Computer solidification modeling speeds design-to-production cycle.

Pouring molten metal into a mold and allowing it to freeze and solidify might seem to be a simple exercise, and the design of a mold into which to pour the metal may at first glance seem trivial. After all, what more is needed than a negative image of the part to be produced? These points of view would be valid were it not for the tendency of: * metal to contract and densify upon solidifying; * alloying elements to come out of solution in various undesirable ways when metal cools and freezes slowly; * molten metal to entrap air and form oxides when sloshing into the mold.

These problems have plagued foundrymen since castings were first poured in the Bronze Age, and they still continue today. The design of gating systems to allow metal to flow into the mold cavity at a controlled rate, and of risering systems to properly feed molten metal as the casting freezes and contracts, remains a central issue in foundry engineering.

Fortunately, our ability to predict the consequences of natural law has increased dramatically. While most gating and risering are still done by rule of thumb and trial-and-error, many quantitative methods have been developed that allow engineering principles to be applied to the founder's art.

Bernoulli's equations for flow along streamlines for example give rise to elegant and effective gating systems. Chvorinov's principle relating freezing time to section modulus(volume divided by surface area) allows an estimation of directional solidification for riser sizing and placement.

While such approaches have been helpful, a newer tool, the computer, has emerged to assist the foundry engineer in ways previously unimagined. Using today's powerful computers and specially designed software, one can open a "window" into any imaginable casting shape and watch simulations of what happens as the casting cools and solidifies. The formation of sound casting or defect-prone areas can be watched on the computer screen before any actual tooling is built or castings poured. Riser and part geometry or mold thermal characteristics can be varied and optimum designs reached early in the life cycle of the part. This can have dramatic effects on quality, cost and delivery time of new parts.

The process of predicting casting freezing patterns on a computer is referred to as solidification modeling. The basis of most of these systems is a geometric representation of a casting and a mold in the computer memory. This computer model is broken down into many small elements, or nodes, to which linear approximations of differential equations describing energy balances can be applied. Through thousands or millions of calculations performed by the computer, the response of each node can be predicted and the behavior of the casting as a whole emerges on the computer screen.

We have developed, and continue to refine, a system for modeling castings up to medium complexity. The development of this system was in response to a search for a simple, effective way to model castings using a personal computer. The absence of such software led to the decision to write our own. As a medium-volume aluminum foundry in the jobbing market, we have been in the somewhat unique position of being able to refine our system as befitted our needs.

Through an arrangement with the American Foundrymen's Society, this system is available to the foundry industry under the name of AFSolid. Feedback from users at other foundries has been invaluable in the application and continued development of the software. Beginning the Analysis

Using AFSolid, the analysis of a casting begins with a drawing of the part, the riser and gating systems, and the mold. This can be done with a special routine that uses a digitizing tablet to trace a part blueprint, or, alternatively, any CAD representation imported into the system using a DXF neutral file format.

The casting and mold material properties must be specified (density, specific heat, thermal conductivity and initial temperature). For the casting material, latent heat of fusion and the freezing range also must be specified. A castingmold system with materials assigned is shown in Fig 1.

The system allows heat transfer coefficients to be applied to individual surfaces such as the interface between metal and mold. This is particularly useful in permanent mold simulations where mold coatings are used to insulate specific surfaces and control heat flow.

AFSolid is essentially a two-dimensional system that considers heat flow picture of the actual behavior of the casting than is possible with heat transfer equations alone. An example is given in Fig 5. Predicting Shrinkage Porosity The objective of all of this analysis is to predict the presence or absence of areas of potential shrinkage porosity in a casting. If such areas are indicated, the user can redefine any of the inputs (casting or riser geometry, material temperatures or properties,surface heat transfer coefficients, etc.) and run the model again to see if the change has been beneficial. This simulates the trial-and-error technique that has been used for years on the foundry floor, but does not involve physical changes to molding equipment. It can be done much more quickly and cheaply than the traditional method.

There are many benefits in using these methods in foundry engineering, the most obvious being its application to new part design and development. The time from concept to production can be shortened and the likelihood of good parts early in the production cycle increased. Troubleshooting on existing parts is made easier because potential changes can be evaluated before they are implemented.

Use of a modeling system can result in better communication with customers when design changes to enhance castability can be demonstrated with an animated display rather than handwaving. These systems can also be used to train and educate foundry personnel on the principles of heat transfer and defect formation in the casting process; being able to actually see what is going on inside the casting can greatly increase one's awareness and understanding of the process.

The systems in use today obviously have a long way to go before they can predict all of the various phenomena which can occur in the multitude of al loys and processes which are in use in the foundry today. Even within their limitations, however, they can offer a wealth of information to the foundry engineer who is willing to try these new tools. Those who become familiar with this technology today will have the advantage tomorrow as the systems inevitably increase in power and sophistication.

The systems of the next century can take us farther in a decade than we have come since the Bronze Age.