The Structure of Al – Mg – Si Casting alloys Modified by Lithium and Scandium

Abstract

The structure of hypoeutectic Al – Mg – Si alloys cast in permanent mold, as well as using a high-pressure die-casting unit (HPDC), was investigated. Changes in the structure after additional alloying of the base alloy (Al – 5.5Mg – 2.5Si – 0.6Mn) with lithium and scandium were studied. For a more detailed description of the structure of the alloys, phase diagrams and the volume fractions of the phases in the Thermo-Calc software were calculated. Thermo-Calc results are in good agreement with metallographic studies. The microstructure of the alloys was investigated using a scanning electron microscope (SEM) on polished and deeply etched microsections. The article also shows the strong influence of the crystallization rate on the eutectic morphology and the secondary dendrite arm spacing (SDAS). Alloying the base alloy with lithium leads to a change in the eutectic morphology from lamellar to fine fibrous. Alloying with scandium not only changes the eutectic morphology from lamellar to the fine fibrous but also reduces the SDAS. Scandium addition leads to the formation of primary Al3Sc crystals, which act as a substrate for the heterogeneous nucleation of α-Al dendrites and Al – Mg2Si eutectic. To determine the effect of alloying elements on the properties of alloys, hardness, tensile strength, and elongation were measured. Local mechanical properties were measured as microhardness of α-Al dendrites, Al – Mg2Si eutectic, and Al3Sc phase. Alloys with lithium and scandium show increased properties in the cast state compared to the base alloy.