Laser-assisted plasma arc welding of stainless steel

JOURNAL OF LASER APPLICATIONS

VOLUME 25, NUMBER 3

MAY 2013

Laser-assisted plasma arc welding of stainless steel Achim Mahrle Institut f€ ur Fertigungstechnik IF, Technische Universit€ at Dresden, 01062 Dresden, Germany and Fraunhofer IWS Dresden, Winterbergstrasse 28, 01277 Dresden, Germany

Sascha Rose and Michael Schnick Institut f€ ur Fertigungstechnik IF, Technische Universit€ at Dresden, 01062 Dresden, Germany

Eckhard Beyer Institut f€ ur Fertigungstechnik IF, Technische Universit€ at Dresden, 01062 Dresden, Germany and Fraunhofer IWS Dresden, Winterbergstrasse 28, 01277 Dresden, Germany

€ssel Uwe Fu Institut f€ ur Fertigungstechnik IF, Technische Universit€ at Dresden, 01062 Dresden, Germany

(Received 29 October 2012; accepted for publication 12 March 2013; published 27 March 2013) A plasma arc and a low-power/high beam quality laser beam were coaxially combined into one process in order to obtain a more efficient and stable arc for thin sheet welding applications. Theoretical discussion of interaction mechanisms between the laser beam and the plasma arc and results of bead-on-plate welding trials carried out on AISI 304 stainless steel will be presented. Measurements were made of electrical and geometrical arc properties with and without assistance from the laser beam. Additionally, the impact of the laser beam on the weld seam geometry was evaluated. The results showed that the use of the additional low-power laser beam is capable of producing significant process improvements in comparison to the individual plasma arc process C 2013 Laser Institute of America. alone with respect to arc stability and welding performance. V Key words: plasma arc welding, laser-assisted arc welding, hybrid welding, fiber laser

I. INTRODUCTION

Plasma arc welding (PAW) is a well-established fusion welding process for joining different kinds of metals in a broad range of sheet thicknesses. It is closely related to gastungsten arc welding (GTAW) as both methods use a nonconsumable tungsten electrode to ignite and maintain an arc to the weld zone, which is preferably shielded from the surrounding atmosphere by inert gases such as argon, helium, or a mixture of both.1,2 The main feature of the PAW process is the specific torch design with an internal plasma gas nozzle that contains the retracted and centrally positioned tungsten electrode as schematically shown in Fig. 1. This nozzle, through which the (inner) plasma gas is flowing, constricts the arc, and this effect is additionally supported by the converging action of the cold (outer) shielding gas that concentrically surrounds the nozzle exit. As a result, the plasma arc column diverges very slowly causing a highly concentrated arc.3 Nevertheless, and in particular in case of low and medium arc currents up to 100 A, the arc tends to become unstable above a critical welding speed. This behavior can be considered as a limit for the development of thin section high-speed arc welding applications. The main objective of the current study was to find ways to overcome this deficiency by combining the PAW process with a low-power laser beam. The resultant hybrid process is referred to as laser-assisted PAW. The idea to combine a welding arc together with a laser beam in a common process zone can be traced back to the work done by Steen et al. in the early 1980s where a 2 kW-CO2 laser beam was used in combination with a gas-tungsten arc 1042-346X/2013/25(3)/032006/8/$28.00

process.4,5 These investigations were primarily motivated by the purpose to improve the process efficiency of the expensive laser beam source for welding and cutting applications. This aim was achieved at that time and after some further primary works, among others by Hamasaki,6 Matsuda et al.,7 Beyer et al.,8 Seyffarth et al.,9 and Maier et al.,10 Steen’s idea gave rise to the development of high-power hybrid laser-arc welding processes, which are nowadays well accepted as a suitable technology for joining different types of metals with potential for improved capabilities in comparison to laser beam welding as an individual process.11–14 The achievable benefits include higher productivity, an increased gap bridging capability as well as improved weld properties as a result of modified heat input and the resulting cooling regimes, and the inherent capability of the combined technique to provide adapted filler metal by applying arc processes with consumable electrodes. On the other hand, Steen et al. also discovered that some of the characteristics of electric arcs can be beneficially influenced by the action of the focused laser beam. They demonstrated that a laser beam in interaction with an electric arc can have a stabilizing and guiding effect on the arc formation. These effects motivated Cui et al. in the early 1990s to use a low-power laser beam in order to improve the efficiency of arc welding.15,16 By combining a CO2 laser beam and a gas-tungsten arc, they could show that even low laser powers in the range of some 100 W were sufficient for achieving positive effects with respect to arc stability and welding performance. In addition, significantly increased weld penetration for welding stainless steel

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C 2013 Laser Institute of America V

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FIG. 1. Schematic illustration of the cross-sectional view of a conventional plasma torch design with its main components.

was demonstrated. Subsequent work also confirmed synergetic interactions between a laser beam and gas-tungsten arcs for welding aluminum alloys.17 Hu and den Ouden later demonstrated that a Nd: yttrium aluminum garnet (YAG) laser beam with a tenfold shorter wavelength than a CO2 laser is also capable of stabilizing the arc root and column of a gas-tungsten arc in the investigated cases of bead-on-plate welding at low welding currents, bead-on-plate welding under asymmetrical magnetic field conditions, and welding of corner joints.18 The experiments were performed on mild steel with a laser power of 500 W. Alternating current GTAW in combination with a lowpower pulsed Nd:YAG laser was applied by Liu et al. for welding magnesium alloys.19,20 The laser system was working with average laser powers in the range of 50–400 W, pulse frequencies between 1 and 50 Hz, and pulse durations between 0.1 and 4.0 ms. The combined welding process offered a two times deeper penetration in magnesium alloy AZ31B in comparison to the individual laser and arc welding processes. It was suggested that the laser welding process transforms from the heat conduction into the keyhole mode during the hybrid welding process. Experimental investigations on laser-assisted plasma arc welding were first presented by Fuerschbach who applied a pulsed Nd:YAG laser as beam source with 400 W output power.21 Fuerschbach developed an unconventional plasma torch design enabling a coaxial combination of the focused laser beam and the plasma arc. It was found out that the fusion zone dimensions of the combined process were wider than laser welds and deeper than plasma arc welds in the case of

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welding aluminum alloys and stainless steel. However, in steel welding, the synergy of the laser beam and the plasma arc was not obvious for all process conditions and the arc weld appeared sometimes to be merely superimposed to the underlying laser weld. The plasma arc welding process in combination with a low-power laser beam has been also the subject of our own experimental investigations.22–24 Initially, both heat sources were arranged in a separate order but inclined to each other in such a way that a common operation point of the arc root and the laser spot resulted. The most distinguishing feature of this set-up was the use of a highly focused single-mode fiber laser beam with a focus diameter of only 40 lm and a resultant intensity of 8 106 W cm2 in the case of 100 W applied laser power. A significant increase in melting efficiency of the combined process in comparison to single arc and laser beam welding trials was achieved using a number of different process parameters. However, these improvements did not generally correlate with measurable improvements of the arc properties. Indeed, a clear stabilization of the arc root and column was only observed in the case of aluminum welding. In the case of steel welding, the use of the focused laser beam actually induced an increase in arc voltage and a slight destabilization of the arc column.24 These experimental findings demonstrated that the often observed effect of increased process efficiency cannot be considered as a direct consequence of adequate and measurable improvements of the arc characteristics. II. THEORETICAL BACKGROUND

The development of appropriate laser-assisted arc welding processes remains a challenging task. This is mainly caused by the lack of a profound understanding of the involved physical relationships. The observed effects were often justified by phenomenological arguments without an adequate and consistent explanation of all the experimental experiences described in the published work so far, e.g., it is not fully understood why the stabilizing effect of the laser beam on the arc and the increase in welding efficiency only arise for certain laser material and parameter combinations. The physical understanding of laser-arc phenomena is also still evolving for the closely related high-power laser-arc welding processes. Although corresponding welding applications are well-established, the fundamental physical relationships are not fully understood. A comprehensive review of problems and issues related to laser-arc hybrid welding was recently given by Ribic et al.25 One discussed an interesting point with respect to our investigations on laser-assisted arc welding. This is the crucial role of an appropriate spacing between the laser beam and the arc. It is reported that the optimum performance of the combined process will be achieved at medium distances between the two involved heat sources, and it is still an unresolved question of scientific interest why the penetration depth is not maximized when the separation between the heat sources is at its minimum. In the case of laser-assisted arc processing, the laser spot should be, however, aligned to the arc axis by definition in order to reach the arc root guiding effect at this particular location. Any insights



about the physical phenomena that determine whether arc stabilization occurs or not in that case can also be of beneficial importance for the interpretation of observations in highpower hybrid laser-arc processing. From a theoretical point of view, only three primary interaction mechanisms can be thought of being responsible for the observed advantageous effects in laser-arc processing. These are (i) a direct interaction by absorption or conversion of laser energy during its way through the arc plasma, (ii) an indirect interaction by changes of the plasma composition as a result of a laser-induced evaporation of base material, and (iii) an indirect thermal interaction as a result of the superposition of two heat sources with different interaction areas and intensities. The possibility of a direct interaction by absorption of laser energy was clearly demonstrated by Albright et al.26 who stimulated this effect by using a particular shielding gas mix with selected molecules offering a preferential absorption for the applied laser beam wavelength and thus inducing a so-called cold-ionization process. Albright used a CO laser producing a 7 W laser beam with 1 mm diameter to excite the CO molecules in gas mixtures of 1% CO in argon. A series of interesting experiments were conducted to demonstrate the influence of the laser-induced cold plasma formation on discharges and arcs under conditions of a vacuum chamber backfilled with the CO/Ar shielding gas. It was, however, stated that the electron density in such a plasma is orders of magnitude lower than the electron density found in welding arcs and that the cold ionization has no noticeable effect on established continuous welding arcs at currents of 10 A or higher. Energy transfer measurements by Hu and den Ouden18 during the common laser-arc interaction in case of Nd:YAG laser radiation and gas-tungsten arc plasma showed that only a small amount of less than 1% of the applied laser power is directly absorbed by the arc plasma, i.e., the direct interaction between laser radiation and arc plasma seems to be only weak at wavelengths around 1 lm. Spectroscopic measurements by the same authors, however, showed that the plasma composition was considerably changed by the laser-induced evaporation of workpiece material and that the metal atoms become ionized and then replace some of the ions of the shielding gas due to their lower ionization potential. It was concluded that the vapor flow produces a conductive plasma channel being able to overcome disturbances by external forces and thus to stabilize the arc root and column. The experimental experience, however, shows that the impact of metal vapor on the arc characteristics is probably much more complex, and it is not self-evident that the laserinduced evaporation of base material stabilizes the arc or even causes a constriction and guiding of the arc attachment. Those effects were preferably observed and described for low arc currents and relatively low laser intensities. In contrast, Maier et al.27 investigated the arc behavior under conditions of higher arc currents and laser powers. In case of steel welding with a 200 A gas-tungsten arc in combination with a 1.9 kW Nd:YAG laser beam, they clearly observed that the arc was pushed away from the laser beam interaction zone. These results conform with the authors own experience that a high-intensity laser beam is capable of destabilizing a plasma arc during steel welding.28,29

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The simple presence of metal vapor is consequently not considered as being a consistent reason for synergistic effects in laser-arc processing. Instead, the thermal interaction between a focused laser beam and an electric arc may play a more crucial role. The laser focus is very capable of generating a localized hot spot at elevated temperatures above the melting point which might be offering good conditions for the formation of a stable arc that has a tendency to be rooted in this hot spot. In fact, it was already described in the early work by Steen that a guiding effect of the laser spot on the arc root also occurs if both heat sources are arranged on opposite sides of thin sheets of metals.4,5 Furthermore, the superposition of two heat sources with different levels of intensity and extension may have a favorable impact on the subsequent heat and fluid flow within the fusion zone. It is also very probable that the preheating of the surroundings of the laser-induced spot favors the absorption of laser energy and reduces heat losses into the base material due to the pre and post heating effect of the arc. III. EXPERIMENTAL SET-UP

In the past, most of the experimental work on laserassisted arc welding was performed with serial arrangements of arc and laser beam as this is typical for high-power hybrid laser-arc welding applications. The serial arrangement offers the advantage of possessing a simple possibility to control the order and spacing of and between the both heat sources. It was found out that in most cases an intermediate distance in the range of a few mm gives the best results with respect to process stability, weld penetration, and weld properties. The idea to develop a coaxial set-up for the purpose of laserassisted arc processing was mainly motivated by the consideration that the applied low-power laser beam as secondary heat source is intended to improve the performance of arc welding and that such an improvement can only be achieved by beneficial interactions between both heat sources. Those interactions, however, should be more likely to occur in a coaxial arrangement by maximizing the overlapping position of the two heat sources at the workpiece. Coaxial variants have been also presented for high-power laser-arc combinations with the aim to make hybrid welding applicable to three-dimensional joining tasks, e.g., for materials of complicated shape or with changing welding directions. As an example, Ishide et al.30 presented an integrated hybrid welding head with advanced optical system. The laser beam was initially split into two parts which were guided around the centrally positioned electrode and then reunified to get a common operation point with the arc. Most interestingly, this approach is applicable for arc welding processes with nonconsumable and consumable electrodes as well. Doi31 prepared a special GTAW torch with a hollow cathode having an inner diameter of 2 or 4 mm, respectively, in coaxial combination with a high-power YAG laser beam. Synergistic effects were found to be dependent on welding speed for this particular arrangement with an applied laser power of 4 kW and an arc current of 500 A. The idea of coaxial laser-arc combinations for different applications and with various objectives was also described in some patents,


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e.g., by Hoshinouchi et al.32 who applied an annular electrode at the end of a nozzle through which the laser beam was transmitted or by Dykhno et al.33 who introduced conical or annular cathodes for high-power laser-arc processing with plasma welding torches. The most distinctive feature of our developed experimental set-up with coaxial arrangement of plasma arc and laser beam is the utilization of a commercially available standard plasma torch. This was enabled by the high laser beam quality or the low beam parameter product which can now be obtained from the latest single-mode fiber lasers that offered in combination with an appropriate optical set-up the possibility to guide the beam through thin electrodes usually applied in plasma arc welding. Only a few technical changes in comparison to the original state of the plasma torch were made as schematically indicated in Fig. 2. First, the usually solid tungsten electrode was replaced by a hollow one. Second, a hole was manufactured into the head of the plasma torch as an opening for the incident laser beam. This opening was sealed against the surrounding atmosphere by a transmissive protection glass. The most serious constriction for the laser beam is the hollow tungsten electrode with an internal diameter of 1.2 mm and a length of about 60 mm. This challenge was mastered by use of the nearly diffraction-limited fiber laser beam emitted from a core fiber with a diameter of 14 lm. This beam was shaped to a focus diameter of 240 lm and a corresponding Rayleigh length of 32 mm. With a fixed position of the focal plane inside the plasma torch, the laser beam intensity at the material surface changes with the working distance, i.e., the distance between plasma torch nozzle and workpiece. Calculated beam radii and resultant intensities for laser powers of 100 and 200 W as a function of the working distance are shown in Fig. 3. These intensity values in the range of some 105 W cm2 are commonly not sufficient to cause a deep-penetration effect in stainless steel sheets if the laser beam is applied alone for a welding process. Only the direct current electrode negative polarity mode being the most common current mode in PAW and GTAW

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FIG. 3. Beam radius and resultant beam intensity as a function of working distance and laser power.

of ferrous alloys was considered. In this mode, the tungsten electrode acts as a cathode, whereas the workpiece is connected to the positive terminal of the power supply. Argon with a purity of 99.996% was applied as both shielding and plasma gas at flow rates between 10 and 12 l min1 for shielding and between 0.5 and 0.7 l min1 as plasma gas. The plasma nozzle diameter was kept constant for all experiments and amounted to 2.6 mm. Welding trials were focused on medium plasma arc processes. The material taken into account was American Iron and Steel Institute (AISI) 304 stainless steel. The manufactured specimens had the dimensions of 300 50 1 mm3. Linear bead-on-plate welds with a total length of 200 mm were carried out. The first third of the weld path was processed by the plasma arc alone. During the period of the middle third, both plasma arc and laser beam were operating together. Finally, the weld of the last third was made by the laser alone. During the experiments, the arc voltage was recorded as function of time with an exposure rate of 100 kHz. High-speed camera recordings with a spatial resolution of 320 160 pixels at frame rates of 2 kHz simultaneously enabled observations of the arc shape and size without and with laser beam support. IV. RESULTS AND DISCUSSION

FIG. 2. Schematic of the modified plasma torch with hollow tungsten electrode and coaxially arranged laser beam.

First, the effect of the laser beam on the arc voltage and arc behavior is discussed. Figure 4(a) shows the arc voltage as a function of time for a 40 A plasma arc during the activation phase of the laser beam with an optical output power of 100 W. It is clear that the laser beam activation causes a sharp drop in arc voltage of about 1 V for the given parameter set. This phenomenon was, however, only observed in case of low values of the arc current. For higher arc amperages, the effect disappeared as shown in Fig. 4(b) for an arc current of 160 A and a laser power of 400 W. Despite the ratio of arc and laser power and the applied linear energies being quite similar to the previous example, the laser beam activation did not have any measurable impact on the arc voltage in this case. In order to find out reasons for this different behavior, high-speed camera recordings of the welding process were analyzed. Figure 5 shows corresponding arc shapes without and with laser beam assistance for the considered cases of



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FIG. 4. Arc voltage during bead-on-plate welding AISI 304 stainless steel with and without laser beam support under different welding conditions. (a) Arc current ¼ 40 A, laser power ¼ 100 W, welding speed ¼ 0.75 m min1, and sheet thickness ¼ 1 mm. (b) Arc current ¼ 160 A, laser power ¼ 400 W, welding speed ¼ 2.00 m min1, and sheet thickness ¼ 3 mm.

“low” and “high” arc currents. In the case of the low arc current and the nonassisted arc, the arc column clearly kinks backwards just above the surface of the material being welded and a clear lag between the arc axis and the position of the arc impingement results [Fig. 5(a)]. This lag is clearly reduced by the action of the laser beam during laser-assisted

FIG. 5. Arc shape during bead-on-plate welding AISI 304 stainless steel with and without laser beam support. Welding conditions: Arc amperage ¼ 40 A, laser power ¼ 100 W, welding speed ¼ 0.75 m min1, and sheet thickness ¼ 1 mm.

plasma arc welding, and it can be reasoned that the observed drop in arc voltage is a consequence of this change in arc attachment location. In case of higher arc currents, the plasma arc is obviously stiff enough to burn in a straight way between tungsten electrode and workpiece by itself, and the additional laser beam has no effect on arc shape and voltage. This may be an acceptable answer to the asked question why the effect of laser radiation can be only ascertained for particular laser, material and parameter configurations. The conclusion can be drawn that the effect of the laser beam on arc voltage is only observable for arcs lacking a certain amount of stiffness that gives rise to the effect that the arc impingement of the individual arc welding process is trailing behind the arc axis. A laser beam which is aligned to the arc axis does cause the displacement of the arc root into the beam spot under those conditions. This phenomenon is considered as the main reason for the measured voltage drop. An open issue remains, however, the question of what exactly causes the arc to favorably root into the beam spot location. It should be noted that only moderate laser intensities were applied to achieve this effect. Indeed, these comparatively low laser intensities are even found to be much more effective for the stabilization of the arc attachment than the high intensities of the much more focused laser beam used in a previous study, which were causing an increase in arc voltage.23,24 These results may be regarded as additional proof of our argumentation that the arc stabilization is not caused by the laser-induced evaporation of base material. Instead, it is concluded that the molten surface region within the beam spot region does already offer preferred conditions for arc rooting. Irrespective of the physical reasons, the capability of the laser spot to fix and guide the arc impingement point in line with the torch axis can be considered as an effect of high practical importance. This is due to the fact that such a fixation can be also achieved for welding speeds at which a conventional arc typically becomes unstable. Such a stability loss is accompanied by erratic deviations of the arc column from the elongated torch axis causing initially irregular weld seams and at further increased travel rates even discontinuous weld


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FIG. 6. Surface appearance of plasma arc welds without and with laser beam support as a function of welding speed. Arc amperage ¼ 40 A, laser power ¼ 100 W, and sheet thickness ¼ 1 mm.

seams as a consequence of leaps between two successive arc impingement points. The corresponding surface appearances of plasma arc weld seams with and without laser support are shown in Fig. 6 for different welding speeds. Under the chosen processing conditions, the plasma arc welding process becomes unstable for a welding speed between 1 and 2 m/min. It is demonstrated that the volatile behavior of the arc behind this limit in welding speed is completely suppressed after the laser beam is turned on. What is really impressive in this respect is the fact that only 100 W of laser power is sufficient to achieve this effect. Thus, a low-power laser beam can be favorably applied for a considerable extension of the working area of electric arcs with respect to the maximum welding speed. Another aspect of high practical importance is the possible impact of the laser beam on the weld seam shape and size of plasma arc welding processes. Figure 7 shows crosssections of bead-on-plate welds made with the plasma arc, the laser-assisted plasma arc, and the laser beam alone for the given parameter set. It is worth to have a close look at the weld seam geometries produced by the three different kinds of processes with consideration of the available heat source power. The arc power amounts to about 820 W for the applied current of 40 A and a measured arc voltage of 20.6 V. The resultant plasma arc weld seam possesses only a shallow penetration but offers a considerable width which might correspond to the effective diameter of the arc root. In the case of the laser welding process alone, the weld seam area is much smaller due to the comparatively lower linear energy. The absorptivity of iron to laser radiation with 1070 nm wavelength may be estimated of being about 40% in case of ferrous alloys, i.e., in case of 100 W laser power only 40 W and in case of 200 W laser power only 80 W effective (absorbed) laser power is available for the welding process. The measured voltage drop of the arc was 1.0 V for 100 W incident laser power and 1.4 V in case of 200 W laser power. This corresponds to a decrease of the arc power of 40 and 60 W, respectively, i.e., the combined process offers

nearly the same value of linear energy as the pure plasma arc welding process. Nevertheless, the impact of the additional laser beam on the weld seam shape is crucial. In the case of only 100 W of additional laser power, the weld seam depth is more than doubled and a finger-shaped penetration results. This considerable increase in weld penetration finally gives rise to a full-penetration weld for a laser power of 200 W. In order to quantify the improvement of the welding performance of the laser-assisted plasma arc welding process in comparison to the single processes, the energy conversion or thermal welding efficiency gth can be determined. This quantity is defined as the product of heat transfer efficiency gIn and melting efficiency gm and is consequently equal the theoretical power needed to melt the weld seam area to the total power delivered by the applied heat sources according to PIn Pm Ptotal PIn q vW AS ½cP ð#liquidus #0 Þ þ DhS=L ¼ Ptotal vW AS fmaterial ¼ : Ptotal

gtotal ¼ gIn gm ¼

(1)

In this equation, PIn denotes the part of the delivered total power Ptotal that is coupled into the workpiece and Pm is the theoretical minimum of power to melt the weld seam area; q denotes the mass density of the base material, vW the welding speed, AS the cross-sectional area of the weld seam, cP the heat capacity, #liquidus the liquidus point temperature, #0 the ambient temperature, DhS/L the enthalpy of melting, and fmaterial a corresponding material function. In case of the investigated stainless steel with q ¼ 8000 kg m3, cp ¼ 500 J kg1 K1, #liquidus ¼ 1455 K, and DhS/L ¼ 274 103 J kg1, the value of the material function follows to 7.932 J mm3. The relevant data for the five investigated process variants, namely (i) pure plasma arc welding with 40 A, (ii) laser beam welding with



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FIG. 7. Comparison of weld seam shapes during bead-on-plate welding AISI 304 stainless steel with plasma arc welding (left), laser-assisted plasma arc welding (middle), and laser beam welding (right). Welding conditions: Arc amperage ¼ 40 A, laser power ¼ 100 W, welding speed ¼ 0.75 m min1, and sheet thickness ¼ 1 mm.

100 W laser power, (iii) laser beam welding with 200 W laser power, (iv) laser-assisted plasma arc welding with 40 A and 100 W laser power, and (v) laser-assisted plasma arc welding with 40 A and 200 W laser power are summarized in Table I. The determined thermal welding efficiencies show a clear increase in the case of laser-assisted plasma arc welding in comparison to the single processes. With respect to experimental results by Hu and den Ouden34 who demonstrated by caloric measurements that the interaction of the laser and the arc did not lead to a noticeable change in heat transfer efficiency, this increase can be only thought of as a result of favorable changes of the heat flow conditions. It is obvious form the cross-sectional areas shown in Fig. 7 that the top surface width of the laser-assisted plasma arc weld seam remains nearly the same as in case of pure plasma arc welding. Thus, the effective diameter of the arc should be of comparable size for both the plasma and the laser-assisted plasma arc welding process. With respect to the particular weld seam geometries, it is however probable that the superposition of both heat sources causes a transition from the pure heat conduction into a deep-penetration or keyhole-like welding process as proposed by Liu and Hao.20 The obvious synergistic effect during laser-assisted plasma arc welding can then be reasoned by beneficial interactions of two

different mechanisms. First, the laser beam is capable of directing the arc impingement point to that of its own surface location on the workpiece. Second, the combined action of laser beam and arc changes the heat flow conditions which favor the occurrence of the deep-penetration effect. With respect to our own experience and other published work on this topic, it is expected that such a synergistic interaction can only be achieved for particular or optimized parameter sets. The most important parameter seems to be the applied laser intensity. High intensities causing high evaporation rates favor a deep-penetration weld but possibly have a detrimental effect on arc stability. This may be one of the reasons why in high-power laser-arc processing with a serial arrangement of both heat sources the penetration depth is not maximized when the separation is at its minimum, i.e., both processes focused at same location or impingement point. V. SUMMARY AND CONCLUSIONS

A new plasma arc welding method with coaxial laser beam support was developed. Corresponding experimental investigations revealed that the low-power laser beam is very capable of improving the stability and performance of the plasma arc welding process. First, it was demonstrated that

TABLE I. Penetration depth, seam width, aspect ratio and cross-sectional area of plasma, laser and laser-assisted plasma welding for bead-on plate welds in 1 mm thick AISI 304 stainless steel sheets. Welding parameters: Arc amperage ¼ 40 A, welding speed ¼ 0.75 m min1.

Plasma 820 W/Laser 0 W Plasma 0 A/laser 100 W Plasma 0 A/laser 200 W Plasma 780 W/laser 100 W Plasma 760 W/laser 200 W

Penetration depth (lm)

Seam width (lm)

Aspect ratio ()

Cross-section (mm2)

Thermal efficiency ()

290 90 219 714 1000

1930 218 434 1755 1577

0.15 0.41 0.50 0.41 0.63

0.59 0.02 0.08 0.94 1.50

0.07 0.02 0.04 0.11 0.15


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the laser beam can direct the arc impingement point to the laser-generated hot spot in case of arcs lacking either stiffness or stability. This effect is accompanied by a clear drop in arc voltage as well as a reliable stabilization of the arc root. Second, the combination of the low-power laser beam and the plasma arc gives rise to significantly improved melting capabilities for optimized parameter sets. The technique developed during this study using a coaxial laser beam and plasma arc offers the potential advantage of being able to weld thin sheet metals at higher speeds than would otherwise be possible using the conventional plasma arc process alone. ACKNOWLEDGMENTS

The authors appreciate the financial support given by the German Research Foundation DFG within the project “Experimentelle und theoretische Untersuchungen zur Verfahrensoptimierung beim Wolfram-Plasmaschweißen durch Laserstrahlung geringer Leistung,” Contract Nos. FU 307/4-1 and BE 1875/19-1. 1

R. W. Messler, Principles of Welding (Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2004). R. Killing, Kompendium der Schweißtechnik, Verfahren der Schweißtechnik, Bd. 1 (Deutscher Verlag f€ ur Schweißtechnik, DVS-Verlag, D€ usseldorf, 1997). 3 K. Weman, Welding Processes Handbook (Woodhead Publishing Limited, Cambridge, 2003). 4 W. M. Steen and M. Eboo, “Arc augmented laser welding,” Met. Constr. 11(7), 332–335 (1979). 5 W. M. Steen, “Arc augmented laser processing of materials,” J. Appl. Phys. 51(11), 5636–5641 (1980). 6 M. Hamasaki, “Welding method combining laser welding and MIG welding,” U.S. patent 4,507,540 (Mar. 26, 1985). 7 J. Matsuda, A. Utsumi, M. Katsumura, M. Hamasaki, and S. Nagati, “TIG or MIG arc augmented laser welding of thick mild steel plate,” Join. Mater. July, 31–34 (1988). 8 E. Beyer, U. Dilthey, R. Imhoff, C. Maier, J. Neuendorf, and K. Behler, “New aspects in laser welding with an increased efficiency,” in Proceedings of ICALEO (Laser Institute of America, Orlando, FL, 1994), pp. 183–192. 9 P. Seyffahrt, B. Anders, and J. Hoffmann, “Combination of laser beam arc welding,” in Proceedings of the 5th European Conference on Laser Treatment of Materials, ECLAT’94 (DVS-Verlag, D€ usseldorf, 1994), pp. 377–397. 10 C. Maier, J. Neuenhahn, K. Behler, and E. Beyer, “Laserstrahlschweißen in Kombination mit konventionellen Techniken—Hybridschweißen,” in Proceedings of 4. Konferenz Strahltechnik (SLV Halle GmbH, Halle, 1996), pp. 14–28. 11 P. Seyffahrt and I. V. Krivtsun, Laser-Arc Processes and Their Applications in Welding and Material Treatment (Taylor & Francis, London, 2002). 12 C. Bagger and F. O. Olsen, “Review of laser hybrid welding,” J. Laser Appl. 17, 2–14 (2005). 13 A. Mahrle and E. Beyer, “Hybrid laser beam welding—Classification, characteristics and applications,” J. Laser Appl. 18, 169–180 (2006). 2

Mahrle et al. 14

F. O. Olsen, Hybrid Laser-Arc Welding (Woodhead Publishing Limited, Oxford, 2009). 15 H. Cui, I. Decker, and J. Ruge, “Wechselwirkungen zwischen WIGSchweißlichtbogen und fokussiertem Laserstrahl,” in Proceedings of Laser’89 (Springer-Verlag, Berlin, 1989), pp. 577–581. 16 H. Cui, I. Decker, H. Pursch, J. Ruge, J. Wendelstorf, and H. Wohlfahrt, “Laserinduziertes Fokussieren des WIG-Lichtbogens,” in Proceedings of Schweißen und Schneiden’92 (DVS-Verlag, D€ usseldorf, 1992), pp. 139–143. 17 I. Decker, J. Wendelstorf, and H. Wohlfahrt, “Laserstrahl-WIGSchweissen von Aluminiumlegierungen,” (DVS-Berichte, Band 170, DVS-Verlag GmbH, D€ usseldorf, 1995), pp. 96–99. 18 B. Hu and G. den Ouden, “Laser-induced stabilization of the welding arc,” Sci. Technol. Weld. Join. 10(1), 76–81 (2005). 19 L. Liu, X. Hao, and G. Song, “A new laser-arc hybrid welding technique based on energy conservation,” Mater. Trans. 47(6), 1611–1614 (2006). 20 L. Liu and X. Hao, “Improvement of laser keyhole formation with the assistance of arc plasma in the hybrid welding process of magnesium alloy,” Opt. Lasers Eng. 47, 1177–1182 (2009). 21 P. Fuerschbach, “Laser assisted plasma arc welding,” in Proceedings of ICALEO’99 (LIA, Orlando, FL, 2000). 22 S. Rose, M. Schnick, A. Mahrle, F. Kretzschmar, C. Demuth, U. F€ ussel, and E. Beyer, “Beeinflussung von Plasmaschweißprozessen durch Faserlaserstrahlung geringer Leistung—Teil 1: Experimentelle Untersuchungen,” Schweißen und Schneiden (DVS Media GmbH, D€ usseldorf, 2011), pp. 170–175. 23 A. Mahrle, M. Schnick, S. Rose, C. Demuth, E. Beyer, and U. F€ ussel, “Process characteristics of fibre-laser-assisted plasma arc welding,” J. Phys. D: Appl. Phys. 44(34), 345502–345513 (2011). 24 M. Schnick, S. Rose, U. F€ ussel, A. Mahrle, C. Demuth, and E. Beyer, “Experimental and numerical investigations of the interaction between a plasma arc and a laser,” Weld. World 56(03/04), 93–100 (2012). 25 B. Ribic, T. A. Palmer, and T. DebRoy, “Problems and issues in laser-arc hybrid welding,” Int. Mater. Rev. 54(4), 223–244 (2009). 26 C. E. Albright, J. Eastmann, and W. Lempert, “Low power lasers assist arc welding,” Weld. J. 80(4), 55–58 (2001). 27 C. Maier, J. Beersiek, J. Neuenhahn, K. Behler, and E. Beyer, “Kombiniertes Lichtbogen-Laserstrahl-Schweißverfahren—Online Prozess€ uberwachung,” (DVS-Berichte, Band 170, DVS-Verlag, D€ usseldorf, 1995), pp. 45–50. 28 M. Schnick, U. F€ ussel, S. Rose, A. Mahrle, C. Demuth, and E. Beyer, “Numerical and experimental investigations of the interaction between a plasma arc and a laser,” in IIW Study Group 212, Physics of Welding, IIWDoc. 212-1164-10, International Institute of Welding, Istanbul Turkey, 2010. 29 M. Schnick, S. Rose, U. F€ ussel, A. Mahrle, C. Demuth, and E. Beyer, “Numerische und experimentelle Untersuchungen zur Wechselwirkung zwischen einem Plasmalichtbogen und einem Laserstrahl geringer Leistung,” in Tagungsband zur Großen Schweißtechnischen Tagung, DVS Verlag GmbH, D€ usseldorf, 2011. 30 T. Ishide, S. Tsubota, and M. Watanabe, “Latest MIG, TIG arc—YAG hybrid welding systems for various welding products,” Proc. SPIE 4831, 347–352 (2003). 31 M. Doi, “Coaxial hybrid process of hollow cathode TIG and YAG laser welding,” Weld. Int. 24(3), 188–196 (2010). 32 S. Hoshinouchi, M. Kanaoka, and A. Fukada, “Laser-beam operated machining apparatus,” U.S. patent 4,689,466 (Aug. 25, 1987). 33 I. S. Dykhno, I. U. Krivtsun, and G. M. Ignatchenko, “Combined laser and plasma arc welding torch,” U.S. patent 5,700,989 (Dec. 23, 1997). 34 B. Hu and G. den Ouden, “Synergetic effects of hybrid laser/arc welding,” Sci. Technol. Weld. Join. 10(4), 427–431 (2005).
