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The present work investigates the influence of adding Mo to an Al-Mn-Mg 3004 alloy on elevated-temperature properties as well as their thermal stability during longterm thermal holding at 350°C and 400°C. In as-cast and heat-treated conditions, both microhardness and yield strength increase with increasing Mo contents and reach peak values at 0.3 wt. % followed by a plateau. With an optimized Mo content (0.3 wt. %), the volume fraction of dispersoids is increased while the volume percentage of the dispersoid-free zone is greatly reduced compared to the base alloy free of Mo, resulting in the remarkable increases in elevated-temperature strength and creep resistance. The results of the long-term thermal holding show that compared with the rapid drop of elevated-temperature strength and creep resistance in the base alloy, the Al-Mn-Mg alloy with 0.3% Mo is thermally stable up to 350°C, exhibiting a slight decrease of stability at 400°C. The combination of high elevated-temperature properties and their excellent thermal stability at 350-400°C with Mo addition makes Al-Mn-Mg 3xxx alloys the promising candidates for elevated-temperature applications.

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Heat treatment for precipitation hardening is known to have a large effect on the nano/micro-structure of cast aluminum alloys, and hence its properties. In the present work, precipitation kinetics after solutionizing and water quenching has been characterized by differential scanning calorimetry, transmission electron microscopy observations (TEM) and microhardness evaluations at different aging conditions. The Kissinger methodology was applied with the Lee-Kim-Starink-Zahra (LKSZ) kinetic equation in order to extract the kinetic parameters from DSC runs at constant heating rates, assuming that the precipitates have an ellipsoidal shape. TEM results showed evidence of semi-coherent θ' precipitation in accordance with the microhardness evolution during isothermal aging at 190°C and kinetic analysis from DSC data. The size and number density of precipitates were measured and counted on bright field TEM images taken on specimens aged at two different times. Activation energies for the precipitation kinetics of θ and θ' were found to be equal to 330 kJ/mol and 114 kJ/mol respectively. Finally, values for the interfacial mobility have been determined from the kinetic parameters derived from the DSC results and the TEM observations.