Abstract
Rotating backward extrusion (RBE) is known as a new severe plastic deformation technology that effectively combines the features of conventional backward extrusion (CBE) and torsion deformation. In this study, 7075 Al alloy shell parts were successfully prepared by CBE and RBE with a different number of revolutions (N = 5, 10, 15, 25, 50) at 410 °C. The effects of the RBE process on the grain refinement, precipitates and properties of extruded parts were revealed, and the deformation characteristics were compared with CBE. The results showed that the RBE process could greatly eliminate the dead deformation zone at the bottom of the CBE section and significantly improved the comprehensive strain level of the material due to the addition of severe torsional deformation via an open punch. The grain refinement feature of RBE parts showed a gradient distribution that continuously weakened from the inner wall to the outer wall with decreasing compressive and torsional stress. Increasing the number of revolutions significantly promoted the level of grain refinement, the grain refinement range, and effectively broke down and refined the coarse insoluble Fe-rich phases of the extruded parts. It was revealed that the finest grain size of approximately 1.3 μm could be obtained in the inner wall region when N was increased to 25, which was linked to the comprehensive effects of continuous dynamic recrystallization and geometric dynamic recrystallization. RBE greatly promoted an improvement in properties of the extruded parts. After T6 treatment, the microhardness of the fine-grained region of the RBE (25 N) part increased to ~192–197 HV, compared with ~180 HV in the initial T6-extruded state.
Highlights
Aluminum (Al) alloys have become one of the important structural materials used in transportation, aerospace, military production and other industrial fields due to its high specific strength, low density, good thermal conductivity and excellent comprehensive properties [1,2]
It was reported that the severe plastic deformation (SPD) methods such as equal channel angular pressing (ECAP), equal channel forward extrusion (ECFE), planar twist channel angular extrusion (PTCAE), high pressure torsion (HPT) and repetitive upsettingextrusion (RUE), etc., [6,7,8,9,10,11]) could be applied to impose high hydrostatic pressures to the sample, which ensured the metal was sufficiently strained at a lower temperature or room temperature without fracture defects, thereby effectively preparing ultra-fine grained materials within micron or even nanometer size scales and high strength
The results showed that when compared with conventional backward extrusion (CBE), rotating backward extrusion (RBE) could increase the deformation uniformity of material and greatly reduced the axial forming load due to the application of torsion deformation, where the forming load was related to the die structure, billet size and rotation speed
Summary
Aluminum (Al) alloys have become one of the important structural materials used in transportation, aerospace, military production and other industrial fields due to its high specific strength, low density, good thermal conductivity and excellent comprehensive properties [1,2]. Among the forging processes, the conventional backward extrusion (CBE) is the most effective way to prepare shell components with convenient operation, simplified mold design, higher dimensional accuracy and surface quality of the product It carries with it the characteristics of large uneven deformation and lower strain levels at the bottom of the part, leading to differences and deficiencies in mechanical properties [15]. With reverse movement of the punch, the bottom metal flows to the wall, and a fine-grained or ultra-fine-grained part is extruded This method inherits the operation mode of CBE in its structure, but can obtain high effective strain through only one-pass deformation, providing a short-process method for the preparation of high performance shell components. The results showed that RBE could greatly refine the grain size of AZ80 Mg alloy to approximately
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