Dynamic behavior of molten pool backside during full-penetration laser welding of Ni-based superalloys

The dynamic behavior of the backside of a molten pool was observed by high-speed imaging during the full-penetration laser welding (FPLW) of the Inconel 617 superalloy, and the influence of the welding parameters on the dynamic behavior was studied. Furthermore, the relationship between the keyhole penetration status and the formation of defects, such as spatter and porosity, was investigated. The results showed that during FPLW, the backside of the molten pool was unstable and fluctuated periodically due to the dynamic pressure balance acting on the keyhole wall. The liquid metal under the keyhole was pushed out by the metal vapor in the keyhole to form a bulge protruding from the molten pool surface. The size and height of the bulge were not stable. The keyhole periodically penetrated through the bulge and sprayed spatter outward. Sometimes a sharp increase in the metal vapor recoil pressure in the local area under the keyhole would cause the liquid metal droplet to be pushed away from the molten pool. The proportion of the keyhole penetration time should be controlled within a moderate range (process window) to inhibit processing defects formation during the FPLW process. When the proportion exceeded this range, excessive spatter would lead to weld underfill. When the proportion was below this range, the keyhole-induced porosity in the weld would be generated. Increasing the welding speed helped to improve the stability of the keyhole. Compared with thick plate welding, the process window of thin plate welding was wider.