This study by Yang Tao et al. investigates the effects of aging conditions on the microstructure and mechanical properties of the Heat-Affected Zone (HAZ) in welded joints of Al–Mg–Si–Cu alloys. The study examined extrusions treated with T4, T5, and T6 heat treatments and analyzed the mechanisms behind the observed softening in the HAZ region.

Findings:

HAZ I:

Hardness increase was linked to the solution hardening mechanism.

In the T4 and T5 conditions, precipitate phases dissolved into the matrix.

In the T6 condition, the insoluble Q phase remained.

HAZ II:

Decrease in hardness was attributed to the limited precipitation of β'' and Q' phases in the T4 condition.

In the T5 and T6 conditions, the dissolution of β'' phase followed by precipitation of Q' and Q phases was the main cause of softening.

HAZ III:

In this region near the base metal, hardness gradually recovered.

This study provides theoretical guidance for the development of high-strength and heat-resistant aluminum alloys by understanding the mechanisms behind the hardness changes in the HAZ under different aging conditions.

For a detailed review, you can access the related study here.