in aluminum alloy die casting production of the production of workpiece often due to the existence of pores and lead to scrap, but there are many reasons for porosity, in the solution to this product quality problem is often unable to start, how to quickly and correctly take measures to reduce the porosity caused by scrap rate, this is the aluminum alloy die casting manufacturers concerned.
first, aluminum alloy die casting pores in the classification
in aluminum alloy die casting production, people often call the holes of the product generally called the pores, the pores produced by what kind of pores, is not known in detail, here we put the pores do a simple classification:
1. pores produced by poor quality of refining and degassing
in aluminum alloy die casting production, the pouring temperature of molten liquid aluminum is usually between 610oc and 660oc. at this temperature, a large amount of gas (mainly hydrogen) is dissolved in liquid aluminum alloy. the solubility of hydrogen aluminum alloy is closely related to the temperature of aluminum alloy, and about 17.25pxj/100g in liquid aluminum alloy at 660oc. at about 660oc, it is only 0.036cm3/100g in solid aluminum alloy. at this time, the hydrogen content in liquid aluminum is about 19-20 times that in solid. so when the aluminum alloy solidifies, there is a lot of hydrogen produced in the form of bubbles in the aluminum die casting.
reduce the aluminum water gas content, prevent a large number of gas in aluminum alloy solidification precipitation and porosity, this is the purpose of aluminum alloy smelting refining degassing process. if the content of the gas in the liquid aluminum is reduced, the amount of gas precipitated during solidification will be reduced, resulting in fewer bubbles, and significantly reduced. therefore, aluminum alloy refining is a very important means of technology, refining quality is good, there will be fewer pores, poor refining quality, there will be more pores. measures to ensure the quality of refining is good refining agent, refining agent is good about 660 oc can react to produce bubbles, the bubbles generated is not too sharp, but even have bubbles, by physical adsorption, the bubbles and good contact with aluminum liquid, a very long time, generally have 6 to 8 minutes of bubbling.
when aluminum alloy is cooled to 300oc, the solubility of hydrogen in aluminum alloy is only 0.001cm3/100g below, at this time only 1/700 when it is liquid. the pores produced by hydrogen precipitation after solidification are dispersed, small pinholes, which do not affect air leakage and processing surface, and the naked eye can not see.
the bubbles produced by hydrogen precipitation during the solidification of liquid aluminum are relatively large, mostly in the final solidification part of liquid aluminum, although also scattered, but these bubbles often lead to leakage. serious often results in the scrap of the workpiece.
2, due to poor exhaust of the air hole
in aluminum alloy die casting, because of the exhaust channel of the die is not smooth, the die exhaust design structure is bad, the gas in the cavity of die casting can not be completely smoothly discharged, resulting in the existence of pores in some fixed parts of the product. the pores formed by the gas in the mold cavity are sometimes large and sometimes small. the inner wall of the pores is the oxidation color of aluminum and air, which is different from the pores produced by hydrogen precipitation. the inner wall of the pores produced by hydrogen precipitation is not as smooth as the air holes, and there is no oxidation color, but the inner wall of the gray light.
for the air holes caused by poor exhaust, the exhaust channel of the mold should be improved, and the residual aluminum skin on the exhaust channel of the mold can be solved in time.
3, due to improper die casting parameters caused by the air hole
in the die casting production die casting parameters are not chosen properly, aluminum water die casting filling speed is too fast, so that the gas in the cavity can not be completely timely and stable extrusion cavity, and the liquid flow of liquid aluminum is involved in the liquid aluminum, because of the rapid cooling of the surface of aluminum alloy, wrapped in the solidified aluminum alloy shell, can not be discharged to form a larger hole. such pores are often under the surface of the workpiece, the aluminum water inlet is less than the final confluence, pear-shaped or elliptical, and more and more large in the final solidification.
for this type of porosity should be adjusted filling speed, so that the aluminum alloy liquid flow smoothly advance, do not produce high-speed gas.
4, aluminum alloy shrinkage porosity and shrinkage
aluminum alloy with other materials, in the solidification of shrinkage, the higher the pouring temperature of aluminum alloy, the greater the shrinkage, a single volume shrinkage pores exist in the final solidification part of the alloy. irregular shape, severe mesh. in the product, it exists at the same time with the pores precipitated by hydrogen during solidification. there are contraction pores around the hydrogen precipitated pores or coils, and filamentous or reticular pores around the bubbles extend to the outside.
for this kind of porosity, we should start to solve the casting temperature, and try to reduce the casting temperature of molten aluminum when the casting process conditions allow. this can reduce the volume shrinkage of the casting, reduce the shrinkage porosity and shrinkage porosity.
if such pores often appear in the same part, you can consider increasing core pulling or cold iron, so that it changes the final solidification position, to solve the problem of leakage and defects.
5, because of the product wall thickness difference is too large and caused by the air hole
product shape often wall thickness difference is too large, often can not change the shape of the product, in the center of the wall thickness is the final solidification of aluminum water, but also the most prone to produce pores, this wall thickness of the pores is a mixture of pores and shrinkage pores, not general measures can prevent.
the shape of the product in the design should be considered to reduce the uneven wall thickness, or too thick problem, adopt hollow structure, in the mold design should consider the addition of core pulling or cold iron, or water cooling, or increase the cooling speed of the mold here.
in the production of die casting, pay attention to the amount of undercooling in this part and reduce the pouring temperature appropriately.
