Abstract
The relationship between precipitation and coefficient of thermal expansion of Al–6.5Si–0.45Mg–0.9Cu–(Ti) alloys (in wt.%) after various heat treatments has been studied by thermomechanical analyzer (TMA) and differential scanning calorimetry (DSC). The solution treatment of alloys was performed at 535 °C for 6 h followed by water quenching and the samples were aged in air at different temperature from 180 °C to 220 °C for 5 h. The coefficient of thermal expansion (CTE) curves exhibited some residual strain, which was decreased with the increase in the aging temperature. In temperature range from 250 °C to 400 °C, the value of CTE dramatically changed and those peaks are shifted in case of the aged samples because of the change of the precipitation behavior of secondary phases. These transformation peaks of aged samples are associated with the volume of the precipitation of Si phases confirmed by DSC analysis. The peak size and the total enthalpy decreased when the aging temperature and Ti contents were increased. The alloys showed the length changes by approximately 0.07% after water quenching followed by isothermal annealing at 300 °C identical to the result of dilatometric tests. In conclusion, the change of CTE is mainly caused by the precipitation of Si phases in Al–Si alloys and the size of the change is concurrent with the size of the precipitates.
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