Papers by Author: Jeremy S. Robinson

Abstract: The residual stresses in heat treated 7075 aluminium alloy blocks have been characterised using two neutron diffraction strain scanning instruments. The influence of uniaxial cold compression (1-10%) on relieving the residual stress has been determined. Increasing the magnitude of cold compression from 1 to 10% has been shown to have a beneficial effect on the residual stress distribution by reducing the range between the maximum and minimum residual stresses. The effect of over aging 7075 on residual stress has also been characterised using neutron diffraction and this was found to reduce the residual stress by 25-40%. A relationship between {311} peaks widths and amount of cold compression was also observed.

Abstract: Experimental errors in neutron residual strain imaging are often underestimated, mis-understood or simply ignored. The choice of beam defining optics can play a large role in the magnitude of these errors. In this paper we show that the use of a traditional slit beam defining system with large specimens can lead to several sources of errors. Large specimens usually require necessarily large distances between the slits and the instrument reference point (gauge volume). At large distances, the slit system can lead to significant underestimation of the residual strain magnitude as well as poor definition of the gauge volume. We show that for large specimens, the use of radial focussing collimators reduces these effects, leading to significant improvement in measurement reliability. Specifically, we show application of radial collimators at the SALSA instrument of the Institut Laue Langevin neutron source in Grenoble, France.

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. During cold water quenching of thick sections, the thermal gradients are sufficient to cause inhomogeneous plastic deformation which in turn leads to the development of large residual stresses. Two 215 mm thick rectilinear forgings made from 7075 and 7010 were heat treated, and the through thickness residual stresses measured by neutron diffraction and deep hole drilling. The distribution of residual stresses was found to be similar for both alloys varying from highly triaxial and tensile in the core to a state of biaxial compression in the surface. The 7010 forging exhibited significantly larger tensile stresses in the core. 7075 is a much more quench sensitive alloy when compared to 7010. This results in loss of supersaturation by second phase precipitation during quenching in the core of the 7075 forging.

Abstract: 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. The surface and through thickness residual stress magnitudes present in heat treated high strength aluminium alloy components are frequently reported to exceed the uniaxial yield stress of small specimens of the same alloy measured immediately after quenching. In thick section plate and forgings it is proposed that these high residual stress magnitudes are a consequence of hardening precipitation that occurs during quenching which allows for a greater elastic stress to be supported. To investigate this theory, thick sections of the quench sensitive alloy 7175 and the less quench sensitive alloy 7010 were heat treated in such a way as to allow the internal hardness to be measured immediately, after quenching. The rate of cooling was also monitored during quenching and these data were used in conjunction with time temperature property data to predict the degree of precipitation and subsequent loss of hardening potential in the fully heat treated condition. The magnitudes of the residual stresses induced during quenching were determined using standard x-ray diffraction techniques.