January 11, 2005
Plasma arc welding (PAW) uses plasma, a gas that has been heated to a high temperature and ionized. The plasma becomes electrically conductive, transferring an electric arc to the workpiece through a small orifice.
This ensures a high concentration of heat to a small area. The shielding gas enters the arc zone and channels the plasma to the workpiece in a precise column.
Because of the advanced level of arc control and accuracy, PAW is suitable for use in precision welding applications. The weld that results is a product of high heat concentration delivered through the plasma torch and the shielding gas that protects the molten weld puddle against oxidation. Additionally, the arc is columnar, which reduces the chances of damage to adjoining components of the workpiece. The welding arc transfer allows thin sheets, fine wire, and miniature components to be welded.
The narrow columnar arc, however, accepts fewer tolerances than the wider arc of gas tungsten arc welding (GTAW). Even the slightest adjustment or process variation can cause premature torch failure or arc-starting difficulty, the most common problems associated with PAW.
For high-production work, it can be frustrating, not to mention expensive, if you have to stop production to replace a damaged torch. While your first instinct may be to blame the torch manufacturer, the problem actually may be associated with the setup and maintenance of your equipment or with your procedure.
Generally, torch failure is caused by a short within the torch that causes it to arc internally rather than arcing through the electrode to the workpiece. Maintaining a consistent procedure is key to improving arc starting, minimizing torch failure, and, most important, minimizing downtime. You can optimize the performance of your plasma torch by incorporating the following steps into your setup and maintenance procedures.
A common problem with a plasma torch is that it can be hard to start. In an optimal setup, the pilot arc is initiated between the torch electrode and nozzle tip. The arc then is transferred to the weld piece, delivering intense heat to a small area by forcing the gas and arc through a small orifice in the nozzle and tip. If the transfer is not initiated, you'll need to examine several factors.
Start problems often are caused by inconsistent gas flow rates. Again, buy gas from a trusted supplier and ensure that your equipment is leak-free. The plasma arc nozzle or tip orifice also can cause gas flow problems. It's important to make sure you're using the right size orifice for your welding application.
A leaking O-ring or backcap also can cause starting problems. Look for blueing on the tip of the tungsten, which indicates that outside air is leaking into the torch. This also will be apparent in the weld itself, which will have a grayish tint and may fail an X-ray inspection. Additionally, make sure to verify that the power supply's pilot board and pilot arc resistor are operating properly.
Poor-quality electrodes also can contribute to arc-starting problems and may not last as long. Again, always buy from a trusted supplier.
You may also consider using some of the new tungsten formulas, which are blends scientifically designed to balance migration and evaporation rates. Other new formulas are cryogenically treated to produce a more dense structure.
Finally, a simple step like having a check list on the production floor and making sure your welding engineers and shift supervisors adhere to it will go a long way toward building the kind of consistent procedures required by the precision of PAW.
Jim Watson is a torch designer and Jennifer Simpson is marketing manager at Arc-Zone.com Inc., 2091 Las Palmas Drive, Suite C, Carlsbad, CA 92009-1551, 760-931-1500, fax 760-931-1504, firstname.lastname@example.org,www.arc-zone.com.
Practical Welding Today® was created to fill a void in the industry for hands-on information, real-world applications, and down-to-earth advice for welders. No other welding magazine fills the need for this kind of practical information. Subscriptions are free to qualified welding professionals in North America.