1. Introduction
Fig. 1 displays the existing space facilities supporting human habitation, including the International Space Station1), which began construction in 1998 and is managed by an international partnership involving the United States and Russia, as well as the Tiangong space station2), operated solely by China, with its core module, Tianhe, completing its orbital testing in 2021. All of these facilities operate in low Earth orbit, at altitudes between 340 and 450 km. This area, however, is densely populated with micrometeoroids and space debris, posing substantial risks to spacecraft, especially to capsules of space stations in long-term orbits, with the potential to lead to catastrophic failures. With advancements in aerospace technology, the number of orbiting space vehicles is rising, which in turn increases the risk of collisions. Recently, the Starlink-2305 satellite came close to the Chinese space station twice, in July and October 2021, while debris from the Kosmos-1408 satellite-a Russian weapons test aimed at destroying satellites-also posed a threat to the International Space Station. Damages in manned spacecraft caused by impacts with micrometeoroids or space debris are typically linked to perforation, crack formation, and the rapid spread of cracks in capsules3), as illustrated by real-world examples4,5) in Fig. 2. Welding has proven to be a reliable method for repairing defects in these capsules, providing an effective solution to ensure the long-term safety and stability of structures in space6). Table 1 summarizes the benefits of welding conducted in space.
Table 1
Research on space welding has been conducted by the former Soviet Union and the United States since the 1960s, with a focus on electron beam welding and arc welding. The key achievements in this field are outlined below. The first space welding experiments were carried out in 1965 in the former Soviet Union, where microgravity conditions were created for 25-30 seconds aboard a Tu-104 aircraft. These experiments involved welding using the Vulkan device9,10), which includes electron beam welding, gas metal arc welding, and plasma arc welding machines11). The aircraft carries out parabolic flights, as depicted in Fig. 3, to achieve this microgravity condition12). Table 2 provides a list of research platforms for microgravity experiments, including drop towers and parabolic flights. The Vulkan device, which had been tested on aircraft on Earth, was first utilized for space welding and cutting in 1969 by Soviet cosmonauts Georgy Shonin and Valeri Kubasov aboard the Soyuz 6 capsule. This experiment confirmed that electron beams are effective for use in space welding. Similar to the former Soviet Union, the United States conducted welding experiments under microgravity conditions using parabolic flights in 1972. Additionally, bead-on-plate electron beam welding and brazing joining experiments were conducted aboard the Skylab space station in 197319,20). Samples of stainless steel (304) and aluminum alloy (2219-T87) produced on the space station demonstrated metallurgical and mechanical properties comparable to those of samples produced on Earth. In 1984, Soviet cosmonauts Svetlana Savitskaya and Vladimir Dzhanibekov performed the first-ever manual electron beam welding test during a spacewalk, with Fig. 4 depicting this historic moment9,10). This experiment further confirmed the similarity between samples produced in space and those made on Earth, while also demonstrating the feasibility of space welding. During the 1990s, NASA in the United States and the Paton Welding Institute (PWI) in Ukraine planned manual welding experiments in space with an electron beam welding gun developed by PWI. However, the project was permanently canceled due to concerns over astronaut exposure to X-rays21). Russia also had planned a manual welding experiment in space, but it was never conducted. As a result, no further welding experiments have been performed in space to date. In Japan, gas tungsten arc (GTA) welding of aluminum alloys was conducted in 1998 under microgravity conditions, achieved through free fall in a drop tower22). In 2002, gas hollow tungsten arc (GHTA) welding experiments on aluminum and titanium alloys were conducted under microgravity conditions achieved via parabolic flight23). Despite China’s operation of its own space station, research on space welding remains largely limited. Similarly, in South Korea, there is a significant lack of research, with the exception of the study by Lee et al. on GHTA welding24). Fig. 5 provides a summary of the achievements in space welding by the major countries mentioned above.
Table 2
Microgravity research platforms13)
| Platform | Gravity level (g) | μg duration | Examples |
|---|---|---|---|
| Drop tower14,15) | 10-6 - 10-2 | 1.3 - 9.5 s | HITec Einstein-Elevator, NASA Glenn Drop Tower |
| Parabolic flight12,16) | 10-3 - 10-2 | 15 - 25 s | Airbus A310 ZERO-G NASA KC-135 |
| Sounding rocket17,18) | 10-5 - 10-3 | 3 - 20 min | MASER 11, TR-IA-3 |
| Satellite12,18) | 10-6 - 10-5 | month - year | International Space Station SJ-10 Recoverable Satellite |
As previously noted, the majority of space welding research thus far has concentrated on electron beam and arc welding from a process-oriented perspective. Table 3 outlines the reasons suggested by Siewert et al. in 19937). The evaluation criteria listed in the table are as follows:
1) Operator/Mission Safety: It shall align with the safety of astronauts and the success of their missions.
2) Micro-G Weld Quality: Weld quality shall not be compromised under microgravity conditions.
3) IVA & EVA Flexibility: It shall operate both in a pressurized life support compartment and outside in a high-vacuum environment.
4) Workpiece Variety: High-quality welding of various aerospace materials in diverse geometries shall be achievable.
5) Operation Mode Flexibility: It shall be easily adaptable to manual, semi-automatic, remote-controlled, and robotic operation modes.
6) Tolerance Flexibility: The tolerance for joint mismatch and fit-up shall be high.
7) Power Requirement: The process shall have low power requirements.
8) Energy Efficiency: The process shall have high energy efficiency.
9) Consumables Requirements: The use of consumables, including electrodes, shall be minimized.
10) Equipment Serviceability: The equipment shall be dependable and simple to repair.
Table 3
Candidate processes for in-space applications7)
As indicated in the table, laser welding was categorized as a process that did not meet the evaluation criteria in the 1990s. However, there is now a renewed need for research on space welding using lasers, due to technological advancements in lightweight, high-power, and high-efficiency laser sources-particularly fiber lasers-that enable laser welding to meet most of the existing evaluation criteria. Especially, laser sources offer several advantages, including the ability to weld in both vacuum and atmospheric pressure, a high degree of flexibility due to beam transmission via fibers, no need for welding electrodes or shielding gas, and applicability in other manufacturing processes such as cutting, drilling, bending, and additive manufacturing. Since many of the limitations of traditional laser welding have been overcome, it appears to be more suitable for space applications than electron beam and arc welding, highlighting the growing need for research in this field. Recently, NASA has been preparing for space welding experiments using fiber lasers, since 2024, in collaboration with Ohio State University. Fig. 6 shows the vacuum chamber and welding system that will be utilized during parabolic flight26).
Fig. 6
A self-contained vacuum chamber laser welding system aboard a parabolic variable flight to reproduce in space conditions26)
Space presents a vastly different environment from Earth, leading to changes in physical phenomena during the welding process. First, the majority of the buoyancy effect in the molten pool is eliminated due to the microgravity environment (1 × 10-4 ~ 1 × 10-3 g). In other words, convection in the molten pool caused by buoyancy is removed, which may lead to changes in the final solidification characteristics. A high vacuum (~10-4 Pa) environment can slow down the solidification of the weld by removing the convective heat transfer phenomenon, while boosting the vaporization of the parent material to increase the recoil pressure. All of these factors can influence the characteristics of the fusion zone during the welding process. Additionally, temperature fluctuations ranging from extremely low to high temperatures (-130℃ to 170℃) can cause substantial changes in welding characteristics. Solar ultraviolet radiation and oxygen atoms can also influence the process. This review paper examines existing research on laser welding conducted in space environments and explores the technologies required for performing laser welding in space.
2. Behavior of the Molten Pool in Microgravity and High Vacuum Conditions
In laser welding, the formation of the weld is influenced by the formation and behavior of the molten pool and keyhole. In laser keyhole welding under gravity, forces such as surface tension, hydrostatic pressure in the molten pool, and recoil pressure from metal evaporation exert on the inner wall of the keyhole. The keyhole shape is retained when these forces are in dynamic equilibrium. Under these conditions, the surface tension and hydrostatic pressure in the molten pool tend to collapse the keyhole, whereas the recoil pressure causes it to expand. The curvature of the free surface forming the keyhole increases if the keyhole becomes unstable, thus causing the surface tension to act in an effort to straighten the curvature. If the laser beam is primarily absorbed in the valleys of these curves, it can accelerate the spatter formation due to the increased recoil pressure. Fig. 7 demonstrates that a molten pool column forms at the keyhole entrance under partial penetration conditions, with the spatter being significantly produced due to the recoil pressure and shear force from the plasma jet. Spatter is produced beneath the keyhole under full penetration conditions, with the intense flow from the recoil pressure and the shear force from the plasma jet being identified as key contributing factors27,28). The hydrostatic pressure effect is removed in microgravity conditions (1 × 10-4 ~ 1 × 10-3 g), achieving a dynamic equilibrium state within the keyhole. As a result, the recoil pressure has a relatively stronger effect, making spatter generation more likely. Additionally, keyhole collapse caused by periodic fluctuations in the laser beam can also significantly contribute to spatter formation29). Katayama et al. used a high-speed camera to observe the laser spot welding process of A5083 aluminum alloy and found that the metal plume (plasma) was generated more intensely under microgravity than under Earth’s gravity, resulting in more severe spatter formation30). Spatter produced during laser welding decreases the total mass of the metal. In the welding of aluminum alloys, spatter formation is a significant factor contributing to underfilled weld bead31). Aluminum alloys, such as 5A06 and 2219, are the most commonly used materials for fabricating the casings of modern space station capsules32). Thus, it is crucial to minimize spatter generation. Furthermore, under the microgravity conditions of space, hot metal droplets, such as randomly scattered spatters, can be a significant risk to the carbon fiber composite shield on the outermost layer of the space station capsule. Consequently, critical challenges that must be addressed to enable laser welding in space include comprehending the underlying mechanism of spatter formation during laser welding in microgravity and effectively controlling the molten pool and keyhole behavior to minimize spatter generation.
The primary factors contributing to porosity during laser welding of aluminum alloys include gases dissolved in the molten pool, vaporization of alloying elements, and keyhole collapse. As the molten aluminum alloy pool solidifies, the solubility of hydrogen (H) drops drastically, with approximately a 20-fold reduction. As a result, hydrogen gets trapped at the solidification interface in the fast-solidifying aluminum alloy, thus forming pores during laser welding. Furthermore, the unstable keyhole, which constantly fluctuates during laser welding, plays an important role in the formation of pores. If the keyhole depth abruptly increases, its instability also rises, making the keyhole more susceptible to collapse. The gas bubbles generated after the collapse are enclosed by the molten pool. If there is insufficient time for them to escape, they become trapped at the solidification interface, leading to the formation of pores33). Both microgravity and high vacuum conditions can amplify the effect of recoil pressure from metal evaporation, potentially causing the keyhole to deepen11,34). Moreover, due to the low melting and boiling points of aluminum alloys, intense plasma is generated during laser welding, causing fluctuations in the keyhole that increase its susceptibility to instability. Keyhole instability is significantly amplified in a high- vacuum environment35), which in turn raises the likelihood of porosity formation. Moreover, under the microgravity conditions of space, the effects of buoyancy and the resulting convection are nearly eliminated, which hinders the escape of bubbles from the molten pool, as depicted in Fig. 8,36). The combination of high vacuum and microgravity conditions can increase the probability of porosity defects during laser welding in space. Thus, developing a method to control the behavior of the molten pool to aid gas evacuation is essential for enabling laser welding in space.
The surface tension of molten metal is greatly influenced by its chemical composition. In other words, adding an element with a high surface tension coefficient to the aluminum alloy is expected to increase the surface tension of the molten pool. 1) The surface tension coefficient of molten aluminum alloy is approximately 0.65 N/m, whereas molten titanium (Ti) has a surface tension coefficient of about 1.65 N/m. The addition of Ti to aluminum alloy can enhance the surface tension as depicted in Fig. 9(a)37). 2) The surface tension of the ternary alloy Al-Mg-Er ranges from 0.65 to 0.73 N/m; the surface tension increases with increasing Er content and then begins to decrease again, as indicated in Fig. 9(b). It is observed that the surface tension reaches its maximum value when the Er content is 0.4%38). Rare earth elements have a strong interaction with hydrogen, forming compounds such as REH2 and REH3. Furthermore, since rare earth elements have a much higher solubility for hydrogen compared to aluminum alloys, they can effectively assist in removing hydrogen from aluminum alloys. This is expected to lower the solubility of hydrogen at high temperatures, thereby reducing pore defects caused by hydrogen.
Aluminum has a reflectivity of approximately 90% for a laser beam with a wavelength of around 1 ㎛ (micrometer), classifying it as a material with high reflectivity. When performing laser welding on materials with high reflectivity, 1) the laser power needed to initiate a keyhole is greater compared to standard steel. 2) Additional laser beam energy is absorbed through multiple reflections once the keyhole is formed, but energy efficiency decreases as a significant portion of the energy escapes from the keyhole. 3) As a result of low absorptivity and wall focusing effects, the amount of energy absorbed at the top of the keyhole decreases, while the energy absorbed at the bottom increases. Consequently, the absorbed energy is unevenly spread throughout the depth of the keyhole, leading to reduced stability in both the keyhole and the molten pool. Microgravity and high vacuum conditions in space enhance the effect of recoil pressure during laser welding, which can address the challenges of keyhole formation in high-reflectivity materials such as aluminum alloy and improve energy efficiency. However, deepening the keyhole during laser welding of high-reflectivity materials can exacerbate the imbalance in energy absorption along the keyhole’s depth39). Such imbalance can be corrected through the use of laser power modulation techniques. In power modulation, lowering the power after the peak helps reduce the excessive concentration of laser energy at the bottom of the keyhole, which improves the unbalanced laser energy absorption along the keyhole’s depth, as shown in Fig. 10,34,39). Thus, power modulation can enhance keyhole stability and prevent spatter and porosity defects during welding.
3. Microstructures Under Microgravity and Extreme Temperature Conditions
In the molten pool of an alloy, a temperature gradient exists due to the difference in temperature, along with a concentration gradient resulting from the variation in concentration. These temperature and concentration gradients indicate density irregularities in the molten pool, which are accompanied by physical phenomena such as buoyant convection, micro-convection, and layered settlement under the influence of gravity. In the molten pool, buoyant convection driven by gravity and Marangoni convection caused by surface tension gradients at the free interface are present40). In microgravity conditions, buoyant convection driven by gravity becomes negligible, leaving only convection induced by surface tension gradients on the free surface. These variations alter the temperature distribution in the molten pool and the shape of the solid-liquid interface, while also significantly influencing the alloy’s solidified microstructure41). Under gravity, the laser-welded fusion zone of 5A06 aluminum alloy features a structure composed of dendritic crystals and clusters of multi-dendritic crystals. Low-angle grain boundaries are found between the dendritic crystals, and the variation in brightness between the crystals results from the segregation of magnesium42). The 5A06 aluminum alloy is a hypoeutectic alloy with a magnesium content of 6% as illustrated in Fig. 11. Microgravity has minimal impact on the proportion of eutectic structures during solidification but notably enlarges the spacing between the primary and secondary dendritic arms. For the Al- 4%Cu alloy, the spacing between the primary and secondary dendritic arms under microgravity is 40% and 85% greater compared to unit gravity, as shown in Fig. 12,43). For Al-Si alloys, the spacing between the secondary dendritic arms also grew considerably under microgravity conditions44). It has been reported that an increase in the spacing between secondary dendrite arms leads to a decrease in the hardness, tensile strength, and elongation of aluminum alloys45). Previous studies conducted on the impact of microgravity on the microstructure during the solidification of aluminum alloys involved placing the alloy ingot in a crucible, heating it until it melted, and then transferring it to a microgravity device to naturally allow solidification during free fall. The solidification rate is significantly slower compared to that during laser welding under these conditions. Thus, additional research is necessary to examine the effect of microgravity on the microstructure of aluminum alloys at high solidification rates.
Fig. 12
Comparison of secondary dendrite arm spacing in the solidified microstructures of Al-4%Cu alloy under microgravity and unit gravity43)
For a spacecraft orbiting the Earth, the side facing the sunlight heats up to 157°C, whereas the side blocked from sunlight by the Earth undergoes a drastic temperature drop, reaching -121°C. Such extreme temperature conditions profoundly affect the melting and solidification properties of metals. Simultaneously, laser welding is a process characterized by rapid and highly uneven temperature variations, resulting in significant temperature gradients around the weld. As a result, laser-welded structures may experience varying levels of residual stress and strain. Furthermore, the fast cooling rates in laser welding can cause quenching effects, leading to higher hardness and, as a result, an increased risk of cold cracking46). Extreme external temperatures can intensify residual stresses, strains, and the vulnerability to cold cracking. Thus, understanding the microstructure of the weld joint and optimizing the mechanical characteristics during laser welding under extreme temperature conditions are essential for producing high- quality laser-welded joints in space.
4. Numerical Analysis of Laser Welding in Space Conditions
The distinct conditions of the space environment pose serious challenges to conducting welding experiments directly in space. As a result, current overseas research on space welding primarily relies on ground-based simulations, replicating the microgravity, high vacuum, and extreme temperature conditions of the space environment here on Earth. Specifically, replicating microgravity conditions is the most challenging, typically achieved using the drop tower or aircraft described in Table 2 above. Consequently, this requires the establishment of large-scale facilities and substantial financial resources. Despite the substantial costs, these facilities offer limited gravity levels and durations during experiments. These limitations hinder advancements in space welding research. Furthermore, the diverse range of materials, thicknesses, and joint types in space structures necessitates extensive experimentation to address all potential scenarios. Additionally, the microgravity, high vacuum, and extreme temperatures of the space environment significantly influence the dynamic behavior of the molten pool and keyhole during laser welding. However, experimental techniques have limitations in fully capturing the multiphase physical phenomena that occur during the welding process. Reliable analytical models for laser welding in typical Earth environments have been developed owing to advancements in computational fluid dynamics (CFD). These models enable the study of phenomena that are challenging to observe through experiments, such as temperature history, as well as the formation and behavior of keyhole and molten pools47,48). This numerical analysis technique is anticipated to be an effective tool for fully replacing or complementing laser welding experiments in space environments, which are both costly and time-intensive. Recently, a numerical model for laser keyhole welding in a high vacuum environment has been established (Fig. 13,49)). It includes a recoil pressure model that varies with ambient pressure and vaporization temperature, as well as a model for laser power attenuation caused by the scattering and absorption of the laser beam by metal particles in the laser-induced plume. This method demonstrated that absorption and scattering coefficients were determined across different pressure conditions, including space environments, to heighten the accuracy of calculations.
5. Conclusion
This paper presents the technologies necessary for implementing laser welding in space environments by reviewing previous studies, with the main points summarized as follows.
1) The hydrostatic pressure effect is eliminated in microgravity conditions, while the recoil pressure effect is relatively enhanced, promoting spatter formation. Because spatter poses a risk to weld quality and space structures, controlling the dynamic behavior of the molten pool and keyhole to suppress spatter during laser welding is a critical challenge that must be addressed to enable laser welding in space.
2) Keyhole instability is intensified in microgravity and high vacuum environments. The reduction in gravity eliminates buoyancy-driven convection, hindering gas evacuation and thereby increasing the likelihood of pore formation in the joint. Enhancing keyhole stability and managing the molten pool flow to ensure efficient gas evacuation are further challenges associated with laser welding in space.
3) Enhancing the surface tension of molten metal through the addition of alloying elements can help reduce spatter formation in space environments. Additionally, incorporating rare earth elements can serve as a solution to mitigate pore formation caused by hydrogen. Also, laser power modulation techniques can help reduce spatter and pore formation by enhancing the stability of keyhole.
4) The microgravity and extreme temperature conditions in space significantly impact the molten pool behavior and temperature profile, ultimately affecting the weld’s microstructure and mechanical properties. Thus, predicting the microstructure of the weld joint and optimizing the mechanical characteristics during laser welding under the microgravity and extreme temperature conditions are essential for producing high-quality laser-welded joints in space.
5) Numerical analysis can be an effective tool for understanding the fundamental phenomena of space welding and investigating methods to control molten pool behavior. Numerical analysis is more time- and cost-efficient than experimental methods, making it likely to replace or complement experiments in the future. Therefore, it is necessary to conduct research on numerical modeling specifically designed for the space environment for simulating space welding.
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