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The use of Different Computer Simulation Software Packages to Predict Casting Filling and Solidification

S Oxley and P M Haigh, Castings Technology International

Abstract A series of filling and solidification simulations have been carried out using different simulation programs to evaluate the effectiveness of simulation for precision castings. A four casting geometry was designed to be a problematical system to create a very poor filling pattern and with a feeding method which will produce areas containing shrinkage defects. Results were determined using six different commercially available software programs: Thermophysical data sets for the aluminium LM25 alloy were used from data obtained through the National Physical Laboratory in London. These datasets have full traceability to measurement techniques and data calculation methods. The results are presented as snapshots at specific positions throughout the simulations. Castings were also produced and inspected to assist with the verification of the simulations carried out. The paper reviews the results obtained. Key words (5 maximum) Simulation, Solidification, Castings, Aluminium.

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Introduction A series of filling and solidification simulations have been carried out using different simulation programs to produce information to evaluate how simulation usage may be better adopted in the European precision casting industry. This activity was carried out within the European Thematic Network “SARE”. A four casting geometry was designed to be representative of a component that will provide information related to the effective use of the simulation programs. The component and production method was deliberately designed to be a problematical system using a gating system designed to create a very poor filling pattern and a feeding method which would produce areas containing shrinkage defects. [1] The geometry was supplied to all of the SARE project partners interested in conducting the simulation. The simulations run using the following software programs:

• MAGMAsoft - Cti • ProCAST – Inasmet, Calcom, VTT • Flow3D - HUT • PAM-CAST - CTIF • MAVIS – Swansea University • View Cast – WTCM

The list order above was not intended to represent any preference or ranking order of the simulations carried out. Each simulation result was assessed against a particular the stage of the simulation, e.g. bottom of the downsprue, the first metal entering cavity, the last part of the cavity to fill, etc. It was never the intention to directly compare each simulation programs results against each other, only to collect information to allow better use of simulation programs. Experimental The models to be used were supplied to the project partners as a casting and gating system component and a ceramic shell model. The property dataset for the LM25 alloy were supplied from data obtained through the National Physical Laboratory in London. The dataset has a full traceability to measurement techniques. There is currently little measured property data available for ceramic shell materials. The property data for the ceramic shell mould was therefore supplied from data developed and used by Cti. Cti also supplied the material boundary conditions as being representative of those that should be used to undertake a simulation of this nature. [2]

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The simulation results were obtained as snapshots at specific positions throughout the simulations. A screen image from each simulation program was takenat the most representative image nearest to the time of the event being monitored. Due to the simulations being carried out by several partners it was not possible to display images at exactly the same time in each case. The results are therefore the closest available fit between each simulation, as the examples shown in figures 1 and 2. The resuts of the filling simulation were evaluated and at the “bottom of the downsprue” position:

• All simulations filled the downsprue, and begin to fill the top runner bar.

• All simulations begin to top fill the casting cavity • There were significant differences in the amount of the pouring cup

that has been filled At the position of the first metal to enter the cavity:

• There were significant differences in the amount of the pouring cup that was filled

• There were significant differences between the simulations when comparing the amount of metal which entered the casting cavity related to the amount of metal in the lower runner bars.

• There was a general agreement between the simulations that top filling of the cavity was occurring

• Differing amounts of metal were predicted to have entered the casting cavity

At the last part of the casting cavity to fill

• There were significant differences in the amount of the pouring cup which was filled

• There were significant differences between the simulations when comparing the amount of metal which entered the casting cavity related to the amount of metal in the lower runner bars.

• There were significant differences in the amount of turbulent metal which was predicted during the filling cycle

• There were differences between the simulations related to the filling pattern

• There were disagreements between the simulation related to whether top filling, or top & bottom filling of the cavity was occurring

• Differing amounts of metal were predicted to have entered the casting cavity

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At the end of the filling cycle:

• There were significant differences in the amount of the pouring cup which was filled

• The predictions at fully filled position show differences in the actual temperatures predicted at the end of the filing cycle

• There was a general agreement of the overall temperature distribution within the mould cavity

The results obtained for the solidification simulation were evaluated in a similar manner. For the first part to solidify:

• There was a general, but not unanimous, agreement that solidification begins in the thin side walls of the casting

• Some results were not available for two of the simulation programs The isolated sections within the casting revealed:a general agreement between the simulation programs of the solidification pattern during the initial stages of solidification [3] The results for the prediction of shrinkage defects showed:

• There was a general agreement that the majority of shrinkage defects were predicted in the middle of the casting as shown in figure 3

• There were differences between the number of shrinkage defects predicted in the middle of the casting

• There were significant differences between the shrinkage defects predicted in the top section of the casting

• There were significant differences between the shrinkage defects predicted in the lower shaft guide boss area

In order to verify the results of the computer simulations, a set of verification castings were produced using a ceramic shell process to make test castings, figure 4. These castings were subjected to radiographic and metalllurgical examinations. The radiographic examination did not exhibit any severe gross shrinkage cavities, however microporisity defects were distributed throught a number of regions of the casting, figure 5. The metallurgical examination was carried out to further investigate the shrinkage defects predicted in the simulations carried out. Micro porosity defects were clearly seen in the sections taken through the casting, figure 6. It should be noted that the defects predicted in the simulation programs should be referenced as micro shrinkage, and not macro shrinkage

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There was a general agreement between the simulation programs relating to the distribution of shrinkage defects predicted across the central section of the casting.These were in agreement with the micro porosity found in the casting. There were also predictions of defects in the top and bottom sections of the casting, for which there is little common agreement between the simulations. These defects shown to be in the actual castings were present as distributed micro porosity only. Conclusion The results have show that whilst there is a general agreement between the overall predictive capabilities of the simulations there are a number of potentially significant differences between the simulations. The rate at which the pouring cup is filled differs in all of the simulations. This suggests that the program is treating the pressure buildup at the bottom of the downsprue in different ways. Some are allowing metal to pass through this ‘choke’ and others are using this are as a flow restrictor. The filling cycle behaves differently in many programs relating to the severity of the turbulence predicted during the filling cycle. [4,5] The test geometry was deliberately designed to produce a very turbulent fill, and has produced significant oxide defects in the test castings. Some of the programs predict a very turbulent fill, whilst others do not show significant metal stream breakup. It is doubtful that all of the simulations would have predicted the severity of the surface defects achieved. The last areas to fill in the mould cavity also differ across the simulations. The solidification cycle predicts differences in the areas that become isolated during solidification, which therefore influence the prediction of defects. Shrinkage in all the simulations was shown to be generally concentrated around the middle of the casting but the programs all differ in the severity of defects predicted in the area. The simulations also predicted of defects in the top and bottom sections of the casting for which there was little common agreement between the different software rprogrmmes.

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References 1 R.W. Wlodawer. (1966) Directional Solidification of Steel Castings. Pergammon press. 2 D.G.R. Sharma and M. Krishnan (1991) Simulation of Heat Transfer at the Casting Metal-Mould Interface. AFS Transactions. 3 P.N. Hansen and P.R. Sahm, (1998) How to Model the Feeding Behaviour in Casting to Predict Shrinkage and Porosity. Modelling of Casting and Welding Process IV. The Minerals and Materials Society. 4 B.D. Nichols, C.W. Hirt, R.S. Hotchkiss. (1980) SOLA-VOF: A Solution Algorithm for Transient Fluid Flow with Multiple Free Boundaries. Technical Report LA8355, Los Alamos Scientific laboratory. 5 J.Wang, S.F. Hansen, P.N. Hansen. (1988) 3D Modelling and Simulation of Mould Filling using PC Computers. Modelling of Casting and Welding Processes IV. The Minerals and Metals Society. s.oxley@castingstechnology.com; pm.haigh@castingstechnology.com

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Acknowledgements The computer simulation and casting manufacture were carried out within the frame of the EU Thematic Network SARE (GTC2-2000-33028) as part of the European Community funding project “ Competitive and Sustainable Growth”(1998-2002) The project was coordinated by INASMET and included a large number of partners across the EU.

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Figures

Figure 1 Filling simulation at the bottom of the downsprue

Figure 2 Filling simulation at the botom of the downspr

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Figure 3 Prediction of shrinkage defects

Figure 4 LM25 verification test castings

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Figure 5 X-ray image of test casting showing porosity defects

Figure 6 Metallurgical examination of LM25 test castings