Abstract
AA6061 and AA7075 aluminum alloys were successfully joined by using single-pass and multiple-pass friction stir welding techniques after which the effects on the nugget zone evolution from a second overlapping pass and its welding direction, were investigated. In comparison to single-pass friction stir welding, the application of a second overlapping pass prolonged the dynamic recrystallization time, and the grains of the nugget zone became finer with increased high angle grain boundaries. Moreover, reversing the welding direction of the second overlapping pass enhanced the vertical flow of materials, increasing the strain of the friction stir welding in the nugget zone. As a result, the efficiency of the grain refinement and mixture of dissimilar materials during the second overlapping pass were significantly elevated. The tensile strength of the nugget zone was improved after the second overlapping pass due to both the grain refinement and mechanical interlocking of the AA6061/AA7075 alloys. The nugget zone, which was fabricated via the multiple-pass friction stir welding technique using an opposite welding direction, exhibited a 23% increase in yield strength as compared to the sample using the single-pass friction stir welding technique.
Highlights
IntroductionDuring Friction stir welding (FSW), the melting of the base metal (BM) does not occur with the BM being joined below the melting point, and as a result, the problems (e.g., porosity and liquation cracking) caused by conventional fusion welding can be avoided [4]
Friction stir welding (FSW) is a solid-state joining technique which was developed by The Welding Institute (TWI) in 1991 [1,2,3]
The AA6061 and AA7075 were first friction stir welded using a single-pass, a second overlapping pass was conducted with the welding direction of the second overlapping pass being the same as (Figure 1b) or opposite to (Figure 1c) the first pass
Summary
During FSW, the melting of the base metal (BM) does not occur with the BM being joined below the melting point, and as a result, the problems (e.g., porosity and liquation cracking) caused by conventional fusion welding can be avoided [4]. Some researchers have performed multiple-pass FSW on aluminum alloys and found that, except for the defect repairing effect, the microstructures and mechanical properties of the nugget zone (NZ) might be varied during the subsequent overlapping pass [9,10,11,12]. Dissimilar AA6061 and AA7075 alloys were first joined by using single-pass FSW, and a second overlapping pass using both the same and opposite welding directions was conducted The influences of both the second overlapping pass and its welding direction on the NZ evolution were investigated, aiming to reveal the multiple-pass FSW behavior in depth
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