Research On Physical Process Of Arc In Micro-plasma Welding Arc

The micro-plasma arc has the characteristics of high energy density, high restraint and good straightness, so it is especially suitable for the fine and high precision welding of ultra-thin fine structural parts. Arc, as a heat source for arc welding, its temperature and velocity distribution determine the movement of all particles in the plasma and temperature distribution of anode work-piece, which directly affect the microstructure and properties of welded joint. Therefore, simulation study on physical process of the arc in micro-plasma arc welding(MPAW) is put forward here, to provide theoretical guiding for the MPAW process.Firstly, according to the thermal-magnetic fluid kinetics, the numerical model of three-dimensional arc in micro plasma arc welding was established. The temperature field, flow field, electromagnetic field, and pressure of arc in MPAW with direct current were studied. The distribution of the axial and radial temperature of arc, fluid velocity of plasma, electromagnetic force, and arc pressure were obtained. The results show that from the tungsten rod end by 0.25 mm, the arc temperature reaches maximum, arc flow reaches stable in computation time reached 0.8 s, the electromagnetic force reaches the maximum at the nozzle outlet, and pressure reaches 40 Pa in stable anode.Because there are many parameters for the numerical simulation of arc in MPAW, the different setting of the parameters have great influence on the physical process of the arc. Therefore, the distribution of arc temperature, plasma flow velocity, and electromagnetic force were studied with different welding current, plasma gas flow, shielding gas flow, radius and angle of tungsten tip, welding distance height, and radius of nozzle. When tungsten rod radius of 0.03 mm, the arc axis at the center of the maximum temperature reached 15000 K, tungsten rod taper is 45 degrees, the radial highest temperature reaches 8000 K, ion gas flow rate of 0.6 L / min, central axis of maximum plasma velocity reaches 4 L / min, the nozzle diameter is 0.8 mm, the maximum velocity of the radial plasma reaches 6.5 L / min, when the welding current is a 3 A, the maximum electromagnetic force to 0.04 N.Secondly, based on the analysis of the physical process in the micro plasma arc welding with direct current, the dynamic physical processes of the arc with pulse current were studied. Because the pulse current varies periodic-alternately between base current and peak current, the dynamic process of temperature field, plasma flow field, and electromagnetic force and pressure of arc were studied, and the effects of different pulsed current parameters on the dynamic physical process of the arc in microplasma arc welding were analyzed. The results showed that the arc had certain shrinkage when the base current was changed to the peak current. The changes of the physical process of the arc with different pulse parameters were lagged behind the changes of the welding current. The flow field, temperature field, electromagnetic field and pressure of arc in MPAW had the feature of inertia.Finally, the temperature distribution characteristics of visible arc(the arc outside of nozzle) in micro plasma arc welding had high agreement with distribution results by using spectrum detection and image processing of high-speed photography, respectively.