<p>In this work, the effect of the age hardening process on the heat transfer rate from vertical short cylinders made from AA (aluminum association) 6061 aluminum alloy has been investigated. Four aluminum alloy workpieces were single age hardened at 140 °C for 4, 8, 12, and 16 hours. Furthermore, a double aging process was performed on the considered aluminum alloy at 120 °C for 4 hours and at 160 °C for 4 hours. The heat transfer coefficient has been experimentally obtained using the lumped capacity method. The novelty of this work is that it was found that as the material becomes softer its ability to exchange heat decreases and when the material becomes harder its ability to exchange heat increases. The pronounced finding of this study is that the heat transfer rate decreased for all the single age hardening regimes as well as for the double aging process. There was a particular characteristic of the relationship between the heat transfer rate and aging time, i.e., the heat transfer rate decreases at varying rates as the aging time increases. In addition, the heat transfer rate decreased more significantly in the case of a double aging process than in the case of a single aging process.</p>