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UBC Theses and Dissertations

The effect of quench rate and initial grain structure on the mechanical behaviour of an Al-Mg-Si-Mn aluminum alloy Sarmady, Neda


The mechanical behaviour of AA6082 is a function of the extrusion conditions and in particular, the quench rate after extrusion. Controlling the quench rate after extrusion can affect the microstructure evolution to produce desirable mechanical properties for application in structural components for the automotive industry, and is therefore a key parameter of interest. In this study, a near industry alloy similar to automotive grade AA6082 containing 0.5 wt.% Mn and 0.15 wt.% Cr was direct chill (DC) cast, homogenized for 2 hours at 550 °C, and extruded at a temperature of 500 °C with a ram speed of 8 mm/s to form 3 mm x 42 mm strips. The microstructure in the as-extruded strip was unrecrystallized due to the Smith-Zener drag from the Mn/Cr dispersoids. Furthermore, when the as-extruded strip was cold rolled prior to heating, recrystallization occurred concurrently with the solution treatment. This allowed for 3 initial microstructures to be produced, i.e. unrecrystallized, and recrystallized with a grain size of 9 and 40 µm. The aim of the study was to measure the quench sensitivity for the 3 different initial grain structures after solution treatment at 560 °C for a sufficient time to dissolve the Mg-Si precipitates, followed by cooling at rates between 4 and 2000 °C/s. Controlled cooling experiments within this temperature range were conducted using the Gleeble 3500 thermomechanical simulator. The relationship between quench rate and precipitation of Mg-Si phases on heterogeneous nucleation sites were analyzed qualitatively by FEGSEM, as well as their effect on mechanical properties such as yield stress, ultimate tensile stress and fracture properties which were characterized by tensile tests. It was found that the yield stress decreased as the quench rate decreased, and that the unrecrystallized material had a much larger quench sensitivity with respect to the recrystallized initial microstructures, speculated to arise from its high density of heterogeneous nucleation sites.

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