1. Limitations of near-infrared wavelength high power lasers
In the past decades, high-power CW laser has become a general tool in the modern manufacturing industry, covering the application fields of welding, cladding, surface treatment, hardening, brazing, cutting, 3D printing and additive manufacturing.
The first development peak of high-power CW laser technology appeared before the year 2000, when high-power 10.6 µm wavelength carbon dioxide (CO2) laser and near-infrared 1064nm wavelength semiconductor pumped Nd: YAG solid-state laser were developed.
However, due to its wavelength, carbon dioxide laser is difficult to transmit through optical fiber, which makes it difficult for industrial application;
Solid-state lasers are limited by brightness and power amplification.
After 2000, high-power industrial fiber lasers began to appear as a solution that can be transmitted through optical fibers and has high brightness and high-power lasers.
Nowadays, fiber laser has replaced carbon dioxide laser in most applications and has been effectively used in many industrial processing applications.
Especially in recent years, it has become the main force of industrial lasers, such as laser welding and cutting.
It has higher speed, efficiency and reliability than carbon dioxide laser.
However, these continuous high-power fiber lasers generally work at near-infrared (NIR) wavelength, and their wavelength is less than 1µm, which is no problem for many applications.
For example, it is applicable to the processing of steel with an absorption rate of more than 50%, but it is limited because some metals will reflect 90% or more of the near-infrared laser radiation incident on their surface.
In particular, yellow metals such as copper and gold are welded with near-infrared laser.
Due to the low absorption rate, it means that a large amount of laser power is required to start the welding process.
There are usually two laser welding processes: heat conduction mode welding (in which the material is only melted and refluxed) and deep penetration welding (in which the laser vaporizes the metal and the vapor pressure forms a cavity or keyhole).
Deep penetration mode welding causes the laser beam to be highly absorbed because the laser beam will interact with metal and metal vapor many times when propagating through the material.
However, starting the keyhole with a near-infrared laser requires considerable incident laser intensity, especially when the welded material has high reflectivity.
Moreover, once the keyhole is formed, the absorption rate will rise sharply.
The high metal vapor pressure generated by high-power near-infrared laser in the molten pool will lead to spatter and pores.
Therefore, it is necessary to carefully control the laser power or welding speed to prevent excessive spatter from spraying out of the weld.
When the molten pool solidifies, the “bubbles” in the metal vapor and process gas may also be captured, forming pores at the welding joint.
This porosity will weaken the welding strength and increase the joint resistivity, resulting in the reduction of the quality of the welded joint.
Therefore, the near-infrared laser is a great challenge for processing materials with an absorptivity of less than 5% at 1µm, such as copper.
In order to better process these high reflectivity materials, people have adopted methods such as generating plasma on the processed materials to increase the laser absorptivity of the materials.
However, because these methods will limit the material processing to the range of deep penetration process, it is impossible to weld thin materials in heat conduction mode.
At the same time, there are inherent risks such as sputtering and controlling energy deposition.
Therefore, the existing wavelength 1µm laser system has its limitations in the processing of high reflective materials such as non-ferrous metals and underwater applications.
In order to develop these near-infrared laser applications, people must study new laser sources.
In addition, in order to reduce greenhouse gases, new energy vehicles are using electric engines instead of gasoline engines and internal combustion engines.
Electric engines, especially power batteries, use a lot of copper materials, which creates a huge demand for reliable copper processing solutions.
At the same time, it also has the same wide application demand in other renewable energy systems such as wind turbines.
2. The birth of high power blue laser
The development of industrial laser technology has been developing along with the road map of production technology and new social requirements.
In the past 60 years, from the digital economy and society to sustainable energy and healthy life, laser technology has made great contributions to solving the important tasks of mankind in the future.
Today, from production technology to automotive engineering, medical technology, measurement and environmental technology, and then to information and communication technology, laser technology has become an indispensable part of many core areas of China’s economy.
With the continuous progress of metal processing technology and the continuous improvement of user requirements, lasers need to innovate in cost, energy efficiency and laser system performance.
The market demand for effective processing of high reflection metals has stimulated the development of blue high-power laser technology and will certainly open the door to new metal processing technologies.
For non-ferrous metals, the absorption of light energy increases with the reduction of light wavelength.
For example, the light absorption of copper at wavelengths below 500 nm will increase by at least 50% compared with infrared light, so the short light wavelength is more suitable for copper processing.
The problem is that it is difficult to develop short-wavelength high-power lasers for these industrial applications.
There are few high-power options available. Even the existing options are expensive and inefficient.
For example, there are some frequency doubling based solid-state laser sources on the market that can be used in this wavelength range to produce 515nm and 532nm (green spectrum) lasers. However, these laser sources rely on their nonlinear optical crystals to convert the pump laser energy into the energy of the target wavelength.
The conversion process will lead to high power loss.
At the same time, the laser needs complex cooling system and complex optical settings.
In order to meet this challenge, people pay attention to blue semiconductor lasers.
First, blue light has its specific properties.
High reflectivity metal materials have high absorptivity of blue light, which means that blue light has great advantages in metal processing of high reflection materials (such as copper).
As shown in Fig. 1, the absorption of blue light by copper is 13% higher than that of infrared light × (13 times) or more.
In addition, the absorptivity changes little when copper melts. Once the blue laser starts welding, the same energy density will continue the welding.
Blue laser welding has inherent good control and few defects, resulting in fast and high-quality copper welds.
At the same time, blue light is less absorbed in seawater, so the transmission range is long, which makes it realistic to explore the field of underwater laser material processing.
In addition, blue light is relatively easy to convert into white light, so it is possible to use blue laser to realize floodlights and other lighting applications very compactly.
Second, the semiconductor laser based on gallium nitride can directly produce a laser with a wavelength of 450 nm without further frequency doubling, so it has higher energy conversion efficiency.
Source: NASA 1969
a) The performance advantages of blue laser stem from the basic physical principles
|Blue light absorption|
b) Comparison of blue light absorption and infrared (NIR) absorption of copper
Fig. 1 Physical properties of blue light
The processing efficiency of 450 nm laser is expected to be nearly 20 times higher than that of 1µm.
Compared with the traditional near-infrared laser welding process, high-power blue laser has advantages in quantity and quality.
Quantitative advantages: it improves the welding speed, widens the process range, can be directly transformed into faster production efficiency, and minimizes production downtime.
Quality advantages: it can obtain a larger process range, high-quality welds without splash and porosity, higher mechanical strength and lower resistivity.
The consistency of welding quality can greatly improve the production yield (see Fig. 2).
In addition, the blue laser can also carry out thermal conduction welding mode, which can not be realized by near-infrared laser (see Fig. 3).
Fig. 2 Cross-section of deep penetration welding on 254 µ m thick copper foil
Fig. 3 Section of heat conduction welding mode in copper sheet with a thickness of 500 µm
3. Development of high power blue laser
With the 2014 Nobel Prize in physics and the increasing awareness of global environmental protection, gallium nitride (GAN) light-emitting devices have attracted extensive attention, especially in the field of lighting.
By continuously improving the high brightness and high output of blue semiconductor devices, blue semiconductor lasers have entered the era of mass production, but they are mainly used for projector light sources, replacing lamps in projectors, and used together with phosphors that produce green or red light.
Because blue semiconductor lasers have a longer life and smaller size than bulbs, they have been rapidly popularized in lighting and display applications in recent years.
However, for laser processing, it is necessary to have higher power than these blue lasers for illumination.
Because blue laser has many advantages as described above, people have been trying to develop high-power blue laser for laser processing.
Because a single blue laser semiconductor chip has only a few watts of output power, it is very time-consuming and expensive to increase the power to a higher power range.
In order to develop the high power required by the great application potential of blue laser, new technical methods will be needed.
So far, the actual power of each chip of blue semiconductor laser is about 5W at a single wavelength, so the beam combination technology of combining multiple chip outputs is essential to obtain higher power output.
The methods of beam combination are divided into coherent method and incoherent method. Among them, the incoherent method is more practical, and there is no need for fine phase control between lasers.
The incoherent method includes a spatial combination method of combining a plurality of beams in space, a polarization combination method of combining orthogonal polarized light in a polarization beam splitter, and a wavelength combination method of combining different wavelengths on coaxial.
Each method has its advantages and disadvantages, and each method can also be used in combination.
Among them, the spatial combination is suitable for combining multiple laser chips with the same wavelength to obtain high power output.
So far, two high-power synthesis methods have been the most successful. The following is a brief introduction.
The first method is to use the laser bars technology, that is, to systematically generate a laser single emitter on a wafer of indium gallium nitride (InGaN).
Firstly, a plurality of individual laser chips are efficiently integrated into a so-called laser bar, and each laser bar can produce at least 50W blue light.
Then, a plurality of semiconductor laser bars are installed and combined into a semiconductor laser stack through appropriate electrical connection, cooling and heat dissipation, and the use of special optical devices.
The whole semiconductor laser can be combined with one or more semiconductor laser stacks, as shown in Fig. 4.
At present, the laser bar technology can reach 2kW blue light power.
a) Synthesis process of bar instrument
b) Bar beam diagram
Fig. 4 beam synthesis of semiconductor laser bars Technology
The second method is to use semiconductor laser single emitter technology.
These lasers have a unique “single tube chip based” design function to collimate the output of each gallium nitride (GAN) laser single tube.
If all laser single tubes are collimated together with one lens, like the bar technique, the combined beam divergence (BPP) will inevitably increase.
By collimating each laser single tube with its own special lens, the divergence of the combined beam can be kept unchanged as far as possible, and the beam BPP can be minimized, so as to improve the brightness of the laser (see Fig. 5).
Moreover, when the gallium nitride laser single tube follows its expected development route and continuously improves the single tube laser power, this unique “single tube chip” design provides the best way to improve the power of the overall laser system.
Moreover, the laser single tube technology produces the best beam quality with an output power of 1.5KW, which provides a guarantee for the laser remote processing of galvanometer scanning.
This scanning system is commonly used in the manufacture of batteries, electric vehicles and consumer electronics.
The laser output power and residence time can be adjusted during the scanning operation to maximize productivity by allowing different joint geometry and material thickness to be solved in a single scanning pattern.
Table 1 shows the advantages of blue semiconductor laser compared with near-infrared semiconductor laser and green solid-state laser.
Fig. 5 Beam synthesis of semiconductor laser single tube technology
Table 1 Comparison of blue semiconductor laser with near-infrared semiconductor laser and green solid-state laser
|Project||Blue semiconductor laser||Near infrared semiconductor laser||Green solid state laser|
|Wavelength||Blu ray||Near infrared||Green light|
|Anti reflection ability||strong||commonly||weak|
|Service life / h||>10000||>10000||>5000|
|Fault type||Service wear||random||random|
|Ease of use and operation||good||good||commonly|
4. Application cases of blue light semiconductor laser material processing
1) Fig. 6 shows a blue semiconductor laser and galvanometer scanning system for power battery manufacturing.
Blu ray has a wide process window, which can handle each stage of battery manufacturing and can weld thicker and a variety of materials, such as copper, gold and stainless steel a few millimeters thick.
It is an ideal choice for manufacturing prismatic batteries, battery housings, and battery packs and battery integration.
a) 70 pcs of 8 µm foils welded to 254 µm copper lugs
b) Connection of two copper lugs
c) Connect two copper lugs to the steel battery housing
Fig. 6 the wide process window of the blue laser can handle each stage of battery manufacturing
2) Using a blue semiconductor light source with a wavelength of 450 nm, the copper material can be melted in the heat conduction mode, so that the molten pool geometry of the thin copper material can be accurately adjusted (see Fig. 7).
Stable energy absorption and accurate control of the heat conduction process are particularly important for deep penetration welding of thin copper materials, mainly because it helps to prevent cutting or splashing of thin materials due to high pressure.
These phenomena are particularly likely to occur when welding stacked thin copper foils, which may produce uncontrollable irregular gaps due to the warpage of the stacked foils (see Fig. 8).
When butt welding is carried out on 34 stacked copper foils with 580W blue light semiconductor laser and a speed of 2m/min, a weld width > 0.8mm can be formed with minimum porosity and low undercut.
For fillet welding on the edge of the foil stack, the end of the foil is successfully melted into a high cross-sectional area and completely attached to the solid foil. Perfect mechanical connection and very good conductivity can be achieved in butt welding and edge welding.
Fig. 7 Molten pool geometry of copper material
a) Edge welding structure
b) At 580W blue laser power and 2m / min welding speed
Fig. 8 joint cross section between 34 stacked copper foils (11 µ m thick each) connection welds
3) Fig. 9 shows the results of lap welding copper foil with 100W blue laser.
When three copper foils are stacked at a thickness of 30μm, the copper foil is scanned by laser from the top surface at a speed of about 10mm / s.
Because the output of the optical fiber with a core diameter of 100 μm is concentrated at the projection ratio of 1 ∶ 1, the laser spot diameter on the sample surface is also 100, which obtains good welding quality and suppresses the influence of heat on debris and surrounding environment.
a) Top view of weld seam
b) Weld seam cross section
Fig. 9 results of lap welding of pure copper sheet
4) Figure 10 shows an example of a 3D printer that can be made of pure copper using a blue light semiconductor laser developed by Osaka University.
The laser focusing spot diameter of 100μm is realized on the powder bed, and pure copper with high conductivity and high thermal conductivity can be laminated. Before that, it is difficult to melt with near-infrared laser.
It is expected that this technology will be applied to industrial fields such as aerospace and electric vehicles.
a) SLM machine with 100W blue laser
b) 3D prototype sample made of pure copper powder
Fig. 10 3D printing application
5) Greater penetration has also opened up the field of electric vehicle applications. Electric vehicle manufacturers are turning to rod winding design to maximize thermal and electrical efficiency.
The three blue laser hairpin welds show consistent quality, which is very important to improve production efficiency, as shown in Fig. 11.
The blue laser can produce hairpin welding, which is very important for high-density and high-intensity motor manufacturing.
Fig. 11 Application in electric vehicle manufacturing
6) High power and high brightness also increase the flexibility of the welding process, making it possible to expand the range of processing materials.
For example, copper and zinc in brass have significantly different thermal properties, which poses a challenge to high-quality welding, but the blue industrial laser is easy to handle.
Now it can weld brass materials commonly used in household appliance production, as shown in Fig. 12.
Preliminary research shows that blue laser can effectively solve the problem of welding dissimilar metals.
Dissimilar metal welding is a challenge because each material has unique thermal, optical and mechanical properties.
Welding of dissimilar metals usually leads to the formation of intermetallic compounds, i.e. areas of different alloys, which damages the mechanical and electrical properties and consistency of the joint.
The latest generation of blue semiconductor laser has a wide range of process parameters, can weld different materials, and has the least defects.
Although copper and zinc in brass have significantly different thermal properties, which poses a challenge to high-quality welding, it is easy to handle for blue semiconductor laser.
Fig. 12 Application in brass welding
2KW blue semiconductor laser has shown its advantages in metal processing, especially in the processing of high reflection metal materials.
The brightness and power of blue semiconductor lasers are still increasing to new limits, which will also lead to more and wider applications.
For example, the additive manufacturing capacity of blue laser continues to be explored (see Figure 10).
In addition, in addition to efficient metal material processing, blue light semiconductor lasers look forward to cross sectoral applications, especially the mechanical engineering department will be able to process laser materials with blue light underwater.
For the manufacturing industry, this is certainly a huge advantage. In addition, the lighting industry can also use high-quality lighting technology based on blue semiconductor laser.
The rise of the Internet of things and artificial intelligence has prompted new model changes in the industrial field.
Because laser processing technology has the natural advantages of integrating numerical control technology and remote processing, and there is no need to replace tools, it will play a leading role in the field of next-generation intelligent manufacturing.
The rise of high-power blue semiconductor laser has brought another surprise to laser technology. Although the processing application based on high-power blue semiconductor laser has just started, with the development and progress of future technology and technology, it may become one of the core tools of the next generation of cutting-edge intelligent manufacturing.