When the concentration of gas in metal exceeds its solubility, or when the invading gas is not dissolved by the metal, it will exist in the liquid metal as bubbles in molecular state. If the bubbles do not have time to float up and be discharged before the solidification is completed, holes will be formed in the metal. Such holes caused by the aggregation of gas molecules are called pores.
The general characteristics of pores are: holes are formed on the surface or inside of the casting, the hole walls are smooth, and they have metallic luster or oxidation color.
Due to the different sources and causes of formation of gas, the manifestations of pores are also different. Generally, casting pores are divided into three types: precipitation pores, intrusion pores and reaction pores.
1. Precipitation pores
During the metal smelting process, gas is dissolved in the molten metal. When the temperature of the molten metal decreases and during the solidification process, the gas solubility decreases, resulting in supersaturated precipitation and growth of gas, forming bubbles that cannot be discharged and generating pores. Such pores are called precipitation pores.
The characteristics of precipitation pores are: dispersed distribution on the cross section of the casting, especially in the last solidification part, near the riser and the center of the hot spot, the shape is spherical, polygonal, intermittent crack or mixed.
Precipitation pores are closely related to the smelting quality of the metal. They often occur in all or most castings cast in the same furnace or the same package of molten metal. Aluminum alloy castings and steel castings are more likely to have such pores. After pouring, the metal liquid level in the cavity often rises. Precipitation pores are mainly hydrogen pores and nitrogen pores.
2. Intrusion pores
Pores formed by the gas generated by the high temperature of the liquid metal in the mold and core intruding into the metal, and pores formed by the gas drawn into the metal liquid during the filling process are called intrusion pores.
The characteristics of intrusion pores are: less in number, larger in size, mostly produced in certain parts of the outer surface of the casting, pear-shaped or spherical, the small head of the pore often points to the gas invasion part, and the pore surface is smooth. Because the density of bubbles is small, they will float in the molten metal and move with the flow of the molten metal, so they are rarely found in the lower part of the casting.
The main sources of intrusive gas are as follows:
1) The gas generated by the vaporization, decomposition or combustion of water, additives or resin binders in the mold (core) under the action of high-temperature molten metal invades the molten metal. Most of the intrusive pores are caused by this reason.
2) The design of the pouring system is unreasonable, and the gas is drawn into the molten metal by the gate and cavity during the pouring process.
3) The gas generated by the interaction between the molten metal and the moisture and oxide scale on the surface of the cold iron, core support, etc. invades the molten metal.
According to statistics, the castings scrapped due to pores in wet casting account for about half of the total scrap volume, and the intrusive pores account for about 50% of the pore defects.
3. Reaction pores
The pores produced by the chemical reaction between the molten metal and the mold, slag or certain elements and compounds inside the molten metal to produce gas and form bubbles are called reaction pores.
The pores generated by the reaction between metal and external factors such as sand molds, sand cores, and chills are usually distributed 1~3mm below the surface of the casting. After cleaning or machining, they will be clearly revealed. The shape is spherical or pear-shaped, with a pore diameter of 1~3mm, but most of them are long strips perpendicular to the surface of the casting, and the depth can reach 10mm. This type of pore is also called subcutaneous pores or subcutaneous pinholes.
The reaction between chemical elements inside the metal or between components and non-metallic inclusions will also produce reactive pores, which are generally honeycomb-shaped and evenly distributed in pear or pellet shapes. This type of reactive pore usually appears on the bottom surface of thin-walled castings or the upper surface of thick-walled castings, and is more common near the sand-adhering parts and pouring risers of the castings.
Post time: Apr-30-2025