Selective Laser Sintering (SLS). Control of laser sintering of metal powder mixtures

SLS technology

SLS prototyping allows you to explore the aerodynamic characteristics of racing cars

Selective laser sintering (SLS) is an additive manufacturing method used to create functional prototypes and small batches of finished products. The technology is based on sequential sintering of layers of powder material using high-power lasers. SLS is often mistaken for a similar process called Selective Laser Melting (SLM). The difference is that SLS provides only the partial melting needed to sinter the material, while Selective Laser Melting involves the full melting needed to build monolithic models .

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Operating principle of SLS printers

Selective laser sintering (SLS) technology was developed by Carl Deckard and Joseph Beeman at the University of Texas at Austin in the mid-1980s. Research was funded by the Defense Advanced Defense Agency research projects USA (DARPA). Subsequently, Deckard and Beaman were involved in a company called DTM, formed to bring SLS technology to market. In 2001, DTM was bought out by a rival company. The last of the SLS technology patents was filed on January 28, 1997. It expired on January 28, 2014, making the technology generally available.
A similar method was patented by R. F. Householder in 1979, but was not commercialized.

Technology

The technology (SLS) involves the use of one or more lasers (usually carbon dioxide) to sinter particles of powdered material to form a three-dimensional physical object. As Supplies plastics, metals (see), ceramics or glass are used. Sintering is carried out by drawing the contours contained in the digital model (the so-called “scanning”) using one or more lasers. Once scanning is complete, the work platform is lowered and the new layer material. The process is repeated until a complete model is formed.

The specificity of the technology allows you to create parts of almost unlimited complexity from various materials

Since the density of the product does not depend on the duration of irradiation, but on the maximum laser energy, pulsating emitters are mainly used. Before printing begins, the consumable is heated to just below its melting point to facilitate the sintering process.

Unlike additive manufacturing techniques such as Stereolithography (SLA) or Fused Deposition Modeling (FDM), SLS does not require the construction of support structures. The hanging parts of the model are supported by unused material. This approach makes it possible to achieve virtually unlimited geometric complexity of manufactured models.

Materials and Application

New Balance uses SLS technology to create shoes for professional athletes

Some SLS devices use a homogeneous powder (see Direct Metal Laser Sintering (DMLS)) produced using drum-ball mills, but most use composite pellets with a refractory core and a shell of material with low temperature melting.

Compared to other additive manufacturing methods, SLS is highly versatile in terms of the choice of consumables. These include various polymers (for example, nylon or polystyrene), metals and alloys (steel, titanium, precious metals, cobalt-chromium alloys, etc.), as well as composites and sand mixtures.

SLS technology has become widespread around the world due to its ability to produce functional parts with complex geometric shapes. Although the technology was originally created for rapid prototyping, Lately SLS is used for small-scale production of finished products. A rather unexpected but interesting application of SLS was the use of technology in the creation of art objects.

Printer: EOSINT P395
Camera size: 340 x 340 x 620 mm
Layer thickness: 120 microns

This technology, on the one hand, is fundamentally different from the layer-by-layer deposition method, on the other hand, it has much in common. As there, the model is created layer by layer based on a computer description. However, the key principle here is to use powder rather than meltable filament. The powder is sprayed in an even layer over the entire area, after which the laser bakes only those areas that correspond to the cross-section of the model on this layer at this height.

The method was invented by a group of students led by Dr. Carl Descartes at the University of Austin, Texas. It was first patented in 1989 by DTM Corporation, which was purchased by 3D Systems in 2001.

Today, the variety of materials used as powder is truly great: particles of plastic, metal, ceramics, glass, nylon.

So, the technology consists of two parallel processes: first, an even thin layer of powder is prepared over the entire possible area. Here you cannot do without a roller, feeding and leveling the powder like a roller. After this, a powerful laser turns on and bakes those areas that correspond to the cut of an imaginary object. The model is then lowered down a distance equal to the thickness of the layer, and the algorithm is repeated until the process reaches the highest point of the model.

As you would expect, there are many options at each stage of such production. There are two baking algorithms: in one case, only those areas that correspond to the transition boundary are melted, in the other, they are melted throughout the entire depth of the model. In addition, the baking itself can vary in strength, temperature and duration.

An important feature of selective laser sintering is that there is no need for supporting structures, since the excess of surrounding powder throughout the entire volume prevents the model from collapsing until the final shape has not yet been achieved and the strength of the target object has not been achieved.

The last stage is finishing. For example, immersion in a special furnace for burning technological polymers, which are needed at the sintering stage if composite metal powders were used. Polishing is also possible to remove visible transitions between layers. Technologies and materials are constantly being improved and, thanks to this, the finishing stage is minimized.

In this review, I tried to present in a popular form basic information about the production of metal products using laser additive manufacturing - a relatively new and interesting technological method that arose in the late 80s and has now become a promising technology for small-scale or single-piece production in the field of medicine, aircraft - and rocket science.
The operating principle of a laser-assisted additive manufacturing installation can be briefly described as follows. A device for applying and leveling a layer of powder removes a layer of powder from the feeder and distributes it evenly over the surface of the substrate. After which the laser beam scans the surface of this layer of powder and forms the product by melting or sintering. At the end of scanning the powder layer, the platform with the product being manufactured is lowered to the thickness of the applied layer, and the platform with the powder is raised, and the process of applying the powder layer and scanning is repeated. After the process is completed, the platform with the product is raised and cleared of unused powder.

One of the main parts in additive manufacturing installations is the laser system, which uses CO 2 , Nd:YAG, ytterbium fiber or disk lasers. It has been established that the use of lasers with a wavelength of 1-1.1 microns for heating metals and carbides is preferable, since they absorb laser-generated radiation 25-65% better. At the same time, the use of a CO 2 laser with a wavelength of 10.64 microns is most suitable for materials such as polymers and oxide ceramics. Higher absorption capacity allows you to increase the depth of penetration and vary the process parameters within a wider range. Typically, lasers used in additive manufacturing operate in continuous mode. Compared to them, the use of lasers operating in pulsed mode and in Q-switched mode due to their high pulse energy and short pulse duration (nanoseconds) makes it possible to improve the bond strength between layers and reduce the thermally affected zone. In conclusion, it can be noted that the characteristics of the laser systems used lie within the following limits: laser power - 50-500 W, scanning speed up to 2 m/s, positioning speed up to 7 m/s, focused spot diameter - 35-400 microns.

In addition to the laser, electron beam heating can be used as a source of heating the powder. This option was proposed and implemented by Arcam in its installations in 1997. An installation with an electron beam gun is characterized by the absence of moving parts, since the electron beam is focused and directed using magnetic field and deflectors, and the creation of a vacuum in the chamber has a positive effect on the quality of products.

One of important conditions in additive manufacturing, it is the creation of a protective environment that prevents oxidation of the powder. To fulfill this condition, argon or nitrogen is used. However, the use of nitrogen as a shielding gas is limited, which is associated with the possibility of the formation of nitrides (for example, AlN, TiN in the manufacture of products from aluminum and titanium alloys), which lead to a decrease in the ductility of the material.

Laser additive manufacturing methods according to the characteristics of the material compaction process can be divided into selective laser sintering (Selective Laser Sintering (SLS)), indirect laser sintering of metals (Indirect Laser Metal Sintering (ILMS)), direct laser sintering of metals (Direct Laser Metal Sintering (DLMS) ) and selective laser melting (SLM). In the first option, compaction of the powder layer occurs due to solid-phase sintering. In the second, due to the impregnation of a porous frame previously formed by laser radiation with a binder. Direct laser sintering of metals is based on compaction using the liquid-phase sintering mechanism due to the melting of a low-melting component in a powder mixture. In the latter option, compaction occurs due to complete melting and spreading of the melt. It is worth noting that this classification is not universal, as one type of additive manufacturing process may exhibit compaction mechanisms that are characteristic of other processes. For example, DLMS and SLM may exhibit solid phase sintering, which occurs with SLS, while SLM may exhibit liquid phase sintering, which is more common with DLMS.

Selective Laser Sintering (SLS)

Solid-phase selective laser sintering is not widely used, since a relatively long exposure under laser radiation is required for a more complete occurrence of volumetric and surface diffusion, viscous flow and other processes that take place during powder sintering. This leads to long laser operation and low process productivity, which makes this process economically unfeasible. In addition, difficulties arise in maintaining the process temperature in the range between the melting point and the solid-phase sintering temperature. The advantage of solid-phase selective laser sintering is the ability to use a wider range of materials for the manufacture of products.

Indirect Laser Metal Sintering (ILMS)

The process, called indirect metal laser sintering, was developed by DTMcorp of Austin in 1995, which has been owned by 3D Systems since 2001. The ILMS process uses a mixture of powder and polymer or powder coated with a polymer, where the polymer acts as a binder and provides the necessary strength for further heat treatment. At the heat treatment stage, the polymer is distilled off, the frame is sintered, and the porous frame is impregnated with a binder metal, resulting in a finished product.

For ILMS, powders of both metals and ceramics or their mixtures can be used. The preparation of a mixture of powder and polymer is carried out by mechanical mixing, while the polymer content is about 2-3% (by weight), and in the case of using a powder coated with a polymer, the layer thickness on the surface of the particle is about 5 microns. Epoxy resins, liquid glass, polyamides and other polymers are used as binders. The temperature of polymer distillation is determined by the temperature of its melting and decomposition and averages 400-650 o C. After polymer distillation, the porosity of the product before impregnation is about 40%. During impregnation, the furnace is heated 100-200 0 C above the melting point of the impregnating material, since with increasing temperature the contact angle of wetting decreases and the viscosity of the melt decreases, which has a beneficial effect on the impregnation process. Typically, impregnation of future products is carried out in a backfill of aluminum oxide, which plays the role of a supporting frame, since during the period from the distillation of the polymer to the formation of strong interparticle contacts there is a danger of destruction or deformation of the product. Protection against oxidation is organized by creating an inert or reducing environment in the furnace. For impregnation, you can use quite a variety of metals and alloys that satisfy following conditions. The material for impregnation must be characterized by the complete absence or insignificant interfacial interaction, a small contact angle and have a melting point lower than that of the base. For example, if the components interact with each other, then undesirable processes may occur during the impregnation process, such as the formation of more refractory compounds or solid solutions, which can lead to the stop of the impregnation process or negatively affect the properties and dimensions of the product. Typically, bronze is used to impregnate a metal frame, and the shrinkage of the product is 2-5%.

Method Selective Laser Sintering or selective (selective) laser sintering , was invented by Dr. Carl Descartes together with a group of students at the University of Austin, Texas. It was first patented in 1989 by DTM Corporation, which was purchased by 3D Systems in 2001.

What is laser sintering?

The technological process consists of two stages: first, an even thin layer of powder is evenly placed throughout the entire working area, after which the laser is turned on and bakes the areas that correspond to the cut of an imaginary object. The model is then lowered down a distance equal to the thickness of the layer, and the algorithm is repeated until the process reaches the highest point of the model.

At each stage of SLS printing, you can choose how best to proceed. The powder can be sprayed or applied with a roller. Baking can be carried out only in the area that corresponds to the transition boundary, or it can be melted throughout the entire depth of the model. In addition, the baking itself can vary in strength, temperature and duration.

An important feature of selective laser sintering is that there is no need for supporting structures, since the excess of surrounding powder throughout the entire volume prevents the model from collapsing until the final shape has not yet been achieved and the strength of the target object has not been achieved.

Materials

The list of materials used is gradually growing; today the following particles can be used as powder:

• plastic;
• metal;
• ceramics;
• glass;
• nylon.

The finished product is often processed. For example, they are immersed in a special furnace to burn technological polymers, which are needed at the sintering stage if composite metal powders are used. Polishing is also possible to remove visible transitions between layers. Technologies and materials are constantly being improved, due to which the finishing stage is becoming less and less relevant.

The main manufacturers of SLS printers are EOS (Germany) and 3D Systems (USA). They offer serial installations to create the largest objects: 730x380x580mm and 550x550x750mm respectively. However, in 2011, the world's largest SLS machine, capable of synthesizing objects measuring 1200x1200mm, was built at Huazhong University in China.

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Classmates

3D printing- this is the performance of a series of repeating operations associated with the creation of three-dimensional models by applying a thin layer of consumables to the desktop of the installation, moving the desktop down to the height of the formed layer and removing waste waste from the surface of the desktop. Printing cycles continuously follow each other: the next layer is applied to the previous layer of materials, the table is lowered again and this is repeated until elevator(this is the name of the desktop that the 3D printer is equipped with) there will not be a finished model.

There are several 3D printing technologies that differ from each other in the type of prototyping material and methods of its application. Currently, the most widespread 3D printing technologies are: stereolithography, laser sintering of powder materials, inkjet modeling technology, layer-by-layer printing with molten polymer filament, powder gluing technology, lamination of sheet materials and UV irradiation through a photomask. Let us characterize the listed technologies in more detail.

Stereolithography

Stereolithography– also known as Stereo Lithography Apparatus or abbreviated as SLA, due to the low cost of finished products, has become the most widespread among 3D printing technologies.

SLA technology consists of the following: a scanning system directs a laser beam at the photopolymer, under the influence of which the material hardens. The photopolymer is a brittle and hard translucent material that warps under the influence of atmospheric moisture. The material is easy to glue, process and paint. The desktop is located in a container with a photopolymer composition. After passing the laser beam and curing the next layer, its working surface moves down by 0.025 mm - 0.3 mm.

SLA technology

Equipment for SLA printing is manufactured by F&S Stereolithographietechnik GmbH, 3DSystem, as well as the Institute for Laser and information technologies RAS.

Below are chess pieces created using the SLA printing method.

Chess pieces created using SLA printing

Laser sintering of powder materials

Laser sintering of powder materials– also known as Selective Laser Sintering or simply SLS is the only 3D printing technology that can be used to produce metal molds for metal and plastic casting. Plastic prototypes have good mechanical properties, thanks to which they can be used for the manufacture of fully functional products.

SLS printing uses materials similar in their properties to structural grades: metal, ceramics, powder plastic. Powder materials are applied to the surface of the desktop and baked with a laser beam into a solid layer that corresponds to the cross-section of the 3D model and determines its geometry.

SLS technology

Equipment for SLS printing is manufactured by the following factories: 3D Systems, F& S Stereolithographietechnik GmbH, The ExOne Company / Prometal, EOS GmbH.

The picture shows the sculptural model “Keep it Up,” made using SLS printing.

Sculptural model “Keep it up”, made using SLS printing, by Luca Ionescu

Layer-by-layer printing with molten polymer filament

Layer-by-layer printing with molten polymer filament– also known as Fused Deposition Modeling or simply FDM, is used to obtain individual products that are close in their functionality for serial products, as well as for the manufacture of lost wax molds for metal casting.

FDM printing technology is as follows: an extruding head with a controlled temperature heats filaments made of ABC plastic, wax or polycarbonate to a semi-liquid state, and with high precision delivers the resulting thermoplastic modeling material in thin layers onto the working surface of the 3D printer. The layers are applied to each other, connected to each other and hardened, gradually forming the finished product.

FDM printing technology

Currently, 3D printers with FDM printing technology are manufactured by Stratasys Inc.

The picture shows a model printed by a 3D printer using FDM printing technology.

Model printed by a 3D printer using FDM printing technology

Inkjet modeling technology

Simulation technology or Ink Jet Modeling has the following proprietary subtypes: 3D Systems (Multi-Jet Modeling or MJM), PolyJet (Objet Geometries or PolyJet) and Solidscape (Drop-On-Demand-Jet or DODJet).

The listed technologies operate on the same principle, but each of them has its own characteristics. Supporting and modeling materials are used for printing. Supporting materials most often include wax, and modeling materials include a wide range of materials that are similar in their properties to structural thermoplastics. The print head of a 3D printer applies supporting and modeling materials to the working surface, after which they are photopolymerized and mechanically leveled.

Inkjet modeling technology makes it possible to obtain colored and transparent models with different mechanical properties; these can be either soft, rubber-like products or hard, plastic-like products.

Inkjet modeling technology

Printers for 3D printing using inkjet modeling technology are manufactured by the following companies: Solidscape Inc, Objet Geometries Ltd, 3D Systems.

Powder bonding technology

– aka Binding powder by adhesives allows you not only to create three-dimensional models, but also to paint them.

Printers with binding powder by adhesives technology use two types of materials: starch-cellulose powder, from which the model is formed, and water-based liquid glue, which glues the layers of powder. The glue comes from the print head of the 3D printer, binding the powder particles together and forming the outline of the model. After printing is complete, excess powder is removed. To give the model additional strength, its voids are filled with liquid wax.

Powder bonding technology

Legend:

1-2 – the roller applies a thin layer of powder to the working surface; 3 – the inkjet print head prints with drops of a binding liquid on a layer of powder, locally strengthening part of the solid section; 4 – process 1-3 is repeated for each layer until the model is ready, the remaining powder is removed

Currently, 3D printers with powder bonding technology are manufactured by Z Corporation.

Lamination of sheet materials

Lamination of sheet materials– also known as Laminated Object Manufacturing or LOM, involves the production of 3D models from paper sheets using lamination. The outline of the next layer of the future model is cut out with a laser, and unnecessary trimmings are cut into small squares, which are subsequently removed from the printer. The structure of the finished product is similar to wood, but is susceptible to moisture.

Sheet materials lamination technology

Until recently, 3D printers for laminating sheet materials were produced by Helisys Inc, but the company has now stopped producing such equipment.

An object printed on a 3D printer using sheet material lamination technology is shown in the photo below.

Model printed with a 3D printer using LOM technology

Ultraviolet irradiation through a photomask

Ultraviolet irradiation through a photomask– also known as Solid Ground Curing or SGC, involves the creation of ready-made models from layers of photosensitive plastic sprayed onto the working surface. After applying a thin layer of plastic, it is treated with ultraviolet rays through a special photomask with an image of the next section. Unused material is removed using a vacuum, and the remaining hardened material is re-irradiated with hard ultraviolet light. The cavities of the finished product are filled with molten wax, which serves to support the following layers. Before applying the next layer of photosensitive plastic, the previous layer is mechanically leveled.

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