Promising welding innovations

WWW.THEFABRICATOR.COM NOVEMBER 2006

November 7, 2006

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Translating any good idea into a practical innovation through research and development can require much persistence, dedicated effort, essential investment, and a long time. This article looks at three welding and joining innovations—hybrid laser arc welding, electron beam welding in air, and NanoFoil® joining—with decades-old roots that still are struggling for widespread acceptance.

Welding sparks image

Sometimes a creative person who is deeply involved with welding comes up with a bright new concept that holds promise for solving problems that still are being addressed in practice with unsatisfactory procedures.

From the raw concept to successful realization, the path to innovation is difficult and extenuatingly long. One has to consider that, generally speaking, there are no short cuts, no quick fixes. Almost always the search for a new way of doing something differently takes years, if not decades, before the idea is brought to satisfactory exploitation.

The unrelated welding innovations discussed in this article all share common backgrounds in that the idea for each was formulated sometime ago, and development still continues long after the original concept was verified as feasible and advantageous.

Hybrid Laser-Arc Welding

It may seem misleading to characterize hybrid laser-arc welding (HLAW) as innovative, since the original idea—essentially combining two well-known and -established unrelated processes, laser beam welding and gas metal arc welding—was first proposed almost 30 years ago.

In fact HLAW already has achieved resounding successes in many areas as a demonstrated, mature, and robust welding process. However, it is not yet an off-the-shelf technique. Note: At least one manufacturer does offer a hybrid laser welding head. However, before adopting this equipment, fabricators should investigate if it compares favorably with standard processes in terms of productivity, quality, and economy.

For each potential industrial hybrid laser-arc welding application, a specific procedure with optimized hardware and control capability has to be developed and tested, quality has to be assessed, and the economic advantage has to be validated.

It appears that the feasibility study alone is a major and costly project, and probably is the main deterrent to a larger acceptance. Researching and planning this solution for specific applications may seem daunting to all but established and resourceful industrial operations.

Another reason likely to limit a larger acceptance is that the high number of parameters to control to manage the combined processes is more feasible for a research institute's capabilities than a manufacturer's.

As stated in the conference paper "Recent progress and innovative solutions for laser-arc hybrid welding" by Dirk Petring and Christian Fuhrmann, "In spite of the simplicity of the basic idea of hybrid welding, the technical rules of arranging laser and arc in a proper way are quite complex, and their understanding is indispensable, if the full benefit of hybrid is to be realized."

An overview HLAW for shipbuilding was presented in the article "Shipyard uses laser-GMAW hybrid welding to achieve one-sided welding" on thefabricator.com in 2003.

The benefits of HLAW exceed the advantages laser welding and GMAW offer individually. These benefits are:

  • Welding from one side
  • Increased tolerance to small but variable gap in joint fit-up
  • Increased penetration
  • Reduced filler metal
  • Higher welding speed
  • Higher productivity
  • Lower net heat input
  • Reduced distortion
  • Improved microstructure and properties
  • Improved weld quality and process reliability

It seems that these benefits would be powerful incentives to dedicate the required attention to the necessary process development and introduce the hybrid process in more numerous and diversified applications.

Plasma Window for Electron Beam at Atmospheric Pressure

It is well-known that an electron beam (EB) is generated in a high-vacuum enclosure. Because electron beams scatter when they bounce into air molecules and lose energy and focus, they cannot be transmitted usefully through the air.

Therefore, classic EB welding is performed in a vacuum chamber where its most appreciated characteristics (high weld depth-to-width ratio; high energy efficiency; low distortion; and the ability to weld deep, square butt joints without filler) are exploited.

Nonetheless EB welding still can be used for many applications in which the degraded quality (versus that obtained in vacuum) is still acceptable, because of the high productivity gains (no lost pump downtime).

A new method for performing EB welding at atmospheric pressurewas demonstrated successfully at the Brookhaven National Laboratory as being capable of producing vacuum-quality welds. It now is in the prototype stage at a company that acquired the exclusive rights.

The device that permits this achievement is a vacuum-atmosphere interface, called a plasma window. It is a small differential pumping chamber, mounted at the bottom of an electron beam column through a tubular passage called a constrictor, which restricts the inert gas flow. Plasma is generated between a cathode placed at the vacuum side and an anode at the other side.

Hot ionized gas particles are trapped by electric and magnetic fields in the plasma window, also called the plasma valve, which prevents air from rushing into a vacuum chamber.

The plasma temperature (in degrees Kelvin) is about 50 times the surrounding temperature. Therefore, a much lower pressure of inert gas is needed to balance atmospheric pressure. However, to protect the cathode from burning, a pressure higher than the theoretical minimum must be maintained in the plasma window. Also, the fact that inert gas viscosity increases with high temperature reduces the gas flow in the same favorable direction.

Researchers found that the plasma current effectively pinches the electron beam, thereby contributing to its continued focus even when it exits to atmospheric pressure.

A safety limitation of the process is the formation of X-rays that operators must be protected from with movable screens.

The preliminary results found that consistent, good welding was achieved at substantially lower energies than had been predicted, with a gap of 50 mm to 100 mm (2 in. to 4 in.) at atmospheric pressure between exit point and weld.

The energy gains are so great with respect to the original vacuum electron beam machines (no powerful pumps to feed) and also versus laser beam (notoriously of limited efficiency) that these gains alone would make this innovation competitive. Furthermore, space requirements and noise reduction are quite remarkable.

As the first demonstrations were performed with a limited-power unit, the current project is to retrofit a full-power EB welder with a proper plasma window and to explore welding results to compete directly with laser beam applications.

A few companies already have expressed an interest in purchasing operational EB welding machines with plasma windows. Therefore, this innovation is bound to leave an important mark in the welding industry as soon as the new machines are available and the promises are confirmed.

NanoFoil® Joining

Many joining processes could be performed more effectively and with less damage to surrounding delicate microstructures if a localized and controllable heat source could be placed exactly where required—in the joint itself.

It appears that an innovative material capable of doing just that—NanoFoil—has been developed from long academic research (Livermore Labs) and industrial (RNT) development. It now is being introduced for high-temperature brazing or soldering critical heat management components in electronic applications. (A list of downloadable technical papersis available from RNT.) Other uses currently are in development for military and space applications.

At the heart of the method is a foil that comprises hundreds of alternate layers of aluminum and nickel, each layer a few nanometers thick. The foil is obtained by vacuum deposition using physical vapor techniques, such as magnetron sputtering and EB evaporation.

When the foil is ignited by one of several means, it develops an exothermal self-propagating reaction at controlled speed and temperature, depending on its exact makeup. This precisely localized heat source can be used for joining applications, in a way similar to other, long-known thermite reactions.

If the foil is sandwiched tightly between preplaced solder or brazing alloy layers deposited on the surfaces of metallic or ceramic elements to be joined together, the localized heat completely melts the filler metals and completes the joining process without heating the adjacent delicate structures.

Although this process seems limited at present to high-tech, demanding applications because of the high cost of the foil, industrial welding of more common material can be envisioned, if the correct forging temperature can be provided locally and a suitable upset pressure cycle can be applied.



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