Preface
With the rapid development of shipbuilding, nuclear power, and other industries, higher requirements are being placed on the welding of medium and thick steel plates.
Currently, narrow gap arc welding is primarily used for welding medium and thick steel plates. Narrow gap submerged arc welding and narrow gap TIG are the most commonly used techniques, with narrow gap laser welding also being explored.
Compared to traditional arc welding, narrow gap arc welding can significantly reduce the number of welding passes, decrease welding deformation, and improve welding efficiency.
Moreover, narrow gap laser wire filling welding and narrow gap laser arc hybrid welding are also crucial research directions for medium and thick steel plate welding.
See also:
Whether it’s narrow-gap arc welding or narrow-gap laser welding, meeting the demand for more efficient welding remains challenging due to the need for multi-layer filling.
In recent years, 10000-watt high-power laser welding has become one of the most popular cutting-edge welding technologies worldwide. The ultra-high power density of a 10000-watt laser enables welds with a greater depth-width ratio.
When using 10000-watt laser welding, the number of weld bead layers can be significantly reduced, and unnecessary groove processing and interlayer cleaning can be avoided, thereby increasing welding efficiency, especially for medium and thick steel plates.
This welding technology boasts high efficiency and high quality, making it significant in solving the issue of high-quality and efficient processing of medium and thick plates in fields such as domestic ships and nuclear power. Therefore, research into 10000+ watt laser welding technology holds important scientific significance and application value.
This article summarizes and analyzes the domestic and foreign research status of related ten thousand-watt fiber laser welding technology. It mainly covers the characteristics of the related technology, the behavior of the molten pool, the physical characteristics of the plume, the suppression of welding defects, and the development and application of the welding process.
1. Characteristics of 10000 watt laser welding technology
The process window for laser welding with a power of 10000+ watts is narrower compared to kilowatt laser welding. This is because the power density of a 10000+ watt laser is higher, with the laser beam’s power density ranging from 1 × 107 to 1 × 108 W/cm2.
The thermal welding process is more complex as the extremely high laser power rapidly heats, melts, and vaporizes the material. The evaporation of metal in the weld pool is more intense, making the welding process challenging to control, especially in achieving a stable penetration state. Additionally, surface collapse and bottom hump defects are prone to occur.
Figure 1 shows the formation and common defects of a 10000+ watt laser weld.
Furthermore, the shielding effect of the welding plume on laser light increases significantly with the laser power increase. In the 10000+ watt laser welding process, a large amount of plume is generated, which strongly interferes with the incident laser light. This interference reduces the stability of laser energy transmission and, consequently, leads to poor welding process stability.
2. Research progress on weld pool behavior and plume characteristics of 10000+ watt laser welding
2.1 Research status of weld pool behavior in 10000-watt fiber laser welding
In the 10,000-watt laser welding process, the occurrence of welding defects, such as spatter, is closely related to the behavior of the welding pool. Specifically, the violent fluctuation of the laser keyhole directly affects the stability of the welding process.
Figure 2 shows the longitudinal section of the laser keyhole.
Therefore, scholars both domestically and abroad have conducted extensive research on the dynamic behavior of the keyhole. Common methods include high-speed imaging technology, real-time X-ray detection, and the “sandwich” observation method.
Among these methods, the Sanming method allows for intuitive observation of the internal state of the keyhole and is one of the main means of researching keyhole characteristics.
Relevant research indicates that in the non-penetrating state, when the laser power is relatively low (below 20 kW), spatter is mainly generated at the rear edge of the keyhole. The early form of spatter in this case is primarily a metal liquid column with a raised rear edge, as shown in Figure 3.
Li used the “sandwich” observation method to analyze the formation process of the liquid column along the back edge of the keyhole, as shown in Fig. 4.
The author explains that the formation of the back edge liquid column is primarily related to the “boss” formed on the inner wall of the keyhole.
The “boss” on the back wall moves upward due to viscous friction and rises to the opening of the keyhole, where it forms a liquid column.
Furthermore, Li notes that since the pressure beneath the boss is low and not directly exposed to the laser, some of the steam flows downward along the front edge of the keyhole.
When the steam erupts upward at the bottom of the keyhole, it generates a steam vortex that causes irregular movement of the molten pool metal and creates a gasification wave on the rear wall of the keyhole.
The gasification wave often results in splashing and a liquid column when it breaks at the opening of the keyhole.
It has been reported that the main generating position of spatter changes when the laser power exceeds 20 kW.
For instance, when the laser power was increased to 30 kW, Feng observed a significant increase in the probability of liquid column and splashing in the front area of the keyhole.
However, the author did not provide a regular and detailed understanding of the reasons for the difference in the probability of metal liquid column in different areas along the keyhole edge, nor the evolution law of liquid column.
Furthermore, using the “sandwich” observation method, Zhang et al. studied the formation cause of spatter on the lower surface of the weld during full penetration welding from the angle of internal stress of the keyhole, as depicted in Fig. 5.
They pointed out that the key driving force of the bottom splash was the viscous frictional resistance caused by the high-speed movement of the steam flow.
Based on the above analysis, Zhang et al. suggested that more spatters were generated on the upper and lower surfaces of the weld, leading to insufficient weld filling, which was one of the reasons for the depression on the weld surface.
Chen and his colleagues believe that the formation of a hump at the bottom of the weld is due to the “boss” created by the front wall of the keyhole, which has a significant impact on its formation.
Moreover, several studies have suggested that the application of an electromagnetic field can help reduce the formation of hump on the back.
Qi et al. explained how the application of an electromagnetic field can prevent the sagging and falling off of the weld root during the 10000 watt level laser welding of medium and thick plates by analyzing the metal flow in the weld pool.
The study highlighted that the electromagnetic force can compensate for the lack of surface tension by weakening the static pressure on the liquid at the back of the weld, thereby altering the fluidity of the weld pool metal.
The combination of surface tension and electromagnetic force enhances the stability of high power laser penetration welding.
However, the use of “sandwich” has some limitations as it is bonded with high-temperature glass, preventing the keyhole from forming a complete closed loop, leading to a different stress state than the actual welding state.
2.2 Research status of plume characteristics in 10000 watt fiber laser welding
During high-power laser welding, the welding plume refracts and scatters the incident laser, significantly reducing the laser energy that reaches the workpiece surface. This reduction results in inadequate welding penetration and has a considerable impact on the stability of the welding process and the quality of the weld.
Therefore, it is crucial to gain a deep understanding of the plume characteristics and its influence mechanism during 10000-watt laser welding.
Currently, researchers mainly study the plume using spectrometers, laser probes, and high-speed imaging technology. The analysis is based on plume temperature, electron density, refractive index, and attenuation coefficient of the incident laser light, as shown in Fig. 6.
In tests, the detection laser’s wavelength typically differs from that of the welding laser. Therefore, to calculate the attenuation rate of the welding laser energy, it’s crucial to convert the change in the detection laser energy into the attenuation coefficient of the welding laser energy.
Formula (1) represents the common conversion formula used for this purpose.
Where:
- Qext is the absorption coefficient of the plume to the laser energy;
- m is the complex refractive index of the plume to the laser;
- λ is the laser wavelength.
Based on the analysis method described above, the researchers measured that when the laser power was 10 kW, the attenuation coefficient of the local plume above the keyhole to the incident laser energy was less than 5%. However, when the laser power was increased to 15 kW and 20 kW, it rose to about 12%.
The spectral information of plume radiation showed that the ionization degree was less than 2%. However, in actual welding processes, differences in laser spot size, focus position, shielding gas, welding base metal, measurement position, and detection laser wavelength, as well as irregular fluctuations in welding plume, can lead to varying measurement results.
Therefore, to effectively and quantitatively analyze the composition of the plume and its influence on laser energy, research on related physical and chemical phenomena needs to be further strengthened. Additionally, it is important to investigate how to eliminate the plume’s influence on laser energy. This is an important research direction for Wanwa laser welding, and the main methods for doing so are the side-blowing method, local negative pressure method, and vacuum method.
3. Research progress on defect suppression of 10000 watt laser welding
The development of 10,000-watt laser welding technology is being hindered by the appearance of surface depressions and bottom humps in the welds. Therefore, it is crucial to effectively control these defects to promote the development of this technology.
Currently, the main methods used to suppress these defects include the electromagnetic support system, changing welding posture, bottom air pressure method, and forced forming of the weld back.
Avilov et al. successfully suppressed the formation of bottom humps by using the “induction electromagnetic welding weld pool support system” to prevent sagging and falling of the weld pool, which provided a theoretical basis for this technology.
Guo et al. from the University of Manchester in the UK, prevented the molten pool from falling off due to gravity during the welding process by using horizontal welding, achieving full penetration welding of 13mm thick S700 steel at a laser power of 13 kW with good weld quality.
Matsumoto et al. from Osaka University conducted the first study on the influence of the focusing performance of high-power fiber laser on weldability. They pointed out that when using a high-power laser to weld thick plates, a long focal depth is more helpful in obtaining good welds.
Chen Fei and others successfully suppressed the formation of relevant welding defects when welding 20mm thick 316L stainless steel under the condition of a laser power of 16 kW based on the bottom pressure method.
Chen Genyu proposed a method to suppress the surface collapse of 10,000-watt laser welding. By synchronously transporting metal particles to the molten pool during welding, the material loss during spatter can be compensated for and weld collapse reduced. Additionally, side-blowing shielding gas can better suppress spatter of the nail head weld and the upper surface.
Deep air holes can be generated during 10,000-watt laser welding. When Kawahito et al. welded 304 stainless steel with a 10 kW fiber laser, they found that pores would occur in the middle and lower part of the keyhole during ultra-high power laser deep penetration welding. They pointed out that when the welding speed was lower than 3 m/min, changing the spot diameter could not effectively reduce the generation of pores.
Through research, Minhyo et al. of Osaka University pointed out that unlike CO2 laser and YAG laser welding, air holes only appear in a large number of incomplete welds. When high-power fiber laser welding medium and thick plates, even under the condition of full penetration, a large number of air holes will appear in the welds. Through analysis of the composition of air holes, it was pointed out that the air holes were caused by air entering the keyhole from the back of the weld.
In the case of incomplete penetration, Zhao Lin et al. studied the influence of process parameters on small hole porosity, hot cracks, and spatter in the process of 20mm low carbon steel fiber laser welding. They pointed out that the tendency of weld porosity and hot crack decreased with the increase of welding speed. At zero defocus, the stomatal tendency is the largest.
Sun et al. pointed out that when using a 10 kW fiber laser for transverse welding of 304 stainless steel, nitrogen has a better inhibitory effect on welding porosity compared to argon and helium.
4. Research progress of 10000 watt laser high efficiency welding technology
In practical engineering applications, most materials require double-sided plate forming during the welding process.
For medium-thickness materials, the traditional welding method involves multi-layer filling. However, using 10000 watt laser welding technology, single-sided welding and double-sided forming of 32mm thick stainless steel can be achieved.
For materials with greater thickness, double-sided welding is typically used. Thus, achieving single-sided welding and double-sided forming of larger plates is a current research focus in the 10000 watt laser welding field for medium and thick steel plates. This includes the 10000 watt fiber laser self-fusion welding process and the laser arc composite welding process.
4.1. Research status of 10000 watt laser self fusion welding process
Avilov et al. achieved single-sided welding and double-sided forming of 12 mm and 15 mm steel using a laser power of 10.9 kW and 10 kW, respectively.
Sokolov from Laplanta University in Finland, along with others, studied the deep penetration welding ability of a 30 kW fiber laser and accomplished penetration welding of S355 with a thickness of 25 mm at a welding speed as high as 2.4 m/min.
The German IPG company group and others completed one-time welding of 32 mm stainless steel under the same laser power.
Katayama et al. from Osaka University realized double-sided single-pass welding of 70 mm thick 304 stainless steel using a 100 kW fiber laser, and the weld quality was good. The weld surface and cross-sectional morphology are shown in Figure 7.
In recent years, there has been a rapid development in fiber laser technology. Consequently, domestic researchers have carried out application research of medium and thick plate 10000 watt fiber laser welding technology.
Using a 15 kW laser power, Chen Genyu and his team successfully accomplished single side welding and double side forming of 18 mm thick stainless steel through horizontal welding and multi-channel side blowing at a welding speed of 0.6 m/min.
Meanwhile, Xin Jijun and his team performed process parameter fluctuation tests for 20 kW laser welding and obtained full penetration weld for 20 mm thick stainless steel. The surface morphology of the weld is shown in Fig. 8.
4.2. Research status of 10000 watt laser arc hybrid welding process
Laser arc hybrid welding has greater adaptability to working conditions and a wider range of engineering applications compared to laser self-fusion welding.
A significant amount of research has been conducted on relevant welding technologies internationally. Among them, Turichin et al. from St. Petersburg State University of Marine Technology achieved butt welding of 14mm RS E36 marine high-strength steel, as well as one-sided welding and two-sided forming of T-joints.
Furthermore, Turichin et al. from St. Petersburg State University of Technology successfully accomplished one-time welding of 14mm X80 pipeline steel at a welding speed of up to 3m/min using 20kW laser composite welding.
Wahba and Mizutani from Osaka University, among others, were able to achieve one-sided welding and two-sided forming of 20 mm and 25 mm SM490A low alloy structural steel using a laser GMAW composite welding method with 16 kW laser power. They pre-cut the welding wire and added submerged arc flux or glass fiber under the test plate to improve the back shape.
The welding process of a 50 mm thick test plate was then completed using two-sided single pass welding, and the weld performance met the relevant industrial standards. Fig. 9 shows the test device and some weld sections.
The Fraunhofer Institute Stündag in Germany studied the adaptability of compound welding to groove gap and misalignment. They successfully achieved single-sided welding and double-sided forming of 20 mm, 25 mm, and 28 mm marine S355J2 low alloy steel and 20 mm X20 pipeline steel using an AC oscillating electromagnetic system and a 20 kW fiber laser GMAW compound welding method, as shown in Fig. 10.
Composite welding schematic diagram and weld cross-section
Bunaziv, along with other research institutions at the Norwegian University of Science and Technology, has conducted a significant number of experimental studies using fiber lasers with more than 15 kW of power, which have yielded fruitful results.
Currently, there are few studies on 10,000-watt fiber laser arc hybrid welding of medium and thick plates in China. However, in recent years, Jing Zhicheng of Shenyang University of Technology and other researchers have achieved one-time welding of 18mm-thick marine high-strength steel using a 10 kW fiber laser MAG composite welding technique, and the mechanical properties of the welded joints meet the requirements of classification societies.
Additionally, Huang Ruisheng and others have studied the characteristics of 30 kW fiber laser arc composite welding by using different arc heat sources. The research indicates that the stability of the laser TIG composite wire filling welding process is significantly better than that of laser MAG composite welding and laser MAG composite wire filling welding when using high-power laser arc composite welding.
5. Engineering application of 10000 watt fiber laser welding technology
The 10,000-watt fiber laser welding technology can significantly improve the efficiency of welding medium and thick steel plates, and it has significant application value in fields such as shipbuilding, oil and gas pipelines, and nuclear power.
As early as 2005, Vollertsen and other researchers from the Bremen Research Institute in Germany utilized a 17 kW fiber laser to achieve welding of an 11.2mm gas transmission pipeline. In 2011, General Motors developed a composite welding system based on a 20 kW fiber laser, which was utilized in infrastructure manufacturing for the oil, aviation, railway, and other industries.
In 2015, the Russian JSC company developed welding technology for a 20 mm thick marine steel plate using 16 kW fiber laser arc composite welding equipment, which was approved by the Maritime Registration Bureau. In collaboration with German IMG company, an automatic production line was designed and built for cutting and welding a 12m x 12m marine steel plate.
Furthermore, JSC started developing the laser arc composite vertical welding technology for a 40 mm thick marine high-strength steel based on the existing welding technology and the highest power 25 kW fiber laser. They also designed a trial production model for the industrial application of relevant automatic vertical welding technology.
Units such as the German Fraunhofer Institute and Aachen University of Technology conducted extensive research on high-power laser composite welding applications for shipbuilding steel with a thickness of over 20 mm, utilizing laser power over 15 kW.
The development and application of high-power fiber laser and laser arc hybrid welding technology in China started later. However, in recent years, scientific researchers have made significant efforts, and relevant technologies have begun to be applied to nuclear power and shipbuilding.
In 2013, Hunan University developed the 18mm thick stainless steel single-sided welding and double-sided forming technology using a 15 kW fiber laser, which was successfully applied to the stable welding production of a nuclear reactor core barrel.
In 2019, Harbin Welding Research Institute Co., Ltd. and Yantai CIMC Raffles Marine Engineering Co., Ltd. successfully applied the 10,000 watt laser arc composite welding technology to the production of 20m long welds. The speed of one-time welding of 10 mm thick steel plates can reach 1.2-1.5 m/min, making it the first application of a laser composite welding production line in the domestic shipbuilding industry.
6. Development trend
With the rapid advancement of new technologies and materials, high-end manufacturing products are trending towards being high load, high strength, and lightweight.
In critical industries such as ships and nuclear power, welding of medium and thick plates are required to meet higher standards.
In recent years, the development of 10,000-watt fiber lasers with better beam quality and higher processing efficiency has further enhanced the technology of 10,000-watt laser welding for medium and thick plates.
Currently, the maximum laser power used in research and application has reached 100 kW. According to the official report of IPG company, they can produce fiber lasers with a maximum output power of 500 kW.
In the future, laser power is expected to increase while costs continue to decrease, making it an inevitable trend to use medium thickness materials in ten-thousand-watt level light welding.
Furthermore, in the development of 10,000-watt fiber laser welding technology, Osaka University, Aachen Technical University of Germany, Fraunhofer Research Institute, St. Petersburg State University of Technology of Russia, and Lappeenranta University of Technology in Finland, among other research institutions, have made 10,000-watt high-power laser welding technology a significant research area, with the laser power used being over 15 kW.
However, in this regard, domestic research lags behind foreign developed countries, and it is necessary to adopt high-power lasers of more than 15 kW and carry out systematic research and development and engineering applications in this technical field as soon as possible to solve the issue of high-quality and high-efficiency laser welding of medium and thick plates in major equipment manufacturing industries.
Conclusion
To summarize, 10000-watt laser welding is an advanced welding technology that can effectively and efficiently weld medium and thick steel plates with high-quality results. It is considered one of the crucial areas for the development of laser welding technology in the future, with significant strategic and application value.
Thanks to the tireless efforts of scholars from around the world, research on this technology has achieved some industrial applications and certain advancements. This progress has provided a foundation for further in-depth research and widespread application of relevant technical achievements.
However, several aspects still require further investigation, such as keyhole and plume characteristics of high-power laser welding, weld formation, effective control of welding defects, and key engineering application technologies. Research in these areas can help to promote the application and popularization of this technology.
