Measurements and Modeling of Stress in Precipitation-Hardened Aluminum Alloy AA2618 during Gleeble Interrupted Quenching and Constrained Cooling

Denis Carron, Nicolas Paul Henry Chobaut, Jean-Marie Drezet
Springer Verlag, 2016
Journal paper

Abstract

Solutionizing and quenching are the key steps in the fabrication of heat-treatable aluminum parts such as AA2618 compressor impellers for turbochargers as they highly impact the mechanical characteristics of the product. In particular, quenching induces residual stresses that can cause unacceptable distortions during machining and unfavorable stresses in service. Predicting and controlling stress generation during quenching of large AA2618 forgings are therefore of particular interest. Since possible precipitation during quenching may affect the local yield strength of the material and thus impact the level of macroscale residual stresses, consideration of this phenomenon is required. A material model accounting for precipitation in a simple but realistic way is presented. Instead of modeling precipitation that occurs during quenching, the model parameters are identified using a limited number of tensile tests achieved after representative interrupted cooling paths in a Gleeble machine. This material model is presented, calibrated, and validated against constrained coolings in a Gleeble blocked-jaws configuration. Applications of this model are FE computations of stress generation during quenching of large AA2618 forgings for compressor impellers.

Official source

https://infoscience.epfl.ch/record/222455?ln=en

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

Denis Carron, Nicolas Paul Henry Chobaut, Jean-Marie Drezet

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.

Elsevier2017

In the fabrication of heat treatable aluminium parts, solutionising and quenching are key steps in order to obtain the required mechanical characteristics. Fast quenching is necessary to avoid coarse precipitation as this one reduces the mechanical properties after heat treatment. However fast quenching gives birth to high residual stresses, which cause unacceptable distortions during machining and can reduce service life drastically. For this reason, plates and forgings such as those considered in this thesis are subjected to a stress relieving operation, removing most of the residual stresses after quenching. For thick products, fast quenching throughout the thickness is no longer possible owing to the high but still limited thermal diffusivity of aluminium alloys, but residual stress occurrence remains problematic. The objective of this work is to develop a comprehensive model of quenching including precipitation effects for understanding the development of residual stresses in thick heat treatable aluminium components. Two industrial contexts are studied in this work: - the cold-water quenching of hot-rolled AA7040 and AA7449 thick plates for aerospace applications, - the boiling-water quenching of large AA2618 compressor wheel forgings used in turbochargers. Besides an accurate knowledge of the thermal field in the components during quenching, this work has involved an extensive thermo-mechanical characterisation of the three alloys in different precipitation states by interrupted quench-tests achieved in a Gleeble machine. Plastic strain recovery at high temperature is considered in a simple way in the description of the material behaviour and the Bauschinger effect is neglected as suggested by dedicated experiments. Three numerical models are developed to predict as-quenched residual stresses: - a thermo-mechanical model ignoring precipitation, - a physically-based thermo-metallurgical-mechanical model where a yield strength model is coupled with a precipitation model, - a thermo-mechanical model accounting for precipitation in a simple but realistic way. Instead of modelling precipitation that occurs during quenching, the model parameters are identified using a limited number of tensile tests achieved after representative interrupted cooling paths in a Gleeble machine. The results are compared to as-quenched residual stress measurements achieved by neutron diffraction and layer removal in thick AA7449 and AA7040 plates and in large AA2618 forgings. The thermo-mechanical model ignoring precipitation is found to be sufficient for relatively thin products. For thicker products, precipitation hardening by small precipitates is found to increase as-quenched residual stresses. Since the thermo-mechanical model ignoring precipitation underestimates these residual stresses, precipitation must thus be accounted for. The second model gives an excellent agreement between measurements and simulations provided that the precipitation model is well calibrated. This physically-based model is necessary for modelling complex parts with different thicknesses. Nevertheless, a lot of effort has to be dedicated to the characterisation and modelling of the precipitation of metastable non-stoichiometric phases and GP zones during quenching. This tedious work is not needed for the third model which requires only a few interrupted quench-tests while giving an excellent agreement between measurements and simulations for all the investigated components. In particular, it is verified that the two models taking into account precipitation give identical residual stress profiles in thick AA7449 plates.

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Quench-Induced Stresses in AA2618 Forgings for Impellers: A Multiphysics and Multiscale Problem

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In the fabrication of heat-treatable aluminum parts such as AA2618 compressor impellers for turbochargers, solutionizing and quenching are key steps to obtain the required mechanical characteristics. Fast quenching is necessary to avoid coarse precipitation as it reduces the mechanical properties obtained after heat treatment. However, fast quenching induces residual stresses that can cause unacceptable distortions during machining. Furthermore, the remaining residual stresses after final machining can lead to unfavorable stresses in service. Predicting and controlling internal stresses during the whole processing from heat treatment to final machining is therefore of particular interest to prevent negative impacts of residual stresses. This problem is multiphysics because processes such as heat transfer during quenching, precipitation phenomena, thermally induced deformations, and stress generation are interacting and need to be taken into account. The problem is also multiscale as precipitates of nanosize form during quenching at locations where the cooling rate is too low. This precipitation affects the local yield strength of the material and thus impacts the level of macroscale residual stresses. A thermomechanical model accounting for precipitation in a simple but realistic way is presented. Instead of modelling precipitation that occurs during quenching, the model parameters are identified using a limited number of tensile tests achieved after representative interrupted cooling paths in a Gleeble machine. The simulation results are compared with as-quenched residual stresses in a forging measured by neutron diffraction.

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