Ultrafine-keyhole welding process using single-mode fiber laser

Deep penetration welding process with ultra-fine keyhole of thin stainless steel foil with thickness between 10μm and 100μm was studied using a single mode Yb-fiber laser with a CW maximum output of 40W. The spot size of approximately 10 m at 1/e²-power point of the maximum value was obtained, which provides power density of 10⁸W/cm² at 40W. Ultra-fine keyhole process was discussed based on energy coupled to the specimen W_C, the laser energy passing though the metal foil W_P and the back-reflected laser power W_R and keyhole process was compared with that of electron beam welding. It was found that defocusing by the refraction in the plume cannot be neglected without assist gas in laser welding with such a fine spot diameter at especially low welding speeds. Although the critical power density for keyhole welding was 1 to 2 orders larger higher than the case of macrowelding, keyhole was maintained up to at least 1m/s at 25W, providing penetration depth of 50μm, if the keyhole condition is once fulfilled in microwelding. Humping is suppressed in microwelding because the flow path beside the keyhole is kept larger even at very high welding speeds resulting in reduced flow velocity of the molten metal.