Investigation On Laser-Arc Interaction And Novel Laser-TIG Arc Hybrid Welding Processes

Laser-arc hybrid welding is an advanced welding technology by combining two heat sources with different physical characteristics and energy transfer mechanisms. Laser-arc hybrid welding can make up for the deficiencies of individual laser process and take great effects on increasing the joint fit-up tolerance, enhancing the process stability, eliminating the weld defects, and improving the welding efficiency. Therefore, the laser-arc hybrid welding is receiving intensive study all over the world in recent years. The hybrid of the laser and the arc is not just a simple combination, so the investigation and comprehension on the physical phenomena and mechanism of the interaction between the laser beam and the arc plasma are significant for understanding of the hybrid process essentially.In laser-arc hybrid welding process, the thermal effect and the laser-induced plasma which generated during laser radiation on metal have significant influences on laser-arc interaction. At the same time, the local electron density and the grads of electron density increase with more laser energy absorbed by arc, which induces more absorption and the refraction. In this thesis, the theory of laser propagation in arc plasma is described and the inverse bremsstrahlung absorption coefficient and the refractive index of arc for incident laser beam are calculated according to classic electron-ion collision theory without taking the thermal effect and the laser-induced plasma into consideration. At the same time, the electron energy equation of arc is amended by introducing A(z) of inverse bremsstrahlung absorption term. The variation trends of the characteristics of beam and arc in laser-arc interaction are analyzed in theoretical, and the physical mechanism and the influence factors are investigated.Then, the characteristics of laser and arc during the interaction between high power laser and DC TIG arc are investigated in many aspects. The heat sources used in the experiment are Rofin DC035 Slab CO2 laser, DS 040 HQ DISC Yb:YAG laser and Fronius TS5000 digital arc welding system. The measurement instruments are Molectron 3sigma laser power meter, Prometec UFF100 beam diagnosis, PHOTRON Fastcam 1024R2 fast camera and Research Spectra Pro 2500i spectrometer. In order to eliminate the influence of the laser-induced thermal effect and directly measure the laser beam parameters and the laser power absorbed by the arc, the laser beam and the TIG arc are vertically set up in the experiment, with the focal spots set at the central axis of the TIG arc. During the experiment, the arc configuration is taken with the high speed camera, the spectrum of the arc plasma is acquired by the spectrograph, the local arc temperature is calculated by Boltzmann plot and the electron density of arc plasma is computed by the Saha equation. The effects of laser power, arc current and laser incident position on the characteristics of beam and arc plasma, such as laser power transmit ratio, beam diameter, laser propagating direction, voltage-current characteristic, arc configuration, arc power, arc temperature and electron density, are investigated in detail and the interaction mechanisms are analyzed. Furthermore, the influences of arc atmosphere and laser wavelength on the laser-arc interaction are analyzed, and the causes for the different interaction phenomena are investigated.After the laser-arc interaction experimental investigations, some novel CO2 laser-TIG arc hybrid welding processes for thin walled butt joint, lap joint and T-joint and heavy section butt joint are exploited. In order to protect the optics lens from the welding spatter and smoke in laser-arc hybrid welding process, an aerodynamic theory-based ultrasonic cross-jet with a non-symmetric Laval cavity is designed according to the moving status and the pattern of spatter particles. Compared with laser welding with filler wire, the characteristics of laser-TIG hybrid welding with filler wire are investigated, which include appearance of the welds, the decrease of defects and the tolerance in gap and unfitness. By optimizing the process parameters, sound thin walled T-joint and heavy section butt joint can be obtained by CO2 laser-TIG hybrid welding process. The laser-arc hybrid welding process with magnetohydrodynamics reinforcing is developed with an integration of laser-arc hybrid welding and laser welding with external electrical current.