1. Aluminum alloy castings structural reasons Aluminum alloy castings structural reasons Aluminum alloy castings structural reasons Aluminum alloy castings structural reasons due to the aluminum alloy castings section is too thick, resulting in the formation of poor feeding shrinkage. The thickness of the aluminum alloy castings is uneven, and it shrinks at the hot section of the wall thickness. The sand core with too small hole diameter is heated by high temperature liquid metal for a long time, which reduces the solidification rate of metal on the surface of the casting hole. At the same time, the sand core provides a channel for gas or atmospheric pressure, resulting in shrinkage and loosening of the hole wall. The small radius of concave fillet of aluminum alloy castings makes the heat transfer ability of molding sand at the sharp corner decrease and the solidification speed at the concave corner decrease. At the same time, because the molding sand at the sharp corner is heated strongly, the pressure of gas emission is high, the gas precipitated can infiltrate into the non-solidified liquid metal, resulting in the gas shrinkage of aluminum alloy castings. 2. The reason of smelting is the reason of smelting. The reason of smelting is that the gas content of liquid metal is too high, which leads to the precipitation of bubbles in the cooling process of aluminum alloy castings, and prevents the adjacent liquid metal from flowing to the place for feeding, resulting in shrinkage or reduction. When the carbon equivalent of gray cast iron is too low, the precipitation of eutectic graphite will be reduced, the expansion of graphitization will be reduced, the solidification shrinkage will be increased, and the flowability of molten iron will be decreased. To reduce the self filling ability of molten iron, aluminum alloy castings can be reduced or reduced easily. When the content of phosphorus or sulfur in molten iron is on the high side, phosphorus is an element that enlarges the solidification temperature range and forms a large number of low melting point phosphorus eutectic, which reduces the feeding capacity during solidification. Sulfur is a barrier to graphitization, and sulfur can also reduce the fluidity of molten iron. At the same time, the serious oxidation of molten iron also reduces the fluidity of liquid metal, resulting in shrinkage or reduction of aluminum alloy castings. When inoculant such as ferrosilicon is used to inoculate inoculant of inoculated cast iron or nodular cast iron before pouring, if inoculant is not good, a large amount of cementite will be precipitated during solidification of molten iron, which will increase solidification shrinkage and produce shrinkage or shrinkage.
There is only one reason for the shrinkage of die-casting holes in aluminum alloy, that is, the shrinkage due to phase transformation when the liquid phase changes to solid phase after the filling of the metal melt.
Therefore, for die-casting aluminum alloy, especially for heavy die-casting aluminum alloy, the shrinkage problem is inevitable and can not be solved.
2. the only way to solve the shrinkage defects of aluminum alloy die casting holes.
The shrinkage of aluminum alloy die casting hole can not be completely solved by the die casting process itself. To solve this problem thoroughly, we can only go beyond the process, or seek a solution from outside the system.
What is the solution?
From the point of view of technological principle, the solution to the shrinkage defect of aluminum alloy castings can only be carried out according to the technological idea of feeding.
3. ways to supplement two
There are two ways to compensate for aluminum alloy castings. One is natural feeding, the other is forced feeding.
Many people intuitively think that low-pressure casting can solve the shrinkage defects of aluminum alloy castings, but this is not the case. Using low-pressure casting technology does not mean that aluminum alloy castings can be solved. If there is no feeding process in the low pressure casting process system, the blank produced by this low pressure casting method may also have a shrinkage defect of 100%.
Because of the characteristics of the die casting process, it is difficult and complicated to set up a natural "sequential solidification" process. The most fundamental reason may be that the "sequential solidification" process requires a longer solidification time for aluminum alloy castings, which is somewhat inconsistent with the die casting process itself.
The biggest characteristic of forced solidification feeding is that the solidification time is short, generally only one fourth or less of the "sequential solidification". Therefore, on the basis of the die casting process system, the addition of mandatory feeding process measures is compatible with the characteristics of die casting process, which can solve the problem of shrinkage of die-casting aluminum alloy.
4. the two degree of compulsory shrinkage: extrusion and shrinkage.
There are two ways to achieve mandatory feeding of aluminium alloy castings. One is to basically eliminate the reduction of defects in aluminium alloy castings, and the other is to break grains or forged structures in the blank. If the two different degrees are to be expressed in different terms, the former can be used as "extrusion feeding". "Shrink" to express the latter, we can use "forging and feeding" to express.
One of the concepts to be fully aware of is that feeding is a direct means, and it cannot be accomplished indirectly.