Mechanical properties of 7A04-T6 high strength structural aluminium alloy at elevated temperatures and after cooling down

Abstract

Recent years have witnessed an increasing use of high strength aluminium alloys in structural applications, however, the mechanical properties of this type of materials are rather susceptible to elevated temperatures. This paper presents an experimental study on the mechanical properties of 7A04-T6 high strength aluminium alloy at elevated temperatures and after cooling down. Tensile coupon tests were first carried out on the specimens at high-temperature steady state conditions and post-fire conditions for temperatures ranging from 20 to 400 °C. The effect of cooling methods (cooled by air/water) was taken into account in the post-fire tests. Key mechanical properties, including Young’s modulus, yield strength, ultimate strength and ultimate strain, as well as the stress–strain responses obtained from both tests were fully reported and the microstructures of the post-fire material were examined. It is shown that the Young’s modulus and strengths decrease dramatically when temperatures are higher than 200 °C and the strengths decrease to less than 5% of the room temperature values at 400 °C, but a large proportion (up to 90%) of them regains after cooling down. In addition, the post-fire mechanical properties of the material exposed to 400 °C was found to be higher than those exposed to 300 °C; this was further explained through the microstructure examination. The test data were then utilised to assess the accuracy of existing codified design methods and the comparisons revealed that all of them provide inaccurate and scattered predictions. Finally, design formulae for the elevated-temperature retention factors as well as the post-fire residual factors and a two-stage Ramberg–Osgood material model were proposed. The design proposal was found to provide accurate predictions for the full-range stress–strain responses of the high strength aluminium alloy at both high-temperature steady state and post-fire conditions.