Experimental study of the evolution of tertiary dendritic arms and microsegregation in directionally solidified Al–Si–Cu alloys castings

Abstract

Abstract It is widely recognized that moderate addition of copper to aluminum, especially when added together with silicon, significantly improves the resulting mechanical properties. Hypoeutectic ternary Al–Si–Cu alloys were used in the present experimental study to investigate the effect associated with the Cu content and the solidification thermal parameters on the microstructural and microsegregation features. Al–9\xa0wt%Si–2\xa0wt%Cu and Al–9\xa0wt%Si–4\xa0wt%Cu alloys were directionally solidified under transient heat flow conditions under a range of cooling rates from 0.2 to 9\xa0°C/s. A quite complex dendritic arrangement prevailed along the entire length of both examined castings, giving rise to well-defined tertiary dendritic arms (λ3). Experimental growth laws relating λ3 to the cooling rate have been determined indicating that the increase in the Cu alloy content from 2 to 4\xa0wt% has induced a thickening effect leading to increase of about 50% in λ3. The effect of the solidification kinetics on microsegregation has been experimentally investigated for different positions along the castings length. The Cu profiles were shown to move upwards with the increase in the solidification velocity for both examined alloys. In contrast, an inverse trend was shown to occur with the Si segregation profiles for both alloys, which moved downward with the increase in the solidification velocity.