Compared with traditional welding technology, laser welding (see Figure 1) has the following advantages:
- Centralized and adjustable energy density
- No contact with the welded workpiece
- High welding efficiency
- Narrow seam and high strength after welding
Therefore, it is actively used in equipment manufacturing fields such as automobiles, ships, aerospace, etc., and continues to expand to more material processing terminal fields.
Figure 1 Principle of laser welding
In order to better respond to future manufacturing competition, the world’s major manufacturing countries have successively proposed national strategies for upgrading and upgrading their manufacturing industries.
The more famous ones are the German Industrial Manufacturing 4.0 and the American Industrial Internet, which actively introduced new policies for the upgrading of the manufacturing industry, encouraging technological innovation in the manufacturing industry and giving key funding.
Among them, laser welding has attracted much attention as an important part of high-end equipment technology.
According to the actual needs of welding, laser welding has proposed a variety of new technologies by solving practical problems. For example, Professor W. Steen of Imperial College London proposed the idea of laser-arc hybrid welding.
The development of laser-arc hybrid welding technology has made up for the shortcomings of single laser welding to some extent and expanded the application range of laser welding.
The interaction between the laser and the arc exerts the advantages of both, reduces the requirements for the size of the welding gap, reduces the cracks and pores that appear during welding, and helps to improve the performance of the welding part.
So far, laser welding technology has developed into many types, such as:
- Heat conduction laser welding
- Laser deep penetration welding
- Laser wire filler welding
- Laser-arc hybrid welding
- Remote laser scanning welding
- Laser brazing
Intermediate process control such as laser welding seam tracking and high-speed camera real-time monitoring of the welding seam process, as well as laser welding defect processing, have been developed to jointly solve the related limitations and deficiencies of laser welding.
2. Research progress at home and abroad
In recent years, domestic and foreign research teams have continuously explored and studied the most suitable process parameters from the perspective of laser movement and heat source combination, and improved the technology of a variety of laser welding methods, including laser deep penetration welding and laser-arc hybrid welding.
The research of laser welding is not only about appearance, but researching the process characteristics of welding through modern characterization methods such as high-speed cameras and spectral analysis, trying to explore the formation mechanism of weld defects.
On the other hand, the internal changes of laser welding are more complicated.
Each research team tried to apply external energy such as magnetic field, multi-arc and electric field to the laser welding process, focusing on the improvement of weld defects, improving its mechanical properties and welding quality.
2.1 Research on laser welding technology
Laser welding can obtain high-quality joint strength and large depth ratio.
Compared with traditional welding technology, it has a larger power density, has a better welding effect on materials that are difficult to weld, and can weld materials with different properties.
Therefore, domestic and foreign scholars have conducted a lot of research on it.
The research on laser technology in China mainly focuses on the welding speed, laser power, defocusing amount, laser pulse waveform and shielding gas flow and other parameters of each welding process, and further the mechanical properties, structure evolution and regulation of welded joints. In-depth research.
Laser pressure welding is a unique laser welding technology.
This technology combines laser-induced heating with traditional flat seam welding.
The working principle of laser pressure welding is: the workpiece to be welded is partially melted with a laser beam, and then rolled under high pressure to produce a welded joint.
Because the melting zone is relatively narrow, welding defects such as shrinkage and gas cavities are avoided, and this technology can also be used to connect thin plates.
The research team studied the evolution of the structure during laser pressure welding of pure aluminum, as shown in Figure 2.
The team studied the basic aspects of microstructure evolution during pure aluminum welding.
Through in-depth analysis of the microstructure of the specimen during the laser pressure welding process, it is inferred that the solidification process started before the rolling, so the newly crystallized material experienced plastic strain.
Figure 2 Principle diagram of laser pressure welding
Laser-arc hybrid welding (see Figure 3), as a promising processing method in the 21st century, has been deeply studied by many scholars.
The researcher studied the welding process of 50CrV/SPHE dissimilar steel by adjusting the process parameters, and analyzed the influence on weld formation and droplet transfer.
The research results show that the laser power is in the range of 2800～3400W, the welding wire is heated uniformly, and the welding process is stable.
Combine oscillating scanning with laser-arc hybrid welding to make up for the defects of the weld.
Weld aluminum alloy materials with horizontal, vertical and circular oscillation scanning methods, and use high-speed cameras and spectroscopy to analyze the changes of the droplets.
The results show that the optimized parameter range of the circular scanning method is much larger than that of the horizontal and vertical directions, and it can promote the interaction with the plasma to form droplets with a smaller diameter, which is beneficial to grain refinement.
The energy of plasma arc is more concentrated in comparison, and it is found that laser-plasma arc welding has good adaptability to gap and wrong edge in flat welding.
Figure 3 Laser-arc hybrid welding
Research on welding technology abroad has focused on improving welding conditions and introducing external energy.
In order to explore the potential of this process to connect large, safety-critical nuclear components, such as steam generators or pressurized water reactors (PWR) boosters.
Using vacuum laser welding technology, at a speed of 150mm/min, using a 16kW laser to produce 80mm thick welds of SA5083 grade steel in two weld passes.
And introduced the advantages of vacuum laser welding, and compared with electron beam welding in terms of process physics.
It is concluded that vacuum laser welding is worthy of further development, because it provides important hope for future nuclear energy construction plans.
Bunaziv I et al. considered the cold metal transfer pulse (CMT+P) arc mode while using fiber laser-MAG hybrid welding, and used metal-cored wire to weld 45mm thick high-strength steel (butt double-sided welding), and compared different pulses. The influence of the method and the front and rear pilot arc on the weld.
Compared with traditional pulse arc welding, it is found that both can provide high-quality welding.
But CMT+P mode can provide more stable droplet transfer within a limited range of feed speed.
2.2 Laser welding process control
Laser welding technology is a kind of welding technology that does not require contact. It has a faster speed and higher welding efficiency. The intermediate process treatment plays an important role in the performance of the welded joint.
The domestic laser welding process control (see Figure 4) mainly focuses on monitoring the welding process with the help of optical devices, such as the use of laser welding seam tracking and high-speed cameras to monitor the weld seam in real time.
For example, through a high-speed camera monitoring system, real-time online monitoring of the formation process of pores and splashes of laser welding DP780 galvanized high-strength steel, and the escape route of pores was studied from the perspective of dynamics.
Figure 4 Layout of the welding test process
The laser welding head is integrated with the CCD video tracking module, and a method of automatic welding seam detection using a line laser is proposed.
This method uses laser triangulation to obtain shape information such as the height and width of the weld.
The principle of straight-line laser detection is shown in Figure 5.
During laser welding, a straight-line laser beam hits the welding seam vertically, and the image is imaged on the CCD image plane by the diffuse reflection of the upper surface of the workpiece to be welded.
Each weld feature point on the image plane will uniquely determine a point on the surface of the workpiece to be welded.
In terms of tracking algorithm, a high-precision and fast-speed nuclear-related filter target tracking algorithm is used to track the positions of common straight and curved welds respectively.
The error between the data fitting curve and the weld shape obtained in the experiment is within 5%, and the agreement is high, and the real-time tracking effect is good.
Figure 5 Principle of straight line laser detection
Foreign research mainly conducted detailed research on adding external energy in the welding process and using artificial intelligence models to simulate and predict welding.
By using additional parameters, oscillation frequency and amplitude, combined with the spatial power modulation method of linear feed with superimposed circular motion, the welding of copper materials used in the interconnection of lithium-ion batteries and high-power electronic devices has been studied.
The results show that not only can the connection area be increased, but also the stability of the laser welding process and the quality characteristics of the weld can be increased.
When welding some special metals, the solder cannot be fully mixed in the molten pool, resulting in uneven distribution of elements in the weld.
Based on this research, the oscillating magnetic field is used to form a non-conservative Lorentz force component in the molten pool to improve the element distribution across the thickness of the material.
The distribution of the two tracking elements (Ni, Cr) was analyzed by spectroscopy (EDS), and the results showed that when the magnetic field was rotated 30° to the welding direction, the solder distribution was fundamentally improved.
This research provides data support for the use of magnetic fields in welding.
Belitzki proposed a method that can minimize the deformation of the complex frame structure with multiple welds. The meta-model established by the artificial neural network is applied to the laser welding process to predict the local area based on the welding parameters in the sub-area. Deformed.
The genetic algorithm is used to effectively find the welding parameters suitable for the global structure.
The results show that the method can effectively and reliably identify the least distortion parameter among more than 1 billion potential parameter combinations.
2.3 Laser welding defect treatment
The application of laser welding is very wide, but the welding process is often accompanied by welding defects such as cracks, welding pores and spatter.
A lot of research has been done on it at home and abroad. They use oscillation, pulse and other methods to combine with laser welding.
While studying the principle, it also attaches importance to the integration with industrial equipment, and actively uses new products to promote its own research, and its research has high practicality.
Domestic research is mainly focused on how to solve the welding joint defects of laser welding, and the formation mechanism of welding defects has also been studied in detail.
Many research teams use simulation analysis, scanning electron microscopy and other methods to study issues such as molten pool splash and Fresnel absorption effect.
The high-power laser is irradiated on the work surface to quickly vaporize the material and produce a keyhole, so the Fresnel absorption effect of the molten pool and the keyhole determines the quality of the welding.
Welding defects are produced during the laser welding process. Figure 6 shows the porosity defects caused by laser welding of galvanized DP780 high-strength steel.
Research on the keyhole and Fresnel absorption of laser deep penetration welding found that the multiple reflections of the laser in the keyhole caused the total power density of Fresnel absorption to be uneven, and the density near the bottom of the keyhole was greater than the upper hole. And the important factor affecting the density distribution is the reflection of the laser.
The single-focus laser welding method still has certain limitations.
For example, the temperature cycle during welding cannot be controlled, and when welding materials with high thermal sensitivity, cracks are prone to appear inside the weld.
In order to stabilize the welding process, many scholars have studied dual-focus laser welding.
Some scholars have studied the stability of the keyhole and the flow in the molten pool of aluminum alloys in the serial arrangement of the laser double focus.
It established a coupling model for the welding transient molten pool and the internal flow of the molten pool for dual-focus laser welding of aluminum alloy, and used the ray-tracing method to establish the heat source model, taking into account the Fresnel absorption effect, steam recoil force, and internal flow of the molten pool influences.
Research results show that dual-focus laser welding is more stable and controllable, and the fluctuation of the keyhole is significantly weaker than that of single laser welding.
Figure 6 The principle of pore defects in laser deep penetration welding
Compared with foreign countries, domestic research focuses on the change of the laser beam’s beam morphology, and most of them focus on the research of laser welding defects by changing the number of laser beams.
Foreign research teams tried to use new optical components to explore the formation mechanism of keyhole collapse and molten pool splash.
Some foreign scholars have also tried new techniques to improve the deficiencies of laser welding, such as the use of beam oscillation or laser power modulation to reduce the occurrence of defects.
VolppJ. used a newly developed multifocal beam shaping optical element.
This component can generate multi-beam laser in the axial direction, which can be used to modify the energy input in the key hole in the additional area to explain the mechanism of spatter formation and evaluate the potential of axial beam shaping to suppress defects during laser deep penetration welding.
The results show that under high-intensity light irradiation, the number of splashes can be effectively reduced, the keyhole collapse is avoided, the upper keyhole section has sufficient energy input, and the liquid splash can be reduced.
3. Application status of laser welding
After years of research and development, laser welding technology has been applied to equipment manufacturing industries such as automobiles, oil and gas pipelines, and tram equipment.
This article mainly introduces the application of the core components of the laser welding system and its engineering application in material processing.
3.1 The core components of the laser welding system
(1) Laser generator
In the laser welding system, the core component is the laser generator, which is used to generate laser light.
There are many types of lasers, but their structure is the same, that is, they are composed of three parts: excitation system, laser active medium and optical resonant cavity.
After years of development, the performance of lasers has been greatly improved.
There are many kinds of lasers, such as fiber lasers, semiconductor lasers, CO2 lasers, etc., as shown in Figure 7.
Figure 7 Laser generator
Foreign excellent laser companies include Coherent, Trumpf, etc., whose lasers have inherent advantages.
After years of research and development and improvement, its beam quality is high, photoelectric conversion efficiency is high, and stability is good.
The spot of a semiconductor laser is more concentrated than that of a fiber laser, the power distribution is more uniform, and the energy consumption is lower.
For example, the TruDiode series of high-efficiency semiconductor lasers have won the favor of users with the best application results, extremely low investment costs and operating costs.
The laser provides stable laser power up to several kilowatts.
Typical applications are deep penetration welding, heat conduction welding, laser metal cladding, brazing and plastic welding, with a high efficiency of up to 40% to reduce production operating costs.
Since there is no need for extra resonant cavity structure, TruDiode laser is very delicate.
CO2 lasers are common gas lasers, which can use the energy level structure of CO2 molecules to obtain spectral output of different wavelength bands.
It is superior to solid-state lasers in thermal performance, and can accumulate a large amount of heat depending on the flow of gas, and is suitable for use as high-power lasers.
The development of domestic lasers has the advantage of backward mobility. After years of technical research, a large number of outstanding laser companies have emerged in China, such as Raycus Laser, Chuangxin Laser and other outstanding domestic laser brands.
With excellent laser products, affordable price and product localization strategy, it quickly gained a large domestic laser market share.
Figure 7b shows the quasi-continuous fiber laser produced by Raycus. Its power is small, covering 75~300W, with better compatibility and higher electro-optical conversion efficiency, better beam quality, and less maintenance cost. Ideal for industrial applications requiring long pulse width and high peak values such as laser spot welding and laser seam welding.
(2) Laser welding head
With the development of laser welding technology, laser welding heads have also introduced various types of laser welding heads according to their functions and needs, as shown in Figure 8.
From left to right, there are welding head, laser galvanometer scanning head, welding swing head double spot & beam shaping head, which can withstand up to 50kW power.
Figure 8 Common laser welding head
According to the actual welding requirements, the welding head is designed and applied to the actual welding processing place, which provides solutions for different welding requirements.
For example, the laser needs to split multiple beams to improve welding efficiency. At this time, the application of scanning galvanometer welding head can effectively solve the requirement of high efficiency.
The swing welding joint as shown in Figure 8, can effectively improve the internal and appearance quality of the weld, and improve the weldability of materials prone to defects.
3.2 Engineering application of laser welding technology
From the beginning, laser welding has been applied in automotive (see Figure 9) manufacturing and other fields, and has gradually expanded to shipbuilding, aerospace, semiconductor, electronic industries and consumer products, from traditional fields to more in-depth and diverse material processing terminal applications.
Figure 9 Laser welding applications in the automotive sector
In the automobile manufacturing process, laser welding technology is mainly used for tailor welding of body plates with different thicknesses, body welding and welding of auto parts.
Through the use of laser welding technology, the weight of the car body can be reduced and the effect of energy-saving and emission reduction can be achieved, the stamping and assembly costs in the automobile manufacturing process can be reduced, the assembly accuracy of the car body, the rigidity of the car body and the degree of integration of the car body can be improved, thereby improving the car The comfort and safety.
Laser welding is widely used in the automotive industry.
Figure 9b shows the workshop of a domestic auto parts company. Its car door is laser brazed and welded. It uses a larger laser spot (2 ~ 4mm), the laser power is 2 ~ 4kW, and the contact tracking is used to test the edge nodes. After calibration, it was found that the weld seam was narrower than other welding methods, which effectively improved the overall appearance of the car body.
After testing, it is concluded that compared with ordinary welding, its strength has been greatly improved.
Laser welding needs to select the corresponding shielding gas according to the nature of the actual connection material, and the laser welding speed is faster, the welding efficiency is higher, the working area is small, and the deformation of the processed workpiece is small.
In some cases, heat treatment to eliminate residual stress is not required. The use of laser welding technology in mechanical manufacturing can greatly improve the quality of welded products and improve the work efficiency of the manufacturing industry;
Laser welding technology meets the requirements of high cleanliness in the manufacturing process of medical devices. There is no need to add any adhesive during the welding process, and almost no welding slag and debris are generated. Therefore, the emergence of laser welding technology has greatly promoted the development of medical devices;
There is a big difference between the plates used in ships and the plate selection of ordinary mechanical products. The use of laser welding technology can effectively solve the problems of longer weld seams and warpage of ship plates.
The engineering application coverage of laser welding process processing is relatively wide, and it can be applied to welding seam positioning, cross-sectional scanning, and online monitoring of surface formation.
Figure 10 shows the new welding whole-process monitoring system LDD-700 based on coherent interference imaging technology. Its 3D imaging mode enables LDD-700 to adapt to the changes in keyhole geometry of different processes, which is a basic ability for accurate depth measurement.
Powerful software supports customized monitoring solutions to meet different process requirements.
Figure 10 Engineering application of laser welding monitoring process
Laser welding is also widely used in the connection of petroleum pipelines.
The use of robot laser welding can not only improve welding efficiency and reliability, but also improve the quality of welded joints.
As an advanced high-energy beam welding technology, laser welding has the characteristics of no need for a vacuum environment, concentrated heat input, small thermal deformation, large aspect ratio of the weld, high accuracy, and easy realization of automatic welding. It was finally determined as the most suitable for CC The best way to seal the coil box.
4. Summary and outlook
Laser welding has obtained great development from the research and engineering application of process processing, welding process treatment and welding defect resolution.
From the current research and engineering practice, domestic and foreign scholars mainly deepen the research of laser welding and solve the problems of industrial application from the following two aspects.
First, based on actual industrial needs, study the causes of defects in the laser welding process, and continuously improve and optimize the processing parameters to improve or eliminate laser welding defects;
Second, try to combine external energy such as magnetic field and oscillation with laser energy to explore new processing techniques to improve welding stability and try to solve laser welding defects and improve the performance of welded joints.
From the initial heat conduction laser welding to the current multi-field coupling laser welding research, laser welding has continuously expanded the application field of laser.
Laser innovation is also constantly being carried out. For example, semiconductor lasers have improved their photoelectric conversion efficiency, with lower energy consumption and more concentrated light spots, which have gradually become the development trend of new lasers. Scholars at home and abroad are also constantly studying new ones. Laser equipment.
With the continuous breakthrough and innovation of new laser equipment, it is foreseeable that in the near future, the application fields of laser welding technology will continue to be applied to more material processing terminal fields, helping the industrial upgrading of manufacturing.