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[Display omitted] Shrinkage flow is very important for porosity formation during solidification since it has the feeding ability. In this study, the shrinkage flow is investigated by Phase-Field (PF) and Lattice Boltzmann method (LBM). It is found that the feeding velocity is positively related to the dendrite growth rate. They both reach their maximum values at the dendrite tip. Subsequently, we calculated six cases under various initial conditions. It is found that the dendrite growth rate and feeding velocity both increase with solute supersaturation. Finally, we compared the feeding velocities under different solid densities. It is found that both the feeding velocity and solute segregation increase with the density difference between solid and liquid. The current work can be further applied in the hot tearing research of solidification. [ABSTRACT FROM AUTHOR]

A mathematical model for coupling prediction of inverse segregation and porosity for up-vertical unidirectional solidification of Al–Cu alloys is established, based on the analysis of the redistribution behaviors of both gas element and the alloying elements in the mushy zone. The model first investigates the mass variation during solidification, including the specific mass variation and the volume shrinkage during the solidification of both primary phase and eutectic, which give rise to the back flow towards the dendrite roots. Then, the porosity fraction is obtained by combing hydrogen segregation and pressure depression associated with the feeding flow through a porous media being solved with Darcy's equation. Consequently, both the feeding flow and the 'local solute redistribution equation' are modified with the presence of porosity. Finally, the solute distribution is obtained by coupling a finite difference solidification model with the modified segregation model. Numerical results show that the overall back-flow, which is needed to compensate the volume shrinkage, decreases with the increase of initially dissolved hydrogen concentration. As a result, the inverse segregation induced mainly by the back-flow of liquid enriched with solute, decreases with the increase in porosity fraction. The numerical predictions show rather good agreement compared with the experimental results reported in literature with the same condition. This model is capable of predicting inverse segregation with the presence of porosity in vertical unidirectional solidification of Al–Cu alloys. A mathematical model for coupling prediction of the inverse segregation and porosity for up-vertical unidirectional solidification of Al-6.2 wt% Cu alloys has been established. This model can be considered as a modification to the 'local solute redistribution equation' while accounting for the presence of porosity. The numerical prediction shows a reasonable good agreement with the experimental measurement acquired from literature. Image 1 • Porosity affects the redistribution behavior of solute elements in the mushy zone. • A model for coupling prediction of porosity and inverse segregation is established. • Increase in initial hydrogen content decreases the level of inverse segregation. [ABSTRACT FROM AUTHOR]

Simulation software dedicated for design of casting processes is usually tested and calibrated by comparisons of shrinkage defects distribution predicted by the modelling with that observed in real castings produced in a given foundry. However, a large amount of expertise obtained from different foundries, including especially made experiments, is available from literature, in the form of recommendations for design of the rigging systems. This kind of information can be also used for assessment of the simulation predictions. In the present work two parameters used in the design of feeding systems are considered: feeding ranges in horizontal and vertical plates as well as efficiency (yield) of feeders of various shapes. The simulation tests were conducted using especially designed steel and aluminium castings with risers and a commercial FDM based software. It was found that the simulations cannot predict appearance of shrinkage porosity in horizontal and vertical plates of even cross-sections which would mean, that the feeding ranges are practically unlimited. The yield of all types of feeders obtained from the simulations appeared to be much higher than that reported in the literature. It can be concluded that the feeding flow modelling included in the tested software does not reflect phenomena responsible for the feeding processes in real castings properly. Further tests, with different types of software and more fundamental studies on the feeding process modelling would be desirable.

The effects of the melt temperature on the relative densities, microstructures and mechanical properties of vacuum counter-pressure casting A357 alloys solidified under grade-pressurising (200kPa) were investigated. The results indicated that the relative densities and mechanical properties of samples solidified under grade-pressurising were affected by the melt temperature (580-610°C). The optimal relative density and mechanical properties were achieved at a melt temperature of 590°C. The microstructures varied with the melt temperature as a result of the differences in the strengths of the feeding flow and dendrites. The powerful feeding flow caused by grade-pressurising solidification can induce stress on the dendrites, resulting in the collapse of the primary dendrite network to form equiaxed dendrites. Decreasing the feeding pressure proportionally to the solid volume fraction was proposed to obtain a suitable melt temperature for improving the density and refining the microstructure. [ABSTRACT FROM AUTHOR]

Centreline macrosegregation is often observed in continuous slab casting of steel. Two of the main macrosegregation formation mechanisms are bulging and feeding. Both were studied and compared in the current work by using a two-phase volume averaging model considering only columnar solidification. The casting of the strand itself is modelled by applying a predefined velocity following the casting speed and solid shell deformation (e.g. bulging). Three different cases are simulated and discussed. (i) The first case considers the influence of the feeding flow during solidification without taking bulging into account. Negative macrosegregation is observed in the centre of the casting in this case. (ii) The second case takes the flow caused by series of bulging along the solidifying strand shell into account, and is, therefore, representative for an ideal situation where bulging takes place without solidification shrinkage. In this case positive centreline segregation is found. (iii) The last case shows the results of a simulation which combines both shrinkage- and bulging-induced flows. It is found that under the current casting conditions the bulging effect dominates over the shrinkage effect, and so positive centreline segregation is predicted. [ABSTRACT FROM AUTHOR]

The article studies the macrosegregation phenomena that occur during solidification of steel ingot castings using a computational multi-fluid dynamics approach. The authors describe four different cases, which include feeding-induced macrosegregations in one-dimensional unidirectional solidification, macrosegregations caused by thermosolutal buoyancy driven flow, macrosegregations caused by grain sedimentation and macrosegregations caused by thermosolutal convection.

Based on the model of microstructure formation of spheroidal graphite iron (SGI) casting which includes continuous nucleation and growth of graphite nodules, carbides and austenite, a method was proposed to calculate the feeding flow during solidification processes by considering mass conservation and using Darcy's laws. The method takes into account liquid shrinkage as well as shrinkage and expansion caused by different phase formation, and can calculate the flow in liquid and mushy regions. According to calculation of the flow, macro porosity formation could be described through treating free surfaces and boundary of the regions. The proposed method has been applied to an automotive wheel hub SGI casting with two different processing cases. The feeding flow during the casting solidification with two cases, especially in isolated regions, was investigated and discussed. With the flow calculation, expansion or shrinkage of liquid or mushy regions might be judged, which could offer important information for evaluating formation of macro porosity. The results of macro porosity prediction were compared with results of practical castings. It is indicated that two results were in quite good agreement, and the method could predict macro porosity formation more accurately. [ABSTRACT FROM AUTHOR]