Research On Residual Stress Relief Countermeasures In Al-Zn-Mg-Cu Alloy Thick Plates

Thermal gradients between the surface and intermediate layer of thick plates during rapid cooling from solution heat treatment temperature result in inhomogeneous plastic flow in quenched Al-Zn-Mg-Cu alloy thick plat, which leads to compressive residual stresses on the surface and tensile stresses in the interior. During later monolithic or structural parts manufacturing processes, up to 95% of the raw material could be machined away. In those manufacturing processes, residual stress plays an important role that can cause distortion or cracking. Therefore, it is of great significance to control the residual stress magnitudes in order to achieve distortion or cracking free machinability and service performance. Traditional and novel treatment were tested to reduce residual stress and the results are shown below.(1) Conventional artificial aging only contributed to a residual stress reduction 10%-35%.The heating process contribute more residual stress reduction than heat preservation stage. Precipitation hardening impedes residual stress reduction.(2) Interrupted quenching had limited effects in residual stress reduction and precipitation hardening was easily affected by interrupted quenching. When applied a extent of temperature variation (80-180?),the thick plates had a residual stress variation of 15-35 MPa.(3) Pulsed electrical current could contribute a significant residual stress reduction about 70-90 MPa by a relatively low ampere density (2.5A/mm2). When use the pulsed electrical current in aging temperature, the residual stress has a further reduction.(4) The results show that residual stress magnitudes were reduced significantly and even eliminated completely by multistage interrupted artificial aging treatment, because an inverse stress was ingeniously employed during rapid heating to release the internal residual stress. Meanwhile, the mechanical properties were maintained, and the corrosion resistance was improved comparing to conventional artificial aging. The maintained mechanical property and improved corrosion resistance were resulted from rearranging of dislocations and precipitates characters in multistage interrupted artificial aged alloys.(5) During quenching process, plenty of dislocations are forced to pile up, which increase the stored elastic energy in aluminum alloys. Hence, the as-quenched alloys with high residual stress were in an unstable and upper state with plenty of lattice defects and high stored elastic energy. To reduce residual stresses, this state should be struck and rebalanced, which means to release the stored elastic energies by annihilating and rearranging the dislocations. Efficient methods of reducing residual stresses should contain a process of inputting energy into the material rapidly to rebalance the upper state of as-quenched conditions.