A process model for friction stir welding of age hardening aluminum alloys
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I. INTRODUCTION
THE interest in friction stir welding (FSW) has gained considerable momentum over the past few years.[1–4] This is because the process has made it possible to implement the advantage of solid-state bonding to aluminum plate and profile joints, thus leading to new product designs previously not feasible. In FSW, the rotating movement of the shoulder and the pin generates the heat. The frictional heating contributes to the formation of a plasticized layer of soft metal beneath the tool shoulder and about the pin. The material is then transported to the flow side of the tool due to the imposed mechanical stirring and forging action before it cools and forms a solid-state joint. At present, the basic mechanism behind the weld formation and the underlying physical processes that lead to microstructural changes during FSW of aluminum alloys appear to be reasonably well established.[5–20] In recent years, significant progress has been made in the understanding of physical processes that take place during welding of aluminum alloys.[21–28] A synthesis of that knowledge has, in turn, been consolidated into process models, which provide a mathematical description of the relation between the main welding variables (e.g., heat input, plate thickness, and joint configuration) and the resulting weld properties, based on sound physical principles.[21,24,25,28] The components of such a model will be as follows: (1) a heat flow model for prediction of the temperaturetime pattern during welding; (2) kinetic models for prediction of the heat-affected zone (HAZ) microstructure evolution (e.g., volume fraction of hardening precipitates) as a function of temperature; and (3) constitutive equations, based on dislocation mechanics,
Ø. FRIGAARD, Senior Engineer Metallurgist, is with the Material Command, Analytical Laboratory, Royal Norwegian Airforce, N-2027 Kjeller, Norway. Ø. GRONG, Professor of Metallurgy, is with Department of Materials Technology and Electrochemistry, The Norwegian University of Science and Technology, N-7491 Trondheim, Norway. O.T. MIDLING, Senior Project Manager, is with Hydro Aluminium Maritime AS, N-4262 Avaldsnes, Norway. Manuscript submitted May 9, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
which provide quantitative information about the resulting HAZ hardness or strength. In the present investigation, this concept is further developed and applied to FSW of AA6082-T6 and AA7108-T79 aluminum alloys. The former alloy is commonly used for structural applications, while the AA7108 has traditionally been utilized in production of autoparts such as bumpers and more recently in load bearing structures such as ship deckings and stiffeners. Included in the investigation is also a characterization of the subgrain structure that forms within the plastically deformed region of the weld HAZ, using the electron backscattered diffraction (EBSD) technique. By combining information about the subgrain size with outputs from the heat flow model, an estimate of the mean strain rate within this region has be
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