During casting, thermally induced deformations give birth to ingot distortions and residual stresses. For some high strength alloys, ingot cracking can happen during casting per se or during cooling down. Ingot distortions such as rolling face pull-in, but curl and but swell are rather easy to quantify as opposed to internal stresses. As aluminium is rather transparent to neutrons, residual stress measurements using neutron diffraction appeared to be a good way to validate the thermomechanical models aimed at simulating the stress build-up during casting. This technique has been applied to DC cast AA7050 rolling plate ingots with special attention to the stress generation in the transient start-up phase, i.e. in the foot of the ingot. Additional results using the hole drilling method complement the measurements. The measured stress distributions are compared with the results of a numerical model of DC casting for ingots cast with and without a wiper.
I IntroductionIn the fabrication of aluminum rolling plates, the first step is the semi-continuous casting of a rectangular ingot. The most commonly used process is known as direct chill (DC) casting [1]. This process gives rise to large thermally induced strains that lead to several types of casting defects (distortions, cold cracks, porosity, solidification cracking, etc.). During casting, thermally induced stresses are partially relieved by permanent deformation. When these residual stresses overcome the deformation limit of the alloy, cracks are generated either during solidification (hot tears) or during cooling (cold cracks). The formation of these cracks results in rejection of the cast part. Furthermore, thermally induced stresses can cause downstream processing issues during the sawing stage prior to rolling. For large ingot formats and high strength alloys, sawing becomes a delicate task owing to the risk of saw pinching or crack initiation ahead of the saw. The use of wipers during casting largely reduces the level of as-cast stresses.The computation of stresses during DC casting of aluminum alloys has been the scope of several studies since the late 90's [2-10] and is a well established technique nowadays. Many numerical models have allowed researchers to compute the ingot distortions and the associated residual stresses. The validation of these models was often done by comparing the computed and measured ingot distortions, e.g. the butt-curl [8] and the rolling face pull-in for rolling sheet ingots produced by DC [9] or electromagnetic casting [11].Validation against the computed room-temperature residual stresses is limited simply owing to the difficulty of measuring the internal strains and the high variability in the measurements. While some measurements are available for quenching [12] or welding [13], they remain rare, uncertain and usually are limited to one or two components of the stress tensor, and to the skin of round billets for as-cast materials [14][15]. In contrast to destructive methods for measuring residual stresses (hole-drilli...