Abstract
In this study, Al-Zn-Mg-Cu alloy was refill friction spot welded, and the precipitates, dislocation, recovery, and recrystallization characteristics were focused. In the stir zone (SZ), continues dynamic recrystallization occurs under the intense plastic deformation. All the original GP (II) zones and η’ precipitates dissolved into the aluminum matrix under the welding heat input, and the stable η and E precipitates remained. In the thermo-mechanically affected zone (TMAZ), high-density dislocations and subgrains boundaries can be observed. The continued dynamic recrystallization was not activated and only dynamic recovery occurs. Sub-boundaries and high-density dislocations in this zone can be observed. In this zone, the η precipitates are coarsened and dissolution is the main evolution mechanism for the GP (II) zones and η’ precipitates. In the heat-affected zone (HAZ), no dislocation was induced and all the initial precipitates were coarsened under the welding heat input. The HAZ, the TMAZ, and the SZ constitute a soft region in the refill friction stir spot welding (RFSSW) joint, and the minimum value located at the interface between the HAZ and the TMAZ.
Highlights
For the Al-Zn-Mg-Cu alloys, the introduction of Zn and Mg makes the formation of MgZn2 strengthening particles in the alloys
In order to realize high quality joining for aluminum alloys, a new solid-state welding technology called refill friction stir spot welding (RFSSW) is developed by Helmholtz-Zentrum Geesthacht [2]
In order to further reveal the microstructural characteristics of the RFSSW joint of the Al-Zn-Mg-Cu alloys, 7B04-T74 aluminum alloy were refill friction stir spot welded in this study
Summary
For the Al-Zn-Mg-Cu alloys, the introduction of Zn and Mg makes the formation of MgZn2 strengthening particles in the alloys. Zhao et al [10] studied the microstructures of RFSSW joints of 7B04-T74 aluminum alloy at different tool rotation speeds They found that the grains in the outer stir zone were equiaxed, and the grains in the inner stir zone were elongated and coarsened. They pointed out that continuous eutectic films can be observed in the stir zone (SZ) when the tool rotation speed was relatively high, and local liquation crack can be formed when the eutectic films were gathered This liquation crack had significant adverse effect on the tensile shear load of the RFSSW joint [5]. In order to further reveal the microstructural characteristics of the RFSSW joint of the Al-Zn-Mg-Cu alloys, 7B04-T74 aluminum alloy were refill friction stir spot welded in this study. The precipitates, dislocation, recovery, and recrystallization characteristics which were rarely concerned before were studied using transmission electron microscopy
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