Orientation microscopy study of cryogenic-temperature friction stir processed AA6xxx sheet: microstructure and second...

Area of Science:

• Materials Science
• Metallurgy
• Mechanical Engineering

Background:

• AA6xxx aluminum alloys are widely used in automotive and aerospace industries.
• Understanding microstructure evolution under cryogenic processing is crucial for optimizing mechanical properties.
• Friction stir processing (FSP) is a solid-state joining and processing technique with potential for microstructural modification.

Purpose of the Study:

• To investigate the effects of cryogenic-temperature friction stir processing (CT-FSP) on the microstructure and mechanical response of AA6xxx sheets.
• To analyze precipitate evolution and grain refinement during CT-FSP.
• To establish microstructure-property linkages in CT-FSPed AA6xxx.

Main Methods:

• Cryogenic-temperature friction stir processing (CT-FSP) at 600 RPM and 500 mm/min with liquid N2 cooling.
• Multiscale characterization using Electron Backscatter Diffraction (EBSD), Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM/EDS).
• Microhardness testing and deep-learning-based image segmentation for particle analysis.

Main Results:

• CT-FSP resulted in a highly refined, equiaxed stir zone (SZ) with grain sizes of 1.6-1.8 µm.
• Dynamic recrystallization (DRX) dominated the SZ, evidenced by high-angle grain boundaries and low Kernel Average Misorientation (KAM).
• Second-phase particles (Al-Fe-Si, Mg2Si) were fragmented, partially dissolved, and reprecipitated, leading to finer, more uniform distributions and reduced particle sizes (0.6-0.8 µm).

Conclusions:

• CT-FSP effectively refines the microstructure of AA6xxx sheets through DRX and particle modification.
• Localized particle refinement and redistribution, along with DRX, explain the observed stir zone softening and thermomechanically affected zone hardening.
• A clear microstructure-property linkage was established, demonstrating the potential of CT-FSP for tailoring mechanical responses in aluminum alloys.