U.S. Navy program advances casting technology: projectiles cast from austempered ductile iron cut fabrication costs more than 50%.

Traditionally, projectile shells are fabricated from forged steel but it has been determined that fabrication costs could be reduced more than 50% by switching to cast austempered ductile iron (ADI).

Before the change, however, several technical problems had to be solved. These included excessive radial expansion caused by a lower Young's modulus and a potentially lower yield strength; and the increased likelihood of premature fracture caused by flaws inherent in the casting process and the potentially lower fracture toughness of ductile iron. To address these issues, the U.S. Navy called on the National Center for Excellence in Metalworking Technology (NCEMT).

Sponsored by the U.S. Navy Manufacturing Technology (MANTECH) Program and operated by Metalworking Technology, Inc. (MTI), the NCEMT's mission was to develop the technical expertise needed to support Navy and Department of Defense requirements, then transfer that technology to U.S. industries.

In this particular case, the NCEMT was asked to determine the maximum tolerable flaw size for 16-in. cast projectile shells during gun launch. This information would be used to establish product specifications for projectile shell manufacturers.

The First Step

NCEMT engineers began by performing finite element analyses (FEA) and fracture analyses of both training and high-explosive projectiles. These analyses were used to determine the displacements, principal stresses, critical flaw sizes and minimum fracture toughness requirements for ADI projectile shells that would be able to withstand gun launch loads.

While this would satisfy the Navy's requirements, the NCEMT took the problem a step further by using its Rational Product and Process Design |Mathematical Expression Omitted~ methodology to obtain information needed to eliminate any process problems, rather than just avoid them.

Product & Process Design

|Mathematical Expression Omitted~ is a comprehensive approach to product and process specifications development that incorporates knowledge from several areas: previous design and manufacturing experience; part and process design and modeling data; material testing information, and process simulation and product analysis; and manufacturing considerations from an environmental point of view in an integrated database.

To model fluid flow, heat transfer, casting solidification and related processes, |Mathematical Expression Omitted~ uses an advanced Computational Fluid Dynamics (CFD) program. This program is based on the SOLA-VOF technique originally developed at Los Alamos National Laboratories and enhanced at the University of Pittsburgh, where it was geared toward modeling of the casting process. It has been modified further at MTI for specific needs. Based on the concepts of |Mathematical Expression Omitted~, an integrated software package |Mathematical Expression Omitted~ was developed that allows the foundry engineer to simulate and visualize the entire casting process.

In the case of the cast projectile, NCEMT engineers used the CFD module to experiment with alternate operating parameters, such as superheat of the molten metal and various ingate and riser designs and casting orientations. |Mathematical Expression Omitted~ visualization software was then used to examine and analyze the results.

For example, the |Mathematical Expression Omitted~ software allowed NCEMT engineers to rotate casting models 360 degrees, cut models open to examine porosity, change display colors to dramatize temperature or other variations, change the background color, magnify sections of a model, and open up as many windows as necessary to compare and contrast different views and, if necessary, different castings. The |Mathematical Expression Omitted~ visualization program even enabled engineers to change processing parameters, run the CFD program and watch the simulation being animated in a three-dimensional, fully rendered form.

Figure 2 shows the projectile being cast in the horizontal position with three risers on the top. The ingate was located at the base of the projectile in the longitudinal direction. This casting orientation was chosen, knowing that it would result in a defective casting, to illustrate the capability of accurately predicting defects. The yellow regions represent isolated pools of liquid metal that formed hot spots after solidification.

Figure 3 illustrates the same casting, cast in a vertical position. The nose of the projectile is situated toward the bottom and the liquid metal is poured from the bottom up. Significant improvements were seen over the previous orientation. Nevertheless, two hot spots were formed just below the riser (dark yellow regions). Hot spots formed from the sudden change in the cross-sectional area of the projectile just below the riser. This forms a constriction and the liquid metal in this region solidifies before the liquid below it, thus choking the riser off from feeding the solidifying liquid below.

Figure 4 displays the same projectile being cast in the vertical direction, with the nose of the projectile up. There is no sudden decrease in the cross-sectional area near the riser. This orientation offers good process parameters for defect-free projectiles.

Figure 5 shows an alternate casting procedure with the nose of the projectile down. The main difference between this and the orientation shown in Fig. 3 is the placement of the ingate and the extra padding placed at the top of the mold (near the base of the projectile). This padding would have to be machined out after removing the solidified casting. The benefit of the padding is that there is no sudden decrease in cross-sectional area to prevent the risers from feeding the hot spots.

To meet the Navy's requirements, NCEMT engineers combined the results of their design analysis and casting simulations and specified minimum fracture toughness and nondestructive evaluation criteria for flaw size. In addition, the NCEMT developed process specifications that delivered high-performance cast projectiles.

After an initial evaluation, the Navy found that the |Mathematical Expression Omitted~ process successfully developed casting parameters that resulted in 16-in. projectile shells meeting the Navy's cost and performance objectives. As a result, |Mathematical Expression Omitted~ methodology is being applied to the design and production of other Navy components. Yet, that is only the beginning.

Commercial Applications

The same process that was applied to the Navy's cast projectiles can also be used to help improve the manufacturing efficiency and product performance of virtually any type of discrete cast object.

For example, the NCEMT already has helped companies develop the process specifications needed to produce several components, including a large marine casting, an exhaust manifold and several automobile pistons. These castings weighed anywhere from a few pounds to more than five tons. In addition, the process has been used on aluminum silicon alloys, ductile iron, nickel aluminum bronze alloys, and various steels and X-D composites. Work also is under way to simulate the expendable pattern casting process.

Quality Castings in Less Time

Using advanced casting process modeling analyses and simulation and |Mathematical Expression Omitted~ technology, the NCEMT has been able to develop an alternative to the "pour and pray" method of casting development. This alternative is estimated to help the Navy save $4.7 million annually. At the same time, it can be used commercially to eliminate the cost and time delays associated with trial and error procedures and prototyping--especially in complex-shaped castings that may require considerable resource to optimize.