Quench sensitivity of a heat treatable aluminum alloy

A series of aging treatments were made on samples cut from bars of a 6063 heat treatable aluminum alloy that were solubilized for 4 h at 520 °C, and were cooled down to room temperature by placing one of their ends into a shallow tank of water to produce a continuous thermal gradient along their length. Heating and cooling cycles were recorded by inserting thermocouples in different positions along the length of the bars, the output of these devices was fed into a data logging system. Aging was carried out at three different temperatures (130, 180 and 230 °C) for periods of time ranging from 0.5 to 64 h on samples cut from positions close to those at which the thermocouples were located in the bars. A continuous cooling precipitation (CCP) diagram was constructed from the cooling curves and, from it, the time available for precipitation during cooling was computed. It was found that the maximum value of hardness after aging depended on the cooling rate, when this was conducted at rates lower than 10 °C/s. It can be concluded that quench sensitivity is caused by incipient precipitation taking place during cooling, and that the maximum value in hardness that can be achieved by aging the material depends on the length of time available for precipitation while cooling.