Microstructures Evolution in Refill Friction Stir Spot Welding of Al-Zn-Mg-Cu Alloy

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.

Friction stir welding of dissimilar alloy combinations—A Review

“…37 In addition, the stable precipitates reduced the migration of dislocations and grain growth, resulting in finer re-crystallized grain at the stir zone. 85 From the previous works, the grain size at the stir zone is listed along with the joined materials, tool shapes and operating parameters for different combinations of alloys are presented in Table 3.…”

Refill friction stir spot welding of AlSi10Mg alloy produced by laser powder bed fusion to wrought AA7075-T6 alloy

“…In contrast to the increase in hardness with increasing distance from the SZ in the upper sheet, a further decrease could be observed in the lower sheet, although hardness in the wrought alloy is still higher than in the printed AlSi10Mg. The minimum hardness in the HAZ of the AA7075 is typical for precipitationhardenable aluminium alloys because coarse precipitates lead to the over-ageing of the material [18]. The measured temperatures in the lower sheet at a distance of 10 mm from the weld centre (TC2), and on the outside of the clamping ring (TC0) increase constantly throughout welding, reaching local peak temperatures of 220 °C and 340 °C, respectively.…”

Mechanical properties and failure analysis of refill friction stir spot-welded 5052/2A12 joints

“…Many recent studies have focused on failure mechanisms of RFSSW aluminium alloy joints [21][22][23][24][25][26][27]. Zou et al [21] found that the weak bonding length at the thermo-mechanically affected zone (TMAZ)/stir zone (SZ) interface and the bending height of hook defects had the strongest effects on shear plug fracture mode.…”

“…Mechanical properties were mainly affected by the joint qualities between the SZ and TMAZ, which was affected by the hook [22,23]. Li et al [24] found that hook defects occurred in the fracture path and tensile shear properties decreased in both fracture modes. Sara et al [25] analysed tensile shear testing using finite-element analysis, and found that a stress concentration exists at the hook during a test; however, the relationship between expansion of cracks and strain distribution during the fracture process is not known.…”