Abstract
Keyhole dynamics in the high power continuous wave laser welding of aluminium alloys, in particular the role of keyhole instability in porosity formation, is discussed on the basis of high speed X-radiography observations. Motion in the weld pool was observed by introducing fine tungsten particles into the weld pool. A depression which moved periodically up and down the back wall of the keyhole gives rise to porosity, through bubbles formed at the bottom of the keyhole. The depression appears to be related to non-uniform evaporation on the front wall of the keyhole; both the keyhole and the weld pool are strongly disturbed by the dynamic pressure of the metallic vapour jet. The characteristic spherical and elongated pores were found to contain predominantly metal vapour and entrained shielding gas. Keyhole fluctuation in continuous wave laser welding can be suppressed by controlled pulse modulation, provided a suitable pulse frequency and duty cycle are selected.
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