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Rheology, Forging and Fatigue Behaviour of Aluminiun Alloys in the Semi-Solid State

Abstract

The objective of the present research proposal is to establish an interdisciplinary group dedicated to the study of aluminium alloys processing in the semi-solid state. The project duration is three years and the activities will be directed to the commercial AA2024 alloy with high solid volume fraction, thus suitable for tixoforging. The research programme is composed by rheological and microstructural studies, the determination of high temperature fatigue strength of the aforementioned alloy and of the A356 alloy, plus the production of a demonstration forged part. The competence thus established will certainly be of practical interest and the interaction with industry will then be a general objective of the group. Semi-solid alloys processing has already achieved commercial status in some areas which were traditionally covered by pressure casting whilst intense development work on forging is underway. As for basic studies the international literature on the subject is growing very fast and semi-solid science and technology has been represented by biannual topical conferences (International Conferences on Semi-Solid Processing of Alloys and Composites). In the last conference (Golden, CO. June 23-25, 1998) almost 100 contributions were presented. In Brasil, the authors of the present proposal have identified a number of industries potentially interested in semi-solid technologies, but the scientific support provided by universities is still limited: there is a very good group at FEA/Unicamp (SP), and some activities are initiating at the Materials Department of UFRGS. Therefore, we think that a new group dedicated to semi-solid science and technology would be welcome. This research proposal brings together professionals with different expertises: material scientists and engineers from UFSCar and metallurgical engineers with emphasis on mechanical properties from USP-SC. Overall nine professionals will be involved, of which three are faculty, the remainder being graduate students and postdoctoral researchers (2). Activities are organized along three sub-projects: sub-project n. 1 deals with the AA2024 commercial alloy whose microstructure will be conditioned by the SIMA process. Simple compression tests (parallel plates) and indentation tests will be employed to determine the viscosity as a function of strain rate, solid volume fraction and temperature. Concurrently, the evolution of microstructural parameters (particle size, contiguity, shape factor) will be quantified and correlated with the rheologic behaviour. Also, the influence of the texture produced during deformation and recrystallization (the steps of the SIMA process) on the microstructural evolution during partial remelting will be studied. The development of a well defined globular structure, with a low degree of agglomeration requires high angle (high energy) grain boundaries, and this situation occurs when there is no strong texture. Conversely, texture should produce some degree of crystallographic alignment of the solid particles, thus leading to their agglomeration which is harmful to formability. Therefore, this effect, still relatively unknown, will be quantified by X-ray diffraction, EBSP, quantitative metallography and compression tests.Sub-project n. 2 consists of studies on the fatigue behaviour of partially remelted material using high temperature fatigue tests. Both isothermic and anisothermic (thermomechanical fatigue) experiments will be carried out, with the objective of comparing conventional whrought (Al-4Cu or 2024) and cast (A356) alloys with the same materials prepared by the SIMA technique. Tixoforging experiments will then be carried out and a demonstration part will be produced using a range of process conditions. The part will be characterized in terms of hardness, soundness, surface finish and microstructural homogeneity. Sub-project n. 3 is directed to the improvement of the SIMA process and aims to the reduction of the recrystallized grain size, hence of the solid particles after partial remelting. As a result, the solid/liquid mixture viscosity will be lower and the final product grain size will be smaller. The project will study the effect of second phase precipitate dispersions (q phase for the 2024 alloy) on the recrystallized grain size. In principle three situations will be studied: (i) no precipitates; (ii) very large precipitates obtained by slow cooling through the solvus, (iii) a bimodal size distribution of precipitades. Recrystallization kinetics, as well as recrystallized grain size and precipitates dispersion parameters (average radius, spacing, volume fraction) will be measured. Results will be correlated and analyzed within the framework of the particles nucleated recrystallization (PNS) existing theories. (AU)

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