Release date: 2016-01-28

  一、 casting alloy shrinkage. Shrinkage of the basic concept of liquid alloy when the temperature decreases, while the by liquid transition for solid, because the metal atomic short-range gradual and orderly transition for the long-range order, and hole decrease and disappear, usually occur volume reduction. After the solidification of the liquid alloy, the distance between the atoms decreases with the decrease of temperature, and the volume is reduced. In the process of casting alloys in the process of liquid, solidification and solid state cooling, the phenomenon of volume reduction due to the decrease of the temperature, the shrinkage of the casting alloy. Shrinkage is the basic cause of many defects, such as shrinkage cavity, shrinkage porosity, stress, deformation and crack, which is one of the important properties of casting alloys. It has a great influence on the casting (such as obtaining the geometry and size of the requirements of the compact and high quality castings). The volume change of the casting alloy from the liquid to the room temperature is indicated by the volume change of the casting alloy. The shrinkage of the alloy in the solid state, in addition to the volume change, the length of the variable to be expressed, known as the line contraction. Because in the design and manufacture of shape, the line is more meaningful contraction. The shrinkage rate of 1/3. alloy is three stages: the liquid phase, the solidification and contraction phase, the solid phase and the solid phase. (1) liquid contraction when liquid alloys from pouring temperature T pouring cooling to start solidification of liquidus temperature T liquid contraction, due to the alloy in the liquid state, so that the liquid contraction, decreased as the cavity surface. (2) solidification shrinkage for a certain temperature range of the alloy, by liquid transition to solid state, due to the alloy in the solidification state, it is referred to as the solidification contraction. The solidification shrinkage of this kind of alloy mainly includes the temperature decrease (Yu Hejin's crystallization temperature range) and the change of state (the volume change of the state change) two parts. For a small number of alloys and metals, the volume increase during solidification is caused by the shrinkage of the solid state. (Bi, Si, Bi-Si alloy and gray cast iron). Shrinkage and shrinkage of castings are the basic causes of shrinkage and porosity. (3) solid shrinkage when casting alloy from the solid phase line temperature T solid cooling to room temperature T ventricular contraction, due to the alloy in the solid state, it is referred to as the solid contraction. But in the actual production, the solid shrinkage is usually shown as the decrease of the shape of the casting, so the linear shrinkage is generally used. If the line of the alloy is not hindered by the external conditions of the casting mold, it is called free shrinkage. Negative, for the blocked line shrinkage. The linear shrinkage of the cast alloy has a direct effect on the dimensional accuracy of the casting, and it is the basic reason of the stress, crack and deformation of the casting. 2 the shrinkage rate of the casting line is discussed. The shrinkage rate of the casting alloy is only related to the chemical composition, the shrinkage coefficient, the change of the temperature and the volume change of the phase change. In the process of casting process design, taking into account the contraction, the size of the size should be enlarged, the size of the L model and the size of the casting L parts are as follows. F = l die - casting / L die x 100% castings casting shrinkage rate of not only with the factors of the alloy, but also with the characteristics of casting process, casting structure shape and alloy in smelting process in the quantity of dissolved gas and other factors.

  二、 casting in the shrinkage porosity and shrinkage porosity, shrinkage of the basic concept of casting in the cooling and solidification process, because of the alloy liquid contraction and solidification shrinkage, often in the final solidification of the 1 hole. Large and relatively concentrated holes are called shrinkage holes; small and scattered holes are called shrinkage holes. Shrinkage of the shape of irregular, rough surface, you can see the development of dendritic endings, so it can be significantly different from the pores. If there is shrinkage cavity and shrinkage porosity in the casting, the effective bearing area of the casting can be reduced on one hand, the stress concentration will be caused on the other hand, and the mechanical properties of the casting will be decreased obviously. At the same time, it also reduces the air tightness and physical and chemical properties of the castings. Especially for pressure parts, it is prone to leakage and make the casting scrap. 2 shrink pore formation shrinkage porosity formation. The basic reason and shrinkage as, mainly because the alloy crystallization temperature range is wide, developed dendrite, almost simultaneously liquid alloy solidification, liquid and solidification shrinkage of small, dispersing holes to external molten metal caused by. The tendency of shrinkage cavity and shrinkage and the composition of the alloy are regular. Directional solidification of the alloy tends to produce concentrated shrinkage; paste solidified alloy tends to produce shrinkage, the number of shrinkage and porosity can be converted to each other, but their total volume remains unchanged. 3. The effect of shrinkage hole, shrinkage factors and prevention measures of the size of casting alloy liquid contraction is bigger, shrink pore formation tendency is bigger; alloy crystallization temperature range is wide, the greater the solidification shrinkage, shrinkage porosity formation tendency. Who can make alloy decreases liquid and solidification shrinkage process measures (such as adjusting chemical composition, lower pouring temperature and slowing casting speed, casting the chilling ability increases, increases in casting solidification feeding capacity, promote solidification during graphitization) for grey cast iron, can decrease the shrinkage hole and shrinkage porosity. Alloy contraction and solidification characteristics in making correct casting process, casting in solidification process to establish good feeding conditions, as far as possible to shrink loose into shrinkage, and shrink holes to appear in the final position of solidification. To the casting in solidification process to establish good feeding conditions, mainly by controlling the solidification mode (the riser and chill with) which are in conformity with the directional solidification principle "or" simultaneous solidification principle ".