Materials Science Forum Vols. 794-796

Abstract: In the present paper, an extended age hardening model for Al-Mg-Si alloys is presented. In this new approach the combined precipitation, yield strength and work hardening model, known as NaMo Version 1, has been further developed to account for the effects of room temperature storage and cold deformation on the resulting age hardening behaviour. Incorporation of these two new stages in NaMo largely increases the versatility of the model by allowing simulations of complex multi-stage industrial processing involving thermomechanical treatment as well. Part 1 of this work deals with the theoretical background and experimental validation of the extended version of NaMo, while Part 2 focuses on the new applications of the model by showing some numerical examples related to production of automotive body panels.

Abstract: The present paper describes a novel methodology for optimization of product properties and production costs in fabrication of aluminium alloys. The main idea is to represent each operation along the process chain by predictive tools, which include material-, mechanical-, cost-and logistics models. An optimisation tool is used to collect the simulation models into a common software environment, which allows fully automatic simulations to be carried out. When this coupling is established, the models are run in sequence using different types of optimisation strategies. The methodology has been applied for optimisation of strength, grain structure and costs of 6xxx series aluminium extrusions. The results indicate that the present methodology is sufficiently relevant and comprehensive to be used as a tool in fabrication of various aluminium products, for instance in optimisation of end-user properties and production costs of extruded, rolled or foundry based alloys.

Abstract: A through process model for high temperature extrusion of AA3xxx aluminum alloys is presented. An overview of the various chemistry dependent sub-models, e.g. homogenization, extrusion, cold deformation and annealing, are described with an emphasis on the linkages between the models. Examples are presented to illustrate the importance of including the linkages between the sub-models. For example, there is a strong linkage between dispersoids which form during homogenization on subsequent recrystallization behavior after high temperature extrusion or ambient temperature deformation. Finally, a number of observations are presented on the prognosis (opportunities and challenges) for the future of through process modelling.

Abstract: The Al-Cu-Mg alloys currently used at elevated temperature for aerospace applications, such as 2618 and 2219, were developed in the 1950s. Since then, not only have property requirements evolved significantly with the widespread introduction of damage tolerant design, but also the understanding and modelling capacity of the alloys' property-composition-processing relationships have developed beyond recognition. Moreover there is a renewed need for higher strength/toughness, higher temperature solutions in many aircraft's hot areas.A kinetic model has been developed to predict the strengthening capability and the thermal stability of hardening phases. It is based on a homogeneous nucleation, growth and coarsening model applied to S' (Al₂CuMg) and θ' (Al₂Cu); the yield strength is then calculated from the precipitates' size distribution. It suggests two areas of interest in the Al-Cu-Mg diagram.Three targeted compositions were then explored inside and outside the areas of interest and their thermal stability assessed up to 250°C. Different behaviours were observed and are explained by the strengthening potential and the coarsening resistance of S' and θ'. The two interesting areas for thermally stability are confirmed. An area of poorer thermal stability was also identified, associated with a high Cu content in solid solution which accelerates precipitate coarsening kinetics.

Abstract: The influence of hot deformation on the evolution of size, shape, and fraction of dispersoids has been studied in a simple 3xxx aluminium alloy by means of hot torsion testing. It has been shown that at high strain rates, deformation leads to spheroidization of the dispersoids, an increase in number density, and an increase in volume fraction. The increase in number density and volume fraction are associated with precipitation of new particles. The enhancement of manganese diffusion is a key factor in promoting rapid dispersoid evolution during deformation. A model has been developed to estimate the effect of deformation induced vacancies and dislocations on diffusion. This predicts that an order of magnitude increase in diffusion coefficient between may occur under typical hot deformation conditions, consistent with the rapid microstructural changes measured experimentally.

Abstract: Our former experimental study showed that the addition of Er to Al-Zn-Mg alloys speeded up the aging precipitation, accelerated the precipitation of and enhanced the effect of aging strengthening distinctively. In this paper, the Monte Carlo method was applied to simulate the microstructural evolution of Al-2.6Zn-(2.3Mg)-(0.07Er), Al-2.6Zn-2.3Mg-(0.12Er), and Al-2.6Zn-2.3Mg-0.1(Er,Zr) alloys during aging. The effects of Er addition to Al-Zn-Mg alloys on the clustering of Zn and Mg atoms are studied through analysis of the simulation results and the effects on the subsequent aging process are discussed as well. The results show that the Zn/Mg/Er clusters appear beside the Zn clusters, Mg clusters and Zn/Mg clusters in the Er addition Al-Zn-Mg alloys. The Zn clusters and Zn/Mg clusters are finer in the Al-2.6Zn-2.3Mg-xEr alloys than that in the Al-2.6Zn-2.3Mg alloys without Er addition. The size of the Zn clusters and Zn/Mg clusters in the Al-2.6Zn-2.3Mg-0.07Er is eight percent and nineteen percent smaller than that in the Al-2.6Zn-2.3Mg alloys without Er addition respectively. This precipitation refinement effect of Er addition to the Al-2.6Zn-2.3Mg alloys is enhanced with the increment of Er content. These above results are consistent with the experimental results that the precipitation in the Al-Zn-Mg alloys with Er is finer and denser than that in the Al-Zn-Mg alloys without Er. The Er addition changes the clusters distribution in the Al-Zn-Mg alloys by its interaction with the main solute atoms and the vacancy, and thus influences the precipitations during subsequent aging processing.

Abstract: A new rationale to assess the work-hardening locus for pre-rolled sheets is described based on the realization that since the internal stresses necessarily sum to zero, the mean dislocation density remains the same upon re-pull in the rolling direction. Thus the 0.2 % yield stress as function of thickness strains results in an estimate of the stress-strain relation during rolling. Under plane strain, the thickness strain is negative to that of extension and hence the deduced rolling locus is compared to that of extrapolated tensile one of the start sheet. This comparison indicates that the onset of Stage IV occurs when volume fraction of point defects produced attains about 2 %.

Abstract: In this work, repaired crack in 2024 T3 aluminium plate with composite patch is presented, the problem is handled in plane stress condition under tensile Mode I crack growth. In the numerical simulation, firstly we determine the displacement, strain and stress, also the first six mode shapes of the plate, secondly we compare the first results with result of a cracked plate, thirdly we repair the crack with composite patch for different materials ( Glass, Boron, carbon) and for ( 2 layers, 4 layers). Finally the comparison of stress, strain, displacement and six first natural frequencies between uncracked, crack initiation and composite patch repair crack. The best offer operating according to the need is selected.

Abstract: The present investigation deals with modelling of the age-hardening behaviour of 6xxx series automotive sheet alloys. The basis for this work is the established precipitation model NaMo developed for coupled nucleation, growth, dissolution and coarsening in Al-Mg-Si extrusion alloys. It has recently been extended for applicability for Al-Mg-Si automotive sheet alloys by incorporating the important effects of room temperature (RT) storage and deformation prior to the final artificial ageing of Al-Mg-Si sheet alloys. The 6xxx automotive sheet alloys change due to natural ageing during the time elapsing between their processing and their paint baking in the customers process. This RT storage time has an impact on the artificial ageing response during the OEMs paint baking cycle. A second effect originates from the deformation introduced in the material during the part forming process prior to the artificial ageing in the paint bake cycle. This deformation leads to the introduction of dislocations which further modify the artificial ageing response by providing heterogeneous nucleation sites for nucleation of additional strengthening phases. Part 1 of this work deals with the theoretical background and experimental validation of the extended version of NaMo, while Part 2 focuses on the new applications of the extended model by simulation of ageing during paint baking according to typical customer requirements. The model validation is based on a comprehensive set of tensile tests. A comparison between model predictions and measurements shows reasonable agreement, and it is concluded that, after some further development, the model can be used to model the yield strength response of 6xxx automotive sheet alloys incorporating the (combined) effects of natural ageing, deformation and the accurate heat treatments in the paint bake cycle.

Abstract: Under certain conditions of extrusion temperature and strain rate Al-Mg-Si alloys produce coarse recrystallized grains at and near the surface. Current FEM models are able to analyze grain size evolution for extruded profiles, but cannot predict the coarse recrystallized grains near the surface. A new model using DEFORM 2D and local state variables such as strain, strain rate and temperature is compared with Al-Mg-Si rods extruded at 440°C and 500°C for two extremes of strain rate. The model is found to be sensitive to the processing conditions and to accurately predict the recrystallized grain size and fraction.