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Conformal cooling channels are becoming one of the next big steps in the fabrication of moulds and tools. Mass flow rate and heat transfer are affected by the surface roughness in the cooling channels. The freeform shape of conformal cooling channels makes it difficult to evaluate the internal roughness with respect to classic planar techniques. This work presents a fitted-ellipse method to evaluate internal surface features of helical cooling channels. The investigated cooling channel was made from maraging steel 300 and manufactured with the selective laser melting process. X-ray computed tomography and image analysis were utilized in order to generate a freeform nominal surface by fitting ellipses to the reconstructed surface. The nominal surface was compared to the reconstructed surface and resulted in a point cloud of deviation values. The deviation values were used as input for deviation plots, inner area and volume estimations together with estimations of classic area surface parameters, according to ISO 25178-2:2012. Results showed that the internal surface features were highly orientation dependent, with extreme roughness observed on the downward facing surface of the cooling channel. The arithmetical mean height and average maximum height of the total inner surface were estimated at Sa = 13.7 μm and Sz20 = 251 μm, respectively. The mass distribution was positively skewed, the root mean square height was Sq = 21.8 μm and the peaks observed on the surface were characterized as spiked. The obtained results suggested that the proposed method could evaluate the internal features of a helical cooling channel efficiently and qualitatively, while giving realistic quantitative estimations of the surface roughness characteristics.

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Recent experiments resolved nucleation and growth of graphite during solidification of ductile cast iron in 3D and time using synchrotron X-ray tomography [1]. We use the experimental observations to analyse the relation between graphite growth rate and the state of the particle neighbourhood to pinpoint possible links between growth rate of individual graphite spheres and the overall solidification state. With this insight we revisit existing models for growth of spheroidal graphite and discuss possible modifications in order to describe the critical final stage of solidification correctly.

An accurate prediction of ductile cast iron microstructures is crucial for a science-based optimisation of cast component design. The number density and distribution of graphite nodules critically influence the mechanical performance of a component in service. Although models predicting nodule growth have been researched for many years, recent improvements have been impeded by lack of detailed experimental data on nodule growth kinetics for validation. This data has now been made available through in situ observations of the solidification of ductile cast iron using synchrotron X-ray tomography in combination with a high temperature environmental cell. In the present investigation, a new sphere of influence model for spheroidal graphite growth is proposed. It inherently incorporates the competition for carbon between neighbouring nodules and the depletion of carbon in the matrix.Comparing simulation results to the in situ observations of graphite growth, the sphere of 2 influence model successfully predicts both growth of individual nodules as well as the size distribution of a large nodule population during solidification.