Overview of modern arc processes and their metal transfer methods in the case of GMA welding

Transcription

1 Overview of modern arc processes and their metal transfer methods in the case of GMA welding H. Cramer, D. Böhme, L. Baum, M. Dudziak German Welding Institute, SLV München, NL der GSI mbh, Germany Abstract The utilisation of modern, electronically clock-pulse-controlled welding devices with innovative digital process control and regulation techniques permits a heat input adapted to the joining task and optimised metal transfer methods in the GMA arc. CMT, coldarc, Cold Weld, STT, SpeedPulse, PCS, forcearc, Rapid Weld, SpeedArc, DeepARC etc. can frequently be read in the specialist literature as process names from various device manufacturers and designate modern arc processes and their metal transfer methods in the case of GMA welding in different power ranges. Due to the large number and diversity of the process names, even the well-versed users have difficulty in finding an overview and, above all, a classification and assignment of the new arc technologies for the previously wellknown arc types and metal transfer methods since the modified metal transfer methods have not yet been taken into consideration in standards and technical bulletins at the moment. In conjunction with the lecture, the present manuscript is intended to give an overview and possible classifications in an independent and neutral form. It also provides assistance with regard to the assignment of the new process names to the previously well-known arc types such as the short, pulsed and spray arcs. Rapid Weld, SpeedArc, DeepARC etc. and promise the users significantly better welding results and savings with regard to the fabrication by means of welding technology. Prozess variants and metal transfer methods on the basis of the short arc The arc process in which the metal is always transferred in the short circuit is designated as the GMA short arc, i.e. the arc is extinguished during the droplet transfer into the weld pool. In the case of the classical welding power sources (transformer, rectifier and choke), the dynamics of the welding process are determined by the adjustment of the electric circuit and by the design-related structure of the welding device. Due to changing boundary conditions (e.g. alteration in the arc length caused by wire feed irregularities), this may lead to disturbances with spatter. In particular, mains voltage fluctuations give rise to process irregularities. Nevertheless, good welding process sequences can be achieved by exerting an active influence on the choke properties. Introduction The general trend towards lightweight construction with complex thin-walled structures in conjunction with highstrength and ultrahigh-strength materials and heat-sensitive mixed joints, the pressure resulting from the global competition to organise the fabrication by means of welding technology in a more efficient and thus more economically viable way and the constantly rising quality demands on welded joints are setting ever more stringent requirements on the GMA welding process. Modern electronic welding devices with innovative, digital control and regulation concepts permit optimised and modified metal transfer methods in the GMA arc and thus offer new solution approaches for these challenges. Numerous manufacturers of GMA welding devices advertise these modified metal transfer methods on the basis of the short, pulsed and spray arcs with new process names such as CMT, coldarc, Cold Weld, STT, SpeedPulse, PCS, forcearc, Figure 1. Current/voltage/power course and metal transfer of a conventional short arc tima12 1

2 Because of their high switching (clock) frequency (up to over 100 khz) and the utilisation of very quick digital signal processors, modern clock-pulse-controlled welding devices with computer-controlled power sections permit optimised, event-oriented current/voltage regulation and thus the outstanding adaptation of the welding process to the joining task. Therefore, they can compensate for certain disturbances. Building upon this, a number of device manufacturers have redefined arc processes and have given them catchy process names such as CMT, coldarc, ColdMIG, Control Weld, STT etc. All these processes have one thing in common. They make use of the principle of metal transfer with short circuiting while exploiting individually adapted, mostly patented regulation strategies and thus avoid the spattering during the arc reignition to a great extent. According to Fig. 2, this new form of the "electronic" GMA short arc can be divided into two control principles in a simplified form: Short arc with a constant wire feed direction and the utilisation of trigger points in the current/voltage course. Short arc with wire electrode retraction during the reignition phase of the short arc with the aid of a quickly changing wire feed drive on the welding torch for the additional support of the droplet detachment. reignition of the arc is detected via another trigger threshold and the electrical power during the arc reignition is limited by a very quick drop in the welding current. Fig. 3 shows schematised current courses of triggered short arc variants with a constant wire feed direction using a wire electrode with continuous positive polarity (representative of Variant B on Fig. 2). Figure 2. Possible classification of the modern process variants and metal transfer methods on the basis of the short arc The use of additional trigger points exerts a defined influence on the current/voltage course. For example, the beginning of the short circuit is detected via a minimal voltage threshold. The current rise can be adjusted in this way. If the short circuit bridge is ripped open because of the increasing current, the quickly rising voltage after the Figure 3. Schematised current courses of modern short arc variants with the utilisation of trigger points in the process sequence (representative of Variant B on Fig. 2) The particular advantage of the GMA short arc with trigger points primarily relates to the fact that very good adaptation 2 June 14-15, 2012

3 of the process to the joining task is achieved in this way. The energy input and the pressure on the weld pool can be controlled in a better manner and, if necessary, can also be reduced in this way. The welding spatter can thus be avoided in most cases (Fig. 4). cycle (Variants C and E on Fig. 2). During the negative polarity, the arc literally envelops the end of the wire electrode, thus causing the formation of larger droplets at the end of the wire (Fig. 6). The heat input and the penetration effect on the component are consequently lower in the negative phase. a) Reduced power b) High power peak Figure 3. Schematised current courses of modern short arc variants with the utilisation of trigger points in the process sequence (representative of Variant B on Fig. 2) If additional trigger points are used in order to control a quickly changing wire feed, this provides direct mechanical support for the droplet detachment in the short circuit phase (Variant D on Fig. 2). The electrical energy required for the dissolution of the short circuit bridge can be reduced further in this way. The CMT process is the practical implementation of this sequence. Immediately after the occurrence of the electrical short circuit, the wire electrode is retracted as far as a stipulated value using a special wire feed on the welding torch and thus supports the droplet detachment. After the arc reignition, the wire feed direction is reversed once again and the wire electrode is fed towards the weld pool for another short arc cycle and droplet transfer operation (Fig. 5). In addition, the changing movement of the wire electrode leads to improved electrical contacting. The exact and reproducible forward and retraction movements of the wire electrode guarantee a high process stability and a constant arc length. Figure 6. Influence of the wire electrode polarity during GMA welding with alternating current Tables and Figures Fig. 7 shows a current/voltage course of a short arc variant using a wire electrode with intermittent negative polarity. Due to the infinitely variable negative polarity proportions, the heat input and the deposition rate can be adjusted to the joining task in a targeted way. Stromstärke [A] Spannung [V] Figure 5. Wire electrode movement and schematised current/voltage course of the CMT process (figure from Fronius) For the GMA welding of thin sheets and/or for the bridging of wider gaps during the welding, a few manufacturers offer short arc variants with alternating current proportions (wire electrode with intermittent negative polarity) in the short arc Figure 7. Schematised current/voltage course of a short arc variant using a wire electrode with intermittent negative polarity (figure from Cloos) Finally, Fig. 8 provides an overview and assignment of modern short arc designations of Variant B on Fig. 2 on the arc working range diagram. tima12 3

4 consequently with a higher pressure on the weld pool. However, the continuously acting arc leads to a somewhat higher heat input than with the AC short arc variants. Figure 8: Overview and assignment of modern short arcs of Variant B (Fig. 2) on the arc working range diagram Process variants and metal transfer methods on the basis of the pulsed arc The pulsed arc of modern electronic welding devices can also be controlled and regulated very much more precisely today because of the high switching (clock) frequencies. The essential pulsed arc parameters such as the level and duration of the background and pulsed currents, the pulsed current rise and drop rates as well as the pulse frequency can be adjusted infinitely. The very quick digital signal processors permit different regulation strategies and modulation types (I/I and U/I modulations or their combination) for a high process stability in the background and pulsed current phases and ensure low-spatter droplet transfer even in the case of pulsed arcs with very short settings. In order to raise the deposition efficiency, it is possible to program and implement current courses with (for example) pulses which do not detach any droplets and with subsequent metal transfer using short circuits or also hold times or delays in the pulsed current drop rate for reliable droplet detachment in the case of particular materials and/or for multidroplet metal transfer methods. The manufacturers advertise these special current programs using a wire electrode with continuous positive polarity and constant deposition efficiencies with names such as SpeedPulse, Speed Weld, Turbo-Pulse, WiseFusion etc. and, with their directionally stable arcs with very short settings in most cases, these programs extend the previously classical working range of the pulsed arc not only towards the short arc but also towards the spray arc (Variant B on Fig. 9). Similar to the case of a few short arc variants, alternating current proportions in the pulsed arc cycle are also utilised for the pulsed arc welding in the lower power range, thus making use of the already mentioned advantages of a wire electrode with intermittent negative polarity for the GMA welding of thin sheets and for a better gap bridging capacity (AC pulsed arc welding, Variant C on Fig. 9). Depending on the duration of the negative polarity, the deposition rate, the penetration effect and the heat input into the component can be controlled very well. Compared with the AC short arc variants, the droplets in the case of the AC pulsed arc are transferred with an additional accelerating force because of the pinch effect in the pulsed phase (a minimum current density must be exceeded for droplet transfer without any short circuits) and Figure 9. Possible classification of the modern process variants and metal transfer methods on the basis of the pulsed arc A certain special position is occupied by pulsed arc variants with periodically changing deposition efficiencies (Fig. 10). While periodically changing pulsed arc powers, i.e. alternating pulses with higher and lower energies ("high and low current phases"), have been successfully utilised for the MIG welding of aluminium materials with characteristic rippling on the weld surface for many years already (Variant D on Fig. 9), it has, in most cases until now, only been possible to implement periodically changing metal transfer 4 June 14-15, 2012

5 types such as short arc / pulsed arc by means of constant job changeovers with the aid of a robot controller. Figure 10. Overview and assignment of modern pulsed arcs with a periodically changing deposition efficiency on the arc working range diagram Modern electronic power sections with digital signal processors now permit nearly any combination possibilities and delay-free switching operations between various metal transfer types. The process names listed in Variant E on Fig. 9 are mostly periodically changing short and pulsed arcs in already prepared welding programs (characteristics). The energy-richer pulsed arc ("high current phase") is responsible for the sufficient heat input and the penetration and the subsequent short arc ("low current phase") for the intermediate cooling of the weld pool. These alternating combinations serve to improve the control of the weld pool especially for GMA welding out of position (PF, PE and PC). The duration of the short and pulsed arc cycles and, in the case of a few manufacturers, even the exact sequence (number) of the droplet transfer operations with and without short circuits can be altered variably (Fig. 11). The combined welding process can thus be optimally adapted to the joining task. Process variants and metal transfer methods on the basis of the spray arc Because of the high attainable deposition efficiencies, the spray arc is often utilised for the GMA welding of filler and cover passes in favourable welding positions on higher sheet thicknesses. The conventional spray arc is characterised by metal transfer with fine droplets and practically no short circuits. With regard to the parameter setting of the spray arc, the specialist literature and the practical welder training already draw attention to a spray arc with a short setting and a typically crackling arc noise in order to avoid any undercuts, any excessive magnetic arc blow especially at the end of the component and any excessive alloy burn-out. This arc noise is characteristic of droplet transfer methods with some short circuits. As a consequence of the high current density, the droplet necks (short circuit bridges) are already constricted extremely by the acting pinch forces so that the duration of the short circuits and the welding current rises associated with these are relatively short and hardly any firmly adherent welding spatter arises during the arc reignition. With the correctly set spray arcs of conventional rectifiers (step-switched or thyristorised), it has already been possible to obtain very good welding results in relation to the deposition efficiency, the penetration and the spatter avoidance. Because of the high regulation speed of computer-controlled power sections, the arc voltage of modern spray arc variants can be lowered a little further without any significant increase in the spattering. This results in very short, directionally stable spray arcs with a high plasma pressure on the molten pool. The spray arc literally burns in a molten pool crater so that any welding spatter which may arise is also collected in the weld pool in most cases (Fig. 12). Figure 11. Schematised current/voltage course of periodically changing short and pulsed arcs, in each case with ten droplet transfer operations without a short circuit and four to five droplet transfer operations with a short circuit a) Shortly before the short circuit bridging b) Shortly after the short circuit dissolution Figure 12. Typical molten pool crater and metal transfer methods of spray arcs with very short settings In the case of longerlasting, more massive short circuiting operations between the droplet or the droplet chain and the weld pool, the digital regulators intervene in an eventoriented way after the so-called trigger thresholds have been reached and limit the electrical energy during the arc reignition due to a very quick welding current drop (welding current smoothing). Those manufacturer-related regulation strategies for gentle, low-spatter short circuit dissolution tima12 5

6 described in Point 2 are often utilised in the case of the short spray arc variants as well. A few manufacturers superimpose a more or less higherfrequency pulsed current on the actual welding current or use a direct current with a certain harmonic content and thus promise an additional constriction of the spray arc and the formation of small-volume droplet necks and droplet chains for easier short circuit dissolution. Fig. 13 shows a selection of modern spray arc variants available on the market at the moment and their assignment to the well-known arc working ranges. The modern process variants and arc technologies are useful tools in order to raise the quality and productivity during GMA welding. However, they cannot alter the physical fundamentals and laws of the arc or of the droplet detachment or replace any external disturbing influences (e.g. deficient surfaces of the components and of the wire electrodes, excessive tolerances of the workpieces, of the weld preparation and during the torch manipulation, draughts etc.) or the corresponding training and qualification of the welders and installation operators. The users continue to be located in the "field of tension" between the lowest possible heat input (e.g. for material and/or distortion reasons) and a welded joint nevertheless without any lack of fusion. We thank all the device manufacturers for the provided information. References Figure 13. Overview and assignment of selected modern spray arc variants on the arc working range diagram If their spray arc variants are utilised, the manufacturers promise the users a significantly higher productivity, amongst other reasons due to the reduction in the weld volume and to the number of required welding passes and beads as a consequence of very acute weld preparation and bevel angles Summary and prospects [1] 3. Lichtbogenkolloquium des FA3 und des V2 des DVS, in der SLV München [2] Erfahrungsaustausch MSG-Schweißen "Die 'kalten' Verfahren", in der SLV München [3] Baum, L.: Der Schutzgasschweißer, Teil II: MIG-/MAG- Schweißen, 4. vollständig überarbeitete Auflage, DVS-Verlag GmbH, Düsseldorf 2007 [4] Schmidt, Klaus-Peter: Lichtbogenschweißen und -Löten: Modulationsarten bestimmen den Werkstoffübergang, den Lichtbogen, die Wärmeeinbringung und das Nahtaussehen, DVS- Jahrbuch 2006 [5] Platz, J., Wiegand, C.: MIG-MAG Schweißen und -Löten mit modernster Gerätetechnik, DVS-Berichte Band 267, DVS Media GmbH, Düsseldorf 2010 [6] Jaeschke, B.: Der wirtschaftliche MSG-Lichtbogenschweißprozess durch moderne Gerätetechnologien, DVS-Berichte Band 267, DVS Media GmbH, Düsseldorf 2010 [7] Budig, B.: EWM-forceArc - ein kraftvolles Werkzeug zum MIG- /MAG-Schweißen, Erstveröffentlichung DVS-Jahrbuch 2005 From the wide range of design possibilities offered by the GMA process because of the electronic control and regulation concepts of modern welding power sources and devices, only a selection of the modern process variants available on the market at the moment are introduced here. However, they already show very clearly with what high precision an arc with a consumable electrode can also be implemented. The device manufacturers offer these, in part, very complex process variants with a large number of variable parameters for the users in already prepared programs (characteristics) with relatively simple operability. The new generation of devices with power electronics contributes to the general improvement in the process stability from the arc ignition to the end crater filling. 6 June 14-15, 2012