Mechanical behaviour of similar and dissimilar AA5182-H111 and AA6016-T4 thin friction stir welds C. Leitao a , R.M. Leal a,b , D.M. Rodrigues a, * , A. Loureiro a , P. Vilac ßa c a CEMUC, Department of Mechanical Engineering, University of Coimbra, Portugal b ESAD.CR, Polytechnic Institute of Leiria, Caldas da Rainha, Portugal c IST, Technical University of Lisbon, Lisbon, Portugal article info Article history: Received 18 December 2007 Accepted 16 April 2008 Available online 28 April 2008 Keywords: Taylor welded blanks Friction stir welding Aluminium alloys Thin sheets abstract The tensile behaviour of similar and dissimilar friction stir welds in 1 mm thick sheets of two aluminium alloys (AA5182-H111 and AA6016-T4) is analysed in this paper. The heterogeneity in properties across the welds was studied by performing microhardness tests and microstructural analysis. The tensile tests were performed in samples extracted longitudinal and transverse to the weld direction. It was found that the tensile behaviour of the welds depends mainly on the grain size in the TMAZ, for the AA5182-H111 alloy, and on precipitate distribution, for the AA6016-T4 alloy. In all types of welds, the HAZ preserves the same properties of the base materials. The global mechanical behaviour of the AA5182-H111 similar welds is very similar to that of the base material. However, for the AA6016-T4 similar welds and for the AA6016-T4-AA5182-H111 dissimilar welds a 10–20% strength reduction relative to the base materi- als and important losses in ductility were reported. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Increasing industrial concern with ambience and energy is becoming notorious. In this context, friction stir welding (FSW) ap- peared as an easy, ecologic and promisingly productive weld meth- od that enables to diminish material waste and to avoid radiation and harmful gas emissions, usually associated with the fusion welding processes. The FSW tools are mainly constituted by a small diameter entry probe and a concentric larger diameter shoulder, both usually made of high strength steel. During the weld process, the FSW tool is rotated and the probe is plunged into the boundary of the adjoining plates. Penetration depth of the probe is controlled by its length and by the tool shoul- der, which should be in intimate contact with the plates during welding. The heat generated by friction between the rotating tool and the plates promotes a local increase in temperature and soft- ens the materials under the tool shoulder. At the same time, the plunged rotating probe moves and mixes the softened materials, by intense plastic deformation, joining both in a solid state weld. According to the temperature attained and the volume of mate- rial which is plastically deformed during the welding process, it is usually possible to distinguish two main zones, with different characteristics, in the FS welds: the thermomechanically affected zone (TMAZ), that is constituted by the material plastically de- formed during the welding process, and the heat affected zone (HAZ), comprising the material affected by the weld thermal cycle but not plastically deformed [1–4]. Frequently, part of the TMAZ presents a recrystallized fine-grained microstructure, resulting from the combination of extremely high plastic deformation and temperature, which is usually called as Nugget. The HAZ of the fric- tion stir welds is of the same nature of the heat affected zone of welds resulting from the fusion welding processes [5]. Despite the large amount of published literature about the FSW process, systematic information does not exist on the influence of the tool and the process parameters on the weld quality for a large range of materials, thicknesses and joint configurations. Until now, FSW industrial application had mostly been restricted to the con- struction of large components in shipbuilding and aerospace and aeronautics industry [6,7]. The application of this process in the automotive industry is relatively recent and has one of its main fields of interest for the production of aluminium tailored welded blanks (TWB) from very thin sheets [8–13]. In fact, some difficul- ties continue to restrict the application of TWBs in industry [14– 17] such as, the difficulty in welding some materials (Al alloys and HSS), the strength reduction in the weld line and the poor formability of the TWBs. The FSW process diminishes some of the weldability problems usually associated with fusion welding processes, due to its low heat input [6]. However, FSW process has limitations in butt-joining thin sheets. The thickness reduction resulting from the forging effect of the shoulder can significantly reduce the mechanical resistance in thin plates (1 mm or less). The presence of micro defects, usually acceptable in thick welds, also pose serious problems in thin plate sheet welds [11]. 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.04.045 * Corresponding author. Tel.: +351 239 790 700; fax: +351 239 790 701. E-mail address: dulce.rodrigues@dem.uc.pt (D.M. Rodrigues). Materials and Design 30 (2009) 101–108 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes