Evaluation of the stress ralaxation technique for measuring softening kinetics in aluminium alloys

Master Thesis


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University of Cape Town

The development of the microstructure during thennomechanical processing (TMP) is critical in determining the final properties and quality of metal strip. In the particular case where aluminium sheet is used for lithographic applications, the surface appearance after electro-etching should be devoid of any streaking or inhomogeneous discolouration. The cause of possible streaking effects can be related to poor microstructure development during TMP and often arises as a result of inadequate recrystallisation. To avoid the deleterious effects, it is important to implement the appropriate rolling conditions in order to control the processes of recovery and recrystallisation. The means by which the correct rolling conditions can be established is usually by extensive laboratory simulations and concomitant microstructural analysis. In view of the fact that this approach is often tedious, the present study has investigated the use of the stress relaxation technique to provide rapid data on the recovery and recrystallisation kinetics for commercial purity aluminium under defonnation conditions that closely simulate hot rolling operations. Stress relaxation (SR) curves have been generated for AA1200 aluminium, as well as for two magnesium containing alloys, namely AA5182 (5wt% Mg) and an experimental alloy (Al-l %Mg). Fully recrystallised microstructures were subjected to uniaxial compression in the temperature range of 300-400?C. Strains and strain rates were up to 0.7 and ls respectively. Stress relaxation was measured for intervals up to 15 minutes and the evolved microstructures were examined after fixed intervals using polarised light microscopy and electron backscatter diffraction.

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