A Review on Friction Stir Welding of High-Strength Al-Zn-Mg Alloy: Insights on Second-Phase Particles

Abstract

The friction stir welding (FSW) process is a unique combination of deformation and high temperature, which provides opportunities to modify microstructures through the adjustment of the processing parameters and is an ideal way to join non-weldable aluminum alloys by avoiding the formation of a molten pool. The 7xxx series heat-treatable aluminum alloys are widely used in the aerospace field as high-performance structural materials. The microstructure evolution and mechanical performance of these alloys are affected by the effects of thermomechanical processing, which provides opportunities to optimize the material properties by controlling microstructural features such as intermetallic constituent particles, dispersoids and nanoscale precipitates. This paper focuses on the basic principles of the thermal and mechanical effects generated during FSW on the evolution of second-phase particles in different zones of the weld.

Keywords: Al-Zn-Mg alloy; dispersoids; friction stir welding; intermetallic constituent particles; microstructure evolution; strengthening precipitates.

Figure 1

Schematic processing route of Al-Zn-Mg heat-treatable aluminum alloy sheets.

Figure 2

HAADF-STEM images of dispersoids and precipitates in single-peak aged 7075 aluminum alloy: (a) precipitates (green box) and dispersoids (red arrows); (b) MgCu_xZn_2−x nucleated on Al₁₈Mg₃Cr₂ dispersoids; (c) an η precipitate (yellow arrow) among a high density of fine η′ precipitates; (d) a large quantity of fine η′ precipitates (green arrows); (e) elemental mapping (EDS) of (b) [28].

Figure 3

Principle of FSW process and weld zones.

Figure 4

Morphologies of Al₇Cu₂Fe in 7050-T651 base metal (a,b) and in stir zone of FSWeld (c,d) [43].

Figure 5

TEM images of SZ-FSW 7075Al-T651 join: (a) randomly distributed dispersoids (Mg₃Cr₂Al₁₈); (b) dispersoids pinning dislocations [48].

Figure 6

Precipitation behavior of 7075-T6 in SZ of FSWeld (a) and IFSWeld (b) [49].

Figure 7

Precipitate evolution in 7050-T651 FSWeld: (a) base metal, (b) HAZ, (c) TMAZ I, (d) TMAZ II, and (e) SZ [43].

Figure 8

Precipitate distribution: (a) 7055-T6 base metal; (b) in HAZ of FSWeld [77].

Figure 9

Schematic diagram of the details of the evolution of the microstructure of the HAZ during FSW of AA7108-T6 [69].

Figure 10

Schematic diagram of the strengthening phase evolution in different zones during FSW.

Figure 11

Schematic diagram of the relationship between the hardness distribution and the strengthening precipitates in a FSW joint.