Stability is a key indicator of the efficiency of the gas metal arc welding (GMAW) operation,and it is linked to other quality indicators like spattering and weld formation. This thesis work describes methods of assessing arc stability for selected GMAW conditions. It focuses on stability characterisation and defines the relationships between process stability and influencing factors.

Welding tests were performed using two selected transfer modes being the spray and the pulse transfer modes to characterize stability and develop processes further for better overall performance. Short circuit was not included because it exceeds the scope of thesis even though it is also of interest for ESAB to characterize stability using this mode. For the spray transfer, stability characterization was carried out by analyzing voltage disturbances (explosions) in the form of voltage peaks (negative and positive) since the explosions occurring in the arc corresponded to the voltage fluctuations during welding. These tests were performed on a sandblasted carbon steel plate, using carbon steel wire in diameter 1,0 mm, gas mixture of 82%CO22/18%Ar, and with three different voltage values (29V, 31,5V and 32,5V) and inductance control settings (0, 60 and 95). This scope of parameters was selected in order to simulate certain range of stability behavior, so it was possible to analyze the factors correlated to stability, their relevance in influencing stability, and consequently to define a calculation algorithm for the determination of stability.Also, stability was calculated and scored based on two scoring indices. The first was stability scoring based on performance character (spatter presence, hardness of arc, smoothness of process) The other was stability scoring based on regularity of the process (repeatability and steadiness of electrical parameters, variations of process disturbances, uniformity of the process). These two different stability characteristics are recognized in industry within ESAB applications, and the process regularity represents the major proportion of this characterisation.

For the pulse transfer, stability characterization was carried out by analyzing and comparing two different current waveforms in terms of evaluating effects on stability. The linear waveform, which is the conventional type of waveform, was compared to an exponential waveform (with optimised droplet detachment). Results obtained from spray arc analyses showed that stability has certain indicators that are measurable by signal and image processing, and these were further analysed. Voltage disturbances were correlated to explosions observable during the process. Actual values of positive and negative voltage peaks, voltage amplitudes, and their average values and standard deviations in selected analysed time showed to be relevant indicators to characterise stability of the process. Variations of indicated voltage signals related to explosions showed to have complex patterns in terms of their waveforms. So, all these were analysed step by step and consequently selected for characterisation (calculation of stability) in the welding conditions that were selected in the scope of this project. Exact determination of stability formula was established, and this can be further developed in ESAB conditions in terms of automating the calculation with possibility of variable setting of calculated factors and ranges for specific analysed welding conditions.

The results for the pulse arc confirmed that the exponential waveform when compared to the linear waveform showed better stability conditions. It also showed that an important factor is the duration of the base current time and its standard deviation over selected analysed time, which optimised the droplet detachment mechanism.I

n conclusion, this approach of characterizing stability has been shown to be successful in terms of having a reasonable capacity to evaluate distinct elements that are acting in the process and determining overall stability.