The aluminum heat treatment industry has a great variety of products and processes designed to enhance the material’s physical properties. Quenching is a process that involves cooling the workpiece at a rapid rate to increase its hardness. An important aspect of this process is the uniformity with which the workpiece is cooled, since an uneven cooling results in distortions and flaws in the material.
Quenching with air results in a mostly uniform, but slow, process. The long cooling times affect the workpiece’s hardness. Another popular method is using a water tank, which gives faster cooling rates given the calorific capacity and thermal conductivity of the medium, and thus, better hardness properties.
However, the formation of a vapor layer around the workpiece results in an uneven cooling rate, which produces undesired distortion and a more brittle material. Quenching with a mixture of pressurized air and atomized water combines the cooling times of water-quenching and the uniformity of air-quenching.
An experimental design was set up in Nutec Bickley’s testing lab to obtain the cooling curves for an aluminum workpiece heated to 525°C and then quenched with different media: in a water tank, with air, and with an air-mist mixture. Figure 1 shows the resulting cooling curves, and Table 1 shows the average cooling rates and the time it took for the piece to reach 100°C for each method.
Figure 1. Cooling curves for an aluminum piece quenched in a water tank, with air, and with air-mist.
Table 1. Average cooling rates and times for each quenching method.