Interrelation of Solidification Processing Variables and Microstructure of a Horizontally Solidified Al-based 319.1 Alloy

In this paper, primary (lambda(1)) and tertiary (lambda(3)) dendritic arm spacings of a ternary Al-7wt.% Si-3 wt.% Cu alloy casting were characterized and correlated with solidification processing variables: growth rates (V-L), cooling rates (T-C) as well as local solidification times (t(SL)). Horizontal directional solidification experiments were carried out under transient heat extraction undergoing cooling rates varying from 0.9 degrees C/s to 22 degrees C/s to be associated with samples having quite different microstructural length parameters. Techniques of metallography and optical microscopy were applied in order to have lambda(1) and lambda(3) measured. The obtained as-cast microstructures consisted of dendritic alpha-Al, with Si particles in the aluminum-rich matrix as well distributed along the interdendritic regions in the eutectic mixture interlinked with theta(Al2Cu) intermetallic phase developing the microstructure alpha-Al+theta+Si. The results showed that power laws -1.1, -0.55 and 0.55 express the variations of both lambda(1) and lambda(3) with V-L, T-C and t(SL), respectively, for investigated alloy. A comparative study with the Al-3wt.% Cu alloy from literature was also performed and the results show that the growth law of lambda(1) as a function of TC is represented, for both the investigated alloys, by the mathematical expression given by lambda(1) = constant (T-C)(-0.55).