Shrinkage of cast alloy
1. The basic concept of contraction
When the liquid alloy changes from a liquid state to a solid state when the temperature drops, the volume reduction is generally caused by the gradual transformation of the metal atoms from the short-range order to the remote order, and the reduction and disappearance of the holes. After the liquid alloy solidifies, as the temperature continues to decrease, the distance between atoms is shortened and the volume is further reduced. In the process of liquid, solidification and solid state cooling of the cast alloy, the volume reduction occurs due to the decrease of temperature, which is called the shrinkage of the cast alloy.
Shrinkage is one of the main causes of many defects in castings, such as shrinkage, shrinkage, stress, deformation, and cracking, and is one of the important casting properties of cast alloys. It has a great influence on castings, such as obtaining the required geometry and dimensions, and dense, high-quality castings.
The casting alloy is expressed by the volume change amount when the liquid state is changed to the normal temperature, and is called volume shrinkage. The shrinkage of the alloy in the solid state, in addition to the volume change amount, can also be expressed by the length of the variable, called line shrinkage. Because the line shrinkage makes more sense when designing and manufacturing the pattern. The line shrinkage is generally 1/3 of the body shrinkage. The alloy undergoes three stages of shrinkage: the liquid shrinkage stage; the solidification shrinkage stage; and the solid state shrinkage stage.
(1) Liquid shrinkage When the liquid alloy is cooled from the pouring temperature t to the liquidus temperature t liquid which starts to solidify, since the alloy is in a liquid state, it is called liquid shrinkage, which is manifested as a decrease in the liquid level in the cavity.
(2) Solidification shrinkage When an alloy having a certain temperature range is converted from a liquid state to a solid state, since the alloy is in a solidified state, it is called solidification shrinkage. The solidification shrinkage of such alloys mainly includes two parts: temperature reduction (related to the crystallization temperature range of the alloy) and state change (volume change when the state changes). For a small number of alloys and metals, since the solidification rate is negative, the volume increases when solidified. (Bi, Si, Bi-Si alloy and gray cast iron).
Liquid shrinkage and solidification shrinkage are the basic causes of shrinkage and shrinkage in castings.
However, in actual production, since the solid state shrinkage tends to be a reduction in the outer dimensions of the casting, it is generally expressed by the linear shrinkage ratio.
If the linear shrinkage of the alloy is not hindered by the external conditions of the mold, it is called free shrinkage. Negative, shrinks the blocked line.
The linear shrinkage of the cast alloy not only has a direct influence on the dimensional accuracy of the casting, but also is the basic cause of stress, crack and deformation in the casting.
2. Casting line shrinkage
The shrinkage of the cast alloy discussed above is only related to the chemical composition, shrinkage factor, temperature change, and volume change during phase change. In the design of the casting process, in consideration of shrinkage, it is necessary to enlarge the size of the pattern, and the following relationship exists between the pattern size L mold and the casting size L piece. £=L mode-L casting/L mode×100%
The casting shrinkage of the casting is not only related to the factors of the alloy used, but also related to the characteristics of the casting process, the shape of the casting, and the amount of dissolved gas in the smelting process.
Second, shrinkage holes and shrinkage in castings
1. The basic concept of shrinkage and shrinkage
During the cooling and solidification of the casting, due to the liquid shrinkage and solidification shrinkage of the alloy, holes are often found in the final solidification of the casting. Large and relatively concentrated holes are called shrinkage holes; small and scattered holes are called shrinkage holes. The shape of the shrinkage hole is irregular, the surface is rough, and the developed dendrite tip can be seen, so it can be clearly distinguished from the pore.
If there are shrinkage holes and shrinkage in the castings, on the one hand, the effective bearing area of the castings will be reduced, and on the other hand, the stress concentration will be caused, and the mechanical properties of the castings will be significantly reduced. It also reduces the airtightness and physicochemical properties of the casting. Especially for pressure-resistant parts, leakage is likely to occur and the casting is scrapped.
2 formation of shrinkage holes
The basic reason for shrinkage formation is the same as that of shrinkage hole. It is mainly due to the wide crystallization temperature range of the alloy, the development of dendrites, the almost simultaneous solidification of the alloy liquid, and the small, dispersed pores formed by liquid and solidification shrinkage cannot be supplemented by external molten metal. And caused.
The tendency to form shrinkage hole and shrinkage in the casting has a certain regularity with the composition of the alloy. Directionally solidified alloys tend to produce concentrated shrinkage hole; paste-solidified alloys tend to produce shrinkage, and the number of shrinkage and shrinkage can be converted to each other, but their total volume remains substantially unchanged.
3. Factors affecting shrinkage and shrinkage and prevention measures
The greater the liquid shrinkage of the cast alloy, the greater the tendency for the formation of shrinkage hole; the wider the crystallization temperature range of the alloy, the greater the solidification shrinkage, the greater the tendency for shrinkage to form. Any measure that promotes the reduction of liquid and solidification shrinkage of the alloy (such as adjusting the chemical composition, lowering the casting temperature and slowing the casting speed, increasing the chilling capacity of the casting, and increasing the feeding capacity during the solidification of the casting. Castings can promote graphitization during solidification, etc., all of which are advantageous for reducing the formation of shrinkage hole and shrinkage.
A correct casting process is established for the shrinkage and solidification characteristics of the alloy, so that the casting can establish a good feeding condition during the solidification process, and the shrinkage is converted into a shrinkage hole as much as possible, and the shrinkage hole appears in the final solidification position.
In order to establish a good feeding condition for the casting during the solidification process, it is mainly controlled by the solidification method of the casting (using the setting of the riser and the cold iron) to conform to the "directional solidification principle" or the "simultaneous solidification principle".