Titanium as an intermetallic phase stabilizer and its effect on the mechanical and thermal properties of Al-Si-Mg-Cu-Ti alloy

Abstract

The effect of precipitation of intermetallics on the mechanical and thermal properties of Al-6.5Si-0.44Mg-0.9Cu-(Ti) alloys (in wt%) during various artificial aging treatments was studied using a universal testing machine and a laser flash apparatus. The solution treatment of the alloy samples was conducted at 535°C for 6h, followed by quenching in warm water. The solution-treated samples were artificially aged for 5h at different temperatures ranging from 170°C to 220°C. After the artificial aging treatment, the Al-6.5Si-0.44Mg-0.9Cu alloy (the Ti-free alloy) had a lower ultimate tensile strength (UTS) than the Al-6.5Si-0.44Mg-0.9Cu-0.2Ti alloy. The UTS response of the alloys was enhanced by the addition of Ti, with the maximum UTS showing an increase from 348MPa for the Ti-free alloy to 363MPa for that containing 0.2wt% Ti, aged at 180°C. The Ti-free alloy had a higher thermal diffusivity than the Ti-containing alloy over all temperature ranges. Upon increasing the temperature from 180°C to 220°C, the room temperature thermal diffusivities increased because the solute concentration in the α-Al matrix rapidly decreased. In particular, the thermal diffusivity increased significantly between 200°C and 400°C. This temperature range matched the range of intermetallic phase precipitation as confirmed by differential scanning calorimetry and measurement of the coefficient of thermal expansion. During the artificial aging treatment, the intermetallic phases precipitated and grew rapidly. These reactions induced a reduction of the solute atoms in the solid solution, thus producing a more significant reduction in the thermal diffusivity. As the temperature was increased to above 400°C, the formation of intermetallic phases ceased, and the thermal diffusivity showed a steady value, regardless of the aging temperature.