Implementation of Sub-Sized Fracture Specimen for Characterization of Hot Forming Experiments on 7050 Aluminium Alloy

The microstructure of 7xxx-alloys after hot rolling is known to have major influence on the mechanical properties, especially on the fracture toughness. A standard determination of fracture toughness in accordance to the current specifications is only possible with relative large Compact-Tension-samples (1.5 to 2.0 CT). The material has to be stress relieved by stretching to achieve valid pre-cracking. That means, that a systematic investigation of the influence of hot rolling parameters on microstructure and fracture behaviour can be investigated only at industrial produced material. The influence of forming parameters (true strain, strain rate and temperature) on the microstructure is typically investigated by hot compression tests. But these samples are usually too small for fracture toughness measurements. It permits only the examination and characterization of the microstructure. Yet it would be very desirable to understand how the microstructure affects the fracture behaviour and thus to get additional information about the mechanical properties from hot compression test specimen. Therefore the geometry and the dimension of the hot compression test specimen were adjusted to serve as miniaturized Charpy specimen that can be used for the characterization of the fracture behaviour. In a first investigation the correlation between fracture toughness and Charpy impact energy of pre-cracked samples of 7050 aluminium plates were measured in different over-aged conditions. The results demonstrate that the Charpy impact test is capable of the characterization of the fracture behaviour. In a second step hot compression tests were carried out using different hot forming parameters, which cover the real conditions during industrial processing. Detailed microstructural and fractographic analysis together with the results of Charpy impact tests allow quantifying the effect of hot forming parameters on fracture behaviour of 7050 aluminium alloy.