Mechanical Properties and Microstructural Evolution of Friction-Stir-Welded Thin Sheet Aluminum Alloys
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TRODUCTION
FRICTION stir welding (FSW) is a solid-state joining method particularly suited for aluminum alloys, which are often difficult to fusion weld without hot cracking, porosity, or distortion. During welding, the material is heated frictionally to a temperature at which it becomes extremely plastic. The heat of friction and plastic flow arising from the rotating tool produces significant microstructural changes, which lead to local variations in the mechanical properties of the weld.[1–5] FSW is being targeted by the industry for structurally demanding applications to provide high-performance benefits.[6] The weld zone consists of a stir zone, a thermomechanically affected zone (TMAZ), and a heataffected zone (HAZ). The grain size in the stir zone is fine and equiaxed, resulting in a higher mechanical strength and ductility.[7–13] In FSW, the work piece does not reach the melting point, and the mechanical properties such as ductility and strength of the welded zone are much higher compared with the traditional welding techniques.[14] The attractive mechanical properties are principally a result of the strong grain refining effect of the process. EMANUELA CERRI, Associate Professor, and PAOLA LEO, Research Assistant, are with the Department of Innovation Engineering, University of Salento, via per Arnesano, 73100 Lecce, Italy. Contact e-mail: [email protected] XIANG WANG, Research Associate, and J.D. EMBURY, Professor Emeritus, are with the Department of Materials Science and Engineering, McMaster University, Hamilton, Ontario L8S 1XS, Canada. Manuscript submitted September 10, 2009. Article published online November 10, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
The data available in the literature suggest that the mean grain size resulting from the FSW process is at least 10 times smaller than that measured in the undeformed parent material.[15,16] Thus, the results of these previous studies suggest that FSW can cause grain refinement through severe plastic deformation. In particular, a single pass of FSW produces an estimated effective strain >40,[17] resulting in a mean size of 0.5 to 10 lm. The strong plastic deformation of the material during welding operation and the advantage that a FSW process may reach 573 K to 673 K (300 C to 400 C) in the nugget, lead to a strong recrystallized structure with high performances in terms of strength and potential superplastic properties. From this point of view, some authors have suggested that the microstructure in the weld zone evolves through a continuous dynamic recrystallization process.[10] Actually, this is a core demand of aircraft and car industries to substitute the traditional joining technologies with low-cost and high-efficiency technologies such as FSW in the future advanced design. Recent literature reports examples of FSW of dissimilar aluminum alloys,[18–20] as well as aluminum-steel,[21,22] aluminum-magnesium,[23] and aluminum-silver.[24] FSW is well established for making butt joints in aluminum alloys in material thicknesses from 1.3 to 15 mm
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