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Correspondence between sound emissions generated in the GMA welding process and signals registered in the arc circuit

A fundamental bonding method for carbon and low-alloy steels is that of welding with consumable electrodes in the shield of an active gas (GMA). In the construction welding industry, this particular method is replacing the shielded-electrode manual welding operation in both the stationary work-stands, and in situ building site situations. Its undoubted advantages of high efficiency, the possibility of welding in all positions, low costs of equipment and filler materials, and the satisfactory forming of welds that are free from defects ensure that the method will find – in the foreseeable future – a permanent place in the industry. Among the many proposals for further evolution of welding by the GMA method, an important one – which is being introduced by the manufacturers of equipment – is that of the concept of integrating the welding process with the processes of quality control over both the welding itself and the welded joint. This is not only a direct consequence of the adoption of quality control systems, but also of the requirements of a technoorganisational nature which face the process of welding; one of the fundamental technological production methods used in the welded-construction, machine building, and other industries. The conduct of the process is subject to a certain degree of risk that is difficult to assess because even a correctly conducted welding operation does not automatically ensure that the required quality level of the produced joint will be attained. The control of the welding process which would guarantee the required level of both the process and joint qualities calls for the assembly of a number of items of information about both the operation and the joint. Examination of information will make it possible to estimate – for the assumed degree of probability – the likely quality of the joint, and to assess the stability of the welding process. It may also be possible to initiate a corrective procedure if any irregularity occurs. For the monitoring of the welding operation – with a view to gathering information – recorders with sensors are used. These will transform signature signals emitted by the process into the signals which can be conveniently recorded. In the majority of cases, the sensors in question will be located in the electric circuit of the welding arc. Only to a relatively low degree is use made, for this purpose, of those data carriers which since the beginning of the history of welding have been utilised by the welder, i.e. the visible arc radiation, and the appearance of the welding zone. These data carriers possess, however, a significant advantage. For instance, their sensors, similar to those used in the monitoring of the welding parameters, can be located outside the areas affected by the arc radiation, molten pool, fumes, gases, and other hazards which create difficult working conditions for the sensor. The tracking of the axis of a joint is often carried out by means of optical signal monitoring systems. This method calls, however, for the analysis of a large volume of information in real time while the current availability of sufficiently fast image algorithms still lags behind the rates at which welding processes are conducted. The acoustic signals emitted by the welding process also fail to be used in the monitoring systems because it is not, as yet, clear how to extract the information, which, after all, is so easily and intuitively made known to the welder.