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JMEPEG https://doi.org/10.1007/s11665-019-04060-9

Effect of Through-Thickness Friction Stir Processing Parameters on Weld Repair and Modification of Fusion-Welded AA6061 Aluminum Alloy Saeed Aliakbari, Mostafa Ketabchi, and Seyyed Ehsan Mirsalehi (Submitted July 23, 2018; in revised form March 17, 2019) Through-thickness friction stir processing was successfully carried out for microstructure modification and elimination of fusion defects of the gas metal arc welds by applying the process on as-welded joints. Variation in rotational speed from 750 to 1050 rpm and traveling speed from 50 to 200 mm/min resulted in minor changes (approximately 1 lm) in the grain size of nugget zone, while this variation in elongation result was up to 40%. The hardness profile, the width of softening zone, minimum hardness and fracture location also changed for different processing conditions. Furthermore, the defects were vanished efficiently across the welds profile by this method. The results also verified the geometric dynamic recrystallization phenomena as a dominant mechanism of microstructural modification. Keywords

AA6061 alloy, friction stir processing, tool rotational speed, tool travel speed, weld modification, weld repair

1. Introduction AA6061 is a heat treatable aluminum alloy and is extensively used in aerospace, automotive and shipbuilding industries (Ref 1). Solid-state bonding methods and fusion welding techniques are adopted for joining this alloy. The friction stir welding (FSW) is a prominent solid-state welding technique that has been widely used. Furthermore, some new joining techniques such as compressing process (Ref 2), reshaping process (Ref 3), twostep clinching process (Ref 4) have been promoted for this alloy. But based on thickness and technical consideration, in many cases, it is necessary to use fusion welding techniques. Gas metal arc welding (GMAW) is a fusion welding technique which is widely used for joining these alloy components. It is due to the high welding rate and automatic welding capability. However, fusion-related defects and imperfections reduce the mechanical properties by up to 43% in welded joints (Ref 5). The wide range of solidification temperatures and large volume shrinkage during solidification make this alloy susceptible to hot cracking (Ref 6). In addition, the welding processes cause overaging in the HAZ and hydrogen porosity in the weld zone (Ref 7, 8). High reactivity and sticky oxide layer also make this alloy susceptible to lack of fusion (LOF). Several studies have used friction stir processing (FSP) for fusion welding modification and defect removal. Nevertheless,

their studies have focused on surface and near-surface treatment and modification, rather than on the bulk of the welds (Ref 9). The studies have suggested that FSP of GMA welds improves fatigue life due to surface geometry and microstructural modification and also reduction in weld defects. However, it does not modify hardness and mechanical strength significantly (Ref 10, 11). Friction stir processing is technically the