Constitutive flow curve approximation of commercial aluminium alloys at low temperatures

Highlights: • We examine flow curve approximations of AA5182 and AA6016 at sub-zero temperatures. • The Hockett–Sherby formulation shows the best extrapolation quality. • Modified layer compression tests are carried out at low temperatures. - Abstract: Cryogenic forming, as a novel process route for enhancing the formability of aluminium alloys, requires new methods for modelling the flow behaviour. To date, the hardening behaviour of commercially used aluminium alloys is not known at sub-zero temperatures if higher strains are considered. Based on uniaxial tensile tests of EN AW-5182-H111 and EN AW-6016-T4, conducted at temperatures ranging from 298 to 77 K, five common empirically based flow curve approximation models are presented and fitted to the experimental test data. All five mathematical models are shown to be in good agreement with the experimental data up to uniform elongation. However, for a description of the flow behaviour beyond this point, an extrapolation of the test data using the flow curve approximation models up to higher strain levels is then shown. A support point is introduced for each material which was obtained with the aid of layer compression tests at sub-zero temperatures. These points provide a means of assessing the appropriateness of the mathematical models and this is demonstrated by showing the deviation from the flow curve approximations. Consequently, a device was developed and demonstrated, where layer compression tests can be performed at very low temperatures. Only two of the empirically based flow curve approximation models reflect the strain hardening behaviour of observed aluminium alloys with any accuracy if higher strain values are considered.