In this work, the effect of the growth rate (V L ) and cooling rate (T R ), primary dendritic arm spacing ( 1 ) and Al 2 Cu intermetallic phase on the microhardness was investigated during transient horizontal directional solidification of Al-3wt%Cu and Al-8wt%Cu alloys. Microstructural characterization of the investigated alloys was performed using traditional techniques of metallography, optical and SEM microscopy and X-Ray diffraction. The microhardness evolution as a function of the thermal and microstructural parameters (V L , T R , and  1 ) was evaluated using power and Hall-Petch type experimental laws, which were compared with other laws in the literature. In order to examine the effect of the Al 2 Cu intermetallic phase, microhardness measurements were performed in interdendritic regions. Finally, a comparative analysis was performed between the experimental data of this work and theoretical models from the literature that have been proposed to predict primary dendrite arm spacing, which have been tested in numerous works considering upward directional solidification.

In this paper, primary (λ1) and tertiary (λ3) dendritic arm spacings of a ternary Al -7wt.% Si -3 wt.% Cu alloy casting were characterized and correlated with solidification processing variables: growth rates (VL), cooling rates (TC) as well as local solidification times (tSL). Horizontal directional solidification experiments were carried out under transient heat extraction undergoing cooling rates varying from 0.9 o C/s to 22 o 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 λ1 and λ3 measured. The obtained as-cast microstructures consisted of dendritic α-Al, with Si particles in the aluminum-rich matrix as well distributed along the interdendritic regions in the eutectic mixture interlinked with θ (Al2Cu) intermetallic phase developing the microstructure α-Al + θ + Si. The results showed that power laws -1.1, -0 .55 and 0.55 express the variations of both λ1 and λ3 with VL, TC and tSL, 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 λ1 as a function of TC is represented, for both the investigated alloys, by the mathematical expression given by λ1 = constant (TC) -0.55 . thermal parameters during transient horizontal direction solidification of Al -7 wt.% Si -3 wt.% Cu alloy.

No abstract

Transient horizontal directional solidification (THDS) experiments have been carried out with Al–7wt.%Si–0.15Fe, Al–7wt.%Si–3wt.%Cu–0.15wt.%Fe and Al–7wt.%Si–0.3wt.%Mg–0.15wt.%Fe alloys, to identify experimental relationships between growth rates ( GR), cooling rates ( CR), tertiary dendrite arm spacings (λ3) and microhardness (HV). Optical microscopy and scanning electron microscopy/energy-dispersive spectrometry (SEM/EDS) were used to perform a comprehensive microstructural characterisation of the β-Al5FeSi, ω-Al7Cu2Fe, θ-Al2Cu, π-Al8Mg3FeSi6 and α-Mg2Si intermetallic phases. The addition of Cu and Mg to the Al–7wt.%Si–0.15wt.%Fe alloy led to the precipitation of ω and π phases from the β phase. It has been found for all analysed alloys that power experimental functions given by λ3 = constant.( GR)-1.1 and λ3 = constant.( CR)-0.55 best describe the variation of λ3 with corresponding thermal and microstructural parameters.