SpeedPulse a productivity and efficiency increasing further development of MSG- Pulse welding

Transcription

1 SpeedPulse a productivity and efficiency increasing further development of MSG- Pulse welding The supplementation of metal shielding gas welding by pulse arc process is well proven, accepted and used in many applications, bearing however due to applicationand quality advantages also certain performance disadvantages. That is why for certain applications, such as the processing of steel, still the classical MSG process is dominating. The novel SpeedPulse- arc presents an extended and improved process variation of pulse arc welding. The most significant difference to conventional one droplet per pulse philosophy is the precise controlled trailing of an additional secondary spray arclike material transition following a pulsed primary lead droplet. This results in many practical advantages such as increased welding speed, improved seam quality, deeper penetration and increased ergonomics. To adapt to the many operation tasks various different work processes are applied, MSG welding procedure (that is according to shielding gas in use metal inert gas, abbr. MIG welding, and metal active gas welding, abbr. MAG welding). Their typing is done according to the characteristics of the forming arc, picture 1. In the lower performance range, meaning at low currents and voltages, there is so-called short arc. Characteristics for the short arc is relatively low energy input and regular short circuits with moderate spatter formation. Whereas in the upper performance range the welding wire is melted without any contact to the workpiece and transforms fine droply to the workpiece. This process is without any short circuits and thus low in spatter. In between there is the transitional arc which is quite distinct with mixed gases. In this range occur more high-energy short arc reactions and accordingly numerous spatter which worsen the result. Furthermore there are still more process variations in the upper performance range, such as working with rotating arcs but we won t go into further detail here. The Pulse arc procedure In practical applications boundary conditions of the welding process quite often force work in the range of the transitional arc. To avoid environmental disadvantages the controlled arc was already developed in the 1960 s and since then continuously in numerous variations refined. Thereby the welding current in form of consecutive pulses between high and low levels vary. During the high current phase it exceeds the critical current to spray arc clearly. Due to the Electro magnetic pinch effect 1) one drop detaches from the melting wire electrode. The basic approach at pulse arc is that ideally one droplet per pulse from the melting wire electrode goes spatter-free over into the weld pool, picture 2. However, there are special forms such as more pulses per drop. This conformation occurs regularly for example during the start phase of the (still) not tuned pulse arc welding process, but is usually unnoticed by the operator. There are also versions where inter- alt. additional pulses are integrated, which do not detach material from the electrode but should cause special effects. Furthermore there

2 are approaches to produce multiple droplets per pulse. This is the case with the intermediate spray arc, which is supposed to cause certain advantageous effects such as oscillations of the weld pool for improvement of outgassing the melt and to influence the chryztallisation. In general the tendency of one droplet per pulse has mostly been prevailed, not least because it is precisely controllable due to suitable pulse parameter and therefor adaptable in very well fulfilling various demands regarding material, shielding gas, etc. As from the output range, the pulse arc completely covers the range of the classical transitional arc. picture 3, in the lower cold performance it competes with the short arc due to the almost spatter-free performance and in the upper performance range sufficient melting performances are largely obtainable. The characteristic advantages of this procedure are amongst others a high seam quality, little rework and improved arc controllability. This is the reason for conventional pulse welding being established as advantageous classical MSG-welding despite of the more expensive basic investment for a pulse-welding unit especially in the aluminium and stainless steel range. Performance limits due to single droplets The qualitative advantage of an almost non-spatter welding process stands in contrast to the disadvantage of a limited melting performance and thus working speed. Because the wire is not melted continuously but only drop by drop, the targeted melting performance at conventional pulse arc is capped". For each given diameter there is a top pulse frequency from where the time between the pulses is no longer sufficient to differentiate between controlled or spray-like arc uncontrolled drop detachment due to a sufficient low current level. The process degenerates but does not go over into a clean, pure spray arc. The welder says that the wire is at its limit. Depending on application, the user has to decide at conventional pulse arc if the low spatter qualitative advantage countervails the disadvantage of slow working speed. On the basis of this history numerous companies still prefer the classical MSG process, especially for joining carbon steels. SpeedPulse facilitates more melting efficiency Due to the recent further development of pulse arc procedures this borderline could be shifted upwards in the meantime. The developed and implemented procedure SpeedPulse by Lorch was first introduced to the professional trade public in 2008 at the "Euroblech 2008" exhibition. Compared to the conventional pulse procedure the increase in performance is reached due to the transfer of more droplets per pulse. This takes place because firstly a higher current pulse detaches a primary droplet, the so-called lead droplet, from the wire electrode. Due to the detaching process molten material will be removed from the wire electrode. Thus giving the basic approach for a normal pulse arc. Contrary to the conventional pulse arc, the lead droplet is followed by a controlled secondary spray arc-like workpiece transition, picture 4. This is only temporarily distinct because it is controlled ended to substantially maintain the characteristic of a pulsed arc. The whole procedure may be best described descriptively with a further material is seemingly pulled behind, picture 5.

3 Physical effects with SpeedPulse Immediately after interrupting the brigade between wire electrode and primary lead droplet, the surface tension has not yet drawn together the remaining part of molten material to spherical segments. Due to the actual reduced diameter this section is for a short time in the spray arc range, whereby more material is detaching. To these acting forces and contributory factors belongs also the pinch effect 1). This secondary detachment is also supported by inertial effects because the molten remaining material is being speeded up before detachment of the lead droplet towards weld pool. By this the SpeedPulse arc obtains a higher melting performance and allows basically a faster wire feed speed, which results directly in a faster welding speed. Basics for this a highly concentrated arc generating a deep penetration, picture 6. This ensures a safe root capture even at higher welding speeds. Furthermore can the arc at SpeedPulse be kept very low reducing thus the risk of undercuts. The combination of these positive features allow during practice a faster welding but keeping all application- and quality benefits of the pulse process. Alternatively may a lower current at unchanged welding speed and given a-value be used and thus reducing the energy input onto the workpiece. From the then colder welding process result various positive effects such as i.e. less distortion of the workpiece. Procedure s performance range The SpeedPulse procedure has firstly been tested in various performance ranges for unalloyed steel. From low to medium and up to high performance ranges the effects of additional secondary material transition can be increased further, picture 7. The inital serving wire feed speed of the conventional pulse arc process (and thus welding speed) can be increased further without having to rise the pulse frequency. Within the line of tests executed on a straight-line travel carriage with a Lorch S8 SpeedPulse it was possible with wire electrodes G3Si1 having 1.2mm diameter and using 82% Ar + 18% CO 2 shielding gas, to increase the wire feed speed in the horizontal fillet weld (welding position PB) from 13.5m/min to 20m/min. This equals an increase of 48%. The characteristic positive features of pulse arc have been retained in full. Experience from operational use show that at manual torch guidance, depending on the individual welder s ability, increases of up to 35% are feasible. Reduction of input per length in the upper performance range Measurements have been taken for the study of relation of melting efficiency and input of electrical performance, whereby the electrical arc s performance has been exactly identified (as intermediate multiplication of the measured arc current with measured arc voltage in sufficient small time intervals), picture 8. On the basis of high pulsed currents would a particularly performance calculation with the results of a normal average value or effective value readings of power and voltage lead to incorrect values. The average values provide too small performance values and the effective value reading too large performance values especially in the lower range. The increase of melting performance for wire electrodes G3Si1 with a 1.2 mm diameter and 82% Ar + 18% CO 2 shielding gas begins the increase of melting performance by SpeedPulse compared to conventional pulse welding approximately from the upper half of the usable performance range and is getting more distinct when increasing electrical performance. Partly the increase is due to an improved utilization of the input of electrical energy for melting the wire electrode. It is recognizable that i.e. at an electric performance of approx. 10 kw the con-

4 ventional pulse arc melts about 10m/min wire whereas the SpeedPulse does already 11m/min, thus already an increase of 10%. Inversely it can be concluded that for a required melting speed of 12m/min a conventional pulse arc needs 13 kw, whereas SpeedPulse arc doesn t need more than 10,5 kw, thus about 20% less electrical performance. The positive effects tend to pick up at increasing performance. On the one hand this leads to an advantageous reduced input of energy per unit length whilst welding, on the other hand is power consumption reduced. This statement is supported by practical experiences from applying operating businesses, where after changing the welding process from conventional pulse welding to SpeedPulse noticeable lower temper colours by welding stainless steel became apparent. Additional significant improvements are relating to the distortion of workpieces and the attached straightening rework. Ergonomic optimization by SpeedPulse In comparison to the classical MSG welding working with the SpeedPulse procedure tends to be more simple. Already purely from a visual point of view the arc appears more pointed and concentrated thus making the higher wire feed and welding speeds well manageable. In practical use this partly called needle effect phenomenon, picture 9, as a rule leads to an automatically faster welding operation because it just runs well. After first purely subjective statements of applicants that the SpeedPulse arc would particularly sound more pleasant when working with steel compared to conventional pulse, comparable sound measuring have been carried out. Under laboratory conditions (wire electrode G3Si1, diameter 1.2 mm, shielding gas 82% Ar + 18% CO 2, welding position PB, 1 m microphone distance) it was found that the acoustic emission during welding with SpeedPulse was approximately 10 db(a) less compared to conventional pulse welding. This corresponds in human perception to halving the noise load (physically already a reduction of 6 db(a) leads to a halving of sound intensity). Even if in practical use strong different conditions for arc s sound generation, propagation of sound at the workplace and exposition of humans are given, this SpeedPulse feature can be evaluated positively also from the occupational safety point of view. Advantages in practical use Especially advantageous are the SpeedPulse features when welding steel and stainless steel. For the welder it becomes faster, more safe and simpler whereby all existing advantages of pulse arcs are maintained. The result is a highly efficient, easy to manage welding process. Also the processing of aluminium profits from the special arc characteristic of the SpeedPulse, although not to the same extent. In comparison to classical MSG welding with short-, transitional- and spray arc covers the SpeedPulse arc a more comprehensive performance range than the conventional pulse arc and thus creating in the upper power range more application possibilities for pulse welding technology. Particularly to emphasize in this connection is the improved use at end craters. In the past, if it was to be filled especially by working in range of spray arc with higher performance the power had to be gradually lowered whereby necessarily the ranges of transitional arc with its spatter down to short arc had to be passed. With the procedure innovation by Lorch this is now possible without any spatter because SpeedPulse arc characteristics allow to pass the necessary performance range almost spatter free, picture 10. Positively noticeable is the increased performance range of the new procedure also in such cases where the performance of conventional pulse arcs is just not sufficient, so

5 that conventional MSG power sources had to be used furthermore. Due to industrial workflow a once set wire diameter can not be constantly changed thus welding in this range is done in spray arc and partly in the with spatter associated transitional arc. Especially here in all performance ranges allows the transitional and almost spatter free SpeedPulse arc, besides the speed increase, more and partly essential increase in productivity by reducing alt. complete avoidance of elaborately rework. Availability The performance versions of the new S SpeedPulse series by Lorch range from 320 to 500 A. The S SpeedPulse is available as mobile unit, compact unit or with separate wire feeder. The synergic SpeedPulse process guides for steel, stainless steel and aluminium are serially integrated in all models and quickly and easily accessible by the operator panel. Dr.-Ing. Birger Jaeschke, Auenwald,/Germany and Dipl.-Ing. Klaus Vollrath, Aarwangen/Switzerland 1) The pinch effect is caused by the interaction of the own magnetic field of a current passed conductor with the moving carriers within it. At sufficient size and exposure time this leads to transport of meltable metal from the wire electrode away in the direction of the widening arc. The so-called pinch power increases approximately squarely proportional to the current and decreases proportional to the increasing cross-section of the carrier. Picture 1. Typing of the observed arcs at metal shielding gas welding (all pictures and graphics by Lorch). Picture 2. At the conventional pulse arc procedure a precisely controlled detachment of single droplets takes place during the pulse phase. Picture 3. The performance range of conventional pulse arc procedure covers the short arc range as well as the classical transitional arc and parts of the spray arc. Picture 4 Compared to the conventional pulse procedure the increase in performance at SpeedPulse ac is reached due to the transfer of more droplets per pulse (graphic top left), the lead droplet is followed by a controlled secondary material transition to maintain substantially the characteristics of the pulse arc. (Principle top right and high speed capture below) Picture 5. The individual phases of SpeedPulse arc in high speed capture. Picture 6. The direct comparison (here with steel) shows that SpeedPulse compared to conventional pulse welding produces a deeper, high quality penetration improved root capture. Picture 7. The SpeedPulse procedure covers almost the entire performance range of the three arc ranges. Picture 8. The increase of melting performance with SpeedPulse begins at the upper half of the useable performance range and rises becomes with increasing arc performance more apparent (wire electrode G3Si1 with a 1.2 mm diameter and 82% Ar + 18% CO2 shielding gas) Picture 9. The high speed camera makes the needle effect clearly and shows the focus of SpeedPulse arc. Picture 10. A comparison of the procedures shows intensive spatter formation at MIG-MAG welding in the range of the transitional arc (left), the SpeedPulse provides an almost spatter free welding in the complete performance range. (right). Picture 11. Welding power sources of the new series "S SpeedPulse" by Lorch.