Hot tearing study of aluminium alloys above the solidus temperature with the aid of a direct chill casting surface simulator (DCSS) = : Étude sur la fissuration à chaud d'alliages d'aluminium au-dessus du solidus à l'aide d'un simulateur de surface de la coulée semi-continue
|Other Title(s):||Étude sur la fissuration à chaud d'alliages d'aluminium au-dessus du solidus à l'aide d'un simulateur de surface de la coulée semi-continue|
|Abstract:||Hot tearing is a severe defect in aluminum castings which is produced during solidification when a certain amount of liquid phase remains and weakens the tensile resistance of the alloy. In direct chill casting of aluminum alloys, hot tears initiate at the surface of sheet ingots just after the primary cooling zone, where the microstructure is particularly vulnerable. In order to study the thermomechanical properties of these alloys and the effect of grain refiners on their thermo-mechanical behaviour, tensile tests were carried out on specimens in the semi-solid state (~90-95% solid fraction) and at low strain rates using an apparatus called Direct Chill Surface Simulator (DCSS). This apparatus is an instrumented rig test reproducing the conditions prevailing during the primary cooling stage of the DC casting process. The thermomechanical behavior of solidifying shells and the hot tear formation under applied tensile loads was analyzed and the occurrence of hot tearing was observed. The temperature in different locations of the casting, applied load and surface strain were monitored during the tensile tests conducted on aluminum alloys AA5182, AA6111 and AA3104. The microstructure of the tested specimens was examined using the optical microscope to evaluate the columnar or equiaxed aspect of grains and their effect on the thermomechanical response of the alloy. A special emphasis has been given to the evaluation of the solid fraction existing in the castings at the start of the tensile tests, taking into account the local temperature, thermal gradient and cooling rates experienced. It was found that excessive grain refiner additions decreased the strength of solidifying shells because of the increased level of porosity induced by easier nucleation of new phases (gas included). It was also observed that cooling rates increased with the level of grain refiner, a phenomenon that was associated to the higher number of contact points with the mould obtained with numerous small equiaxed grains. Finally, the concept of a stress based criterion for hot tearing was reinforced by the strengths obtained on not grain refined AA5182 and AA6111 alloys showing similar solid fraction near the surface, but with significantly different grain sizes.|
|Document Type:||Mémoire de maîtrise|
|Open Access Date:||17 April 2018|
|Collection:||Thèses et mémoires|
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