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Oxide fragments in friction stir welds - Distribution and effects on crack initiation

: Jene, T.; Dobmann, G.; Wagner, G.; Eifler, D.

Welding Institute -TWI-, Cambridge:
Proceedings of 6th International Friction Stir Welding Symposium. CD-ROM : 10 - 13 October 2006, Saint Sauveur, Canada
Cambridge: TWI, 2006
ISBN: 1-903761-05-0
Paper 65, 6 pp.
International Friction Stir Welding Symposium <6, 2006, Saint Sauveur>
Conference Paper
Fraunhofer IZFP ()
Rührreibschweißen; Oxidschichtbildung; Rißeinleitung; mechanische Eigenschaft; Partikel; mechanische Festigkeit; Young-Modul; Schweißparameter; Zugprüfung; Oberflächenbeschaffenheit; Spannungs-Dehnungs-Diagramm; Streckgrenze; Aluminiumlegierung; Schweißtechnik; Schweißen

In the present work, friction stir welds of the Al-alloy AA5454 are investigated. The examined micrographs show the mixture of Al-hydroxides and Al-oxy-hydroxides arranged along characteristic lines. Moreover the present work suggests an approach how to avoid this conglomeration of oxide fragments in the weld zone in order to improve the mechanical properties especially the fatigue properties of FSW-joints. The aim of this work is to determine the characteristics and influences of embedded oxide particles as well as to evaluate possibilities to reduce the oxide fragments in the welding zone. In the cross section these particles seem to be arranged in a continuous line and were identified by as a mixture of aluminum hydroxides and oxy-hydroxide. It is assumed that such particles reduce the mechanical strength because of their considerable higher Young's modulus and the resulting strain incompatibility to the surrounding Al-matrix. The oxide particles, often referred to as 'Root Flaw', 'Kissing Bond' or 'Joint Line Remnant', are arranged along a characteristic pattern transversely and longitudinally to the weld. The formation of the pattern is influenced by the welding parameters welding force as well as by the design of the FSW-tool. In order to reduce or even to avoid the presence of such oxide fragments in the welding zone, first investigations concerning the surface finish of the samples were carried out. In order to investigate the influence of the oxide fragments on crack initiation the face and root of the weld were removed before tensile tests were performed to avoid cracks caused by the surface structure of the FSW-seam. In particular the ratio of the feed and rotational speed was correlated with the periodical length of the measured forces and the distribution of the oxide bands within the seam. Furthermore the detrimental effect of the oxide particles within the welding zone could be shown.
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