The effect of time and temperature on the precipitation behavior and hardness of Al–4 wt%Cu alloy using design of experiments

The effect of time and temperature in precipitation heat treatment (artificial aging) on the hardness of Al–4 wt% Cu alloy was studied. Time was varied from 5 to 48 h and temperature was varied from 120 to 220 °C. Hardness data were obtained from standard Rockwell hardness tests using 75 randomly selected specimens. A linear statistical model, which is based on the design of experiments method, was applied to determine the importance of each variable. To optimize the process and determine the time and temperature levels at which a desired value of hardness can be achieved, a second-order regression model that captures most of the curvature in the actual relationship between hardness, time, and temperature was developed. Results showed a maximum increase of 29.3% in hardness of Al–Cu alloy when time was increased (temperature is constant), and changed between −3.6% and +21.1% when temperature was increased (time is constant). However, when both time and temperature were increased, a maximum increase of 36.0% in hardness was obtained due to a significant interaction effect between time and temperature. A contour plot was produced from the second-order regression model which showed that many time and temperature conditions at which a desired value of hardness can be achieved may exist.