This paper addresses the problems involved in the automatic monitoring of the weld quality produced by robotized short-arc welding. A simple statistical change detection algorithm for the weld quality, the repeated Sequential Probability Ratio Test (SPRT), was used. The algorithm may similarly be viewed as a cumulative sum (CUSUM) type test, and is well-suited to detecting sudden minor changes in the monitored test statistic. The test statistic is based on the variance of the weld voltage, wherein it will be shown that the variance decreases when the welding process is not operating under optimal conditions. The performance of the algorithm is assessed through the use of experimental data. The results obtained from the algorithm show that it is possible to detect changes in weld quality automatically and on-line.

On-line quality monitoring in short-circuit gas metal arc welding

The dynamic characteristics of the globular and spray metal transfer modes are simulated by adopting the Volume of Fluid (VOF) method. The electromagnetic force due to the welding current is included in the formulation of the VOF algorithm. The effect of the welding arc is considered under the assumptions of a uniform and linear current density on the pendant drop surface. The free surface profiles, pressure and velocity distributions within the drop are calculated numerically. It is found that axial flow and radial pinch force promote drop detachment, and the taper of the elongated molten drop in the spray mode is maintained by the electromagnetic force. The current density distribution on the drop surface has significant effects on the molten drop profile, detaching drop volume and taper formation at the molten tip. Predicted results are in reasonably good agreement with the experimental data.

Dynamic simulation of metal transfer in GMAW, Part 1: Globular and spray transfer modes

On-line quality control in automated welding operations is an important factor contributing to higher productivity, lower costs and greater reliability of the welded components. However, the development of on-line inspection techniques and feedback control is still in its infancy. Sound signature produced by GMAW contains information about the behavior of the arc column, the molten metal and the metal transfer mode. Manual welders rely on sound signature as important feedback for production of good welds. High-speed data acquisition and computer-aided analysis of the sound signature may indicate conditions that generate weld defects. This paper presents an experimental investigation that attempts to establish correlations between sound signature, welding parameters and weld transfer mode with a view toward detection of weld defects. The results indicate that the sound signature is a usable, practical information source for process-linked quality control of welds.

An investigation of gas metal arc welding sound signature for on-line quality control

Compared with using semi-automatic gas shielded arc welding, using automatic TANDEM twin wire welding and twin wire gas metal arc welding (GMAW) to weld Q690 steel, a low-alloy high-strength structural steel used in the hydraulic support in the fully-mechanized mining face, the welding speed, deposition rate, production environment and welding quality can be obviously improved. Compared with GMAW twin wire welding, a refined micro-structure in the weld and heat-affected zone (HAZ), narrow HAZ and improved joint strength were achieved with TANDEM on Q690. Also, due to the push-pull pulsed way in TANDEM welding, the droplet transfer, distribution on heat flow and interaction between two arcs were completely different from those in GMAW twin wire system. The heat input of TANDEM is only about 76.6% of GMAW, and correspondingly, the welding speed and welding seam can be obviously improved. The complete oscillation caused by TANDEM pulsed current occurred in the welding pool, which refined the grains in the microstructure. The results show that TANDEM twin wire welding is very suitable in the welding of Q690 used in the hydraulic support.

TANDEM and GMAW Twin Wire Welding of Q690 Steel Used in Hydraulic Support

Improved sensors are needed to enable automatic GMAW to accommodate a greater range of disturbing inputs. A low-cost, non-intrusive sensing technique, known as through-arc sensing, involves collecting and analyzing welding current and voltage signals. Through-arc sensing can be used to detect various GMAW quality indicators. It is shown that through-arc sensing can detect arc-start quality, steady-state arc stability, and the mode of metal transfer. Furthermore, the technique shows promise for detecting the onset of GMAW process disturbances including variations in tip-to-work distance, insufficient shielding gas coverage, electrode feeding problems, joint fit-up problems, and contact tip wear. Additional work is needed to refine and implement these techniques into a production-worthy system.

"Through-arc" process monitoring: techniques for control of automated gas metal arc welding

Metal droplet transfer in flux cored arc welding (FCAW) was studied using electrical arc signals and droplets collected from the welding process. A large number of metal droplets from the FCAW experiments was collected. According to the size distribution of the droplets, several metal transfer modes could be identified amongst which spray transfer predominated. The electrical arc signals, in particular, voltage, were processed using the fast Fourier transform (FFT) technique. Characteristic spectral frequencies corresponding to different metal transfer modes were identified. The size distribution of the collected droplets correlated extremely well with these characteristic frequencies. The electrode melt rate, calculated using the characteristic frequencies identified from the FFT analysis, agreed closely with the measured melt rate. Results from the arc signal analysis and the FFT analysis showed that both arc voltage fluctuations (Δu) and characteristic frequencies of the FFT spectra were adequate to distinguish the different kinds of metal transfer modes in FCAW. Metal transfer mode maps, constructed using the two sets of results, were used to determine the optimal parameters for E71T-1, 1/16-in.-diameter electrode, and Ar-25%CO 2 shielding gas.

S. Liu

Papers

Flux cored arc welding : arc signals, processing and metal transfer characterization