Flowformed nickel-base alloy mortar barrelshave been fabricated in a program jointly

funded by the U.S. Army’s Armament ResearchDevelopment and Engineering Center (ARDEC)headquartered at Picatinny Arsenal, N.J., and theU.S. Marine Corps, Office of Naval Research, Ar-lington, Va. The goal of the joint program was todemonstrate that lightweight flowformed mortarbarrel tubes could be quickly produced, at a cost50% to 75% less than existing steel barrels, and withlower weight.

Two years after the project was approved, proto-type flowformed cannon barrels were successfullyfatigue-tested and live-fire tested (Fig. 1) at theYuma Proving Grounds in Arizona, and moreArmy personnel were added to the developmentprogram.

Steel mortar systemsToday, the United States Armed Forces operate

three different types of mortar systems. • The 60 mm M224 has a range of up to 3490 me-

ters (about two miles). • The 81 mm M252 is the preferred system for

long-range, indirect fire. • The 120 mm M120/M121 is used for close and

continuous fire. Each of these mortar systems is comprised of

four sections: the cannon or gun tube, bipod, base-plate, and sight unit.

The 60 mm and 81 mm mortar cannon barrelsare carried, but the 120-mm barrels are vehicletransported. For this reason, it was decided to firstfocus re-engineering efforts on reducing the weightof the 60 mm and 81 mm barrels.

Today, these two mortar barrels are made viaconventional manufacturing processes that includemachining solid steel forgings into a thick-wallcannon tube. After that, many hours of machiningare required to remove steel on both the inner andouter diameters before the mortar tubes are ma-chined to net shape.

The incumbent steel has been used for mortartubes for decades. This steel has excellent fatiguecharacteristics, but its strength wanes at elevatedtemperatures. Consequently, thick walls have al-ways been required on steel mortar cannon tubesbecause, under heavy fire, the mortar tube canreach maximum temperatures of 1100°F, and thesteel’s strength is significantly reduced.

Erosion of the steel tubes has also been a problem.Each time a round is fired, the wall thickness isscrubbed off on a micro-level. Eventually, the wallthickness is worn so thin that the cannon tube iscondemned and pulled from the battlefield. In spiteof great progress in both manufacturing technolo-gies and advanced materials over the past century,these steel cannon tubes had not changed much.

Lightweight tube developmentFinite element analysis (FEA) computer mod-

eling was used to numerically simulate the dy-namic firing conditions in which the mortar cannontubes function during worst-case scenarios. Fromthere, the design began to be optimized by takingweight out of the tubes. This was done by reducingthe wall thickness and by utilizing advanced mate-rials that are very strong and light and retain theirstrength at 1100°F.

• Titanium: Initially, prototype flowformedmortar tubes were made from an alpha-beta tita-nium alloy. The advantage to the titanium alloywas that it offered a great strength-to-weight ratio,reducing the barrel weight by almost 75% com-pared to the existing steel barrel. However, likesteel, titanium’s strength also drops off at 1100°F,not offering great weight advantages. Addition-ally, it was believed that the steel mortar roundswould score or gall the inner diameter of the tita-nium tubes when the material became hot underfiring conditions.

• Nickel-base superalloys: Nickel-base super-alloys that serve as shaft and turbine blade ma-terials in hot zones of jet aircraft engines were thentried. Although twice the density of titanium, theyretain their strength at the higher operating tem-

CASE STUDY FLOWFORMING MORTAR

CANNON BARRELSMatthew

Fonte*Dynamic

Flowform Corp.Billerica,

Massachusetts

*Member of ASM International

Fig. 1 — The flowformed mortar tube was live-fire tested atthe Yuma Proving Grounds in Arizona by U.S. Army personnel.

ADVANCED MATERIALS & PROCESSES/MAY 2008 47

peratures, and provide high resistance to corrosionand wear.

The team decided to try alloy 718 per AerospaceMaterial Specification 5662 (AMS 5662). The abilityfor this material to retain its strength during tem-perature changes from 30 to 1200°F is critical to re-ducing wall thickness of the mortar barrel.

After test firing the nickel mortar tubes in excessof 5000 shots, the wall thicknessremained essentially the same,and was not thinned from theshot wear. Therefore, nickelmortar tubes would not bepulled from the field because ofwear; instead, the life-limitingfactor would be its fatigue life.

To evaluate fatigue life, theflowformed alloy 718 tubeswere fatigue tested at BenetLabs, located at Watervliet Ar-senal near Albany, New York.Both the 60 mm and 81 mmflowformed, nickel, thin-wallmortar tubes passed the min-imum acceptance criteria of30,000 fatigue cycles, and wenton to pass the maximum fa-tigue criteria of 75,000 fatiguetest cycles.

Because the weight of thecannon barrels was reduced, en-gineers were also able to reduce

the weight of the bipod and base plate.The combined weight reduction forthe three major components for bothmortar systems are significant to thesoldiers and Marines carrying them.As shown in Fig. 2, the weight of thesteel 60 mm mortar is reduced from44 to 32.5 pounds.

Flowforming processFlowforming is a chipless, cold-

metal forming process for net shapeor near net shape seamless, thin wall,tubular hardware. Flowforming oftencreates dimensionally precise diame-ters and concentric wall thickness, andforms parts that are very straight andround. This either eliminates or sub-stantially reduces the need for expen-sive post-forming machining opera-tions, saving material and minimizingmanufacturing costs.

After a cylindrical workpiece is fittedover a rotating mandrel, hydraulicforce is applied to the outside diam-eter by a set of three CNC-controlledrollers. The correct geometry isachieved when the preform is com-pressed above its yield strength andplastically deformed.

As the preform wall thickness is re-duced by the set of three rollers, thematerial is lengthened and formedover the rotating mandrel. The process

is carried out at room temperature, but due to thelarge wall deformations, adiabatic heat is gener-ated. Refrigerated coolant floods the workpiece todissipate the heat.

This cold forming process causes the material’sstrength to increase, often a beneficial condition.The dimensional accuracies that are consistentlyachieved are well beyond accuracies possiblethrough hot-forming processes. Additionally, thelarge deformations during flowforming wall re-ductions of the material refine the microstructure,thus improving fatigue life. In the context of amortar barrel, which is a highly pressurized vesselthat is fatigue-cycled with every shot, having a fineand uniform microstructure is a significant en-hancement to the material’s characteristics.

For more information: Matthew Fonte, Dynamic Flow-form Corp., 12 Suburban Park Drive, Billerica, MA01821-3998; tel: 978/667-0202; mfonte@flowflorm.com; www.flowform.com.

Resources:http://americanhistory.si.edu/militaryhistory/collection/object.asp?ID=441http://www.army.mil/factfiles/equipment/indirect/m224.htmlhttp://en.wikipedia.org/wiki/M224_Mortarhttp://www.dtic.mil/ndia/2005smallarms/tuesday/mozeson.pdfhttp://www.strategypage.com/htmw/htweap/articles/20080201.aspx

Fig. 2 — The total weight of the 60-mm mortar was reduced from 44 to 32.5pounds by replacing steel (inset) with flowformed nickel-base alloy 718. Thisweight savings is a combination of lighter flowformed cannon tube, bipod,and base plate.

In October 2007, the U.S. Marine Corps and the U.S.

Army’s Product Manager forMortar Systems named

Dynamic Flowform Corp.“Small Contractor of the Year2007.” The award was givenin recognition of Dynamic’s

“outstanding efforts to develop lightweight 60 mm

and 81 mm flowformedcannon barrels.” In January,

2008, the Marine Corps Systems Command in

Quantico, Va., awarded afirm-fixed-price contract to

Dynamic Flowform Corp. toproduce the 60 mm

Lightweight CompanyMortar System and 81 mm

Lightweight Mortar Systemcannon assemblies and spare

components.

48 ADVANCED MATERIALS & PROCESSES/MAY 2008