3D printing
3D Printing Manufacturing Technology for Metal Materials
Generally speaking, laser rapid prototyping needs to use high-power laser to illuminate the surface of the test piece, melt the metal powder, form a liquid molten pool, and then move the laser beam to melt the powder in front and let the metal liquid behind cool and solidify. The surrounding area needs to be equipped with powder feeding devices, inert gas protection, nozzle control, etc.
The reason why 3D printing manufacturing technology of metal materials is difficult is that the melting point of metal is relatively high, involving a variety of physical processes such as solid-liquid phase transition, surface diffusion and heat conduction of metal. The issues that need to be considered also include whether the generated crystal structure is good, whether the entire specimen is uniform, the size of internal impurities and pores, and so on. In addition, rapid heating and cooling will also cause significant residual stress in the specimen. To solve these problems, it is generally necessary to cooperate with various manufacturing parameters, such as the power and energy distribution of the laser, the movement speed and path of the laser focal point, the feeding speed, the protective pressure, external temperature, and so on.
1.3D Rapid Manufacturing Technology for Metal Parts
1.1 Selective Laser Sintering (SLS)
The Selective laser sintering (SLS) technology was originally proposed by Carl Deckard of the University of Texas at Austin in the United States in his master's thesis in 1989. As the name implies, the metallurgical mechanism used in selective laser sintering is the liquid phase sintering mechanism. During the forming process, the powder material is partially melted, and the powder particles retain their solid core, and the powder is densified through the subsequent solid phase particle rearrangement and liquid phase solidification bonding. DTM Corporation of the United States launched the commercial production equipment SinterSation for this process in 1992. German EOS company has also done a lot of research in this field and developed corresponding series of forming equipment. Several domestic institutions, such as Huazhong University of Science and Technology, Nanjing University of Aeronautics and Astronautics, Northwest University of Technology, Central North University, and Beijing Longyuan Automatic Forming Co., Ltd., have conducted research on SLS and achieved significant results.
1.1.1 SLS technology principles and characteristics
The entire process device consists of a powder cylinder and a forming cylinder. The working powder cylinder piston (powder feeding piston) rises, and the powder is evenly laid on the forming cylinder piston (working piston) by a powder laying roller. The computer controls the two-dimensional scanning trajectory of the laser beam based on the prototype's slicing model, selectively sintering solid powder materials to form a layer of the part. After completing one layer, the working piston drops by one layer thickness, and the powder laying system lays new powder, controlling the laser beam to scan and sinter the new layer. This cycle repeats itself, layer by layer, until the three-dimensional part is formed.
The SLS process adopts a semi solid liquid phase sintering mechanism, where the powder does not completely melt. Although it can reduce the thermal stress accumulated in the forming material to a certain extent, the formed part contains unmelted solid particles, which directly leads to process defects such as high porosity, low density, poor tensile strength, and surface roughness. In the SLS semi solid forming system, the viscosity of the solid-liquid mixture system is usually high, resulting in poor fluidity of the molten material, There will be a metallurgical defect unique to SLS rapid prototyping process - the "spheroidization" effect. The phenomenon of spheroidization not only increases the surface roughness of the formed part, but also makes it difficult for the powder spreading device to evenly lay the subsequent powder layer on the surface of the sintered layer, thereby hindering the smooth progress of the SLS process. Due to the low strength of sintered parts, post-processing is required to achieve high strength, and 3D parts manufactured generally have problems such as low strength, low accuracy, and poor surface quality.
At the beginning of SLS, compared with other mature rapid prototyping methods, Selective laser sintering has the advantages of wide selection of molding materials, simple molding process (without support), etc. However, due to the fact that the energy source during the forming process is laser, the application of lasers has led to a higher cost of forming equipment. With the significant progress of laser rapid forming equipment after 2000 (manifested by the use of advanced high-energy fiber lasers and the improvement of powder laying accuracy), the metallurgical mechanism of complete powder melting has been used for laser rapid forming of metal components. Selective laser sintering (SLS) has been replaced by similar and more advanced technologies.
1.2 Direct Metal Laser Forming (DMLS)
There are usually two methods for manufacturing metal components using SLS. One is the indirect method, which is SLS with polymer coated metal powder; The second method is direct metal powder laser sintering (DMLS). Since the research on direct laser sintering of metal powders was conducted at ChatOfci University in Leuvne in 1991, the use of SLS technology to directly sinter metal powders to form three-dimensional components has been one of the ultimate goals of rapid prototyping manufacturing. Compared with indirect SLS technology, the main advantage of DMLS process is the elimination of expensive and time-consuming pre-treatment and post-treatment process steps.
1.2.1 Characteristics of Direct Metal Powder Laser Sintering (DMLS)
DMLS technology, as a branch of SLS technology, has similar principles. However, DMLS technology has great difficulty in accurately forming metal components with complex shapes. Ultimately, it is mainly due to the "spheroidization" effect and sintering deformation of metal powder in DMLS. The spheroidization phenomenon is to minimize the free energy of the system formed by the surface of molten metal and surrounding media. Under the interfacial tension between liquid metal and surrounding media, A phenomenon in which the surface shape of molten metal transforms into a spherical surface. Spheroidization causes the metal powder to melt and fail to solidify into a continuous and smooth molten pool, resulting in loose and porous parts that fail to form. Due to the relatively high viscosity of single component metal powder during the liquid phase sintering stage, the "spheroidization" effect is particularly severe, and the spherical diameter is often larger than the diameter of the powder particles, which leads to a large number of pores existing in the sintered part. Therefore, The DMLS of single component metal powder has obvious process defects and often requires subsequent treatment, which is not truly a "direct sintering".
To overcome the "spheroidization" phenomenon in single component metal powder DMLS, as well as the resulting sintering deformation, density porosity and other process defects, it is currently generally achieved by using multi-component metal powders with different melting points or using pre alloy powders. A multi-component metal powder system is generally composed of a mixture of high melting point metals, low melting point metals, and certain added elements. As a skeleton metal, high melting point metal powder can retain its solid core in DMLS; Low melting point metal powder, as a bonding metal, melts in DMLS to form a liquid phase, which encapsulates, wets, and binds solid metal particles, thereby achieving sintering densification.
1.2.2 Problems with Direct Metal Powder Laser Sintering (DMLS)
As an important branch of SLS technology, DMLS technology is still in the process of continuous development and improvement. The physical process and densification mechanism of its sintering are still unknown, and the laser sintering process parameters of different metal powder systems still need to be explored. The development and development of specialized powders still need to be breakthrough. Therefore, the establishment of mathematical and physical models of metal powder direct laser sintering process and the quantitative study of sintering behavior and microstructure changes in the process of DMLS sintering densification have become one of the important contents of powder metallurgy science and engineering research.
In DMLS, the physical properties of metal powders have a significant impact on the sintering quality. Under the same process parameters, the sintering effect of different powder systems often varies greatly. Grasping the physical properties of the powder system and selecting the optimal process parameters for it is the most basic and important requirement of DMLS. Numerous studies have shown that the three key physical parameters that affect the quality of DMLS are mainly sintering characteristics, paving characteristics, and stability.
1.3 Selective Laser Melting (SLM)
The idea of SLM was first proposed by the Fraunhofer Institute in Germany in 1995, and its research on SLM technology achieved great success in 2002. The world's first SLM device was launched by the German MCP-HEK branch under the British MCP (Mining and Chemical Products Limited) group at the end of 2003. In order to obtain fully dense laser formed parts and benefit from the significant progress of laser rapid forming equipment after 2000 (manifested by the use of advanced high-energy fiber lasers and the improvement of powder laying accuracy), the metallurgical mechanism of complete powder melting is used for laser rapid forming of metal components. For example, the famous rapid prototyping company EOS in Germany is one of the earliest specialized companies in the world to carry out metal powder laser sintering, mainly engaged in SLS metal powder, process and equipment research and development. The company's newly developed EOSINTM270/280 equipment, although continuing to use the term "sintering", has been equipped with a 200W fiber laser and formed metal components using a fully melted metallurgical mechanism, significantly improving its formability. Currently, as an extension of SLS technology, SLM technology is thriving in European countries such as Germany and the United Kingdom. Even if the term "selective laser sintering" (SLS) continues to be used, the actual forming mechanism used has transformed into a fully melted powder mechanism.
1.3.1 Principle of selective laser melting
SLM technology is developed on the basis of SLS, and the basic principles of the two are similar. SLM technology requires the complete melting of metal powder and the direct formation of metal parts. Therefore, before starting scanning with a high-power density laser beam, the horizontal powder roller first spreads the metal powder onto the substrate in the processing room. Then, the laser beam selectively melts the powder on the substrate according to the contour information of the current layer, processing the contour of the current layer, and then the system can be lowered by a distance of one layer thickness, The rolling powder roller then lays metal powder on the current layer that has been processed, and the equipment transfers it to the next layer for processing. This layer by layer processing is carried out until the entire part is processed. The entire processing process is carried out in a vacuum or gas shielded processing chamber to avoid metal reacting with other gases at high temperatures. The boundary between SLM and DMLS is currently very vague, and the difference is not obvious. Although DMLS technology is translated as metal sintering, most of the actual forming process has completely melted the metal powder. DMLS technology uses a mixture of materials composed of different metals, and each component compensates with each other during the sintering (melting) process, which is conducive to ensuring manufacturing accuracy. The SLM technology mainly uses single component powders as the material, and the laser beam quickly melts the metal powder and obtains continuous scanning lines.