Formability and failure mechanisms of AA2024 under hot forming conditions

Aluminium alloy 2024 (AA2024) is extensively used as a structural material in the aircraft industry because of its good combination of strength and fatigue resistance. However, complex shaped components, particularly those made from sheet, are extremely difficult to form by traditional cold forming due to its low ductility at room temperature. A possible solution of this problem is to form sheet workpieces at elevated temperature. The aim of the work described in this paper is to determine the relationship between formability and temperature for AA2024 by conducting a series of tensile tests at elevated temperatures ranging from 350 to 493 °C. Ductility of AA2024 was found to increase gradually with increasing temperature up to 450 °C, followed by a sharp decrease with further increase in temperature. So-called cup tests confirmed that the formability of AA2024 is very high at a temperature of about 450 °C. Fracture surfaces and longitudinal sections of formed samples were examined by scanning electron microscope. It was found that fracture occurred in three different modes depending upon the temperature, and the sharp decrease in ductility when the temperature exceeds 450 °C was caused by softening of grain boundaries by solute enrichment (at higher heating rates liquation may be involved) and softening of the matrix around inclusion particles.