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SLS-Technology. Selective Laser Sintering (SLS)

Selective Laser Sintering (SLS)

This method appeared at about the same time as SLA, and even has much in common with it, only instead of liquid, a powder with a particle diameter of 50-100 microns is used, distributed in thin uniform layers in a horizontal plane, and then a laser beam sinteres the areas to be curing on this layer of the model.

The starting materials can be very different: metal, plastic, ceramics, glass, foundry wax. The powder is applied and leveled over the surface of the work table with a special roller, which removes excess powder during the reverse pass. Then a powerful laser works, sintering the particles with each other and with the previous layer, after which the table is lowered by an amount equal to the height of one layer. To reduce the laser power required for sintering, the powder in the working chamber is preheated to almost the melting temperature, and the laser itself operates in a pulsed mode, since peak power is more important for sintering than the duration of exposure.

Particles can melt completely or partially (along the surface). The unbaked powder remaining around the cured layers serves as support for creating overhanging elements of the model, so there is no need to form special support structures. But at the end of the process, this powder must be removed both from the chamber, especially if the next model will be created from a different material, and from the cavities of the already made model, which can only be done after it has completely cooled.

Finishing, such as polishing, is often required as the surface may be rough or have visible lamination. In addition, the material can be used not only pure, but also in a mixture with a polymer or in the form of particles coated with a polymer, the remains of which must be removed by burning in a special oven. For metals, the resulting voids are simultaneously filled with bronze.

Since we're talking about O high temperatures necessary for sintering, the process occurs in a nitrogen environment with a low oxygen content. When working with metals, this also prevents oxidation.

Serially produced SLS units allow you to work with fairly large objects, up to 55×55×75 cm.

The dimensions and weight of the installations themselves, as well as the SLA, are quite impressive. Thus, the Formiga P100 device shown in the photo, with the rather modest dimensions of the manufactured models (working area 20×25×33 cm), has dimensions of 1.32×1.07×2.2 m with a weight of 600 kg, and this does not take into account options such as installations for mixing powder and cleaning and filtration systems. Moreover, the P100 can only work with plastics (polyamide, polystyrene).

Technology options are:

a. Selective Laser Melting (SLM), which is used to work with pure metals without polymer impurities and allows you to create a finished sample in one step.

b. Electron Beam Melting (EBM) using an electron beam instead of a laser; this technology requires working in a vacuum chamber, but even allows the use of metals such as titanium.

There are also names such as Direct Metal Fabrication (DMF), and also Direct Manufacturing.

The SPRO 250 Direct Metal printer produced by 3D Systems, which, as the name implies, can work with metals using SLM technology, with a working chamber of 25×24×32 cm, has a size of 1.7×0.8×2 meters and a weight of 1225 kg. The stated speed is from 5 to 20 cubic centimeters per hour, and we can conclude that a model with a volume of a glass will take at least 10 hours to produce.

· wide range materials suitable for use;
· allows you to create very complex models;
· the speed is on average higher than that of SLA, and can reach 30-40 mm per hour vertically;
· can be used not only for creating prototypes, but also for small-scale production, incl. jewelry;

· a powerful laser and a sealed chamber are required, in which an environment with a low oxygen content is created;
· lower maximum resolution than SLA: minimum layer thickness 0.1-0.15 mm (depending on the material, it may be slightly less than 0.1 mm); horizontally, as in SLA, accuracy is determined by focusing laser beam;
· it takes a long time preparatory stage to warm up the powder, and then you need to wait for the resulting sample to cool so that the remaining powder can be removed;
· in most cases finishing is required.

The price of SLS installations is even higher than SLA and can reach millions of dollars. However, we note that in February 2014 the patents for SLS technology expired, so it is quite possible to predict an increase in the number of companies offering such equipment, and, accordingly, a noticeable reduction in prices. However, it is unlikely coming years prices will drop so significantly that SLS printing will become accessible to at least small businesses, not to mention private enthusiasts.

Due to the wide variety of materials, we do not provide indicative prices.

DMLS technology

Direct metal laser sintering (DMLS) is an additive manufacturing technology for metal products developed by Munich-based EOS. DMLS is often confused with the similar technologies Selective Laser Sintering (SLS) and Selective Laser Melting (SLM).

The process involves using 3D models in STL format as blueprints to build physical models. 3D model subject to digital processing to be virtually separated into thin layers with a thickness corresponding to the thickness of the layers applied by the printing device. The finished “construction” file is used as a set of drawings during printing. Fiber optic lasers of relatively high power - about 200 W - are used as a heating element for sintering metal powder. Some devices use more powerful lasers with increased speed scanning (i.e. moving the laser beam) for higher productivity. Alternatively, productivity can be increased by using multiple lasers.

DMLS allows you to create solid metal parts with complex geometric shapes

Powder material is fed into the working chamber in quantities necessary to apply one layer. A special roller levels the fed material into an even layer and removes excess material from the chamber, after which laser head sinteres fresh powder particles among themselves and with the previous layer according to the contours determined by the digital model. Once the layer has been drawn, the process is repeated: the roller feeds fresh material and the laser begins to sinter the next layer. An attractive feature of this technology is that it is very high resolution printing – on average about 20 microns. For comparison, the typical layer thickness in amateur and consumer printers using FDM/FFF technology is about 100 microns.

Another interesting feature process is the absence of the need to build supports for overhanging structural elements. Unsintered powder is not removed during printing, but remains in the working chamber. Thus, each subsequent layer has a supporting surface. In addition, unused material can be collected from the build chamber after printing is completed and reused. DMLS production can be considered virtually waste-free, which is important when using expensive materials - for example, precious metals.

The technology has virtually no restrictions on the geometric complexity of construction, and high precision execution minimizes the need for mechanical processing of printed products.

Advantages and Disadvantages

DMLS technology has several advantages over traditional manufacturing methods. The most obvious is the ability to quickly produce geometrically complex parts without the need for machining (so-called “subtractive” methods - milling, drilling, etc.). The production is virtually waste-free, which distinguishes DMLS from subtractive technologies. The technology allows you to create several models simultaneously, limited only by the size of the working chamber. Building models takes about several hours, which is incomparably more profitable than the foundry process, which can take up to several months, taking into account the full production cycle. On the other hand, parts produced by laser sintering are not solid and therefore do not achieve the same strength values as cast samples or parts produced by subtractive methods.

At the moment, DMLS installations are used only in professional environments due to the high cost

DMLS is actively used in industry due to the possibility of building internal structures solid parts, unavailable in complexity traditional methods production. Parts with complex geometries can be made entirely, rather than from components, which has a positive effect on the quality and cost of products. Since DMLS is not required special tools(for example, casting molds) and does not produce large quantity waste (as is the case with subtractive methods), the production of small batches using this technology is much more profitable than through traditional methods.

Application

DMLS technology is used to produce small and medium-sized finished products in various industries, including aerospace, dental, medical, etc. The typical size of the construction area of existing installations is 250x250x250mm, although there are no technological restrictions on the size - it is only a matter of cost of the device. DMLS is used for rapid prototyping, reducing development time for new products, and in manufacturing, reducing the cost of small batches and simplifying the assembly of products with complex geometric shapes.

Parts photos rocket engine Super Draco published by Space X founder Elon Musk

Northwestern Polytechnic University of China uses DMLS systems to produce aircraft structural components. Research conducted by EADS also points to cost and waste reductions when using DMLS technology to produce complex designs in one-off or small batch quantities.

Materials

As consumables Almost any metals and alloys in powder form can be used. Today, stainless steel, cobalt-chromium alloys, titanium and other materials are successfully used.

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 so on 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 technology RAS.

Below are chess pieces created using the SLA printing method.

Chess pieces, created by SLA printing method

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 production 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 powder layers. 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 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.

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 over 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 a 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 has not received widespread, since for a more complete occurrence of volumetric and surface diffusion, viscous flow and other processes that take place during powder sintering, a relatively long exposure under laser radiation is required. This leads to long work laser and low productivity of the process, 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 the following conditions. The material for impregnation must be characterized complete absence or negligible interfacial interaction, low 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%.

One of the disadvantages of ILMS is the inability to regulate the content of the refractory phase (base material) over a wide range. Since its percentage in the finished product is determined by the bulk density of the powder, which, depending on the characteristics of the powder, can be three or more times less than the theoretical density of the powder material.

Materials and their properties used for ILMS

Direct Laser Metal Sintering (DLMS)

The direct laser sintering process for metals is similar to ILMS, but differs in that alloys or compounds with low melting points are used instead of polymer, and there is no technological step such as impregnation. The DMLS concept was created by the German company EOS GmbH, which in 1995 created a commercial installation for direct laser sintering of steel-nickel bronze powder systems. The production of various products by the DLMS method is based on the flow of the resulting melt-binder into the voids between the particles under the action of capillary forces. At the same time, to successfully complete the process, compounds with phosphorus are added to the powder mixture, which reduce the surface tension, viscosity and degree of oxidation of the melt, thereby improving wettability. The powder used as a binder is usually smaller in size than the base powder, as this increases the bulk density of the powder mixture and speeds up the melt formation process.

Materials and their properties used for DLMS by EOS GmbH

Selective Laser Melting (SLM)

Further improvements in additive manufacturing facilities include the ability to use a more powerful laser, a smaller focal spot diameter, and a thinner powder layer, which has made it possible to use SLM for the production of products from a variety of metals and alloys. Typically, products obtained by this method have a porosity of 0-3%.
As in the methods discussed above (ILMS, DMLS), big role wettability, surface tension and melt viscosity play a role in the manufacturing process of products. One of the factors limiting the use of various metals and alloys for SLM is the “beading” effect or spheroidization, which manifests itself in the form of the formation of droplets lying separately from each other, rather than a continuous melt path. The reason for this is surface tension, under the influence of which the melt tends to reduce the free surface energy by forming a shape with a minimum surface area, i.e. ball. In this case, the Marangoni effect is observed in the melt strip, which manifests itself in the form of convective flows due to the surface tension gradient as a function of temperature, and if the convective flows are strong enough, the melt strip is divided into individual drops. Also, a drop of melt, under the influence of surface tension, draws in nearby powder particles, which leads to the formation of a pit around the drop and, ultimately, to an increase in porosity.

Spheroidization of M3/2 steel under non-optimal SLM conditions

The spheroidization effect is also facilitated by the presence of oxygen, which, dissolving in the metal, increases the viscosity of the melt, which leads to a deterioration in the spreading and wettability of the melt below the underlying layer. For the reasons listed above, it is not possible to obtain products from metals such as tin, copper, zinc, and lead.

It is worth noting that the formation of a high-quality melt strip is associated with the search for the optimal range of process parameters (laser radiation power and scanning speed), which is usually quite narrow.

Influence of gold SLM parameters on the quality of the formed layers

Another factor affecting the quality of products is the appearance of internal stresses, the presence and magnitude of which depends on the geometry of the product, heating and cooling rates, coefficient of thermal expansion, phase and structural changes in the metal. Significant internal stresses can lead to deformation of products and the formation of micro- and macrocracks.

Partially reduce negative impact The above mentioned factors can be achieved through the use of heating elements, which are usually located inside the installation around the substrate or feeder with powder. Heating the powder also allows you to remove adsorbed moisture from the surface of the particles and thereby reduce the degree of oxidation.

When selective laser melting of metals such as aluminum, copper, gold, an important issue is their high reflectivity, which necessitates the use of a powerful laser system. But increasing the power of the laser beam can negatively affect the dimensional accuracy of the product, since with excessive heating the powder will melt and sinter outside the laser spot due to heat exchange. High laser power can also lead to changes chemical composition as a result of metal evaporation, which is especially typical for alloys containing low-melting components and having high vapor pressure.

Mechanical properties of materials obtained by the SLM method (EOS GmbH)

If a product obtained by one of the methods discussed above has residual porosity, then, if necessary, additional technological operations are used to increase its density. For this purpose, powder metallurgy methods are used - sintering or hot isostatic pressing (HIP). Sintering allows you to eliminate residual porosity and increase the physical and mechanical properties of the material. It should be emphasized that the formed properties of the material during the sintering process are determined by the composition and nature of the material, the size and number of pores, the presence of defects and other numerous factors. HIP is a process in which a workpiece placed in a gasostat is compacted under the influence of high temperature and compression by an inert gas. The operating pressure and maximum temperature achieved by the gasostat depend on its design and volume. For example, a gasostat having a working chamber dimensions of 900x1800 mm is capable of developing a temperature of 1500 o C and a pressure of 200 MPa. The use of HIP to eliminate porosity without the use of a hermetic shell is possible if the porosity is no more than 8%, since at a higher value, gas will enter the product through the pores, thereby preventing compaction. It is possible to prevent the penetration of gas into the product by making a sealed steel shell that follows the shape of the surface of the product. However, products produced by additive manufacturing generally have complex shapes, which makes it impossible to produce such a shell. In this case, for compaction, you can use a vacuum-sealed container in which the product is placed in a granular medium (Al 2 O 3, BN hex, graphite), which transmits pressure to the walls of the product.

After additive manufacturing using the SLM method, materials are characterized by anisotropy of properties, increased strength and reduced ductility due to the presence of residual stresses. To remove residual stresses, obtain a more equilibrium structure, and increase the viscosity and plasticity of the material, annealing is carried out.

According to the data below, it can be noted that products obtained by selective laser melting are, in some cases, stronger than cast ones by 2-12%. This can be explained by the small size of grains and microstructural components that are formed as a result of rapid cooling of the melt. Rapid supercooling of the melt significantly increases the number of solid phase nuclei and reduces their critical size. At the same time, the rapidly growing crystals on the nuclei, coming into contact with each other, begin to impede their further growth, thereby forming a fine-grained structure. Crystallization nuclei are usually nonmetallic inclusions, gas bubbles, or particles released from the melt with their limited solubility in the liquid phase. And in the general case, according to the Hall-Petch relation, as the grain size decreases, the strength of the metal increases due to the developed network of grain boundaries, which is an effective barrier to the movement of dislocations. It should be noted that due to the different chemical composition of the alloys and their properties, the conditions for carrying out SLM, the above-mentioned phenomena that occur during cooling of the melt manifest themselves with different intensities.

Mechanical properties of materials produced by SLM and casting

Of course, this does not mean that products obtained by selective laser melting are better than products obtained traditional ways. Due to the great flexibility of traditional methods for producing products, the properties of the product can be varied within wide limits. For example, using methods such as changing the temperature conditions of crystallization, alloying and introducing modifiers into the melt, thermal cycling, powder metallurgy, thermomechanical processing, etc., it is possible to achieve a significant increase in the strength properties of metals and alloys.

Of particular interest is the use of carbon steel for additive manufacturing, as a material that is cheap and has a high range of mechanical properties. It is known that with increasing carbon content in steel, its fluidity and wettability improve. Thanks to this, it is possible to obtain simple products containing 0.6-1% C with a density of 94-99%, while in the case of using pure iron the density is about 83%. In the process of selective laser melting of carbon steel, the melt path, when rapidly cooled, is quenched and tempered into a troostite or sorbitol structure. At the same time, due to thermal stresses and structural transformations, significant stresses can arise in the metal, which lead to the product being damaged or to the formation of cracks. The geometry of the product is also important, since sharp transitions along the cross-section, small radii of curvature and sharp edges cause the formation of cracks. If after “printing” the steel does not have a given level of mechanical properties and it must be subjected to additional heat treatment, then it will be necessary to take into account the previously noted limitations on the shape of the product in order to avoid the appearance of hardening defects. This to some extent reduces the prospects of using SLM for carbon steels.
When receiving products using traditional methods, one of the ways to avoid cracks and leads when hardening products complex shape is the use of alloy steels, in which the alloying elements present, in addition to increasing the mechanical and physical and chemical properties, delay the transformation of austenite during cooling, as a result of which the critical hardening rate decreases and the hardenability of alloy steel increases. Due to the low critical quenching rate, steel can be heated in oil or in air, which reduces the level of internal stresses. However, due to rapid heat removal, the impossibility of regulating the cooling rate and the presence of carbon in alloy steel, this technique does not avoid the occurrence of significant internal stresses during selective laser melting.

In connection with the above-mentioned features, martensitic steels (MS 1, GP 1, PH 1) are used for SLM, in which strengthening and increased hardness are achieved due to the release of dispersed intermetallic phases during heat treatment. These steels contain a small amount of carbon (hundredths of a percent), as a result of which the martensite lattice formed during rapid cooling is characterized by a low degree of distortion and, consequently, has low hardness. The low hardness and high plasticity of martensite ensures relaxation of internal stresses during hardening, and high content alloying elements allows steel to be calcined to a great depth at almost any cooling rate. Thanks to this, SLM can be used to manufacture and heat treat complex products without the risk of cracking or warping. In addition to maraging steels, some austenitic stainless steels, such as 316L, can be used.

In conclusion, it can be noted that the efforts of scientists and engineers are now aimed at a more detailed study of the influence of process parameters on the structure, mechanism and features of compaction of various materials under the influence of laser radiation in order to improve the mechanical properties and increase the range of materials suitable for laser additive manufacturing.

SLS technology(Selective Laser Sintering) - selective laser sintering is one of the technologies for producing products of any geometry from powdered material. The technology began its development, like other similar methods, in the 70s of the last century.

So, in 1971, a Frenchman Pierre Ciro(Pierre Ciraud) has filed a patent application describing a method for making products from a powder material based on the curing and bonding of the powder under the influence of a focused beam of energy.

The technology presented has little relation to any of today's commercial additive technologies, but it was a precursor to later developments in laser materials processing technology.

And in 1979, an inventor named Ross Householder(Ross F. Housholder) has filed a patent application describing a system and method for creating a three-dimensional product layer by layer, similar to future laser sintering technologies. But due to the extremely high cost of lasers at the time, Householder was only able to partially test his method.

Commercially successful selective laser sintering technology was developed and patented by a student at the University of Texas at Austin Carl Deckard and his supervisor, professor of mechanical engineering Joe Beaman in the mid-1980s with the support of DARPA (Defense Advanced Defense Agency) research projects) and the NSF agency (an independent agency of the US government responsible for the development of science and technology).

The essence of the technology was the application of the method production of a three-dimensional object from metal powder under the influence of a laser beam, when the powder particles are heated only until the outer layer melts, sufficient for their bonding. The process must be carried out in a sealed container filled with an inert gas to avoid ignition of the powder and leakage of toxic gases released during solid-state synthesis.

Note: the term "sintering" refers to the process by which objects are created from powders using the mechanism of atomic diffusion. Diffusion of atoms occurs in any material at temperatures above absolute zero, but the process occurs much faster at higher temperatures, so sintering is caused by heating the powder at high enough temperatures. Since the first devices used ABS plastic powder to build 3D products, the term “sintering” most technically accurately reflected the processes taking place. However, when plants began to use crystalline and semi-crystalline materials such as nylon and metals that flow during the construction process, the name "selective laser sintering" was already well established and remained, although it became a misnomer.

SLS technology uses multicomponent powders or powder mixtures from different chemical materials, in contrast to DMLS technology (), where single-component powders are mainly used.

In the first prototype of the device, it was not possible to obtain a finished product, since it used a laser with a power of only 2 Watts. After rechecking the mathematical calculations, Karl Deckard found out that when transferring a physical constant from one page to another, he was mistaken by almost 3 orders of magnitude. After which, the laser was replaced with a more powerful one - a 100 W solid-state laser, where yttrium aluminum garnet is used as the active medium. Later, carbon dioxide lasers began to be used.

In late 1986, Deckard, along with Associate Dean Dr. Paul F. McClure and businessman Harold Blair, founded Nova Automation, which was renamed DTM Corp. in February 1989.

The first units developed by DTM corp were called Mod A and Mod B, and the first batch of 4 units was released under the name 125S. In 2001, DTM corp was purchased by 3D Systems, which created a competing technology - .

3D Systems has been and remains one of the leaders in additive manufacturing, and obtaining rights to selective laser sintering technology is an important milestone for the development of commercial applications of additive technologies. Currently, 3D Systems is one of the leaders in the 3D printing market, along with such companies as EOS GmbH and Stratasys Inc.

EOS, after selling its SLA equipment business to 3D Systems in 1997, focused on developing equipment using SLM (selective laser melting) technology.

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