Hot Extrusion
For metals and alloys that do not have sufficient ductility at room temperature (or in order to reduce the forces required), extrusion is carried out at elevated tempera¬tures (Table). As in all other hot-working operations, hot extrusion has special requirements because of the high operating temperatures. For example, die wear can be excessive, and cooling of the surfaces of the hot billet (in the cooler chamber) and the die can result in highly nonuniform deformation .
To reduce cooling of the billet and to prolong die life, extrusion dies may be preheated, as is done in hot-forging operations.Because the billet is hot, it develops an oxide film unless it is heated in an inert-atmosphere furnace. This film can be abrasive, and it can affect the flow pattern of the material. It also results in an extruded product that may be unacceptable when a good surface finish is important. In order to avoid the forma¬tion of oxide films on the hot extruded product, the dummy block placed ahead of the ram (Fig) is made a little smaller in diameter than the container. As a result, a thin shell (skull) consisting mainly of the outer oxidized layer of the billet is left in the container. The skull is removed later from the chamber.
For metals and alloys that do not have sufficient ductility at room temperature (or in order to reduce the forces required), extrusion is carried out at elevated tempera¬tures (Table). As in all other hot-working operations, hot extrusion has special requirements because of the high operating temperatures. For example, die wear can be excessive, and cooling of the surfaces of the hot billet (in the cooler chamber) and the die can result in highly nonuniform deformation .
To reduce cooling of the billet and to prolong die life, extrusion dies may be preheated, as is done in hot-forging operations.Because the billet is hot, it develops an oxide film unless it is heated in an inert-atmosphere furnace. This film can be abrasive, and it can affect the flow pattern of the material. It also results in an extruded product that may be unacceptable when a good surface finish is important. In order to avoid the forma¬tion of oxide films on the hot extruded product, the dummy block placed ahead of the ram (Fig) is made a little smaller in diameter than the container. As a result, a thin shell (skull) consisting mainly of the outer oxidized layer of the billet is left in the container. The skull is removed later from the chamber.
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