In Reverse Extrusion, The State
In reverse extrusion, the state of metal flow is very different from that in forward extrusion. There is no relative displacement between the center layer and the surrounding area in the metal ingot, and the metal flow is much more uniform than in the forward extrusion. Reverse extrusion has the following flow and deformation characteristics:
(1) The metal deformation area is close to the die surface when extruding, and the plastic deformation area is very small. According to experiments, the height of the compression cone in the deformation area is not more than 0.3 Do (D. barrel diameter), and the metal behind the deformation area does not have any Deformation.
(2) Only a very small metal flow dead zone is created near the die surface. The dead zone metal hardly participates in the deformation until the final stage of extrusion, when there are few ingots remaining in the barrel, a significant lateral flow is produced. . Therefore, it is difficult to block the impurities and defects on the surface of the ingot, and the surface quality of the product is easily deteriorated.
(3) The plastic deformation zone in the reverse extrusion is only concentrated in the vicinity of the die hole, which greatly reduces the unevenness of the deformation of the product, particularly along the length direction. Therefore, the cross-section structure of the reverse extrusion product is much more uniform than the forward extrusion product.
(4) Strong additional shear deformation caused by no sharp friction at the edge of the ingot during reverse extrusion.
(5) In the reverse extrusion, the top layer of the ingot is prevented from being in the dead zone near the die surface, and the slightly deeper metal may flow directly into the surface layer of the extruded product, and the back end metal has no back flow phenomenon. Reverse extrusion has little tendency to produce defects such as center-tail shrinkage and ring-tail shrinkage at the end of the article.