The laser beam weldability of AZ31B magnesium alloy was examined with high power pulsed Nd:YAG and continuous wave (cw) CO2 lasers. The low solid absorptivity, liquid viscosity, and liquid surface tension of magnesium make it more difficult to weld than steel. Welding parameters necessary to obtain sound autogenous welds were determined for both pulsed Nd:YAG and cw CO2 lasers. The weldability of magnesium was significantly better with the Nd:YAG laser. This observation was attributed to the higher absorption of the Nd:YAG beam, which in turn reduced the threshold irradiance required for welding and produced a more stable weldpool. The signal from an infrared weld monitor was correlated with both the penetration depth and the quantity of black powder on the workpiece after welding. The fine black powder was shown to result from evaporation of magnesium and zinc. The Nd:YAG welds were only slightly softer than the base material and had very narrow heat-affected zones.
A nonintrusive, solid-state device has been developed to monitor in real time the infrared emissions during laser welding. The weld monitor output is an analog signal ͑100-1000 mV͒ that depends on the beam power and weld characteristics. The dc level of this signal is related to weld penetration, while ac portions of the output can be correlated with surface irregularities and part misalignment or contamination. Changes in dc behavior are also noted for both full and deep penetration welds. Full penetration welds are signified by an abrupt reduction in the weld monitor output. Bead-on-plate welds were made on steel, aluminum, and magnesium with both a cw CO 2 laser and a pulsed Nd:YAG laser to explore the relationships between the weld characteristics and the weld monitor output.
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