Military makeover the investment casting way.
* This article describes the success of a joint U.S. Army/Marine Corp program initiative that integrates castings into the design of a lightweight howitzer system.
* Explained within is how the introduction of thin-wall titanium castings greatly improved the manufacturability, affordability and quality of the Lightweight 155-millimeter Howitzer.
Metalcasting has a great impact on many industries throughout the world, not the least of which is the Defense industry. Castings can be found throughout the spectrum of systems within the U.S. Dept. of Defense (DOD), from small items within handguns to large portions of Naval ships and aircraft.
The emerging security challenges of this century, and the need to respond more decisively and rapidly across the full distribution of operations, requires the U.S. Army Lo transform itself to a lighter, more effective instrument. The current situation in the Middle East, the wars in Iraq and Afghanistan and the attacks of September 11, 2001 provide compelling evidence that the strategic environment remains dangerous and unpredictable.
These examples have highlighted the need for the U.S. Armed Forces to transform Dom a heavy tactical configuration to one that is lighter, with a quicker reaction time and is comparably overpowering against the enemy. The metalcasting industry's role in this transformation is to provide economical, high-strength and lightweight parts to the military.
Through the National Defense Authorization Act in 1998, Congress directed the DOD to identify key additional steps it could take to build on the acquisition reforms of the previous five years. One of the goals of this transformation is civil-military integration, through which the DOD will increasingly rely on ah integrated civilian-military industrial base.
It will seek to insert commercial technology and products into new and current systems to improve reliability through continuous technology refreshment. Another transformation goal of the DOD is to make a weapon system's price and schedule play more of a key role in driving its design development.
The initiatives are helping the DOD achieve the transformation goals of providing high-performance weapon systems and support in less time, at lower costs and with higher performance.
The Lightweight Howitzer Program
The Joint Lightweight Program Management Office was formed in the mid-1990s to assist the DOD's transformation by managing the Lightweight 155 mm Towed Howitzer program, which was created to replace the existing M198 Medium Towed Howitzer.
A howitzer is a piece of cannon artillery that functions in the rear of advancing forces to provide destructive, suppressive and protective indirect and direct tire support ahead of the troops. Howitzers are classified by the size of the projectiles (artillery) they tire.
The M198 Medium Towed Howitzer is a 155 mm field artillery howitzer. It is the standard 155 mm towed howitzer used by the Army and Marines. The M198 is constructed of aluminum and steel, weighing 16,000 lb. The M198 provided better range and improved reliability and maintainability than its predecessor the M114A2.
But, a more efficient model was needed. By incorporating new casting designs into the 155 mm howitzer design, the LW155 was born. Designed to provide close and deep tire support and interdiction tires, it will be lightweight without sacrificing range, stability, accuracy or durability.
BAE Systems RO Defense (formerly the armaments group of Vickers Shipbuilding and Engineering, Ltd.), Barrow-in Furness, England, originally designated the LW155 as the Ultra lightweight Field Howitzer, but it has since been named the M777.
Open competition among defense industry contractors led three manufacturers to submit aluminum and titanium alloy configuration prototypes. BAE Systems' prototype was selected in 1997 by a joint U.S. Army/Marine Corps initiative to replace the existing inventory of M198 155 mm towed howitzers. The XM777 (then an experimental model) matched the firepower of current generation 155 mm towed systems with a weight reduction of more than 6000 lb. The XM777 had a production weight of 9500 lb (4309 kg).
To achieve this weight savings, the M777 design incorporated a significant amount of a titanium/aluminum/vanadium alloy (Ti-6Al 4V) for the major structural components.
Eight Engineering Manufacturing Development (EMD) phase weapons were produced in BAE Systems' Barrow, England facility from 1998 through 2001. The major structures of these weapons were fabricated by welding machined titanium plate-work. These weapons went through significant design and operational testing to verify that all performance requirements were met. More than 500 design changes were implemented to improve the systems performance, reliability, maintainability and human interface while maximizing safety.
The first casting was introduced to the M777 because of problems encountered during testing. A fabrication known as the buffer yoke was developing fatigue cracks well before its life requirement. This fabrication was manufactured from machined sculptured plate and bar, which was welded together, heat treated and machined. Cracks were developing in the heat affected zone of the welds.
The Buffer Yoke was an 11 piece weldment that was converted to a single casting weighing 81 lb. This critical component guides the cannon during recoil and is therefore under a lot of stress. The piece was manufactured by Howmet Castings, Greenwich, Conn.,
BAE worked with Howmet Corp. to redesign this fabrication into an investment easting. The initial prototypes were made using stereolithographic modeling. All eight EMD weapons were retrofitted with this casting and thousands of rounds were fired with no further incidents.
A second casting, the center body, was then introduced into the redesign. The casting is one of five pieces that make up the body assembly, which is the major component of the lower carriage assembly for the gun. The body assembly was 211 pieces welded together. It is now five castings plus six closing plates (11 parts in total). The lower carriage houses the suspension and road arms/wheel assemblies for towing the gun as well as the trails and spades and stabilizers that transfer the firing loads to the ground and stop reward movement of gun. The center body casting was investment cast by PCC Structurals in Portland, Ore.
The EMD phase of the LW155 program proved the design of the weapon could meet the requirements of the Marine Corps and Army, but it also demonstrated the need for more robust manufacturing processes. The large titanium fabrications required an excessive amount of welding hours and were exhibiting distortion issues that required rework. The distortion complicated machining and assembly operations during manufacture and affected the accuracy and reliability of the weapon.
To address these issues, two additional pilot production howitzers were commissioned for manufacture prior to the start of mainstream production. The new howitzers implemented 23 castings into the design and utilized a supply chain with vendors in the U.S., Great Britain, Italy and Canada.
Two titanium casting suppliers, PCC and Howmet Castings, were selected to develop the castings. Engineers from BAE Systems, PCC, Howmet and the Joint Program Office worked together to identify which welded areas would benefit most by being replaced with a casting. The criteria used included part count and weld length reduction, areas with known manufacturing issues and projected mainstream production costs.
The implementation of these castings was a major success. Welding hours and part count were reduced by more than 50%, distortion was significantly reduced and high-stress areas were selectively strengthened. Castings also allowed the free flow of material, giving the designer greater flexibility.
The pilot production guns were then put through strength of design testing to determine the reliability of castings under extreme conditions. Based on their success, an additional five castings were implemented into the manufacture of the M777, bringing the total number of Ti-6Al-4V castings in the howitzer to 28. The additional castings brought the total part count reduction to 80% and reduced the total number of welds by more than 75%, with a 77% reduction in the total length of welds. The improvements saved about 3000 hr of welding for a single howitzer.
The incorporation of these 28 castings also assisted the program in reaching its weight reduction goals. The pre-production guns met the performance criteria, and it is anticipated that the reliability of the M777 will be improved by reducing the potential problem areas associated with welds.
Once the hard tooling for the manufacture of the castings has been purchased and the manufacturing line is under production, a schedule savings also will be experienced when compared to the EMD structures. And by purchasing cast parts instead of manufacturing welded components, the program should experience a reduced time to manufacture the M777 because of the parallel production processes. Although the cast parts also must be manufactured, they can be done at the same time as the howitzer and brought into the production line at the proper time.
Looking to the Future
The Lightweight 155 mm Howitzer joint U.S. Army/Marine Corp program has integrated civilian and military industrial bases by dealing directly with the vendors within the casting industry and has demonstrated that cost and schedule both playa key role in the design development of this weapon system. This program has proven to be a vivid example of the successful application of applied engineering improvements to accomplish DOD transformation objectives.
The successful implementation of thin-walled titanium investment castings has been crucial in achieving full-rate production requirements of the howitzer while maintaining quality. Castings deliver components without the variability associated with welding. In addition, castings area better economical choice than welded fabrications. The up-front cost of hard tooling for castings is offset by the jigs and fixtures, machining of plate-work and the recurring costs associated with welding.
By incorporating lightweight metals and later introducing castings into the redesign, the joint team is an example of how technology refreshment can improve an existing system's performance while lowering production costs.
M198 LW 155 Weight 16,000 lb 9,500 lb Set up time 8 min. <3 min. Mobilization time 11 min. <2 min. Firing range (unassisted) 30 km. 30 km
For More Information
"Mission Accomplished: Advancement in Military of Castings," MODERN CASTING, July 2003. pp. 25 29.
About the Authors
Robert Nestor is a systems engineer (Environmental Technologies) at the Industrial Ecology Center Casting Emission Reduction Program for the U.S. Army Armament Research and Development Center. Christopher Hatch is a program management engineer on the LW155 Joint Program Office for the Marine Corp System Command.
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|Title Annotation:||howitzer design|
|Date:||Dec 1, 2003|
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