Microstructural Characterization of Internal Welding Defects and Their Effect on the Tensile Behavior of FSW Joints of A
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AMONG processes that have been developed to substitute riveting in lightweight structures, friction stir welding (FSW) offers the advantages of avoiding hot cracking and limiting component distortion. Application of this process to aluminum alloys has been extensively reviewed recently.[1] For certain processing conditions, internal flaws may appear. Internal cavities and tunneling defects could be detected at least in certain cases using non-destructive evaluation methods.[2] They may impair the ductility of the welded joints, in particular for high values of the rotational speed.[3,4] In most of aluminum friction stir welds, a discontinuous, wavy surface within the nugget referred to as ‘‘joint line remnant’’ (JLR), ‘‘lazy S,’’ ‘‘zigzag THOMAS LE JOLU, formerly Ph.D. Student with MINES ParisTech, Centre des Mate´riaux, UMR CNRS 7633, BP 87, 91003 Evry Cedex, France, is now Research Engineer with the CEA Saclay, DMN/SEMI/LCMI, 91191 Gif-Sur-Yvette Cedex, France. THILO F. MORGENEYER, Researcher, MOHAMED SENNOUR, Research Engineer, ANNE LAURENT, Technical Assistant, JACQUES BESSON, Researcher, Director, and ANNE-FRANC¸OISE GOURGUES-LORENZON, Professor in Materials Science and Engineering, are with the MINES ParisTech, Centre des Mate´riaux, UMR CNRS 7633. Contact e-mail: [email protected] ANNE DENQUIN, Deputy Director, is with the Department of Metallic Materials and Structures, Onera, 29 Avenue de la Division Leclerc, BP 72, 92322 Chaˆtillon Cedex, France. Manuscript submitted September 6, 2013. Article published online September 6, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
curve,’’ or ‘‘zigzag line’’ may be found after welding.[5– 10] Such a feature is connected to the weld root and may induce fracture after severe bending of the weld. In this case, it is referred to as ‘‘kissing bond’’ (KB), ‘‘weak bond,’’ or ‘‘root flaw’’.[6,11–15] Both JLR and KB seem to originate from insufficient mixing of matter close to the initial butt surfaces,[5–7,13,16] but KB results more specifically from a lack of pin penetration during welding. From TEM studies of 1050-H24 and 5052 alloys,[3,17] the poorly mixed oxide film close to the weld root seems to be discontinuous for the JLR but continuous and intergranular for the KB. Plastic flow and recrystallisation in the nugget have been characterized as a function of welding parameters (see e.g.,[18–22]); however, information about stirring conditions very close to the bottom surface of FSW joints is still lacking. As a whole, only scarce data are available from open literature and still need to be completed by considering other alloys. Another kind of feature may appear if there is some distance (a ‘‘gap’’) left between the two sheets to be butt welded. Metallographic observations of cross sections showed that leaving such a gap caused the presence of cavities, such defects can be remediated by increasing the heat input energy.[12,23] Little data are available about the consequences of such a gap on the mechanical properties of the welded joint, in the c
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