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This work was undertaken to investigate the influences of Mg content on the microstructure and mechanical properties of cast Al-2Li-2Cu-0.2Zr alloy. The addition of increasing amounts of Mg led to continuous grain refinement of as-cast base alloy accompanied by the increased volume fraction of secondary phases distributed along grain boundaries or interdendritic regions. The tensile test results showed that Mg addition could greatly enhance the mechanical properties of the base alloy after subjected to the solution treatments and artificial ageing. The precipitation behavior and microstructural evolution were also investigated. The addition of Mg would facilitate the nucleation of δ′ (Al3Li) phases after quenching. The presence of Mg was observed to lower the growth rate of δ′ and δ′-precipitation-free zones (δ′-PFZs) of the base alloy. Precipitation of θ′ (Al2Cu) phase would be almost suppressed, and S′ (Al2CuMg) phase was consequently introduced due to Mg additions. The precipitation of S′ phases was encouraged at the expense of T1 (Al2CuLi) phases as the Mg content increased. The increased amount of S′ phases tended to coalesce to form coarse laths distributed in uneven manner for alloys with relatively high Mg content (≥ 1 wt%). No improvements in mechanical properties were observed in alloys with heterogeneous distribution of coarse S′ laths. The optimal Mg addition in cast Al-2Li-2Cu alloy was 0.5 wt%. [ABSTRACT FROM AUTHOR]