FASTER WITH TWO WIRE ELECTRODES METAL INERT-GAS WELDING OF ALUMINIUM MATERIALS

Transcription

1 FASTER WITH TWO WIRE ELECTRODES METAL INERT-GAS WELDING OF ALUMINIUM MATERIALS Metal active-gas welding using two wire electrodes has opened up new scope for boosting the efficiency of this version of the process. Following initial successes with simultaneous melt-off from two wire electrodes on unalloyed steel, efforts are now being focused on transferring these results to MIG welding of aluminium materials. However, the first trials with twin-wire welding (N.B. the various versions of MAG multi-wire welding are presented in [1] ) failed to deliver the expected gains in welding speed. The small length of the two arcs repeatedly led to short circuits between one or other wire electrode and the weld pool. This also caused arc extinction on the second wire electrode. The high current density which then occurred on the first wire electrode very quickly broke the short circuit. The result was a large amount of weld spatter and instability of both arcs owing to the wide fluctuations in the length of the arc. Although both the spattering and the instability were reduced when the arc length was increased, this also decreased the welding speeds. For this reason, an entirely different machine concept was used in a new series of trials. Figure 1 Principle of metal inert-gas welding with two electrically insulated wire electrodes and power-source synchronisation REVISED CONCEPT Instead of using a shared contact tube for both wire electrodes, the tandem technique (i.e. of having two insulated contact tubes under a shared shielding-gas atmosphere) was exploited here. (Fig.1) In this variant of the process, each of the power sources had its own control and regulation system, an independently controllable wirefeed unit and a menu-driven graphic user environment. The synchronisation device between the two power sources made it possible to time the instants of metal transfer from each of the wire electrodes during pulsed-arc welding in such a way (Fig. 2) that there was no impairment by magnetic arc-blow effect.

2 Figure 2 Chronologically offset amperage curves on the wire electrodes during pulsed-arc welding Given optimal parameter adjustment (N.B. the power sources permit continuous adjustment of a large number of parameters), the pulsed arc delivers short-circuit-free, low-spatter metal transfer in which one droplet of weld-filler per pulse is shed from the wire electrode and into the weld pool. This pulsing technology enables a virtually constant size of droplet to be set, regardless of the arc power. This is particularly important with regard to magnesium burn-off, which can thus be kept more or less constant over the entire power range. In metal inert-gas welding, there is only limited scope for high arc power to be translated into higher welding speeds. The reason is that when the power is increased, the arc pressure rises very rapidly, which makes the weld pool difficult to control. With a 1.2 mm thick AlMg wire electrode, the borderline beyond which weld-pool control becomes critical will be found between approx. 320 and 350 A at a wirefeed speed of 20 to 22 m/min. This clearly shows the advantage that results from having separately controllable metal transfers when two wire electrodes are being melted off into one shared weld pool namely, that the lengths of each of the arcs can be kept short. When the arcs are short, the weld pool remains narrow. Owing to the longer weld pool that is produced by the consecutively placed arcs, a very considerable part of the available energy can be turned into welding speed. As a rule, a slightly higher power is set for the leading arc. The cold base metal is thus thoroughly melted and exact fusion takes place into the root. The weld-metal deposit from the second electrode fills the weld pool. Also, the trailing arc prolongs the weld-pool degasification time, lessening the pore-formation sensitivity. With electrically insulated contact tubes, precisely controlled current distribution can be achieved across both wire electrodes. The uniform droplet size which results from the exactly predetermined instant of detachment of the filler metal leads to low-spatter weld seams with a high degree of process stability. This is clearly demonstrated by the high-speed photos of the metal transfer shown in Fig. 3. Figure 3 Controlled droplet transfer with synchronised AlSi5 wire electrodes (diam. 1.2 mm)

3 EQUIPMENT TECHNOLOGY For the welding operations described below, field-proven inverter power sources were used (Fig. 4). Together, they deliver a welding amperage of 900 A at 100% duty cycle. Integrated in each power source are quality monitoring systems, a diskette drive for welding-data documentation and an interface for communicating with a welding robot. Figure 4 Co-ordinated inverter power sources for metal-inert-gas welding of aluminium materials with two wire electrodes In this connection, it should be noted that welding with two wire electrodes in a common weld pool is only possible in a fully mechanised configuration. If the welding torch were to be guided by hand, it would not be possible to keep it positioned precisely enough in relation to the weld seam not to mention the acute strains that would be placed on the welder by the high welding speed and the considerable heat radiation emanating from the workpiece. For this reason, the torch, too (see Fig. 5) must also be built to allow for the high deposition rate which is why it has an especially powerful cooling system. This ensures efficient cooling of both the gas nozzle and the contact tubes. Being on the outside, the connections for coolant forward and return flow on the gas nozzle can be designed without any O-ring seals (which risk getting damaged in the course of time), as even the tiniest quantities of leaking moisture may cause pores to be formed in the weld deposit. Figure 5 Tip of the welding torch with two insulated contact tubes and a cooling system for both the gas nozzle and the contact tubes

4 One factor having a major influence on the transfer of the welding current into the wire electrode is the design of the wirefeed mechanism. A special arrangement is used here to lead the wire into the contact tube in such a way that it ensures a defined forced contact. This makes for troublefree current transfer to the wire electrode. Being a good thermal conductor, aluminium tends to encourage undesirable lack of fusion at the start of welding. To counteract this, a function supported by the power sources calls up a higher welding power during the start of the welding operation. This means that the base metal starts to be melted as early as in the ignition phase. Once there has been sufficient heat input into the weld pool, the welding power is decreased to its nominal level. Towards the end of the seam, there is a danger of the weld pool collapsing or dropping through as a result of the heat running ahead in the workpiece. This is counteracted by reducing the welding power to a lower value for filling the crater. EXAMPLES OF APPLICATIONS IN PRACTICE The variant of the GMA welding process outlined here for aluminium materials (i.e. using two wire electrodes and separate contact tubes) is employed to give but one example in the automobile industry and its component suppliers. In the example shown in Fig. 6 a car wheel-rim made of AlMg4.5Mn the weld was executed under argon at a speed of 130 cm/min. This low-spatter weld has a fine appearance which renders any post-weld machining unnecessary. The fillet weld joining the two halves of the rim shows sufficient penetration and notch-free side-wall fusion junctions, as may be seen from the transverse weld sections. Figure 6 Seam welded at 130 cm/min on a wheel rim for a passenger car The facility for melting two wire electrodes in one torch also opens up scope for rationalising the production of weldments in the railway rolling-stock engineering, shipbuilding, machinery, boilermaking and apparatus construction industries. Fig. 7 shows a completely mechanised application in the fabrication of railway rolling stock. Two torches are being used simultaneously here to weld two seams at once. Figure 7 Two torches in simultaneous use in the fabrication of railway rolling stock

5 Further examples of joins executed under argon shielding gas with 1.2 mm diam. wire electrodes are shown in Figs. 8 and 9. The transverse section of a fillet weld executed in the horizontal position (Fig. 8) demonstrates that even a 2 mm wide air-gap between the 6 mm plates can be dealt with satisfactorily. AlMg5 wire electrodes were used as the filler for the AlMg3 base metal. The trial welds shown in Fig. 9 were executed at a welding speed of 200 cm/min on 2.3 mm thick extruded sections made of AlMgSi0.7. Both in the gravity and horizontal-vertical positions, the edges of the base metal were sufficiently fused. In these particular welds, it was not necessary for there to be fusion all the way through to the fixed-on weldpool backing bar. For this reason, the formation of the root areas may not be regarded as fusion defects. Figure 8 Transverse section through a lapped fillet joint with a 2 mm air-gap, welded at 100 cm/min in the horizontal position Figure 9 Square butt joints welded on AlMgSi0.7 extruded sections at 200 cm/min in the gravity position (left) and horizontal-vertical position (right). LITERATURE: [1] R. Killing: MAG multi-wire welding how does it work? (Original title: Das MAG- Mehrdrahtschweissen Wie funktioniert es? ). In Praktiker 49 (1997), II.6, p.243