Pure copper is a widely used material in electronics and power production due to its high thermal and electrical conductivity. Corresponding applications often involve complex geometries combined with fully dense materials to enhance electrical conductivity. For such applications, additive manufacturing (AM) appears to be sufficient for new designs.
More precisely, the high accuracy and spatial resolution offered by Laser Powder Bed Fusion (L-PBF) technology appears to be particularly well suited to making very complex shapes and reducing material waste in the process. However, due to the high reflectivity and high thermal conductivity of copper powder under laser infrared laser radiation, it is still a real technical problem to fabricate low-porosity pure copper materials by traditional L-PBF method.
Powder properties of copper powder
Copper has excellent thermal conductivity, electrical conductivity, and good corrosion resistance and ductility, and in the metal system, copper has a wide range of sources and low cost, and can be widely used in many fields such as electrical and thermal materials, biomedicine, etc. . Copper has a high reflectivity to laser light, with a reflectivity of more than 90% for lasers with a wavelength greater than 1060 nm, and an absorption rate of more than 60% for lasers with a wavelength of 515 nm. In this case, these characteristics of copper bring challenges in the processing of additive manufacturing technology. Copper has a relatively high thermal conductivity. During the forming process, heat will be quickly transferred to the melt area, resulting in Higher local thermal gradients can easily lead to process defects such as layer curl, delamination, and partial part failure. In addition, the high ductility of copper will make it difficult to remove and recycle residual powder from formed parts. In addition, copper powder has high surface activity and is easy to oxidize. Copper powder requires special handling and storage.
The limitations of copper's high thermal conductivity and high reflection of laser light make it difficult to control the forming process of copper powder additive manufacturing technology, and the forming process is difficult. At present, the research and application of 3D printing copper lags behind some other common metal materials. Copper, as a typical structural-functional integration material, has a wide range of additive manufacturing needs and is a research hotspot in the 3D printing industry.
Technical Difficulties of Traditional Laser Powder Bed Fusion Forming Copper
The heat source of the laser selective melting technology is the laser beam. The high reflectivity of copper to the laser causes most of the laser energy to be reflected back to the optical system during the forming process, and only a small part of the energy is absorbed by the copper powder. Xi rock is completely melted, and the parts are prone to defects such as pores and cracks, which brings difficulties to the forming of laser selective melting copper. At present, in the research field of laser selective melting and forming of copper, related research mainly focuses on improving the density of parts.
Early research was limited by hardware facilities such as laser equipment. During the forming process, it was difficult for the laser to completely melt the copper powder, and it was difficult to prepare dense parts. With the continuous development of laser technology, the performance of laser equipment has been continuously improved, and high power can be used to increase the density of parts. However, the laser returned to the optical system will damage the optical components, and then some researchers proposed that methods such as modifying the surface of copper powder and reducing the laser wavelength can improve the high reflectivity of copper. The early laser selective melting forming equipment used lasers with low power, poor stability and low beam quality, so it was difficult to achieve complete melting of copper powder. Only alloy powder with low melting point or high laser absorption rate can be added to the copper powder as a binder. Under laser scanning, the binder melts to form a liquid phase that fills the pores between the copper powder particles and solidifies to achieve sintering Preparation of parts. This method is called "indirect sintering method". Although the complete printing of the entire part can be achieved in this way, some related researchers have found that the obtained parts are less dense.
In academia, Gu Dongdong from Nanjing University of Aeronautics and Astronautics used a CO2 laser with a maximum output power of 1 KW, pre-alloyed CuSn powder as a binder and CuP as a deoxidizer to sinter Cu+CuSn+CuP powder to prepare a dense 82% copper parts. Tang Y et al. used a 200 W laser to laser sinter Cu+Cu3P powder with pre-alloyed metal powder Cu3P as a binder, and finally prepared a part with a density of 76%. In addition, domestic manufacturers such as Shenghua 3D have also made explorations in indirect 3D printing and forming copper materials, and have made breakthroughs.
In summary, it can be seen that the early related research is still limited by the influence of laser power and beam quality, which makes the density of the prepared parts low and the forming quality poor. This requires the use of higher power and better quality lasers to overcome the difficulty of copper's absorption rate of laser light and produce stable forming conditions, so as to improve the quality and performance of laser selective melting and forming copper parts.
With the continuous development of laser technology, the stability and beam quality of lasers have also been continuously improved, and some laser equipment with high beam quality, high stability and high power have been put into use. Some researchers experimented with this type of equipment and found that the density of parts was greatly improved. Lykov P A et al. used Pro DM125 equipment to prepare pure copper samples with different process parameters. Under the conditions of laser power 200 W, scanning speed 100 mm/s, line spacing 0.12 mm, and layer thickness 0.05 mm, pure copper samples with a density of 88.1% were obtained. Copper samples. Ikeshoji T T et al. used 1KW high-power single-mode fiber laser SLM equipment, under the condition of laser power of 800 W and scanning speed of 300 mm/s, obtained a pure copper sample with a density of 96.6%, and studied the effect of scanning distance on forming According to the influence of the quality of the workpiece, it is found that when the scanning distance is about 0.1mm, the density of the sample obtained is the highest. Colopi M et al. used the same laser SLM equipment to prepare pure copper samples with a density greater than 97%. Jadhav S D et al. used high-power fiber laser equipment to obtain a sample with a density of up to 98% under the process conditions of an energy density of 740-1120J/mm3.
Although the densification of formed parts can be achieved by increasing the laser power and optimizing the forming process, the laser reflected back to the optical system will destroy the optical coating and further damage the laser. Therefore, it is not an effective and feasible solution to rely solely on improving the beam quality of the laser and increasing the laser power. Only reducing the reflectivity of copper to laser power is an effective way to solve this problem. Because copper has a laser absorption rate of more than 60% for wavelengths less than 515nm. Therefore, reducing the laser wavelength and increasing the absorption rate of copper to laser is the key to realize laser selective forming of copper.
green laser
In order to solve the problem of copper's high reflection of laser light, some foreign research institutions began to use newly developed high-power laser sources that work in the visible wavelength range, and tried to use laser equipment with a wavelength of 515nm (green laser) for experiments. Improved laser-copper energy coupling.
In 2017, researchers at the Fraunhofer Institute for Laser Technology in Germany took the lead in exploring green laser printing of pure copper. They developed a green laser selective laser melting (SLM) system for pure copper or copper alloys. 3D printing, the technology is named "Green SLM".
In November 2022, Trumpf (TRUMP) demonstrated the latest 3D printer-TruPrint 5000 and green laser technology at the Frankfurt International Formnext exhibition. In 2021, TRUMP launched its 3 kW high-power continuous green disc laser. It is reported that the average output power of this product is as high as 3 kilowatts, which represents the strongest power in the current green laser series, and is very suitable for welding high-reflective materials such as copper and aluminum, especially in the lithium battery industry represented by new energy vehicle power batteries. , Trumpf green laser (1000-3000W) can achieve up to 120 layers of copper foil welding, almost no spatter, and the penetration depth is precise and controllable. In addition, high-power green light also has outstanding advantages in the application of additive manufacturing of pure copper materials - 3D printing.
In 2018, Shimadzu Corporation (Japan) commercialized its BLUE IMPACT blue-impact diode laser, which can produce 100 watts of power at high brightness. This product was developed by Shimadzu Corporation in cooperation with Osaka University in Japan as part of a national project in Japan. The BLUE IMPACT laser combines many gallium nitride (GaN) blue laser diodes from Nichia Chemical Corporation (Japan), doubling efficiency since 2006 and increasing output power by an order of magnitude. A key application for Shimadzu's 450nm blue diode laser is the 3D printing of copper materials.
The above-mentioned green laser was discovered during the 1960s to 1980s. At that time, people used various nonlinear crystal materials to perform intracavity frequency-doubling Nd:YAG lasers to obtain green light sources. In the 1990s, all-solid-state green lasers with high power and high repetition rate, which have the advantages of long life, high reliability, small size, and high efficiency, have achieved unprecedented development. With the improvement of the quality of domestic semiconductor lasers and the reduction of the price of foreign semiconductor lasers, the research of domestic all-solid-state high-power green lasers has also made great progress.
The use of green lasers has proven to couple better to copper in welding applications. In fact, green wavelengths (λ = 532 or 515 nm) are more easily absorbed by pure copper not only in solid state but also in liquid state. The corresponding absorption rates are expected to be between 40% and 60% in solid state and 25% to 50% in liquid state. According to the research results given by the German Institute of Photon Technology, when the copper is in a solid state at room temperature at 20°C, the absorption rate for the green light band is about 40%; Instead, it dropped by about 5%. That is, the absorption of green light decreases slightly after the copper is melted. This feature helps to achieve a stable small hole and almost zero spatter when machining copper. This is the obvious advantage of green laser over infrared laser welding. Therefore, promoting the widespread use of green lasers on L-PBF copper is the main goal of the current research work.
blue laser
A second possible way to improve laser-copper energy coupling is to use a blue laser source, therefore, high-power blue diode lasers at a wavelength of 450 nm are also strong candidates for laser 3D printing of copper.
In the study of pure copper and Cu-6Sn alloy, Hummel et al. pointed out that the absorption rate of copper for blue laser light is even higher than 515–530 nm, and the absorption rate is as high as 80% in the conductive welding state, while at 515 nm 60%. However, even though higher powers are already under development, existing blue laser diodes are still limited in brightness and available focused beam diameter, which limits their possible application in L-PBFs, since this requires higher Higher scanning speed for laser welding.
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△ Copper, gold, aluminum, and other materials absorb blue laser light better than other wavelengths of laser light. Image via NUBURU/NASA 1969
In May 2022, Antarctic Bear learns that Essentium, the original equipment manufacturer behind High Speed Extrusion (HSE) 3D printing technology, and NUBURU, an industrial laser specialist, have teamed up to develop a new blue laser-based metal 3D printer that can Solve the pain points of easy reflection and difficult forming in the traditional metal 3D printing process of copper/gold/aluminum/stainless steel and other metals. It is reported that the new laser metal 3D printing machine will integrate NUBURU's proprietary blue laser technology and be able to process materials in the form of wire feeding, so we can infer that it operates on the principle of directed energy deposition (DED). Additionally, NUBURU claims that blue laser technology can enable 3D printing up to 10 times faster than competitors, while also printing metal at very high densities.
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△A NUBURU blue laser. Photo via NUBURU.
NUBURU, another company focused on high-power blue laser technology, has raised $20 million to develop industrial production lines and develop markets for energy storage, electric vehicles and 3D printing. Laser cladding and laser metal deposition (LMD) are two applications where the raw material is heated to its melting point and adhered to the surface. According to NUBURU, the advantages of its blue laser technology allow for the cladding of copper onto stainless steel (and vice versa). Industrial blue lasers can deposit copper metal layer by layer. This advantage extends to the laser metal deposition additive manufacturing process (LMD). For gold, copper, aluminum and other reflective metals, the blue laser can build faster than Infrared lasers are 10 times faster and deliver higher quality.
polar bear summary
The above research proves that both green laser and green laser can be used as the preferred light source for 3D printing of highly reflective metal materials, and 3D printing of pure copper materials can solve related problems well and achieve higher density. However, the cost of these two lasers is still high at present, and the improvement and cost reduction of green/blue lasers are still problems to be solved in the future. It is foreseeable that if laser 3D printing technology can be applied to pure copper materials on a large scale, the market size of 3D printing copper materials is expected to be further expanded.
