Abstract
Sheets of AZ31B wrought magnesium alloy were welded by CO<sub>2</sub> laser. Welds of "partial penetration", "weld pool penetration", "keyhole penetration" and "excessive penetration" were obtained, and the "keyhole penetrated" weld possessed the best quality. The light emission of welding plasma was monitored by photo-diode, and it was found that the photo-diode signal varied with different penetration statuses. When the welds were "weld pool penetrated" and "keyhole penetrated", the average signal amplitude A reached its minimum value. R, which defined as ratio of A and standard deviation <sup>δ</sup><i>s</i> of signal was introduced to describe the fluctuation tendency of plasma, and the ratio reached its maximum value (which meant the lowest plasma stability) when welds were "weld pool penetrated". By detecting the minimum value of A and R, "keyhole penetration" can be differentiated from "weld pool penetration" and "excessive penetration".
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