Abstract
An attempt has been made to quantify the effects of composition, and aging time and temperature, on the mechanical properties of AI-Zn-Mg-Cu alloys, with the help of statistically designed experiments; for this, the method of design of 2n factorial experiments is used to obtain response surface equations. The coefficients in the regression equations obtained are corrected with the help of the t test. The equations obtained are checked by Fischer's F test and are found to be quite adequate in predicting the mechanical properties with 95% accuracy. The equations developed agree with the results obtained by the usual methods. The equations show that the main effects of Mg, Zn, and Cu are to contribute positively to the strength and hardness above the base level. The interaction between Zn and Mg representing the η′-phase shows an increase in strength, whereas cit does not appear to contribute to the strength by any kind of binary or ternary interaction. Mg contributes most among the three elements to the strength for the range of compositions studied. The equations indicate that the highest levels of time and temperature lead to a decrease in the strength properties of the alloys. All three elements reduce the elongation, with Mg having the most effect; Cu does not, however, measurably reduce the percentage elongation; although it is found to help in retaining the peak strength values at the temperatures of aging studied in this investigation.
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