Features of the Microstructural Composition of Low-Alloyed Aluminum Alloys of the 6XXX Series with Small Additions of Zr and Sc

Abstract

The study investigates the effect of Zr and Sc on the dendritic structure and phase composition during casting of a silicon rich Al0.3Mg1.0Si alloy. Optical microscopy, computations using Thermo-Calc software and electron microscopy were applied. It showed that scandium and zirconium additions facilitate dendritic structure refinement, however, the maximum effect is achieved by their combined addition. Computations demonstrated the occurance of three main phases in case of Sc (without zirconium) addition to Al0.3Mg1.0Si alloy: i) the strengthening Mg2Si phase, ii) the harmful ScSi phase and iii) free silicon. In case of only Zr addition, the same phases are observed, except for ScSi replaced with Zr2Si, which has not been studied in Al-Mg-Si alloys. Computations predict the occurance of all three phases in the case of the combined Sc and Zr addition, besides, strengthening Al3(ScZr) can be expected. Electron microscopy shows, that all alloys contain Mg2Si phases as well as Al12Fe3Si and Al9Fe2Si2 type phases, formed due to the iron impurity. The main Zr and Sc fractions occur in solid solution, thus, a quenching rate of 1 oС/s is sufficient to achieve a self-hardening effect.