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Casting alloy contractionary impact on porosity shrinkage

by:BoQiao Machinery     2020-12-22
one The shrinkage of casting alloy 1. The basic concept of contraction liquid alloy and the liquid into a solid when the temperature falls, because the metal atoms by short-range order gradually into the remote and orderly, as well as the decrease of hole and disappeared, reduced in volume generally occur. After liquid alloy solidification, temperature continues to fall, to shorten the distance between atoms, also further reduce the volume. Casting alloy in the process of liquid, and solid state cooling solidification, due to lower temperature shrinkage phenomenon, called shrinkage of casting alloy. Shrinkage and many defects in castings, such as shrinkage cavity, shrinkage porosity, stress, deformation and crack, etc the basic cause, is one of the important casting alloy casting performance. It is for the castings ( With the geometric shape and size of meet the requirements, the density of high quality castings) There is a big impact. Casting alloy by liquid into normal temperature when the amount of volume change, called the volume contraction. Alloy in the solid state of contraction, besides said by the volume change, also can be said with the variable length, called line contraction. Because in the design and manufacture of shape, line contraction more meaningful. Line shrinkage rate generally is 1/3 of the volume shrinkage rate. Alloy in contraction has three stages: liquid contraction phase; Solidification shrinkage stage; Solid-state contraction phase. ( 1) Liquid contraction when liquid alloy from pouring temperature t water the liquid cooled to solidifies the liquidus temperature t of contraction, because of the alloy in a liquid state, therefore, referred to as liquid contraction, experience on the surface of the liquid inside the cavity is reduced. ( 2) Solidification shrinkage for alloy has a certain temperature range, from a liquid into a solid, because of the alloy in the frozen state, therefore, called the solidification shrinkage. This kind of alloy solidification contraction mainly includes the body temperature ( In the alloy crystallization temperature range) And state changes ( State change of volume change) Two parts. For a handful of alloy and metal, because of its solidified body shrinkage rate is negative, so the solidification occurs when volume increase. ( Bi、Si、Bi- Si alloy and cast iron) 。 Liquid and solidification contraction is basic reason of shrinkage cavity and shrinkage formation of castings. ( 3) Solid-state contraction from the solidus temperature t when casting alloy is cooled to room temperature t chamber contraction, due to the alloy in the solid state, so called solid-state contraction. But in actual production, the solid-state contraction is often characterized by the decrease of the casting dimension, so the general line shrinkage rate are used to indicate. If line contraction of alloy is hampered by mould external conditions, referred to as the contract freely. Negative, as blocked line contraction. Line contraction of casting alloy not only have a direct impact on the dimension precision of casting, and the castings produced in the basic cause of stress, crack and deformation. 2. Casting line shrinkage has been discussed above the alloy casting shrinkage rate and the chemical composition of alloy, the contraction coefficient, temperature change and phase change volume change and other factors. In the casting process design, considering the contraction, need to amplify the shape size, shape size L exist the following relation between the mould and casting size L. On = L die - L/L casting mould & times; 100% of the casting casting shrinkage rate is not only related to the factors of alloy used, but also with casting process characteristics, structure shape and alloy castings are related to the amount of dissolved gas in the process of smelting, etc. Second, the shrinkage cavity and shrinkage of casting 1. Shrinkage cavity, shrinkage porosity in the process of cooling solidification, the basic concept of casting due to the liquid contraction of alloy and solidification shrinkage, often appear in the last place the casting solidification holes. Large volume and relatively concentrated hole called shrinkage cavity; Small and scattered holes called shrinkage cavity. The irregular shape of shrinkage cavity, the surface is rough, you can see developed dentrite endings, it can clearly distinguish from the pores. If there is a shrinkage cavity, shrinkage porosity in the casting, on the one hand can make the effective bearing area of the casting is reduced, on the other hand caused by stress concentration, and the mechanical properties of castings to be significantly reduced. At the same time also reduce the air tightness and physical and chemical properties of casting. Especially for the pressure parts, prone to leakage and make the casting scrap. 2. The basic reasons for the formation of shrinkage cavity formation of shrinkage and shrinkage cavity, mainly due to the crystallization temperature range wide, alloy dendrites developed, liquid alloy solidification, almost at the same time liquid formation and solidification shrinkage of small, scattered holes without external metal caused by the supplement. Formed in the casting shrinkage and shrinkage tendency between alloy elements and has certain regularity. Directional solidification of alloys tend to produce concentrated shrinkage cavity; Paste solidification of alloys tend to produce shrinkage, the shrinkage cavity and the amount of shrinkage can be mutual conversion, but their total volume basic remain unchanged. 3. Factors affecting the size of shrinkage cavity, shrinkage porosity and prevention measures of casting alloy liquid contraction is larger, the greater the tendency of shrinkage cavity formation; The more alloy crystallization temperature range wide, the greater the solidification shrinkage, the shrinkage tendency to form.
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