Calcium-aluminum alloy can absorb a large amount of hydrogen gas during melting and casting, and continuously precipitates during cooling due to the decrease in solubility.

　　When casting calcium-aluminum alloys, there may be the occurrence of gas holes. What is the reason for this, and how can it be prevented? The author will analyze this for everyone, hoping it will be helpful to you. Let's take a look together. Due to the severe oxidation and gas absorption tendency of calcium-aluminum alloys, they come into direct contact with furnace gas or the external atmosphere during the smelting process. Therefore, if the control during the smelting process is slightly improper, calcium-aluminum alloys can easily absorb gas and form gas holes. The most common type is pinholes. Pinholes usually refer to precipitated gas holes smaller than 1mm in the casting, mostly round and unevenly distributed across the entire cross-section of the casting, particularly in the thicker sections and areas with slower cooling rates. Based on the distribution and morphological characteristics of the precipitated gas holes in calcium-aluminum alloys, pinholes can be divided into three categories:

　　(1) Dot pinholes: In low magnification tissue, pinholes appear as dots, with clear and non-continuous outlines, allowing for counting the number of pinholes per square centimeter and measuring their diameter. These pinholes can be easily distinguished from shrinkage holes and shrinkage porosity.

　　(2) Network pinholes: In low magnification tissue, pinholes are densely connected in a network, with a few larger holes, making it inconvenient to check the number of pinholes per unit area and difficult to measure the diameter of the pinholes.

　　(3) Comprehensive gas holes: This is an intermediate type between dot pinholes and network pinholes. From low magnification tissue, there are more large pinholes, but they are not dot-shaped; instead, they are polygonal. Production practices of calcium-aluminum alloys have confirmed that the main gas component forming gas holes due to gas absorption is hydrogen, and its occurrence does not follow a certain pattern. Often, all or most castings in a furnace batch exhibit pinhole phenomena; materials are no exception, as various compositions of calcium-aluminum alloys easily produce pinholes.

　　The formation of pinholes occurs when calcium-aluminum alloys absorb a large amount of hydrogen gas during smelting and pouring, and during cooling, hydrogen continuously precipitates due to the decrease in solubility. Some data indicate that the solubility of hydrogen dissolved in calcium-aluminum alloys increases with the rise in alloy liquid temperature and decreases with the drop in temperature. When transitioning from liquid to solid state, the solubility of hydrogen in calcium-aluminum alloys decreases by 19 times. Therefore, during the cooling and solidification process of the calcium-aluminum alloy liquid, at a certain moment, the hydrogen content exceeds its solubility and precipitates in the form of bubbles. The hydrogen bubbles formed due to supersaturated hydrogen, which do not have time to rise and be expelled, create small, dispersed gas holes during the solidification process, commonly referred to as pinholes. The degree of supersaturation reached before the formation of hydrogen bubbles is a function of the number of hydrogen bubble nuclei, while oxides and other inclusions act as nuclei for bubble formation.

　　Under general production conditions, especially in thick sand mold castings, it is very difficult to avoid the formation of pinholes. When smelting and pouring calcium-aluminum alloys in an atmosphere with high relative humidity, the pinholes in the castings are particularly severe. This is why we often wonder why there are fewer pinhole defects in calcium-aluminum castings during dry seasons compared to rainy and humid seasons.

　　Generally speaking, for calcium-aluminum alloys, if the crystallization temperature range is large, the probability of forming network pinholes is much greater. This is because under general casting production conditions, castings have a wide solidification temperature range, making it easy for calcium-aluminum alloys to form developed dendritic crystals. In the later stages of solidification, the residual aluminum liquid in the gaps of the dendritic crystals may be isolated from each other, existing in nearly closed small spaces. Due to the relatively small external atmospheric pressure and static pressure of the alloy liquid acting on them, when the residual aluminum liquid further cools and shrinks, it can create a certain degree of vacuum (i.e., the compensation shrinkage channel is blocked), causing the supersaturated hydrogen in the alloy to precipitate and form pinholes.