Microstructure and Cyclic Deformation Behavior of a Friction-Stir-Welded 7075 Al Alloy
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THE 7xxx-series aluminum alloys are precipitationhardened Al-Zn-Mg-(Cu) alloys that have been used extensively in the aircraft structural components, mobile equipment, and other highly stressed applications.[1] Zinc and magnesium are the main alloying elements. The undesirable iron and silicon impurities are present in the form of coarse constituent particles, i.e., Al7Cu2Fe, Al2CuMg, and Mg2Si.[2,3] Zirconium or chromium is added to retard recrystallization and control grain size.[4] Most 7xxx-series aluminum alloys can be categorized in two groups, depending on the type of grain structure and controlling element: Zr-containing alloys (Al3Zr) and Cr-containing alloys (Al7Cr).[5] Friction stir welding (FSW), invented at The Welding Institute (Cambridge, United Kingdom) in 1991, is an environmentally friendly, energy-effective, and versatile solid-state joining technique capable of welding lightweight materials, i.e., 7xxx-series aluminum alloys, which are relatively difficult to weld using conventional fusion welding techniques.[6,7] It is thus considered to be the most significant development in materials joining in the past two decades. It is therefore essential to identify A.H. FENG, Postdoctoral Fellow, and D.L. CHEN, Professor and Ryerson Research Chair, are with the Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, ON M5B 2K3, Canada. Contact e-mail: [email protected] Z.Y. MA, Professor, is with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Manuscript submitted July 8, 2009. Article published online January 22, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
the relationships among joining parameters, microstructures, and mechanical properties. Extensive studies of the FSW/friction stir processing of 7xxx-series aluminum alloys have been reported, including studies of the precipitation sequence,[8–10] recrystallization mechanisms,[4,11,12] superplasticity,[13,14] material flow,[15] residual stresses,[16] and mechanical properties.[17–26] While the FSW of aluminum alloys has engendered considerable scientific and technological interest, material property data are still limited, especially on fatigue properties that directly limit the widespread applications of the FSW process.[7,18–26] The 7xxx-series aluminum alloys exhibit very high strength but also poor ductility and high notch sensitivity.[27] The fatigue of the commercial 7075Al-T6 alloy has received considerable attention.[28] The FSW offers high joint quality and good fatigue performance.[22] From an engineering design perspective, the fatigue properties of the friction-stir-welded aluminum alloys are of particular importance. This has led to increasing research interest in evaluating the fatigue resistance of the friction-stirwelded joints, including the stress number of cycles to failure behavior[20,22,25,26] and fatigue crack propagation behavior.[7,18,19,21] Previous studies indicated the following results.[7] First, the
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