1. Limitations of high-power lasers with near-infrared wavelengths
Over the past few decades, high-power continuous wave (CW) lasers have become a ubiquitous tool in modern manufacturing industries. These lasers are used for a wide range of applications, including welding, cladding, surface treatment, hardening, brazing, cutting, 3D printing and additive manufacturing.
The first significant development in high-power CW laser technology occurred before the year 2000, with the creation of the 10.6 µm wavelength high-power carbon dioxide (CO 2 ) and the solid-state Nd:YAG laser pumped by semiconductor of near-infrared wavelength of 1064 nm.
However, due to its wavelength, the carbon dioxide laser is difficult to transmit through optical fibers, making it challenging for industrial applications. Likewise, solid-state lasers are limited by brightness and power amplification.
After 2000, high-power industrial fiber lasers were developed as a solution that can be transmitted through optical fibers and still have high brightness and power. Nowadays, fiber lasers have replaced carbon dioxide lasers in most applications and have proven to be highly effective in many industrial processing applications. In recent years, fiber lasers have become the main industrial laser used for processes such as welding and laser cutting, as they offer greater speed, efficiency and reliability than carbon dioxide lasers.
However, these high-power continuous fiber lasers generally operate at near-infrared (NIR) wavelengths of less than 1 µm. While this is suitable for many applications, some metals will reflect 90% or more of the near-infrared laser radiation falling on their surface, limiting their effectiveness. Yellow metals, such as copper and gold, are particularly challenging to weld with near-infrared lasers due to their low absorption rates, which require a large amount of laser power to begin the welding process.
There are two main laser welding processes: heat conduction welding, which involves melting and solidifying the material, and deep penetration welding, which vaporizes the metal and forms a cavity or keyhole.
Deep penetration welding requires a highly absorbed laser beam, as the laser interacts with the metal and metal vapor several times as it propagates through the material.
Starting a keyhole with a near-infrared laser requires a high incident laser intensity, especially when the material to be welded has high reflectivity. Once the keyhole is formed, however, the rate of absorption increases markedly.
High-power near-infrared lasers generate a high pressure of metallic vapor in the molten pool, causing spatter and pores. Therefore, laser power or welding speed must be carefully controlled to avoid excessive spatter.
When the molten pool solidifies, “bubbles” in the metal vapor and process gas can become trapped, forming pores in the welding joint. These pores weaken the welding strength and increase the resistivity of the joint, reducing the quality of the welded joint.
Processing materials with absorptivity less than 5% at 1 µm, such as copper, with near-infrared lasers is a significant challenge. Methods such as plasma generation in processed materials can increase the laser absorptivity of materials. However, these methods limit material processing to the deep penetration process and carry inherent risks, such as sputtering and energy deposition control.
Therefore, existing 1 µm wavelength laser systems have limitations in processing highly reflective materials such as non-ferrous metals and underwater applications.
To advance near-infrared laser applications, researchers must investigate new laser sources. Additionally, the shift to electric motors in new energy vehicles as a way to reduce greenhouse gases has resulted in significant demand for reliable copper processing solutions. Electric motors, especially batteries, require large amounts of copper materials. This demand extends to other renewable energy systems, including wind turbines.
2. The birth of the high-power blue laser
The development of industrial laser technology followed the script of production technology and social demands. Over the past 60 years, laser technology has significantly contributed to solving important future tasks such as the digital economy, sustainable energy and healthy living.
Today, laser technology has become an indispensable component in several core areas of China's economy, including production technology, automotive engineering, medical technology, measurement, environmental technology, and information and communications technology. As metal processing technology continues to progress and user requirements continue to improve, lasers must innovate in cost, energy efficiency and laser system performance.
Market demand for effective processing of highly reflective metals has spurred the development of high-power blue laser technology, which will undoubtedly open the door to new metal processing technologies. In non-ferrous metals, the absorption of light energy increases as the wavelength of light decreases. For example, the light absorption of copper at wavelengths below 500 nm increases by at least 50% compared to infrared light, making shorter wavelengths more suitable for processing copper.
However, developing high-power, short-wavelength lasers for industrial applications is challenging due to the limited options available. Even existing options are expensive and inefficient. For example, there are frequency-doubling-based solid-state laser sources on the market that can produce 515 nm and 532 nm (green spectrum) lasers in this wavelength range, but these sources rely on nonlinear optical crystals to convert the bomb laser energy on target. wavelength energy, resulting in high power loss.
Furthermore, such lasers require complex cooling systems and complex optical configurations.
To meet this challenge, attention has turned to blue semiconductor lasers. Blue light has unique properties that make it advantageous for metal processing of highly reflective materials such as copper. Figure 1 shows that copper absorbs blue light 13% more efficiently than infrared light, up to 13 times greater absorption.
Additionally, copper's absorption of blue light remains consistent even as the metal melts, providing a stable energy density for welding. As a result, blue laser welding offers precise control, few defects and produces high-quality copper welds quickly.
Blue light also has long transmission ranges in seawater because it is less absorbed, making it suitable for exploring underwater laser processing of materials.
Additionally, blue light is relatively easy to convert to white light, allowing the compact use of blue lasers for spotlights and other lighting applications.
Gallium nitride-based semiconductor lasers can directly produce a laser with a wavelength of 450 nm without the need for additional frequency doubling, thus achieving higher power conversion efficiency.
Source: NASA 1969
a) The performance advantages of the blue laser arise from basic physical principles
| Key metals | Blue light absorption |
| Gold | 66X |
| Copper | 13X |
| Aluminum 1100 | 3X |
| Nickel | 1.5X |
b) Comparison of blue light absorption and infrared (NIR) absorption of copper
The 450 nm laser is expected to have a processing efficiency almost 20 times greater than that of the 1 µm laser. When compared to the traditional near-infrared laser welding process, the high-power blue laser offers quantitative and qualitative advantages.
In terms of quantitative advantages, blue laser improves welding speed, expands process range, increases production efficiency, and reduces production downtime.
Regarding qualitative advantages, the blue laser allows for a greater process range, produces high quality welds, without spatter or porosity, provides greater mechanical resistance and reduces resistivity. Consistent welding quality greatly improves production yield (see Fig. 2).
In addition, blue laser can also realize thermal conduction welding mode, which is impossible for near-infrared laser.
3. Development of high-power blue laser
Gallium nitride (GaN) light-emitting devices have gained significant attention, especially in the field of lighting, due to the 2014 Nobel Prize in Physics and growing global awareness about environmental protection.
With the continuous improvement of high brightness and production of blue semiconductor devices, blue semiconductor lasers have entered the era of mass production. They are commonly used as light sources for projectors and, in combination with phosphors that produce green or red light, are replacing projector lamps.
In recent years, blue semiconductor lasers have gained popularity in lighting and display applications due to their longer lifespan and smaller size compared to light bulbs. However, for laser processing, a power greater than that of these blue lasers is required.
Despite the advantages of blue lasers, which include longer lifetime and smaller size, developing high-power blue lasers for laser processing requires greater output power than a single blue laser semiconductor chip, which has only a few watts of power. output power. Increasing power to a higher range is a time-consuming and expensive process.
To meet the high power requirements of blue lasers, new technical methods are needed. Currently, the actual power of each blue semiconductor laser chip is about 5W at a single wavelength. Therefore, beam combining technology is essential to obtain higher power, which can be achieved by combining the outputs of multiple chips.
Beam combining methods are divided into two types: coherent and incoherent methods. The incoherent method is more practical as it does not require fine phase control between the lasers.
The incoherent method includes several techniques for combining multiple laser beams, such as the spatial combination method that combines multiple beams in space, the polarization combination method that combines orthogonal polarized light using a polarization beam splitter, and the polarization combination method. wavelength that combines different wavelengths in coaxial.
Each technique has its own advantages and disadvantages and can be used in combination.
The spatial combination method is particularly suitable for combining multiple laser chips with the same wavelength to obtain high power.
So far, two high-potency synthesis methods have had the most success. Here is a brief introduction to them:
The first method uses laser bar technology to systematically generate a single laser emitter on an indium gallium nitride (InGaN) wafer.
Initially, individual laser chips are efficiently integrated into a “laser bar”, and each laser bar can produce at least 50W of blue light.
Then, multiple semiconductor laser bars are installed and combined into a semiconductor laser stack through appropriate electrical connections, cooling and heat dissipation, and the use of special optical devices.
The entire semiconductor laser can be combined with one or more semiconductor laser stacks, as shown in Figure 4.
Currently, laser bar technology can generate up to 2kW of blue light power.
a) Bar instrument synthesis process
b) Bar beam diagram
The second method involves the use of semiconductor laser single-emitter technology. These lasers utilize a unique “single-tube chip-based” design that is intended to collimate the output of each single gallium nitride (GAN) laser tube.
If all single laser tubes are collimated together with a single lens, as in the bar technique, the combined beam divergence (BPP) will inevitably increase. However, by collimating each single laser tube with its own special lens, the divergence of the combined beam can be kept as unchanged as possible, and the beam BPP can be minimized, which improves the laser brightness.
Furthermore, as the single-tube gallium nitride laser continues to improve the power of the single-tube laser along its expected development path, this unique “single-tube chip” design provides the best way to improve the power of the overall laser system.
Furthermore, single-tube laser technology offers the best beam quality with an output power of 1.5KW, ensuring remote laser processing of galvanometer scanning. This scanning system is widely used in the production of batteries, electric vehicles and consumer electronics.
During the scanning operation, laser output power and dwell time can be adjusted to maximize productivity, allowing different joint geometries and material thicknesses to be resolved into a single scan pattern.
Table 1 illustrates the advantages of the blue semiconductor laser compared to the near-infrared semiconductor laser and the solid-state green laser.
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 |
| Wave-length | Blu-ray | Near infrared | Green light |
| Metal absorption | good | commonly | preferably |
| Shine | good | commonly | good |
| Anti-glare ability | strong | commonly | weak |
| Useful life/h | >10,000 | >10,000 | >5000 |
| Failure type | Service wear | random | random |
| Ease of use and operation | good | good | commonly |
4. Application cases of blue light semiconductor laser material processing
1) Figure 6 illustrates a scanning system composed of a blue semiconductor laser, used to manufacture energy batteries. The advantage of using a blue laser lies in its wide process window, which allows it to handle each step of battery manufacturing.
Additionally, it can weld thicker materials such as copper, gold and stainless steel that are a few millimeters thick. This makes it an ideal choice for manufacturing prismatic batteries, battery compartments and battery packs with integrated batteries.
a) 70 pieces of 8 µm foil soldered to 254 µm copper terminals
b) Connection of two copper terminals
c) Connect two copper terminals to the steel battery compartment
2) Using a blue semiconductor light source with a wavelength of 450 nm, it is possible to melt copper material in heat conduction mode, allowing precise adjustment of the molten pool geometry of thin copper materials (see Fig. 7) .
In deep penetration welding of thin copper materials, stable energy absorption and precise control of the heat conduction process are especially important, as they help to avoid cutting or spattering the materials due to high pressure.
These occurrences are more likely when welding thin stacked sheets of copper, which can result in uneven gaps due to warping of the stacked sheets (see Fig. 8).
When butt welding is performed on 34 stacked copper sheets with a 580 W blue light semiconductor laser at a speed of 2 m/min, a weld width of >0.8 mm can be formed with minimal porosity and low cut.
In fillet welding at the edge of the sheet stack, the edge of the sheet can be successfully fused into a high cross-sectional area and completely fixed to the solid sheet. Perfect mechanical connection and excellent conductivity can be achieved in butt welding and edge welding.
a) Edge welding structure
b) With blue laser power of 580W and welding speed of 2m/min
Fig. 8 Cross section of the joint between 34 stacked copper sheets (11 µm thick each) connection solders
3) The results of overlay welding of 30 μm thick copper sheets using a 100W blue laser. The welding process involved scanning the top surface of three stacked copper sheets at a speed of approximately 10 mm/s with the laser.
The diameter of the laser spot on the sample surface was 100μm due to the concentration of the optical fiber output with a central diameter of 100μm at a projection ratio of 1:1. This resulted in excellent welding quality while minimizing the impact of heat on the surrounding environment and debris.
4) Figure 10 shows an example of a 3D printer made entirely of pure copper, using a blue light semiconductor laser developed by Osaka University. The laser has a focusing spot diameter of 100μm, which allows the lamination of pure copper with high thermal and electrical conductivity in the powder bed. Previously, this was difficult to achieve with near-infrared lasers.
It is anticipated that this technology will have a wide range of applications in industrial fields, including aerospace and electric vehicles.
5) Increased penetration has also opened up the field of electric vehicle applications, with electric vehicle manufacturers turning to rod winding design to maximize thermal and electrical efficiency. As shown in Figure 11, the consistent quality of the three blue laser hook welds is crucial to improving production efficiency.
The blue laser's ability to produce hook welding is particularly important for manufacturing high-density, high-intensity motors.
6) High power and high brightness can increase the flexibility of the welding process, expanding the range of materials that can be processed. For example, brass, which consists of copper and zinc with significantly different thermal properties, can be challenging to weld with high quality. However, industrial blue laser technology can easily accomplish this task, enabling the welding of brass materials commonly used in the production of household appliances, as shown in Fig.
Preliminary research suggests that blue laser technology can effectively solve the challenge of welding dissimilar metals. Welding dissimilar metals is difficult because each material has unique thermal, optical, and mechanical properties. When different metals are welded, it can lead to the formation of intermetallic compounds, which are areas of different alloys that impair the mechanical and electrical properties and consistency of the joint.
The latest generation of blue semiconductor lasers has a wide range of process parameters, allowing the welding of different materials with minimal defects. Although the copper and zinc in brass have different thermal properties, making high-quality welding difficult, blue semiconductor laser technology can easily address this challenge.
Conclusion
The 2KW blue semiconductor laser has demonstrated its superiority in metal processing, especially for high-reflection metal materials.
The brightness and power of blue semiconductor lasers continue to increase, opening up new possibilities and applications. For example, the additive manufacturing potential of blue lasers is still being explored (see Figure 10).
Furthermore, in addition to efficient processing of metallic materials, blue light semiconductor lasers are expected to be employed in cross-industry applications, especially in the mechanical engineering department, enabling laser processing of blue light materials underwater.
This advantage is significant for the manufacturing industry. Furthermore, the lighting industry can take advantage of high-quality lighting technology based on blue semiconductor laser.
The emergence of the Internet of Things and Artificial Intelligence is leading to new paradigm shifts in the industrial sector.
Laser processing technology naturally integrates numerical control technology and remote processing, eliminating the need for tool replacement, and will take a leading role in next-generation intelligent manufacturing.
The rise of the high-power blue semiconductor laser has also brought a new surprise to laser technology. Although high-power blue semiconductor laser-based processing applications are still in their infancy, with future technological advancements, it could become one of the key tools for the next generation of cutting-edge smart manufacturing.
