Abstract
The microstructures and mechanical properties of friction stir welded (FSW) Al–Zn–Mg alloy plate under different travel speeds were investigated. Both the average grain sizes (AGSs) of the shoulder affected zone (SAZ), nugget zone (NZ), and the widths of thermo-mechanically affected zone (TMAZ) decreased with the increase of travel speed. Moreover, the AGSs of NZ are always about 60% of that of SAZ at different travel speeds. The fractions of high-angle grain boundaries (HAGBs) in the FSW joints reduce with the distance away from the stir zone (SZ). Furthermore, the initial η’ strengthening precipitates in NZ and TMAZ dissolve and GP zones form during subsequent natural aging, so that the hardness is similar in the two zones. The precipitate evolution in the heat-affected zone (HAZ) at hardness minima are affected by travel speeds, which induce the hardness minima and ultimate tensile strength (UTS) of FSW joints and increase with the increase of travel speed, and a fracture tends to occur at hardness minima location of HAZ during tensile testing.
Highlights
Friction stir welding (FSW) is a solid-state joining technique invented by The Welding Institute of UK in 1991 [1,2]
This novel solid-state FSW process can be widely applied to the joining of high-strength Al–Zn–Mg alloys, which are difficult to be welded by conventional fusion welding techniques due to the solidification of cracks, severe softening behavior, and severely degraded mechanical properties within the joints [14,15]
The results showed that precipitation strengthening in thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) was weakened with the increase of welding speeds, leading to a narrower softened region and an increase in the lowest hardness value
Summary
Sakthivel [32] found that the fine equiaxed grains in NZ were more homogeneous at low welding speed than high welding speed In these investigations, little attention has been paid to the evolution of grain structures and precipitates in different zones of FSW joint under different travel speeds, and the correlations between microstructures and mechanical properties under different travel speeds need further study. The effects of tool travel speeds on microstructure and the mechanical properties of FSW joints of 7N01 aluminum alloy have been investigated by means of optical microscopy, scanning electron microscopy electron backscattered diffraction (SEM-EBSD), transmission electron microscopy (TEM), hardness testing, and tensile testing. This can help to achieve a better understanding of the correlations between microstructure and mechanical properties and further improve the mechanical properties of FSW joints of Al–Zn–Mg alloys
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