Process assurance and monitoring in GMA welding Heinz Hackl, Wels INTRODUCTION Today s ever-more stringent demands regarding weldment quality and life-span, intense cost pressure, the rapid propagation of the DIN EN ISO 9000 Standard and tougher product liability legislation are all new challenges which welding firms are having to face. The increasing mechanisation and automation taking place in this sector lends particularly great importance to process-integrated quality assurance, which is achieved by on-line monitoring of the relevant welding parameters. POWER SOURCE Unregulated systems Power sources of this type are still widely used. The typical representative of this generation is the tapswitched MIG/MAG power source (Fig. 1). To adjust the operating point, the primary transformer tap is switched over. The desired operating point is then set in accordance with the transformation ratio of the welding transformer. Fig. 1: Tap-switched MAG machine The welding characteristics of this type of system are determined by the design of the welding transformer and the output inductance. This means that the rate of current rise for the short-circuit treatment can only be changed to one or another of the available steps. A disadvantage of unregulated systems is that every fluctuation in mains voltage has a direct influence upon the secondary side of the welding circuit. This means that the welding current and voltage change in proportion to the mains-voltage fluctuation occurring on the primary side. Changes in the deposition rate are the consequence. Fig. 2 shows the displacement of the operating point resulting from naturally occurring fluctuations in the mains voltage.
Fig 2: Displacement of operating point as a result of fluctuation in mains voltage Systems with closed feedback control circuit The increasing use of electronics in the field of power-source technology has prompted the development of regulated systems that keep the welding current and/or the welding voltage constant, irrespective of changes in the mains voltage and in the lengths of the mains cables. Fig. 3 shows the typical representative of this generation of power sources - i.e. an inverter power source (e.g. FRONIUS TPS 330/450). The central element is the closed feedback control circuit, consisting of sensors for welding current and welding voltage. The actual values from the welding process are constantly compared with the preselected command values (welding parameters), and any deviations are immediately corrected via the actuator. In this way, the fundamental precondition for the replicability of welding results is realised. As well as these advantages, inverter power sources also have the advantage that the welding characteristics do not depend upon the design of the welding transformer and output inductance. Fig 3: Block diagram of a modern inverter power source This opens up unprecedented scope for using electronics to influence the quality of the weld seam and of the welding process.
By having a large number of continuously adjustable parameters (around 30 parameters), it is possible to improve droplet detachment in pulsed-arc welding, and the short-circuit treatment in dip-transfer welding, for a wide spectrum of filler metals. However, these additional parameters would make the power sources very much more difficult to operate and would mean that only a handful of experts would be able to use them. Fig. 4: Fronius GMA machine (TPS 330/450) with process-integrated quality assurance, used here in robot welding By providing pre-programmed parameters for any combination of wire and shielding gas, synergic (or single-dial ) operation makes the machine very easy for the welder to operate. In effect, the job of optimising the parameters for many different base and filler metals and shielding gases is done for the user by the equipment manufacturer. These empirical results are stored in an EPROM - an electronic memory module - in the form of a databank. The user selects the filler metal directly on the power source, and the integrated microprocessor enables the desired power to be selected on a continuous scale from minimum through to maximum. In order for there to be consistently high and reproducible weld quality, the machine technology must fulfil the following requirements: - Replicability of the parameters for every single weld seam - Continuous monitoring of these parameters during welding Q-MASTER These requirements are fulfilled by storing the optimised operating points (machine settings) for this particular application in the power source itself. The robot or automatic welder retrieves these operating points via either an analogue or a digital interface. Only authorised persons are able to change the machine settings, as access privileges can be defined with the aid of a key-switch. In addition, those parameters having a particularly great influence on the welding process because they - are subject to short-term changes - cause welding defects - affect mechanical-technological properties are all continuously monitored.
Fig. 5: Adjustable limit values in the Q-Master menu This is mainly concerned with welding current, welding voltage and the wirefeed speed. The evaluation of the electric signals permits conclusions to be drawn as to the metal transfer, the melting behaviour and the process stability. In turn, these conclusions enable inferences to be made as to the quality of the welded joint. The maximum permitted deviations are defined for the parameters to be monitored (Fig. 5), and are stored in the power source together with the respective operating point. The internal microprocessor of the power source monitors these parameters on-line and reports on any limit violations during the welding process that persist for longer than a defined time. The algorithm for calculating limit violations takes account not only of the duration but also of the extent and the frequency of the violations. Fig. 6 shows a graphic representation of the monitoring function. Fig. 6: Warning limits and limit signals Many of the welding-data documentation systems currently available on the market work with an external PC and an interface to the power source. The Q-Master function integrated within the Fronius TPS 330/450 GMA machines logs, monitors and documents the welding parameters actually inside the power source (see Fig. 7).
Fig. 7: QA system for Fronius TPS 330/450 GMA machines Among the advantages of this system are: - High operational reliability, as there is no additional external cabling - Easy to use, thanks to graphic menu-driven screen (see also Figs. 4 and 5) - DOS-compatible 3.5 diskette drive for data storage - Operating points can be transferred by diskette to several different power sources The welding data can either be printed out on paper (via RS 232 interface) or stored on diskette via the integral disk drive. The facility for saving data to a low-cost mass storage medium opens up the convenience of processing and archiving the data in a database on an IBM-compatible personal computer. Q-VISION The PC software is marketed under the name of Q-Vision. Developed by Fronius International GmbH, it runs under Windows 3.1. or higher. The data gathered during the welding process can be presented either in tabular or graphic (i.e. diagrammatic) form. Zoom and selection functions help the user to select the most suitable presentation format on the screen or in the print-out (Figs. 8, 10). Fig. 8: Graphic representation of weld-seam data Extensive search criteria are provided, enabling the user to retrieve information on weldments from the databank quickly and easily.
The welding engineer will find the Q-Vision software package a most useful data-analysis tool - not least because of the help it gives in determining the most suitable limit values for the Q-Master function. Q-Vision also analyses the consumption of e.g. shielding gas, filler metal and electricity, or the machine utilisation factor (see Fig. 9). These data provide a valuable basis for costing. As well as these analysis functions, it is also possible to compute the energy input into the base metal per unit length. Fig. 9: Q-Vision: Consumption menu SUMMARY With its Q-Master function and the Q-Vision software, Fronius offer a highly practical tool for comprehensive process-integrated quality assurance of GMA welding performed with the TPS 330/450 power sources. The integration of the logging, monitoring and documentation functions inside the power source makes the system easy to operate and gives it a high degree of operational safety. The Q-Vision software package helps the welding engineer to evaluate and archive welding data on a PC. Fig. 10: Weld seam data in tabular form