Weld Pool Impedance for Pool Geometry Measurement: Stationary and Nonstationary Pools

Abstract

The most elusive quantity in describing weld pool geometry is the depth, since it cannot be directly observed; yet it is the most important quantity to be regulated during welding. This paper addresses the problem of depth feedback measurement for full penetration welds, where the objective is to completely melt the cross section. It has been demonstrated that the existence and size of a full penetration weld can be detected by measuring the mechanical impedance of the resulting weld pool. Previous work in modeling this phenomenon has been limited to stationary welds, and experiments have either used impractical measurement methods or have not provided conclusive results. In this paper, a model of pool motion is developed that applies to both the stationary and moving weld case, and the pool motion is detected directly from changes in the arc voltage. A description of pool motion is derived from an elliptical membrane model, and the total system transfer function, including arc and pool dynamics is derived. A series of experiments demonstrates that the pool motion can indeed be detected for the moving pool case. However, the exact determination of pool oscillation frequencies requires knowledge of the pool perimeter geometry, since the elliptical system has many closely spaced eigenvalues arising from both symmetric and antisymmetric mode shapes.