Residual Stresses in As-Cast Billets: Neutron Diffraction Measurement and Thermomechanical Modeling

Abstract

Stress relief treatment is often required prior to sawing aluminum DC cast products in order to prevent crack formation and significant safety concerns due to the presence of high residual stresses generated 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 variation in residual strains and stresses have been measured using neutron diffraction in two AA6063 grainrefined cylindrical billet sections cast at two casting speeds. The measured residual stresses compare favorably 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. I IntroductionIn the fabrication of aluminum extrusion products, the first step is the semi-continuous 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, 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 usually results in rejection of the cast part. Furthermore, thermally induced deformations can cause downstream processing issues during the sawing stage prior to 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 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 the billet for as-cast materials [14][15]. In ...

Neutron Diffraction Measurement of As-Cast Residual Stresses in Aa7050 Rolling Plate Ingots: Influence of A Wiper

“…SALSA [20] is a neutron diffraction instrument designed for strain measurements through the accurate determination of lattice spacing. In a stressed material, the lattice spacing acts as a kind of strain gauge.…”

“…In DC cast extrusion billets [20], the elastic stress and strain tensors have only four components due to the axisymmetric billet geometry and casting conditions. For rolling slabs, this is no longer the case and the stress/strain tensor has 6 components.…”

“…Further details on the FE model of casting can be found in [1,2,8,9,20]. Figure 4 shows the computed temperature distribution and extension of the mushy zone (solid volume fractions between 0.0 and 1.0) in the absence of a wiper (left) and when a wiper is used (right).…”

“…The authors showed that a minimum billet sectionlength of a least three times the billet radius was required to ensure that the residual stresses at its mid-height are not relaxed during sawing. The validation of the FE casting model against residual stresses in aluminum round billets is fully detailed in [18,20]. The aim of the present work is to extend this validation to the case of rolling plate ingots and to assess the benefits of using a wiper during casting.…”

Neutron Diffraction Measurement of As‐Cast Residual Stresses in AA7050 Rolling Plate Ingots: Influence of a Wiper

“…The heat transfer to the dummy block was simplified and modeled using a mean heat transfer coefficient of 200 W/m 2 K. The mechanical properties of the AA7050 alloy were taken from the work of Lalpoor et al [25,26]. Further details on the FE model of casting can be found in [1,2,8,9,20]. Figure 4 shows the computed temperature distribution and extension of the mushy zone (solid volume fractions between 0.0 and 1.0) in the absence of a wiper (left) and when a wiper is used (right).…”