1. Selective Laser Sintering (SLS)
Selective laser sintering, as the name suggests, the metallurgical mechanism used is a liquid phase sintering mechanism, the forming process of powder material partial melting, powder particles retain their solid phase core and through the subsequent solid phase particle rearrangement, liquid phase solidification bonding to achieve powder densification.
SLS technology principles and characteristics
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 laying roller evenly spreads the powder on the forming cylinder piston (working piston). The computer controls the two-dimensional scanning trajectory of the laser beam according to the slice model of the prototype and selectively sinters the solid powder material to form a layer of the part. After the completion of a layer, the working piston is lowered one layer thick, the powder laying system is coated with new powder, and the laser beam is controlled to scan the new sintered layer. This cycle is repeated, layer upon layer, until the three-dimensional part is formed.
SLS process adopts a semi-solid liquid phase sintering mechanism, the powder is not completely melted, although it can reduce the thermal stress accumulated in the forming material to a certain extent, but the formed part contains unfused solid phase particles, which directly leads to high porosity, low density, poor tensile strength, and high surface roughness process defects. The viscosity of the solid-liquid mixing system is usually high, resulting in poor fluidity of the molten material, which will appear as the unique metallurgical defect of the SLS rapid prototyping process – the “periodization” effect. Spheroidization not only increases the surface roughness of the formed parts but also makes it difficult for the powder-laying device to uniformly lay the subsequent powder layer on the surface of the sintered layer, thus hindering the smooth development of the SLS process.
Because the sintered parts have low strength, they need to be post-processed to achieve high strength, and the manufactured 3D parts generally have problems such as low strength, low precision, and poor surface quality. In the early stage of SLS, compared with other mature rapid prototyping methods, selective laser sintering has the advantages of a wide selection of forming materials and a relatively simple forming process (without support). However, because the energy source in the molding process is laser, the application of laser makes the cost of its molding equipment higher, with the rapid progress of laser rapid forming equipment after 2000 (manifested as the use of advanced high-energy fiber lasers, the improvement of powder laying accuracy, etc.), the metallurgical mechanism of powder melting is used for laser rapid forming of metal components. Selective laser sintering (SLS) has been replaced by similar, more advanced techniques.
2. Selective Laser Melting (SLM)
Principle of selective laser melting:
SLM technology is developed based on SLS, and the basic principles of the two are similar. SLM technology requires the metal powder to be completely melted and directly formed into metal parts, so before the laser beam of high power density laser starts scanning, the horizontal powder laying roller will first lay the metal powder on the substrate of the processing chamber, and then the laser beam will selectively melt the powder on the substrate according to the profile information of the current layer to process the outline of the current layer. Then the lifting system can drop the distance of a layer thickness, roll the powder roller, and then spread metal powder on the current layer that has been processed, the equipment is transferred to the next layer for processing, and so on, until the whole part is processed. The entire process is carried out in a vacuum or gas-protected processing chamber to avoid metal reactions with other gases at high temperatures.
Advantages of selective laser melting technology
In principle, selective laser melting is similar to selective laser sintering, but because of the use of higher laser energy density and smaller spot diameter, the mechanical properties and dimensional accuracy of the molded parts are better, only a simple post-processing can be put into use, and the raw materials used for molding do not need to be specially prepared. The advantages of selective laser melting technology can be summarized as follows:
1. Direct manufacturing of metal functional parts without intermediate processes;
2. Good beam quality, can obtain a fine focus spot so that you can directly manufacture a higher dimensional accuracy and better surface roughness of the functional parts;
3. The metal powder is completely melted, and the metal functional parts directly manufactured have a metallurgical bonding structure, high density, good mechanical properties, and no post-treatment.
4. The powder material can be a single material or a multi-component material, and the raw material does not need to be specially prepared;
5. Functional parts can be directly manufactured with complex geometric shapes;
6. Especially suitable for single or small batch of functional parts manufacturing. Selective laser sintering has poor density and mechanical properties. It is difficult to obtain high dimensional precision parts in electron beam melting forming and laser cladding manufacturing. In contrast, selective laser melt forming technology can obtain the formed parts with metallurgical bonding, dense structure, high dimensional accuracy, and good mechanical properties, which is the main research hotspot and development trend of rapid prototyping in recent years.
3. Electron Beam Melting (EBM)
Electron beam selective melting (EBSM) principle
Similar to laser selective sintering and laser selective melting, electron beam selective melting technology (EBSM) is a rapid manufacturing technology that uses high energy and high-speed electron beam to selectively bombard metal powder, to melt the powder material into shape. The process of EBSM technology is: first spread a layer of powder on the powder surface; Then, the electron beam is selectively melted under the control of the computer according to the information of the cross-section profile, the metal powder is melted together under the bombardment of the electron beam, and the following parts have been bonded, piled up layer by layer, until the whole part is melted. Finally, the excess powder is removed to obtain the desired three-dimensional product. The real-time scanning signal of the upper computer is transmitted to the deflection coil through digital-analog conversion and power amplification, and the electron beam is deflected under the magnetic field generated by the corresponding deflection voltage to achieve selective melting. After more than ten years of research, it is found that some process parameters such as electron beam current, focusing current, action time, powder thickness, acceleration voltage, and scanning mode are orthogonal experiments. Action time has the greatest influence on molding.
Advantages of selective melting of electron beams
Electron beam direct metal forming technology uses a high-energy electron beam as a processing heat source, scanning forming can be carried out by manipulating a magnetic deflection coil, there is no mechanical inertia, and the vacuum environment of the electron beam can also avoid the oxidation of metal powder during liquid phase sintering or melting. Compared with a laser, the electron beam has the advantages of high energy utilization, large depth of action, high material absorption rate, stability, and low operation and maintenance cost. The advantages of EBM technology are high efficiency in the forming process, small deformation of parts, no metal support in the forming process, and more dense microstructure, and the deflection focusing control of electron beams is more rapid and sensitive. The deflection of the laser requires the use of a galvanometer, which rotates at a high speed when the laser is scanning. At higher laser power, the galvanometer requires a more complex cooling system, and the weight of the galvanometer also increases significantly. Therefore, when using high-power scanning, the scanning speed of the laser will be limited. When scanning a large forming area, the focal length of the laser is also difficult to change quickly. The deflection and focusing of the electron beam are completed by the magnetic field, and the deflection amount and focusing length of the electron beam can be quickly and sensitively controlled by changing the intensity and direction of the electric signal. The electron beam deflection focusing system is not disturbed by metal evaporation. When a laser and electron beam are used to melt the metal, the metal vapor is dispersed throughout the forming space and coated with a metal film on any object it touches. The electron beam deflection focusing is completed in the magnetic field, so it is not affected by metal evaporation. Optical devices such as laser galvanometers are easily polluted by evaporation.
4. Laser Cladding Forming Technology (LMD)
Laser Metal Deposition (LMD) was first proposed by Sandia National Laboratory in the United States in the 1990s and then developed in many places around the world because many universities and institutions are independently conducting research, so the name of this technology is many. Although the names are not the same, their principle are the same, in the molding process, the powder is gathered to the working plane through the nozzle, and the laser beam is also gathered to the point, where the powder light application point is overlaps, and the covered entity is obtained by moving through the workbench or the nozzle.
LENS technology uses a kilowatt-level laser, due to the use of the laser focusing spot is large, generally more than 1mm, although you can get a dense metal entity of metallurgical bonding, its dimensional accuracy and surface finish are not good and needs to be further machining before use. Laser cladding is a complex physical and chemical metallurgical process, and the parameters in the cladding process have a great influence on the quality of the cladding. The process parameters of laser cladding mainly include laser power, spot diameter, defocusing amount, powder feeding speed, scanning speed, melting pool temperature, etc., which have a great influence on the dilution rate, crack, surface roughness, and densification of the cladding parts. At the same time, the parameters also affect each other, which is a very complicated process. Appropriate control methods must be adopted to control various influencing factors within the permissible range of the coating process.
5. Direct Metal Laser Forming (DMLS)
SLS manufacturing metal parts, there are usually two methods, one is the indirect method, that is, polymer-coated metal powder SLS; The second for the direct method, namely direct metal laser sintering (DMLS DirectMetalLaserSintering). Since the direct laser sintering of metal powders began in 1991 at Chatofci University in Leuven, the direct sintering of metal powders to form 3D parts using the SLS process has been one of the ultimate goals of rapid prototyping. The main advantage of the DMLS process over indirect SLS technology is the elimination of costly and time-consuming pre-treatment and post-treatment process steps.
Characteristics of direct metal powder laser sintering (DMLS)
DMLS technology is a branch of SLS technology, the principle is the same. However, the DMLS technology makes it more difficult to accurately form metal parts with complex shapes. In the final analysis, it is mainly due to the “periodization” effect and sintering deformation of metal powder in DMLS. The spheroidization phenomenon is to make the system formed by the molten metal surface and the surrounding medium surface have minimum free energy. A phenomenon in which the surface shape of a liquid metal changes to a spherical surface. Modularization will make the metal powder can not solidify after melting to form a continuous 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, so the “modularization” effect is particularly serious, and the spherical diameter is often greater than the diameter of the powder particle, which will lead to a large number of pores in the sintered parts, therefore, The DMLS of single-component metal powders has obvious process defects and often requires follow-up treatment, which is not “direct sintering” in the true sense.
To overcome the “periodization” phenomenon in single-component metal powder DMLS, the resulting process defects such as sintering deformation and loose density, can generally be achieved by using multi-component metal powders with different melting points or using pre-alloying powders. The multi-component metal powder system is generally composed of high melting point metal, low melting point metal, and some additional elements, in which the high melting point metal powder as the skeleton metal, can retain its solid phase core in DMLS. As a bonding metal, the low-melting metal powder is melted in DMLS to form a liquid phase, and the generated liquid phase covers, wets, and bonds the solid metal particles to achieve sintering densification.
