Controlled precipitation of intermetallic (Al,Si)3Ti compound particles on double oxide films in liquid aluminum alloys

Abstract

Entrained double oxide films, or bifilms, can seriously alter the structural integrity, microstructure, and in this way, the mechanical performance of aluminum alloys. Bifilms, which usually preexist in suspension in the liquid metals, are known to be potential heterogeneous nucleation sites for certain intermetallic phases during the solidification of the alloys. However, the investigations on the possibility of the nucleation of titanium-containing intermetallic phases on double oxide films, the possible effects of this phenomenon on the melt quality, as well as on the resulting microstructure, were absent to date. In this work, a novel melt treatment technique is proposed, which was used to induce the precipitation of (Al,Si) 3 Ti particles in a liquid multicomponent Al Si alloy. Differential thermal analysis (DTA), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were used to characterize the phases formed during the experiment. SEM-EDS investigations proved that the (Al,Si) 3 Ti intermetallic phase precipitated on MgAl 2 O 4 double oxide films and the wetted side of the surface oxide layer of the melt. As revealed by glow discharge optical emission spectroscopy (GDOES) and optical microscopic investigations of the microstructure, the majority of the precipitated (Al,Si) 3 Ti particles sedimented to the bottom region of the melt. Based on the computed tomographic (CT) analysis of reduced pressure test (RPT) samples, this sedimentation extensively reduced the bifilm content in the upper regions of the melt. The theoretical basis of a new melt treatment technique is laid down. • The heterogeneous nucleation of TiAlSi intermetallics on bifilms was investigated. • A novel, controlled precipitation-based melt treatment technique is proposed. • The sedimentation of TiAlSi compounds can significantly improve melt quality. • Nucleation on the surface oxide layer causes Ti-macrosegregation.