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Abstract
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The possibility of underwater dissimilar friction stir welding of AA6061 and AA7075 aluminum
alloy was explored to overcome the problem of hardness loss in different microstructural
zones. Optical microscopy and electron backscattered diffraction were employed to characterize the
microstructure of the joint. Vickers hardness measurements were conducted on the cross-section of
the joint to evaluate the mechanical strengths. The results showed that the microstructure of the
AA7075 side had undergone the same mechanisms as those occurring during conventional friction
stir welding. In the case of the AA6061 side, in addition to typical restoration mechanisms, the grain
subdivision was observed. The AA7075 side had finer grains compared to the AA6061 side, which
may be related to the different morphology and size of precipitates. Moreover, friction stir welding
caused a reduction in the hardness values in all the microstructural areas compared to those of
corresponding base materials. For example, it caused a reduction in the hardness of a thermomechanically
affected zone from 105 HV to 93 HV in the AA6061 side, and from 187 HV to 172 HV in
the AA7075 side. The underwater media improved the overall hardness values in thermo-mechanically
affected zones (13% reduction in hardness) compared to those reported in literature (57% reduction
in hardness).
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