Precision Micro-Welding Using Single-Mode Fiber Laser

Welding process of thin stainless foil, the thickness of which was 40 μm, was studied using a single mode Yb-fiber laser with a CW output of 40W at maximum. The spot size of the laser beam used for a series of welding was measured by a beam profiler equipped with a rotating knife-edge and a photodiode. The spot size of approximately 10 μm in diameter at 1/e2 power density of the value at beam center was obtained by a focusing lens, the focal length of which was 55mm. The power density up to 108W/cm2 was provided by a laser power of only 40W. The transition of beam absorption process between conduction type and keyhde type was discussed based on back-reflected laser energy, which was estimated by a laser energy passing through the metal foil, WP, and a laser energy coupled to the specimen, WC. The former was directly measured using an integration sphere. The latter was estimated by a calculation based on the line heat source model. The threshold power density by the single mode fiber laser for deep penetration welding was 2 orders higher than that of macro welding, however, deep penetration was maintained up to at least 920mm/s at 25W resulting efficient and ultra-fine keyhole welding of stainless foil, the thickness of which was 40 μm, without humping.