After pouring the liquid metal into the mold, the temperature of the metal drops due to the heat dissipation of the mold, the distance between the atoms decreases, and the volume of the liquid metal decreases. As the temperature continues to decrease, the liquid metal solidifies, atoms change from near-range order to long-range order, change from liquid to solid state occurs, and the distance between atoms is further shortened. After the metal is solidified, the interatomic distance is shortened when the solid is cooled. The volume reduction that occurs during the cooling of a casting in its liquid, solidified, and solid state is called shrinkage. Therefore, shrinkage is the physical property of the casting alloy itself
With the decrease of the temperature and the change of the condensed state, the mechanical behavior of the casting material also changes, and produces different reflections on different casting process conditions, such as the plasticity and strength of the casting, and the casting due to different degrees of shrinkage resistance. Internal stress and deformation
The shrinkage property is one of the casting properties of the cast alloy. The alloy shrinkage is an important reason for the occurrence of deficiencies such as necking, hot cracking, stress, deformation, and cold cracking in the casting.
Therefore, in order to obtain high-quality castings with accurate dimensions and internal compactness, it is necessary to understand the shrinkage characteristics.
The volume of any object is related to its temperature and the pressure applied to it. In general casting conditions, the change in pressure is negligible, and the change in the size of the casting depends only on changes in temperature (not considering the state of matter and allotropic changes). The volume change of the metal from liquid to room temperature is called Body shrinkage; The amount of line size change in the solid state of the metal is called linear shrinkage; in practice, the relative shrinkage is usually expressed as the shrinkage property of the metal. This relative shrinkage is called the shrinkage rate.
After any kind of liquid metal is injected into the mold, three interconnected shrinkage stages are required to cool the casting temperature to room temperature (Fig. 3 - Coin: 1 liquid contraction stage (I) 12 solidification shrink stage (II), 3 solid state shrinkage Stage (III)
Fig. 3-4 I. At the moment when the liquid shrinks and fills the mold, the body contraction of the liquid metal from the temperature at which it has cooled to the liquidus temperature at which it starts to solidify is a liquid contraction. Since the metal is in the liquid state at this stage, the volume reduction only shows a decrease in the liquid level in the cavity. There are many factors that affect the liquid shrinkage, such as alloy composition, temperature, gas and inclusion content, etc.
Second, the shrinkage of the solidification shrinkage of the molten metal during the solidification phase is the solidification shrinkage. For pure metals and eutectic alloys, the bulk shrinkage during solidification is only due to the change of state, and is independent of temperature, so it has a constant value. Alloys with a certain crystallization temperature range change from a liquid state to a solid state. The shrinkage rate is related to the change in volume when the state is changed, and to the crystallization temperature range. In the solid shrinkage phase, the casting shows a reduction in the dimensions of the wire in all directions. Therefore, this stage has the greatest influence on the accuracy of the shape and size of the casting.
After the liquid metal is injected into the mold, a solidified layer is first formed on the surface, and when the metal in the liquid state is cooled in the housing, the volume is reduced due to the liquid contraction and solidification shrinkage. If the reduced volume does not result in the replenishment of the foreign metal, a hole or shrinkage that concentrates somewhere or in the casting is formed in the casting. Therefore, liquid shrinkage and solidification shrinkage are the basic causes of shrinkage and shrinkage in castings.
In some alloys, the volume not only does not shrink during solidification, but instead expands, such as certain Ga alloys, Bi-Sb alloys, so the solidification shrinkage rate is negative
Third, the solid shrinkage alloy solid shrinkage refers to the alloy from the solidus temperature cooling to room temperature shrinkage, the performance of a three-dimensional shrinkage, that is, the three-directional shrinkage
For pure metals and eutectic alloys, the line shrinkage begins after the metal has completely solidified. For alloys with a certain crystallization temperature range, when the temperature of the liquid metal is slightly lower than the liquidus temperature, the crystallization begins. However, due to the relatively low dendrites and the inability to form a continuous skeleton, the liquid metal shrinkage property still appears. As the temperature continues to decrease, the number of dendrites increases and they are connected to each other to form a continuous skeleton. The alloy then begins to exhibit a solid-state nature, that is, the starting line shrinks. For alloys with a crystallization temperature range, the linear shrinkage does not start after it has completely solidified, but starts at a certain temperature in the crystallization temperature range, which is an important concept for the formation mechanism of thermal cracking in castings.
The total volume shrinkage of the alloy is the sum of the above three stages of shrinkage. Liquid shrinkage and solidification shrinkage can cause a decrease in the liquid level in the cavity, which is manifested by the shrinkage of the volume of the alloy, which is represented by the shrinkage of the body. They are the basic cause of shrinkage and shrinkage in castings. Solid shrinkage is generally expressed intuitively as a reduction in the size of the casting, which is often expressed as the shrinkage of the wire. It is the basic cause of the internal stress, deformation, and cracking of the casting.
Fourth, the resistance of the casting shrinkage The above analysis of the contraction of the alloy, only involves the alloy composition, temperature and other factors on the impact of the contraction, did not consider the shrinkage encountered in the process of various obstacles, this kind of contraction is called free shrinkage. In fact, when the casting shrinks in the mold, it is subject to various obstacles so that the shrinkage cannot be freely performed. At this time, the resulting shrinkage is called blocked shrinkage. The blocked shrinkage rate is always less than the free shrinkage rate, cracks will appear when the resistance is greater
Due to differences in structure, wall thickness, and mold environment, different parts of the casting will not have the same state and shrinkage process. Even if the same part, the surface and the center of the casting are different, so that the shrinkage of each part is affected by each other. contain. In addition, due to the contact between the casting and the mold, the casting may be hindered by the cavity surface and the core.
Resistance to casting shrinkage is as follows
1 frictional resistance. The resistance created by the relative movement between the surface of the casting and the surface of the cavity when the casting shrinks. The amount of frictional resistance is related to the weight of the casting and the smoothness of the cavity surface. When the cavity or sand core has a smooth surface or a smooth coating or dressing, the frictional resistance is small and can be ignored.
2 thermal resistance. Due to the different temperatures of the various parts of the casting, the shrinkage is not completely synchronized, and the resistance to each other is constrained by each other when shrinking. Thermal resistance is related to casting structure, temperature distribution and material properties
3 mechanical resistance. Castings are subjected to resistance from walls, cores, etc.
The compactness, strength, and concession of molds and cores, the position of box stops and cores, and the thickness or length of castings all affect the size of mechanical resistance.
In actual production, the solidification process of castings is generally subjected to a variety of shrinkage resistance at the same time, and the determination of the size is difficult. All kinds of resistance are not harmful to the formation of the castings. Sometimes the rational use of resistance to design the casting process can improve the quality of castings. Specific examples are described in the casting process design at the back of this book.
V. Factors Influencing Alloy Shrinkage Alloy shrinkage is related to its chemical composition, casting temperature, casting structure and casting conditions
1 chemical composition. As the carbon content of carbon steel increases, the solidification temperature range expands and the shrinkage increases. In grey cast iron, C and Si are elements that promote graphitization. When the content is increased or the carbon is more likely to exist in the form of graphite, the shrinkage is reduced. Due to the small graphite density and large specific volume, the gray cast iron is offset. Partial contraction reduces the total shrinkage, while the inhibition of graphitized elements increases the shrinkage. For example, So, different alloys have different chemical compositions and different shrinkages. The shrinkage of several cast alloys is given in Table 3-1 o Table 3-1 2 Pouring temperature. The higher the pouring temperature is, the greater the degree of superheat is, the more the liquid shrinkage increases, and the total shrinkage of the alloy increases.
3 mold conditions and casting structure. Castings are shrinking rather than shrinking in the mold, and their resistance comes from the mold and the core. The wall thickness of the casting is different, and the position of the wall in the type is different. The cooling rate is also different. During the condensation, resistance of each part of the casting will also be generated. These will affect the actual shrinkage of the alloy
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