Study of the influence of focal position on back-reflected radiation during deep penetration laser welding and its simulation

A penetration laser welding mode characterized by formation of keyhole is often used. A small portion of the laser beam is reflected back from the keyhole and returned to the laser through the optical system of the welding head and optical fiber. The reflected radiation is monitored in the laser for the safety of the optical resonator and its signal can be read and used for process monitoring. The experiment was conducted to relate the intensity of the back-reflected radiation with depth and width of the weld for the variable focal position. The experiments were performed for two variants of the optical focusing system and materials with different thermo-physical properties - carbon steel and stainless steel. Furthermore, a mathematical model for back-reflected radiation was created using ray tracing. The shape of the keyhole was approximated as an inclined conical cavity. The results of the simulation were compared with experimentally measured data and it was found that the proposed model is most suitable for the description of stainless steel.