Itsuo Ohnaka

In situ observation of solidification phenomena in Al–Cu and Fe–Si–Al alloys

Abstract Synchrotron radiation enables the observation of solidification in metallic alloys. In situ observations of solidification for Al–Cu alloys (5, 10 and 15 wt-%Cu) are reported. Nucleation and fragmentation of dendrite arms were often observed in the 15 and 10%Cu alloys when unidirectional solidification was performed from the planar interface. In contrast, nucleation and fragmentation were rarely observed in the 5%Cu alloys. The nucleation ahead of the solidifying front and the fragmentation in the mushy region strongly depended on alloy composition. This paper also presents in situ observation of solidification of Fe–10Si–0·5Al (at-%) alloys. The dendritic growth of δ-Fe was clearly observed using this technique. The development of X-ray imaging techniques enables the solidification of various conventional cast alloys such as Al, Ni and Fe alloys to be observed and will be increasingly used to investigate solidification phenomena.

In situ observation of nucleation, fragmentation and microstructure evolution in Sn–Bi and Al–Cu alloys

Abstract This paper presents recent progress of in situ observation for the microstructure evolution during solidification. Nucleation and fragmentation of dendrite arms are important issues for controlling microstructure during solidification. However, there are few studies on in situ observation of nucleation and fragmentation in metallic alloys. Time resolved X-ray imaging technique has been developed to observe solidification of metallic alloy systems in situ. Fragmentation of dendrite arms often occurred at the root after growth velocity was reduced for the Sn–13 at.-%Bi alloys and the Al–15 mass%Cu alloys. In the Al–15 mass%Cu alloys, both of nucleation and fragmentation contribute to formation of grain structure. The result suggested that fragmentation should be considered for controlling grain structure.

Mechanism and estimation of porosity defects in ductile cast iron

Abstract Although there are many works on the formation mechanism of porosity defects due to solidification in ductile cast iron, the formation mechanism is still not clear and decreasing the porosity defects is still a main issue in the industry. This paper critically reviews conventional explanations for the porosity formation including estimation methods. Based on the discussion the authors propose a formation theory where it considers gas and oxide entrapment during mould filling, expansion of outer part of casting due to graphite formation and pressure decrease in the inner part, followed by the growth of entrapped small gas bubbles. This mechanism can explain various facts in practice and be usable to estimate the defects. It also gives a good way to design effective risers. Future challenges are also discussed including the effect of inoculation on the fluidity of the mushy region.

Mold filling simulation with consideration of gas escape through sand mold

Although there are some simulation codes that can consider the backpressure in mold cavity, they cannot handle the sand casting and are applied only to metallic mold casting such as die-casting. In the case of sand casting, however, it is important to consider the gas generation on the mold surface and escape through the mold during mold filling. This paper presents a method to simulate the mold filling in sand molds with consideration of backpressure and the gas escape. The governing discrete equations for the momentum and mass conservation laws have been derived by the DFDM (Direct-Finite-Difference-Method). The momentum conservation equation is considered in the staggered elements. It is assumed that the gas in the cavity is an ideal one, and the gas-flow in the mold follows the DArcy law. Simulated mold filling patterns agreed rather well with the results directly observed by an X-ray apparatus.

Numerical simulation and X-ray direct observation of mould filling during vacuum suction casting

Abstract The mould filling of the aluminium casting alloy AC4C was directly observed by X-ray radiography in order to evaluate the accuracy of simulated results for the mould filling behaviour of a vacuum suction casting process. A numerical method has been applied to simulate mould filling behaviour, in which evacuation of gas through the suction vent is taken into account and the backpressure of gas is coupled with the pressure field of the melt. Backpressure of gas in the empty mould cavity was also measured using a semiconductor pressure sensor. Experiments have been carried out at two different suction pressures (10 and 20 kPa) and three different decompression rates (1·2, 4·2 and 80 kPa s-1), controlled by the aperture of the valve between the gas pipe of the vacuum suction system. Experimental results showed that the effects of suction pressure and decompression rate on the mould filling behaviour were significant. The higher the suction pressure and decompression rate, the more turbulent the free surface of molten metal, resulting in earlier occurrence of gas entrapment, larger volumes of entrapped gas and higher risk of misrun. By comparing simulated results with experimental ones, it was found that good agreement could be obtained in terms of mould filling patterns, positions of gas entrapments and mould filling time. It was also found that results simulated by the conventional method, in which gas evacuation and the effects of backpressure on mould filling are not taken into account, showed poor agreement with experimental values.

A regular-irregular mixed meshing method for solidification simulation of castings

A Regular-Irregular Mixed meshing method has been developed for the solidification simulation of castings. In this method, the mold and cavity are enmeshed with regular and irregular elements, and the irregular elements are only used for curved or inclined casting/mold boundary, resulting in better shape modeling than the regular meshing method. Furthermore the cost of calculation and storage in the mixed meshing method is low as compared with those in the irregular meshing method because the regular elements are used inside of the boundary. The discrete governing equations for the regular and irregular elements have been derived with the Direct Finite Difference Method (DFDM). Where, the implicit difference format was adopt6ed to avoid the small time step problem of irregular elements, and the implicit matrix equation was solved by using Incomplete Cholesky Conjugate Gradient (ICCG) method. Simulation results of temperature have been compared with those of other methods using iso-parametric elements, showing that the proposed method is a promising one.

Challenging issues in computer simulation of casting

Abstract Although computer simulation has progressed rapidly and widely in the casting industry over the last decades, many problems still remain challenging. This paper overviews the problems being faced most frequently, including estimation of material properties and boundary conditions, simulation of mould filling and solidification, prediction of porosities and optimisation of casting. The authors emphasise, in particular, the importance of estimating the material properties of sand moulds and boundary conditions with consideration of the oxide film on the molten metal, and the comparison of simulation results with observations of real casting processes.

Computer Simulation of Lost Foam Process of Cast Iron

Although several computer simulation codes have been developed to predict and prevent defects of lost foam castings, most of them are focused on aluminum alloy castings. This paper presents a new simulation method suitable for not only aluminum alloy but also cast iron. First of all, main assumptions and numerical schemes are as below: 1) Gas flow in the mold follows the D’Arcy’s law. 2) The gas pressure in the kinetic zone (gas gap) is uniform in each gas group and is solved by using the flow field of melt one time-step before. 3) The gas obeys the ideal gas law. 4) Latent heats of evaporative pattern degradation are simplified by an overall latent heat in which both melting and evaporation are considered. 5) The pattern absorbs heat of melt through conduction, radiation and convection. Further, X-ray direct observations of mold filling behaviors of cast iron were carried out in order to better understand the physical fundamentals and to evaluate simulated results. It was found that: 1) Mold filling behaviors of cast iron are quite different from those of aluminum alloy castings depending on the gating system and thickness of the casting. 2) Bubble floating-up often occurred in the case of the downward flow. 3) Although the newly developed simulation code could predict the mold filling to some extent, further work needs to be done, including better understanding of the heat transfer mechanism between the melt and pattern.

Modeling of mold filling in gating systems with filters

A numerical method has been developed to simulate mold filling in gating systems with filters. The discretization method of flow and temperature fields is based on the DFDM (Direct-Finite-Difference-Method), which directly derives the discrete governing equations from physical phenomena. For the simulation of fluid flow through filters, in addition to the momentum convection, pressure and gravity terms, the Darcy force term is incorporated into the momentum-conservation equation to account for the filter flow resistance. Furthermore, for the simulation of heat transfer between the melt and the filter, both the melt- and the filter- temperatures were solved by applying the energy conservation law to each part of a mass region (element) individually. Simulated results showed the possibilities of the decrease of the melt front turbulence and less entrapments of gas in the downstream runner and cavity. Simulated results also showed that the permeability and the thickness of the filter significantly influence both the mold filling behavior and the temperature distribution. Simulated mold filling patterns were validated against the experimental ones observed directly by X-ray radiography, and good agreements were obtained.