The solidification method of the casting and the direction of solidification in the solidification direction are different, and the morphology and progress of the solidification zone are different, that is, the solidification mode is different.
First, the solidification of the casting method The solidification method of the casting section is divided into three types: layer-by-layer solidification, pasty solidification, and intermediate solidification. The solidification of the alloy depends on the width of the solidification zone
1. Layer-by-layer solidification Figure 3-1 shows the solidification of a pure metal or eutectic alloy crystallized at a constant temperature. t. Is the crystallization temperature, T1, and T2 are casting profiles
Figure 3-1 Temperature field at two different times
From Figure 3-1, it can be observed that the metal crystallized at a constant temperature has a solidification zone width of zero at the casting cross section during the solidification process. The solids and liquids on the cross-section were clearly separated from each other by a boundary line. As the temperature drops, the solid layer thickens and gradually reaches the center of the casting. This is a typical layer-by-layer solidification method. Figure 3-1 also belongs to the layer-by-layer solidification method. The alloy crystallization temperature interval At. Very small, the temperature in the solidification zone decreases by 5, and the solidification zone is very narrow. The solidified layer in the solidification process is gradually thickened up to the center of the casting.
2. Volume Solidification Figure 3-2
It is a schematic diagram of volume solidification. Alloy crystallization temperature interval. Large, the temperature drop SL on the section is relatively small. In Figure 3-2(a), â–³t. Smaller, but the temperature drop S, also small in Figure 3-2, Figure 3-2 (b) â–³ t. Larger and larger SL. During the solidification of the casting, the solidification zone is very wide at all times, even throughout the entire casting section. That is, the solidification process may exist in all sections at the same time, and the liquid-solid coexisting paste area is filled with the casting section. This kind of solidification is called paste solidification or volume solidification
3. Intermediate solidification When the solidification zone width is between the above two conditions, it belongs to the intermediate solidification mode, as shown in Figure 3-3. The crystallization temperature interval is smaller and the temperature drop across the section is smaller, as shown in Figure 3-3(a).
Or the crystallization temperature interval is larger and the temperature drop of the section is larger. See Figure 3-3. The solidification region has a certain width and does not occupy the entire casting section. Figure 3-3 The solidification zone width can be determined based on the longitudinal distance between the liquid phase boundary and the solid phase boundary on the solidification dynamic curve. Therefore, the size of this distance is the criterion for distinguishing the solidification mode. If the two curves coincide, ie the metal or alloy crystallized at a constant temperature, or the distance between them is very small, then it tends to solidify layer by layer; if the two curves have a large distance, it tends to volume solidification; if the two curves are Small, it is the middle solidification method
Second, the factors affecting the solidification method The solidification method of the casting depends on the width of the solidification region. The width of the solidification region is determined by two factors: the temperature interval of the alloy crystallization and the temperature gradient of the casting section. These two factors affect the solidification method.
1 alloy crystallizing temperature interval effect. In the case where the temperature gradient of the casting section is similar, the larger the crystallization temperature interval, the wider the solidification area.
2 The effect of temperature gradients. After the crystallization temperature interval of the alloy is determined, the width of the solidification region mainly depends on the temperature gradient. When the temperature gradient is large, alloys with wide crystallization temperature intervals can have smaller solidification regions, tend to solidify in the middle, and even solidify layer by layer; when the temperature gradient is small, the solidification region generally has a large width, and even tends to solidify.
III. Direction of solidification of castings In the above classification of solidification methods of castings, the section referred to is perpendicular to the wall or axis of the casting. It is characterized by solidification starting from the two sides of the casting in contact with the casting and gradually advancing toward the center until the end of solidification. . In fact, in the case that both the alloy and the casting mold have been determined, the order of solidification of the various parts of the casting and the solidification process may be due to the difference in the casting structure and the casting position, the introduction position of the gate, the setting of the appetite or the cold iron and the like. different. This is the direction of solidification of the casting
The study of the solidification direction of the casting is based on the longitudinal section of the casting, that is, the length of the casting or the section of the axis. When the adjacent parts of the casting are solidified in a certain order and direction, the solidification process is called sequential solidification; if the casting is adjacent to each part or part of the casting At the same time, the beginning and the end of the solidification time are the same or similar, and even the solidification process is completed at the same time. There is no successive difference and obvious directionality. It is called simultaneous solidification.
The solidification method and solidification direction are two different concepts, but they are related to each other.
Both are starting from the temperature field. The former studies the process and characteristics of the solidification of the casting from the surface to the center. The latter studies the overall solidification characteristics of the casting. In general, castings that tend to solidify on a layer-by-layer basis may achieve sequential solidification, tending to volume solidification or uniform solidification of thin-walled castings.
Fourth, the solidification method and solidification direction and the relationship between the quality of the casting 1. The relationship between the solidification method and the casting quality When the casting section temperature gradient has been determined, for example, in general casting conditions, pure metal, eutectic alloy and other crystallization temperature interval is very narrow Alloys such as low-carbon steel, aluminum bronze, etc., are often layer-by-layer solidification. In the liquid metal filling process, the metal is crusted in the flow path, the layers are thickened, the passage is smooth, the resistance is small, the flow velocity is large, and the solid phase content precipitated before flow is stopped due to the flow path obstruction is large, that is, released The latent heat of crystallization is long and the flow time is long. Therefore, the filling ability of layer-by-layer solidification is good.
During the layer-by-layer solidification, the solidification zone is narrow, the solidification frontier is smooth or serrated, the flow path for liquid feeding is short, the resistance is small, and the feeding is easy. In the solid-liquid zone, it is less likely to be separated by a feeding boundary. A small molten pool, which is isolated and not sourced, shrinks during the solidification process and can easily be replenished with liquids to become a concentrated shrinkage cavity. Concentrate shrinkage holes are in the final solidification of the castings. If they are properly positioned, the riser can exclude the shrinkage holes from the casting itself. When the late shrinkage of the casting is blocked and hot cracking occurs, the solidification area is narrow, the crack is close to the liquid phase, and the fluidity is also good. The crack is likely to heal by liquid refilling.
When the volume solidifies, at the front end of the liquid stream, the crystals are distributed over the entire section, the dendrite is developed, the feeding channel is long, the flow resistance is large, and the flow velocity is small, so the filling ability is poor.
The volumetric solidification shrinks loosely and has a wide area. It is difficult to eradicate under ordinary casting conditions and affects the compactness of the casting. The hot set cracking tendency of the volume solidified casting is also large, because the dendrite begins to shrink after reaching the skeleton. The greater the shrinkage, the greater the shrinkage resistance and the greater the risk of hot cracking. Due to the small amount of liquid phase in the late stage of solidification, they are clustered between dendrites and even divided into isolated baths. The resulting hot cracks are difficult to bridge by the liquid metal.
2. The relationship between the solidification direction and the quality of the casting In the region of the solidified casting or casting, there is always a large temperature gradient, and the solidification has successive points. The solidified portion can be obtained by feeding the adjacent solidified portion. Therefore, under the condition of sequential solidification, if the riser design is reasonable, the defects of shrinkage and shrinkage of the casting can be prevented, and the compactness of the casting is also good.
The disadvantage of the sequential solidification is that due to the large longitudinal temperature gradient of the casting and the solidification is different, the hot cracking defect may occur in the dangerous sections with relatively small strength and plasticity. If a large temperature gradient is maintained during cooling after solidification, stress or deformation will occur in the casting. In addition, risers, subsidies, and cold irons are needed to achieve sequential solidification, which increases the loss of metal materials and production costs.
The simultaneous solidification of the castings, the longitudinal temperature gradient is very small, adjacent parts or the whole casting, the solidification process is carried out simultaneously, the feeding conditions are poor, and the compactness of the castings is poor. The distribution of shrinkage varies with different solidification methods. When the volume is solidified, the cross-section of the casting will have shrinkage at various places. When solidified layer by layer, the shrinkage is mainly concentrated in the center of the cross section and is called axial shrinkage. Due to the uniform temperature throughout the casting at the same time, the risk of thermal cracking is small, and it is not easy to cause large stress and deformation. Under normal circumstances, at the same time the solidification is used even if there is no need for feeders or subsidies, thus saving metal materials and production costs.
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