I. The Difference between Porosity and Pores
Defect characteristics
Porosity is a fine and scattered porous defect, while porosity is a spherical porous defect with a smooth inner surface. From the perspective of appearance and form, the loose holes are distributed rather scattered and have no regular shape. The pores present a relatively regular spherical shape.
2. Causes of formation
Porosity occurs when the volume shrinkage of the alloy in both liquid and solid states is not replenished. During continuous casting, the residual melt between the crystalline frameworks in the transition zone of the ingot is separated. When the melt further crystallizes, the voids caused by volume shrinkage cannot be filled with new liquid, thus forming porosity. The main factors influencing the formation of porosity are the content of gas in the melt and the size, shape and structure of the transition zone in the ingot. Porosity is formed when gases that precipitate in the form of bubbles during the cooling and solidification of molten metal cannot escape from the liquid surface in time and remain in the ingot. For a gas to precipitate in the form of bubbles, three conditions must be met: First, the dissolved gas is in a supersaturated state; The second is that the sum of the partial pressures of all gases within the bubble is greater than the external pressure acting on the bubble. The third is the presence of bubble nuclei larger than the critical size.
3. Influencing factors
The formation of porosity is mainly influenced by the gas content in the melt and factors related to the transition zone. The higher the gas content in the melt, the greater the possibility of porosity formation. Widening the transition zone will increase the tendency to form looseness. The formation of pores is related to factors such as the supersaturated state of gas in the melt, the formation of bubble nuclei, and external pressure. There are a large number of non-metallic inclusions, crystals and bubbles that did not escape during refining in the melt, which is prone to form non-spontaneous bubble nuclei. The local hydrogen content supersaturation of the liquid at the crystallization front during the solidification process creates conditions for the formation of bubbles.
Ii. Measures to be taken during the casting process to avoid porosity and gas hole defects
1. Measures for loosening
Controlling the gas content in the melt: The gas content in the melt can be reduced through processes such as refining to alleviate the difficulty of feeding caused by gas precipitation. For instance, methods such as vacuum refining and gas blowing refining are adopted to remove gases from the melt. Optimize the size, shape and structure of the transition zone; Control the casting process parameters, such as cooling rate and pouring temperature, to avoid widening the transition zone. A reasonable cooling rate can make the crystallization of ingots more uniform and reduce the adverse effects of the transition zone. An appropriate pouring temperature helps ensure the fluidity of the melt and improve the feeding conditions.
2. Measures for pores
To prevent gas supersaturation, strictly control the quality of raw materials and reduce the gas content in them. During the smelting process, avoid excessive contact between the melt and gas-containing media such as air to prevent gases from dissolving and entering the melt. At the same time, the melting time and temperature should be reasonably controlled to avoid excessive dissolution of gases in the melt.
Reduce the probability of bubble formation
To reduce non-metallic inclusions and crystals in the melt, impurities in the melt can be removed through processes such as filtration. During the refining process, ensure that all bubbles are fully released to prevent any remaining bubbles from becoming bubble nuclei.
Promote the expulsion of bubbles
Increase the rising speed of the bubbles to enable them to be promptly discharged from the liquid surface. The rising speed of the bubbles can be made greater than the crystallization speed of the ingot by adjusting the crystallization rate of the ingot. The upward movement of bubbles can also be promoted through methods such as stirring. At the same time, reduce the adhesion force of the solid surface to bubbles, for example, improve the properties of the mold surface and reduce the retention of bubbles.
