Instability phenomena in the gas-metal arc welding self-regulation process

Arc instability is a very important determinant of weld quality. The instability behaviour of the gas-metal arc welding (GMAW) process is characterized by strong oscillations in arc length and current. This phenomenon was reported by Halmoy [1] for the short electrode extension and by Lebedev [2] for the long electrode extension, particularly as related to the arc voltage-current characteristics and the thermal inertia of the electrode wire respectively. However, with these studies, a complete theoretical explanation of the instability phenomena is lacking since the process is not modelled dynamically. In the present work, a model of the GMAW process is developed using an exact arc voltage characteristic. This model is used to study stability of the self-regulation process and to develop a simulation program that helps to understand the transient or dynamic nature of the GMAW process and relationships among current, electrode extension and contact tube-work distance (CTWD). The process is shown to exhibit instabilities at both long electrode extension and normal extension. The experimental results agree closely with both Lebedev's and Halmoy's results. Results obtained from simulation runs of the model were also experimentally confirmed by the present author, as reported in this study. In order to explain the concept of the instability phenomena, the metal transfer mode and the arc voltage-current characteristic were examined. Based on this examination, the conclusion of this study is that their combined effects lead to the oscillations in arc current and length.