During the solidification process of a casting, there are generally three areas on its cross section, namely the solid area, the solidification area, and the liquid area.
The solidification zone is the area where “solid and liquid coexist” between the liquid zone and the solid zone. Its width is called the solidification zone width. The width of the solidification zone has a great influence on the quality of the casting. The solidification method of the casting is based on the width of the solidification zone presented on the cross section of the casting, and is divided into layer-by-layer solidification, paste solidification, and intermediate solidification.
Let’s take a look at the characteristics of solidification methods such as layer-by-layer solidification and paste solidification.
Layer-by-layer solidification: When the width of the solidification zone is very narrow, it belongs to the layer-by-layer solidification method. Its solidification front is in direct contact with the liquid metal. Metals belonging to the narrow solidification zone include pure metals (industrial copper, industrial zinc, industrial tin), eutectic alloys (aluminum-silicon alloys, near-eutectic alloys such as gray cast iron), and alloys with a narrow crystallization range (such as low carbon steel). , aluminum bronze, brass with small crystallization range). The above metal cases all belong to the layer-by-layer solidification method.
When the liquid solidifies into a solid state and shrinks in volume, it can be continuously replenished by the liquid, and the tendency to produce dispersed shrinkage is small, but concentrated shrinkage holes are left in the final solidified part of the casting. Concentrated shrinkage cavities are easy to eliminate, so the shrinkage properties are good. Intergranular cracks caused by hindered shrinkage are easily filled with molten metal to heal the cracks, so castings have little tendency to hot crack. It also has better filling ability when solidification occurs during the filling process.
What is paste coagulation: When the coagulation zone is very wide, it belongs to the paste coagulation method. Metals belonging to the wide solidification zone include aluminum alloys, magnesium alloys (aluminum-copper alloys, aluminum-magnesium alloys, magnesium alloys), copper alloys (tin bronze, aluminum bronze, brass with a wide crystallization temperature range), iron-carbon alloys (high carbon steel , ductile iron).
The wider the solidification zone of a metal, the harder it is for bubbles and inclusions in the molten metal to float and remove during casting, and it is also difficult to feed. Castings are prone to hot cracking. When cracks occur between crystals, they cannot be filled with liquid metal to heal them. When this type of alloy solidifies during the filling process, its filling ability is also poor.
What is intermediate solidification: The solidification between the narrow solidification zone and the wide solidification zone is called the intermediate solidification zone. Alloys belonging to the intermediate solidification zone include carbon steel, high manganese steel, some special brass and white cast iron. Its feeding characteristics, thermal cracking tendency and mold filling ability are between layer-by-layer solidification and paste solidification methods. The control of solidification of this type of casting is mainly to adjust the process parameters, establish a favorable temperature gradient on the cross section of the casting, reduce the solidification area on the casting cross section, and change the solidification mode from pasty solidification to layer-by-layer solidification to obtain qualified castings.
Post time: May-17-2024
