This study was carried out to know the laser weldability of Ni-base superalloy, Inconel 718. Inconel 718 whose thickness and grain size were 2 mm and 84μm were prepared for a head on plate test using CO{sub}2 laser welding with a power of 1 to 3kW and with a traveling speed of 0.5 to 5 m/min. The testing plate was solution heat-treated at the temperature of 1340 K for 3.6 ks before welding.The shape of penetration depended on traveling speed and was typically classified into two types of Type C and Type N by specialized shape. Type C had low depth to width ratio like a champagne glass and Type N had high depth to width ratio like a nailhead. Type C and Type N weld beads are seen at a side of low welding speed and at a high welding speed side, respectively and the critical welding speed between Type C and Type N is 1.5 m/min in the present study.Liquation cracks easily occurred at HAZ in the case of nail head like penetration. We used a radius of curvature of fusion boundary at neck zone of penetration to represent a feature of penetration. It can be seen that a critical radius of curvatureexists by which whether cracks occur or not in HAZ can be judged. No weld cracks were found when the radius of curvature exceeded the critical value ofρ{sub}cr. Furthermore, as a countermeasure welding cracks can be prevented by increasing the laser heat input over the valus of 100 kJ/m in the present study. According to observation of grain boundary liquation at HAZ, liquation crack was related to grain boundary liquation.
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