Product Code: ICAL05_2101

Importance of the Coupling Between the Induced Vapor Plume and the Melt Pool, for Nd-Yag CW Laser Welding
Authors:
Remy Fabbro, LALP (CNRS) / Gip Gerailp; Arcueil Cedex France
Francis Briand, Air Liquide - CTAS; St OUEN L4AUMONE France
Sonia Slimani, Air Liquide - CTAS; St OUEN L'AUME France
Frederic Coste, LALP (CNRS) / Gip Gerailp; Arcueil France
Presented at ICALEO 2005

We have analyzed the effect of the induced vapor plume on the metallic pool during CW laser welding. We used diagnostics based on fast video imaging (4 kHz frame rate) of the vapor jet, the melt pool surface and the keyhole geometry (front wall inclination and the top and bottom dimension apertures). An integrating sphere allowed us to determine the laser reflectivity of the front keyhole wall.
As a function of the main operating parameters (incident laser intensity and welding velocity), the following results were obtained: The determinant parameter is the inclination angle of the front keyhole wall. When the welding speed increases, this inclination angle also increases and so the local absorbed laser intensity strongly increases. Therefore, the evaporation process on the front wall is important and leads to a very intense vapor jet, always emitted perpendicularly from the front wall, whose dynamic pressure expels efficiently the rear keyhole wall.
At low welding speed (so for deep penetration conditions), the absorbed laser intensity on the front wall is much smaller and the dynamic pressure of the vapor is rather low. The effect on the rear keyhole wall is then different: the expansion of the vapor gases induces a shearing stress along the rear keyhole wall surface that produces a liquid bump at the rear keyhole exit. Resulting oscillations of the melt pool surface intercept the exhaust gases and local droplets emission ensues.
Finally the consequences of the very high front keyhole wall laser absorptivity we measured (around 70 to 80%) will be discussed for the keyhole generation and its dynamical behavior.