The influence of quench sensitivity on residual stresses in the aluminium alloys 7010 and 7075

Abstract

The most critical stage in the heat treatment of high strength aluminium alloys is the rapid cooling necessary to form a supersaturated solid solution. A disadvantage of quenching is that the thermal gradients can be sufficient to cause inhomogeneous plastic deformation which in turn leads to the development of large residual stresses. Two 215 mm thick rectilinear forgings have been made from 7000 series alloys with widely different quench sensitivity to determine if solute loss in the form of precipitation during quenching can significantly affect residual stress magnitudes. The forgings were heat treated and immersion quenched using cold water to produce large magnitude residual stresses. The through thickness residual stresses were measured by neutron diffraction and incremental deep hole drilling. The distribution of residual stresses was found to be similar for both alloys varying from highly triaxial and tensile in the interior, to a state of biaxial compression in the surface. The 7010 forging exhibited larger tensile stresses in the interior. The microstructural variation from surface to centre for both forgings was determined using optical and transmission electron microscopy. These observations were used to confirm the origin of the hardness variation measured through the forging thickness. When the microstructural changes were accounted for in the through thickness lattice parameter, the residual stresses in the two forgings were found to be very similar. Solute loss in the 7075 forging appeared to have no significant effect on the residual stress magnitudes when compared to 7010.

Residual Stress Analysis in AA7449 As‐Quenched Thick Plates Using Neutrons and Fe Modelling

“…The difference in RS between single (311) diffraction peak and multiple peaks analysis is within the experimental uncertainty. This is in agreement with other studies [4]. The major difference between the three cases is the experimental uncertainty; it is around 10 MPa at SALSA, 30 MPa at POLDI using one peak and 50 MPa using Pawley analysis.…”

“…Robinson et al [4] who reported a decrease in stress free lattice parameter from the surface to the core of AA7075 thick forgings attributed to solute loss, no such variation is observed in the present work. 7449 and 7075 alloys being both quench sensitive, the difference is thought to originate from the different thickness of the components: 75 mm in the present case and 215 mm in [4].…”

“…7449 and 7075 alloys being both quench sensitive, the difference is thought to originate from the different thickness of the components: 75 mm in the present case and 215 mm in [4]. The shallow difference between measured and calculated 0 d is an experimental verification of equation [1.6] which means that no reference specimen is needed in the case of large plates where edge effects are negligible.…”

“…As a consequence, the ideal quench of heat treatable AA is always a compromise between final properties and RS. Many researchers have built thermo-mechanical (TM) models to predict RS build-up in thick AA components [3][4][5]. To validate and improve the numerical models, results were compared with experimental measurements of RS using a wide range of methods such as hole drilling, layer removal, crack compliance, ultrasounds and diffraction.…”

Problems in the robot path control caused by AL-7075 chassis distortion and methods of improvement through non-conventional machining process

“…When an area of the part which is on residual stress mode, is removed this causes an imbalance of the tensile/compressive layers (Hatamleh et al, 2009), thus, the part distorts to maintain a balance of compressive and tensile stresses. If the resulting residual stresses in the part are zero or very low, no movement occurs (Mandy et al, 2007); however, to achieve such a state, almost all the machining conditions must be chosen correctly (Robinson, 2012). Distortion is problematic in several ways.…”