1 Selective Laser Sintering (SLS) was originally proposed by Carl Deckard of the University of Texas at Austin in his master's thesis in 1989. After the United States DTM company in 1992 launched the process of commercial production equipment Sinter Sation. For decades, Austin and DTM have done a lot of research work in the field of SLS, and have achieved fruitful results in equipment development, process and material development. Germany's EOS has also done a lot of research work in this field and developed a series of molding equipment.

There are also a number of units in China that conduct research on SLS, such as Huazhong University of Science and Technology, Nanjing University of Aeronautics and Astronautics, Northwestern Polytechnical University, North University of China, and Beijing Longyuan Automated Molding Co., Ltd., etc., and have also achieved many major achievements, such as Nanjing Aerospace. The RAP-I laser sintering rapid prototyping system developed by the university and the commercial equipment of AFS-300 laser rapid prototyping developed by Beijing Longyuan Automatic Molding Co., Ltd.

2 How SLS technology works

Selective laser sintering is a method in which a laser selectively stratifies a solid powder and superimposes the sintered solidified layer to form a desired shape. The entire process includes the establishment of CAD models and data processing, powdering, sintering and post-processing. The working principle of the rapid prototyping system of SLS technology is shown in Figure 1.

Figure 1 The working principle of the rapid prototyping system of SLS technology

The whole process device consists of a powder cylinder and a forming cylinder. When working, the powder cylinder piston (powder piston) rises, and the powder is evenly layered on the forming cylinder piston (working piston) by the powder roller, and the computer is based on the prototype slicing model. The two-dimensional scanning trajectory of the laser beam is controlled to selectively sinter the solid powder material to form a layer of the part. After the powder is completed, the working piston is lowered by one layer thickness, and the powder coating system is coated with new powder. The laser beam is controlled to scan the new layer. This cycle reciprocates and layers are stacked until the three-dimensional part is formed. Finally, the unsintered powder is recovered into a powder cylinder and the molded part is taken out. For metal powder laser sintering, the entire table is heated to a certain temperature before sintering, which reduces thermal deformation during molding and facilitates layer-to-layer bonding.

The most outstanding advantage of the 5LS compared to other rapid prototyping (RP) methods is that it uses a wide range of molding materials. Theoretically, any powder material capable of forming an interatomic bond after heating can be used as a molding material for SLS. At present, materials that can be successfully processed by SLS include paraffin, polymer, metal, ceramic powder and composite powder materials thereof. Due to the variety of SLS molding materials, material savings, extensive distribution of molded parts, suitable for a variety of applications, and SLS without the need to design and manufacture complex support systems, SLS is becoming more widely used.

Metal powder sintering method of 3SLS technology

3. 1 metal powder and binder mixed and sintered

First, the metal powder and a certain binder are uniformly mixed in a certain ratio, and the mixed powder is selectively scanned by a laser beam, and the action of the laser melts the binder in the mixed powder and bonds the metal powder together to form a metal. The blank of the part. The metal part blank is then subjected to appropriate post-treatment, such as secondary sintering to further improve the strength and other mechanical properties of the metal part. This process is relatively mature and has been able to manufacture metal parts and is used in practice. Nanjing University of Aeronautics and Astronautics uses metal powder as the base material (iron powder), adding appropriate amount of deadener, and sintering to obtain the prototype, and then carry out subsequent treatment, including loss of binder, high temperature roasting, metal infiltration (such as seepage In the process of copper), the EDM electrode was finally fabricated (see Figure 2). And use this electrode to machine a three-dimensional mold cavity on the EDM machine (see Figure 3).

3.2 Metal powder laser sintering

The process of laser direct sintering of metal powder to make parts is not very mature. At present, more research is done on the mixing and sintering of two kinds of metal powders, one of which has a lower melting point and the other has a higher melting point. Laser sintering melts the low melting point powder, and the molten metal bonds the high melting point metal powder together. Due to the low strength of the sintered parts, post-treatment is required to achieve higher strength. At the University of Texas at Austin, a SLS forming study of a metal powder without a polymer binder such as CuSn NiSn bronze nickel powder composite powder was carried out, and metal parts were successfully fabricated. In recent years, they have studied the laser sintering of single metal powder, and successfully manufactured the metal parts of NCONEL625 superalloy and Ti6A 14 alloy for F1 fighter and AIM9 missile. American Aerospace Materials has successfully developed laser rapid prototyping technology for advanced alloy components. At present, the Institute of Metals of the Chinese Academy of Sciences and the Northwestern Polytechnical University are working on laser rapid prototyping of high melting point metals. Nanjing University of Aeronautics and Astronautics has also conducted research in this area. The experiment of sintering forming with Ni-based alloy mixed copper powder has been successful. Metal parts with an inverted cone shape with a large angle are manufactured (see Fig. 4).

Figure 4 Metal parts sintered by nickel-based alloy mixed copper powder

3. 3 metal powder compact sintering

Metal powder compact sintering is to pre-press two kinds of metal powders with high and low melting points into a thin sheet blank, laser sintering with appropriate process parameters, melting of low melting point metal, flowing between the pores of high melting point particles, so that high melting point The powder particles are rearranged to obtain a sample having a high density. Guo Zuoxing of Jilin University used this method to test FeCu, Fe C and other alloys. It was found that compacted laser sintering has a different densification phenomenon than conventional sintering. The microstructure after laser sintering varies with cooling mode, and air-cooled to obtain fine pearl light. Body, martensite and granular after quenching.

4 SLS technology metal powder molding problems

SLS technology is a very young manufacturing field, and it is not perfect in many aspects. For example, the three-dimensional parts currently manufactured generally have problems such as low strength, low precision and poor surface quality. The SLS process involves many parameters (such as the physical and chemical properties of the material, laser parameters and sintering process parameters) that affect the sintering process, molding accuracy and quality. During the forming process, due to various material factors and process factors, the sintered parts may cause various metallurgical defects (such as cracks, deformation, pores, uneven structure, etc.).

4. 1 Influence of powder materials

The physical properties of the powder material, such as powder particle size, density, coefficient of thermal expansion, and fluidity, have an important influence on the formation of defects in the part. The particle size and density of the powder not only affect the formation of defects in the molded part, but also have a significant influence on the precision and roughness of the molded part. The effect of the expansion and solidification mechanism of the powder on the sintering process can result in increased pores and reduced tensile strength of the molded part.

4. 2 Influence of process parameters

Laser and sintering process parameters such as laser power, scanning speed and direction and spacing, sintering temperature, sintering time and layer thickness can affect the adhesion between layers, the shrinkage deformation of the sintered body, warpage and even cracking. . The above various parameters often interact with each other during the molding process. For example, Yong Ak Song and other studies have shown that reducing the scanning speed and scanning pitch or increasing the laser power can reduce the surface roughness, but the reduction of the scanning pitch leads to the warping tendency. Increase.

Therefore, in the optimization design, it is necessary to consider the optimization of each parameter as a whole to obtain the most effective parameter set for improving the quality of the molded part. At present, the manufactured parts generally have some problems such as low density, strength and precision, mechanical properties and thermal properties that cannot meet the requirements of use. These molded parts cannot be directly used as functional parts, and need to be post-treated (such as hot isostatic pressing HIP, liquid phase sintering LPS, high temperature sintering and melting) before they can be put into practical use. In addition, it should be noted that since the SLS temperature of the metal powder is high, in order to prevent oxidation of the metal powder, the metal powder must be enclosed in a container filled with a protective gas during sintering.

5 Summary and outlook

Among the rapid prototyping technologies, metal powder SLS technology is a hot spot in recent years. The direct sintering of molded parts using high-melting-point metals makes it difficult to manufacture high-strength parts by conventional cutting methods, which is of particular importance for the wider application of rapid prototyping technology. Looking into the future, the research direction of SLS-shaped technology in the field of metal materials should be the sintering of metal parts of unit systems, the sintering of multi-component alloy parts, the laser sintering of advanced metal materials such as metal nano-materials, amorphous metal alloys, etc. It is especially suitable for the molding of micro-components of cemented carbide materials. In addition, parts with functional gradients and structural gradients are sintered according to the specific function and economic requirements of the part. We believe that with the mastery of laser sintering metal powder forming mechanism, the acquisition of optimal sintering parameters for various metal materials, and the emergence of special rapid prototyping materials, the research and reference of SLS technology will surely enter a new realm. .