Effect of solution time in T6 heat treatment on microstructure and hardness of a directionally solidified Al–Si–Cu alloy

Abstract

An experimental investigation on the role of solution time during the T6 heat treatment on the scale of the secondary dendrite arm spacing (λ2), morphology and distribution of Si particles and Al2Cu intermetallics and the resulting effect on microhardness of Al-5.5 wt%Si-3.0 wt%Cu alloy samples is performed. A directionally solidified (DS) casting was previously obtained using a water-cooled apparatus, which permitted a wide range of cooling rates (T˙) to be associated with samples having quite different microstructural parametric features. An experimental growth law relating λ2 to T˙ is proposed. The T6 heat treatment was then carried out on the DS samples for different solution times: 8 h and 5 h at 490 ± 2 °C, followed by quenching in warm water (60 ± 2 °C), ageing for 5 h at 155 ± 2 °C and air-cooling. The microstructure characterization (optical and scanning electron microscopies) has shown that λ2 was only affected (when compared with those of the DS casting samples) when the solution time increased from 5 h to 8 h. In contrast, significant influences on the morphology and distribution of both Si and Al2Cu particles are shown to occur. The highest hardness is shown to be associated with a sample subjected to the 5 h treatment, having lowest λ2 and appropriate modification of Si and Al2Cu particles.