Reasons for Pores on Aluminum Alloy Die Casting
Posted: 07/11/2016 04:57:44  Hits: 20
Aluminum alloy die casting is scraped because of pores existing. There are various of reasons causing pores. However, figuring out the product quality is quite difficult. How to quickly and correctly take measures for reducing rejection rate due to pores attracts attention from all of aluminum alloy die casting manufacturers.

During production of aluminum alloy die casting, reasons causing pores have the following categories:

1. Caused by poor degasification during refining
During the production of aluminum alloy die casting, the casting temperature of molten aluminum is general 610~660℃. Under such temperature, molten aluminum dissolves with amount of gas (hydrogen mainly). Solubility of hydrogen has closed relationship with temperature of aluminum alloy. In the molten aluminum whose temperature is about 660℃, dissolution of hydrogen is 0.69cm3/100g while dissolution in solid aluminum alloy is 0.036cm3/100g. Then, hydrogen content in liquid aluminum is about 19~20 times of that in solid aluminum. Therefore, when aluminum alloy solidifying, plenty of hydrogen separates out as form of bubble and exist in aluminum die casting. 

Reducing air content in molt aluminum can prevent large numbers of gas from separating out and forming pores when aluminum alloy solidifies. That is the objective of refining degasification during aluminum alloy smelting. If the gas content in molten aluminum decreases, separating gas content is decreased and caused bubble is also decreased significantly. So, refining of aluminum alloy is very important technological mean. Good refining quality, pores must be few while poor refining quality, pores must be many. Measure ensuring refining quality is choosing good refining agent. Good refining agent can react and cause bubbles under temperature about 660℃. Caused bubble is not drastic. The bubbles are caused evenly and constantly. Through physical absorption, these bubbles fully contact with molten aluminum, absorbing hydrogen contained in molten aluminum and take it out of aluminum. So, bubbling time should not be too short. Generally, 6~8 min is appropriate.

When aluminum alloy is cooled to 300℃, dissolution of hydrogen in aluminum alloy is below than 0.001cm3/100g, only 1/700 of that in liquid aluminum. The pores caused by hydrogen separating out after solidification are dispersive tiny pinholes, which cannot affect air leakage and processing surface. And those pores also cannot basically be seen by using naked eyes. 

When molten aluminum solidifies, caused bubbles are larger due to hydrogen separate out, mainly concentrating in finally solidified center of molten aluminum. Although those bubbles are also dispersive, they usually result in leaks, or even causing scrap more seriously. 

2. Caused by poor venting
In aluminum alloy die casting, gas inside cavity cannot be ejected completely and smoothly, which causes that pores exist in some fixed parts. The pores caused by gas inside cavity of mold may be large or small. Inner wall of pores shows oxidation that aluminum oxidizes with air. Different from pores caused by hydrogen separating out, inner wall of pores caused by hydrogen separating out is not as smooth as that of ventage. It is grey light without oxidation tint. For pores caused by poor venting, exhaust passage of mold should be improved and clean up remained aluminum sheet on exhaust passage. 

3. Caused by air entrapment due to improper die casting parameters
Due to improper die casting parameters, mold filling velocity of molten aluminum die casting is too fast so that air in cavity cannot be extruded out from cavity completely, timely and steadily while it was involved into molten aluminum by liquid flow of molten aluminum. Because of rapid cooling on aluminum alloy surface, air is packages inside solidified aluminum alloy shell, which cannot be discharged, forming larger pores. Those pores are under surface of workpiece. For such pores, mold filling velocity should be adjusted, making aluminum alloy liquid advance steadily, without causing high speed flowing and entrapment.

4. Shrinkage porosity of aluminum alloy
Just as other materials, aluminum alloy shrinks during solidification. The higher pouring temperature of aluminum alloy is, the larger shrinkage is. Pores solely caused by volume shrinking exist in finally solidified parts of alloy, showing irregular shape or net more seriously. In most cases, the pores coexist with pores caused by hydrogen during solidification. There are shrinkage porosities around the pores caused by hydrogen separating out or volume porosity. There are filiform or net pores to the outside around the bubbles.

For such pores, pouring temperature should be started. Permitted by die casting technological conditions, pouring temperature during die casting should be reduced as much as possible, which can reduce volume shrinkage of castings, decreasing shrinkage porosity and loosen. If the pores usually appear in hearing parts, increasing loose core or chiller can be considered, changing the final solidified parts and figuring out leakage.

5. Caused by oversize wall thickness
The center of wall thickness is the final solidified site, also the part which causes pores the most easily. Pores in wall thickness are mixture of precipitation pores and shrinkage pores, which cannot be prevented by using general measures. 

When designing shape of product, uneven or oversize wall thickness should be considered to reduce as much as possible. Adopt hollow structure. When designing mold, increasing loose core, chiller, water cooling or cooling velocity on the mold should be considered. During die casting, cold quantity on large parts of wall thickness should be noticed. Reduce pouring temperature appropriately.

Reasons Causing Pores on Aluminum Alloy Die Casting
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