Arc welders are flexible and inexpensive but don't have the precision and power density of high-power laser welding systems. Now, an idea in its early stages at Ohio State and Princeton Universities takes the precision of a laser, used as a sensor, to guide an arc welder. The goal is low-cost precision arc welding.
Welding researchers there have found a way to guide the position of welds with a special low-power laser beam and have just received a patent on their technique-Laser Assisted Arc Welding (LAAW). It is the first significant improvement in arc welding since World War II.
Cheap and widely used, arc welding is still hard to control and sometimes damages metal parts. Multi-kW lasers are much more precise and have found wide use in the automobile and aerospace industries. Switching back to arc welding through the LAAW technology, which uses only a 7-W laser, could cut costs to one-tenth of a typical laser welding system. With further development, LAAW should get arc welding to the same precision as a laser beam.
Arc welding, aside from being cheap, creates very strong material connections. Electrical arcs melt metal at temperatures as high as 3500 degrees C. But the arcs, streams of electric current resembling small lightning bolts, can move unpredictably. They tend to follow the path of least electrical resistance, sometimes in lightning-bolt-like jagged patterns.
It was realized more than 20 years ago that laser beams could be used to create a path of electrons for arcs to follow, but generating enough electrons to attract a welding arc requires a high-power laser and a high-temperature laser beam path.
The goal at Ohio State and Princeton was to find a way to create an electron path with much more finesse than brute force. After infusing small amounts of carbon monoxide gas into a welding gas such as argon, they use their low-power laser at just the right frequency to cause molecules of the gas to vibrate. The laser cuts a glowing blue trail through the weld chamber as the vibrating molecules shed electrons to become ions, creating an attractive path for the welding arc to follow. The effect-cold ionization-ionizes the carbon monoxide gas without generating heat.
Arc influence has been demonstrated at about 1 kW/square centimeter with a 7-W laser beam, about two orders of magnitude lower than past laser-arc processes that typically used 1-kW lasers.
Cold ionization works because in a shielding gas mix, selected molecules have a preferential absorption for a specific laser beam wavelength. When carbon monoxide molecules are excited with a carbon monoxide laser beam, photons are absorbed directly into the molecular vibration modes of the CO molecule. Collisions between the vibrating CO molecules can then raise a small fraction of molecules to energies high enough for ionization.
The fact that a low level of ionization (some 10 to the tenth electrons per cubic centimeter) will do the job is important-it means that a low-power laser producing a low-power density beam is all that's necessary. The gas remains cold. Power source systems' cost for a LAAW system composed of a low-power laser and an arc-welding power supply will be extremely competitive.
The electron density in the laser induced plasma-the glowing blue path-is orders of magnitude lower than the electron density found in welding arcs, but is enough to establish initial current flow. Short pulses of power are switched through plasma-positioned high-frequency discharges (AC current with a 50 kHz frequency and voltages ranging from 5 to 10 kV, delivered in 5 to 10 microsecond burst every 50 microseconds), providing enough power to make small bead-on-plate spot welds in 0.25-mm-thick AISI type 304 stainless steel.
For a small fraction of a second, the DC welding current pulse follows the established plasma-high frequency path. The plasma provides direction and possibly a degree of arc concentration for short pulses. For very short current pulses, very small spot welds 2 mm in diameter or less are made through a 12-mm arc gap.
Saving power at seven to 15 cents a kilowatt-hour can be important to large manufacturing operation. But capital savings could be more important. A typical multi-kilowatt industrial laser costs $200,000 to $300,000. With a $5000 power supply and a $20,000 laser, LAAW does the work of a laser welding system costing 10 times as much.
The group at Ohio State's Edison Welding Institute is now looking for a commercial partner to help develop the system. Contact Charles Albright, Professor, Industrial, Welding, and Systems Engineering, 111 Edison Joining Technology Center, Ohio State University, 1248 Adams Dr., Columbus, OH 43221. Phone: 614-292-2570. Fax: 614-292-6842. E-mail: [email protected].
Source: Sensor Technology, Technical Insights, Frost & Sullivan, NY, NY (July 2001). For more information on the newsletter, go to Frost & Sullivan.