OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 The equiaxial shape of the die channels ensures a uniform stress distribution in each macro-zone of deformation. Fig. 1, a, b on the right shows a diagram of the device in question, showing the mutual position of the parts at the intermediate moment of deformation of the blank 7 to obtain the bars 8. It should be mentioned that this device is mounted and fixed on the press table, and the punch 1 (fig. 1, a) interacts with the press slide. The structural elements fastening the plate 6 with the table and the punch 1 with the slide are not shown in fig. 1. Preparation for the pressing process of a cylindrical blank 7 with a diameter D is the application of a lubricant on its ends and side surface. The first step of the pressing cycle is the placement of the blank 7 into the container channel 2. Further, the punch 1 is actuated by the press drive and lowered until it comes into contact with the upper end of the blank 7. Fig. 1a, b on the left shows the relative position of the parts at this moment. At the next stage of the process, the punch 1 moves under the action of the press force P, while the lower end surface of the blank 7 gets deformed, and the main metal flows into the channels of the die 5. This pressing cycle results in the production of three bars with a diameter d, the length of which depends on the size of the initial blank and the material strain degree. The experiments on cold pressing of magnesium were carried out on a press with a nominal force of 10 MN [17] in the conditions of the scientific laboratory of the Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences. The nominal force of the press lifting cylinders was 2 MN. The nominal pressure of the power fluid was 32 MPa. The stroke of the moving crosshead was 1,000 mm. The maximum distance between the table and the moving crosshead was 1,800 mm. The pressing tool assembly corresponded to the non-equal channel angular pressing method, and a die with a single rectangular channel of 40×1 mm was used. The punch was subjected to sufficiently high force during deformation and it was necessary to prevent its destruction, so the punch was made of steel R18. The choice of this grade is explained by its high fracture resistance and hardness. Steel R18 was subjected to quenching in a vacuum chamber at 1,290 °C and triple tempering for 1 hour at a temperature of 550 °C. Such heat treatment resulted in a fairly high hardness value (64 HRC). Pressing was carried out from a container with a round cross-section consisting of two liners tightly inserted one into the other with interference fit. The inner liner, compared to the outer one, is made of stronger steel, since it takes up most of the pressure in the process. The inner diameter of the container was 40 mm. The elongation ratio when producing a strip of 40×1 mm was 2 / ( ) 31 4 D bh π λ = = . It was revealed that with such technological parameters there is no damage to the press tooling. It was concluded that this technique is workable and the value of the elongation ratio λ ≤ 31 in the described deformation technique is acceptable. When using the device in question, which includes a die, it is possible to produce bars (instead of strips) with a diameter d equal to the number of the die channels (n). In the example under consideration, n = 3. To calculate the diameter of the resulting bars, we determine the total cross-sectional area of the channels 2 4 d F n π = and calculate the elongation ratio using the Eq. (1): 2 2 2 2 4 / ( ). 4 D D nd D n π λ = = π , (1) Hence, the diameter of the resulting bars is calculated using the Eq. (2): 2 2 / ( ). d D n = λ (2) Thus, for λ = 31 and n = 3 we get d = 4.1mm. This is the smallest diameter value at the specified elongation ratio and the number of die channels. Reducing the diameter may lead to exceeding the permissible stresses in the tool.
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