10000+ Watt Fiber Laser Welding: Depth Analysis

Preface

With the rapid development of shipbuilding, nuclear power and other fields, higher requirements are put forward for the welding of medium and thick steel plates.

At present, narrow gap arc welding is mainly used for the welding of medium and thick steel plates, among which narrow gap submerged arc welding and narrow gap TIG are the main ones, and narrow gap laser welding is also explored.

Compared with traditional arc welding, narrow gap arc welding can obviously reduce the number of welding passes, reduce the welding deformation and improve the welding efficiency to a certain extent;

In addition, narrow gap laser wire filling welding and narrow gap laser arc hybrid welding are also important research directions of medium and thick steel plate welding.

See also:

10000+ Watt Fiber Laser Welding: Depth Analysis 1

However, whether it is narrow gap arc welding or narrow gap laser welding, it is difficult to meet the needs of more efficient welding due to the need of multi-layer filling.

In recent years, 10000 watt high-power laser welding has become one of the most popular cutting-edge welding technologies in the world.

The ultra-high power density of 10000 watt laser can be used to obtain welds with greater depth width ratio.

Under the same plate thickness, the use of 10000 watt laser welding can greatly reduce the number of weld bead layers, thereby reducing the number of interlayer cleaning and reducing unnecessary groove processing, and greatly improve the welding efficiency, especially in the welding of medium and thick steel plates.

It has the advantages of high efficiency and high quality welding technology.

Therefore, the research of 10000+ watt laser welding technology has important scientific significance and application value for solving the problem of high-quality and efficient processing of medium and thick plates in major fields such as domestic ships and nuclear power.

In this post, the domestic and foreign research status of the related ten thousand watt fiber laser welding technology is summarized and analyzed, mainly involving the characteristics of the related technology, the behavior characteristics of the molten pool, the physical characteristics of the plume, the suppression of the welding defects and the development and application of the welding process.

1. Characteristics of 10000 watt laser welding technology

The process window of 10000+ watt laser welding is narrower because the power density of 10000+ watt laser is higher than that of kilowatt laser welding (the power density of laser beam can reach 1 ×  10^7 ~ 1  ×  10 ^ 8 w/cm2).

The welding thermal process is more complicated, the extremely high laser power makes the material heated, melted and vaporized violently in a very short time, the metal evaporation in the weld pool is more intense, the welding process is difficult to control, especially when the penetration is difficult to reach a stable state, and the surface collapse and the bottom hump are easy to occur.

The formation and common defects of 10000+ watt laser weld are shown in Fig. 1.

10000+ Watt Fiber Laser Welding: Depth Analysis 2
Fig. 1 common defects of 10000 watt laser welding

In addition, since the shielding effect of welding plume on laser light will increase sharply with the increase of laser power, a large amount of plume generated in the process of 10000 watt laser welding will strongly interfere with the incident laser light, reduce the stability of laser energy transmission, and then lead to poor stability of the welding process.

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 10000-watt laser welding process, the occurrence of welding defects such as spatter is closely related to the behavior of the welding pool, especially the violent fluctuation of the laser keyhole directly affecting the stability of the welding process.

The longitudinal section of the laser keyhole is shown in Fig. 2.

Therefore, scholars at home and abroad have done a lot of research on the dynamic behavior of keyhole.

Common methods include high-speed imaging technology, real-time X-ray detection and “sandwich” observation method.

Among them, Sanming method can observe the internal state of keyhole intuitively, and is one of the main means of keyhole characteristics research.

10000+ Watt Fiber Laser Welding: Depth Analysis 3
Fig. 2 Schematic diagram of longitudinal section of laser keyhole

Relevant research points out 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, and its early form is mainly a metal liquid column with a raised rear edge, as shown in Fig. 3.

10000+ Watt Fiber Laser Welding: Depth Analysis 4
Fig. 3 appearance of liquid column behind keyhole

For the formation of liquid column along the back edge of keyhole, Li analyzed the formation process by “sandwich” observation method, as shown in Fig. 4.

The author points out that the formation of the back edge liquid column is mainly related to the “boss” formed on the inner wall of the keyhole.

The “boss” of the back wall is constantly transferred upward under the action of viscous friction.

When it is transferred to the opening of the keyhole, it rises upward and forms a liquid column.

In addition, Li also pointed out that because the pressure under the boss is low and is not directly irradiated by the laser, part of the steam flows downward along the front edge of the keyhole.

When the steam erupts upward at the bottom of the keyhole, the steam vortex will be generated, which will cause the irregular movement of the molten pool metal and generate the gasification wave on the rear wall of the keyhole.

When the gasification wave is broken at the opening of the keyhole, it is often accompanied by splashing and liquid column.

10000+ Watt Fiber Laser Welding: Depth Analysis 5
Fig. 4 process of liquid column and splashing at the rear edge of keyhole

It is also reported that when the laser power exceeds 20 kW, the main generating position of spatter will change.

For example, when the laser power was increased to 30 kW, Feng observed that the probability of liquid column and splashing in the front area of the keyhole was significantly increased.

However, the author has not given a regular and detailed understanding of the reasons why the probability of metal liquid column is different in different areas along the keyhole edge and the evolution law of liquid column.

Also based on 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 shown in Fig. 5.

It is pointed out that the key driving force of the bottom splash is the viscous frictional resistance caused by the high-speed movement of the steam flow.

Based on the above analysis, Zhang et al. pointed out that more spatters were generated on the upper and lower surfaces of the weld, resulting in insufficient weld filling, which was one of the reasons for the depression on the weld surface.

10000+ Watt Fiber Laser Welding: Depth Analysis 6
Fig. 5 Schematic diagram of formation of small holes and bottom flying in 10000 watt laser penetration welding

For the formation of hump at the bottom of the weld, Chen and others believe that the “boss” produced by the front wall of the keyhole has an important impact on the formation of the hump at the bottom.

In addition, some studies have pointed out that the generation of hump on the back can be reduced to a certain extent by applying electromagnetic field.

Qi et al. explained how the applied 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 from the aspect of metal flow in the weld pool.

The study pointed out that the electromagnetic force can compensate for the lack of surface tension by weakening the static pressure on the liquid on the back of the weld, thereby changing the fluidity of the weld pool metal.

Under the combined action of surface tension and electromagnetic force, the stability of high power laser penetration welding is improved.

However, the use of “sandwich” also has certain limitations, because it is bonded with high-temperature glass, the keyhole does not form a complete closed loop, and its stress state will be different from 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 will refract and scatter the incident laser, greatly reducing the laser energy reaching the workpiece surface, resulting in the welding penetration not reaching the expected depth, and has a great impact on the stability of the welding process and the quality of the weld.

Therefore, it is of great significance to deeply understand the plume characteristics and its influence mechanism on the welding process during 10000 watt laser welding.

At present, the research on plume is mainly carried out by means of spectrometer, laser probe and high-speed imaging technology, and the analysis is carried out in terms of plume temperature, electron density, refractive index and attenuation coefficient of incident laser light.

The common methods are shown in Fig. 6.

10000+ Watt Fiber Laser Welding: Depth Analysis 7
Fig. 6 schematic diagram of common plume analyzer

Since the wavelength of the detection laser used in the test is usually different from that of the welding laser, when calculating the attenuation rate of the welding laser energy, it is necessary to convert the change value of the detection laser energy into the attenuation coefficient of the welding laser energy.

The common conversion formula is shown in formula (1).

10000+ Watt Fiber Laser Welding: Depth Analysis 8

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 above analysis method, when the laser power is 10 kW, the researchers measured that the attenuation coefficient of the local plume above the keyhole to the incident laser energy is less than 5%, while when the laser power is increased to 15 kW and 20 kW, it is about 12%;

The ionization degree is less than 2% by the spectral information of plume radiation.

However, in the actual welding process, due to the difference of laser spot size, focus position, shielding gas, welding base metal, measurement position and detection laser wavelength, especially the irregular fluctuation of welding plume, the measurement results will be different.

Therefore, how to effectively quantitatively analyze the composition of plume and its influence on laser energy, and the research on related physical and chemical phenomena need to be further strengthened.

In addition, how to eliminate the influence of plume on laser energy is also an important research direction of wanwa laser welding.

The main methods are side blowing method, local negative pressure method and vacuum method.

3. Research progress on defect suppression of 10000 watt laser welding

The development of 10000 watt laser welding technology is seriously affected by the appearance of weld surface depression and bottom hump.

Therefore, the effective control of relevant defects is of great significance to promote the development of 10000 watt laser welding technology.

At present, the main methods to suppress relevant defects are electromagnetic support system, changing welding posture, bottom air pressure method and forced forming of weld back.

In order to prevent the sagging and falling of the weld pool and increase the weld penetration, avilov et al.successfully suppressed the formation of the bottom hump by using the “induction electromagnetic welding weld pool support system”, providing a theoretical basis for this technology.

Guo et al. Of the University of Manchester, UK, effectively prevented the molten pool from falling off due to gravity during the welding process by means of horizontal welding, and realized the full penetration welding of 13 mm thick S700 steel at the laser power of 13 kW, with good weld quality.

Matsumoto et al. of Osaka University conducted the first study on the influence of focusing performance of high-power fiber laser on weldability, and pointed out that when high-power laser is used to weld thick plates, the long focal depth is more helpful to obtain good welds.

Chen Fei and others successfully suppressed the formation of relevant welding defects when welding 20 mm thick 316L stainless steel under the condition of laser power of 16 kW based on the bottom pressure method.

Chen Genyu proposed a method to suppress the surface collapse of 10000 watt laser welding.

By synchronously transporting metal particles to the molten pool during welding, the material loss during the spatter can be compensated and the weld collapse can be reduced;

In addition, the side blowing shielding gas can better suppress the spatter of the nail head weld and the upper surface.

Deep air holes will be generated during 10000 watt laser welding.

When Kawahito et al. welded 304 stainless steel with 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, and 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 the 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 crack and spatter in the process of 20 mm low carbon steel fiber laser welding, and 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 10 kW fiber laser is used for transverse welding of 304 stainless steel, compared with argon and helium, nitrogen has better inhibition effect on welding porosity.

4. Research progress of 10000 watt laser high efficiency welding technology

In practical engineering applications, the welding process of most materials needs to realize the double-sided forming of plates.

For medium thickness materials, the traditional welding method requires multi-layer filling, while the 10000 watt laser welding technology can realize single-sided welding and double-sided forming of 32 mm thick stainless steel.

However, for materials with larger thickness, double-sided welding is often used at present.

Therefore, how to realize single-sided welding and double-sided forming of larger plate thickness is one of the research hotspots in the 10000 watt laser welding field of medium and thick steel plates, including 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. realized single-sided welding and double-sided forming of 12 mm and 15 mm steel at laser power of 10.9 kW and 10 kW.

Sokolov of laplanta University in Finland and others studied the deep penetration welding ability of 30 kW fiber laser, and realized the penetration welding of S355 with a thickness of 25 mm when the welding speed was as high as 2.4 m / min;

The German IPG company grupp and others completed one-time welding of 32mm stainless steel under the same laser power.

Katayama et al. of Osaka University realized the double-sided single pass welding of 70mm thick 304 stainless steel by using 100 kW fiber laser.

The weld quality was good.

The weld surface and cross-sectional morphology are shown in Fig. 7.

10000+ Watt Fiber Laser Welding: Depth Analysis 9
Fig. 7.70mm stainless steel weld appearance

In recent years, with the rapid development of fiber laser, domestic researchers have also carried out the application research of medium and thick plate 10000 watt fiber laser welding technology.

When the laser power was 15 kW, Chen Genyu and others successfully realized single side welding and double side forming of 18 mm thick stainless steel at the welding speed of 0.6 m / min by means of horizontal welding and multi-channel side blowing.

For 20 kW laser welding, Xin Jijun and others carried out process parameter fluctuation test and obtained 20 mm thick stainless steel full penetration weld.

The weld surface morphology is shown in Fig. 8.

10000+ Watt Fiber Laser Welding: Depth Analysis 10
Fig. 8. Surface morphology of 20mm stainless steel weld (P = 19kw)

4.2. Research status of 10000 watt laser arc hybrid welding process

Compared with laser self fusion welding, laser arc hybrid welding has strong adaptability to working conditions and a wider range of engineering applications.

A great deal of research has been carried out on relevant welding technologies abroad, among which turichin et al. of St. Petersburg State University of marine technology realized the butt welding of 14 mm RS E36 marine high-strength steel and one-sided welding and two-sided forming of T-joints;

Turichin et al. of St. Petersburg State University of technology successfully realized one-time welding of 14mm X80 pipeline steel at a welding speed of up to 3m / min by using 20kW laser composite welding.

10000+ Watt Fiber Laser Welding: Depth Analysis 11
Fig. 9 schematic diagram of test device and macro image of joint cross section

Wahba and Mizutani of Osaka University, etc. realized one-sided welding and two-sided forming of 20 mm and 25 mm SM490A low alloy structural steel under 16 kW laser power through laser GMAW composite welding method by pre cutting welding wire and adding submerged arc flux or glass fiber under the test plate to improve the back shape.

Then, through two-sided single pass welding, the welding process of 50 mm thick test plate was completed, and the weld performance met the relevant industrial standards.

The test device and some weld sections are shown in Fig. 9.

Fraunhofer Institute Stündag, Germany studied the adaptability of compound welding to groove gap and misalignment.

Through the AC oscillating electromagnetic system and the 20 kW fiber laser GMAW compound welding, they successfully realized the 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, as shown in Fig. 10.

10000+ Watt Fiber Laser Welding: Depth Analysis 12
Fig. 10 laser GMAW based on AC oscillating electromagnetic system
Composite welding schematic diagram and weld cross-section

Bunaziv and other research institutions of the Norwegian University of science and technology have successively carried out a large number of experimental studies using fiber lasers of more than 15 kW, and achieved fruitful results.

At present, there are few researches on 10000 watt fiber laser arc hybrid welding of medium and thick plates in China.

In recent years, it can be seen that Jing Zhicheng of Shenyang University of technology and others have realized one-time welding of 18 mm thick marine high-strength steel with laser power of 10 kW by using fiber laser MAG composite welding, and the mechanical properties meet the requirements of classification society;

In addition, 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 laser TIG composite wire filling welding process is obviously 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 10000 watt fiber laser welding technology can greatly improve the welding efficiency of medium and thick steel plates, and has important application value in shipbuilding, oil and gas pipeline, nuclear power and other fields.

As early as 2005, Vollertsen of Bremen Research Institute in Germany and others have used 17 kW fiber laser to realize the welding of 11.2mm gas transmission pipeline;

General Motors has also developed a composite welding system based on 20 kW fiber laser in 2011, which has been applied to the infrastructure manufacturing of oil, aviation, railway and other industries.

In 2015, the welding technology of 20 mm thick marine steel plate developed by Russian JSC company using 16 kW fiber laser arc composite welding equipment was approved by the maritime Registration Bureau, and the automatic production line for cutting and welding of 12 m × 12 m marine steel plate was designed and built in cooperation with German img company;

In addition, JSC started to develop the laser arc composite vertical welding technology of 40 mm thick marine high-strength steel based on the existing welding technology and the highest power 25 kW fiber laser, and designed a trial production model for the industrial application of the relevant automatic vertical welding technology.

German Fraunhofer Institute, Aachen University of technology and other units have conducted a lot of research on the application of high-power laser composite welding of shipbuilding steel with a thickness of more than 20 mm.

The laser power used is more than 15 kW.

The development and application of high-power fiber laser and laser arc hybrid welding technology in China started late.

In recent years, with the efforts of scientific researchers, relevant technologies have also begun to be applied to nuclear power and shipbuilding.

In 2013, the 18mm thick stainless steel single-sided welding and double-sided forming technology developed by Hunan University using 15 kW fiber laser has been successfully used in the stable welding production of nuclear reactor core barrel;

In 2019, the 10000 watt laser arc composite welding technology developed by Harbin Welding Research Institute Co., Ltd. and Yantai CIMC Raffles Marine Engineering Co., Ltd. was successfully applied to the production of 20 m long welds.

The speed of one-time welding of 10 mm thick steel plates can reach 1.2-1.5 m / min, which is also the first application of the laser composite welding production line in the domestic shipbuilding industry.

6. Development trend

With the rapid development of new technologies and new materials, high-end manufacturing products are developing towards the trend of high load, high strength and light weight.

In important fields such as ships and nuclear power, higher requirements are also put forward for the welding of medium and thick plates.

In recent years, with the development of 10000 watt fiber lasers with better beam quality and higher processing efficiency, the 10000 watt laser welding technology for medium and thick plates has been further improved.

At present, the maximum power of the laser put into scientific research and application has reached 100 kW.

According to the official report of IPG company, it has the ability to produce fiber lasers with the maximum output power of 500 kW.

In the future, the laser power will be further increased and the cost will be continuously reduced.

Therefore, it is an inevitable trend to use the medium thickness material in the ten thousand watt level light welding.

In addition, in the research and development of 10000 watt fiber laser welding technology, Osaka University, Aachen Technical University of Germany, Fraunhofer Research Institute, St. Petersburg State University of technology of Russia and laplanta University of Finland and other research institutions have taken 10000 watt high-power laser welding technology as an important research direction, and the laser power used is more than 15 kW.

In this regard, the domestic start is late and the research level is far behind the foreign developed countries.

The research shows that in order to solve the problem of high-quality and high-efficiency laser welding of medium and thick plates in the major equipment manufacturing industry, it is necessary to adopt high-power lasers of more than 15 kW, and systematically carry out research and development and engineering application in this technical field as soon as possible.

Conclusion

To sum up, the 10000 watt laser welding is an advanced welding technology to solve the high-quality and efficient welding of medium and thick steel plates.

It is one of the important research directions for the development of laser welding technology in the future and has important strategic significance and application value.

With the unremitting efforts of scholars from all over the world, the research on wanwa laser welding technology has made certain achievements and achieved some industrial applications, which lays a foundation for further in-depth research and wide application of relevant technical achievements.

However, the keyhole and plume characteristics of high-power laser welding, weld formation and effective control of welding defects, key engineering application technologies and other aspects still need to be further studied to promote the application and popularization of this technology.

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