Large aluminium monolithic parts used in the aeronautic industry frequently show significant geometric distortions after the machining process. These distortions are the consequence of the initial residual stresses of the raw material as well as machininginduced residual stresses. Therefore, to minimise distortions it is fundamental to understand the effect of the different parameters of the manufacturing process and define the optimum manufacturing strategy.This work studies the effect of initial residual stresses of aluminium plates and the residual stresses generated by the machining process on the final geometric distortions of the part. First, 7175-T7351 aluminium bars (40 mm wide × 38 mm thick × 400 mm long) were face milled at two different cutting conditions reducing the thickness to 6 mm. The distortions were measured in a CMM machine. The results revealed that the machining strategy significantly influenced the distortions, as a difference of 65% on distortions was found between the two cutting conditions. In addition, an FEM model to predict distortions was developed. This model considers initial residual stresses (measured by the contour method) and residual stresses induced by the machining process (measured by the hole drilling technique). Once the FEM model was validated, the study was extended to more complex geometries. These new studies revealed that final distortions are sensitive to machining-induced residual stresses. Furthermore, this finding indicates that it is possible to define machining conditions which generate desirable residual stress profiles to minimise part distortion.
This work consisted of the characterisation of star cracks found on the surface of continuously cast steel slabs solidification experiments on molten steel in Cu moulds, with and without ceramic insulation. Cracks located at the slab surface were propagated through the grain boundaries, with the presence of Cu particles observed along the crack paths. Zones normal to the cracks exhibited an iron oxide layer followed by Cu particles and the steel matrix. Specimens solidified in Cu moulds showed an iron oxide/Cu/steel layer arrangement extending from the sample surface towards the steel matrix, as well as a penetration of Cu through the grain boundaries. In contrast, in samples solidified in ceramic insulated Cu moulds, Cu particles were neither observed at the surface nor at the grain boundaries. Thus, it is thought that the star cracks are due to liquid steel/ Cu mould interaction during casting and that they are formed during primary cooling in the mould in the continuous casting machine and then they propagate during secondary cooling and bending operations. It is also thought that cracking susceptibility is promoted by peritectic transformation which occurs at high solid fractions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.