Simulation bridges metal gap: working from its knowledge of aluminum, General Aluminum Manufacturing Co. utilized simulation software to produce a magnesium automotive casting.

Casting magnesium automotive components in the low pressure permanent mold (LPPM) process offers unique technical challenges due to the low volumetric heat content and the high hot tear tendency of magnesium alloys. Using its experience producing safety-critical aluminum automotive components with the LPPM process, General Aluminum Manufacturing Co., Madison Heights, Mich., developed filling and solidification simulation models for a magnesium control arm to assist the tooling design and casting process. Simulation analysis results obtained with the company's software assisted with the following aspects of the casting process:

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1. Maintaining a stable mold temperature for a steady production process.

2. Achieving and maintaining a warm mold temperature to avoid non-fills and reduce hot tear risks.

3. Modifying part design to facilitate feeding and remove solidification shrinkage.

4. Optimizing mold design for hot tear risk control and better part ejection Magnesium casting content in automobiles has increased annually at a rate of 16% in the last decade and is predicted to continue growing at an annual rate of 11.5% for the next decade. LPPM has proven to be a competitive process for producing high volume, high quality aluminum automotive castings, and the technical and economic guidelines for aluminum castings are also applicable to magnesium component castings. However, many technical challenges still remain before a stable LPPM process can produce high volume safety-critical magnesium components, including efforts to address some of these challenges based on filling and solidification analyses.

The Part

The front lower control arm originally was designed by DaimlerChrysler as a steel stamping. The magnesium casting geometry was optimized with about 30% weight reduction from the original design (Fig. 1). The control arm geometry was converted first into surface mesh with MSC-Patran and then into solid mesh. A conceptual mold was created for the initial filling and solidification analyses and gradually improved to prepare it for tool making. Alloy AM60 was used in the model, as well as the following casting process data:

* 1,300F (704.4C) initial metal temperature;

* 700F (371.1C) initial mold temperature;

* 180-second cycle time.

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Only air cooling was used to cool the spreader located directly above the spree.

In Fig. 2, filling patterns of the control arm are shown, where molten metal is represented by the colored range and the unfilled cavity is in black. Solidification patterns are shown in Fig. 3, where the solidified part area is in black. Smooth filling was obtained with the help of pressure control of the low pressure machine and a warm mold. Shrinkage risk did not exist, as indicated by the absence of the hot spots in the solidification patterns, a result of the casting process design and part modifications for improved feeding.

Magnesium has a lower volumetric heat content than aluminum. Compared with LPPM casting of aluminum, magnesium casting requires additional measures to prevent process interruptions caused by plugged sprues or overcooled molds. Without these additional measures, mold temperature can drop quickly (Fig. 4).

The results in Fig. 4 were obtained with a block mold ideal for aluminum casting. A mold cooling rate such as this must be avoided when casting magnesium to maintain stable production and prevent non-fills and other casting defects. In the process of mold design, measures are needed to reduce heat loss from the mold to achieve a stable mold temperature during casting.

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Several mold designs were evaluated using simulation software. The first design (Case 1) included a thin layer of insulation applied to the back of both mold halves. The second design (Case 2) reduced the mold mass by creating pockets in the mold and filling them with insulation material. The last design (Case 3) added insulation blankets to the sides of the mold of the second design.

Mold temperature variations for the three designs are shown in Figs. 5-7. The temperature variations were measured from the same mold locations for each of the designs. In all three cases, the mold variation starts with a preheated temperature of 700F (371.1C). The simulation results indicate that a steady and warm mold temperature, which helps reduce hot tear risks, was only achievable in Case 3.

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Optimizing the Mold

For a typical aluminum casting, the initial mold designs are based on automotive functional requirements and usually need further modification for improved castability. The same procedure was followed to produce the magnesium casting mold design.

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Magnesium alloys are more prone to hot-tears than aluminum alloys, and the AM series of magnesium alloys, which were used in the production of the control arm, are more hot-tear prone than the AZ series. Among the factors that contribute to hot tears are mold temperatures and constraints in areas with sharp corners, both of which were addressed in the mold design process. To determine an appropriate mold temperature, previous research results were consulted. Data from that research indicated that to prevent hot tear occurrences, minimum mold temperatures at around 752 F (400C) are preferred, and alloys that are more hot-tear prone require a higher mold temperature. For the current work, mold temperatures at likely hot tear locations were targeted at above 752F.

The initial mold design produced several hot spots in the solidification pattern (Fig. 8). To remove these potential shrinkage risks, the design was modified at critical feeding locations. Pull cores also were added to the tooling to address mold constraints that might cause hot tears in the locations indicated by the simulation. The cores made it possible to remove the constraints of the mold by pulling them at appropriate times during the solidification process.

"Magnesium Casting Process Development: Designing an Engine Cradle for Magnesium Semi-Permanent Mold Casting," 2005 AFS Transactions, 15-217.

Yuanyi Sheng, Sarah Chert and Jagan Nath, General Aluminum Manufacturing Co., Madison Heights, Michigan 

Yuanyi Sheng is senior engineer, Sarah Chen is program manager and Jagan Nath is vice president of advanced engineering for General Aluminum Manufacturing Co., Madison Heights, Mich.