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Mar 20, 2025

Laser arc weak coupling achieves porosity free welding and stable droplet transfer for high nitrogen steel | Scientific Reports

Scientific Reports volume 15, Article number: 9474 (2025) Cite this article Metrics details To address the challenge of porosity suppression in laser-arc hybrid welding of high-nitrogen steel, a

Scientific Reports volume 15, Article number: 9474 (2025) Cite this article

Metrics details

To address the challenge of porosity suppression in laser-arc hybrid welding of high-nitrogen steel, a weakly coupled laser-arc welding method suitable for high-nitrogen steel is proposed. A test platform for composite and “sandwich” welding of high-nitrogen steel was constructed, and this weakly coupled laser-arc welding method was thoroughly analyzed in terms of droplet transfer, arc morphology, keyhole stability, electrical signal stability, weld formation, and porosity rate. The research results show that when the distance between the laser and arc is 6 mm, the droplet transfer mode is jetting, fine droplet, or mixed, with stable, uniform transfer, a stable keyhole, minimal electrical signal fluctuations, and good weld formation quality. The arc is elongated due to laser attraction, increasing the molten pool area and reducing its solidification rate, which provides sufficient time for bubbles to escape, resulting in a pore-free weld. This proves the effectiveness of the weakly coupled laser-arc welding method for high-nitrogen steel, achieving a high-quality, pore-free weld with excellent formation quality. These findings provide guidance and reference for composite welding processes of high-nitrogen steel.

High-nitrogen austenitic stainless steel (High nitrogen steel) achieves an austenitic structure by partially or completely substituting nitrogen for nickel. Nitrogen provides solid-solution strengthening, giving the material excellent toughness and corrosion resistance. As a result, high-nitrogen steel is widely used in industries such as aerospace and marine equipment1,2,3. The superior properties of high-nitrogen steel are attributed to the dissolved nitrogen content. However, nitrogen loss to varying degrees occurs in the weld seam after fusion welding, which significantly affects the mechanical properties of the welded joints and limits the further application of high-nitrogen steel4,5,6. Consequently, finding effective measures to reduce nitrogen loss in high-nitrogen steel welds and improve the mechanical properties of welded joints has become a focal point of research for scholars both domestically and internationally.

Laser-arc hybrid welding is the primary welding method for high-nitrogen steel. This technique combines laser and arc heat sources to achieve a “1 + 1 > 2” effect, providing advantages such as high welding speed, low heat input, strong bridging capability, and deep penetration7,8. In hybrid welding, the distance between the laser and the arc is particularly important; the laser can attract and stabilize the arc, while the arc can dilute laser-induced plasma, improving energy efficiency9,10. The degree of interaction between the laser and arc depends on their separation, impacting droplet transfer, molten pool temperature, weld seam formation, and plasma shape. Liu et al. recorded the motion trajectories of tracer particles in the molten pool during hybrid welding using a high-speed camera and studied surface flow patterns under different laser-arc distances. They found that at a 5 mm distance, a distinct low-temperature region formed between the arc interaction zone and laser keyhole11. Liu et al. also studied the effect of distance between the laser and TIG electrode on penetration depth and melting efficiency in different hybrid welding modes. When using TIG-assisted laser hybrid welding, penetration depth initially increased with the distance (DLA) and then decreased, achieving a 53% improvement in melting efficiency at DLA = 4 mm12. This indicates that adjusting the laser-arc spacing can effectively control weld formation, molten pool temperature, plasma shape, and laser-arc coupling13,14,15. At normal temperature and pressure, nitrogen’s solubility in the weld pool is much lower than in high-nitrogen steel production, closely linked to alloy composition and nitrogen partial pressure16,17. During solidification, excess nitrogen escapes from the weld pool, forming weld porosity due to rapid cooling, leading to nitrogen loss in the weld seam and reducing joint performance17,18,19. To address nitrogen loss, researchers have conducted extensive studies on welding techniques, shielding gases, multiphysics fields, and specially designed filler wires20,21,22,23,24,25,26. Ning et al. used gas metal arc welding on high-nitrogen steel with a nitrogen content of 0.56%, achieving weld joint tensile strength comparable to the base metal but with significantly reduced toughness27. Bo et al. suppressed weld porosity and nitrogen loss by adjusting shielding gas ratios, modifying filler wire composition, and applying ultrasonic vibration. Adding nitrogen and oxygen to the shielding gas increased nitrogen content in the weld seam, while ultrasonic vibration helped release trapped gases, reducing porosity28. Li et al. proposed a CrN powder-feeding laser-CMT welding method, which increased weld nitrogen content while reducing ferrite content in the arc and laser regions to 1.3%, enhancing joint tensile strength by 24.5%29. Liu et al. improved weld joint performance by adding different nitrides, finding that MnN had the best nitrogen-adding effect, increasing nitrogen by 43% and weld impact toughness by 28.1%. While adding nitrides can raise nitrogen levels in the weld, it also increases the tendency for porosity formation30. Currently, the main approach to reducing nitrogen loss involves increasing nitrogen content in filler materials and shielding gases, but no satisfactory solution has been found for nitrogen-induced porosity in welds. Thus, developing a simple and effective method to suppress nitrogen porosity in high-nitrogen steel welds is urgently needed.

In summary, research on laser-arc hybrid welding of high-nitrogen steel has mainly focused on the strong coupling effect between the laser and the arc. This strong coupling effect can accelerate the formation of the weld pool, increase peak temperatures, and improve welding efficiency. However, due to the unique behavior of nitrogen in high-nitrogen steel compared to other alloying elements, strong coupling also accelerates nitrogen escape, leading to porosity and reducing weld quality. This paper proposes a laser-arc weak coupling welding technique, where adjusting the distance between the laser and the arc achieves a weak coupling effect. The study evaluates the stability of the weak coupling welding process, arc shape, weld formation, and porosity rate, demonstrating the advantages of laser-arc weak coupling welding for high-nitrogen steel and offering a new approach for high-quality welding of this material.

The laser-arc hybrid welding system consists of an Nd:YAG continuous solid-state laser and a MIG welding machine. The laser has a maximum output power of 6 kW, a wavelength of 1064 nm, and a spot diameter of 0.6 mm. The welding machine uses the YD-350AG2HGE model MIG/MAG welder produced by Japan’s Panasonic company, and is equipped with a dedicated wire feeding mechanism. The MIG/MAG welder has a rated output current of 350A and a rated output voltage of 31.5 V. Prior to welding, the oxide layer on the surface of the high-nitrogen steel was removed by grinding, followed by acetone cleaning to eliminate oil and impurities. After thorough drying, the specimen was clamped onto the welding platform and butt-welded in a flat position. The sample dimensions were 400 mm × 100 mm × 8 mm, featuring a single-sided Y-groove with a 15° groove angle, a butt gap of 0.6 mm, and a root face of 4 mm, as shown in Fig. 1c. A stainless-steel wire with a diameter of 1.2 mm was used as the filler material, and the main chemical compositions of the base material and filler wire are listed in Table 1. The schematic diagram of the laser-arc hybrid welding process is illustrated in Fig. 1a, where the welding torch and laser beam are set at an angle of 30°, as shown in Fig. 1b. The optimal welding parameters determined from previous studies were as follows: laser power 2.2 kW, welding current 240 A, voltage 28 V, welding speed 0.6 m/min, defocus distance − 2 mm, with pure Ar used as the shielding gas at a flow rate of 18 L/min for the welding torch and 30 L/min as the external shielding gas11. To prevent oxidation of the weldment, argon shielding gas was supplied 0.2 s before welding and stopped 0.5 s after welding.

Schematic diagram of high nitrogen steel composite welding.

The droplet transition images are captured using a CMOS-CR5000 × 2 high-speed camera, which is placed on the side of the composite weld joint. During the welding process, a red laser with a wavelength of 808 nm is used as the background light source. The high-speed color camera records images at a sampling frequency of 5000 frames per second. Due to the high brightness in the arc zone during welding, an interference filter with a wavelength of 808 nm is added in front of the high-speed camera lens. The electrical signals during the welding process are recorded using a Hannover arc quality analyzer with a sampling frequency of 5 kHz. To observe the keyhole morphology, a “sandwich” welding technique, widely used and improved by various researchers, was employed. The setup and principle of the “sandwich” welding method are shown in Fig. 1d. The 3D image in Fig. 1 was generated using the 2020 version of SolidWorks. A custom fixture was used to clamp an 8-mm-thick high-nitrogen steel plate together with a 5-mm-thick quartz glass plate, with the laser spot focused on the high-nitrogen steel surface at a distance of 0.2 mm from the quartz glass. The transparency of the glass allowed direct observation of the keyhole morphology and the dynamic evolution of the hybrid welding molten pool during the welding process. A high-speed camera, positioned perpendicular to the quartz glass, recorded the process. After welding, non-destructive X-ray testing equipment was used to detect defects, and Image Pro Plus 6.0 software was utilized to analyze radiographic images and calculate the porosity of the weld. Samples with dimensions of 40 mm × 2 mm × 2 mm were extracted from the center of the arc zone and laser zone of the weld using a wire electrical discharge machine. These samples were polished to remove surface oil and impurities. The cross-sectional morphology of the weld was observed and analyzed using metallographic samples prepared by cutting 15 mm × 5 mm specimens perpendicular to the weld center. After embedding, the specimens were polished stepwise with sandpaper ranging from 180 to 2000 grit on a UNIPOL-80Z grinding machine and subsequently polished to a mirror finish. A ME61 stereomicroscope was used to capture macro images of the weld cross-sections, and SPOT software was employed to measure the weld bead width and reinforcement height. The microstructure of the welded joint was observed using a Leica DM2700M optical microscope. After welding, porosity within the weld was inspected using X-ray radiography. To avoid the start and end phases of welding, a 200 mm long section of the weld was selected for radiographic testing. The porosity ratio was calculated using Image Pro Plus 6.0 to measure the areas of the weld and pores on the radiographic film. According to ISO 5871-2003 (Welded joints in steel, nickel, titanium, and their alloys—Quality levels for imperfections), the porosity ratio was defined as the ratio of the total pore area to the total weld area on the radiographic film.

The characteristics of droplet transfer as the heat source distance varies from 4 to 8 mm are shown in Fig. 2. With the change in heat source distance, significant differences are observed in droplet transfer modes and welding process stability. When the heat source distance is 4 mm, the laser exerts a strong attraction and compression effect on the arc, causing the arc to noticeably deflect toward the laser action point. Under the thermal radiation of the laser, the wire melts rapidly, forming large-sized droplets. These droplets generate intense impact waves when transferring into the molten pool, leading to temporary closure of the laser keyhole, which disrupts the welding process stability and is prone to spatter and pore defects. As the heat source distance increases to 5 mm, the droplet size decreases and exhibits a more regular spherical shape. The transfer process is primarily controlled by surface tension, resulting in reduced impact on the molten pool and improved welding stability. When the heat source distance further increases to 6 mm, the larger spacing results in smaller droplets, exhibiting a fine particle transfer mode. The droplet transfer position is farther from the laser keyhole, significantly reducing the disturbance to the keyhole, which remains stable, thereby lowering the risk of process-related pores. This contributes to high welding stability and weld quality.

Droplet transition characteristics under different heat source spacing.

However, when the heat source distance exceeds 6 mm, the attraction and compression effects of the laser on the arc gradually weaken. The droplet formation and growth rates slow down, and the transfer cycle becomes longer, leading to potential adhesion between successive droplets. During droplet transfer, noticeable arc column oscillations occur, causing significant deformation and oscillation on the molten pool surface. This results in a substantial decline in welding process stability and makes it prone to spatter and other defects. Additionally, the weakened interaction between the laser and arc leads to an increase in weld width. Thus, the heat source distance has a critical impact on droplet transfer modes and weld quality. Optimizing the heat source spacing is essential to achieving high-quality welding.

From the images of droplet transfer characteristics, it can be observed that the entire droplet transfer process involves multiple forces. Figure 3 shows a schematic diagram of the forces acting during droplet transfer. The droplet is primarily influenced by a combination of gravity, surface tension, plasma drag force, electromagnetic contraction force, and the reactive force of metal vapor. These forces collectively impact the characteristics of droplet transfer during the welding process.

Schematic diagram of droplet force.

In the arc space, the gravity acting on the droplet is represented by the following equation:

where R is the droplet radius, \(\rho\) is the droplet density, and g is the gravitational acceleration. From the formula, it can be seen that the gravity of the droplet is directly proportional to its diameter. When the welding wire melts into droplets and hangs at the end of the wire, the droplets are hindered from growing and detaching by surface tension, which can be expressed as:

Among them, \(R_{w}\) is the radius of the droplet shrinkage, and \(\sigma\) is the surface tension coefficient of the droplet. The plasma flow force (\(F_{p}\)) plays an important role in the process of droplet transfer. The high-temperature airflow (plasma flow) in the arc space rapidly flows from the end of the welding wire to the molten pool. The plasma flow has a significant impact on the motion state of the droplet before and after separation from the welding wire. \(F_{p}\) can be represented by an equation:

Among them, \(C_{d}\) is the plasma flow coefficient (\(C_{d}\) = 0.45), \(A_{p}\) is the area affected by the plasma flow, \(\rho_{f}\) is the density of the plasma flow, and \(v_{f}\) is the plasma flow velocity.

Electromagnetic contraction force (\(F_{em}\)) is the Lorentz force exerted on the current in a magnetic field within a droplet. When the current flows from the welding wire to the molten pool, \(F_{em}\) promotes droplet transition; When the current flows from the molten pool to the welding wire, \(F_{em}\) hinders the transition of the molten droplet. The electromagnetic contraction force can also be represented by the current parameter in the droplet, as shown in the equation:

In the formula, \(u_{0}\) is the vacuum magnetic permeability, I is the welding current, and \(R_{d}\) is the equivalent radius of the droplet. When laser arc hybrid welding is performed, the metal vapor sprayed from the laser hole will generate a reaction force on the droplet, and the reaction force of the metal vapor on the droplet can be expressed as \(F_{RL}\):

The coefficient of flow resistance related to the Reynolds number of metal vapor, denoted as \(C_{D}\), is 0.45. A is the projected area of the object perpendicular to the flow direction. \(N_{a}\) is the Avogadro constant, \(k_{B}\) is the Boltzmann constant, \(T_{s}\) is the surface temperature of the molten metal, \(v_{0}\) is a constant of 340 m/s, \(B_{0}\) is the evaporation constant of the molten material, and \(r_{o} \left( z \right)\) is the area radius of the heat flux.

The relationship between metal vapor reactive force and the laser-wire distance can be calculated using Eq. 5. Calculations show that the metal vapor reactive force is greatest when the laser-wire distance is 0 mm. For sufficient penetration depth, most welding is performed with strong coupling between the laser and arc, typically with a laser-wire distance of less than 4 mm. In this case, the metal vapor reactive force acts on the droplet, hindering its transition into the molten pool. When the laser-wire distance exceeds 4 mm, the metal vapor reactive force is nearly 0 N, meaning the effect of metal vapor ejected from the laser keyhole on droplet transfer is almost negligible, which to some extent promotes the droplet’s transition into the molten pool. At a laser-wire distance of 6 mm, although the laser and arc are in a weak coupling state, the laser still helps stabilize the arc, preventing it from drifting over the molten pool. The radiative heat on the welding wire decreases, slowing the melting rate, and the droplet grows at the wire tip, increasing the droplet’s neck radius \(R_{w}\) and slightly reducing the electromagnetic contraction force \(F_{em}\). With high current and voltage, plasma drag force becomes the primary force; the larger the droplet, the greater the area impacted by the plasma drag force, which prevents excessive droplet growth and promotes higher-frequency droplet transfer. This results in a high-frequency, stable jet or droplet transfer, improving the stability of droplet transfer. When the laser-wire distance is greater than 6 mm, there is virtually no coupling between the laser and arc, and the laser cannot stabilize or guide the arc, causing the arc to drift over the molten pool randomly, thus reducing welding stability. This observation is consistent with the droplet transfer behavior observed in the experiment.

To investigate the stability of the keyhole, a “sandwich” welding method was employed to observe the morphological changes of the keyhole during the laser-arc hybrid welding process. Under different heat source coupling modes, the shape and stability of the keyhole exhibited significant variations. Figure 4 shows the longitudinal images of the laser keyhole and molten pool at different heat source distances. A keyhole with a high aspect ratio can be observed, along with the typical molten pool morphology of arc welding, characterized by shallow penetration and a wide weld pool in the arc-affected region. As the heat source distance changes, the keyhole morphology also varies. At a heat source distance of 4 mm, the keyhole connects with the arc melting zone, resulting in a large keyhole opening, low stability, and significant depth fluctuations. At this distance, the strong laser-arc coupling draws the arc toward the keyhole, effectively reducing the gap between the arc and the laser. This causes the keyhole to collapse, generating considerable metal spatter. Droplets fall directly into the keyhole, and at time t1 + 17.2 ms, a liquid bridge forms on the keyhole wall, connecting the front and rear of the droplet. When the laser acts on the liquid bridge, instantaneous vaporization occurs, leading to spatter, collapse, or keyhole closure, which results in the formation of pores.

Longitudinal images of laser keyhole and molten pool at different heat source spacings.

At a heat source distance of 5 mm, the keyhole depth exhibits significant variations, with droplets flowing along the front wall of the keyhole. At 6 mm, the laser keyhole is positioned at the edge of the arc molten pool, where molten metal accumulates between the keyhole and the arc zone to form a protrusion. Droplets land on this protrusion, reducing their impact on the molten pool. At this distance, the pressure of metal vapor and the surface tension of the molten pool reach a balance, maintaining keyhole stability. As the heat source distance increases, the width of the protrusion also increases. When the heat source distance exceeds 7 mm, a large amount of metal vapor and spattered metal accumulates above the keyhole. The keyhole becomes shallower and wider, indicating reduced stability. These findings highlight that the heat source distance significantly affects the stability and morphology of the keyhole during laser-arc hybrid welding, making it a critical parameter for optimizing welding performance.

To evaluate the stability of the keyhole at different heat source distances, a statistical analysis of the keyhole depth was conducted, with measurements taken every 5 ms, totaling 30 data points per group. Figure 5 shows the average and variance of keyhole depth for various heat source distances. When the heat source distance is 4 mm, the average keyhole depth is 6.11 mm, which is the deepest. As the heat source distance increases, the weld depth gradually decreases, reaching 4.6 mm at an 8 mm distance, a reduction of 1.51 mm compared to the 4 mm distance. This change in welding depth is related to the coupling of the two heat sources. When the heat source distance is small, the two sources have strong coupling; the arc can dilute the laser-induced plasma, allowing most of the laser energy to be absorbed by the base material, creating a deeper keyhole. When the coupling between the two sources is weak or nonexistent, the plasma plume and metal vapor remain in the laser beam path, causing scattering and absorption that lead to laser energy attenuation29,30. This results in reduced laser energy and shallower keyhole depth. From the variance curve, it is evident that the variance of keyhole depth is smallest at a heat source distance of 6 mm, indicating minimal depth fluctuation and the best stability at this distance. When the heat source distance exceeds 6 mm, the variance of the keyhole depth fluctuates, indicating poorer stability.

Keyhole stability analysis.

Figure 6 shows the current and voltage waveforms at different heat source distances, with each curve taken from a 0.7 s interval that excludes the arc initiation and termination stages. The figure reveals that at heat source distances of 4–6 mm, the current and voltage waveforms are dense and uniform, with relatively small fluctuations and a consistent pattern. At a 6 mm heat source distance, the waveform shows small spikes. In laser-arc hybrid welding, the addition of laser-generated plasma alters the physical characteristics of the composite arc plasma. When the laser and arc are in close proximity, the laser strongly guides and compresses the arc, reducing the drift and disturbance of charged particles from the arc root to the laser’s impact point, resulting in a more stable welding signal. According to the droplet transfer analysis, at a laser-wire distance of 6 mm, the droplet transfer mode is jet or stream transfer with a high transfer frequency. This mode creates a more stable molten pool flow, effectively maintaining the stability of the laser keyhole and minimizing waveform fluctuations in the welding signal. When the heat source distance exceeds 6 mm, the current and voltage fluctuations increase, with distinct spikes and irregular patterns in the waveform. At larger distances, the constriction force on the arc weakens, the arc volume expands and disperses, and arc root drift leads to fluctuations in the welding signal. In this condition, the droplet transfer mode shifts to large-particle transfer, reducing welding stability.

Instantaneous current and voltage waveforms at different heat source distances.

Figure 7 shows the probability density distribution curves of instantaneous welding current at different heat source distances. During welding, the arc discharge remains steady most of the time, which occupies a large proportion of the welding process and corresponds to the higher middle region of the curve. However, the arc does not always remain stable; influenced by laser plasma and droplet transfer modes, the welding process may experience arc extinction and droplet short-circuiting, represented by the relatively flat regions on the left and right sides of the curve. If the curve covers a broad high-current region, this indicates frequent short-circuiting, making the welding process less stable. Conversely, if short-circuiting events are infrequent, the overall probability density curve of the instantaneous current converges more toward the central setpoint. Comparing the shapes and positions of each curve reveals that at a 6 mm heat source distance, the curve converges most closely to the center. At this distance, the droplet transfer mode is spray transfer, making the welding process the most stable, followed by 5 mm and 4 mm. When the heat source distance is 7 mm or 8 mm, the curve becomes more dispersed with a higher probability of high-current areas, indicating a particle transfer mode that tends to cause welding signal fluctuations. Based on the comparison of droplet transfer characteristics, arc shape, keyhole stability, and welding signal, a heat source distance of 6 mm yields the most stable welding conditions.

Probability density distribution of instantaneous welding current under different heat source spacing.

Figure 8 illustrates the surface and cross-sectional morphologies of welds under varying heat source distances. When the heat source distance ranges between 4 and 6 mm, the weld surface appears relatively smooth with no noticeable spatter, indicating effective coupling between the laser and arc, as well as stable and orderly droplet transfer. However, when the heat source distance exceeds 6 mm, a small amount of spatter appears on the surface. This is attributed to the weakened attraction of the arc to the laser at greater distances, causing intermittent separation between the two heat sources and leading to transient short-circuit transfer, which results in droplet spatter. The cross-sectional morphology of the welds reveals that all welds exhibit the characteristics of deep penetration welding. The upper portion is the arc-affected zone, while the lower portion is the laser-affected zone, forming a fan-shaped profile. The volume of the laser-affected zone decreases as the heat source distance increases. The weld top exhibits excessive reinforcement without undercuts or collapse defects, while the weld surface demonstrates good spreading, continuity, and high-quality formation. Full penetration is achieved for all samples at different heat source distances.

Surface morphology of weld under different heat source spacing.

However, when the heat source distance is set to 5 mm, porosity appears in the excess weld reinforcement at the top of the weld. This may result from gases produced by metallurgical reactions in the molten pool that are insoluble in liquid metal, or from bubbles generated by transient instability of the laser keyhole that fail to escape and remain trapped in the weld. When the heat source distance exceeds 6 mm, the volume of fully penetrated metal in the welds significantly decreases. Examination of the backside of the test plates reveals intermittent lack of penetration in the welds. At this distance, the separation between the two heat sources is too large, leading to a weakened dilution effect and reduced laser-induced plasma intensity. Consequently, the laser energy on the workpiece surface decreases, resulting in reduced weld penetration depth.

The quality of weld seam formation can be quantitatively analyzed through weld width, penetration depth, and reinforcement. Under full penetration conditions, the lower the reinforcement height, the better the spread ability of the weld surface. The formation quality of the weld surface is evaluated by the weld surface formation coefficient; the smaller its value, the better the formation quality. Figure 9 shows the effect of different heat source distances on weld width, reinforcement height, and weld surface formation coefficient. As shown in Fig. 9a, the weld width increases continuously as the heat source distance increases, while the reinforcement height first rises and then falls, reaching a maximum of about 1.6 mm. Figure 9b shows that, with increasing heat source distance, the formation coefficient initially increases and then decreases, following the same trend as the reinforcement height. When the heat source distance is 6 mm, the weld surface formation coefficient reaches its maximum, indicating the best formation quality.

The Effect of heat source spacing on weld seam width, weld reinforcement, and surface forming coefficient.

The microstructures of the weld and transition zone under different heat source spacings are shown in Fig. 10. The base metal (BM) primarily exhibits an austenitic structure with uniformly distributed lath morphology, characterized by a fine grain size and a small amount of twinning. The heat-affected zone (HAZ) also displays an austenitic structure but with coarser grains compared to the base metal, while the weld zone (WZ) consists of a mixed microstructure of ferritic dendrites and austenite. Since the microstructures of the base metal and HAZ remain identical across different spacings, the following discussion focuses specifically on the weld microstructure. When the heat source spacing is 4 mm or 5 mm, the weld contains a high density of dendrites with excellent growth continuity and elongated lengths, showing distinct directional alignment. Numerous dendrites exhibit outward expansion tendencies, accompanied by the formation of tertiary dendrite arms. This configuration arises from excessive energy concentration at the heat source, leading to overheating tendencies in the weld. Accelerated atomic diffusion rates promote continuous adsorption of atoms onto dendrite arms, driving their growth toward the weld center. At a 6 mm spacing, dendritic growth becomes discontinuous, with significantly reduced dendrite density and length. The number of tertiary dendrite arms decreases, while the secondary dendrite arm spacing increases. In this scenario, the increased distance between the dual heat sources enables the laser to act as a secondary stirring mechanism within the molten pool, transforming coarse dendritic structures into uniformly refined lamellar dendrites with random growth orientations. When the spacing exceeds 6 mm, decoupling between the laser and arc results in unstable welding conditions, manifesting as pronounced porosity within the weld due to insufficient molten pool dynamics.

Microstructure of welds at different heat source spacings.

Figure 11 shows the results of X-ray non-destructive testing for weld seams at different heat source distances. As shown, when the heat source distance is 4 mm, pores are distributed in both the middle and edge areas of the weld. At this distance, the laser and arc are close together, concentrating energy and forming a small, high-temperature composite molten pool. This results in precipitated pores in the weld, as they do not have enough time to grow and escape during the solidification process. Analysis of droplet transfer shows that when the heat source distance is 5 mm, the droplet transfer mode is spray transfer, making the welding process relatively stable. Most pores at this distance are likely nitrogen pores formed due to a decrease in solubility as the weld solidifies. When the heat source distance is 6 mm, the composite weld pool area increases, allowing gases sufficient time and space to escape to the pool surface, resulting in almost no pores in the weld. When the heat source distance exceeds 6 mm, the number of pores in the weld increases. Small pores appear in the middle of the weld in a chain-like pattern, while near the fusion line, the pores are larger, irregularly shaped, and more dispersed. The formation of large pores in the weld can be attributed to two main factors: first, the larger pool volume allows small bubbles to gradually merge into larger bubbles as they rise, but increased viscous resistance from the liquid metal prevents timely escape, leaving some bubbles trapped in the weld. Second, the increased distance between the heat sources reduces the laser keyhole effect, while the droplet transfer mode shifts to a large-particle transfer, easily causing turbulent disturbance in the molten pool. This disturbance can lead to laser keyhole collapse, resulting in process-induced pores.

X-ray non destructive testing of weld seam at different heat source spacing.

The porosity of the weld seam was calculated based on the X-ray inspection images in Fig. 11, and the results are shown in Fig. 12. As the heat source distance increases, the weld porosity initially decreases and then increases. When the heat source distance is 6 mm, a pore-free weld was achieved. Under other parameters, the weld porosity remained above 3%, reaching a maximum of 4.6%, indicating poor pore suppression. This suggests that increasing the heat source distance to 6 mm can alter the interaction between the laser and the arc, placing them in a weak coupling state. This increases the molten pool area, allowing more time for bubbles in the weld to escape and effectively maintaining the stability of the laser keyhole. Consequently, process-induced pores are reduced, lowering the weld porosity.

The effect of heat source spacing on weld porosity.

This study conducted butt welding and “sandwich” experiments on 8 mm thick high-nitrogen steel plates using a laser-arc hybrid heat source. By adjusting the distance between the laser and arc heat sources, the coupling intensity between the laser and arc was modified. The process stability of laser-arc weak coupling welding for high-nitrogen steel, along with its effects on weld morphology and porosity, was analyzed. The main conclusions are as follows:

Heat source distance is a key factor affecting welding stability. Analysis of droplet transfer, keyhole characteristics, and electrical signals showed that a heat source distance of 6 mm provides the best weak coupling effect, resulting in the most stable welding process.

At a 6 mm heat source distance, the droplet transfer mode is characterized by jetting, fine droplet, or mixed transfer, all of which are uniform and stable. The keyhole and electrical signal fluctuations are minimal, indicating a stable welding process. At this distance, the arc is elongated by laser attraction, reducing its shielding effect on the laser, which enlarges the molten pool area and lowers the solidification rate, allowing sufficient time for bubbles to escape.

As the heat source distance increases, the weld porosity initially decreases and then increases. At a heat source distance of 6 mm, a pore-free weld is achieved. The weld surface formation coefficient reaches its maximum at this distance, indicating good weld formation quality.

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

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This research is financially supported by the Jilin Provincial Department of Education’s “13th Five Year Plan” Science and Technology Project (Grant No. JJKH20190561KJ). We would like to thank Editage (www.editage.cn) for English language editing.

College of Mechanical and Electric Engineering, Changchun University of Science and Technology, Changchun, 130022, China

Fengde Liu & Kai Ning

Optical International Cooperation Base of the Ministry of Science and Technology, Changchun University of Science and Technology, Changchun, 130022, China

Fengde Liu & Kai Ning

Wuxi InfiMotion Propulsion Technology Co., Ltd, Wuxi, 214000, China

Xiaojun Liu

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Kai Ning: Conceptualization, Writing—original draft. Fengde Liu: Conceptualization, Writing—review and editing. Xiaojun Liu: Project administration, Resources, Methodology.

Correspondence to Fengde Liu.

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Liu, F., Ning, K. & Liu, X. Laser arc weak coupling achieves porosity free welding and stable droplet transfer for high nitrogen steel. Sci Rep 15, 9474 (2025). https://doi.org/10.1038/s41598-025-92670-w

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Received: 17 November 2024

Accepted: 03 March 2025

Published: 19 March 2025

DOI: https://doi.org/10.1038/s41598-025-92670-w

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