Prediction of quench-induced residual stresses in Al-Cu-Mg impeller forgings

Due to its high creep resistance, the AA2618 heat treatable Al-Cu-Mg alloy is used for the fabrication of impellers for turbochargers. Small impellers are directly machined from heat treated forgings whereas large ones undergo an additional step of pre-machining before heat treatment. Quenching is a key step of the heat treatment since the final mechanical properties strongly depend on the cooling rate. Fast quenching is needed to avoid detrimental coarse precipitation that reduces mechanical characteristics after heat treatment. But fast quenching induces residual stresses that can cause unacceptable distortions during machining and unfavourable stresses in service. Control and prediction of quench-induced residual stresses is thus of particular interest for the industry to prevent such negative impacts. However the problem is complex since some locations in large impellers experiment a too low cooling rate to prevent any precipitation. Heat transfer during quenching, precipitation phenomena, thermally induced deformations and stress generation are interacting and therefore need to be taken into account. In this work a finite element analysis is used for the calculation of residual stresses in AA2618 impellers. Heat transfer during quenching is determined from temperature measurements on forgings equipped with thermocouples. Rather than a complex and time-consuming precipitation characterisation and modelling task, a thermomechanical model accounting for precipitation in a simple but realistic way is used. Precipitation is accounted for through an uniform (not space dependant) and temperature dependent yield strength in an elasto-viscoplastic constitutive model with additive hardening. A limited number of thermomechanical Gleeble tests have been used to identify the model parameters. Calculated stresses are compared to neutron diffraction measurements.