Effect of Primary and Eutectic Mg2Si Crystal Modifications on the Mechanical Properties and Sliding Wear Behaviour of an Al-20Mg2Si-2Cu-xBi Composite

This work investigated the microstructure evolution, tensile, impact, hardness, and sliding wear properties of an Al-20Mg2Si-2Cu in situ composite treated with different Bi contents. The desired modification of primary Mg2Si particles was achieved with the addition of 0.4 wt% Bi. Increasing Bi beyond 0.4 wt% resulted in a loss of modification, possibly due to the formation of Al8MgBiSi4 compound before the precipitation of the primary Mg2Si. Additionally, the structure of the pseudo-eutectic Mg2Si was transformed from plate to fibrous, which was consistent with decrease of growth temperature extracted from the cooling curve thermal analysis. Addition of Bi had an effect on the morphology of Al5FeSi (β), Al2Cu (θ) and Al5Cu2Mg8Si6 (Q) intermetallic compounds. The tensile strength, elongation percentage, impact toughness, and hardness increased by 6%, 13%, 75%, and 23%, respectively, due to modification of both the primary and eutectic Mg2Si crystals. The tensile and impact fracture surfaces showed fewer decohered particles in the Bi-treated composite. The enhancement in wear resistance of the Bi-treated composite could be attributed to solid lubricant function of insoluble soft Bi phase and modification effects on Mg2Si particles.