OBRABOTKAMETALLOV technology Vol. 25 No. 4 2023 Introduction In order to process low-ductile materials, it is often necessary to use the methods that provide a high level of compressive stresses [1–3]. For example, the authors of works [4–5] demonstrated that it is possible to deform low-ductile magnesium and its alloys on a par with ductile materials by means of compressive stresses achieved during equal channel angular pressing (ECAP) [6–8]. However, the ECAP devices have a drawback: the final product produced by this method has a cross section similar to the initial blank [9–11]. In addition, the final product has a short length due to the short length of the initial blank, which is limited by the friction conditions on the container wall. Besides, with a one-time application of the ECAP process, the final product demonstrates non-uniform deformed state [12]. In the case of non-equal channel angular pressing (NECAP), unlike ECAP, the change in the shape of the initial blank is implied by the process itself [13]. The NECAP device consists of a punch and a container resting its lower part on a plate, with a rectangular groove gap between the lower end of the container and the flat surface of the plate, which acts as a die. As a result of the metal flowing through the gap, a sheet stock is produced in the shape of a thin strip having a profile similar to the gap. It is not possible to obtain a different profile, which is a drawback of this device. However, there is a need for press products with round, square and other cross-sections. Thus, this device is limited in its manufacturing capabilities. There are multichannel pressing devices of non-angular type, where the travel direction of a punch coincides (direct method) or does not coincides (indirect method) with the movement direction of the extruded profile during deformation [14–15]. However, these devices also have a drawback – it can be used only with horizontal presses, which ensure the acceptance of long products in the production areas, while on vertical presses the acceptance of such products is impossible. Therefore, in this case it is advantageous to use an angular pressing scheme, where the axis of the press is vertical and the axis of the pressed product is perpendicular to it. It is possible to place products produced by this method on a rack using, for example, additional tensioning devices for finished profiles [16]. Typically, the maximum permissible strain level during pressing is determined either by the ductility of the metal or by the load on the pressing tool. Unlike conventional structural analysis, where a permissible safety factor is limited to values between 5 and 10, a pressing tool often operates at the limit of its capabilities with a safety factor slightly higher than 1. In this case such a tool is used for one pressing cycle, in the next cycle it has to be replaced in the next cycle due to loss of shape. For example, this approach is used when pressing titanium, tungsten, molybdenum and other alloys. Due to the different thermomechanical processing parameters used for different alloys, the allowable strain degree turns out to be different. There are recommendations for the use of different elongation ratios under production conditions for different materials, e.g., 30 to 50 for bronzes, 60 to 100 for magnesium alloys. Thus, the maximum permissible elongation ratio for magnesium alloys is 100, which corresponds to a strain degree about ln(100) @ 5. This value will be taken into account in further calculations. The purpose of this study is to evaluate the multichannel angular pressing of bars, which combines a change in the shape of the initial blank in the cross section, as well as the accumulation of a high level of strain during the deformation process. To achieve this purpose, the following objectives are set: 1. To describe the design of the device for multichannel angular pressing of bars, including the die design features; 2. To build two variants of computer models for magnesium cold deformation by the method of multichannel angular pressing of bars with a diameter d = 4.1 mm with the number of matrix channels n = 3 (according to the first variant, the axes of the matrix channels are located along the axis of the rectangular groove; according to the second variant, the axes of the matrix channels are located along the container radius) and to run simulation with the aid of the DEFORM-3D software package; 3. To analyze the stress-strain state of the metal using computer modeling for two die variants and, in particular, to compare the mean stresses, axial stresses, strain degree and strain rate.
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