3D Metal Printing Materials

The whole process device consists of a powder cylinder and a forming cylinder. The piston of the working powder cylinder rises, and the powder spreading roller evenly spreads a layer of powder on the working piston. The computer controls the two-dimensional scanning trajectory of the laser beam according to the slicing model of the prototype. There are options. Solid powder material is sintered to form a layer of the part. After completing one layer, the working piston descends one layer thickness, the powder spreading system spreads new powder, and the laser beam is controlled to scan and sinter the new layer. This cycle repeats, layer by layer, until the three-dimensional part is formed.

The SLS process adopts a semi-solid liquid phase sintering mechanism, and the powder is not completely melted. Although the thermal stress accumulated by the forming material can be reduced to a certain extent, the formed parts contain unmelted solid phase particles, which directly lead to high porosity and density. Low, poor tensile strength, high surface roughness, high process defects, in the SLS semi-solid forming system, the viscosity of the solid-liquid mixed system is usually high, resulting in poor fluidity of the molten material, and there will be metallurgical defects unique to the SLS rapid prototyping process – ” spheroidization” effect. The spheroidization phenomenon not only increases the surface roughness of the formed parts, but also makes 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 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 such as low strength, low precision and poor surface quality. In the early days of SLS, compared with other more mature rapid prototyping methods, selective laser sintering has the advantages of a wide selection of molding materials and a relatively simple molding process. However, since the energy source in the forming process is laser, the application of laser makes the cost of forming equipment high. With the great progress of laser rapid prototyping equipment after 2000, the metallurgical mechanism of complete melting of powder is used for the laser of metal components. Rapid prototyping. Selective Laser Sintering (SLS) has been replaced by similar more advanced technologies.

SLM technology is developed on the basis of SLS, and the basic principles of the two are similar. SLM technology needs to completely melt the metal powder and directly form the metal parts, so it requires a high power density laser before the laser beam starts to scan, the horizontal powder roll first spreads the metal powder on the substrate of the processing chamber, and then the laser beam will press the current layer. According to the contour information, the powder on the substrate can be selectively melted to process the contour of the current layer. Then, the lifting system can be lowered by a distance of one layer thickness, and the powder spreading roller can be rolled to spread metal powder on the processed current layer. The equipment adjusts. Enter the next layer for processing, and so on layer by layer until the entire part is processed. The entire processing process is carried out in a vacuum or gas-protected processing chamber to prevent the metal from reacting 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, and only simple post-processing is required. It can be put into use, and the raw materials used for molding do not need special preparation. The advantages of selective laser melting technology can be summarized as follows:

1. Directly manufacture metal functional parts without intermediate processes;
2. With good beam quality, fine focused spots can be obtained, so that functional parts with higher dimensional accuracy and better surface roughness can be directly manufactured;
3. The metal powder is completely melted, and the directly manufactured metal functional parts have metallurgical bonding structure, high density, and good mechanical properties, without post-treatment;
4. The powder material can be a single material or a multi-component material, and the raw materials do not need to be specially formulated;
5. Functional parts with complex geometric shapes can be directly manufactured;
6. It is especially suitable for the manufacture of single or small batches of functional parts. The compactness and mechanical properties of selective laser sintering moldings are poor; electron beam melting molding and laser cladding are difficult to obtain parts with high dimensional accuracy; in contrast, selective laser melting molding technology can obtain metallurgical bonding, dense Molded parts with high microstructure, high dimensional accuracy and good mechanical properties are the main research hotspots and development trends of rapid prototyping in recent years.

There are usually two methods for SLS to manufacture metal parts, one is an indirect method, that is, SLS of polymer-coated metal powder; the other is a direct method, that is, direct metal laser sintering (Direct Metal Laser Sintering, DMLS). Direct sintering of metal powders to form three-dimensional parts using the SLS process has been one of the ultimate goals of rapid prototyping since 1991, when metal powder direct laser sintering research was carried out at the Chatofci University in Leuvne. The main advantage of the DMLS process compared to indirect SLS technology is the elimination of expensive and time-consuming pre- and post-processing steps.

As a branch of SLS technology, DMLS technology has basically the same principle. However, DMLS technology is difficult to accurately form metal parts with complex shapes. In the final analysis, it is mainly due to the “spheroidization” effect and sintering deformation of metal powder in DMLS. The spheroidization phenomenon is to make the molten metal surface and surrounding The system formed by the surface of the medium has the minimum free energy. Under the action of the interfacial tension between the liquid metal and the surrounding medium, the shape of the liquid metal surface changes to a spherical surface. Spheroidization will make the metal powder unable to solidify to form a continuous and smooth molten pool after melting, so the formed parts are loose and porous, resulting in failure of molding. Due to the relatively high viscosity of single-component metal powder in the liquid phase sintering stage, “spheroidization” The effect is particularly serious, and the spherical diameter is often larger than the powder particle diameter, which causes a large number of pores to exist in the sintered part. Therefore, the DMLS of single-component metal powder has obvious process defects, and often requires subsequent processing, not in the true sense of ” direct sintering”.

In order to overcome the “spheroidization” phenomenon in single-component metal powder DMLS, and the resulting process defects such as sintering deformation and loose density, it can generally be achieved by using multi-component metal powders with different melting points or using pre-alloyed powders. . The multi-component metal powder system is generally composed of high melting point metal, low melting point metal and some additive elements. The high melting point metal powder is used as the framework metal and can retain its solid core in DMLS; Metals are melted in DMLS to form a liquid phase, and the resulting liquid phase coats, wets and binds the solid-phase metal particles, thereby achieving sintering densification.