The causes and considerations of porosity defects in precision castings

The causes and considerations of porosity defects in precision castings

The occurrence of casting defects in precision castings is not accidental, and there must be an inevitable cause. Therefore, when encountering casting defects, we must look at the essence through the phenomenon and identify the fundamental reasons behind it, in order to thoroughly eliminate the obstacles on the production path of castings. Today, precision casting manufacturer Tianyao Metal will analyze the cause of porosity in a casting, identify the root cause of defects, and attempt to completely eliminate future quality issues in the casting.

Precision castings use a single sprue to form a tree, with three pieces in each group. After pouring, 40 pieces were subjected to radiographic testing, leaving six pieces with a qualification rate of only 15%, while the rest were all porosity. The position is at the top of the casting, which is equipped with an exhaust groove, and the position of the air hole is at the bottom of the exhaust groove. According to common sense, for medium frequency furnace pouring, the pouring system of the casting should be designed with an exhaust groove to prevent the occurrence of porosity defects inside the casting. This product is also designed with an exhaust groove, why does it still produce porosity defects?

Firstly, we will qualitatively characterize this porosity defect. This porosity defect is located inside the casting, and the casting adopts a side injection pouring system with an exhaust groove at the top. According to convention, the probability of porosity in the casting is very low. Furthermore, based on our previous analysis of pore defects, it can be concluded that entangled pores generally exhibit as individual, large pores. And dispersed pores, especially those located under the casting skin, are generally pores from the molten metal. In this casting example, the porosity is located inside the casting and at the root of the exhaust groove. Therefore, we qualitatively conclude that this porosity comes from the molten metal and is caused by unclean raw materials or inadequate deoxidation treatment.

Regarding the need for clean treatment and thorough deoxidation before remelting of metal materials, let's discuss why castings can still produce pores even after designing exhaust grooves.

1、 The pores only escape to the top of the casting without entering the exhaust groove, which first indicates that the gas escape ability is insufficient, that is, the pouring temperature may be too low. Because this piece is relatively thick, in order to avoid shrinkage of the casting after pouring, the pouring temperature is relatively low. In fact, I would like to say that according to common sense, a high pouring temperature will cause shrinkage in the casting. However, if the pouring temperature is too low, shrinkage will still occur. The reason is that the fluidity of the metal liquid decreases, and the filling and shrinking force decreases. This gas is also the same. If the temperature is low, its escape rate will be greatly reduced. It is likely that it will soon reach the exhaust groove, and the metal liquid solidifies, leaving the gas inside the casting, resulting in a failure of the quality of the casting.

2、 I have another opinion on the design of the exhaust groove for this casting. The function of the exhaust groove is to exhaust and collect gas, so its design purpose is to allow gas to gather in a small range. However, the designer's design of the exhaust groove in the picture undoubtedly provides more places for gas escape, which also increases the range of gas hole defects. Therefore, I think this design of the exhaust groove seems inappropriate.

Based on the above views, I think the process of this product should be adjusted appropriately: first, the pouring temperature should be appropriately increased, such as adjusting from the original 1600 ℃ to 1610-1620 ℃, so that the gas has the power to escape; Secondly, the design of the exhaust groove should be small and high, because the gas is designed to go higher. You can take a look at the exhaust groove design of this casting. The gas really runs into the exhaust groove, and the exhaust groove plays its due role.

Precision machining