Plasma Spectroscopy Monitors Welds

Scott M. Aldis-Wilson

Researchers from Università degli Studie Politecnico di Bari and from Istituto Nazionale per la Fisica della Materia, both in Bari, Italy, have developed a nonintrusive sensor system to monitor laser welding in real time. By spectroscopically examining the welding plasma, the system identifies more- and less-energetic regions near the laser-material interaction zone. Results of experiments with the sensor suggest that a lower standard deviation of the plasma temperature signal correlates with higher weld quality, enabling the researchers to predict the size of defects based on the welding speed.

A plasma spectroscopy system monitors plume emissions for the real-time analysis of CO₂ laser welding. A quartz collimator is fixed to the laser head to collect the 390- to 575-nm radiation, and a 50-mm optical fiber transmits the signal to a miniature spectrometer. An acquisition card translates the data to a personal computer.

Previous systems have relied on the temperature of the welding pool, the electric potential between the nozzle and target, and acoustic data from the keyhole plasma. These approaches offer only qualitative or quantitative data for one parameter, however, and they require postprocess tests -- most of which are destructive -- to determine weld quality.

Antonio Ancona, a researcher on the project, said that the new system detects weld defects in real time for different types of metal alloys and can also be implemented in manual, semiautomated and fully automated arc-welding procedures.

"Therefore, it can be applied not only to auto and aero businesses," he said, "but also naval carpentry, piping and structural steelwork, down to laser precision machining."

In tests with a Rofin-Sinar 2.5-kW C0₂ laser, the system examined laser plasma during the welding of AISI 304 stainless steel. It examined iron, chromium and manganese emission lines between 390 and 575 nm to determine the electron temperature of the plasma for different incident powers, shielding gases, gas fluxes and beam focus positions. The bead-on-plate tests were made at a translation speed of 10 mm/s.

Software added

Ancona said that the work, in progress since 1998 and patented by Istituto Nazionale per la Fisica della Materia, began under a national project to apply academic research to the industrial environment. "We decided to exploit our broad spectroscopic background to study the optical plasma emission produced during industrial welding processes," he said.

The system detects surface imperfections such as lack of penetration, weld disruptions and plate undulations as small as 1 mm. Software has also been developed for the sensor, he said, including a self-detection algorithm that searches for and flags defects according to standards set by the operator.