Development of innovative diagnostics for characterization of material transfer and plasma properties in tungsten inert gas welding (TIG) and gas metal arc welding (GMAW)

摘要

In this paper two different techniques analyzing the welding process are described. First a new approach allowing non-intrusive measurement of the current density distribution in a current-carrying plasma is introduced. The measurement of the magnetic flux density over a fine mesh of locations around the plasma and the application of Ampere's law allow to cross sections out of a measurement volume which contains the current density distribution. Collecting different cross sections and solving a system of linear equations or applying reconstruction algorithms for computed tomography can yield the spatial distribution of current. The feasibility is shown for a simplified axisymmetric system. Therefore the experimental realization is described and an optical diagnostic for the validation of axial symmetry is introduced. Spatial current densities were measured for a 20 mm long argon arc with an overall current of 50 A. The current density distribution in z- and r-direction was in the range of 2e4 A/m~2 to 6e5 A/m~2 and 1e3 A/m~2 to 2e4 A/m~2, respectively. A way to determine the current density distribution applying the new approach without simplifications is discussed. In addition we present an adaptation of a drop oscillation technique that enables in situ measurements of thermophysical properties of an industrial pulsed gas metal arc welding (GMAW) process. Surface tension, viscosity, density and temperature were derived expanding the portfolio of existing methods and previously published measurements of surface tension in pulsed GMAW. Natural oscillations of pure liquid iron droplets are recorded during the material transfer with a high-speed camera. Image processing algorithms were employed to derive parameters such as oscillation frequencies and damping rates along different dimensions of the droplet. Accurate surface tension measurements were achieved incorporating the effect of temperature on density. Surface tension and viscosity of a sample droplet were 1.83 N/m and 2.9 mPa·s, respectively. The corresponding droplet temperature and density are 2,040 K and 6,830 kg/m~3, respectively.