The magnitude of heat treatment induced residual stresses and the thermal stress relief of aluminium alloys

To produce useful strengthening, precipitation hardenable aluminium alloys rely on rapid quenching from the solution heat treatment temperature to suppress the formation of coarse equilibrium second phases. An unavoidable consequence of the rapid quenching of thick sections is the severe thermal gradients that quickly develop in the material. The attendant inhomogeneous plastic flow can then result in the establishment of residual stresses. Established procedures exist to minimise residual stress by quenching into boiling water or organic quenchants at the expense of ageing response. Residual stresses can also be relieved after solution heat treatment by the application of plastic deformation in a controlled manner. A limited degree of thermal stress relief is also reported to occur during subsequent artificial ageing treatments, especially duplex ageing treatments. It is generally accepted that the size of the residual stresses induced during quenching cannot exceed the yield strength of the material. However, for precipitation hardened aluminium alloys, stress magnitudes as measured by standard techniques can exceed the uniaxial stress required to cause plastic flow during tensile tests conducted immediately after quenching. An investigation to explain these observations involving measuring as-quenched tensile properties and room temperature stress relief in heat treatable and non-heat treatable aluminium alloys has been conducted. Two alloys were investigated: 7010, an Al-Zn-Mg-Cu precipitation hardenable alloy and 5251, a non-heat treatable medium strength Al-Mg-Mn alloy. Tensile properties were determined by heat-treating test specimens at 475°C, cold water quenching and then testing without delay to avoid significant microstructural modification. The progress of stress relief at room temperature was then monitored utilising test coupons and standard x-ray diffraction techniques. Natural ageing of 7010 leads to a rapid increase in strength and a subsequent locking in of residual stresses (this cannot occur in 5251) and the change in residual stress is monitored as a function of time. An attempt has also been made to determine the efficacy of x-ray diffraction to monitor thermally induced stress relief. X-ray diffraction and hole drilling techniques to ASTM E837 were utilised to follow the progress of isothermal stress relief at room temperature and 200°C in both 7010 and 5251.