Effect of Alclad Layer on Material Flow and Defect Formation in Friction-Stir-Welded 2024 Aluminum Alloy
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pical high-strength aluminum alloy, 2024 aluminum (2024Al) alloy is gaining commercial importance in aerospace industries as wing and fuselage structures where high specific strength and fatigue resistance are required.[1] It is usually clad with pure aluminum or Al-1Zn alloy to improve the corrosion resistance. However, its industrial application is restricted by the process technology, especially welding technology, because it is unweldable using the conventional fusion welding techniques because of its hot cracking sensitivity. The structures are fastened together usually using rivets that not only increase the weight but also reduce the fatigue properties.[2] Therefore, a new welding method that can join the high-strength aluminum alloys is highly desirable. Friction-stir welding (FSW), which is a relatively new solid-state joining process that uses a nonconsumable rotating welding tool to generate frictional heat at the welding location without material melting, produces pore-free joints and smaller temperature gradient than conventional arc processes.[3,4] Consequently, FSW is considered an ideal process for joining high-strength aerospace aluminum alloys that typically are difficult to weld.[5] In the previous studies,[5–7] Unclad 2024Al plates were subjected to extensive FSW investigations, and it was reported that the Unclad 2024Al plates could be welded successfully by FSW. However, the FSW of an Alclad 2024Al alloy was seldom conducted.[8] Talwar et al.[8] reported that the Alclad layer on the root surface of the Z. ZHANG, Postgraduate, B.L. XIAO, Professor, D. WANG, Assistant Professor, and Z.Y. MA, Professor, are with the Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: [email protected] Manuscript submitted April 27, 2010. Article published online November 24, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
weld was brought into the interior of the weld along the advancing boundary of the nugget zone (NZ) and thermomechanically affected zone (TMAZ), which deteriorated the mechanical properties of the weld. Machining the root side to remove the Alclad layer increased the ductility and tensile strength of the weld. However, the effect of the top Alclad layer was not mentioned in their research. It should be pointed out that for the engineering applications, it is not a wise method to remove the surface layer by machining. Thus, the FSW of the Alclad 2024Al alloy becomes of practical significance. During FSW, heat is generated by the friction between the welding tool and the workpiece, and by the plastic deformation around the tool, which determines the weld microstructure and properties.[3] Thus, the tool/workpiece interface plays an important role during FSW. Reynolds[9] reported that the condition of the tool/workpiece interface affected the material flow strongly, but they did not give the details. For FSW of the Alclad 2024Al alloy, the Alclad layer may change the tool/workpiece interface condition. Therefo
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