Materials Science Forum Vols. 794-796

Abstract: A new atomic mobility database for Fcc_A1, L1₂, Bcc_A2, Bcc_B2, and liquid phases in the Al-Cu-Fe-Mg-Mn-Ni-Si-Zn system has been established via a hybrid approach of experiment, first-principles calculations and DICTRA (DIffusion Controlled TRAnsformation) software, focusing on the atomic mobility parameters in ternary systems. Various diffusivities can be computed as a function of temperature and composition. The reliability of this diffusivity database is further validated by comparing the calculated and measured diffusion properties in a series of ternary and quaternary diffusion couples, including concentration profiles, diffusion paths, interdiffusion fluxes, and so on. The effect of the diffusivity database on microstructure evolution during solidification is demonstrated by the phase field simulation of primary (Al) grains in Al356.1 alloy. The simulation results indicate that such accurate diffusivity database is highly needed for the quantitative simulation of microstructural evolution during solidification.

Abstract: Aluminium cast alloys are used for engine components, such as pistons and cylinder heads. The micromechanical properties of an AlSi12 cast alloy under monotonic and cyclic loadings are investigated. Therefore a microstructure-based two dimensional finite element model is generated. The characteristic shape of primary precipitates is analyzed and translated into an artificial microstructure. The quality of the generated microstructure is evaluated based on the stress distribution along the primary particle boundaries. The effect of the temperature dependent material behavior of the aluminium matrix is studied with respect to the resulting stress distribution along the particle boundaries. The results are discussed in terms of a possible change of fracture mechanisms from a brittle type fracture at low temperatures to an increasingly ductile fracture at high temperatures.

Abstract: Resistivity against intergranular corrosion (IGC) is one of the major requirements for AlMgMn 5xxx-series alloys for automotive chassis applications. In 5xxx alloys IGC is caused by the formation of β-Al₅Mg₃ precipitates along the grain boundaries. Todays 5xxx alloys for chassis applications have been developed such that under specific test conditions they will not exceed a certain mass loss by IGC. However, current developments in the automotive industry will lead to an increased temperature load on chassis parts, in particular for front axle applications in the vicinity of the car engine. Therefore it is to be expected that the properties of the existing 5xxx series alloys will not be sufficient any more. Certain alloy elements, including Mn, Cr, Cu and Zn, alter type and morphology of the Mg-bearing precipitates and, hence, reduce the sensitivity against IGC. The present study was aimed at developing a series of Zn-containing Al alloys which are free of IGC, while maintaining mechanical properties of current 5xxx chassis alloys. Alloy development was performed by micro-chemistry simulation with the aim to avoid the formation of the detrimental β-Al₅Mg₃ precipitates. Eventually a series of three alloys was cast and processed on lab scale and tested for mechanical properties and resistivity against IGC after application of various critical time/temperature scenarios in order to validate that the newly developed alloys are free of IGC.

Abstract: Aluminium cast products are becoming more and more interesting for energy absorbing applications, as a higher functional integration can be achieved with casting processes. Therefore, it is required to find a way to characterise different aluminium alloys regarding their energy absorption behaviour. Energy absorption phenomena in materials depend on the combination of material and geometry on a macro scale level. One of the main contributions of the current research work is to show that the full realization of material absorbing capacity may not be achieved by more complex geometries. Consequently, for the characterisation of cast material under crash load, it is very important to keep the geometry influence on the energy absorption behaviour as low as possible. The ultimate aim herein is to determine an optimised geometry setup to characterise different aluminium casting materials. Three different test geometries were chosen for numerical investigations. All specimens possess the same cross-sectional area and also the same second moment of inertia. The specimens have been tested under an axial crash load at constant speed. Failure has been simulated using a Johnson-Cook damage and failure model. Their absorbing behaviours will be compared and based on the existing literature a theoretical discussion about the geometrical influence will also be given.

Abstract: Calphad type thermodynamic assessment of Al-Zn binary system was performed to calculate the metastable phase diagram including not only miscibility gap but also spinodal lines. The Gibbs free energy for liquid, fcc and hcp phases was evaluated by taking into account available experimental data, and most of them were satisfactorily reproduced by our thermodynamic descriptions. Furthermore, the Gibbs free energy for GP zones was also expressed by combining the chemical energy for solid solution of fcc-Al with appropriate elastic strain energy, in good agreement with experimentally reported solubility limits. Therefore, the spinodal lines derived from the differential of the Gibbs free energy could be also reasonably estimated, although those are quite difficult to be measured experimentally.

Abstract: Precipitate-host lattice interface studies have not traditionally been viewed as requiring hybrid model schemes for accurate determination of the interfacial and strain energies. On the other hand, the interfaces of main hardening precipitates of age hardenable alloys are often characterized by both high levels of coherency and considerable subsystem misfits. Near the interface, linear elasticity theory evidently fails in such cases to fully correctly predict the subsystem strains. Further, density functional theory based studies on isolated supercells may prove inadequate in capturing strain influences on the chemical interactions underlying the interfacial energy. Recent work within the group has focussed on the implementation of a first principles based hierarchical multi-scale model scheme, capable of determining the interfacial and strain energies for the same model system. Choosing the fully coherent Al-Mg-Si alloy main hardening phase β'' as our test system and limiting our studies to 2D, we discuss the variation in these energies with changing precipitate cross-section morphology and size.

Abstract: There have been many efforts to investigate and develop a mechanical plasticity, damage and failure models for metal alloys in the last couple of decades. These models (single and multi-damage parameters) are generally based on energy and constitutive equations to simulate the fracture and failure processes in metal alloys. The conventional fracture mechanics theory and its applications have been successfully employed to study fracture and failure processes. However, these methods have serious short comes in predicting the damage and failure in metal alloys where the fracture is dominated by the presence of defects like micro-voids (and their growth, nucleation and coalescence), oxides and inclusions. In the present study, following the in-depth study of damage initiation and progression in aluminium alloys, a frame work has been setup to develop a numerical model for damage accumulation. Based on the existing phenomenological damage theory, a mathematical basis for damage initiation and also damage accumulation under wide range of stress triaxiality (including near pure shear) has been developed. The damage model has been checked and verified using a result of experimental-simulation comparative study. The experiments have been carried out using samples made from squeezed and high pressure casting step plates. One of the main contributions of this paper is to show the advantages of using plasticity-based modified damage models to investigate the damage accumulation in cast aluminium alloys.

Abstract: During reversible break-down hot rolling, recrystallization can take place during inter-pass annealing and even after the final pass, because the material is kept at high temperature throughout the process. A partially recrystallized microstructure is quite often obtained during inter-pass annealing and is the starting state for the subsequent rolling pass. This is in particular the case when fine particles are already present or have just precipitated in the microstructure. The particles can pin the moving grain boundaries in the hot deformed material with low energy stored during deformation. To transfer the partly recrystallized microstructure from the recrystallization model to the deformation model for simulation of the subsequent pass, a new link method has been developed for multi-pass through-process modeling. This method allows a direct data flow between the deformation model GIA-3IVM+ and the recrystallization model CORe. This new model framework was used to simulate the microstructure evolution during break-down hot rolling of precipitates containing alloy AA6016. The predicted microstructure agrees well with experimental observations.

Abstract: Predicting yield strength of the cast is difficult, mainly due to inherent chemical inhomogeneity of the microstructure and metal matrix composite nature of the cast. In our approach to predict the yield strength of as cast material AlSi9Cu3(Fe), Scheil-Gulliver model has been used to calculate the phase fraction and chemical composition of each phase during solidification and at each temperature step. Inhomogeneity of the microstructure has been taken into account by considering the evolution of pre-eutectic and eutectic fractions separately. The solidification time-temperature data and cooling to room temperature are recorded using thermocouples and serve as input for the thermo-kinetic software “MatCalc”, that has been used for Scheil simulation and takes into account the evolution of microstructure after solidification and during any arbitrary cooling rate. The strengthening model takes into account the contributions of the intrinsic yield strength of the aluminum matrix, solid solution strengthening, precipitation hardening, effect of eutectic silicon portion and dendrite arm spacing size effect. The phases taken in to consideration include α-Al, Intermetallics, Si and Cu-rich precipitates. The predicted yield strength values are validated by comparing with the experimental values. This approach is extendable to calculate yield strength of the as-cast and heat-treated aluminum alloys.

Abstract: A combination of numerical simulation using the finite element method (FEM) and experimental characterization was used to study material flow and grain deformation during the hot extrusion process for an AA3003 aluminum alloy. The grain structure of the extrudate was experimentally studied using optical microscopy and Electron Back-Scattered Diffraction (EBSD) methods. Using the FEM model predictions of material flow, a simple procedure was used to predict the spatial variation of grain thickness both through the extrusion and along its length. Experimental measurements using EBSD of the grain thickness at the center of the extrudate show that the model predictions are in good agreement with the measurements. The model was then used to calculate the grain thickness changes along the extrudate.