Influence of heat treatment on the microstructure, texture and formability of 2024 aluminium alloy

We have investigated the effect of heat treatment on the microstructure, texture and formability of high-strength aluminium Al2024 sheets of gauge thicknesses 1.27 mm and 2.03 mm. Both optical and electron microscopy were employed to characterise the microstructure. Tensile tests performed at 0°, 45° and 90° to the rolling direction were used as an indication of the anisotropic behaviour of the sheets. The formability of the sheets was assessed by performing stretch forming tests over a hemispherical punch. Comparison of microstructure and material properties indicated an effect of precipitation hardening on the overall anisotropy of the investigated materials. We report an improvement in the total elongation under uniaxial tension with a loss in strength for 2.5 h and 2 days ageing while the ageing treatment for 1 week (peak hardness) resulted in increased strength with a decline in total elongation. The 1.27 mm thick sheet showed better drawability and least tendency to earing than the thicker sheet. The drawability was the highest at 45° to the rolling direction for the as-received material and those that had been aged for 2.5 h and 2 days. Forming limit diagrams derived from the stretch forming tests showed that the 2 days aged sample had the highest plane strain limit making it the most appropriate condition considering that the plane strain is usually the most critical forming strain in stamping applications. In addition, the 2 days aged sample had its plane strain shifted towards the biaxial stretching area which is likely to have a positive effect on some sheet forming applications. Finally, a formability index was calculated and compared against the hardness plot.