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  • Welding System Monitors Gas Shield

Photonics Spectra
Apr 2002
Gary Boas

Although laser welding is widely used in manufacturing, it has not yet found a home in the aerospace industry. The technique would be an attractive tool in this sector if its reliability could be improved, but aerospace alloys present problems that make in-line process monitoring and control difficult. These materials are highly reactive when hot and quickly oxidize and absorb gases from the air if unprotected during fusion welding.

Laser welding has not become common in the aerospace sector because of the reactivity of the materials involved. A new system monitors the spectra of the welding plume to detect if the integrity of the gas shoe has been compromised.

A gas shoe helps prevent such problems and the consequent risk to the weld if one is working with flat components. But with three-dimensional components, air can seep beneath the layer of inert gas. Now a group at Heriot-Watt University in Edinburgh and at GSI Lumonics Ltd. in Warwickshire, both in the UK, has developed a real-time, nonintrusive system to monitor the condition of the gas shield, enabling the user to detect oxidation before the weld is compromised.

The system features a GSI Lumonics Nd:YAG laser that provides 2 kW of 1.06-µm, continuous-wave radiation. A 1000-µm-diameter step-index silica fiber delivers the output to the surface to be welded, which two achromatic lenses focus to a 500-µm-diameter spot. The broadband light from the resulting welding plume is gathered on-axis using a 600-µm step-index fiber and is analyzed with a miniature spectrometer from Ocean Optics Inc. of Dunedin, Fla.

The researchers determined that, when the weld zone becomes oxidized, the plume increases in both size and intensity. Closer inspection of its spectrum revealed a peak at 426 nm when the gas shield's condition is poor, and that the peak increases in amplitude as oxidation increases.

One of the most likely applications of the system is as a real-time monitor that would shut down the welding process if the gas shield became compromised. It also could be used in a closed loop, in which the gas feed would be adjusted to maintain the optimum flow.

The closed-loop-scenario is not likely to be used with critical aerospace components because of the risk of compromising the quality of the weld when the spectrum changes, but such systems could be used with noncritical components.

This automation of the monitoring process would be a boon to manufacturers using reactive materials, said Mahlen D.T. Fox, a researcher at Heriot-Watt who helped to develop the system. Currently, the detection of oxidation is a manual process that can spot extreme problems.

Assuming the necessary industry interest, a commercial product could be available within 12 months. "The working system is envisaged as a low-cost component in an integrated laser welding process-control system," Fox said, "including focus control constructed from two detectors with narrowband filters rather than a spectrometer, and simple, analog electronics rather than a PC." Besides a cost savings, such an arrangement would offer faster response because it would not rely on the sample rate of the spectrometer.

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