A study on internal quenching of hollow extrusions to reduce distortion and increase the energy to failure of aluminum profiles

Abstract

Lightweight automotive extrusions are increasingly complex, thin-walled, multi-hollow profiles made from quench-sensitive alloys like AA6082. These profiles require rapid (water) quenching as they leave the press in preparation for age-hardening. Conventional rapid quenching, which only directly cools the profile’s extremity, can distort the part. Lower quenching rates reduce distortion but may compromise the mechanical properties. We test three hypotheses: (1) That the different cooling rates across the section during quenching induce varying mechanical properties as well as distortion; (2) That this temperature differential can be minimized by combining novel internal profile quenching with conventional quenching; and (3) That internal quenching can be achieved using insulated channels in the extrusion die to convey the quenchant to the profile’s interior. The first hypothesis is tested experimentally by taking tensile specimens from a AA6082 multi-hollow profile. The second is examined experimentally using a lab-built quench box and theoretically using thermo-mechanical finite element simulations. The third hypothesis is tested by conducting a hollow profile extrusion trial using a specially designed porthole die. The testing shows that conventional quenching results in reduced mechanical properties in the profile’s internal walls but that combined external/internal quenching alleviates this problem and reduces distortion. The extrusion trial on internal quenching demonstrates die survivability, an acceptable die temperature drop during quenchant flow, and effective quenchant disposal via evaporation and capture of liquid at the end of the profile. This study suggests that internal quenching is a promising technology option for reducing scrap and improving mechanical properties of hard-to-quench aluminum profiles.