Effect of Si content on solidified and heat treated structure of Al–Li–Mg casting alloys

Abstract

The effects of Li and Si contents on the solidified and heat treated structures of Al–2.5~5 mass%Li–O.7~7 mass%Si–1 mass%Mg alloys cast into metal and sand molds were investigated by scanning electron microscopy, specific heat curve measurement, X-ray diffraction and micro-vickers hardness measurement. AlLiSi compounds crystallize as primary and eutectic phases and disperse at the inter-dendrite arm, and they remaind intact after solution treatment at 803 K for 36 ks. The crystallization of primary and eutectic Si phases, and Mg₂Si phases were not observed. The morphology of the AlLiSi phase is triangular plate-like in the alloys contained more than 3 mass%Si, is needle-like in the alloys contained 1~3 mass%Si and changed to small rod-like in the alloys contained less than 1 mass%Si. By the amount decrease in the solubility of Li in the matrix the crystallization of AlLiSi phase decreases and brings the decrease or retardation in age hardening by δ' (Al₃Li) precipitation. The mechanical strength becomes higher with decreasing the amount of AlLiSi phase and δ (AlLi) equilibrium phase.