THEORETICAL STUDIES OF THE JOINT EXTRUSION-ROLLING PROCESS AIMED AT MAKING SUB-ULTRA FINE GRAINED STRUCTURE METAL

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1 THEORETICAL STUDIES OF THE JOINT EXTRUSION-ROLLING PROCESS AIMED AT MAKING SUB-ULTRA FINE GRAINED STRUCTURE METAL Lezhnev S.N., Naizabekov A.B., Panin Ye.A. RSE Karaganda state industrial university, 30, Republic avenue, Temirtau, Republic of Kazakhstan, Abstract Studies and developments aimed at making high strength metal have always been of great scientific and practical interest and even now are topical problems. New unique metal properties can be obtained in the process of realization of high rate plastic deformation through the whole bulk of the deformed billet. A number of new metal forming processes based on the use of active friction forces have been developed lately. The most promising and the least theoretically studied is the joint extrusion-rolling process which exceeds well known metal forming processes in many parameters. The joint extrusion rolling process is realized due to the use of the reserve friction forces arising on the contact surface of metal and spinning rolls. At Metal Forming chair of Karaganda state industrial university there has been developed and proposed a new joint process of rolling and extrusion in the equichannel step die which provides continuity of the process and removes limitations on the sizes of initial billets in comparison with the extrusion in the equichannel die. Theoretical studies of the joint process are given in this work. Keywords: deformed billet, joint extrusion - rolling process, continuity of the process 1. INTRODUCTION Development of new technological processes aimed at obtaining metal of ground structure is a very important and urgent problem because making high strength metal by grinding grains to nano sized level is one of the prior importance problems in the field of nano science of many countries. One of the most promising and superior metal forming techniques is the joint rolling extrusion process using the grooved rolls and equichannel step die for extrusion (fig.1) which was developed at Metal Forming chair of Karaganda state industrial university [1]. The advantage of this process in comparison with simple step extrusion consists in the fact that it provides continuity of deformation process and removes limitations on the sizes of initial billets. Fig.1. Joint rolling extrusion process

2 The essence of the proposed process consists in the following. The billet preheated to the temperature of the beginning of deformation is fed to the rolls which grip it into the roll gap due to the contact friction forces and on leaving it push the billet through the channels of the equichannel step die. When the billet leaves the die channels completely it is gripped by the second pair of rolls which also due to the contact friction forces grip the billet into the gap of the second pair of rolls and draw out the billet from the channels completely. Theoretical studies of this process were made in the works [1-2]. A comparative analysis of using plain and grooved rolls in this process showed that using of the grooved rolls in this process is more expedient as far as their use with similar initial data makes it possible to realize the rolling extrusion process with a less joint angle in the equichannel step die at a much less reduction. Another indisputable advantage of using grooved rolls instead of plain ones is a possibility of controlling broadening of the billet during its deformation in the grooved rolls. Besides, there were also received theoretical dependencies for overcoming overpressure forces in the equichannel step die when pushing the billet through it due to the active friction forces created by the rolls. On the basis if these studies there have been created the program to determine rational geometrical and technological parameters of this process for deformation square and rectangular cross section billets of any size. In case deformation of some profile size is impossible the program gives information about it. Besides there have been made the simulation of the rolling extrusion process using the grooved rolls and equichannel step die [3]. Simulation of the rolling extrusion process was made in 3D-DEFORM program complex. After the completion of calculations and checking the results the model was considered successful if the billet was gripped and rolled in the first pair of rolls, then pushed through all the die channels and at the exit from the die gripped by the second pair of rolls and completely drawn out from the die (fig.2). The analysis of the effect of various factors on the conditions of the running of this process showed that such factors as the angle of the die channels joint, friction coefficient, the temperature, the extent of the die channels have a substantial effect on the possibility of realization of the joint rolling - extrusion process. Only the choice of rational modes of deformation for the given profile size of the billet and consideration of all the factors will allow to realize this complicated process. Tables of optimal geometrical and technological parameters for various cross sections of billets were made for this. The tables data make it possible to choose precisely the values of all the recessary parameters for realization of the rolling extrusion process using the grooved rolls and the equichannel step die. When a successful simulation of the joint rolling - extrusion process was made it was decided, to study the stressed and strained state of the deformed billet during realization of this process. This is connected with the fact that the study of the stressed state allows to study the distribution of the stresses through the whole body of the billet during deformation and to detect the zones which are more subjected to the formation of defects brought about by great tensile stresses. It gives the opportunity to make necessary corrections and thus to prevent the formation of defects. And the study of the strained state makes it possible to study the distribution of the accumulated strain through the whole body of the deformed billet and to detect the zones which are more subjected to strain. On this bases it is possible to determine rational geometrical and technological parameters of deformation. When studying stressed and strained state (SSS) during the joint rolling - extrusion process of deformation of billets there have been analysed the following stages of this process: 1) The billet is only rolled in the first pair of rolls: 2) The billet is rolled in the first pair of rolls and passes through the die channels 3) The billet is rolled in the first pair of rolls, passes through the die channels and then rolled in the second pair of rolls. To analyse SSS in these stages there have been studied the following parameters: equivalent strain ε equ ; equivalent stress σ equ ; main stresses σ 1, σ 2, σ 3.

3 c) d) e) Fig.2. Stages of the process of a successful model Study of the metal SSS during realization of the joint rolling - extrusion process using equichannel step die and grooved rolls was made with the help of DEFORM 3D program (USA). When analyzing SSS of the first stage it was found that the distribution of equivalent strains through the whole body of the deformed billet is of uniform character. The average value of the accumulated strains is about 0,7 0,8, with the exception of the front end of the billet where ε equ. 0,2. 03 which is the result of the recesses made during the construction of the model to facilitate the grip of the billet by rolling mills. The 1st stage Fig. 3. Equivalent strain, ε equ (a) and stress σ equ (b)

4 c) a,b,c - main stresses σ 1, σ 2, σ 3.accordingly Fig.4. Main stresses The analysis of the equivalent stresses was carried out in the site of the strain only at the given moment of time, i.e. without considering the accumulation of stresses, like studying equivalent strain. Equivalent stress σ equ. during rolling in the rolls reaches the value of 384 MPa, the greatest values of equivalent stresses arise in the places of the contact of the billet with the rolls. The analysis of the distribution of the main stresses shows that on the whole compressive stresses predominate in the site of the strain. The scheme of all round compression guarantees the absence of macro and micro cracks in the metal and promotes maximum ductility of the deformed billet. In the second stage the billet is simultaneously rolled in the rools and by its front end passes through the channels of the equichannel step die. It results in arising of two sites of stress concentration. Here the equivalent stress σ equ. as before has maximum value in the strain site during rolling the billet in the rolls 386 Mpa, and in the zone of the die channels joint it is about MPa. The analysis of the main stresses predominate through the whole body of the billet. This is exsplained by the fact that the billet is subjected to a great resistance from the die when passing the zone of the die channels joint which prevents the movement of the billet and this creates substantial compressive stresses along the all three directions. The 2nd stage Fig.5. Equivalent strain ε equ (a). and stress σ equ (b)

5 c) a,b,c, - the main stresses σ 1, σ 2, σ 3 accordingly Fig.6. Main stresses The analysis of the distribution of equivalent strains in the second stage is similar to the first stage. But in the zone of the channels joint there arise extra shift strains which results in accumulation of ε equ from 0,7 to 1. The 3-d stage Fig.7. Equivalent strain ε equ (a) and strees σ equ (b) In the 3-d stage the front end of the billet is rolled in the second pair of rolls while the back end is still in the site of the strain of the 1-st pair of rolls. Thus three zones of the local strees are formed. Here the equivalent stress σ equ reaches maximum values (648 MPa) in the sites of the strain during rolling in the rolls and in the zone of the channels joint it is about MPa. The analysis of the main stresses shows that in this stage compressive stresses predominate through the whole body of the billet, however their concentrations are centered in the strain sites of both pairs of rolls and in the zone of the channels joint. In the section from the zone of the joint up to the strain site of the second pair of rolls there arises tensile stresses due to the tension of the billet by the second rolls.

6 c) a,b,c, - main stresses σ 1, σ 2, σ 3 accordingly Fig.8. Main stresses The equivalent strains up to the zone of the channels joint are equally distributed through the whole body (ε equ = 0,7). After passing the zone of the channels joint there arise extra shear strains which results in the accumulation of the equivalent strains (ε equ = 0,7 1). When rolling in the second pair of the grooved rolls the value ε equ reaches 2. Thus the analysis of the stressed and strained state of the billets during their deformation in the joint rolling extrusion process showed that during realization of this process a favourable SSS for obtaining a sub ultra fire grained structure metal is realized. References [1.] Naizabekov A.B., Lezhnev S.N., Panin E.A. Theoretical studies of the joint rolling extrusion process using equichannel step die. //Higher school proceedings Ferrous metallurgy, Moscow, р [2.] Naizabekov A.B., Lezhnev S.N., Panin E.A. Kinematic calculation of the joint rolling extrusion process realized by using grooved rolls and equichannel step die. //Technology of making metals and secondary materials. Temirtau, , p [3.] Naizabekov A.B., Lezhnev S.N., Panin E.A. Simulation of the joint rolling - extrusion process using grooved rolls and equichannel step die. //Technology of making metals and secondary materials. Temirtau, 2008, 1 р