The dynamics of the melt pool and keyhole was investigated during CO2 laser welding using high-speed video photography and the laser reflectometer technique. A low-power argon laser beam, focused on the weld pool, provided illumination to obtain a direct image of the weld pool surface. The near-surface plasma emission background was decreased by using a narrow-bandwidth interference filter centred at the argon laser wavelength (514 nm). A variation in the shape of the keyhole opening with a characteristic frequency higher than 1 kHz was observed both during spot welding and during welding with a moving beam. For the case of spot welding with a 20 ms laser pulse, long-wavelength (about 1 mm) oscillations of the weld pool were observed with a frequency during the laser pulse and the first 5 ms after the laser pulse in the range 200-500 Hz. In the time interval starting at 25 ms and ending at approximately 40 ms from the beginning of the laser pulse, the long-wave oscillation frequency increased up to 1.3 kHz. The solidification time was determined to be approximately equal to the pulse duration for the spot welding. Surface deformation during cooling was also observed. This information is used to develop a model illustrating the dynamics of the post-pulse weld pool.

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Preliminary studies were conducted on the laser processing of SiC/A356-Al alloy metal matrix composite (MMC) for such applications as welding/joining and cutting. The SiC/A356-Al MMC was processed using several different laser specific energies. Microstructural observations after laser processing revealed that the extent of reinforced material (SiC)-matrix (A356-Al) reaction is directly proportional to the laser energy input. As energy input increased, SiC particle dissolution became greater and aluminum carbide formation increased in both size and quantity. It appears possible to control substantial change (physical and chemical) in SiC particles during processing by controlling the amount and mode of energy input.

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The microstructural changes and the tensile behavior of laser processed A356-A1 alloy matrix composites reinforced with 10 and 20 vol. % SiC particulates are characterized. The autogenous bead-on-plate welds were made using a pulsed CO 2 laser operating at a peak power level of 3.2 kW. The pulse on-time was constant at 20 ms and the off-time was varied from 20 to 2 ms (duty cycles of 50-91%). The microstructure of the laser melted region was investigated by optical, scanning, and transmission electron microscopy, and x-ray microchemical analysis techniques. The extent of microstructural changes varied directly with duty cycle, i.e., being a maximum for the longest (91%) duty cycles. Pulsed laser processing produced partial to complete dissolution of SiC particles and sometimes resulted in the formation of aluminum carbide. The associated rapid cooling also produced a fine distribution of nonequilibrium complex precipitates. In addition, the laser energy modified the SiC surface both physically and chemically. The results of tensile tests indicated that the modified SiC and the distribution of fine nonequilibrium precipitates enhance the mechanical properties of the laser processed composites. Optimum changes in microstructure and mechanical properties were obtained in the composites processed with intermediate (67 and 74%) duty cycles; therefore pulsed processing appears to be a strong candidate for successful joining of these MMCs. 514

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