Effect of Solution Heat Treatment on the Hot Workability of Al–Mg–Si Alloy

Abstract

The current work presents a detailed study on the high temperature processing of solution treated Al–Mg–Si alloy in the temperature range of 623 K to 773 K and at different strain rates in the range of 5 × 10⁻⁵ to 6 × 10⁻² s⁻¹. A constitutive relation that can be used in modeling the forming process of this alloy under similar hot working conditions is established. Also, the prevailing deformation mechanism was investigated through relations of the steady state stress dependence on strain rate which revealed a stress exponent of 8.5 (strain rate sensitivity; m ∼ 0.12). This stress exponent is higher than what is usually observed in Al and Al–Mg alloys under similar experimental conditions. This high stress exponent may arise from the presence of threshold stress that results from dislocation interaction with second phase particles (Mg₂Si), precipitating during the deformation at high temperatures. The values of threshold stress showed an exponential increase with decreasing temperature and a dependence with an energy term Qo = 38 kJmol⁻¹. The apparent activation energy for solution treated condition was calculated to be about 320 kJmol⁻¹, which is higher than the activation energy for self-diffusion in Al (Qd = 143 kJmol⁻¹) and for the diffusion of Mg in Al (115–130 kJmol⁻¹). By incorporating the threshold stress in the analysis, the true activation energy was calculated to have a value of 111 kJmol⁻¹, and the normalized strain rates can be represented by a power function of the effective stress with stress exponent of ∼3. Ductility was documented to reveal the best working condition for this alloy in solution treated condition. The ductility exhibited a maximum value of about 120% at 773 K at a strain rate of 0.064 s⁻¹. The results of the current work is, also, compared to the results of another heat treatment condition (T4-naturally aged) to reveal which ever condition holds better hot forming characteristics.

Keywords:

• Al–Mg–Si alloy
• Apparent activation energy
• Elongation
• Normalized strain rate
• Solution heat treatment
• Strain rate sensitivity
• Stress exponent
• Threshold stress
• True activation energy
• Workability

ACKNOWLEDGMENTS

This work was supported by the Research Center, College of Engineering, King Saud University, and SABIC under grant no. 428/38. This support is highly acknowledged.