Control of chaos in laser-induced vapor capillaries

Vapor capillaries associated with deep penetration laser welding (keyholes) are inherently unstable and it is well known that the bounded evolution of unstable modes of nonlinear dynamic systems can lead to complex fluctuations of state and/or output variables. In the case of keyholes, the magnitude of self-induced fluctuations is sometimes large enough to cause occasional closure (“collapse”) and trapping of voids in the solidifying weld metal. Since voids are serious and frequent defects in partial penetration laser welds, methods to improve the stability of the keyhole, and thereby reduce or eliminate their incidence are of considerable practical interest. One technique that is known to simplify the complicated behavior of chaotic systems involves the application of periodic perturbations to accessible system inputs or boundaries. Consequently, this work investigated the effect of power modulation on porosity formation in laser welds. Frozen glycerin was used as the medium for welding since its transparency allowed visualization of the keyhole, facilitating control parameter development. Statistical and spectral analysis of penetration depth signals for the uncontrolled system was performed to characterize the nonlinear keyhole dynamics. In the control studies, it was found that the optimum frequency of power modulation at 400 Hz was very effective for reducing porosity. A 15° leading beam incidence angle was found to provide further improvements. Three-dimensional phase portraits of the keyhole depth for uncontrolled and controlled signals showed that the control did not convert depth fluctuations to a completely predictable limit cycle but that the remaining fluctuations were more predictable than those of the uncontrolled system.