1. Introduction
Recently, due to the intensification of international environmental regulations to combat global warming, there has been a growing interest in reducing greenhouse gas emissions and improving fuel efficiency in the transportation industry. As a result, the pace of technological development for creating components and manufacturing processes related to high fuel efficiency and emission-free transportation devices is accelerating. Specifically, in the automotive industry, there is ongoing development of materials and component production technologies for eco-friendly vehicles designed to achieve high fuel efficiency. This includes the thinning of steel materials or the application of non-ferrous materials like aluminum, magnesium, and titanium in car bodies1). Among lightweight non-ferrous materials, the use of aluminum alloys is expected to grow at an average annual rate of 12% in the global automotive market by 2028, as they continue to be adopted for lightweighting. Aluminum is a widely used structural material with low density, high specific strength, good processability, excellent corrosion resistance, and low-temperature characteristics. However, the welding technology required to assemble components using aluminum alloy materials faces challenges due to high thermal and electrical conductivity, as well as the presence of oxide film with a high melting point (2,060 °C). This necessitates intense heating and makes it difficult to achieve uniform welding quality1-5). First, arc welding is one of the most widely used welding techniques due to its low investment and operational costs and efficient joining. When applying arc welding to aluminum materials, the high heat input results in a wide heat-affected zone and the low energy density causes low weld penetration, which can lead to deformations. However, the use of DCEP (Direct Current Straight Positive) welding removes the oxide film, improving weld quality. Additionally, using filler material can result in high welding efficiency and effective gap bridging6,7). When using arc welding methods, issues related to weld quality such as thermal deformation, insufficient penetration, high-temperature cracking, and internal pore formation can occur. Laser welding is a more expensive option, but it offers the benefit of fast welding speeds and minimized weld area due to keyholes, resulting in high-quality welds. When applying laser welding to aluminum materials, although the high reflectivity of the laser reduces weldability and its gap-bridging capability is weak, the method does result in less deformation, deeper penetration due to keyhole formation from the high energy density, and narrower beads. Furthermore, laser welding can be performed at speeds about ten times faster than arc welding5-7). Laser welding techniques can lead to issues such as high-temperature cracking, underfill, and internal pore formation or defects due to rapid cooling and solidification8,9). Defects arising in arc welding and laser welding are influenced not only by alloy composition but also by various process variables, either singly or in combination. These defects significantly impact mechanical properties like tensile strength and fatigue strength of the weld. As the porosity within the weld increases, mechanical properties such as tensile strength and fatigue strength drastically decline10,11).
Laser-arc hybrid welding combines the high-density, low-heat input of the laser with the low-density, high- heat input of the arc to mutually compensate for the drawbacks and maximize the advantages of each, leading to improved weld quality and speed in aluminum alloys. Due to the characteristics of aluminum materials, active research is underway to optimize laser-arc hybrid welding processes and ensure quality. L. Huang et al.13) found that using a laser-leading welding process results in more stable keyhole formation and less weld porosity due to better molten pool convection, compared to when arc-leading welding is used. I. Bunaziv et al.14) reported that when the distance between the laser and the arc is short, collisions between the laser beam and pool can cause keyhole collapse and increased porosity. H. Miao et al.15) described a relationship formula where the keyhole could be stabilized under various pressures. They also observed that as welding speed increases, the distribution of porosity within the weld decreases. Y. Zhao et al.19) found that the escape of pores is hindered when oxides attach to metallurgically-formed pores, causing them to remain in the molten pool. Y. Li et al.18) reported that bubbles can escape due to the thermal buoyancy and flow formed on the upper surface of the molten pool. From both a process and metallurgical perspective, pores that form during the welding process and fail to escape the molten pool have a negative impact on weld quality. Such defects degrade mechanical properties, which is why active research has been conducted to optimize the process. In particular, methods have been used to improve weldability by reducing defects, not only based on individual variables like the laser and arc but also complex variables such as laser-leading, the distance between laser and arc, welding speed, and heat source output12-16). Using both laser and arc simultaneously exhibits different characteristics or properties due to the synergistic effects between the two heat sources. Therefore, it is necessary to examine the impact of these complex variables mentioned earlier. Recently, active research has been underway to control defects and porosity through various processes, such as controlling laser beam oscillation17).
This paper summarizes studies that address the need for laser-arc hybrid welding in aluminum alloys and improving weld integrity through process variable optimization. It aims to provide relevant information for further research by investigating domestic and international research on welding improvements from a process perspective, which is easier to control compared to other variables.
2. Main Text
2.1 Laser-Arc Hybrid Welding
Laser-arc hybrid welding, as seen in Fig. 1, is a welding technique where the laser beam and the arc heat source are simultaneously applied, moving in the direction of the weld to form a molten pool. By combining laser and arc heat sources, it achieves high efficiency and ensures high weld quality. Fig. 2(a) shows that a broad area can be welded due to the arc zone in GMAW, confirming excellent gap-bridging ability; (b) a narrow area can be welded with the laser’s high-density energy to achieve deep penetration; and (c) in hybrid welding, excellent gap-bridging can be obtained due to deep penetration by the high-density energy laser and the low-density, high-heat arc source. It also has the advantage of preventing thermal distortion and reducing deformation and stress in complex-shaped welds4).
Due to these advantages, laser-arc hybrid welding can be applied to a wide range of plate thicknesses, from thin sheets to thick plates, as well as various materials including metal and non-metal alloys. Moreover, various heat sources (energy sources - laser source: CO2, Nd:YAG, diode, disk, fiber, arc source: MIG/MAG, TIG, SAW, plasma) can be used, making hybrid welding versatile through numerous combinations. Despite this, research is ongoing to reduce defects that occur when welding aluminum alloys using hybrid methods. Fig. 3 shows a schematic of the molten pool and pore formation mechanisms during hybrid welding. It illustrates that pores form when the laser’s keyhole becomes unstable and the keyhole wall collapses, as well as when the arc’s molten pool forms and causes a rapid increase in hydrogen solubility. These pores fail to escape due to inadequate molten pool convection, causing issues in weld quality. Many research is ongoing on the weldability of aluminum alloys, including studies that suggest pores can easily escape from the molten pool due to the behavior of the laser heat source’s keyhole, the volumetric transfer of the arc heat source, and the convection action at the top of the molten pool18,19).
2.2 Characteristics of Aluminum Hybrid Welding
When applying hybrid welding to aluminum materials, Han et al. found that as the porosity increases, both the tensile strength and fatigue strength of the weld decrease. They reported that the speed of gas flow impacts pore formation. Table 1 shows that an increase in porosity and pore size results in a decrease in tensile strength and elongation. Fig. 4 confirms that as porosity and pore size increase, fatigue strength decreases. Wu et al. explained that an increase in pores formed by gas leads to more fatigue cracks. Research by Zhan et al. on the distribution of pores and microstructure within aluminum welds reported that pore formation is influenced by the increase in gas solubility in the molten pool and the vaporization of specific elements. They found that both the size and rate of pores adversely affect the quality of the weld12,20-22).
Table 1
Tensile properties of the laser-MIG hybrid welded joints with different porosity distributions21)
Fig. 4
S-N fatigue curves of the laser-MIG hybrid welded fatigue testing specimens with five typical porosity distributions: (a) P1#, (b) P4#, (c) P5#, (d) P6#, and (e) P7#21)
2.3 Comparison of Pore Characteristics Based on Process Variables in Hybrid Welding
2.3.1 Impact of Heat Input from Laser/Arc Sources on Pores
Pores in hybrid welding are predominantly formed in the lower part of the keyholes created by the laser23,24). Most of these pores become trapped within the weld as it solidifies. Large pores that remain trapped in the weld can be reduced through process optimization by adjusting the laser and arc heat sources. As the laser power decreases, the height-to-width ratio of the keyhole shrinks, and as the arc current increases, the molten pool enlarges23,25,26). This lowers the chance of keyhole collapse, and the molten pool is pressed down by the arc pressure, allowing the pores to rise to the surface of the molten pool and escape. Yan et al. compared the porosity of welds using 6xxx series aluminum alloys, depending on the magnitude of laser power and arc current16). Fig. 5 and Fig. 6 show that an increase in laser output leads to an increase in porosity, while an increase in arc power results in a decrease in porosity. The cross-section of the weld in Fig. 6 shows that penetration has not been achieved. The conditions for achieving complete fusion not only involve controlling the heat input from the laser and arc but also require consideration of phenomena affecting the molten pool, such as surface tension and recoil pressure15,18).
Fig. 6
Surface appearances, cross sections and X-ray inspection results of YAG laser and YAG-MIG hybrid weld beads25)
2.3.2 Impact of Welding Speed
Welding speed is one of the variables that significantly affects the shape and size of the welding bead and the porosity. As the welding speed increases, the size of the molten pool decreases, the keyholes become smaller, and the fusion depth becomes shallower, allowing pores to be distributed smaller and escape more easily toward the edge of the weld. Miao et al. used 7xxx series aluminum alloys and 5xxx series filler wires and found that in both laser-leading and arc-leading hybrid welding, an increase in welding speed led to a reduction in porosity, as shown in Fig. 7. This is due to the reduction in the height of the molten pool, which shortens the path for residual gases to escape from the weld, thus facilitating their release and reducing porosity15,27,28).
Fig. 7
X-ray inspection, cross-section observation results and porosity area fractions under different welding speeds in different configurations15)
2.3.3 Impact of Distance Between Laser and Arc
The positions and distance differences between the laser and the arc also have a significant impact on welding characteristics. In laser-leading welding, when the distance between the laser and the arc is short, the pool from the arc torch collides with the laser beam, leading to unstable interactions that frequently cause keyhole collapse. This keyhole collapse is due to aluminum having a lower surface tension than iron; the expansion phenomenon closes the keyhole with the molten pool, leading to the formation of pores of various sizes within the weld. These large and small pores in the weld significantly degrade the overall welding quality17,23). Bunaziv et al. used 5xxx series aluminum alloys and filler wires and compared the porosity in the weld by setting the laser-arc distance to 1mm, 3mm, and 5mm under the same conditions, while also analyzing the mechanical properties of the weld. As shown in Fig. 8, as the distance between the laser and arc increases when the laser leads, there is a noticeable reduction in porosity17). However, optimal conditions for the laser and arc heat sources may vary depending on the distance between the laser and the arc, so optimizing the heat sources based on this distance is necessary.
Fig. 8
The effect of separation distance and MIG torch direction on weld appearance, quality and porosity level in fiber laser-MIG hybrid welding of AA5083 with pure argon shielding gas(100%) of cross section and fractures workpieces parallel to the weld line14)
2.3.4 Impact of Laser/Arc leading mode
In laser-arc hybrid welding, if the molten pool formed by the laser and arc heat sources is narrow, it becomes difficult for bubbles to escape, resulting in a higher rate of defects such as pores and cracks20,25). Huang et al. used 5xxx series aluminum alloys and filler wires to analyze the molten pool size, spatter phenomenon, and microstructure based on whether the laser or arc leads. They measured the porosity and confirmed the mechanical properties. When the laser leads, it forms a larger molten pool compared to when the arc leads, allowing for the easier escape of pores and stable pool flow and convection, resulting in lower pore formation16). As seen in Fig. 9, when the laser leads, the size and number of pores in all areas of the weld are reduced.
Fig. 9
Macrograph of joint and longitudinal section of the weld beads (a) ALHW, (b) LAHW, (c) The porosity distribution in the welds correspond to (a) and (b)13)
2.3.5 Impact of Laser Oscillation
Laser beam oscillation plays a significant role in promoting stable keyhole formation and inhibiting pore formation through stirring the molten pool. Wang et al. used 6xxx series aluminum alloys and 5xxx series filler wires, measuring and comparing the porosity and weld penetration depth based on the amplitude and frequency of the oscillations. Fig. 10(a) shows a reduction in porosity as the amplitude increases. Fig. 10(b) shows that the porosity dramatically decreases when the amplitude is above 0.5mm, and at amplitudes above 1.5mm, the welding transitions to conduction mode, resulting in shallower penetration. Fig. 11(a) shows that the porosity decreases as the frequency of the oscillations increases. Fig. 11(b) shows that the porosity decreases to below 1% when the frequency reaches 300Hz, and the weld penetration depth also decreases as the frequency increases. As the stirring effect strengthens, the inhibition of pore formation increases. These results indicate that laser beam oscillation expands and stabilizes the keyhole and minimizes defects through convective flow in the molten pool by stirring, demonstrating a positive impact17,29,30).
Fig. 10
(a) X-ray NDT results of porosity under different amplitudes, (b) effect of oscillating amplitude on percent porosity and penetration depth, (I=200 A, f=300 Hz)17)
Fig. 11
(a) X-ray NDT results of porosity under different frequencies, (b) effect of oscillating frequency on percent porosity and penetration depth, (I=200 A, A=0.6 mm)17)
3. Conclusion
This paper presents comprehensive research findings on optimizing welding defects that affect mechanical properties through process variables in aluminum alloy laser-arc hybrid welding. We reviewed the current research trends in reducing welding defects from a process-oriented perspective, and the conclusions are as follows.
1) As laser power decreases, the height-to-width ratio of the keyhole shrinks, and as arc current increases, the molten pool expands. Therefore, the probability of keyhole collapse decreases, and defects can be reduced as the molten pool is pressed down by the arc pressure, allowing pores to escape to the surface.
2) As welding speed increases, the size of the molten pool decreases, and penetration due to the keyhole becomes shallower. The shorter path for pore egress within the molten pool due to increased welding speed makes it easier for pores to escape, contributing to improved weld quality.
3) Higher levels of defects lead to a reduction in mechanical properties like tensile strength and fatigue strength in the weld. If the distance between the laser and arc is too short, the pool and laser beam may collide, causing the keyhole to collapse. Therefore, maintaining a certain distance between the laser and arc is advantageous for defect control.
4) When the molten pool formed in hybrid welding is narrow, it is difficult for bubbles to escape, causing defects such as pores and cracks. Laser leading, compared to arc leading, forms a wider molten pool and provides better defect control through stable convective phenomena.
5) Laser beam oscillation significantly influences the formation of stable keyholes and convective flow in the molten pool, which in turn impacts pore formation. An increase in oscillation amplitude and frequency expands the keyhole and enhances the stirring effect, reducing the porosity. Oscillation plays a significant role in inhibiting pore formation.
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