Formation of Nondendritic Primary Aluminum Phase in Hypoeutectic Alloys in Controlled Diffusion Solidification (CDS): A Hypothesis

Controlled diffusion solidification (CDS) is a novel process wherein specific Al alloys can be cast by mixing two precursor alloys of specific compositions and temperature and subsequently casting the resultant mixture. This process enables a nondendritic morphology of the primary Al phase in the cast samples, which is beneficial in mitigating hot tearing tendencies and enabling castability of dilute Al (wrought) alloys to obtain castings with superior mechanical and performance properties. In this study, a hypothesis is proposed to describe the mechanism of the CDS process, specifically the processes of mixing two precursor alloys and a subsequent solidification process. Al – 4.5 wt pct Cu was used as an example alloy system to propose a hypothesis and to verify the various features in the mechanism of mixing two alloys. Experimental results show that the mixing process naturally causes copious nucleation of one of the alloys mixed and that the turbulence energy during mixing distributes these nuclei uniformly to enable a favorable solidification condition for a nondendritic cast microstructure. It is critical that the alloy with the higher thermal mass (mass and temperature) is mixed into the alloy with lower thermal mass to obtain a valid CDS process and that the reverse will not yield a favorable homogeneous cast sample. Certain critical parameters during the CDS process have also been identified and quantified for a favorable microstructure of the casting.