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Additive Manufacturing 101-5: What is powder bed fusion?

(Image: 3D Hubs)

Powder Bed Fusion (Image: 3D Hubs)

  Mechanical Design Engineer and Additive Manufacturing Ph.D. student

This is the sixth article in a series of original articles that will help you understand the origins of the technology that is commonly called 3D printing. First an introduction, followed by the seven main technologies categories (binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, vat photopolymerization) and then a design philosophy for additive manufacturing.

Powder Bed Fusion

ISO/ASTM definition: “powder bed fusion, —an additive manufacturing process in which thermal energy selectively fuses regions of a powder bed.”[1]

Powder Bed Fusion can also be known as (in alphabetical order):

➢ Direct Metal Laser Remelting or DMLR[2]

➢ Direct Metal Laser Sintering or DMLS® (EOS GmbH)[2]

➢ Direct Metal Printing or DMP (3D Systems Corporation)

➢ Electron Beam Additive Manufacturing or EBAM[3]*‡

➢ Electron Beam Melting or EBM (Arcam AB)

➢ High Speed Sintering or HSS[4]

➢ LaserCUSING® (Concept Laser GmbH)[2]

➢ Laser Metal Fusion (TRUMPF Laser Technology)

➢ Micro Laser Sintering or MLS (EOS GmbH)

➢ Selective Electron Beam Melting or SEBM[5], [6]*

➢ Selective Heat Sintering or SHS[7]

➢ Selective Laser Melting or SLM[2]

➢ Selective Laser Sintering or SLS® (3D Systems Corporation)[2]

* Occasionally used in literature for describing EBM.

‡ EBAM is officially a trademarked DED process.

Powder bed fusion contains a range of technologies, all of which have core similarities, the main three are SLS, SLM and EBM. These originated from work at the University of Texas at Austin in the early 1980s which was awarded a patent in 1989[8], [9]. In 1995, SLM and DMLS were developed in Germany as part of a project between the Fraunhofer Institute, EOS and others[10]. In 1997, Arcam AB of Sweden was founded to commercialize the idea of EBM with their first machine being sold in 2001 and delivered in 2002. HSS was invented in the UK in 2003 and shows promise of being an up and coming AM technology[4].

Figure 1: Powder Bed Fusion example setup[11]

Powder bed fusion methods all start with a powder bed, another parallel to binder jetting. A thin layer of loose powder between 0.001mm (EOS) and 0.2mm (Arcam) with an average of 0.02mm to 0.1mm is smoothly spread flat over a build platform. This layer of powder is then passed over by either a laser or electron beam which supplies significant heat to the powder. The powder is then either partially melted (sintered) or fully melted, to a point where the powder fuses to itself and to the layers below. Then the build platform changes height, a new layer of powder is deposited, and the process is repeated layer by layer until the part is complete. The main differences between all these methods are the heat source that causes melting, the environment in which the melting takes place, and the degree to which things are melted. SLS, DMLS, SLM and all other methods that contain laser in the name, use a laser to provide the thermal energy. SHS uses only a thermal print head. HSS uses an infrared lamp and a radiation absorbent material to do the sintering. EBM uses an electron beam. SLS and HSS generally work only with plastics and operate in a heated nitrogen atmosphere with the powder bed at an elevated temperature. DMLS and SLM primarily work with metals, and due to the reactive nature of some powdered metals, the process takes place in an inert oxygen-free environment with the powder bed being either at room temperature or at a low-temperature set-point. EBM works with metals and takes place in a vacuum and at elevated temperatures much closer to the metals melting temperature. SLS, HSS, and SHS all sinter the material resulting in a final part that has some porosity, but low to no residual stress in the part. Residual stress is energy contained within the material itself that causes it to deform and move[12]. This lack of residual stress allows the parts do not need any type of support system as the powder bed provides the needed supports for overhangs. Although DMLS contains sintering as part of its name, its process does involve melting and not sintering of metals. SLM, DMLS, LaserCUSING and EBM fully melt the material and can result in fully dense parts. SLM, DMLS, and LaserCUSING all can result in significant residual stress depending on materials, geometry, and laser parameters, and thus need significant support structures to hold parts down. EBM utilizes its fast scan speed to preheat the entire layer with the electron beam to just below melting before actually melting the selected portions. This not only preheats the powder but also causes the powder to become loosely aggregated and reduces dust ‘smoking’ which is a repulsive reaction of the powder once it is hit with negatively charged electrons[13], [14]. EBM results in parts that have little to no residual stress and thus uses very little if any supports.

Advantages of this process are mostly about material properties. First, there is a wide range of materials that can be processed from plastic parts like Nylon to a wide range of different metals like copper and Inconel to even ceramics. Next, parts that are fully or even partially melted can have significant strength advantages over non-melted processes as the material properties can be close to that of stock material. Then, depending on the process and material, support structures may not be needed, as the powder bed becomes the support. Build speeds can also be fast depending on materials and process, with processes that preheat the powder bed to just below melting being the quickest because it allows a very fast scan speed. Electron beam scan speeds are the fastest of any process due to the lack of mechanical parts to direct the beam. Processes that use lasers result in a high level of detail and fine features. EBM can process materials that are highly reactive in oxygen, and thus can be made quicker and cheaper than subtractive methods.

Disadvantages depend on the process; however, all of these processes can only utilize a single material in the final part. Some methods require inert gases as an additional consumable material. Layer heights are a function of the powder diameter and thus have a medium resolution in the build direction compared to other processes. EBM surface finish is generally rough and will need some post-processing in order to achieve tight tolerances. Some methods require support structures that need additional manual work to be removed from the final part. EBM requires more than an air blasting to remove unsolidified material as the remaining powder no longer acts as a light metal powder but clings together more like wet sand. Thus it needs either powder blasting, ultrasonic vibration, or mechanical methods to remove the powder, with certain geometries like deep narrow cavities being especially difficult.

References

[1] “ISO/ASTM 52900:2015(en), Additive manufacturing — General principles — Terminology,” International Organization for Standardization (ISO), Geneva, Switzerland, 2015.

[2] Gu D. D., Meiners W., Wissenbach K., and Poprawe R., “Laser additive manufacturing of metallic components: materials, processes and mechanisms,” International Materials Reviews, vol. 57, no. 3, pp. 133–164, May 2012.

[3] Gong X., Anderson T., and Chou K., “Review on Powder-Based Electron Beam Additive Manufacturing Technology,” in ASME/ISCIE 2012 International Symposium on Flexible Automation (ISFA 2012), St. Louis, Missouri, USA, 2012, p. 507.

[4] Hopkinson N. and Erasenthiran P., “High Speed Sintering – Early Research into a New Rapid Manufacturing Process,” in Proceedings of the 15th Solid Freeform Fabrication Symposium (SFF), Austin, Texas, USA, 2004, pp. 312–320.

[5] Heinl P., Rottmair A., Körner C., and Singer R. F., “Cellular Titanium by Selective Electron Beam Melting,” Advanced Engineering Materials, vol. 9, no. 5, pp. 360–364, May 2007.

[6] Lodes M. A., Guschlbauer R., and Körner C., “Process development for the manufacturing of 99.94% pure copper via selective electron beam melting,” Materials Letters, vol. 143, pp. 298–301, Mar. 2015.

[7] Baumers M., Tuck C., and Hague R., “Selective Heat Sintering Versus Laser Sintering: Comparison of Deposition Rate, Process Energy Consumption and Cost Performance,” in Proceedings of the 26th Solid Freeform Fabrication Symposium (SFF), Austin, Texas, USA, 2015, pp. 109–121.

[8] Deckard C. R., “Method and apparatus for producing parts by selective sintering,” U.S. Patent 4,863,538, 05-Sep-1989.

[9] Bourell D. L., Marcus H. L., Barlow J. W., Beaman J. J., and Deckard C. R., “Multiple material systems for selective beam sintering,” U.S. Patent 4,944,817, 31-Jul-1990.

[10] Shellabear M. and Nyrhilä O., “DMLS – Development History and State of the Art,” in Proceedings of the 4th Laser Assisted Net Shape Engineering Conference (LANE 2004): Volume 1, Erlangen, Germany, 2004, pp. 393–404.

[11] Frazier W. E., “Metal Additive Manufacturing: A Review,” Journal of Materials Engineering and Performance, vol. 23, no. 6, pp. 1917–1928, Jun. 2014.

[12] Buchbinder D., Meiners W., Pirch N., Wissenbach K., and Schrage J., “Investigation on reducing distortion by preheating during manufacture of aluminum components using selective laser melting,” Journal of Laser Applications, vol. 26, no. 1, p. 12004, 2014.

[13] Kahnert M., Lutzmann S., and Zaeh M. F., “Layer formations in electron beam sintering,” in Proceedings of the 18th Solid Freeform Fabrication Symposium (SFF), Austin, Texas, USA, 2007, pp. 88–99.

[14] Eschey C., Lutzmann S., and Zaeh M. F., “Examination of the powder spreading effect in Electron Beam Melting (EBM),” in Proceedings of the 20th Solid Freeform Fabrication Symposium (SFF), Austin, Texas, USA, 2009, pp. 308–319.

What’s the Difference Between Stereolithography and Selective Laser Sintering?

By Jeff Kerns Machine Design

Depending on the application, one must weigh the pros and cons of a liquid- or powder-based approach to optimize 3D printing.

With the rapid growth of 3D printing, some people may confuse the different types of printing processes. In fact, the misunderstanding is so great that organizations are creating standards to facilitate better communications when talking about additive-manufacturing (AM) processes. We’ll explore the differences between two of these AM processes: stereolithography (SLA) and selective laser sintering (SLS).
Related

First, there’s a difference between ultraviolet (UV) lamp cured and UV laser cured. The lamp-cured processes such as multi-jet and some fused-deposition-modeling (FDM) processes are not SLA, and should not be confused with it.

SLA Process

The SLA laser travels through the liquid photo-resin, building the part layer by layer.

SLA is a photopolymerization process in which a build tray is submerged 0.002 to 0.006 in. (0.05 to 0.15mm) in a basin of liquid photosensitive material. This depth can vary based on laser strength, material, or tolerance desired. A UV laser (not lamp) solidifies one slice of the part onto the build tray. The tray then submerges, another 0.002 to 0.006 in., and the laser solidifies the next slice of the part. The thickness of the layers can affect the quality of print and tolerances. An industry average tolerance is around 3.9 × 10-3in. (0.1 mm). The laser travels the entire path of the part’s cross-section as it builds up each layer, so speed becomes an important consideration.

Commercial SLA printers generally cost a few thousand dollars.

SLS Process

After the part is formed in the powder-bed process, it’s removed and cleaned for any post processing. The powder that’s left un-sintered acts as a support material; after the process, it can be sifted and reused.

In powder-bed SLS, a layer of powdered material is carefully laid down by a leveler or roller on the build tray. A laser then sinters the cross-section of the part. Subsequently, the tray drops another 0.002 to 0.004 in. (0.05 to 0.10 mm) and the process repeats. Similar to SLA, layer thickness varies based on laser strength, material, or tolerance desired.

A variation of SLS is called the cladding process. One way to clad is via powder jet, whereby a print head sprays powder to form layers as a laser sinters each layer. The powder-jet cladding process may be used to fix broken or damaged parts. Users can even align the grain structure of the new material with that of the existing part, an important consideration for corrosion resistance. Often times, cladded parts require post machining.

Some 3D printer companies offer a three-step process in which one machine clads (puts one material over another), measures, and grids the part to the desired tolerance. These processes often use fiber lasers, rather than CO2 lasers, to lower operating cost and make control simpler.

SLS economical printer can cost anywhere from $12,000 to $33,000.

SLS vs. SLA

One major difference between SLA and SLS revolves around material selection. SLA works with polymers and resins, not metals. SLS works with a few polymers, such as nylon and polystyrene, but can also handle metals like steel, titanium, and others. SLA works with liquids, while SLS uses powders that raise safety concerns. Breathing in fine particulates of nickel, for example, can be harmful. Breathing apparatuses and ventilation should be considered, depending on the type of powder… more

SOURCE – Machine Design