Materials Science Forum Vols. 546-549

Abstract: Removal of Na by an “active” AlF3 filter seemed to be so efficient that all the sodium was removed in the present experiments. The removal of Na is considered to be limited by the resistance in the melt boundary layer only. However this is not the case for calcium. For calcium to be transfered into the AlF3, resistance in the filter grains has to be taken into account in addition to the resistance in the melt boundary layer. This was indicated by the total mass transfer coefficient of only 4x10-5 m/s. A kinetic model was derived that can describe the removal of Ca from molten aluminum in an “active” AlF3 filter by deep bed filtration following first order kinetics.

Abstract: Local orientation distributions of two neighbouring grains initially orientated near (01l)[100] and (210)[001], in a pure Al poly-crystal aggregate rolled by a reduction of 65% at room temperature have been investigated using electron backscatter diffraction (EBSD) technique. It was found that the rotation angles of zones adjacent to both sides of initial boundary (BP) are less than that of zones remote from the boundary (IP), which reveals that rotation incompatibility and heterogeneity of local orientations within individual grains during rolling. A type of improved Taylor type crystal plasticity models, in which an added prescribed shear (APS) depending on distances from grain boundary proposed, was established to illustrate development of micro-textures in each grain. The results simulated by the APS-Taylor models well agree with the micro-textures examined by EBSD.

Abstract: The influence of crystallographic orientation stability on the inhomogeneous deformation on the meso-scale was numerically simulated with the crystal plasticity finite element. The initial microstructures were introduced into the finite element via intra-granular orientation gradient. The results demonstrated that the average orientation for both stable Goss and meta-stable cube oriented grains changed very little in plane strain compression (PSC). And for stable Goss oriented grain, the orientation gradient increased slightly, but for meta-stable cube oriented grain, the orientation gradient increased dramatically. And in spite of the same Taylor factor for both oriented grains, the dissipated averaged energy for cube oriented grain was higher than for Goss oriented grains, and the distribution width of dissipated work in cube oriented grain was also wider than that in Goss oriented grain. Furthermore, there was also strong energy gradient in PSC of cube oriented grain. Both the orientation gradient and energy gradient favor the nucleation of cube grains during recrystallization.

Abstract: The atomic bonding of the segregated cell of Al-Li alloy in earlier ageing condition was calculated according to the empirical electron theory (EET) in solid. The results showed that the strongest covalent bond was the Al-Al bond in the segregated cell without vacancy, while the strongest covalent bond was the Al-Li bond in the cell containing vacancy. Since the difference of electronagativity between the Al and Li atoms was obvious, it was inclined to form the Al-Li segregated cell of short range order structure in the condition of vacancy joining. The short range order structure with vacancy was probably the embryo or precursor structure of the metastable phase ' δ (Al3Li). As the ' δ (Al3Li) was coherence with matrix, the bond net strength was enhanced by the precipitation of ' δ (Al3Li) and hence the alloy was strengthened.

Abstract: The precipitation behaviors and microstructures of nano-precipitates in AA6082 Al-Mg-Si alloy with and without Cu additions during heat treatment process were studied using hardness measurements, TEM, mechanical tests and 3DAP. Meanwhile, the softening process of 6082 alloys with Cu and without Cu, isothermally conditioned at 250°C, has also been investigated. It was found that the rate of age hardening, mechanical properties and thermal stability are higher for the Cu-containing alloy. The TEM and 3DAP observations showed that Q’ precipitates were existed after aged at 170°C for 8h in the alloy with Cu addition. Comparing the hardness, mechanical properties and thermal stability curves, it was concluded that the Q’ precipitates play a major role in improving the age hardening kinetics and properties of 6082 alloy with Cu addition.

Abstract: The clearance and friction have large influence on wrinkling in rotary draw bending process of thin-walled rectangular tube. In this paper, a three-dimension Al FEM model of rotary draw bending of thin-walled rectangular tube is built under the ABAQUS/ Explicit environment. Based on the model, simulation and analysis of rotary draw bending for aluminum alloy 3A21 thin-walled rectangular tube have been carried out. The results show that the height of wrinkling wave decreases obviously with the decrease of the clearance between mandrel and tube or with the increase of the friction coefficient between wiper die and tube; however, the effect of friction coefficient on wrinkling becomes weaker while increasing the clearance between mandrel and tube. The achievements of this study provide a guideline for determining the process parameters for thin-walled rectangular tube in rotary draw bending process.

Abstract: A study was conducted on 7055 Al alloys processed by Equal-channel Angular Pressing (ECAP). The quantitative information about precipitate size, distribution and volume fraction in ECAPed samples was investigated by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). It is found that with the number of ECAP increasing, the quantitative information about volume fraction (fv), precipitate size (R), specific inner surface of particle-matrix (Sp), and integrated intensity (Q0) all increased, indicating that ECAP enhanced the nucleation and growth of precipitates. The microhardness of samples after two passes of ECAP is higher than that of T6 treated samples even after 96 h at 120°C. The formation of fine grains, higher volume fraction of precipitates, together with strain hardening induced by severe deformation are the main factors leading to a high microhardness in ECAPed samples.

Abstract: The mechanical properties and microstructures of different state cryogenic treatment 3102 Al-foil, such as H19，H26 or O state, were studied by tensile testing, optical metalloscopy (OM) and transmission electron microscope(TEM). The results showed that, after deep cryogenic treatment, the strength of H19 state increased and the elongation to failure decreased. In the same time, the yield strength increased and breaking strength decreases and the elongation decreased dramatically for O state, but, for H26 state, the strength and elongation increased dramatically, which correspond to the properties of this state at low temperatures. OM and TEM observation showed cryogenic treatment caused by the fibrous grains broken down and many grains with the size of 0.1~3μm These fine equiaxial grains can improve the strength and elongation of the Al-foil. On the contrary, the atomic shrinkage force slips high density dislocations into interface of the grains and forms terraces and fissure in it. That leads to the elongation decrease and the interface been broaden at very low temperature. The grain size of O state with cryogenic treatment is larger than without cryogenic treatment. As a result, the synthetic mechanical properties of H26 state improved by the way of cryogenic treatment, but the H19 state and O state should avoid use and put at low temperatures.

Abstract: The microstructure after different ageing treatments and its relation to the mechanical properties and corrosion behavior of 7150 alloy have been investigated. The microstructures were studied by optical microscopy (OM) and transmission electron microscopy (TEM). It was shown that suitable heat treatments led to a combination of high strength and corrosion resistance of the alloy. The yield strength of the alloy is 580MPa in peak aged state, which can be achieved by one step (120 °C/28h) or two step ageing (120°C/6h + 155°C/12h) tempers. The dominant matrix precipitates of the alloy for one step ageing temper are GP zones and η' phase. The alloy after ageing at 120°C for 28h is not susceptible to exfoliation corrosion. On the contrary, the alloy after two step ageing (120°C/6h + 155°C/12h) showed worse exfoliation corrosion resistance. The alloy exhibits high strength and good resistance to the exfoliation corrosion after the ageing treatment of 120°C/6h + 165°C/12h. The dominant precipitates within the grains are η' phase and η phase. The η phase at the grain boundaries plays an important role on the corrosion behaviors. The susceptibility of corrosion is associated with the potential difference between grain and grain boundary, and the characterization of grain boundary precipitates.