Superplasticity is the ability of polycrystalline materials to display large tensile elongations during forming without necking prior to failure. Superplastic sheet metals allow products of complex shapes to be produced in a single operation, using relatively inexpensive tools.

The properties of these materials are determined under uniaxial tensile stress: flow stress, true strain dependence, strain at break, strain-rate sensitivity, strain hardening and crystal-grain stability are characterized. The tensile testing machine should therefore be capable applying a tensile load to the specimens with a constant strain rate in the range 1x10^-5 to 1x10^-1 s^-1.

To obtain the precise crosshead speed and accurate control and measurement essential for this complex task, the University of Ljubljana chose Zwick to support it in this research project. The superplastic tensile tests on an aluminium alloy were performed using a Z250 testing machine and a 0.5 kN load cell. To maintain the sample at superplastic temperature the machine was equipped with a three-zone radiant split furnace (max. 900 °C). The test was performed and the results evaluated via testXpert II testing software. The alloy investigated was an ingot-cast aluminium alloy, conventionally processed to a sheet with a thickness of 1.9 mm. Professor Smolej: “The results obtained for the alloy under test confirm the effectiveness of the equipment referred to above in characterizing superplastic materials.”

Figure 1: The test equipment

Figure 2: The sample and the gripping system

Figure 3: True stress, true strain curves for various constant strain rates at 550ºC

Figure 4: Untested and tested sample of investigated alloy at an initial strain rate of 7,5x10^-4 s^-1 and at 550°C