Internal Stress Generation during Quenching of Thick Heat Treatable Aluminium Alloys

Abstract

In the current trend toward thicker aluminium plates, a major concern is the stress build-up during quenching which causes distortions during machining. Indeed, cooling rates are not high enough, especially at the core of such thick plates, to prevent any precipitation and quench induced precipitates lower the hardening potential. Multi-scale modelling is required when predicting macro-scale stresses after quenching for thick heat treatable aluminium components. The reason is the instantaneous strong coupling between phase precipitation at the nano-scale and material hardening due to precipitation or softening owing to solute depletion at the microscale. For thick parts, quenching intensities decrease when going from the skin to the core of the component, thus introducing a gradient of nanostructure and consequently a gradient of mechanical properties. In addition, large thermally induced deformations lead to high macroscale residual stresses although part of them is relaxed by plastic deformation. These stresses have been measured in water quenched thick plates of 7040 and 7449 aluminium alloys using neutron diffraction and layer removal techniques and the results when compared with a thermomechanical finite element model of quenching highlight the influence of precipitation.

Quench induced residual stress prediction in heat treatable 7xxx aluminium alloy thick plates using Gleeble interrupted quench tests

“…It consists of one row of axisymmetric elements through the plate thickness. In this model, only the inplane components xx and yy of the stress tensor are non-zero as verified experimentally by Chobaut et al (2012) using neutron diffraction and by Drezet et al (2013) using Layer Removal. In this model, stress components are:…”

“…Residual strain measurements in 75 mm thick as-quenched AA7040 and AA7449 plates were already described in the work of Drezet et al (2013). Additionally, a 500 mm × 300 mm × 20 mm as-quenched AA7449 plate was measured at the POLDI diffractometer (PSI, Switzerland) and a 600 mm × 400 mm × 140 mm as-quenched AA7040 plate was measured at the SALSA diffractometer (ILL, France).…”

Review of the Quench Sensitivity of Aluminium Alloys: Analysis of the Kinetics and Nature of Quench-Induced Precipitation

“…The UCCR is very relevant in technological applications, and it is shown below that the maximum age-hardening potential can be exploited only if the alloy is cooled at the UCCR or faster. However, faster cooling may result in undesired residual stresses and distortion of age-hardened aluminium components [38,[41][42][43]104,[135][136][137]. Although this is of lower importance for extrusions or rolled products, which are stretched after quenching, residual stresses and distortion are very important for complex cross-section extrusions and net-shaped cast and forged products.…”