worse than that of am91d in the process of die casting. Also because the metal fluid of AM60B solidifies faster (much faster than AZ91D), the surface of AM60B castings solidifies faster than other parts of its castings. In addition, because AM60B has a long solidification area, it takes a long time to achieve complete solidification. The unique defect of magnesium alloy castings is its internal delamination, or called defect band, which is mainly manifested in the differences in the surface and internal structure of the castings. The formation of such defects is also affected by its formlessness and solidification process. The buyer has proved that the above defects can be avoided by selecting a better runner inlet and optimizing the geometric shape design of the casting

am 60m automobile steering casting

this casting is used to fix the steering column housing. Castings need to sacrifice a certain strength in exchange for higher ductility and creep resistance

hot cracking and fracture

hot cracking usually occurs in the T-shaped area. The defect zone in the central area of the casting is an evidence, and more studies show that this defect is the main factor leading to hot cracking of the casting

fluid flow mode in die casting process

when metal fluid is pressed into the cavity at high speed, due to the viscosity of the fluid itself, the resistance of the fluid at the boundary is large, while the resistance at the center of the fluid is small. Therefore, the velocity of the fluid at the most boundary is close to zero, while the fluid at its center progresses rapidly. Figure 4 shows the flow velocity field distribution of the fluid. In fact, the surface layer of the fluid will be filled by reflux, because the surface layer of the fluid has relatively strong thermal conductivity, resulting in the temperature of the surface of the casting being lower than the temperature of the center of the casting, which eventually leads to the formation of two alkali pressed interfaces with different temperatures inside the casting. This kind of interface will directly produce the defect circle inside the casting. The research shows that this kind of defect ring begins at the initial stage of die casting and is further strengthened during the solidification of the casting. It can be concluded that the different solidification rates of the surface and center of the casting will strengthen the generation and strengthening of this defect ring. Practice and theory have proved that the fluid with high Reynolds number (high speed) has a small velocity gradient distribution. Therefore, high speed die casting will be more feasible in magnesium alloy die casting

microstructure

the above conclusion of casting defect circle caused by internal interface is also supported by microstructure photos. Fig. 5 shows the microstructure diagram of this defect by measuring the mold hole number that is continuous, large or less than the average value. An internal split band can be clearly observed. The infiltration of the modified matrix into the filler is the upper part of the surface area of the casting, and the lower part is the central area of the casting. All areas show non dendritic α The primary crystalline phase of magnesium (white) is separated β The eutectic phase (black) is surrounded, which proves that the surface area has a fine Chinese electrolyte output of 63000 tons of crystal particle shape, while the crystal particles in the internal area appear coarse

we also believe that another reason for the source of this non dendritic crystal structure is the forced convection of heat formed by the metal fluid when it passes through the sprue inlet of the hot chamber. EDS (X-ray energy dispersion detector) is used to test whether there is an important alloy separator at the internal interface of casting equipment, key parts and accessories with backward R & D level. EDS can carry out this chemical test in some small areas, and can detect unwanted chemical elements from atoms. The EDS test results show that there are smaller crystal particles on the surface of the casting than the central part, but there is no obvious alloy segregation in the area between the surface and the interior. This conclusion will help to improve the design, that is, to change the fluid model and produce castings without defect layer

how non dendritic crystals are produced

in figures 5 and 6, the morphology of microstructure shows that this non dendritic crystal structure is very different from those formed in other processes. This non dendritic crystal structure actually comes from its rheological characteristics. This principle is currently used in the research and development of semi solidification die casting process. Several different conditions are usually required to produce this non dendritic crystal structure, first of all, rapid cooling, and then the effect of mechanical force or other agitation. These two conditions will produce smaller crystal particles and eliminate this dendritic crystal. Under certain conditions, the gooseneck shaped runner inlet channel of the hot chamber just meets the above two conditions. Figure 7 shows that molten metal fluid must pass through the heating chamber before being cast into the mold cavity

gooseneck shaped sprue inlet, this "Z" shaped sprue inlet enables the metal fluid to exchange heat with the pipe wall through its interface layer at the earliest. Because of the high solidification temperature of alloy AM60, first of all, some primary crystalline phases of magnesium will be produced. Under the dual action of forced convection and "Z" shaped metal flow, the metal fluid in the sprue inlet pipe will be cooled, which destroys the dendritic crystals in the metal fluid and produces nearly spherical crystals. Then, these metal fluids containing some condensed solids are injected into the mold cavity for cooling, and rapid cooling is also under way α Dispersed and isolated eutectic crystals are produced around the primary phase, which can enhance the ductility and creep resistance of the metal. It is worth mentioning that this non dendritic microcrystalline structure is not a true semi condensing solid, and the temperature region it produces is not in the temperature city of the semi condensing solid, and this convection mode is not a laminar state

due to the production of early solidified solids, the metal fluid of magnesium alloy in die casting is not Newtonian fluid flowing in a straight line, but belongs to the category of non Newtonian fluid mechanics. Therefore, the velocity of metal fluid depends on the microstructure of the material. This non dendritic crystal structure has low fluidity, which makes a small amount of metal involved in the well and rolling phenomenon occur in the process of metal flow. This characteristic is very important for non dendritic crystal structure. In addition, the gooseneck shape of the hot chamber also supports this kind of intense convection, which contributes to the formation of this non dendritic crystal structure in magnesium alloy die casting

the occurrence of hot cracks on the surface of castings is due to the different solidification temperatures and shrinkage rates of castings during cooling. Thermal shrinkage is concentrated in the "t" area of the metal that has not yet fully solidified, and thermal cracking usually occurs during the first filling of the mold, because the use of the mold has not yet entered a stable state

increasing the radius does not necessarily reduce the surface hot cracking of castings. In order to achieve better design modification, it is very important to analyze the metal flow direction and solidification temperature in areas prone to defects

alloys with large solidification intervals and some small eutectic, such as AM60B, are more prone to internal and surface hot cracking defects

internal thermal cracks appear at the interface of the "layer" (microstructure crystals with different structures). These interfaces appear in the solidification process or at some places far away from the sprue inlet that are not easy to be filled, which are caused by the lack of filling, or they have been solidified long before the solidification process. Due to the force produced by the different solidification temperature of the alloy and the different shrinkage strength of the mold, the occurrence of surface hot cracks will be delayed. At the same time, the hot cracks on the surface of the casting will also expand outward with some small internal hot cracks

defect ring is produced in the process of die-casting filling and cooling solidification. The improvement method is to modify the design parameters. First of all, in order to obtain low velocity ladder fluid morphology, metal fluid needs to have high velocity and good fluidity. This requires improving the shape of the ingate inlet and the casting position

secondly, by redesigning the shape of the casting, such as increasing or reducing the volume and shape of some castings. The purpose of this is to accelerate the cooling and solidification speed, so as to reduce the accumulation of heat in some parts of the casting

other technologies, such as changing the casting radius, adding side ribs or grooves, large fillets and using local temperature cooling rods, can reduce the accumulation of heat in the brittle position of the casting; at the same time, necessary simulation tests can be made to make the design more perfect. Through such engineering improvements, the defects of the casting can be further reduced

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