THE EFFECT OF STRAIN INTENSIFICATION ON THE SINGLE-OPERATION PROCESS OF EXTRUSION OF DEEP-BOTTOMED SLEEVES

摘要

In the article the authors present the results of a study on solving the problem occurring in the production of deep steel sleeves (H/d > 2.5) from full preforms. The basic problem during the production of deep bottomed sleeves, e.g. during making sleeves by indirect extrusion, is that the punch operates under very difficult conditions (such as high temperature and high unit pressures), which causes a reduction in its lifetime and frequent failures. For this reason, deep sleeves are produced through at least two technological treatments. This involves the necessity of building two technological stands (often using two presses), which considerably increases the cost of production, its labour-consumption and, above all, reduces the productivity. The new method of producing sleeves, proposed by the authors, combines the known and specific pattern of metal flow in the extrusion process, and aims at achieving the following: increasing the lifetime of working tools, increasing the productivity, lowering the production costs, reducing the material losses and decreasing the magnitude of extrusion forces. The method under analysis consists in the simultaneous indirect extrusion and co-extrusion, which results in a semi-finished product in the form of a sleeve with a stem. These are known methods of the plastic working of full preforms. It is these schemes that are applied in the new method as the first stage of producing deep bottomed sleeves. The second stage in the process is a quite specific deformation pattern consisting in the formation of a finished sleeve wall from the stem formed in the first stage. By combining different extrusion schemes, both in the first and in the second stage of the process the strain intensities were increased, and the partitioning of the flowing stream and the change of the metal flow direction caused local temperature increases as a result of strain intensification, which resulted in a drop in the magnitude of forces needed for the single-operation extrusion of deep sleeves. Then, different variants of process rate and degree of deformation resulting from the geometry of the movable ring and its taper angle for four values of temperature were subjected to analysis and their effect on the extrusion force was determined. A preliminary analysis of the process included computer simulations in the FORGE 2D program. As the model material, steel 45 was used. The process was conducted at different tool speeds: (25, 50, 100, 200, 300 mm/s), at different temperatures: (850, 950, 1050, 1150 deg C), for four degrees of deformation: (epsilon = 0.3, 0.4, 0.5, 0.75) and for three taper angles of the movable ring: (2 beta = 120 deg, 140 deg, 160 deg). Maximum extrusion forces were determined, graphs of the relationship of the extrusion force versus the above-mentioned parameters were plotted, and the maximum values of metal pressures on the punch, as dependent on the parameters analyzed, were determined. Also determined during analysis were the sizes of the zones of local increase in temperatures and their values, which are the result of strain intensification and strain rate intensification, the consequence of which is a reduction in the forces of metal pressure on the tools and a reduction in the extrusion force. With the aim of performing the physical modelling of the process, a testing stand was build, which included a tool, whose operation allows the deformation pattern concerned to be carried out, and a modern testing press, type ZWICK Z100. The model material for the extrusion of sleeves on the testing stand was lead. Extrusion operations were performed in the tool, obtaining sleeves of an overall height of H = 150 / 200 mm, an outer diameter of D_z = 50 mm, an inner diameter of D_w = 40 mm, a wall thickness of 5 mm and a bottom thickness of 5 mm. Graphs of the relationship of the force P [kN] versus the punch path S [mm] were also plotted and compared with the graphs from computer simulations.