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3D printing of metal materials manufacturing technology

In general, laser rapid prototyping requires the use of high-power laser irradiation of the specimen surface, melting the metal powder to form a liquid pool, and then move the laser beam to melt the front of the powder and allow the rear of the liquid metal cooling and solidification. Around the need to have powder feeding device, inert gas protection, nozzle control to support.
The reason why the metal material 3D printing manufacturing technology is difficult is because the melting point of the metal is relatively high and involves various physical processes such as solid-liquid phase transition, surface diffusion and heat conduction of the metal. The issues to be considered also include whether the resulting crystalline structure is good, whether the entire specimen is homogeneous, the amount of internal impurities and pores, and so on. In addition, rapid heating and cooling will also cause larger residual stresses in the specimen. In order to solve these problems, generally need to cooperate in a variety of manufacturing parameters, such as the laser power and energy distribution, laser focus point of the moving speed and path, feeding speed, protection pressure, external temperature and so on.
 
1.3D metal parts rapid manufacturing technology

1.1 Selective laser sintering (SLS)

Selective Laser Sintering (SLS) was first proposed by Carl Deckard of the University of Texas at Austin in his master's thesis in 1989. Selective laser sintering, as its name implies, uses a metallurgical mechanism that is a liquid-phase sintering mechanism, During the forming process, the powder material partially melts, the powder particles retain their solid phase core, and the powders are densified by subsequent solid phase particle rearrangement and liquid phase solidification and adhesion. US DTM company in 1992 launched the process of commercial production equipment SinterSation. EOS company in Germany has also done a lot of research work in this area, and developed the corresponding series of molding equipment. Domestic such as Huazhong University of Science and Technology, Nanjing University of Aeronautics and Astronautics, Northwestern Polytechnical University, North University and Beijing Longyuan Automatic Molding Co., Ltd., a number of units SLS-related research work has also made significant achievements.

1.1.1 SLS technical principle and its characteristics

The whole process device consists of a powder cylinder and a forming cylinder. The working powder cylinder piston (powder feeding piston) is raised. The powder is spread evenly on the forming cylinder piston (working piston) by the roller. The computer is controlled according to 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 the completion of a layer, the work of the piston down a layer of thickness, spreading powder system covered with new powder, control the laser beam and then scan the new layer sintering. This cycle of reciprocation, layers, until the three-dimensional parts forming.

SLS process using semi-solid liquid phase sintering mechanism, the powder did not completely melt, although to some extent, reduce the thermal stress of the forming material accumulation, but the shape contains unmelted solid particles, a direct result of high porosity, density Low, poor tensile strength and high surface roughness. In the SLS semi-solid forming system, the viscosity of the solid-liquid mixing system is usually high, resulting in poor flowability of the molten material, and the unique metallurgical defects of the SLS rapid prototyping process will occur. Nodulizing "effect. The phenomenon of spheroidization will not only increase the surface roughness of the formed parts, but also make it difficult for the powder device to uniformly powder the subsequent powder layer on the surface of the sintered layer, thereby hindering the smooth development of the SLS process. Due to the low strength of sintered parts, post-processing is required to achieve high strength and the three-dimensional parts manufactured generally have problems of low strength, low precision and poor surface quality.

In the initial stage of SLS, selective laser sintering has the advantages of wide selection range of forming materials and simple forming process (no need of support), compared with other rapid prototyping methods that have been developed relatively well. However, since the energy source in the molding process is laser light, the application of the laser makes the cost of the molding equipment high. With the rapid development of the laser rapid prototyping equipment after 2000 (the performance of the advanced high-energy fiber laser is improved, Etc.), the metallurgical mechanism of the complete melting of the powder is used for laser rapid prototyping of metal components. Selective laser sintering (SLS) technology has been replaced by more advanced technologies.

1.2 Direct Metal Laser Forming (DMLS)

SLS manufacturing metal parts, there are usually two ways, one of the indirect method, that is, polymer-coated metal powder SLS; the second is the direct method, that is, direct metal laser sintering (Direct Metal LaserSintering, DMLS). Since the Direct Laser Sintering of Metal Powders in 1991 at Chatofci University in Leuvne, direct sintering of metal powders into 3D parts using the SLS process has been one of the ultimate goals of rapid prototyping. The main advantage of the DMLS process compared to indirect SLS technology is the elimination of expensive and time-consuming pre-treatment and post-treatment process steps.

1.2.1 direct metal powder laser sintering (DMLS) features

DMLS technology as a branch of SLS technology, the principle is basically the same. However, it is difficult for DMLS technology to precisely shape metal parts with complex shapes. In the final analysis, it is mainly due to the "spheroidization" effect of metal powders in DMLS and the sintering deformation and spheroidization phenomenon, which make the molten metal surface and periphery The system composed of the medium surface has the smallest free energy and the phenomenon that the surface shape of the metal liquid changes to the spherical surface under the interfacial tension between the liquid metal and the surrounding medium can cause the metal powder to melt and can not solidify to form a continuous and smooth melt As a result, the "spheroidization" effect is particularly serious and the spherical diameter tends to be larger than the diameter of the powder particles, which may cause the molding failure. Since the single component metal powder has a relatively high viscosity in the liquid phase sintering stage, Resulting in a large number of pores present in the sintered part. Therefore, one-component metal powder DMLS has obvious process defects and often requires subsequent processing, which is not a true "direct sintering".

In order to overcome the "spheroidization" phenomenon in single component metallic powder DMLS and the resulting process defects such as sintering deformation and loose density, it is currently generally possible to use multi-component metallic powders with different melting points or to use pre-alloyed powders . Multi-component metal powder system generally consists of high melting point metal, low melting point metal and some additional elements mixed, wherein the high melting point metal powder as a framework metal, can retain its solid phase core in the DMLS; low melting point metal powder as a bond The metal melts in the DMLS to form a liquid phase, and the resulting liquid phase encapsulates, moistens and binds the solid phase metal particles to achieve sintering densification.

1.2.2 direct metal powder laser sintering (DMLS) problems

As an important branch of SLS technology DMLS technology is still in the process of continuous development and improvement, the sintering of the physical process and sintering densification mechanism remains unclear, different metal powder laser sintering process parameters still need to explore, special Powder research and development remains to be done. Therefore, it is one of the important contents in powder metallurgy science and engineering to establish the mathematical and physical model of direct laser sintering of metal powder and to quantitatively study the sintering behavior and structure change during DMLS sintering densification.

In DMLS, the physical properties of metal powders have an important impact on the quality of sintering. Under the same process parameters, the sintering performance of different powder systems tends to be greatly different. Grasp the physical properties of the powder system, to choose the most optimized process parameters, is the most basic and most important DMLS requirements. Numerous studies have shown that the three key physical parameters that affect the quality of DMLS are: sintering characteristics, paving characteristics and stability.

1.3 constituency laser melting (SLM)

The idea of ​​SLM was first proposed by the Fraunhofer Institute in Germany in 1995, and in 2002 the Institute's research on SLM technology was a great success. The world's first SLM equipment was launched by the end of 2003 at the MCP-HEK branch of Germany under the jurisdiction of the UK group Mining and Chemical Products Limited (MCP). In order to obtain full dense laser forming parts, and also benefit from the rapid progress of laser rapid prototyping equipment after 2000, which is manifested in the use of advanced high-energy fiber lasers and the improvement of laying accuracy, the metallurgical mechanism of complete melting of the powder is used Laser rapid prototyping of metal components. For example, Germany's well-known rapid prototyping company EOS company, is the world's earlier laser sintering of metal powder, specialized companies, mainly engaged in SLS metal powder, process and equipment research and development. The company recently developed EOSINTM270 / 280 equipment, while continuing to use the "sintering" of this statement, but has been equipped with 200W fiber laser, and completely melted metallurgical mechanism forming metal components, forming performance can be significantly improved. At present, as an extension of SLS technology, SLM technology is booming in Germany, Britain and other European countries. Even with the statement of "Constituency Laser Sintering" (SLS), the actual forming mechanism has been transformed into a powder fully melted mechanism.

1.3.1 Selection laser melting principle

SLM technology is developed on the basis of the SLS, the basic principle of the two similar. The SLM technique requires that the metal powder be completely melted to directly form the metal part, thus requiring a high power density laser. Before the laser beam starts scanning, the horizontal tamping roller first tows the metal powder onto the substrate of the processing chamber, and then the laser beam advances the current layer Contour information to selectively melt the substrate powder, the outline of the current layer processing, and then the system can be down a layer thickness of the distance, roll the roller and then processed the current layer of metal powder, the device is adjusted Into the next layer for processing, so layers of processing, until the entire part is completed. The entire process is carried out in a vacuum or gas-filled process chamber to prevent the metal from reacting with other gases at elevated temperatures. The boundary between SLM and DMLS is very vague at present, and the difference is not obvious. Although DMLS technology is translated into the sintering of metal, the metal powder has been completely melted most of the time in the actual forming process. DMLS technology materials are used as a mixture of different metals, the ingredients in the sintering (melting) process of mutual compensation, help to ensure the production accuracy. The SLM technology uses a single component of the main material powder, the laser beam quickly melts the metal powder and obtain a continuous scan line.