| The method of ECAP (equal channel angular pressing), which achieves to prepare bulk nanostructured materials, was developed by prof. Segal et al in 1980s. Samples after repeated ECAP can meet a number of requirements as following: (1) ultra fine-grained structure are obtained with prevailing high-angle grain boundaries; (2) The ultra fine structure is uniform in the whole volume and the properties of the processed materials are stable;(3) The ECAPed samples avoid mechanism damage and cracks. Compared with other method of preparing bulk nanostructured materials (such as vapor condensation, ball milling), ECAP method is in possession of several unique advantages. For example, ECAP method results in overcoming a number of difficulties connected with residual porosity in compacted samples, impurities from bail milling, processing large-scale samples and practical application of the given materials. The unique properties in ECAPed materials play significant performance in both practical application and academic research. Therefore, as a SPD process of obtaining bulk submicron or nanometer materials, ECAP method has attract growing interest in material science field and show a possible future to be a practical preparation process. However, so far, ambiguity still exists in details. Firstly, it is still difficult to draw a uniform view on formation mechanism of ultra fine grains and grains transformation in ECAP process. There are arguments in explaining the formation of ultra fine structure and trouble in giving explanation to some experimental phenomenon. Secondly, ECAP process is influenced by several factors, such as the number of pass, the pressing routes, velocity, temperature, and heat transformation etc. Such factors play important role in determining the material properties, refinement and pressing technics. To make an overall and systematical study on ECAP process, conventional experimental method results in a great cost, for the factors such as physical properties of materials and friction between workpiece and die is hard to be exactly controlled. Despite of a lot of experiments, problems still exist. It needs a further study.In this paper experiments are directed at the problems above. AI-3%Mg and 2A12 Al alloy are adopted in finite element analysis and ECAP pressing. The content of this paper is generally shown as following:Firstly, the software Deform-3D is adopted to make numerical calculation in different ECAP conditions. The transformation and distribution of stress, strain and strain velocity are obtained in order to study the influence of ECAP factors. Simulation results illustrate that according to the variation of load with displacement, ECAP process can be divided into five steps: rapid load increase — slow load increase — rapid load increase -? steady load —load decrease. In this process, except velocity, the routes, friction efficiency, channel angle, temperature and the number of pass make great influence on the value, distribution and homogeneousness of the deformation. A die with its inner angle of 90° or near 90° can introduce a higher value of effective stress and effective strain, which bring in a better effect and higher efficiency, while requires a higher value of load. The values of accumulated strain for 8 passes ECAP shows little different from route A, Be, C, however, compared with others the effective strain from route Be distributes homogenously, which brings in the best effect of refinement. ECAP under an elevated temperature requires a relative low load for the deformation drag decreases. The value of deformation has no changes but the degree of uniformity improves. However, given the return action after pressing, ECAP should be conducted under a relative low temperature if the equipment is in condition. For route Be, as the number of pass increasing, the value of strain increases and the degrees of uniformity are enhanced. That is to say the more number of passes is good in improving uniformity and refinement. A high value of friction coefficient introduces higher load. The value of friction coefficient exerts an influence on the distribution of plastic deformation area, the process of deformation, and the structure uniformity. Therefore, the surface of the channel needs finish and lubricate should be used to reduce friction, which is helpful to improving grain refinement and reducing pressing load. Beside, under a lower pressing velocity, when the sample going through the turning point of the channel, the time needs longer, the strain rate is lower and the fluctuation of pressing load smaller, but the distribution of strain almost has no changes. Therefore, thevelocity makes very a little effort on ECAP. So, given to obtaining a high efficiency, ECAP should be conducted under a relative high pressing velocity.Secondly, two sets of ECAP dies, (a) inner angle 0=105° -. out angle w =45° ,(b) inner angle 0=90° ^ out angle v =30° are independently design and manufactured. This two dies possess proper structure and reliable performance, for they have suffered from hundreds of ECAP experiments and now maintained good state. However, there is a shortage in the exit channel of the die. When the sample is pressed out of the exit channel, elastic expand happens. The ignorance of this point results in the low efficiency of the experiment and the troubles in following ECAP pressing. Therefore, a correcting zone should be adopted in the exiting channel according to the size of samples and working accuracy.Thirdly, AI-3%Mg alloy is repeated pressed in ECAP die in room temperature. After four ECAP passes, the grain size decreases from 200 u m to 10 u m, the value of ultimate strength gains 146.8% increase, from 147.3Mpa to 363.5MPa, yield strength 156.3% increase, from 108.8MPa to 279.1Mpa, microhardness 104.7% increase, from 63.9MPa to 130.7MPa, while the elongation keep a almost steady value. Experimental results show that ECAP process is an effective method of refinement and improving mechanical properties. However, compared with the result of ECAPed 2A12 alloy, with the same value of accumulated strain, the refinement of 2A12 Al performs a better effect, which shows that the effect of refinement is closely connected with initial microstructure, especially the initial size of grains.Fourthly, samples of 2A12 Al alloy are subjected to repeated ECAP in room temperature. In this section, the influence of ECAP routes, friction efficiency, velocity, channel angle and pass numbers on mechanical properties is studied. The experimental results are used to certificate the veracity of numerical calculation and try to find out the optimized conditions of ECAP process. Experimental results is as following: for 2A12 Al alloy in room temperature, a die with inner angle 90° prefer to show a good performance rather than that of 105° , route Be provides better effects than route A and route C, velocity makes little effect on mechanical properties. In the early ECAP passes, mechanical properties obtain great improvement, in whichone-pass ECAP efforts mostly. As the pass number increases, the amplitude of improvement becomes small, and even negative. The simulation result is mostly accordant with experimental result. Therefore, in room temperature, ECAP with channel angle of 90° , route Be and a relative fast pressing speed is able to enhance 2A12 Al alloy with best properties.Fifthly, the microstructure of ECAPed 2A12 Al alloy is observed by the method of TEM and HREM etc. Microstructure observation shows that: after eight passes ECAP with a die of 0=90° and route Be, bulk ultra fine-grained material in grain size of 200nm is obtained, in which nanometer A^Cu intracrystalline grains and high grain boundary structure can be observed. In 8-pass-ECAPed materials, there can be found out grain boundaries with high value of internal stress and elastic distortion, and the formation of amorphous structure. The refinement process is as: initial coarse grains — shear deformation in grains—the formation of high density dislocation lines—the transformation from dislocation lines to dislocation cells and walls— the formation of refined subgrains—the breaking and revolution of subgrains—the formation of ultra fine grains with obvious high grain boundaries.Sixthly, influencing factors and mechanism of refinement are analyzed on the base of experiments. The influencing factors of refinement involve shear character, shear strain, crystal structure, deformation texture, heat treatment process, friction condition and pressing temperature. In the ECAP process of manufacturing bulk ultra fine-grained 2A12 Al material, the mainly influencing factors include shear plane, shear strain, crystal structure and deformation texture. The mechanism of refinement mainly embodies mechanisms of dislocations anchoring, second-phase particle shearing and deformation inducing. The combined action of the three kinds of mechanism results from the achievement of preparing bulk ultra fine-grained 2A12 Al alloy by ECAP method.Finally, on the base of experimental results, the mechanism of reinforcement in ECAP process is analyzed as: the mechanism of reinforcement caused by ECAP mainly includes mechanism of refined grains strengthening, high dislocation density strengthening, second-phase strengthening and solution strengthening, in which refined grainsstrengthening and dislocation strengthening make the chief effort. The improvement on strength in ECAPed materials results from the combined action of the kinds of mechanism mentioned above. In early deformation, the effect of grain refinement is obvious, dislocation density is in a high value and the acicular second-phase in unECAPed material transform to granular ones under shear deformation. Therefore, the strength of the samples in this time increases greatly under the combined action of grains strengthening, dislocation strengthening second-phase strengthening. As the ECAP passes going on, though the second-phase particles is subjected to further shearing and distributes more homogenous in the matrix and the grains further refined, the amplitude of the refinement becomes small and the dislocation density in material is continuously decreasing. So, in the middle period of the whole ECAP passes, in spite of the increase in strength, the amplitude of the trend decreases. When the passes going on, the shape of the second-phase particles has no changes and dislocation density keep on decreasing. That cause the saturation of grain refinement. As a result, in the last ECAP passes, the strengthen is almost unchanged, or slightly descending.
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