Influence of a physical constraint combined with uphill quenching on residual stresses in 7000 series aluminium alloys

When quenching heat treatable aluminium alloys, the interior of a sample cools at a different rate than that of the surface. Post quench this usually forms a triaxial tensile stress in the interior and a biaxial compressive stress on the surface of the sample. These residual stresses can have sufficient magnitudes to cause distortion of parts when being machined. Uphill quenching is a thermal method that relieves residual stresses post conventional quenching. This technique maintains the dimensional properties of the sample, unlike mechanical procedures such as cold compression and stretching. Despite uphill quenching being a time consuming and expensive method it does have industrial applications, as it allows parts to maintain all of the desired mechanical properties associated with rapid quenching. Uphill quenching could be used on long aluminium aircraft simple geometric pieces such as rolled wing spars or extruded stringers. The aim of this study is to quantify the magnitude of residual stresses that can be relieved during a variation of uphill quenching heat treatment. The alloys tested are 7000 series aluminium alloys. The uphill quenching method is cooling a test piece to -197 ℃ in liquid nitrogen post solution heat treatment and quenching, then rapidly reheating in boiling water or high pressure steam. This study added a variation to the standard uphill quench. The test piece was thermally constrained along its long axis during the reheating phase of uphill quenching. A simple rectilinear shaped test piece was chosen to ensure uniform stress redistribution. Cold compression tests were also performed as a comparison to the effects of constrained thermal expansion. Post quench delay experiments were done to see at what time would be the most optimal for mechanically relieving the test pieces of the residual stress magnitudes. The surface residual stresses were determined by X-ray diffraction using the sin2Ѱ method. Vickers hardness tests at varying temperatures and artificial ageing response tests were undertaken to study the mechanical properties during the reheating phase. Micro and hot caustic macro etching was carried out to see the shape, size and pattern of the microstructure of a test piece post an uphill quenching process. The thermally constrained uphill quenching methods using boiling water and steam were relieved on the surface about 60 and 66 % in the longitudinal orientation and 30 and 38 % short transverse orientation respectively. The majority of this relief resulted from the physical constraint. Constrained uphill quenching gave better residual stress relief than standard uphill quenching practices. Cold compression results showed that a compression of 0.6 % is enough to relieve 90 % of surface residual stresses. Post quench delay showed no clear results other than keeping the time of mechanical relief constant. During the reheating phase of uphill quenching it is best to relieve as much residual stress within a minute of being subjected to elevated temperatures, as the test pieces start to artificially harden. Results are compared to finite element analysis simulations using Abaqus and available literature data