OBRABOTKAMETALLOV Vol. 24 No. 2 2022 TECHNOLOGY It should be noted that the diagram shows the strain with a light reduction, which is not typical for the extrusion process, where the elongation ratio reaches 1,000 or more. In the extrusion theory, the elongation ratio question assigning at least 5 for semi-fabricated products intended for subsequent forming (the fi rst option) and at least 10 for semi-fabricated products not subsequently processed by forming (the second option) is considered. Such limitations are due to the press product core non-processing possibility due to the localization of plastic deformation in the peripheral layers [20]. Equations are provided for the elongation ratio conversion into other strain indicators, as well as for the relationship between it: – for reduction rate: % =100 ( –1)/ (1) – for strain (sometimes called logarithmic strain) = ln (2) or ln (1 – (3) Calculation by equations (1) and (2) gives the minimum values % = 1.61 for the fi rst option % = 2.30for the second option. The strain indicators are large and often it is not achievable in individual rolling passes, as an alternative process even though the elongation ratio values are minimal. The extrusion process is carried out on horizontal hydraulic presses. The extrusion tool in the form of a pressure pad and an extrusion die form is heated to 380оС in a separate furnace and mounted in the extrusion line. The container temperature is 450 оС and is stabilized by a heating device. After leaving the extrusion die, the rods get on the press rack, and then are cut into measured lengths, while the rod front part is separated. Metal structure is considered insuffi ciently developed by plastic deformation. There are general recommendations in extrusion production, based on which the front part of the extruded press product on a plate approximately equal to two diameters is considered to be defective. For example, if an extruded largesized rod diameter is 360 mm, it is necessary to cut off the front part with a length of 720 mm and a mass of 198 kg. When the mass of the original ingot is about 2 tons, the waste of this type is ~10%. Such large wastes arise precisely when extrusion with low elongation ratio, since the resulting rod length turns out to be low and in comparison with it the metal proportion being cut turns out to be signifi cant. Fig. 2, a show the rod front part as it leaves the extrusion die, and Fig. 2, b shows the rod cutting surface. The DEFORM-2D software module was used to evaluate the situation arising at the extrusion initial stage and the following statement of the boundary value problem was formulated. The stress-strain state is axisymmetric. The extrusion tool material is rigid, the ingot material is considered as ductile. The parameters of the computational statement are set: the mesh elements number is 26,000, the element size in the ingot volume is 1.0–1.2 mm; near the tool is 0.5–0.7 mm. Thermal boundary conditions are as close as possible to production indicators: ingot temperature is 470 оС; container temperature is 450 оС; extrusion die and a pressure pad temperature is 380 оС, environmental temperature at the extrusion die exit is 20 оС; convection coeffi cient into the environment is 0.02 N/s/mm/оС, heat transfer ratio – 11 N/s/mm/оС, the coeffi cients and its dimensions are borrowed from the software module interface. The velocity boundary conditions are also tied to the production environment: the extrusion punch velocity is 3.66 mm/s, another parts of press tool are stationary. The friction boundary conditions are set with Fig. 1. Scheme of direct extrusion: 1 – ingot; 2 – container; 3 – punch with a pressure pad; 4 – die; 5 – the front of the rod; arrow – direction of tool motion
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