P is for plasma, precision

Requirements, qualities that make PAW suitable for some, not all

The FABRICATOR December 2006
December 12, 2006
By: Stephanie Vaughan

Plasma arc welding (PAW) is very similar to gas tungsten arc welding (GTAW), but its highly concentrated arc makes it very different - and not necessarily the best process for every application. A close look at its characteristics and equipment will help you decide if it's right for your applications.

Plasma Arc Welding

A telephoto lens can be a photographer's greatest tool when shooting something far away. But even the photographer's slightest movement—even a breath—can cause a blur in the photo.

The same statement can be used to describe plasma arc welding (PAW).

A process that provides a stable heat source for welding most metals from 0.002 to 0.0375 inches thick, PAW provides a stiff arc with penetration deeper than other arc welding methods. But any variation can cause a change in arc reaction—a change that can mean the difference between a good and a bad weld.

Because variables play such a large role in this process, you should know as much as possible about it before deciding if it's right for your application.

When PAW Is Used

In PAW, a highly focused plasma arc column generates the heat for welding. Most similar to gas tungsten arc welding (GTAW), PAW focuses the arc through a constricting tip orifice. That's what makes its arc stiffer, penetration deeper, and heat-affected zone (HAZ) narrower than other arc welding processes'.

Eighty-five to 90 percent of PAW is automated, according to Gary Reid, plasma welding technician for Thermal Arc. Although PAW can be used in manual applications, Reid said that the process often isn't as flexible when used manually.

PAW's nonkeyhole mode allows the process to be used on thin materials, even foil-thin metals. Industries that use this process include:

  • Biomedical
  • Pharmaceutical
  • Food service
  • Brewery
  • Power generation
  • Aircraft
  • Aerospace
  • Automotive

PAW is used mainly in applications that would otherwise require GTAW, according to Reid. That's because PAW can be described as GTAW with a copper tip surrounding the tungsten electrode with an argon gas passing through the tip, known as plasma gas. The difference is that the weld arc is focused through a copper tip and concentrated into a narrow area. The copper tip concentrates the welding current and the plasma gas to a narrow stream.

Although having a copper tip to restrict the arc is one of the benefits of PAW, it also results in limitations that GTAW doesn't have.

"TIG allows you to adjust the electrode to meet weld joint configurations. The copper tip in PAW is fixed—you can't bring the electrode out past the copper tip," Reid said, adding that this can make certain applications challenging, if not impossible, because the tip length cannot be adjusted to reach into joints that are difficult to access.

However, the restricting copper tip also protects the electrode, which is a benefit of the process, Reid said. The pilot arc provides good arc starting and reduces the chance of tungsten contamination, which is critical for many PAW applications.

According to Reid, when GTAW is used to join metal that's 18 inch thick or thicker, both pieces must be beveled, and welding must be completed with a root pass and a second or third pass to fill the joint. The more weld passes, the more the parts will warp.

Substituted for GTAW in these types of applications, PAW in keyhole mode allows the part to be welded in one pass. The heat is spread evenly through the thickness of the part in one pass, resulting in less warpage.

Modes of Operation

Although some applications are manual, such as turbine blade rebuilding, PAW offers the most benefits when automated, Reid said.

Automated PAW has two modes of operation: keyhole and nonkeyhole.

In nonkeyhole mode, PAW uses the torch as a heat source and melts the surface of the materials to be joined. Nonkeyhole mode is used in automated and robotic welding to fuse two parts together, much like GTAW.

Keyhole welding generally requires larger equipment, Reid said. In keyhole PAW, two plates, 18 in. to 38 in. thick, can be butt-welded in one pass. Materials thicker than 14 in. may require two passes.

"The plasma welding gas pierces a hole through the part. The plasma arc stream is restricted by the tip and you use enough pressure to pierce a hole through the part. Then that material flows around the keyhole and comes back together behind the keyhole as you progress forward through the part," Reid said.

Welding is performed in one pass with a large seamer so that two pieces of metal can be butt-welded together to form one continuous piece. This mode is used when joining large tanks for beer vats and tankers that haul liquids, according to Reid.

PAW Considerations

But PAW isn't the best process for every welding application, Reid said. Jim Watson, welding products specialist at Arc-Zone.com, agrees. He recommends PAW for use on thin-gauge materials and in high-production applications that require precise arc starting.

Reid advises potential PAW users to think about two main factors: fixturing and tooling.

"The part must have good, tight fit-up," he said, particularly in automated PAW. "If you cannot hold the tolerances of the part, then it's not going to be a good plasma application. The torch has to see the same thing each and every time."

The other consideration is tooling. The tooling must make good contact with the part. A variation in tooling contact will cause a difference in weld penetration. If tooling doesn't make good contact with the part, the part will absorb all of the heat, which can cause warpage. If tooling makes good contact with the part, the tooling will absorb the heat and control the heat to a narrow zone, resulting in less warpage.

Watson recommends that potential PAW users also make a plan, the way they might for other processes they introduce into a manufacturing environment, by:

  1. Developing production expectations and quality goals.
  2. Consulting a welding engineer or process specialist to review the application and recommend equipment needed.
  3. Creating a budget for equipment acquisition, maintenance, and replacement parts.
  4. Building proper tooling and fixtures to hold and position the PAW torch, cable leads, and work lead.
  5. Preparing a system check list, including needed replacement parts, documented welding procedures, and a welding system maintenance program.
  6. Designating a qualified individual to follow a system check list, maintain needed parts inventory, document welding procedures, and keep maintenance schedules.

Watson also cautions that PAW is not a process that should be considered lightly. "Shops that have no skilled welding technicians or lack the ability to develop detailed operating procedures" are not good candidates to use this process, he said. Other jobs unsuitable for PAW include hand-held welding of small parts and parts that may be better suited for GTAW.

Photo courtesy of Thermal Arc, a Thermadyne company, 16052 Swingley Ridge Road, Suite 300, St. Louis, MO 63017, www.thermadyne.com/thermalarc

Stephanie Vaughan

Stephanie Vaughan

Contributing Writer

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The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.

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