Selective laser sintering, the metallurgical mechanism adopted is liquid phase sintering mechanism, during the forming process, powder materials are partially melted, powder particles retain their solid phase core, and powder densification is realized through subsequent rearrangement of solid phase particles and liquid phase solidification and bonding.
Principle and characteristics of SLS technology
The whole process device consists of a powder cylinder and a forming cylinder. The piston of the working powder cylinder (powder feeding piston) rises, and the powder is evenly spread on the piston of the forming cylinder (working piston) by a powder spreading roller. The computer controls the two-dimensional scanning trajectory of the laser beam according to the prototype slicing model, and selectively sinter solid powder materials to form a layer of parts. After finishing one layer, the working piston descends one layer thick, the powder spreading system spreads new powder, and the laser beam is controlled to scan and sinter the new layer. This is repeated and stacked layer by layer until the three-dimensional parts are formed.
SLS process adopts semi-solid liquid-phase sintering mechanism, and the powder is not completely melted, which can reduce the thermal stress accumulated in forming materials to a certain extent, but the formed parts contain unmelted solid particles, which directly leads to process defects such as high porosity, low density, poor tensile strength and high surface roughness. In SLS semi-solid forming system, the viscosity of solid-liquid mixed system is usually high, which leads to poor fluidity of molten materials, and the special metallurgical defect of SLS rapid forming process-"spheroidization" effect will appear Spheroidization will not only increase the surface roughness of the formed 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 SLS process.
Due to the low strength of sintered parts, high strength can be achieved only after post-treatment, and the three-dimensional parts manufactured generally have problems such as low strength, low precision and poor surface quality. At the early stage of SLS, compared with other developed rapid prototyping methods, selective laser sintering has the advantages of wide selection of molding materials and simple molding process (without support). However, because the energy source in the forming process is laser, the application of laser makes the cost of its forming equipment higher. With the rapid development of laser rapid forming equipment after 2000 (such as the use of advanced high-energy fiber laser and the improvement of powder spreading accuracy, etc.), the metallurgical mechanism of complete powder melting has been used for laser rapid forming of metal components. Selective laser sintering (SLS) technology has been replaced by similar and more advanced technology.
Advantages of SLS technology:
1. Wide selection of materials
2. The printing process does not need a supporting structure
3. It can be made into any complex structure, including hollow structure and hollow structure
4. The material utilization rate is high, and the unused powder can be continuously utilized
Disadvantages of SLS technology:
1. The surface of powder sintering is rough and needs subsequent treatment
2. The additive manufacturing process is time consuming
3. When polymer materials are sintered and melted by laser, they usually produce peculiar smell
4. The manufacturing and maintenance cost of equipment is high
5. The quality control requirements of solidification structure and internal defects are high, and the auxiliary process is complex
Principle of selective laser melting
SLM technology is developed on the basis of SLS, and their basic principles are similar. SLM technology needs to completely melt the metal powder and directly form metal parts. Therefore, before the laser beam of high power density laser starts scanning, the horizontal powder spreading roller will spread the metal powder on the substrate of the processing room, then the laser beam will selectively melt the powder on the substrate according to the contour information of the current layer to process the contour of the current layer, then the lifting system can be lowered by a distance of layer thickness, and the rolling powder spreading roller will spread the metal powder on the processed current layer, and the equipment will be transferred to the next layer for processing, so that the whole layer will be processed The whole processing process is carried out in a vacuum-pumped or gas-protected processing chamber to avoid the metal reacting with other gases at high temperature.
Development of selective laser melting technology
Among laser selective forming parts, there are many researches on SLM forming of Fe-based alloy (mainly steel), but the SLM forming process needs to be optimized and the forming performance needs to be further improved For the formability of SLM (especially the basic density), it is difficult to achieve full density of steel members formed by SLM at present. Solving the densification problem of steel SLM forming is the key bottleneck problem in rapid prototyping research. The difficulty of laser forming of steel mainly depends on the chemical characteristics of main elements in steel. The matrix element Fe and alloy element Cr have strong affinity for oxygen, so it is difficult to completely avoid oxidation under the conditions of conventional powder treatment and laser forming.
Therefore, in the SLM process, the existence of oxide and other pollution layers on the surface of steel melt will significantly reduce the wettability, and cause the special metallurgical defect spheroidizing effect and solidification microcracks in laser melting, thus significantly reducing the density and corresponding mechanical properties of laser forming. On the other hand, the content of C in steel is another key factor that determines the laser formability. Generally, too high C content will be detrimental to laser formability. With the increase of C content, the thickness of C element layer on the melt surface will also increase. This is similar to the adverse effect of oxide layer, which also reduces wettability, leads to the decrease of melt spreadability and causes spheroidization effect. In addition, complex carbides formed on grain boundaries will increase the brittleness of laser formed parts of steel materials. Therefore, usually for SLM forming of steel materials, it is necessary to increase laser energy density and SLM forming temperature, which can promote the dissolution of carbides and homogenize alloy elements.
Advantages and disadvantages of SLM technology
1) The density of standard metal processed by 1)SLM process is over 99%, and its good mechanical properties are equivalent to those of traditional process
2) The types of machinable materials continue to increase, and the machined parts can be welded later
3) The price is high and the speed is low
4) The precision and surface quality are limited, which can be improved by post-processing
Scope of application of SLM process
1) processing the appearance, assembly and functional prototype of standard metal
2) Supporting parts, such as clamps and fixing devices
3) Small batch parts production
4) Injection mold
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