ABSTRACTThe main objective of this paper is to demonstrate how flowand solidification simulation were used in the development ofa new gating system design for three different magnesiumalloys; and to determine the relative castability of each alloybased on casting trials. Prototype tooling for an existing 3-slide rear wheel drive automatic transmission case designedfor aluminum A380 was provided by General Motors. Flowand solidification simulation were performed usingMagmasoft on the existing runner system design using A380(baseline), AE44, MRI153M and MRI230D. Based on thefilling results, new designs were developed at Meridian forthe magnesium alloys. Subsequent modeling was performedto verify the new design and the changes were incorporatedinto the prototype tool. Casting trials were conducted with thethree magnesium alloys and the relative castability wasevaluated. The conclusion of the study was that all threealloys were castable; however, there were significantdifferences between the alloys with respect to surface andinternal imperfections based on the runner design used in thetrials. In addition, it was found that different gatingtechniques are needed when casting the MRI alloys.

INTRODUCTIONOver the past 10 years, Meridian has conducted extensivecasting trials to develop our knowledge on the castability ofvarious high temperature low creep alloys for powertrainapplications1. This work has enabled Meridian to rank thesealloys on a relative scale and allowed for the proper

characterization, selection and use of these alloys for newapplications.

The purpose of this study was three-fold; 1) to design amagnesium gating system for a 3-slide automatictransmission die-cast tool which was originally designed forA380 aluminum alloy, 2) evaluate three high temperature lowcreep alloys, namely; AE44, MRI230D and MRI153M, interms of overall castability and 3) to correlate and assess thecapability of the current technology to predict castinganomalies. The project description is shown schematically inFigure 1.

Effect of Different Magnesium Powertrain Alloys onthe High Pressure Die Casting Characteristics of anAutomatic Transmission Case

2010-01-0409Published

04/12/2010

John Jekl and Richard D. BerkmortelMeridian Lightweight Technologies Inc.

Paula ArmstrongGeneral Motors LLC

Copyright © 2010 SAE International

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Figure 1. Project description from beginning to end.

GATING DESIGN FOR MAGNESIUMBENCHMARKING THE ORIGINALDESIGNFlow simulation was utilized to benchmark the filling andsolidification of aluminum and magnesium using the originalrunner design as-received. For all simulations, the overflowand venting configuration remained unchanged. The resultsof the initial studies are shown in Figure 2. The aluminumfilling pattern shows a relatively even filling profile anduniform metal temperature; however, the magnesiumsimulation shows a more turbulent filling pattern with areasof low metal temperature relatively early in the sequence.These simulations confirmed a re-design of the runner systemwould be ideal to create a more uniform filling scenario.

<figure 2 here>

GATE AND RUNNER ANALYSISThe new magnesium runner design was progressed usingstandard analysis techniques for AE/AM/AZ alloys. Theiterations of the design are shown in Figure 3 which increasein gate area from the original design on the left (5.6 cm2) tothe middle (8.5 cm2) and the final design (12.0 cm2) on theright. The final design yielded an average gate velocity of48.7 m/s with a cavity fill time of 75ms which was within anacceptable range for conventional magnesium HPDC alloys.

Figure 3. Progression of the magnesium runner design(original aluminum design on left, alternative

magnesium design in middle, final magnesium design onthe right).

Once the runner design was completed and the toolingupdated, the casting trial was conducted in the followingorder; MRI153M, MRI230D and AE44. By the end of theMRI153M trial, the gating had been modified significantly tominimize the soldering and sticking issues at the gate, whichwas not predicted in the flow simulation. The changes to thegating during the trial are shown below in Figure 4 andsummarized in Table 1. They essentially involved increasingthe length and thickness of each feed and correspondingrunner branches.

<figure 4 here>

Table 1. Details of gate / runner modifications during thecasting trials.

FILLING SIMULATION ANALYSISTo understand the effect of the changes made during thetrials, the final geometry was modeled and several iterationsof flow and solidification were completed for each of thealloys. An example of the simulation parameters used forAE44 is shown in Table 2.

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Table 2. Example of simulation parameters for AE44.

The results of metal temperature during filling are shown forthe aluminum, both MRI alloys and AE44 in the next twofigures.

<figure 5 here>

<figure 6 here>

When the overall filling pattern is compared, they appear tobe very similar for all four scenarios. However, it is apparentthat AE44 shows potential for colder metal near the oil panflange and output shaft bore. This would result in increasedpotential for surface discontinuities such as cold flows/laps.The MRI alloys exhibit minimal metal temperature lossduring filling.

SOLIDIFICATION SIMULATIONANALYSISSolidification time results are shown in Figures 7 and 8 foreach of the 4 alloys. It is evident that the aluminum has thehighest solidification time of approximately 20s, followed bythe MRI alloys at 5-7s with the AE44 alloys solidifying thequickest at 2-3s after the cavity has been filled. Since these 3magnesium alloys all have approximately the same latentheat, the low solidification time for AE44 is due to the shortfreezing range of the alloy (590 to 625°C) versus theMRI153M (506-601°C) and MRI230D (522-603°C).

<figure 7 here>

<figure 8 here>

ANALYSIS AND CORRELATIONSURFACE ANALYSISTwenty castings from each group were visually examined todetermine surface quality. The surface imperfections wereclassified into six categories; cracks, sinks, knit lines, lack offill, soldering and blisters. The results of this analysis areshown below in Figure 9 and indicate that AE44 has the leastamount of surface irregularities, followed by MRI153M andMRI230D. Leak testing was also conducted to evaluate thecasting quality. The relative ranking of leak test results forboth as-cast and machined conditions for all three alloys areconsistent with this relative ranking of the surface analysis.

Figure 9. Surface analysis for each alloy group.

AE44 was chosen for a simplified correlation study betweensurface imperfections and flow simulation. Each side (view)of the casting was analyzed and numbered Views 1-6. Foreach view, a grid (not shown) was overlaid to document thelocation of the discontinuity. Table 3 shows the results withall observed imperfection types listed along with selectedflow simulation parameters. For each type/location on thecasting, the parameters were reviewed and a “1” wasrecorded in the table to indicate if that condition existed. Forexample, in View 6, lack of fill at location L10, there was airentrapment and low metal temperature, but no flow frontmixing or hot spot. This is illustrated in Figures 10 and 11.

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Figure 10. View 6, lack of fill at location L10.

<figure 11 here>

<table 3 here>

Based on Table 3, it appears that air entrapment is the mostsignificant parameter related to knit lines. Hot spots arecritical for soldering, while lack of fill is dependant on airentrapment, flow front mixing and low metal temperature.Cracks are considered inconclusive due to only one locationwhere cracks occurred.

INTERNAL ANALYSISTen castings from each alloy were selected and X-rayed toanalyze internal quality in terms of number (density) and sizeof the imperfection. Based on flow simulation analysis(FSA), each casting was analyzed from 5 different views. X-ray results shown in Figure 12 indicate that, generally, AE44castings show smaller imperfection size but higher numbers(density) than MRI alloy castings; whereas MRI153Mcastings show better casting soundness than MRI230Dcastings.

Figure 12. Number and size of shrinkage.

Table 4 shows the correlation between flow simulationanalysis and actual X-ray analysis. Views 1, 3, 4 and 5 werepredicted by flow simulation to have shrinkage porositybased on hot spot analysis, while View 2 was predicted tohave high casting quality. An example of View 2 and View 5are shown below for reference in Figures 13 and 14,respectively.

Table 4. Correlation summary of flow simulation and X-ray.

<figure 13 here>

A reasonable correlation can be seen between predicted “hotspots” and degree of casting shrinkage, indicating thatsimulation is a potential tool to predict internal castingshrinkage during the tool design stage.

<figure 14 here>

SUMMARY / CONCLUSIONSThe effect of alloy type on the casting characteristics of threehigh temperature low creep alloys was studied. It wasdetermined that the MRI alloys require significantly less gatevelocity to avoid excessive overheating/sticking at the gates.The sticking was controlled during the run by adjusting themetal flow rate and cooling (internal / external) in the gate

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area. Within the scope of this project (utilizing an existing Alprototype tooling), we were not able to optimize the toolingfor maximum process control. Therefore, reducing gatevelocity was primarily achieved by increasing the gate area.

As a result, the runner system for all the alloys was notoptimized; however, for this casting study, acceptableprototype parts were still produced with this configuration.Despite the relative differences in these alloys, they can all bedeveloped to produce good quality castings, but will vary intheir process window operating tolerances. AE44 has thelargest process window, followed by MRI153M andMRI230D. It was not determined, within the scope of thistrial, if the observed hot tearing characteristics of theMRI230D castings could be brought to an acceptableproduction level for a large structural casting.

The predictive capability of casting simulation was studied.Flow simulation was capable of predicting internaldiscontinuities but could not predict soldering. It wassomewhat successful in predicting surface imperfections;however, predicting cracks was inconclusive.

REFERENCES1. “Die Castability Assessment of Magnesium Alloys forHigh Temperature Applications”, part of USCAR-MPCCFinal Report, 2002.

ACKNOWLEDGMENTSThe author would like to thank Gerry Wang, Jeremy Bos andthe staff at Meridian Magnesium Products of America fortheir hard work and dedication to completing this project. Inaddition, thank you to Paula Armstrong and General Motorsfor the opportunity to collaborate on this interesting project.

DEFINITIONS/ABBREVIATIONSUSCAR-MPCC

United States Consortium for Automotive Research -Magnesium Powertrain Cast Components.

HPDCHigh Pressure Die Casting

FSAFlow simulation analysis

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Figure 2. Metal temperature at 46% cavity filled; Aluminum (Left) and Magnesium (Right).

Figure 4. Comparison of the magnesium designed runner and the final runner geometry at the end of the MRI153M run.

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Figure 5. Metal temperature for Aluminum (left), AE44 (right), 86% filled.

Figure 6. Metal temperature for MRI230D (left), MRI153M (right), 88% filled.

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Figure 7. Solidification time for Aluminum (left), AE44 (right).

Figure 8. Solidification time for MRI 153M (left), MRI230D (right)

Figure 11. Flow simulation results for metal temperature and air entrapment.

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Figure 13. Flow simulation and X-ray results for View 2, high casting integrity predicted location.

Figure 14. Flow simulation and X-ray results for View 5, hot spot predicted location.

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Table 3. Correlation summary of flow simulation and surface imperfections.

The Engineering Meetings Board has approved this paper for publication. It hassuccessfully completed SAE's peer review process under the supervision of the sessionorganizer. This process requires a minimum of three (3) reviews by industry experts.

All rights reserved. No part of this publication may be reproduced, stored in aretrieval system, or transmitted, in any form or by any means, electronic, mechanical,photocopying, recording, or otherwise, without the prior written permission of SAE.

ISSN 0148-7191

doi:10.4271/2010-01-0409

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