Effect of thermo mechanical treatments and aging parameters on mechanical properties of Al–Mg–Si alloy containing 3 wt.% Li

• TMT studies of 3 wt.% Li on Al–0.5Mg–0.2Si in W, T6 and T8 conditions.
• Artificial aging at 175 °C for 2–12 h with prior cold reductions of 10–60%.
• Hardness surveys, YS and UTS, microscopy, fractography and DSC studies.
• Mechanical properties significantly enhanced in T8 condition.
• Property enhancement attributed to a possible refinement of δ′ (Al3Li) precipitates.

In the present work, microstructure and mechanical properties of 3 wt.% Li addition in a Al–Mg–Si alloy of target composition 0.5 wt.% Mg and 0.2 wt.% Si in W (solution heat treated), T6 (solution heat treated and artificially aged) and T8 (solution heat treated, cold worked and artificially aged) conditions was studied. The age-hardening response of the alloy was determined after systematic cold reductions from 10%, 20%, 30%, 40%, 50% and 60% in quenched condition followed by aging at 175 °C (448 K) for 2, 4, 6, 8, 10 and 12 h (T8 condition). The results were compared with samples aged in the same conditions with 0% cold reduction (T6 condition). The alloy displayed a strong artificial aging response and maximum hardness value achieved was after 60% cold work and 10 h of aging time. Furthermore, the yield strength and the ultimate tensile strength were increased from 123 MPa to 224 MPa and 356 MPa to 540 MPa respectively with a slight decrease in ductility. Scanning electron microscopy (SEM) based fractography showed a uniform network of bigger and deeper dimples with round morphology in T6 condition while a ductile tearing with few discernable cleavage planes was observed in T8 condition. The interplay of various precipitation hardening mechanisms and relevant phases was established by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). It was concluded that the enhancement in mechanical properties, with the degree of cold work, was attributed due to a possible refinement of δ′ (Al3Li) precipitates resulted after aging.