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Investment Castings Pioneer New Applications

June 4, 2004
Titanium components are critical aspects of an updated version of an important artillery system, proving the advantages that investment castings bring to the design process.

Recent developments have expanded the application for investment castings, as the process continues to demonstrate its capabilities in a variety of new part categories and industry segments. An emerging example of the process’s success is the BAE Systems 155-mm Lightweight Towed Howitzer artillery system. This is the first example of titanium investment castings having been selected for use in a land-based armament system of this type.

Changes are in the air, too. This past spring’s publication of the new edition of the Metallic Materials Properties Development + Standardization Handbook — formerly known as Mil-Handbook-5, which establishes a database for titanium Ti-6Al-4V alloy (the alloy for castings used in the new BAE Systems Howitzer) — makes investment castings “design allowable” in fracture-critical airframe applications.

These developments illustrate the attention the process is receiving from system designers who are interested in reducing part count, shedding weight, trimming cycle time, and cutting production costs. All these objectives become more achievable using investment castings.

At the same time, designers are finding that this process can enhance the performance of end-products because it facilitates manufacturing of innovative designs that aggregate multiple, intricate, and asymmetrical features in a single, large, complex component. BAE Systems’ Lightweight Towed Howitzer is an example of these problem-solving capabilities.

BAE Systems has begun “low-rate-initial-production” of the new 155-mm Lightweight Towed Howitzer. This artillery system, scheduled to be field deployed by the U.S. Department of Defense in about one year, has been designated the M777. The artillery pieces BAE Systems is supplying to DOD will provide an array of measurable improvements compared to its predecessor, the M198 Howitzer, which will soon be phased out of service.

Measurable improvements

The improvements of the M777 include: a 42% reduction in the weight of the basic system; a 25% reduction in size; retention of the 30-km firing range using NATO-standard ammunition; and greatly increased performance capabilities in a number of crucial combat-support areas. These include a 25% increase in the ability to destroy enemy vehicles in close-support combat, a 70% improvement in survivability and a 500% increase in counter-fire exchange ratios. The lightweight and more compact configuration of the new Howitzer make it extremely nimble. It can be “emplaced” in three minutes and displaced in two minutes, enabling it to conduct firing missions from four different locations within an hour.

When evaluated by these and other measures, it becomes clear that the M777 has rewritten the effectiveness criteria of a 155-mm Howitzer in such critical categories as transportability, weight and lethality.

The capabilities of the M777 that are redefining performance standards for Howitzers were made possible by a revolutionary design that recruited a material form unprecedented in land-based artillery—titanium Ti-6Al-4V alloy investment castings. By this selection, BAE Systems was able to reach DOD’s weight target, which demanded a new design that supports the military transformation effort. Among many systems under evaluation across all the service branches, the DOD especially wanted a new artillery piece that could be very rapidly deployed in response to threats anywhere on the globe.

TheM777 system designers faced numerous hurdles that made it difficult to meet the weight, transportability, and on-ground mobility requirements set forth for the new system. The M198 (itself just a few decades old) had achieved major improvements over the Howitzer it replaced. Making another round of even more aggressive improvements would strain the limits of existing production technologies for land-based defense systems.

As system designers tried options for a new system, they began to imagine a Howitzer with a significantly smaller “footprint.” Their evaluation of available options led them to conclude that the targets for lightness and compactness could only be achieved by design changes that incorporated new, innovative dual-function structures and pressure vessels. The new Howitzer design resulted in an unprecedented configuration that placed the center of gravity (CG) “out of balance.” Officially, the M777 Howitzer’s design is described as “static-out-of-balance.”

In the new design approach, the CG of the recoiling mass is located well forward of the Howitzer’s trunnions when the gun is at rest. This design approach allowed engineers to position the trunnions close to the ground, and locate the weapon’s overall CG as low as possible. With the trunnions placed just above the ground, the low CG counteracts the right-hand torque that develops when the gun is fired. With this design innovation, the recoiling mass remains within the Howitzer’s structure when firing and keeps the lightweight system highly stable in operation.

Design, then supply

The design held high promise for BAE Systems, but suppliers needed to be selected to manufacture its revolutionary components. Specifically, BAE needed to select a material form for the major structural components and a production process that was reliable, repeatable, and affordable. Bearing the system’s requirements in mind, candidate materials were limited. With so many objectives linked to the transportability of the new Howitzer, particularly by aircraft, it is no surprise the designers turned to an aerospace material — titanium Ti-6Al-4V alloy, a long-established choice among military-aircraft designers for its lightweight and high-strength properties.

Gaining acceptance

As noted, this material recently passed a major design-acceptance hurdle. A consortium of representatives from U.S. government agencies, original equipment manufacturers, and casting suppliers, recently published a static database for cast, HIP’ed, and mill annealed Ti-6Al-4V.

This acceptance represents a milestone in the application of investment castings for commercial and military airframes, because the data give aerospace design engineers the mechanical properties database that makes Ti-6Al-4V investment castings “design allowable.” This removes a barrier for converting fabrications to single-piece investment castings in fracture-critical aerospace applications. The database also documents the high degree of comparability in mechanical properties between investment-cast and wrought Ti-6Al-4V components.

This documentation supplies the room temperature tensile, compression, shear, and bearing information that meet requirements for a static database. In addition to its contribution to the static data, Alcoa Howmet Castings, a supplier of IC components to BAE Systems for the M777, independently developed dynamic data for strain-controlled, low-cycle fatigue, fatigue-crack growth, and fracture toughness. The company expects this data to be incorporated in a future edition of the MPPD+S Handbook.

Already proven in the air, the titanium Ti-6Al-4V alloy is now proving itself on the ground. As noted , BAE Systems’ M777 155-mm Lightweight Howitzer was the first application of investment-cast titanium in a land-based armament system. The M777 currently employs more than two dozen investment-cast, Ti-6Al-4V structural components. Alcoa Howmet Castings, operating from its facilities in Whitehall, MI, and Hampton, VA, supplies 18 of the 29 titanium structural castings to BAE Systems for the M777.

From the outset, BAE Systems’ plan to use titanium structural components had its skeptics. However, the titanium investment castings demonstrated their ability to meet demands for weight reduction and overall ruggedness during a series of tests in the developmental phase — as well as in a series of subsequent operational assessments.

The components measured up to the task of meeting the durability objectives set forth in the joint U.S. Army and U.S. Marine Corps system requirements. For example, the M777 achieved considerably more than a 900-plus, Mean-Rounds-Between-System-Abort (MRBSA) level of performance, exceeding the target performance objective by a reassuring margin. Additionally, the M777 successfully completed a 20-year corrosion test that included firing dozens of rounds through the gun after the corrosion tests were finished.

Durable, too

Further, the investment castings overcame a number of durability issues that have been identified with structural components produced using alternative manufacturing processes. All of these tests confirm that the new Howitzer design, with its reliance on titanium investment castings, delivers superior performance and reliability in demanding field conditions.

Converting fabrications to castings did more than help BAE achieve high performance in the field; it also helped draw superior performance from the production process. The performance benefits include simplifying, streamlining, and standardizing the manufacturing process.

One significant achievement has been consolidating numerous detailed parts into single-piece castings. Specifically, the detail part count for fabricating titanium structures was 973. When converted to single-piece castings, the count dropped to 196 — an 80% improvement. A part-count reduction of this magnitude helps streamline production by reducing or eliminating the numerous manufacturing and administrative tasks that inevitably accompany alternative processes, such as hogouts or fabricated assemblies.

A case in point is labor. Fabricated structural assemblies for the M777 required 2,458 welds. The cast components need only 483 welds — another 80% improvement. In addition, the length of the welds was cut by 77%. As a consequence of consolidating fabricated assemblies into single-piece castings, administrative record-keeping tasks in purchasing, supplier management, accounts payable, shipping, receiving, material handling, inventory management, inspection, and more, were streamlined or eliminated.

Added savings accompanied conversion to the casting process. These include reduced raw-material input requirements and much shorter manufacturing cycle times, typically 25-50% less than the cycle time required for fabricating comparable assemblies. These additional savings are made possible by the tighter control of process variables that now characterizes the investment casting process. Today, the parts that emerge from the process are completely interchangeable. This results from continuous improvements in the precision, reliability, and repeatability of the wax-injection, shell-making, vacuum-melting, casting, and heat-treating phases of the investment casting processes, all of which are now automated. Automation brings a high level of scientific control to the process and simultaneously reduces opportunities for human error.

Cycle time reduction

Reductions in cycle time are helped by vertical integration at Howmet and the expanding role the company plays in managing finish-to-print tasks. By way of example, the majority of material and technical inputs into the casting production process were accomplished at the company’s investment casting campus in Whitehall, MI. Howmet’s research center there used SLA pattern-making in the project’s engineering and manufacturing development stage to make test articles and prototypes rapidly, and relatively inexpensively.

Whitehall also is home to Howmet’s titanium alloy production and specialty material production facilities. The Specialty Materials operation produces the waxes used in the pattern-making process. It is adjacent to the Ti-Alloy and Ti-Cast operations that provide raw material and foundry resources for most of the Howmet castings used in the M777 Howitzer. The castings for the M777 are all HIP’ed in Howmet’s hot isostatic press facility, co-located with the Ti-Cast operation. The company’s Tempcraft operation in Cleveland was enlisted to make tooling for the Howitzer’s castings. Tempcraft also designs, builds, and supplies the microprocessor-controlled wax-injection equipment used to create the wax patterns for investment casting’s “lost wax” process.

As a result of these vertically integrated resources, Alcoa Howmet Castings can be responsive to customer demands for engineering support and rapid manufacturing. As the investment casting process continues to display its problem-solving flexibility, it continues to increase the range of market segments in which it can be applied.

Frank Hoerster is the director of fire support systems with BAE Systems, Rockville, MD. Jeffrey Boulet is a program manager with Alcoa Howmet Castings in Whitehall, MI.