The U.S. Army and American industry come up with titanium-welding solutions to make fighting vehicles for modern warfare.

By Leslie Gordon, associate editor

Lincoln Electric's 120 iLT 5-axis Gantry robot welds parts for a titanium fighting vehicle.

M.W. Technologies' special gas-distribution manifold system incorporates the PURWeld purifier, which traps moisture associated with "bad gas."

Fabricated pieces are mechanically cleaned, tacked, and welded into subassembly sections, which are then joined.

Industry experts often regard welding titanium as a difficult process replete with problems such as high risk of contamination. But the U.S. Army — with the help of American industry — has come up with some creative solutions that have led to the development of a new generation of fast and lightweight fighting vehicles. These vehicles address Iraq's desert conditions, the varied locations of battle, and the speed at which the U.S. now goes to war.

The army's Armament Research, Development, and Engineering Center, Picatinny, N.J., which works with American industry developing products to help the soldier in the field, and M.W Technologies Inc., Elmwood Park, N.J., developed specialized titanium-welding techniques using pulsed gas metal arc robotic welding (GMAW-P) to make new titanium fighting vehicles.

At the start, project engineers had limited information or hard data on welding titanium using GMAW-P. In fact, commercial power supplies didn't include titanium-welding parameters in their preprogrammed databases. But to make the vehicles, the army needed an effective, reliable, and repeatable way to mass-produce high-integrity welds.

To make matters worse, it was assumed that trailing shields, backing gases and, in many cases, a totally enclosed chamber are necessary to provide the correct environment for the weld arc. Also, industry experts typically expect titanium and weld spatter to go hand-in-hand.

A robot does the welding
To do the project welding, engineers use a Lincoln Electric 120 iLT 5-axis gantry robot and a Power Wave 455R GMAW pulse-capable power supply. They also employ 0.045-in. filler metal, Grade 23 (ELI) titanium, and standard 75% argon/25% helium mixture shielding gas (commonly called 75/25), which is readily available throughout the U.S.

They weld in both flat and out-of-position conditions at 14 to 24-ipm-travel speed. The typical weld joint is a 1-in.-thick plate with partial-penetration welds of ³ / 8- in. groove/fillet (a two-pass weld is typical).

The torch is a M.W. Technologies' custom design. It's water-cooled with an aluminum body. In addition, it has a water-cooled contact tip and uses a ⁷ /8-in. gas cap with a specially designed diffuser.

Purifier protects from bad gas
The biggest culprit causing problems with shielding gas is moisture (H2O), especially true when welding titanium. The standard provided by manufacturers is 10 ppm or less of impurities, usually okay for most applications — but for this project, the army wanted extra security.

For this security, M.W. Technologies provided a special gas-distribution manifold system incorporating the PURWeld purifier, which traps moisture associated with the "bad gas" that won't allow proper welding.

Controlling metal transfer
To make the titanium fighting vehicles, the welding robots use Lincoln's WaveDesigner software to get the desired one-drop-per-pulse condition. A modified active control technology proposed by Y. M. Zhang and P.J. Li at the University of Kentucky, Lexington, Ky., is incorporated. According to the researchers, this technology controls peak and transition currents to prevent accidental detachment (spatter). Metal transfer is thus controlled.

The weld-metal chemistry contains oxygen, nitrogen, and hydrogen, which are well below the amounts in the base metal, despite the appearance of weld discoloration.

Assembling the pieces
Engineers model the fighting vehicle's hull using Pro-E software and finite element analysis. Each plate is detailed for outside dimensions, weld bevels (if any), and mounting provisions, along with threaded and through holes. Plates are nested, cut out using abrasive waterjet, and machined as necessary for mounting provisions. Resulting pieces are mechanically cleaned, tacked, and welded into subassembly sections, which are then joined.

Needing no machining or stress relief, the final assemblies are ready to go. The vehicles have armor in all directions (360°) to withstand cannon fire, and titanium overhead cover. Each vehicle holds 11 military personnel. They have rear-facing seats for passengers .and side-by-side crew stations for running.

Using this method, hull structures are fabricated in record time. And, although the new fighting vehicles aren't solely made from titanium, most of their weightsavings is a result of the switch from steel to titanium.

The eight-wheeled vehicles, with independent suspensions featuring adaptive damping, can go up to 75 mph and have a cruising cross-country speed of 42 mph. Their engines develop 31 hp/ton for a short duration and 22 hp/ton continually using a hybrid-electric drive. Vehicles accelerate 0 to 30 mph in 6.5 sec.

Their 36,000- lb weight seems heavy, but it replaces the over-80,000 lb of previous vehicles.

The army's long-term vision is to completely transform its combat vehicles from heavy, reliable, slow, and effective to lightweight, decisive, fast, and strong. The upshot is titanium is a key material to meet the this need, and welding titanium is a key technology to fulfill this vision.

Project successes include: Spatter-free welds and stable arc conditions, while using a conventional power source, standard industrial-grade shielding gas in conjunction with PURWeld, and a double-pulse waveform template. Acceptable out-of-chamber welds with limited or no trailing shielding by using a properly designed gas-distribution manifold and equipment.