Stored elastic energy in aluminium alloy AA 6063 billets: residual stress measurements and thermomechanical modelling

Abstract

Stress relief treatment before machining and sawing aluminium direct chill cast products is required to avoid uncontrolled distortion, crack formation and significant safety concerns due to the presence of thermally induced residual stresses created during casting. Numerical models have been developed to compute these residual stresses and yet have only been validated against measured surface distortions. In the present contribution, the variations in residual strains and stresses have been measured using neutron diffraction and hole drilling strain gage in two AA 6063 grain refined cylindrical billet sections cast at two casting speeds. The measured residual stresses compare favourably with the numerical model, in particular the depth at which the axial and hoop stresses change sign. Such results provide insight into the development of residual stresses within castings and show that the stored elastic energy varies linearly with the casting speed, at least within the range of speeds that correspond to production conditions. Keywords: Aluminium billet casting, Residual stresses, FE modelling, Neutron diffraction, Hole drilling strain gage, Elastic energy IntroductionIn the fabrication of aluminium extrusion products, the first step is the semicontinuous casting of a cylindrical billet. 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, cracks, porosity, residual stresses, 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 during either solidification (hot tears) or cooling (cold cracks). The formation of these cracks usually results in rejection of the cast part. Furthermore, the thermally induced deformations can cause downstream processing issues during the sawing stage before extrusion, when the billet is cut without thermal annealing between casting and sawing. For large diameter and high strength alloys, sawing becomes a delicate task owing to the risk of saw pinching or crack initiation ahead of the saw. Parts might be ejected and injure people or damage the equipment.The computation of stresses during DC casting of aluminium alloys has been the scope of several studies since the late 1990s 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 carried out 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. 11Validation 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 we...

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