Characteristic of Droplet Transfer Behavior: History
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Contributor:
Qingyong Liu
,
Di Wu
,
Qingzhao Wang
,
Peilei Zhang
,
Hua Yan
,
Tianzhu Sun
,
Jie Zeng
,
Mingliang Yan
,
Zhenyu Liu
,
Ruifeng Li

With the synergistic effect of laser and arc heat sources, laser-arc hybrid welding (LAHW) technology can improve welding speed and penetration depth, and enhance gap-bridging ability. LAHW technology is an advanced welding method, which has been applied widely and successfully. Laser-arc hybrid welding combines laser and arc heat sources, acting together on the same location of the workpiece, and a laser beam and arc are simultaneously superimposed on a common interaction zone. It uses the advantages of the laser and arc energy effectively. It is a novel and efficient welding method, and one of the study directions for welding technology all over the world. In arc welding, due to the dispersion of the arc energy distribution, the actual energy involved in material melting is low. The guiding role of the laser on the arc enhances the melting efficiency of the arc. In laser welding, the absorption rate of laser energy by molten metal is higher than that of solid metal, and the preheating effect of the arc on the molten pool also enhances material absorption efficiency effectively. Therefore, the absorption and conversion rate of a hybrid heat source are better than those of single heat source. In comparison to traditional welding methods, the primary advantages of LAHW are high welding speed and penetration, good bridging performance, low assembly accuracy, and process stability, so it is widely used and studied.

  • laser-arc hybrid welding
  • droplet transfer behavior
  • process parameter

1. The Laser-to-Arc Distance

The welding parameters affect the weld morphology, microstructure, mechanical properties, and welding quality of the joint. Laser-to-arc distance (DLA) is the key factor to determine whether the laser and arc heat sources can be coupled optimally. DLA refers to the linear distance from the center of the laser spot to the point where the welding wire is perpendicular to the plate. the size of the DLA determines whether the laser and the arc form a molten pool together. It also has an impact on the coupling effect of the two heat sources. It is one of the pivotal parameters in LAHW, and exerts a significant influence on the cooperation effect of the two heat sources. Many scholars have carried out basic research on the heat source interaction in LAHW.
Firstly, the DLA has a vital influence on the penetration depth of LAHW. Jokinen and Karhu [1] utilized the LAHW method to weld austenitic steel plates with the thickness of 20 mm, and suggested that the main factor to achieve good welding was the DLA. Song et al. [2] studied the overlap weldability of an AZ31B Mg alloy sheet utilizing the hybrid welding process, considering the DLA as the main factor affecting the penetration depth. Liu et al. [3] found that, with increase in DLA, the penetration depth of welding increased firstly and then decreased. This showed that, as arc current increased, the horizontal distance between the deepest surface of the molten pool and the TIG electrode increased. For the sake of ensuring the synergistic effect in the molten pool, the arc electrode was often close to the laser beam. However, if the DLA was too small, the laser energy would be lost and the penetration depth would be reduced [4]. In the LAHW process, when the coupling distance of laser-to-arc was small, the arc would perturb the stability of the keyhole, and the laser would interfere with the stability of droplet transfer [5]. The laser beam shined on the splashed droplet, which hindered the laser beam irradiation, resulting in shallow penetration of the weld [6][7].
Secondly, the DLA also exerts an important effect on the stability of hybrid welding. Moradi et al. [8] proposed that a short DLA would make laser-arc hybrid welding unstable, because the droplet directly interacted with the keyhole, resulting in extra flow fluctuations. Atabaki et al. [9] reported that in the process of welding high-strength quenched and tempered steel, the DLA played an important role in keyhole stability. By monitoring the welding process, less plasma plume led to higher molten pool stability in comparison to the arc-leading mode. Liu et al. [10] studied the influence of different DLAs on process stability and burn-through defects by HIS and spectrometer, and considered that the DLA influenced keyhole behavior, induction efficiency, and burn-though defects.
Finally, the DLA is of great importance to the mode and frequency of droplet transfer. Liu et al. [11] studied the synergistic effect of laser and arc in LAHW. When the D LA was set to be small, because of a strong synergistic effect, the reduction of welding voltage and welding current caused the transfer mode to change from spray transfer mode to globular transfer mode. Campana et al. [12] considered that the DLA must be kept at 2–3 mm in their research of hybrid welding, avoid the disorder of the molten pool and the instability of the keyhole. At the same time, the synergistic influence of the hybrid heat sources can be realized. The transfer mode of the droplet was very important to the stability and repeatability of the welding process, so pulse transfer and spray transfer modes were preferred. Liu et al. [13] investigated droplet transfer modes and weld formation processes in LAHW. The DLA exerted a great impact on arc characteristics, droplet transfer mode, and final weld geometry. With the increase in DLA and when the DLA was smaller than the arc plasma radius, the weld geometry altered from “cocktail cup” to “cone”, droplet transfer frequency in the welding process increased, and the transfer mode altered from globular transfer to spray transfer. Huang et al. [14] utilized hybrid welding to connect 3 mm low-alloy high-strength steel. The results suggested that the change in the DLA could optimize the weld formation. When the DLA was 0 mm, droplet transfer frequency was the fastest, and droplet transfer mode was a mixture of short-circuit transfer and liquid bridge transfer. When the optimal process parameter of DLA was 0.5 mm, the weld penetration value was best. Zhou [15] studied the complex transmission phenomenon in the keyhole of hybrid welding through the developed mathematical model and related numerical technology. It could be seen that the dynamics of the molten pool, cooling rate, and weld morphology were hugely impacted though droplet impact process in LAHW, and it stressed that the homogeneity of weld composition was influenced by the competition between the rate of mixing and the rate of solidification.
Zhang et al. [16] analyzed the influence of the DLA on the heat source coupling effect of laser-MAG welding in the alloy steel. When the DLA was 0 mm, the droplet transfer and laser keyhole were extremely unstable. On the one hand, the arc was located on the laser transmission path, which had a strong shielding effect on the laser energy. In the meanwhile, the laser may irradiate the droplet, further increasing the energy loss, and greatly reducing the stability of the keyhole while generating a large number of spatters. On the other hand, the droplet was very close to the keyhole, and metal vapor reaction force was very strong, leading to unstable droplet transfer. When the DLA was within 2–4 mm, the heat source coupling effect was good, and the laser played a role in compressing and stabilizing the arc. In the meantime, the arc captured some metal vapor, which weakened the shielding effect of laser-induced plasma and increased laser transmission efficiency, and droplet transfer was more stable. When the DLA exceeded 6 mm, the heat source coupling effect weakened rapidly.
The stability of LAHW relies on droplet transfer mode [17]. In LAHW, droplet transfer plays an important role in deciding arc stability. When the welding current is small or the arc length is short, the droplets do not separate until they contact the molten pool, leading to short-circuit transfer and explosion. In low-current LAHW, the laser can provide extra heating input for the workpiece, so as not to explode in flight. Researchers have carried out much research on LAHW, including the coupling of laser welding process and arc welding process.

2. The Heat Source Leading Mode

The location of laser and arc heat sources in the welding direction has a vital influence in LAHW. There are two leading configurations, namely, laser-leading mode [18][19] and arc-leading mode [12][13][20][21], which have a significant impact on welding process, weld geometry, and formation of welding defects [3][10][22]. The different relative positions of the laser and arc have a vital impact on the surface formation and internal performance of the weld. 
Some researchers believed that laser-leading mode was superior to arc-leading mode. When the position of the laser is used for welding before the arc, the laser energy can preheat the workpiece, enhance the fluidity of the molten pool, and make the liquid molten pool easier to spread around. Casalino et al. [23] utilized the laser-leading and arc-leading hybrid welding to weld aluminum alloys, and proposed that the laser-leading mode provided a more solid weld and better penetration depth. The results suggested that the laser-leading mode generated optimal penetration depth and a more complete weld, and it was considered that the laser-leading mode was more convenient than the arc-leading mode. Therefore, under the same welding process parameters, laser-leading mode could achieve greater penetration depth than arc-leading mode. This was because when the position of the laser was before the arc, the laser acted on the front of the molten pool, which was conducive to the formation of greater penetration depth. Huang et al. [24] welded Al alloy in different leading modes in LAHW. The results suggested that a relatively stable arc, low porosity, and good weld appearance were achieved in laser-leading mode. In addition, in the laser-leading mode, the formation of pores could be effectively suppressed, because there was almost no air in the molten pool, and keyhole was not easy to collapse, so bubbles could easily escape from the molten pool without delay. Zhang et al. [25] utilized LAHW to weld steel to study the influence of leading mode on plasma and metal transfer. The results manifested that the laser-leading mode decreased the arc plasma resistance. For the identical pulse duration, the transfer mode was spray transfer under the laser-leading mode. Because of the low arc resistance and spray transfer mode, the laser-leading mode provided a stable process. Miao [26] studied LAHW of A7N01 Al alloy with an X-ray device, and observed that dark gray weld morphology occurred under arc-leading mode, while bright and clear weld morphology was presented under laser-leading mode. Cao et al. [27] welded HSLA-65 steel with a thickness of 65 mm. They proposed that when the position of the laser was before the arc, they achieved better weld quality. They believed that, in comparison to the arc-leading mode, laser-leading mode could form a higher quality weld, because there were fewer filler and pore defects at the bottom. When the heat source mode was laser-leading, the laser energy could preheat the workpiece, improve the fluidity of the arc welding molten pool, and make the liquid molten pool easier to spread around, so the weld width is larger. Zhao [28] used the LAHW method to weld 11 mm SM490A steel. It could be seen that molten metal moved inward during laser-leading mode, improving the uniformity of the weld metal, while the molten metal moved outward during arc-leading mode. It can be seen form the above discussion that huge coupling effects between laser and arc affected the welding thermal cycle and flow of the molten pool in LAHW, thus affecting the microstructure of welding joints.
Other researchers considered that the arc-leading mode was superior to the laser-leading mode. When the position of arc is used for welding before the laser, the heating of the arc enhances the use rate of the laser. The addition of laser plays a crucial role in stabilizing and inducing the arc. The mutual attraction of the laser plasma and arc plasma makes the arc compressed and strengthened. The laser always irradiates the molten pool of liquid metal, and the absorption rate of the liquid metal with the laser is far greater than that of solid metal, so effective laser energy for welding and welding penetration depth can naturally increase. Liu et al. [29] studied the leading mode of welding integrity. The results proposed that, in the arc-leading mode, there was superior weld geometry and a better grain size of the joint in the heat-affected zone than that of laser-leading mode. The ALHW (arc-laser hybrid welding) joint had better weld morphology and a more uniform lath martensite structure, while the LAHW (laser-arc hybrid welding) joint had an asymmetrical lath martensite and austenite structure. Li et al. [30] also applied the method of changing the location of the heat source to research the effect of the location of the heat source on droplet transfer in the LAHW process. Compared with the ultra-high-power laser-leading mode, the ultra-high-power arc-leading mode had formed steady arc characteristics and molten pool flow, and the angle between the droplet radius and conductive surface produced a greater force to promote the separation of the molten droplets, improving welding process stability. Zhang et al. [16] studied the influence of arc morphology and droplet transfer under different heat source modes. The comprehensive comparison of weld morphology suggested that the arc-leading mode was superior to the laser-leading mode.
The welding process is affected not only by the different positions of the laser and arc, but also by the different DLAs. In LAHW, the thermal radiation effect of the laser plasma on the droplet and the absorption effect of the laser plasma on the arc change the shape of the arc and the corresponding stress state of the droplet, which changes the droplet transfer process. For different welding currents, there is an optimal DLA. Under the optimal DLA, the droplet transfer mode is a single stable spray transfer, and the weld formation is good. Bunaziv et al. [31] proposed that increasing the DLA could optimize the melt flow state in the arc-leading mode. However, in the laser-leading mode, the small DLA was preferred, since the excessive DLA would reduce the previous laser–arc hybrid welding effect. Bunaziv [4] utilized a hybrid welding method to weld 5083 aluminum alloy with a thickness of 5 mm. He proposed that, under the arc-leading mode, the porosity decreased as the DLA increased, while under the laser-leading mode, the situation was opposite. Liu et al. [32] studied DC LAHW in the arc-leading mode. They found that with the increase in DLA, the droplet transfer mode shifted from globular transfer to spray transfer. The difference between the heat source positions of the two processes has a direct impact on the thermal phenomenon of LAHW.
Combined with the changes in plasma shape, electron temperature, and electron density, the laser-arc coupling mechanism of two heat source sequences is different under different laser power. When the heat source is in laser-leading mode, due to the energy density of the laser and the formation of a molten pool prior to the arc, a large number of dispersed electrons are generated, making the plasma shape more divergent. With the continuous increase laser power, the evaporation of molten metal in the molten pool on the surface of the base metal is enhanced, the ionization of metal particles is enhanced, the volume of plasma expands, the electron temperature increases, and electron density increases. When the heat source is in arc-leading mode, laser energy density is small, and there are fewer electrons generated in the welding process. When laser is added, the electrons that maintain the arc are mainly provided by the laser keyhole. In the process of laser-leading mode, due to the lack of preheating effect of the arc, the corresponding laser-induced plasma absorbs more energy and generates higher electron temperature, and the plasma morphology is more divergent. The electron density of the divergent plasma is smaller. When laser power is small, the coupling process is dominated by the arc. With the increase in laser power, the plasma electron density increases, the electron temperature increases, the keyhole effect of the laser is suppressed, and the weld penetration decreases. In the LAHW process, heat source mode has a direct impact on plasma characteristics, the dynamic behavior of the droplet and molten pool, and temperature field, thus affecting the welding process and welding quality. Under different heat source positions, the droplet transfer and transfer frequency in LAHW are very significant.

3. The Shielding Gas Composition

In LAHW, the energy absorption of the materials is related to the energy density of the heat source, the thermal conductivity of the materials, hot melting, and other physical properties. The absorptivity of the materials to the arc is affected by electronic circle-circuit, shielding gas, material characteristics, etc. The absorption of laser energy is mainly affected by laser wavelength, workpiece surface state, joint shape, plasma morphology, and properties above the molten pool. Shielding gas is a very important welding parameter. Whether in single laser welding or arc welding, it must be considered that it has an important impact on the joint quality. In laser welding, shielding gas is an effective means to eliminate the plasma shielding effect, improve process stability, and realize deep penetration welding. In arc welding, shielding gas is the key factor to achieve stable arc combustion and determine the arc column heat distribution and droplet transfer mode.
Compared with GMAW, the physical process of LAHW is complicated, because mutual influence between the arc and laser-induced plasma affects the arc and droplet transfer mode stability. In LAHW, shielding gas exerts an impact on improving the weld quality [33][34]. For the sake of obtaining better process stability and appearance of the weld, it is necessary to comprehend the interaction of the shielding gas and its influence on droplet transfer. Zhu et al. [35] reported droplet transfer behavior in LAHW under shielding gas. The results suggested that, when the shielding gas was pure Ar, Ar + 30% He, and Ar + 50% He, the droplet transfer mode was rotary spray transfer. When the shielding gas was Ar + 30% He, the transfer behavior was best. When the shielding gas was pure He, the transfer mode shifted from rotary spray transfer to large globular transfer, which not only decreased stability of the welding state, but also weakened uniformity of weld metal. Therefore, the hybrid influence between laser and arc was improved by adding 30% He, and the matched degree between rotary spray transfer and arc pulse period was improved. When Ar and He were used as shielding gases, their advantages and disadvantages were different. The combination of them made full use of the advantages of “1 + 1 > 2”. When Ar is used as shielding gas, the arc is stable, and the density of Ar is higher than air, so the protection effect is good, but there are many porosities in the weld. When He is used as a shielding gas, the porosity of the weld is low, but the arc stability is poor. The He-Ar mixture gas has the following advantages: stabilizing the arc, increasing the welding speed, increasing the penetration, and reducing the porosity.
Similarly, Campana [12] discussed the influence of the transfer mode of a CO2 laser-MIG hybrid welding process on the weld morphology under a 40% Ar, 57% He, and 3% O2 ternary shielding gas. They believed that, in the LAHW process, though the harmonious control of the laser and MIG arc, there must be a distance of 2–3 mm to avoid the disturbance of the welding pool and holes, and the focal point of the laser should be in a negative defocusing state. The transfer mode of the droplet directly influenced the weld quality, and the pulse or spray transfer mode was better than the short-circuit transfer mode. Cai et al. [36] proved that instability of the shielding gas and high gas flow rate in laser-arc hybrid welding could lead to precarious droplet transfer behavior with spatter. Pan et al. [37] suggested, in comparison, the impact of 20% CO2 + Ar and 100% CO2 utilized as a shielding gas on defects of welding joints. The results showed that the spatter could be significantly reduced by optimizing the composition of the shielding gas to control the transfer mode. Zhang et al. [38] investigated the influence of CO2 content on droplet transfer during LAHW. The increase in CO2 content enhanced the synergistic effect. The laser energy density continuously increased, and the penetration depth became deep. When CO2 content was low (5% CO2 + Ar), the welding state was fluctuant with spatter, and transfer mode was spray transfer. When CO2 content was high (15% CO2 + Ar), the welding state was not fluctuant, the spatter was small, and transfer mode was spray transfer.
Continuous optimization of shielding gas can enhance the stability of welding joints and process stability. The shielding gas not only influences the welding line morphology, welding defects and alloy composition of laser-arc hybrid welding, but also has an important impact on the mechanical properties of the weld. Different kinds of shielding gas and volume fractions of the shielding gas can not only improve weld penetration, but also improve welding defects. Cai et al. [39] reported the influence of Helium content within shielding gas on porosity defects. The results suggested that the penetration depth increased and the pore defects were obviously reduced when 50% He was added. Naito et al. [40] found that in the process of LAHW, when O2 was added into the Ar shielding gas, the penetration depth was increased slightly, which might be due to the melt flow caused by the inward surface tension. Gao et al. [19] proposed that full penetration depth could be achieved when He content in He-Ar mixture shielding gas is more than 50% during hybrid welding. Due to the indispensable role of shielding gas, Kah [34] elaborated on the importance of shielding gas on the experiment research. Tani et al. [41] studied the influence of shielding gas in LAHW, and observed that higher He content and gas flow rate in the mixture of shielding gas would gradually lead to unstable an arc. When He content increased to 30%, the plasma formation was restricted and the absorption of laser power was decreased. When the content of He exceeded 40%, the welding process was not stable, and the weld formation was not significantly improved. They also pointed out that high-density gas was a fine opportunity for a good protection effect, rather than enhancing gas flow rate to a greater level [42]. Yang et al. [43] reported that when the gas flow was too large, the process stability of MIG welding was worse than that of LAHW, and the gas flow force hindered the droplet transfer.
Whether laser welding or arc welding, shielding gas is the vital factor affecting the process characteristics. In laser welding, especially CO2 laser welding, shielding gas is an effective means to eliminate the plasma shielding effect, improve process stability, and realize deep penetration welding. In arc welding, shielding gas is the vital factor to achieve stable arc combustion and determine the arc column heat distribution and droplet transfer mode. Moreover, for the welding pool, shielding gas is a necessary means to prevent the oxidation or pollution of the high-temperature welding pool. Therefore, the shielding gas is also important for laser-arc hybrid welding, which integrates two processes. How to choose the suitable shielding gas parameters is a necessary premise for the research of LAHW.
In solid laser LAHW, because the defocusing shielding effect of laser-induced plasma on the laser beam is very small, the stable process and good hybrid effect can be obtained by using pure Ar as the shielding gas. In CO2 LAHW, because of the strong plasma shielding effect, it is necessary to use He to obtain good hybrid welding effect. However, He is not good for arc stability, especially for droplet transfer, and is expensive. Therefore, He-Ar mixture gas is usually used in CO2 LAHW. In the research of the hybrid welding of low carbon steel and stainless steel, adding a small amount of CO2 and O2 and using He-Ar-O2 or He-Ar-CO2 ternary mixed gas was also shown to improve the stability of CO2 LAHW to a certain extent and reduce welding spatter. The research shows that the volume fraction of He must be higher than 30% for both binary and ternary gas mixtures to ensure effective suppression of the laser-induced plasma shielding effect, and enhance the synergy between the two heat sources and produce greater welding penetration. However, once the plasma shielding effect is effectively suppressed, increasing the He or CO2 content does not significantly help increase the welding penetration, but leads to a decline in process stability and an increase in welding spatter.

4. The Laser Power

4.1. The Addition of Laser

In recent years, domestic and overseas researchers have concentrated on the analysis and discussion of laser-arc droplet transfer, welding process characteristics, and the mechanism of effect in hybrid welding. Researchers have discovered that laser can increase the stability of the arc. However, it is still controversial whether the role of the laser is to hinder or promote droplet transfer.
Regarding the influence of laser on droplet transfer, some scholars considered that laser can hinder droplet transfer in hybrid welding. Sugino et al. [44] found that the laser hindered the droplet transfer because of the reduction in electromagnetic force. Fellman et al. [45] studied the droplet transfer behavior in LAHW. They considered that the high pressure of the laser-induced plasma prevented droplet transfer. The droplet would rotate around the tip of the welding wire before transferring to the molten pool. Liu et al. [13] found that, in comparison to arc welding, in globular transfer mode, mutual influence between arc plasma and laser-induced plasma in LAHW hindered droplet transfer, and the separation speed and transfer frequency decreased. The laser-induced plasma was generated by the addition of laser, which increased the plasma concentration on the surface of the workpiece, and the arc was attracted by plasma, thus changing the arc shape, thereby altering the pressure difference between the upper and lower surfaces of the droplet, so that the droplet close to the surface of the molten pool merged, and transfer frequency slowed down.
However, some other scholars believe that laser can promote droplet transfer in LAHW. Ono et al. [46] utilized 3 kW laser power and a current of 100 A in LAHW. They found that the voltage waveform was more stable after the addition of laser. The droplet transfer frequency was increased nearly four times, compared with arc welding. The author thought that the laser plasma could change the arc shape, and its discharge area was within the range of about 1 mm of the laser spot, so that the arc was compressed, energy was more concentrated, and volume was smaller. Gao et al. [47] also concluded that, in comparison to arc welding, in the large globular transfer mode of hybrid welding, the number and density of plasma increased in LAHW, which changed arc morphology, increased the area of anode spots on the droplet surface, and changed the direction of the ionization force. Therefore, the laser could promote the droplet transfer. In LAHW, due to the addition of laser, the electromagnetic force in arc welding changed from retention force to separating force. At the same time, the increase in arc plasma density increased the size of the ionization force. Therefore, the role of laser promoted the frequency of droplet transfer. Zhang et al. [48] compared the droplet transfer of MIG welding and laser-MIG hybrid welding. The results suggested that laser-coupling with a certain power can stabilize the arc, reduce the arc fluctuation, and reduce the possibility of the droplet flying out of the molten pool to form the spatter. Through researching droplet transfer frequency in the welding progress, it could be seen that laser generated a lot of thermal radiation in metal plasma, which promoted the droplet transfer. Meanwhile, because of the attraction force of the laser plasma on the droplet and the metal vapor reaction force on the droplet, the transfer mode and frequency of the droplet changed [32].
From the above research, it could be found that numerous research focuses on the influence of CO2 and a YAG laser on droplet transfer behavior in short-circuit and globular transfer modes, but the influence of the laser on the droplet is controversial. In fact, compared with a CO2 laser, a fiber laser can improve photo-electric efficiency and retain the advantages of a YAG laser in aspects of reliability [49][50][51][52]. This has been applied far and wide in LAHW [53][54][55][56]. However, the effect of a fiber laser on droplet behavior in short-circuiting, globular, and spray modes is still indistinct. Cai et al. [57] reported on the impact of laser on droplet transfer behavior in the three transfer modes. Compared with arc welding, it could be found that droplet transfer frequency and the position of the falling point of the three transfer modes in hybrid welding had changed. The results suggested that the addition of laser improved the transfer frequency of short-circuit transfer and droplet transfer modes, but hindered droplet transfer under spray mode. The size and direction of the electromagnetic force and plasma flow force affecting the droplet were the key factors. Therefore, the arc stability mechanism of hybrid welding was attributed to two aspects. Firstly, the coupling of laser and arc boosted globular transfer of the droplet. Secondly, the interaction between laser and arc caused the electromagnetic force to change from separation force to retention force.
In conclusion, as an important parameter, the arc energy determines the droplet transfer mode, and the laser energy has a decisive influence on droplet transfer frequency. The addition of laser in the arc welding process causes changes in the arc shape, temperature distribution, and molten pool shape, which inevitably leads to changes in the droplet transfer characteristics. Compared with the droplet transfer characteristics of individual MIG/MAG welding, on the one hand, the metal plasma generated during the laser deep penetration drives the droplet transfer because of thermal radiation of droplet. On the other hand, the attraction of laser plasma to the droplet and the metal vapor reaction force on the droplet hinder droplet transfer, and their combined effects change droplet transfer mode and frequency.

4.2. The Effect of Laser Power

In the LAHW process, the stability of arc morphology and transfer is affected not only by arc parameters, but also by laser parameters. The laser energy affects the arc morphology and the heat conduction of the arc plasma, affects the electron density and current density, and changes the arc length, droplet size, and speed.
Huang et al. [58] reported the influence of the groove constraint of thick plate titanium alloy in LAHW. It was found that groove had a constraint effect on the metal vapor emitted by the laser keyhole. Compared with flat plate welding, the metal vapor forced to escape upwards had a stronger inhibition influence on droplet transfer. The blocking effect continued to increase as laser power increased. Liu et al. [59] thought that the effect of metal vapor reaction force on droplet transfer behavior was based on theoretical calculation. When laser energy increases, the metal vapor reaction force naturally increased. When the DLA exceeded a certain range, its influence on the droplet could be ignored. Mahmoud Moradi et al. [8] investigated droplet transfer stability in LAHW. It was observed that if laser power was properly increased, the compression influence of the laser on the arc would be stronger, and the arc would absorb more laser energy, which was conducive to a more stable droplet transfer. However, when the laser power increased, the metal vapor reaction force increased, which hindered the droplet transfer. They also studied the stability effect of voltage and laser power in hybrid welding. They discovered high laser energy could lead to instability, and high arc voltage could seriously damage welding state, since spray arc became extremely long and the droplet moved laterally. Liu et al. [32] observed that, in CO2 laser-MAG hybrid welding, the droplet diameter increased and transfer frequency decreased with the increase in laser power. Zhang et al. [60] found that high-power laser-induced plasma altered the droplet’s direction of force. This force led the center of mass of the droplet to deviate from the axis, resulting in a weak shrinkage effect and destroying the transfer mode of one droplet per pulse. Therefore, it could be seen that decreasing the arc length though decreasing voltage could deal with this matter. It was shown that a low-power laser and short arc length could improve droplet transfer stability in LAHW.
In order to obtain smooth and steady droplet transfer and reduce the loss of laser heat input, many researchers changed the conventional process parameters (including laser power and laser-to-arc distance) to solve the problem. Due to the narrowing of the window of process parameters, it is difficult to achieve excellent hybrid welding between a high-power laser and an arc with a small DLA. Therefore, some scholars considered adjusting the laser power to synch the arc current and voltage to cope with the above-mentioned matters. Petring et al. [61] firstly reported the coordinating adjustment technology of laser and arc, and forecasted that this method had plenty of advantages. Chen et al. [62] proposed that adding laser could inhibit arc discharge during negative arc current, which led to a greater penetration depth than adding laser during positive arc current. Sugino et al. [44] discovered that, under the invariable 5 kW laser power, unstable droplet transfer was led though the decrease in the arc current in the period of peak value. Therefore, they adopted the means of adjusting the laser energy to synch the arc current and voltage to promote droplet transfer and restrain spatter. Li et al. [63] realized the synchronization of the arc current by modulating the laser power. There were two coupling modes. In the in-phase coupling mode, adding high-power laser in the period of the peak value of arc current would increase the time of the droplet formation and separation, causing the droplet to deviate from the axis, thus decreasing droplet transfer stability. In the anti-phase coupling mode, utilization of a low-power laser with the maximum arc current inhibited the arc length from becoming longer, leading to faster droplet separation, less spatter, and enhanced stability of droplet transfer. It can be seen that many research fellows concentrated on the influence of the synchronous pulse of modulation laser-arc hybrid welding on the penetration and changes in arc current or voltage. They also considered that droplet transfer would reduce the stability of the keyhole for LAHW. Because of the attraction of the laser to the arc, they confirmed through high-speed photography that the droplet might pass through the laser transmission path when passing through the arc space. The droplet absorbed laser energy and explode, resulting in splash. At the same time, the laser keyhole cannot be maintained, the depth decreased, and the fluctuation increased.
When laser acts on the combustion stage of the MIG arc, high temperature plasma attracts and compresses the MIG arc to a laser focal point. The temperature rises sharply, the arc conductivity drops, and electric field intensity drops. At the same time, the arc increases the penetration ability of the laser. The absorption of the laser energy is proportional to the density of the plasma. The high temperature and density plasma produced by individual laser welding has a great absorption effect on the energy. A large amount of low temperature and density plasma is generated during the arc combustion stage, which has a dilution effect on the plasma generated by the laser, decreasing the absorption rate of the plasma on the laser, and increasing laser penetration ability. With the increase in temperature, the laser absorptivity of the material increases, and the preheating effect of the arc also greatly increases the laser penetration ability. The stability of the welding process affects the welding quality, especially in the high-power welding process. The laser beam hitting the material surface causes the material to evaporate rapidly to form a keyhole, which blocks laser energy inside and transmutes it into heat. The process includes huge changes and complex coupling of various physical elements, which causes great challenges in becoming a true stable welding process.
In the LAHW process, the keyhole maintains a high absorption rate of laser energy. Because of high energy density and intense energy conversion, it is easy for energy deviation to occur, affecting the welding stability. In the process of welding, the preheating effect of the backside becomes more obvious, which leads to the occurrence of “quasi-focus reduction”. The preheating effect can increase evaporation, and it is a reason for unstable welding. Therefore, effective control of laser-induced evaporation is considered key to adjusting the distribution and preventing welding instability.

This entry is adapted from the peer-reviewed paper 10.3390/coatings13010205

References

  1. Jokinen, T.; Karhu, M.; Kujanpää, V. Welding of thick austenitic stainless steel using Nd:yttrium–aluminum–garnet laser with filler wire and hybrid process. J. Laser Appl. 2003, 15, 220.
  2. Song, G.; Liu, L.; Wang, P. Overlap welding of magnesium AZ31B sheets using laser-arc hybrid process. Mater. Sci. Eng. A 2006, 429, 312–319.
  3. Liu, L.M.; Yuan, S.T.; Li, C.B. Effect of relative location of laser beam and TIG arc in different hybrid welding modes. Sci. Technol. Weld. Join. 2012, 17, 441–446.
  4. Bunaziv, I.; Akselsen, O.M.; Salminen, A.; Unt, A. Fiber laser-MIG hybrid welding of 5 mm 5083 aluminum alloy. J. Mater. Process. Technol. 2016, 233, 107–114.
  5. Ishide, T.; Tsubota, S.; Watanabe, M. Latest MIG, TIG Arc-YAG Laser Hybrid Welding Systems for Various Welding Products; SPIE: Bellingham, Wa, USA, 2003.
  6. Uchiumi, S.; Wang, J.B.; Katayama, S.; Mizutani, M.; Hongu, T.; Fujii, K. Penetration and welding phenomena in YAG laser-MIG hybrid welding of aluminum alloy. In Proceedings of the ICALEO 2004—23rd International Congress on Applications of Laser and Electro-Optics, San Francisco, CA, USA, 4–7 October 2004.
  7. Thomy, C. Hybrid laser-arc welding of aluminium. In Hybrid Laser-Arc Welding: A Volume in Woodhead Publishing Series in Welding and Other Joining Technologies; Elsevier Ltd.: Amsterdam, The Netherlands, 2009; pp. 216–269.
  8. Moradi, M.; Ghoreishi, M.; Frostevarg, J.; Kaplan, A.F.H. An investigation on stability of laser hybrid arc welding. Opt. Lasers Eng. 2013, 51, 481–487.
  9. Mazar Atabaki, M.; Yazdian, N.; Kovacevic, R. Hybrid laser/arc welding of thick high-strength steel in different configurations. Adv. Manuf. 2018, 6, 176–188.
  10. Liu, L.; Shi, J.; Hou, Z.; Song, G. Effect of distance between the heat sources on the molten pool stability and burn-through during the pulse laser-GTA hybrid welding process. J. Manuf. Process. 2018, 34, 697–705.
  11. Liu, W.; Ma, J.; Yang, G.; Kovacevic, R. Hybrid laser-arc welding of advanced high-strength steel. J. Mater. Process. Technol. 2014, 214, 2823–2833.
  12. Campana, G.; Fortunato, A.; Ascari, A.; Tani, G.; Tomesani, L. The influence of arc transfer mode in hybrid laser-mig welding. J. Mater. Process. Technol. 2007, 191, 111–113.
  13. Liu, S.; Liu, F.; Zhang, H.; Shi, Y. Analysis of droplet transfer mode and forming process of weld bead in CO2 laserMAG hybrid welding process. Opt. Laser Technol. 2012, 44, 1019–1025.
  14. Huang, H.; Zhang, P.; Yan, H.; Liu, Z.; Yu, Z.; Wu, D.; Shi, H.; Tian, Y. Research on weld formation mechanism of laser-MIG arc hybrid welding with butt gap. Opt. Laser Technol. 2021, 133, 106530.
  15. Zhou, J.; Tsai, H.L. Modeling of transport phenomena in hybrid laser-MIG keyhole welding. Int. J. Heat Mass Transf. 2008, 51, 4353–4366.
  16. Zhang, S.; Wang, Y.; Zhu, M.; Feng, Y.; Nie, P.; Li, Z. Effects of heat source arrangements on Laser-MAG hybrid welding characteristics and defect formation mechanism of 10CrNi3MoV steel. J. Manuf. Process. 2020, 58, 563–573.
  17. Wang, X.; Huang, Y.; Zhang, Y. Droplet transfer model for laser-enhanced GMAW. Int. J. Adv. Manuf. Technol. 2013, 64, 207–217.
  18. Shinn, B.W.; Farson, D.F.; Denney, P.E. Laser stabilisation of arc cathode spots in titanium welding. Sci. Technol. Weld. Join. 2005, 10, 475–481.
  19. Ming, G.; Xiaoyan, Z.; Qianwu, H. Effects of gas shielding parameters on weld penetration of CO2 laser-TIG hybrid welding. J. Mater. Process. Technol. 2007, 184, 177–183.
  20. Wu, C.S.; Chen, M.A.; Li, S.K. Analysis of excited droplet oscillation and detachment in active control of metal transfer. Comput. Mater. Sci. 2004, 31, 147–154.
  21. Bang, H.S.; Bang, H.S.; Kim, Y.C.; Oh, I.H. A study on mechanical and microstructure characteristics of the STS304L butt joints using hybrid CO2 laser-gas metal arc welding. Mater. Des. 2011, 32, 2328–2333.
  22. Zhang, Y.; Chen, G.; Zhou, C.; Jiang, Y.; Zhong, P.; Li, S. Pores formation in laser–MAG welding of 42CrMo steel. J. Mater. Process. Technol. 2017, 245, 309–317.
  23. Casalino, G.; Campanelli, S.L.; Maso, U.D.; Ludovico, A.D. Arc leading versus laser leading in the hybrid welding of aluminium alloy using a fiber laser. Procedia CIRP 2013, 12, 151–156.
  24. Huang, L.; Wu, D.; Hua, X.; Liu, S.; Jiang, Z.; Li, F.; Wang, H.; Shi, S. Effect of the welding direction on the microstructural characterization in fiber laser-GMAW hybrid welding of 5083 aluminum alloy. J. Manuf. Process. 2018, 31, 514–522.
  25. Zhang, W.; Hua, X.; Liao, W.; Li, F.; Wang, M. The effect of the welding direction on the plasma and metal transfer behavior of CO2 laser+GMAW-P hybrid welding processes. Opt. Lasers Eng. 2014, 58, 102–108.
  26. Miao, H.; Yu, G.; He, X.; Li, S.; Chen, X. Comparative study of hybrid laser–MIG leading configuration on porosity in aluminum alloy bead-on-plate welding. Int. J. Adv. Manuf. Technol. 2017, 91, 2681–2688.
  27. Cao, X.; Wanjara, P.; Huang, J.; Munro, C.; Nolting, A. Hybrid fiber laser—Arc welding of thick section high strength low alloy steel. Mater. Des. 2011, 32, 3399–3413.
  28. Zhao, L.; Sugino, T.; Arakane, G.; Tsukamoto, S. Influence of welding parameters on distribution of wire feeding elements in CO2 laser GMA hybrid welding. Sci. Technol. Weld. Join. 2009, 14, 457–467.
  29. Liu, S.; Li, Y.; Liu, F.; Zhang, H.; Ding, H. Effects of relative positioning of energy sources on weld integrity for hybrid laser arc welding. Opt. Lasers Eng. 2016, 81, 87–96.
  30. Li, Y.; Geng, S.; Zhu, Z.; Wang, Y.; Mi, G.; Jiang, P. Effects of heat source configuration on the welding process and joint formation in ultra-high power laser-MAG hybrid welding. J. Manuf. Process. 2022, 77, 40–53.
  31. Bunaziv, I.; Frostevarg, J.; Akselsen, O.M.; Kaplan, A.F.H. The penetration efficiency of thick plate laser-arc hybrid welding. Int. J. Adv. Manuf. Technol. 2018, 97, 2907–2919.
  32. Liu, S.; Liu, F.; Xu, C.; Zhang, H. Experimental investigation on arc characteristic and droplet transfer in CO2 laser-metal arc gas (MAG) hybrid welding. Int. J. Heat Mass Transf. 2013, 62, 604–611.
  33. Wahba, M.; Mizutani, M.; Katayama, S. Hybrid welding with fiber laser and CO2 gas shielded arc. J. Mater. Process. Technol. 2015, 221, 146–153.
  34. Kah, P.; Salminen, A.; Martikainen, J. The analysis of shielding gases in laser-arc hybrid welding processes. Proc. Inst. Mech. Eng. Part B J. Eng. Manuf. 2011, 225, 1073–1082.
  35. Zhu, Y.; Cai, Y.; Wang, M. Effects of He content in shielding gases on high-efficient hybrid laser arc welding with C-276 filler metal. J. Mater. Process. Technol. 2022, 299, 117367.
  36. Cai, C.; Li, L.; Tai, L. Narrow-gap laser-MIG hybrid welding of thick-section steel with different shielding gas nozzles. Int. J. Adv. Manuf. Technol. 2017, 92, 909–916.
  37. Pan, Q.; Mizutani, M.; Kawahito, Y.; Katayama, S. Effect of shielding gas on laser–MAG arc hybrid welding results of thick high-tensile-strength steel plates. Weld. World 2016, 60, 653–664.
  38. Zhang, S.; Sun, J.; Zhu, M.; Zhang, L.; Nie, P.; Li, Z. Effects of shielding gases on process stability of 10CrNi3MoV steel in hybrid laser-arc welding. J. Mater. Process. Technol. 2019, 270, 37–46.
  39. Cai, C.; He, S.; Chen, H.; Zhang, W. The influences of Ar-He shielding gas mixture on welding characteristics of fiber laser-MIG hybrid welding of aluminum alloy. Opt. Laser Technol. 2019, 113, 37–45.
  40. Naito, Y.; Mizutani, M.; Katayama, S. Effect of oxygen in ambient atmosphere on penetration characteristics in single yttrium–aluminum–garnet laser and hybrid welding. J. Laser Appl. 2006, 18, 21–27.
  41. Tani, G.; Campana, G.; Fortunato, A.; Ascari, A. The influence of shielding gas in hybrid LASER-MIG welding. Appl. Surf. Sci. 2007, 253, 8050–8053.
  42. Tani, G.; Ascari, A.; Campana, G.; Fortunato, A. A study on shielding gas contamination in laser welding of non-ferrous alloys. Appl. Surf. Sci. 2007, 254, 904–907.
  43. Yang, X.; Chen, H.; Zhu, Z.; Cai, C.; Zhang, C. Effect of shielding gas flow on welding process of laser-arc hybrid welding and MIG welding. J. Manuf. Process. 2019, 38, 530–542.
  44. Sugino, T.; Tsukamoto, S.; Nakamura, T.; Arakane, G. Fundamental study on welding phenomena in pulsed laser-GMA hybrid welding. In Proceedings of the 24th International Congress on Applications of Lasers and Electro-Optics, ICALEO, Miami, FL, USA, 31 October–3 November 2005; pp. 108–116.
  45. Fellman, A.; Salminen, A. A study of the phenomenon of CO2-laser welding with filler wire and MAG hybrid welding. In Proceedings of the 24th International Congress on Applications of Lasers and Electro-Optics, ICALEO, Miami, FL, USA, 31 October–3 November 2005; pp. 117–126.
  46. Ono, M.; Shinbo, Y.; Yoshitake, A.; Ohmura, M. Welding Properties of Thin Steel Sheets by Laser-Arc Hybrid Welding-Laser Focused Arc Welding; NKK Corporation 1 Kokan-cho, Japan; SPIE: Bellingham, WA, USA, 2003.
  47. Gao, M.; Mei, S.; Wang, Z.; Li, X.; Zeng, X. Process and joint characterizations of laser-MIG hybrid welding of AZ31 magnesium alloy. J. Mater. Process. Technol. 2012, 212, 1338–1346.
  48. Zhang, C.; Gao, M.; Zeng, X. Influences of synergy effect between laser and arc on laser-arc hybrid welding of aluminum alloys. Opt. Laser Technol. 2019, 120, 105766.
  49. Mei, L.; Yan, D.; Yi, J.; Chen, G.; Ge, X. Comparative analysis on overlap welding properties of fiber laser and CO2 laser for body-in-white sheets. Mater. Des. 2013, 49, 905–912.
  50. Kwon, H.; Baek, W.K.; Kim, M.S.; Shin, W.S.; Yoh, J.J. Temperature-dependent absorptance of painted aluminum, stainless steel 304, and titanium for 1.07 μm and 10.6 μm laser beams. Opt. Lasers Eng. 2012, 50, 114–121.
  51. Tu, J.F.; Riley, P.E.B. Low-temperature evaporative glass scoring using a single-mode ytterbium fiber laser. Opt. Lasers Eng. 2013, 51, 696–706.
  52. Sinha, A.K.; Kim, D.Y.; Ceglarek, D. Correlation analysis of the variation of weld seam and tensile strength in laser welding of galvanized steel. Opt. Lasers Eng. 2013, 51, 1143–1152.
  53. Unt, A.; Lappalainen, E.; Salminen, A. Autogeneous laser and hybrid laser arc welding of T-joint low alloy steel with fiber laser systems. Phys. Procedia 2013, 41, 140–143.
  54. Lamas, J.; Karlsson, J.; Norman, P.; Powell, J.; Kaplan, A.F.H.; Yañez, A. The effect of fit-up geometry on melt flow and weld quality in laser hybrid welding. J. Laser Appl. 2013, 25, 32010.
  55. Chen, Y.; Feng, J.; Li, L.; Chang, S.; Ma, G. Microstructure and mechanical properties of a thick-section high-strength steel welded joint by novel double-sided hybrid fibre laser-arc welding. Mater. Sci. Eng. A 2013, 582, 284–293.
  56. Casalino, G.; Campanelli, S.; Ludovico, A.D. Hybrid welding of AA5754-H111 alloy using a fiber laser. Adv. Mater. Res. 2013, 628, 193–198.
  57. Cai, C.; Feng, J.; Li, L.; Chen, Y. Influence of laser on the droplet behavior in short-circuiting, globular, and spray modes of hybrid fiber laser-MIG welding. Opt. Laser Technol. 2016, 83, 108–118.
  58. Huang, A.; Zhang, J.; Gao, C.; Hu, R.; Pang, S. Effects of groove constraint space on plasma characteristics during Laser-MIG hybrid welding of Titanium alloy. J. Manuf. Process. 2019, 48, 137–144.
  59. Liu, S.; Zhang, F.; Dong, S.; Zhang, H.; Liu, F. Characteristics analysis of droplet transfer in laser-MAG hybrid welding process. Int. J. Heat Mass Transf. 2018, 121, 805–811.
  60. Zhang, W.; Hua, X.; Liao, W.; Li, F.; Wang, M. Study of metal transfer in CO2 laser+GMAW-P hybrid welding using argon-helium mixtures. Opt. Laser Technol. 2014, 56, 158–166.
  61. Petring, D.; Fuhrmann, C.; Wolf, N.; Poprawe, R. Investigations and applications of laser-arc hybrid welding from thin sheets up to heavy section components. In Proceedings of the ICALEO 2003—22nd International Congress on Applications of Laser and Electro-Optics, Jacksonville, FL, USA, 13–16 October 2003.
  62. Chen, M.; Xin, L.; Zhou, Q.; He, L.; Wu, F. Effect of laser pulse on alternative current arc discharge during laser-arc hybrid welding of magnesium alloy. Opt. Lasers Eng. 2018, 100, 208–215.
  63. Li, F.; Tao, W.; Peng, G.; Qu, J.; Li, L. Behavior and stability of droplet transfer under laser-MIG hybrid welding with synchronized pulse modulations. J. Manuf. Process. 2020, 54, 70–79.
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