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Metal printing, also known as additive manufacturing, is a process for manufacturing three-dimensional parts based on digital files. This technology can use thin-layer metal powder materials to produce complex shapes that cannot be achieved with traditional techniques such as casting, forging, and processing. Additive manufacturing has created new design possibilities, such as integrating multiple components in the production process to minimize material usage and reduce mold processing costs.
When it comes to small batch manufacturing of metal parts, metal 3D printing technology will be regarded as the first choice. The successful application of this technology has greatly promoted the development of the printing market and brought profound changes in manufacturing methods. For a long time, metal printing technology was considered omnipotent, but as the technology itself, it has limitations that cannot be ignored.
Printing technology is different from previous subtractive materials manufacturing. If professionals in the manufacturing field want to successfully use this technology to realize its full potential, they must understand its manufacturing process and adopt additive thinking to avoid design flaws.
Metal printing generally uses laser and electron beam energy sources to melt metal powder, so that the metal powder is sintered and stacked to form an overall structure. There are two ways to input metal powder in the whole process, powder spreading and powder feeding. According to different powder feeding methods, the metal printing process principle is divided into directional energy deposition (also called powder feeding) and powder bed selective area melting (also called powder spreading). Dusting is a means to spread the metal powder onto a substrate to form a thin layer, and then further sintered together by laser melting specific area on the sheet. Compared with dusting powder feeding is not formed in a thin layer directly through the powder into powder puddle formed by the laser nozzle in the matrix, the sintering to form a whole.
Powder bed selection melting
Introduction and principle
Selective laser sintering ( SLS) was originally proposed by Carl Deckard of the University of Texas at Austin in his master thesis in 1989. Selective laser sintering, as the name implies, uses a metallurgical mechanism of liquid phase sintering. During the forming process, the powder material partially melts, and the powder particles retain their solid-phase core, and the powder is densified through subsequent solid-phase particle rearrangement and liquid-phase solidification and bonding.
The whole process device is composed of a powder cylinder and a forming cylinder. The working powder cylinder piston (powder feeding piston) rises, and the powder is spread evenly on the forming cylinder piston (working piston) by a powder spreading roller. The computer controls the prototype slice model the two-dimensional scanning trajectory of the laser beam selectively sinters the solid powder material to form a layer of the part. After finishing one layer, the working piston is lowered by one-layer thickness, the powder coating system is covered with new powder, and the laser beam is controlled to scan and sinter the new layer. This cycle repeats, layer by layer, until the three-dimensional part is formed.
Features
The SLS process uses a semi-solid liquid phase sintering mechanism. The powder is not completely melted. Although the thermal stress of the forming material can be reduced to a certain extent, the formed part contains not melted solid phase particles, which directly leads to high porosity and density Process defects such as low, poor tensile strength, high surface roughness, etc. In the SLS semi-solid forming system, the viscosity of the solid-liquid mixing system is usually high, resulting in poor fluidity of the molten material, and the metallurgical defects unique to the SLS rapid prototyping process will appear balling effect. The spheroidization phenomenon will not only increase the surface roughness of the shaped parts, but also make it difficult for the powder spreading device to evenly spread the subsequent powder layer on the surface of the sintered layer, thus hindering the smooth development of the SLS process.
Due to the low strength of the sintered parts, post-processing is required to achieve higher strength. The manufactured three-dimensional parts generally have the problems of low strength, low accuracy, and poor surface quality. In the early days of SLS, compared with other well-developed rapid prototyping methods, selective laser sintering has the advantages of a wide selection of molding materials and a relatively simple molding process (no support required). However, because the energy source in the molding process is laser, the application of laser makes the cost of the molding equipment higher. With the rapid progress of laser rapid prototyping equipment after 2000 (expressed by the use of advanced high-energy fiber lasers and the improvement of powder coating accuracy Etc.), the metallurgical mechanism of complete melting of the powder is used for laser rapid prototyping of met-al components. Selective laser sintering technology (SLS) has been replaced by similar more advanced technology.
Introduction and principle
Since the direct laser sintering of metal powder was carried out at Catholic University of Leuven in 1991, the use of SLS technology to directly sinter metal powder to form three-dimensional parts is one of the ultimate goals of rapid prototyping. Compared with indirect SLS technology, the main advantage of the DMLS process is the elimination of expensive and time-consuming pre-processing and post-processing process steps. DMLS is to use a high-energy laser beam and then controlled by 3D model data to locally melt the metal matrix, while sintering and solidifying the powder metal material and automatically layer-by-layer stacking, to generate dense geometric solid parts. This part manufacturing process is called "Direct Metal Laser Sintering (DMLS)".
Features
As a branch of SLS technology, DMLS technology has basically the same principle. However, DMLS technology is more difficult to accurately shape metal parts with complex shapes. In the final analysis, it is mainly due to the "spheroidization" effect and sintering deformation of metal powder in DMLS. The phenomenon of spheroidization is to make the molten metal surface and surroundings the system formed by the surface of the medium has the smallest free energy. Under the action of the interfacial tension between the liquid metal and the surrounding medium, the surface shape of the liquid metal changes to a spherical surface. Spheroidization will make the metal powder unable to solidify after melting to form a continuous and smooth molten pool, so the formed parts are loose and porous, resulting in molding failure. Because the viscosity of the single-component metal powder in the liquid phase sintering stage is relatively high, the "spheroidization" The effect is particularly serious, and the spherical diameter is often larger than the diameter of the powder particles , which will cause a large number of pores to exist in the sintered parts. Therefore, the DMLS of the single-component metal powder has obvious process defects and often requires subsequent treatment, not in the true sense Direct sintering ".
In order to overcome the "spheroidization" phenomenon in single-component metal powder DMLS, as well as the resulting sintering deformation, density looseness and other process defects, currently generally can be achieved by using multi-component metal powders with different melting points or using pre-alloyed powders. Multi-component metal powder system is generally composed of high-melting-point metal, low-melting-point metal and some additional elements. Among them, high-melting-point metal powder serves as skeleton metal, which can retain its solid core in DMLS; low-melting-point metal powder acts as bonding The metal melts in DMLS to form a liquid phase, and the resulting liquid phase coats, wets, and binds solid metal particles to achieve sintering and densification.
Introduction and principle
The idea of SLM was originally proposed by the German Fraunhofer Institute in 1995. In 2002, the Institute ’s research on SLM technology achieved great success. In order to obtain fully dense laser formed parts, but also benefit from the rapid progress of laser rapid prototyping equipment after 2000 (expressed by the use of advanced high-energy fiber lasers and the improvement of powder coating accuracy, etc.), the metallurgical mechanism of complete powder melting is used Laser rapid prototyping of metal components.
SLM technology is developed on the basis of SLS, the basic principles of the two are similar. SLM technology needs to completely melt the metal powder and directly shape the metal parts. Therefore, before the laser beam of the high-power density laser starts to scan, the horizontal powder roller rolls the metal powder onto the substrate of the processing room, and then the laser beam will be pressed according to the current layer. The contour information of the substrate selectively melts the powder on the substrate to process the contour of the current layer, then the system can be raised and lowered by a layer thickness distance, the powder roller is rolled and the metal powder is deposited on the processed current layer. Enter the next layer for processing, so that the layers are processed until the entire part is processed. The entire processing process is carried out in a vacuum-evacuated or gas-protected processing chamber to prevent the metal from reacting with other gases at high temperatures. The line between SLM and DMLS is currently vague, and the difference is not obvious. Although DMLS technology translates to metal sintering, most of the time in the actual molding process, the metal powder has been completely melted. The DMLS technology uses a mixture of materials composed of different metals, and each component compensates for each other during the sintering (melting) process, which is beneficial to ensure the manufacturing accuracy. The SLM technology uses a single-component powder, and the laser beam quickly melts the metal powder and obtains a continuous scan line.
Features
In principle, selective laser melting is like selective laser sintering, but because of the higher laser energy density and smaller spot diameter, the mechanical properties and dimensional accuracy of the molded parts are better, and only a simple post-processing is required. It is put into use, and the raw materials used for molding do not need to be specially formulated. The advantages of laser melting technology in selected areas can be summarized as follows:
1. Directly manufacture metal functional parts without intermediate processes;
2. Good beam quality, can obtain a fine focusing spot, so that functional parts with higher dimensional accuracy and better surface roughness can be directly manufactured
3. The metal powder is completely melted, and the metal functional parts manufactured directly have metallurgical bonding structure, higher density, and better mechanical properties, without post-processing;
4. The powder material can be a single material or a multi-component material, and the raw materials need not be specially formulated;
5. Can directly manufacture functional parts with complex geometric shapes;
6. Especially suitable for the manufacture of single parts or small batches of functional parts. Selected laser sintered molded parts have poor density and mechanical properties; electron beam fusion molding and laser cladding manufacturing parts with high dimensional accuracy are difficult to obtain; in contrast, selected laser fusion molding technology can obtain metallurgical bonding, dense structure, Molded parts with high dimensional accuracy and good mechanical properties are the main research hotspots and development trends of rapid prototyping in recent years.
Laser metal deposition technology
Introduction and principle
Laser metal cladding is a creative metal manufacturing process. The international common name is mostly " Laser Metal Deposition", abbreviated LMD. This statement is taken from "Direct Metal Deposition" (DMD) or "Direct Energy Deposition" (DED). The process is quite simple: the laser generates a molten pool on the surface of the part, and automatically injects metal powder through the nozzle to form a welding bead welded to each other, which can form a structure on the existing substrate or form the entire part. The process is used in aerospace, energy technology, petrochemical, automotive and medical technology industries. TRUMPF customers can benefit from the variety of laser devices and laser systems, process knowledge for many services and applications. Thus, for example, LMD technology can be combined with laser welding or laser cutting.
Features
The process includes depositing metal materials layer by layer. For microstructures, it has high accuracy and high resolution. The substrate can be placed in a fixed position ( 3-axis system) or a rotating table (5+ axis system) to improve the machine's ability to handle more complex geometries.
Our so-called laser cladding is more about adding material layers to existing objects, but the process is still the same. This is a good way to improve the corrosion resistance of metal parts. Indeed, as a coating, this technology can get parts with very good mechanical properties. However, repairing worn parts is also great. This metal 3D printing technology can be used to repair parts, even very large parts, and can also be used to completely manufacture parts.
This manufacturing technique can be used to print materials such as steel and aluminum.
Metal printing also has advantages and disadvantages, the important thing is how to develop and use this process. The advantage of metal printing is that it can create complex components with high strength and light weight. The parts are almost completely dense. It can manufacture internal precision structures that cannot be processed by traditional means, and it produces very little material waste.
Another advantage of metal printing is related to its reliability. Although building metal parts requires finding the best parameters, placement angles, and corresponding designs, once determined, the consistency of metal printing is very good, producing the same parts every time. In this way, for a given part, the predictability and efficiency of the performance of metal printed parts cannot be ignored.
Speed and cost cannot guide people to chase this technology, which is also the current disadvantage of metal 3D printing. Slower printing speed and high cost seriously limit the promotion and application of metal 3D printing. Based on this, the aerospace and automotive industries have become early users of the technology, and the high-strength, light-weight structure satisfies the needs of these industries. There is plenty of evidence that high-end applications are driving the demand for metal 3D printing.
Compared to plastic 3D printing, metal printing takes longer. It usually requires multiple printing to verify and repair the rationality of the design in order to achieve later mass production. Even so, this technology still brings a huge breakthrough.
Before the parts are printed, the designer should consider the preferred materials for the parts. Although the equipment manufacturer claims that there are many materials that can be used for 3D printing, the materials available are actually very limited. The development of the material process will take a lot of time and need to be repeatedly verified, and the material and the equipment process are one-to-one matching. If the equipment or material is replaced, the process cannot be used universally, which is also a tricky aspect of the process.
Currently commonly used metal materials include stainless steel, aluminum alloys, titanium alloys, cobalt-chromium alloys, etc. Choosing the most suitable material is critical to whether the parts can meet the requirements for use.
Another issue to be considered for metal 3D printing is the surface quality. Due to the layer-by-layer manufacturing, some locations also need to add support. The parts of metal printing often have poor surface quality, requiring CNC secondary processing or manual grinding and polishing. Moreover, these processes can only be limited to the areas that can be reached by the processing tools. If the parts are too thin or too complex, they cannot even be surface treated.
Although metal 3D printing is not as easy as plastic printing, due to the high process requirements and the obvious advantages, it is worthwhile to improve the design process.
Metal 3D printing is best used to manufacture difficult or very expensive complex custom parts using traditional methods.
Minimizing the need for support structures will greatly reduce the cost of metal printing.
Topology optimization is critical to maximize the additional advantages of using metal printing.
Metal 3D printed parts have excellent mechanical properties and can be manufactured using a variety of engineering materials, including metal superalloys.
