EFFECT OF SOLUTION TIME DURING THE T6 HEAT TREATMENT ON MICROSTRUCTURE AND MICROHARDNESS OF A DIRECTIONALLY SOLIDIFIED Al-Si-Cu ALLOY

摘要

In this work, an experimental investigation on the influence of solution time during the T6 heat treatment on the scale of the secondary dendrite arm spacing (λ_2), morphology and distribution of Si particles and Al_2Cu intermetallics and the resulting effect on microhardness (HV) of an Al-5.5wt.%Si-3.0wt.%Cu alloy is performed. An initial directionally solidified (DS) casting was obtained using a water cooled-upward solidification device, which permitted a wide range of cooling rates (T) to be associated with parametric features of the microstructure along the casting length. An experimental growth law relating the evolution of λ_2 to T is proposed. The T6 heat treatment was then carried out on the DS samples for different solution times: 8 h and 5h at 490 ± 2 °C, followed by quenching in warm water (60 °C± 2 °C), aging for 5 h at 155 ± 2 °C and air cooling. The microstructure characterization (optical and scanning electron microscopies) indicated that the scale of λ_2 has been affected (as compared with those of the DS casting samples) only when the solution time was increased from 5 h to 8 h, despite the significant influences on the morphology and distribution of both Si and Al_2Cu particles. The resulting highest hardness obtained is shown to be associated with the samples subjected to the 5 h treatment, due to the connection of lower λ_2 and appropriate modification of Si and Al_2Cu particles located in the interdendritic regions.