The working principle of Selective Laser Melting (SLM) forming technology is similar to Selective Laser Sintering (SLS). The main difference lies in the different ways in which powders are combined. SLS bonds high melting point metal or non-metallic powders together through the melting of low melting point metals or binders, while SLM technology completely melts metal powders, requiring a significantly higher laser power density than SLS.
To achieve instantaneous melting of metal powder, a high-power density laser is required, and the spot is focused to several tens of degrees μ m. SLM technology currently uses fiber lasers, with laser power ranging from 50W to 400W and a power density of over 5 × 106W/cm2.
Forming principle: Firstly, the 3D model is sliced and layered using slicing software, and the model is discretized into 2D cross-sectional shapes. The scanning path is planned, and then converted into laser scanning information. Before scanning, the scraper evenly spreads the metal powder in the powder feeder into the laser processing area. Then, the calculator controls the deflection of the scanning galvanometer based on the laser scanning information, selectively irradiating the laser beam into the processing area to obtain the two-dimensional solid of the current two-dimensional cross-section. Then, the forming area descends by one layer thickness, repeats the above process, and piles up layer by layer to obtain the product prototype.
SLM technology schematic diagram
Forming process
In order to ensure the rapid melting of metal powder materials, SLM technology requires a high-power density laser with a spot focused to several tens μ M to several hundred μ M. SLM technology currently commonly uses fiber lasers with excellent beam modes, with laser power above 50w and power density of over 5 × 106W/cm2. Under the action of high laser energy density, the metal powder is completely melted, and after heat dissipation and cooling, it can be welded and formed with solid metal metallurgy. SLM technology is a rapid prototyping technology that accumulates and shapes three-dimensional entities layer by layer through this process.
In the SLM molding process, to improve the formability of the powder, it is necessary to enhance the wettability of the liquid metal. During the forming process, if the liquid metal forms balls, it indicates that the wettability of the liquid metal is poor. The wettability of liquid metal to solid metal is influenced by process parameters, so process parameters can be optimized to improve the wetting ability of specific powders.
Technological advantages
(1) CAD models can be directly made into terminal metal products with simple post-processing or surface treatment processes.
(2) Suitable for workpieces of various complex shapes.
(3) The density can almost reach 100%, and the mechanical properties are comparable to those obtained by forging technology.
(4) The obtained metal parts have high dimensional accuracy and good surface roughness values.
(5) The ability to melt high melting point metals at lower power makes it possible to manufacture parts using single component metal powders, and the variety of metal powders available for selection has greatly expanded.
(6) Can use titanium powder and nickel based high-temperature alloy powder for direct processing, solving the problem of difficult processing of complex high-temperature alloy parts with uniform microstructure widely used in aerospace; It can also solve the processing problem of gradient functional materials with continuous component changes in biomedical applications.
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In our industrial field, metal 3D printing can be said to be an important direction for the development of advanced manufacturing, and it is also a very promising technology in the 3D printing technology system. So how does 3D printing turn metal into a physical object? What kind of process does it take to go through?
There are currently five mainstream metal 3D printing technologies: Nanoparticle Jet Metal Forming (NPJ), Selective Laser Sintering (SLS), Selective Laser Melting (SLM), Laser Near Net Forming (LENS), and Electron Beam Melting (EBM). We will mainly introduce the forming principle of selective laser melting (SLM), and you can search and learn more about it yourself.
Selective Laser Sintering (SLS) is the earliest metal 3D printing technology that emerged. Its principle is to use a laser beam to scan the pre laid part powder, raise its temperature to the melting point, and sinter it into shape. After each layer is burned, the platform descends, and new powder is re laid on this layer. The above process is repeated, and finally, it is fully cooled to form a solid model.
What materials can metal 3D printers print
At present, metal powders commonly used in metal 3D printers both domestically and internationally include tool steel, martensitic steel, stainless steel, pure titanium and titanium alloys, aluminum alloys, nickel based alloys, copper based alloys, cobalt chromium alloys, etc.
1. Tool steel and martensitic steel
Taking tool steel and martensitic steel as examples, the applicability of tool steel comes from its excellent hardness, wear resistance, and deformation resistance, as well as its ability to maintain cutting edges at high temperatures. Mold H13 hot working tool steel is one of them, which can withstand uncertain process conditions; Martensitic steel, taking martensite 300 as an example, also known as "martensitic aging" steel, is well-known for its high strength, toughness, and dimensional stability during the aging process. They are different from other steels because they are carbon free and belong to intermetallic compounds that harden through rich metallurgical reactions of nickel, cobalt, and molybdenum. Due to its high hardness and wear resistance, martensite 300 is suitable for many mold applications, such as injection molds, light metal alloy casting, stamping and extrusion, etc. It is also widely used in aerospace, high-strength fuselage components, and racing parts.
2. Stainless steel
Stainless steel is different from carbon steel, and the current chromium content is different. The steel alloy with the lowest chromium content of 10.5% is less prone to rust and corrosion. At present, there are three main types of stainless steel used for metal 3D printing: austenitic stainless steel 316L, martensitic stainless steel 15-5PH, and martensitic stainless steel 17-4PH.
Austenitic stainless steel 316L has high strength and corrosion resistance, and can be lowered to low temperatures over a wide temperature range. It can be used in various engineering applications such as aerospace and petrochemicals, as well as in food processing and medical fields.
Martensitic stainless steel 15-5PH, also known as martensitic aging (precipitation hardening) stainless steel, has high strength, good toughness, corrosion resistance, and can further harden, making it ferritic free. At present, it is widely used in aerospace, petrochemical, chemical, food processing, papermaking, and metal processing industries.
Martensitic stainless steel 17-4PH still has high strength and toughness at temperatures as high as 315 ºC, and has excellent corrosion resistance. With laser processing, it can bring excellent ductility.
3. Alloy
The most widely used metal powder alloys for metal 3D printing materials include pure titanium and titanium alloys, aluminum alloys, nickel based alloys, cobalt chromium alloys, copper based alloys, etc.
1) Pure titanium and titanium alloys
The pure titanium currently used in the market, also known as commercial pure titanium, is divided into grade 1 and grade 2 powders. Grade 2 is stronger than grade 1 and also has corrosion resistance for most applications. Due to its excellent biocompatibility, pure titanium grade 2 has broad application prospects in the medical industry.
Titanium is the key to the titanium alloy industry. At present, titanium alloys used for metal 3D printing are mainly grade 5 and grade 23. Due to their excellent strength and toughness, combined with corrosion resistance, low density, and biocompatibility, they have very ideal applications in aerospace and automotive manufacturing. Moreover, due to their high strength, low modulus, and strong fatigue resistance, they are used in the production of biomedical implants. Titanium alloy grade 23, with higher purity, is a dental and medical titanium grade similar to the divine grade.
2) Aluminum alloy
At present, there are two main types of aluminum alloys used for metal 3D printing: aluminum silicon AlSi12 and AlSi10Mg. Aluminum silicon 12 is a lightweight additive manufacturing metal powder with good thermal performance, which can be applied to thin-walled parts such as heat exchangers or other automotive components, as well as prototype and production components for aerospace and aviation industry; The combination of silicon and magnesium makes aluminum alloys stronger and harder, making them suitable for thin-walled and complex geometric parts, especially in situations with good thermal performance and low weight.
3) Nickel based alloy
In general, nickel based alloys have good tensile, fatigue, and thermal fatigue resistance properties. At present, there are mainly Inconel 738, HastelloyX, Inconel 625, Inconel 713, Inconel 718, etc.
Inconel 738 has excellent high-temperature creep fracture strength and resistance to hot corrosion. It is a low chromium content superalloy that can be exposed to high temperature corrosive environments up to 920-980 ºC for a long time. It is suitable for aircraft engines and gas turbines.
Hastelloy X has high strength and oxidation resistance at high temperatures, and also has good ductility in environments up to 1200 ºC. Currently, it is mainly used in aerospace technology, such as gas turbine components and combustion zone components such as transition tubes, burner tanks, spray rods, exhaust pipes, afterburning chambers, etc; Moreover, due to its resistance to stress corrosion cracking, it is applied in industrial furnaces, petrochemicals, and chemical process industries.
Inconel 625 still exhibits good load performance under high temperatures of about 815 ºC, and has strong corrosion resistance. It is widely used in aerospace, chemical, and power industries.
Inconel 713 has excellent thermal fatigue resistance and special fracture strength at 927 ºC, making it suitable for jet engine gas turbine blades.
Inconel 718 is a superalloy based on iron nickel hardening, which has good corrosion resistance, heat resistance, tensile, fatigue, and creep resistance, and is suitable for various high-end applications, such as aircraft turbine engines and land-based turbines.
4) Cobalt chromium alloy
Cobalt chromium alloy has high strength, strong corrosion resistance, good biocompatibility, and non-magnetic properties. It is mainly used in surgical implants, including alloy artificial joints, knee joints, and hip joints. It can also be used in engine components, as well as in the fashion and jewelry industries.
5) Copper based alloy
Copper based alloys, commonly known as bronze, used in the market have good thermal and electrical conductivity. They can combine design degrees of freedom to create complex internal structures and cooling channels, making them suitable for more effective tool insertion into molds, such as semiconductor devices, and can also be used in micro heat exchangers. They have the characteristics of thin walls and complex shapes.
Detailed introduction to the application field of metal 3D printing:
Industrial sector: Currently, many industrial sectors have made metal 3D printing their daily machines and almost all use metal 3D printing technology in prototype manufacturing and model production. At the same time, when producing some large components, metal 3D printing technology is also used to print out the components and assemble them. Compared to traditional processes, 3D printing technology can achieve larger production volumes while shortening time and reducing costs. In the automotive industry, the application time of metal 3D printing is not too long, but it has enormous potential and rapid development. Currently, many well-known domestic and foreign automobile manufacturers are seriously studying how to use metal 3D printing technology to reform their production methods. At the same time, metal 3D printing can directly print the same product as the drawing without opening the mold. Greatly accelerating the research and development cycle has brought great convenience to product development and design! Whether it is the car engine housing or small parts sheet metal parts, they can be quickly achieved through metal 3D printing.
In the medical field, prosthetic implants are commonly used in metal 3D printing. The biggest advantage of using metal 3D printing technology is naturally customizability. Doctors can design implants based on the specific situation of patients, which reduces pain during the treatment process and makes the implants closer to the original limbs.
Aerospace field: Many countries around the world have begun to use metal 3D printing technology to achieve development in defense, aerospace and other fields. 3D printing production of jet engine manufacturing components has been achieved, which is sufficient to demonstrate the ability of metal 3D printing.
Metal 3D printing, promising for the future
The application fields of metal 3D printing materials are quite extensive, such as petrochemical engineering applications, aerospace, automotive manufacturing, injection molds, light metal alloy casting, food processing, medical, papermaking, power industry, jewelry, fashion, etc. I believe that with the continuous progress of technology, the application of metal 3D printing materials will have an increasing impact on society in the future.
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