A Method to Solve the Porosity of Investment Casting

Abstract:Single-gate, top-injection flow casting, applying repeated use of medium-temperature mould material, using secondary dewaxing, to ensure that all the wax in the shell is removed, to prevent intrusive gases from being produced in the casting.

Keywords:Medium temperature mould material, secondary dewaxing, air holes

The casting porosity is divided into involved porosity, precipitated porosity and intrusive porosity. Precipitation porosity is formed by the liquid metal folding out of the gas; involved in the porosity is casting and pouring system in the atmosphere before pouring occupation, pouring is not smooth metal flow is likely to be the cavity gas involved in the formation of porosity; invasive porosity is flow injection, casting due to the liquid metal thermal effect of water evaporation, organic matter (mould material) combustion produces a large number of gases, when the gas pressure is greater than the liquid metal pressure, the gas will invade the liquid metal and the formation of porosity. When the gas pressure is greater than the liquid metal pressure, the gas will invade the liquid metal and the formation of porosity. In this paper, the melt mould castings due to medium temperature mould material residue and produce intrusive porosity causes and preventive measures to analyse.

I. Causes and analysis of porosity

1. Causes of porosity

Some of our castings using a single gate, top injection casting system, often appear in the casting process of porosity defects (see Figure 1), the problem occurs, we first consider for the molten steel oxidation and shell gas permeability is not good to produce porosity, and take the following measures: ensure that the return material is clean, no oil, no rust; ② to improve the burnout temperature and burnout time to ensure that the shell gas permeability; ③ to reduce the pouring speed well will be casting shells Tilt pouring to ensure that the gas in the steel to float: ④ shorten the high-temperature melting time and fully deoxidise the steel to avoid oxidation well reduce the gas content in the steel. After the test casting porosity defects still exist, there is no obvious improvement, and finally in the roasted shell to find the problem, the destruction of the roasted shell found that there are a lot of carbon dust inside the shell (see Figure 2).

2. Cause analysis

The purpose of top casting is mainly to ensure that the casting filling capacity, reduce the casting shrinkage problem, followed by the shell in the medium temperature mould material will not have a residual phenomenon, even if there is a slight presence of roasting will not appear as shown in Figure 2 large area of carbon grey, the casting using a single gate, the top casting casting system, the casting using high-pressure steam dewaxing. Wax residue is consistent with the height of the inner mouth of the shell inside a small amount of mould material can not be excluded as shown in Figure 3 (caused by the number of groups is not neat), the shell after roasting the shell inside the appearance of carbon ash (hydrogen compounds), when the high-temperature steel poured into the shell and carbon ash contact with a large number of gases, the formation of intrusive air holes.