Application of the Spray Quenching to T6 Heat Treatment of Thick A6061 Hollow Cylinders

Aluminum allay A6061 is commonly applied T6 heat treatment and ultimate tensile strength above 300 MPa is obtained by solid solution strengthening. The T6 treatment consists of solution treatment, water quenching and artificial aging. Lower cooling rate during quenching is very important because impurity phase deposition in solution phasedue to longer quenching delay results in growth of grains in the aging process and less strengthening. The quenching delay time strongly depends on boiling transition from film boiling to nucleate boiling as well as heat capacity of the solid. Stable wetted situation hardly occurs in the film boiling region and film boiling heat transfer coefficient is one or two digit lower than that of the nucleate boilingheat transfer. In this research, dip (bath) and spray quenches were applied to thick A6061 hollow cylinders to get comparable spray cooling rate with the conventional dip quenching. Temperature histories in the cylinder were measured during quenching and inside temperature distribution was estimated with inverse heat conduction analysis. After the T6 treatment, tensile test pieces were cut away from the cylinder and effectiveness of the spray quench was accessed. The spray quenchingtests were done for water temperature of 10-22 oC and the maximum total spray flow rate of 20 L/min with four or eight spray nozzles around the cylinder. For the dip quenching, test piece was soaked into well stirred water pool at a fixed temperature of 18 oC.From the spray quenchtests, the quenching delay time was difficult to keep within the allowable maximum delay time requested from existing metallurgical studies. It was found that higher water flow rate or larger number of nozzle arrangement will be required to obtain comparable cooling rate with the dip quenching result. However, each tensile test showed that the strength of spray quenched materials is almost equivalent to that of dip quenching and satisfied the minimum tensile strength of A6061-T6 regulated by JIS (Japanese Industrial Standards) B 8270.