On the Effect of Fluid Flow on Intermetallic Phases in Cast Aluminium Alloys

Abstract

Technical Al-Si base cast alloys always contain amounts of Fe and Mn, especially alloys extracted from scrap. During casting, Fe and Mn containing intermetallics, such as Al-Fe, Al-Fe-Si and Al-Fe-Mn-Si phases, are formed between the aluminum dendrites. Fe and Mn-rich intermetallic phases with a needle to plate-like morphology strongly affect the interdendritic feeding. They induce porosity and shrinkage holes and also have detrimental effects on the mechanical properties in Al-Si alloys. In the paper we describe new research about the influence of controlled fluid flow on the morphology and spatial arrangement on intermetallic phases in cast Al-Si alloys. A binary Al-7wt.%Si, ternary Al-7wt.%Si-1wt.%Fe and Al-7wt.%Si-1wt.%Mn alloys were solidified over a range of solidification velocities (0.015- 0.18mm/s) at a constant temperature gradient of 3K/mm. Fluid flow was induced by a rotating magnetic field device introducing flow velocities in the range of mm/s. A comparison of alloys processed with and without artificial flow indicates a strong influence of the electromagnetic stirring on the primary dendrite and secondary dendrite arm spacings, beyond current theoretical understanding of flow effects. The paper presents the results and discusses the relevance to casting of scrap alloys.