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Quench sensitivity of an Al-Mg-Si-Cu aluminum alloy Sundaresan Ramesh, Naveen

Abstract

The use of aluminum extrusion alloys in automotive applications is increasing due to the requirement that vehicle weight be reduced. One of the steps involved in the processing of aluminum extrusion alloys is rapid cooling (or quenching) after the solution heat treatment. The term quench sensitivity refers to the relationship between mechanical properties and the cooling rate after solution treatment. In this study, two initial microstructures were produced from the as-received alloy using different thermomechanical processing routes to study quench sensitivity: in the first case, the as-received microstructure underwent a solution treatment of 5 minutes at 550 C to produce a predominantly unrecrystallized grain structure, and in the second case, the material was rolled at room temperature prior to a solution treatment of 5 minutes at 550 C to produce a recrystallized grain structure. The main objective of this study was to measure the quench sensitivity of the two initial microstructures. The variation of hardness with cooling rates was examined using a modified Jominy test, and the variation of yield strength and ultimate tensile strength with quench rates was characterized using tensile tests. Hardness, yield strength, ultimate tensile strength and ductility were found to decrease with decreasing cooling rates. The reduction in the mechanical properties was attributed to the loss of solute atoms which precipitates as coarse precipitates during slow cooling. Slow cooling typically results in a higher number of grain boundary precipitates and a wider precipitate free zone (PFZ) adjacent to the grain boundary, which increases the propensity of intergranular fracture. Finally, using a polycrystal plasticity model, the degree of strain localization within the PFZs was studied for different idealized geometries.

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