Colloquia

Summary

Solid-state additive manufacturing (SSAM) processes such as Friction Screw Extrusion Additive Manufacturing (FSEAM) can build large aluminium components without melting, while potentially re-using machining chips as feedstock. However, it is still unclear whether the very short, locally varying thermal cycles in FSEAM can replace the conventional T6 solutionizing step for Al-Mg-Si alloys.

This thesis investigates the short-time dissolution behaviour of strengthening precipitates in AA6060-T6 during isothermal heat treatments between 280 and 540 °C. Vickers hardness measurements as a function of time are used to track softening. The hardness evolution is well described by a simple exponential decay with a temperature-dependent time constant τ(T). An Arrhenius analysis of τ(T) yields an apparent activation energy of ~115 kJ/mol, consistent with diffusion-controlled precipitate dissolution, and provides a compact expression to estimate the time required to reach a characteristic hardness level for the solution annealed state.

A more detailed temperature-dependent analysis reveals two regimes: at 280–340 °C, over-ageing and partial dissolution dominate, whereas at 390–540 °C the precipitates dissolve rapidly and the base hardness is approached. A piecewise Arrhenius description links this transition to a change in curvature of the Mg₂Si solvus. The resulting kinetics framework enables first-order predictions of precipitate dissolution during arbitrary thermal cycles and forms a basis for designing integrated heat-treatment strategies in FSEAM and related solid-state processes for AA6xxx alloys.