György Krállics

Microstructure and Mechanical Behavior of Ultrafine-Grained Titanium

Ultrafine-grained titanium was processed by severe plastic deformation (SPD). The SPD was carried out by equal channel angular pressing (ECAP) at high temperature. The ECAPprocessed sample was further deformed by conventional techniques such as radial forging and drawing. The microstructure was characterized quantitatively by X-ray diffraction line profile analysis and transmission electron microscopy after each step of deformation. The effect of procesing routes on the mechanical behavior was also studied. It was found that the conventional deformation processes after ECAP result in further increment in dislocation density and strength at the expense of ductility.

Research on the thermo-physical process of laser bending

Abstract This paper presents a new method of real-time measuring technology to reveal the thermo-physical process of laser bending. The dynamic micro-deformation of the specimen is measured using a laser beam reflex amplifier system. Experimental results show that the common action of thermal stress and phase transformation stress makes the specimen bend. The final deformation direction of the specimen depends on the result of the co-operation of the thermal strain and the phase transformation strain, away from the laser beam or towards the laser beam. The conclusion lays the foundation for further research on the process of laser bending.

Evolution of the Microstructure of Al 6082 Alloy during Equal-Channel Angular Pressing

A commercial Al-Mg-Si alloy (Al 6082) was deformed by Equal-Channel Angular Pressing (ECAP) to produce bulk ultrafine-grained microstructure. The crystallite size distribution and the characteristic parameters of the dislocation structure were investigated by X-ray diffraction profile analysis. It was found that the crystallite size decreased and the dislocation density increased during ECAP deformation. The increase of the yield stress of the alloy was related to the increase of the dislocation density using the Taylor model.

Experimental Investigations of the Al 6082 Alloy Subjected to Equal-Channel Angular Pressing

The interest in bulk nanostructured materials (NSM), processed by methods of severe plastic deformation (SPD), is justified by their unique physical and mechanical properties. Equalchannel angular pressing (ECAP) is one of the methods of severe plastic deformation (SPD) that produces ultra fine-grained material. Due to the cyclic nature of the process, it is difficult to produce specimens with a high length to diameter ratio. Ratios of 6-7 have been reported in the literature to date. Longer specimens, however, are useful since the homogenous part is larger and the relative size of end effects is smaller. A new method was developed to obtain length to diameter ratios as high as 8-10. This new technique was developed using the multi-pass finite element simulation. The as-received alloy used in this study was the 6082 commercial Al-Mg-Si alloy. High strength and high ductility phenomenon that was found recently in materials after SPD were reached with the route C. The induced anisotropy of specimens after ECAP was monitored.

Finite Element Simulation of Multi-Pass Equal Channel Angular Pressing

Equal-channel angular pressing (ECAP) is one of the methods of sever e plastic deformation for producing ultra fine-grained material. During the mult i-pass ECAP process the accumulated plastic deformation creates microstructure of equiaxe d fin grains with high angle grain boundaries. This structure is mostly depends on the mechanical sc heme of process. This paper analyses the influence of the die geometry, the friction condition bet ween the die and workpiece and the number of passes for the distribution of plastic deformation. Two-dim ensional elastic-plastic finite element analysis of ECAP was done. It was suggested a numerical parameter in order to estimate the inhomogenity of the strain field. The modelling result s of annealed pore cooper workpieces demonstrate the even-numbered passes can be reach more homoge ne us distribution than odd numbered ones. Introduction Ultrafine-grained (UFG) materials often exhibit high strength at room temperature and superplastic behaviour at elevated temperature. Severe plastic deformation [1] is one of the most efficient methods for producing UFG (nanoand submicrocristalline) structure in m etals, intermetallic compounds and superconductors. Several methods of severe plastic deformation ha ve been developed to process bulk nanostructured materials with a grain size f rom 20 to 200 nm like high pressure torsion (HPT) [2] process, repetitive corrugation and strai ghten ng (RCS) [3], equal channel angular pressing (ECAP) [4], accumulative roll-bonding (ARB) [5] process. The evolution of microstructure depends on the maximum plastic strain, the distribut ion of the deformation and its change during the process. The simple analytical formulas of the equi valent plastic strain, subsequently, for the ECAP process [6], Eq. 1; for the ARB process, Eq. 2; for the CSR proce ss, Eq. 3, are the following: 2cot cosec 2 2 2 2 3 n ε Ψ Φ Ψ Φ = + + Ψ + (1) where Φ − channel angle, Ψ − die corner angle, n − number of passes, 2 1 ln 3 1 100 c n r ε = − (2) where c r − reduction per cycle, n − number of cycles,

Study of Non-Monotonity of Forming Processes Using Finite Element Analysis

The method of severe plastic deformation is often used to produce bulk ultra-fine grained materials. Numerous methods exist, which are able to produce ultra fine grained materials. Investigated the mechanical schema of these procedures, we searched for such attributes, which characterize the mentioned techniques. By our previous researches can be established, that for these techniques shear deformation as well as the so-called non-monotonic deformation are characteristic. In this paper a degree of non-monotonity is presented, which is an appropriate measure to characterize the forming processes. Besides this the combined Euler – Lagrange method is showed, which is appropriate to obtain a continuous velocity field from finite element analysis. Eventually the mechanical analysis of the extrusion, conventional and asymmetric rolling is showed with respect to the non-monotonity.

Mechanical and microstructural characterization of Al-6082 ultrafine-grained alloys processed by ECAP combined with traditional forming techniques

An Al-6082 alloy was subjected to equal channel angular pressing (ECAP) and subsequently to conventional forming methods such as shape rolling and rotary forging. The effect of different deformation techniques on the microstructure and the mechanical properties was studied. It was found that the shape rolling and rotary forging increased further the strength of ECAP-processed samples and induced a loss of ductility.

Simulation of Equal-Channel Angular Extrusion Pressing

Equal-channel angular (ECA) pressing is the main technique of the severe plastic deformation (SPD) method, applied for fabrication of bulk nanostructured metal materials. At the same time the practical realization of this technique is a rather challenging task. This is connected with the fact that the material during the ECA pressing is subjected to large strains under high imposed pressure at relatively low temperatures. Simulation with the help of the finite element method (FEM) or the variation-difference (VDM) method is widely applied to analyze the process of ECA pressing. A variety of as commercial as well as in-house developed programs are used by researches, when conducting this analysis. As a result the correlation between the modeling results, obtained at different laboratories as well as their adequacy, i.e. possibilities of their application for the analysis of the experimental data become topical issues. In order to find answers to the questions put by there has been performed computer simulation of 1st pass of ECA pressing by an example of pure copper at 4 different laboratories, engaged in SPD problems. Meanwhile, the investigators used different software packages, however, initial simulation conditions were set equal. This refers in particular to geometry sizes and the form of the die-set possessing square transverse section of the channels, as well as to the inner and outer curvature radii of the channels in the point where they intersect, and to the form and dimensions of the billet, strain rate, strain curve, isotropic model of the material. The modeling temperature was ambient. The die-set and the punch were assigned as absolutely solid non-deformable bodies. Taking into account the symmetry of the solving task, the modeling was conducted for a half of the billet, cut along the vertical plane, coming through its geometrical center. The friction coefficient was assigned equal to zero, in order to avoid influence of friction on the character of the material flow as well as not to complicate the problem at the given stage of comparison. Other modeling parameters were chosen by each researcher on his own, basing on his experience and conventional approaches to modeling. Comparison of the obtained modeling results was made by means of matching of the calculated values of the level of the accumulated strain along the bulk of the billet, pressing efforts, and the geometrical form of the billet after ECA pressing. Modeling results were compared with the results of the experimental researches.

The Effect of Multiple Forging and Cold Rolling on Bending and Tensile Behavior of Al 7075 Alloy

This work studies the effect of multiple forging (MF), followed by cold rolling, on the tensile and bending behavior of Al 7075 alloy. The raw material was received as a rod shape. The specimens were subsequently cut and annealed at 450 °C for 30 minutes. The cylindrical specimens were subjected to MF at 250°C for three passes, then the bulk of multiple forging specimens were faced rolling at room temperature. Three different base materials were tested, raw material (as received then annealed), MF material, and rolled sheet. Hardness test, tensile test, and bending test were carried out in order to measure the mechanical properties, which are affected by the MF and cold rolling. It has been proved that the MF base specimens have higher tensile strength and better maximum elongation comparing with the raw material specimens. Rolled sheets specimens have got the highest tensile strength and lowest ductility. The result of bending test was similar to the tensile test results, beside these mechanical tests some metallographic samples were performed to help finding an explanation of this properties change.

Mechanical behavior of a multiple forged 6082 Al alloy

Abstract 6082 Al alloy was investigated in its initial state (IS) and in its multiple forged (MF) state. The MF specimens were obtained using multi-step closed die forging. A cold compression test at room temperature was accomplished in order to measure the deformation anisotropy of MF specimens; it was also used to obtain the standard stress-strain curve. The homogeneity and structure were both evaluated by Vickers hardness measurements. While image analysis was based on optical microscopy investigations. Moreover, the open and closed die forging were simulated using Simufact software. The results showed the effects of the multiple forging process on the material. The outcome of the hardness measurement demonstrated the homogeneity of the structure. Whereas the micrographs described the microstructure development (during the multiple forging process) and the change of the grains shape over the cross section of the MF specimen.