In laser and electron-beam welding, a deep cavity called a keyhole or beam hole is formed in the weld pool due to the intense recoil pressure of evaporation. The formation of the keyhole leads to a deep penetration weld with a high aspect ratio and this is the most advantageous feature of welding by high-energy-density beams. However, a hole drilled in a liquid is primarily unstable by its nature and the instability of the keyhole also causes the formation of porosity or cavities in the weld metal. In particular, the porosity formation is one of the serious problems in very high-power laser welding, but its mechanism has not been well understood. The authors have conducted systematic studies on observation of keyhole as well as weld pool dynamics and their related phenomena to reveal the mechanism of porosity formation and its suppression methods. The article will describe the real-time observation of keyhole and plume behaviors in the pulsed and continuous-wave laser welding by high-speed optical and x-ray transmission methods, the cavity formation process and its suppression measures.

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The fibre laser has been receiving great attention due to its advantages of high efficiency, high power and high beam quality, and is expected to be one of the most desirable heat sources for high-speed and deep-penetration welding. In this study, therefore, in bead-on-plate welding of Type 304 stainless steel plates with 6 kW fibre laser, the effects of laser power, power density and welding speed on the formation of sound welds were investigated with four laser beams of 130, 200, 360 and 560 µm in spot diameter, and their welding phenomena were clarified with high-speed video cameras and an x-ray transmission real-time imaging system. The weld beads showed a keyhole type of penetration at any diameter, and the maximum penetration of 11 mm in depth was obtained at 130 µm spot diameter and 0.6 m min−1 welding speed. It was found that the laser power density exerted a remarkable effect on the increase in weld penetration at higher welding speeds, and sound partially penetrated welds without welding defects such as porosity, underfilling or humping could be produced at wide process windows of welding speeds between 4.5 and 10 m min−1 with fibre laser beams of 360 µm or 560 µm in spot diameter. The high-speed video observation pictures and the x-ray images of the welding phenomena at 6 m min−1 welding speed and 360 µm spot diameter show that a sound weld bead was formed owing to a long molten pool suppressing and accommodating spattering and a stable keyhole generating no bubbles from the tip, respectively.

The objectives of this research are to investigate penetration characteristics, to clarify welding phenomena and to develop high quality welding procedures in bead on plate welding of type 304 austenitic stainless steel plates with a 10 kW fibre laser beam. The penetration depth reached 18 mm at the maximum at 5 mm s 21 . At 50 mm s 21 or lower welding speeds, however, porosity was generated at any fibre laser spot diameter. On the other hand, at 100 mm s 21 or higher welding speeds, underfilling and humping weld beads were formed under the conventionally and tightly focused conditions respectively. The generation of spatters was influenced mainly by a strong shear force of a laser induced plume and was greatly reduced by controlling direction of the plume blowing out of a keyhole inlet. The humping formation was dependent upon several dynamic or static factors, such as melt volume above the surface, strong melt flow to the rear molten pool on the top surface, solidification rate and narrow molten pool width and corresponding high surface tension. Its suppression was effective by producing a wider weld bead width under the defocused laser beam conditions or reduction of melt volume out of keyhole inlet under the full penetration welding conditions. Concerning porosity, X-ray transmission in situ observation images demonstrated that pores were formed not only from the tip of the keyhole but also at the middle part because of high power density. The keyhole behaviour was stabilised using a nitrogen shielding gas, resulting in porosity prevention. Consequently, to produce high quality welds in 10 kW high power fibre laser welding, the reduction procedures of welding defects were required on the basis of understanding their formation mechanism, and 10 kW fibre laser power could produce sound deeply penetrated welds of 18 mm depth in a nitrogen shielding gas.

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