Welding in hyperbaric chamber helps keep aluminum-hulled ships afloat
May 3, 2013
The U.S. Navy has been searching for new weld-repair methods on its aluminum-hulled ships beyond dry docking, which is impractical and costly. Phoenix Intl., an underwater services company that has held the Navy’s Diving Services contract for the past 15 years, may have found one: underwater aluminum GMAW in a hyperbaric chamber.
Aluminum vessels are steadily making their way into the U.S. Navy fleet. While on-the-water repairs to steel-hulled ships are nothing new, working with aluminum presents far greater challenges. While dry docking these ships would address these welding challenges, it is not always feasible. With global reach, U.S. Navy ships can be nearly anywhere on earth, and dry docks rarely are nearby.
In addition, the dry docking process wreaks havoc on the Navy’s intricate deployment scheduling. As a war-fighting machine, the Navy and its ships must be at the ready and in position to move at all times. The cost of dry docking is much higher than keeping the ship in the water when performing maintenance and repair.
The Navy has been searching for new weld-repair methods for its aluminum-hulled ships beyond dry docking, which currently is the only option. It may have found one with a promising new technique: underwater aluminum gas metal arc welding (GMAW) in a hyperbaric chamber.
Phoenix Intl., Largo, Md., an underwater services company that has held the Navy’s Diving Services contract for the past 15 years, is performing tests at Bayou Vista, La., with a third-party auditor validating the results.
Aluminum is generally considered much more difficult to weld than steel. For example, aluminum’s high thermal conductivity and low melting point can easily lead to burn-through and warpage problems if proper procedures are not followed. Aluminum’s high thermal conductivity means the material tends to act as a heat sink, making fusion and penetration more difficult.
In terms of chemical composition, aluminum has a high maximum solubility for hydrogen atoms in the liquid form and a low solubility at the solidification point. This means that even a small amount of hydrogen dissolved in the liquid weld metal will tend to escape as the aluminum solidifies, and porosity is likely to occur—a great cause of concern during the welding process.
All-aluminum U.S. Navy littoral combat ships (LCS-2s) began entering service in 2009, creating an urgent need to develop a dependable, certified underwater aluminum welding process.
“Stick welding, or shielded metal arc welding, does not work well on aluminum, so we had to develop a GMAW procedure for performing underwater dry-chamber aluminum welding repairs,” explained Justin Pollack, underwater ship husbandry/underwater welding program manager for the Naval Sea Systems Command and supervisor of salvage and diving for the U.S. Navy.
Phoenix has been developing the new process for more than two years. Ultimately, the company’s procedures must pass three hyperbaric aluminum weld tests: 5083 alloy welded to itself; 5083 alloy welded to a 6000-series alloy; and a 6000-series alloy welded to another 6000 series.
While welding aluminum on dry land poses a number of challenges, welding in an undersea environment vastly complicates the process. Early on in development of the new procedure, aluminum welds performed on the surface passed X-ray tests, but when the same process was performed in the water, the welds experienced significant porosity, Pollack said.
“We realized that we have to control the environment. The welders are working around water, so humidity is an issue, and because they are working at depth, pressure is increased.”
Ken Elliott, welding manager for Phoenix Intl., echoed the concerns about pressure.
“Underwater, a dry environment such as a hyperbaric chamber has an elevated pressure, and elevated pressure will find any route back to ambient pressure,” he said. “The habitat environment is trying to escape. Even through the small space between the wire conduit and the wire itself, pressure tries to escape to the surface, taking contaminants with it.”
Given the pressure variation, impurities in the habitat atmosphere, including moisture, can work their way into the weld, contaminating the weld bead and leading to porosity, which significantly weakens the welding joint. Beyond that, underwater welding in a hyperbaric
chamber presents its own set of challenges.
“The hyperbaric chamber, also known as a habitat, measures about 6 feet tall by 4 feet wide and 5 feet deep,” explained Nathan Martin, welder/diver for Phoenix Intl.
“We are welding in 23 ft. of water with everything around us that a normal welder would have. In the chamber, we have a greater sense of how clean to keep the work area. We are covered in hoses and leads and have to make sure that we can make the necessary body movements to weld comfortably, so we practice those before we actually weld.”
Commercial welder/diver Whitney Ehrgott, Martin’s colleague at Phoenix Intl., agreed, and also has learned to rely on senses beyond sight to create proper welds.
“Our visibility diminishes when welding underwater, so we have to feel and hear the process, and these heightened senses help us to create better welds.”
Phoenix Intl. used a Power Wave® S350 welding machine, a Power Feed® 25M wire feeder, and a SuperGlaze® 5556 3⁄64-in.-diameter wire from Lincoln Electric, Cleveland. The power source and wire feeder are located above the surface of the water on land, connected to a push/pull welding gun with a 50-ft. gun cable located 24 ft. underwater in a hyperbaric chamber. The process and procedures have been carefully selected for easy repeatability anywhere in the world. Accordingly, the team selected procedures that used 100 percent argon as the inert shielding gas.
“One project goal was to develop a way to repair an aluminum ship in any theater throughout the world. So we wanted a welding process that would work without using helium in the gas mix. If we can’t find the right gas mix, we can’t repair the ship,” Elliott explained.
The standard Power Mode™ advanced process on the power source lends itself to using 100 percent argon inert gas very easily, according to Elliott, while providing consistent weld penetration with reduced voltage input. The process uses high-speed regulation of output power to deliver fast response to changes in the arc. The result is improved GMAW performance, including low spatter, uniform and consistent bead-wetting, and controlled penetration.
Getting power and wire to the welding gun, with a feeding distance in this application of 50 ft. from the wire feeder, is another challenge.
“Lincoln has provided true 100-ft. separation between the power source and wire feeder, and by using a 50-ft. push/pull gun on top of that, we can get huge separation from where we need to plug in and get power to the actual arc,” Elliott said.
“Also, we have not experienced wire failure in terms of pushing or pulling it through a 50-ft. gun cable. A 50-ft. gun cable is essential in repairing the ships in the water, and 50 ft. is the extreme distance at which you can push/pull any soft material like aluminum,” Elliott continued.
Elliott reported that the team has experienced smooth wire feeding, with no wire-dross buildup on slave rolls and no undue wear on spring-loaded contact tips that represent the final element of the welding circuit.
To prevent the habitat atmosphere, with its humidity and other contaminants, from contaminating welds, the Phoenix team modified some system features without compromising equipment and consumable performance. For example, though there is no way to completely prevent habitat air and its contaminants from entering the weld area, the crew has placed material at various connecting points of the system’s conduits and gun cables or encased welding leads to keep out unwanted moisture.
“The system runs beautifully. I attribute that to the nice finish on the wire and the speed at which the power system and wire feed can control the process. With this lineup, we can achieve 50-ft. push/pull flawlessly. We’ve experienced no collapse of column strength and have not had feeding issues with any of the wire in the extended gun cable,” Elliott noted.
As the trials head toward completion this spring, equipment and consumable dependability has been noted by Navy and Phoenix Intl. officials.
“Aluminum can be welded in a hyperbaric chamber, but cheap, bottom-of-the-line equipment will not get us there. Lincoln Electric offered the best background and tech support we have seen in a long time, and the machinery is capable of taking some welding nuances out of the formula. Welders can concentrate on their bead profiles instead of thinking about what they have to do because their machines can’t. Factors such as voltage and amperage controls and wire stick-out length come into play, and the Lincoln Electric machinery is well-suited to handle that and perform some very technical welding,” Elliott said.
Following the promising results of this research, the Navy has since purchased several Lincoln power sources and wire feeders to enhance and expand its welding program. In fact, according to Phoenix’s Ehrgott, the machines have been running 24 hours per day for the past six months without a problem.
“We initially chose Lincoln Electric because we were looking for a company that could help support this project,” Naval Sea Systems Command’s
Pollack continued. “It imposed risk on that company in that they would have to let us borrow equipment before we bought. But the equipment and service allowed us to get where we are today. We have bought several machines, including both power sources and wire feeders.”
Elliott agreed with Pollack’s assessment.
“The welding equipment is a huge factor in getting repeatable, 100 percent Class 1 X-ray-quality work. The Lincoln Electric machinery we chose for this project has offered that time