Determining the depth in laser overlay welding is important for achieving the highest weld quality. But since it is not trivial, we searched for a method, that would allow us to determine penetration of both materials in an overlap joint. Aluminum, copper and AISI 304 steel were welded with a fiber laser source, first individually and then in overlapping contact. The resulting plasma light emission was recorded with a spectrometer. We have found, that each material has a typical spectral response and that when the underlying material is aluminum or steel, it is possible to detect their penetration by means of spectral observation.
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