Practical Welding Today - May/June 2009

boats for the legendary Dennis Conner and the New York Yacht Club (see Figure 1).

When working with aluminum, Ekblaw said, “The question is, When you weld it, can you keep it straight? I had a lot of experience doing that with the 12-meter America’s Cup yachts. We had to hold tolerances to ¹⁄ 32 inch, and we did that by equalizing the heat input. It’s just like putting torque on a car’s header; you have to evenly distribute the pressure.”

The basic Nereus design calls for two rectangular-shaped pods joined by a center section. The center section is simply an 86- in. piece of Schedule 80 6061-T6 aluminum pipe. The 6061-T6 plate, in thicknesses from ¹⁄ ³4 to ⁄ 4 in., forms the skeleton of the 160-in.-long pods.

Nereus’ designers also demanded holding alignment to ¹⁄ 32 in. on a centerline. To accomplish this, Ekblaw started by using a high-tolerance, modular fixturing table. The table provides accuracy because the bores holding the clamps and associated tooling have a tolerance of ±0.001 in. All table elements are machined flat and square to a tolerance of 0.004 in.

To maintain part tolerance, WHOI uses an abrasive waterjet to cut all of its materials. Depending on the model, a waterjet typically cuts parts to within tolerances of ±0.003 to ±0.001 in.; leaves no dross on the back side of the cut; and creates no heat-affected zone (HAZ) or warping to affect fit-up.

Ekblaw said once cut, the sections of aluminum fit together like a puzzle. WHOI engineers developed a design where all the pieces mechanically interlock with slots and tabs. When fitted into a slot, each tab has a “shoulder” that creates an absolute dead stop. The end of the tab fits perfectly flush with the outside of the slot.

All welds required 100 percent penetration, so the WHOI fabrication team used routers and carbide cutters to bevel every joint.

“It took us days of routing, but good weld quality demands good weld preparation.”

Another key part of the preparation was selecting the right filler wire. Instead of using the more traditional choice of filler wire for welding 6061-T6 aluminum, 4043, Ekblaw elected to use 4643 aluminum filler

wire because it has lower silicon content and therefore less chance of cracking.

The company chose the Syncrowave^®350 LX AC/DC gas tungsten arc welding (GTAW) power source from Miller Electric Mfg. Co., Appleton, Wis., to weld Nereus’ aluminum frame. For thick sections of aluminum at 200 to 250 amps using a conventional square-wave technology, Ekblaw selected ⁵⁄32-in.-dia. pure tungsten. The high output/high duty cycle welding on Nereus required a water-cooled GTAW torch which he outfitted with a size 7 or 8 alumina cup and a gas lens. A gas lens ensures smooth, even distribution of the argon shielding gas. This prevents turbulence that could pull in atmospheric contaminants, and Ekblaw won’t make critical GTAWs without one.

FIGURE 1

Geoff Ekblaw’s 20 years of experience with welding aluminum told him to shift between the port and starboard sides and make shorter welds to distribute heat evenly around Nereus’ perimeter.