Minimizing Quench Distortion and Improving the Toughness of Complex Hollow Extrusions Using Internal Cooling

Lightweight automotive extrusions are increasingly complex, thin-walled, multi-hollow profiles made from high-strength, quench-sensitive aluminum alloys such as AA6082. These alloys require rapid quenching as the profile leaves the press to prevent the precipitation of undesired phases, to create a supersaturated solid solution, and to prepare them for subsequent age-hardening treatments; e.g., for the T6 temper. However, rapid quenching can cause profile distortion, which leads to high scrap reject rates, increasing costs, environmental impacts, and production lead time. This study tests two hypotheses: (1) That the different cooling rates set-up across the profile section during quenching induces not only distortion but also varying mechanical properties across the section; and (2) That this temperature differential can be minimized by combining (conventional) external quenching with internal quenching supplied by through-die cooling channels. The first hypothesis is tested experimentally by taking tensile specimens from different locations of an AA6082 multi-hollow profile, showing a significant decrease in the ductility and ultimate tensile strength of samples extracted from internal webs. The second hypothesis is tested by performing thermo-mechanical finite element simulations that compare the thermal history, stresses, and strains of simultaneous internal and external quenching in contrast with conventional quenching (external only). The combined quenching approach results in a significant reduction in the residual stress and plastic deformation. This implies lower scrap reject rates, improved internal wall mechanical properties (giving scope for further light-weighting), and a wider profile design space by enabling the extrusion of more challenging profile shapes.