Stress generation during the quenching of large AA2618 forgings: Finite element computations and validation against neutron diffraction measurements

Solutionising and quenching are key steps in the fabrication of heat treatable aluminium parts such as AA2618 compressor impellers for turbochargers. Quenching not only dictates the mechanical characteristics of the product but also induces residual stresses that can cause unacceptable distortions during machining and unfavourable stresses in service. Predicting and controlling stress generation during quenching of large AA2618 forgings is therefore of great interest. Since possible precipitation during quenching may affect the local yield strength of the material and thus impact the level of macro-scale residual stresses, consideration of this phenomenon is required. A phenomenological material model accounting for precipitation in a simple way is used instead of modelling in detail precipitation that occurs during quenching. The required model parameters are identified using a limited number of tensile tests achieved after representative interrupted cooling paths in a Gleeble machine. This model is used in FE computations of stress generation during quenching of large massive AA2618 forgings for compressor impellers. The residual strain and stress profiles are compared with neutron diffraction measurements carried out at SALSA and STRESS-SPEC diffractometers in as-quenched and in T6 conditions. It turned out that the residual stress predictions by FE modelling might be wrong if precipitation is not taken into account properly in the material model.