When the molten metal stays in the mold cavity for a long time, the mold is closed and pressure is applied, and the molten metal is squeezed to fill the mold, forming a cold gap between this part of the metal and the original pouring liquid level.
There is a thick layer of oxide scale on the surface of the sheet metal in the die cavity. After extrusion, the remaining oxide scale is basically still in its original position, resulting in no fusion between the metal in this part, that is, cold separation. The cold gap is formed by clamping the mold and applying pressure, which is related to the part forming method. That is, when the convex punch is pressurized, this kind of cold separation is inevitable.
Preventive measures: increase the mold temperature and pouring temperature; when the process rhythm permits, shorten the residence time before pressurization as much as possible; select alloys that are not easily oxidized, etc. These measures can only reduce the hazards of cold isolation, but cannot fundamentally eliminate it if it is not allowed to exist.
The forming method can only be changed: l) When designing the mold, turn the position of the part upside down so that a flat punch can be used instead of a convex punch;
2) First close the mold, squeeze in the liquid sheet metal, and then apply pressure.
Link to this article：Defects and prevention of liquid die forgings: extrusion cold insulation
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