Laser Welding of Metal

Effect of Focussing Optics on Bead Form

Abstract

This paper describes the effect of focussing optics on the shape and dimensions of laser welded beads, and interaction between the focussed laser beam and material during welding with a 1 KW CO₂ laser. Theresults obtained are summarized as follows:
(1) In laser welding, a "bead transition" occurs, in which the shape and dimensions of the bead change significantly with a slight displacement of focal position relative to the workpiece surface, and is caused by a change in absorption rate of the beum energy depending on whethere or not a cavity is produced; the cavity is formed by the reaction force of evaporation depressing the molten metal surface.
(2) Under optimized focussing conditions, the beam reflection loss during welding can be ignored in a wide range of welding speed, 1 to 40 in/min.
(3) The bead width decreases with increasing welding speed; the width depends on the temperature of molten metal around the cavity at low speeds, and on the spot size at high speeds.
(4) The focal length of lens is evaluated for the deepest penetration depth depending on welding speed at 1 KW power level; 127 mm for v<1 m/min and 64 mm for v>1 m/min are recommended. The distance between focal point and workpiece surface giving the deepest penetration depth is in proportion to the focal length, and decreases exponentially with welding speed.
(5) The range of the focal position within which the penetration depth is deeper than 90 % of the maximum depth is determined. This range of the laser welding is extremely narrower than that of the electron beam welding, owing to the very high reflectivity of the laser beam at metal surface.