Abstract: 2 mm thick low carbon steels are welded with butt joints under air cooling(AC)and water cooling(WC)conditions respectively. Different parameters of laser power, welding speed and off-focus length are selected. The joint formation, microstructure and microhardness of welded joints are analyzed after welding. The results show that the width of weld metal(WM)and heat affected zone (HAZ) in WC are less than that in AC. Weld width increases with the decreasing of focal distance under the same condition of heat input and positive defocus. Microstructure of WM is composed of lath proeutectoid ferrite and pearlite. The grain size of HAZ is refined obviously in water cooling compared with that in air cooling method. The angle between the growth direction of columnar crystal and the center line of weld is about 70°-80°under the air cooling condition. But under water cooling condition, the growth direction is almost perpendicular to the center line. The average microhardness of weld in AC and WC are 316.7 HV0.2 and 331.5 HV0.2, which are both higher than 181.8 HV0.2 of the base metal. Meanwhile, the weld hardness under WC is slightly higher than that under AC.

Key words: air cooling, fiber laser welding, microstructure analysis, water cooling