Norris, J.T., Robino, C.V., Perricone, M.J., Hirschfeld and D.A., “Development of a Time-Resolved Energy Absorption Measurement Technique for Laser Beam Spot Welds”, Welding Journal, Vol. 89(4), pp. 75s-81s, 2010.
A method has been developed to temporally characterize the power and energy absorbed in laser beam spot welding (LBSW). As a spot weld is created, the absorption of laser power changes as the surface of the weld pool changes from initial melting through the development of the keyhole. By relating the instantaneous delivered power and pulse energy to the scattered power during welding, a time-resolved description of the power and energy absorption can be obtained. The method uses two goldplated integrating spheres containing Nd:YAG notch-filtered photodiodes to capture and detect the scattered laser light. Under various welding parameters (pulse energy, duration, and shape), the level of scattered light changes with the y condition of the weld pool. For high depth-to-width aspect ratio keyhole mode welds, power transfer efficiency (or instantaneous energy transfer) ranges from -40 to 80% depending on the state of the weld pool. In contrast, low aspect ratio conduction mode welds maintain less than 50% transfer efficiency throughout the welding process. Overall energy transfer efficiencies measured by this method show good agreement with calorimetric (Refs. 1, 2) and thermal expansion measurements (Ref. 4). Time-resolved energy absorption was also evaluated for square and constant ramp down (CRD) pulse shapes. Through characterization of keyhole formation and transfer efficiency in relation to welding parameters, the laser welding process can be optimized, and insight into keyhole phenomena necessary for developing and improving modeling capabilities can be obtained.
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