Selective laser melting
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Selective laser melting (SLM) is a particular rapid prototyping, 3D printing, or Additive Manufacturing (AM) technique designed to use a high power-density laser to melt and fuse metallic powders together. In many SLM is considered to be a subcategory of Selective Laser Sintering (SLS). The SLM process has the ability to fully melt the metal material into a solid 3D-dimensional part unlike SLS.
Selective laser melting started in 1995 at the Fraunhofer Institute ILT in Aachen, Germany, with a German research project, resulting in the so-called basic ILT SLM patent DE 19649865. Already during its pioneering phase Dr. Dieter Schwarze and Dr. Matthias Fockele from F&S Stereolithographietechnik GmbH located in Paderborn collaborated with the ILT researchers Dr. Wilhelm Meiners and Dr. Konrad Wissenbach. In the early 2000s F&S entered into a commercial partnership with MCP HEK GmbH (later on named MTT Technology GmbH and then SLM Solutions GmbH) located in Luebeck in northern Germany. Today[when?] Dr. Dieter Schwarze is with SLM Solutions GmbH and Dr. Matthias Fockele founded Realizer GmbH.
The ASTM International F42 standards committee has grouped selective laser melting into the category of "laser sintering", although this is an acknowledged misnomer because the process fully melts the metal into a solid homogeneous mass, unlike selective laser sintering (SLS) and direct metal laser sintering (DMLS), which are true sintering processes. A similar process is electron beam melting (EBM), which uses an electron beam as energy source.
The process starts by slicing the 3D CAD file data into layers, usually from 20 to 100 micrometres thick, creating a 2D image of each layer; this file format is the industry standard .stl file used on most layer-based 3D printing or stereolithography technologies. This file is then loaded into a file preparation software package that assigns parameters, values and physical supports that allow the file to be interpreted and built by different types of additive manufacturing machines.
With selective laser melting, thin layers of atomized fine metal powder are evenly distributed using a coating mechanism onto a substrate plate, usually metal, that is fastened to an indexing table that moves in the vertical (Z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, either argon or nitrogen at oxygen levels below 500 parts per million. Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam, usually an ytterbium fiber laser with hundreds of watts. The laser beam is directed in the X and Y directions with two high frequency scanning mirrors. The laser energy is intense enough to permit full melting (welding) of the particles to form solid metal. The process is repeated layer after layer until the part is complete.
Many Selective Laser Melting (SLM) machines operate with a work space of 250 mm (9.842 in) in the x & Y and they can go up to 350 mm (13.779 in) Z. Some of the materials being use in this processed can include copper, aluminium, stainless steel, toll steel, cobalt chrome, titanium and tungsten. In order for the material to be use in the process it must exist in atomized form.
The types of applications most suited to the selective laser melting process are complex geometries & structures with thin walls and hidden voids or channels on the one hand or low lot sizes on the other hand. Advantage can be gained when producing hybrid forms where solid and partially formed or lattice type geometries can be produced together to create a single object, such as a hip stem or acetabular cup or other orthopedic implant where oseointegration is enhanced by the surface geometry. Much of the pioneering work with selective laser melting technologies is on lightweight parts for aerospace where traditional manufacturing constraints, such as tooling and physical access to surfaces for machining, restrict the design of components. SLM allows parts to be built additively to form near net shape components rather than by removing waste material.
Traditional manufacturing techniques have a relatively high set-up cost (e.g. for creating a mold). While SLM has a high cost per part (mostly because it is time-intensive), it is advisable if only very few parts are to be produced. This is the case e.g. for spare parts of old machines (like vintage cars) or individual products like implants.
Tests by NASA's Marshall Space Flight Center, which is experimenting with the technique to make some difficult-to-fabricate parts from nickel alloys for the J-2X and RS-25 rocket engines, show that difficult to make parts made with the technique are somewhat weaker than forged and milled parts but often avoid the need for welds which are weak points.
Selective laser melting or additive manufacturing, sometimes referred to as rapid manufacturing or rapid prototyping, is in its infancy with relatively few users in comparison to conventional methods such as machining, casting or forging metals, although those that are using the technology have become highly proficient. Like any process or method selective laser melting must be suited to the task at hand. Markets such as aerospace or medical orthopedics have been evaluating the technology as a manufacturing process. Barriers to acceptance are high and compliance issues result in long periods of certification and qualification. This is demonstrated[when?] by the lack of fully formed international standards by which to measure the performance of competing systems. The standard in question is ASTM F2792-10 Standard Terminology for Additive Manufacturing Technologies.
Difference between Selective Laser Sintering (SLS) & Selective Laser Melting (SLM)
The use of SLS refers to the process as applied to a variety of materials such as plastics, glass, and ceramics, as well as metals. What sets SLS apart from other 3D printing process is the lacked ability to fully melt the powder, rather heating it up to a specific point where the powder grains can fuse together, allowing the porosity of the material can be controlled. On the other hand, SLM can go one step further than SLS, by using the laser to fully melt the metal, meaning the powder is not being fused together but actually liquified long enough to melt the powder grains into a homogeneous part. Therefore, SLM can produce stronger parts because of reduced porosity and greater control over crystal structure, which helps prevent part failure. However, SLM is only feasible when using a single metal powder.
- List of notable 3D printed weapons and parts
- 3D printing
- Desktop manufacturing
- Digital fabricator
- Direct digital manufacturing
- Rapid manufacturing
- Selective laser sintering
- Solid freeform fabrication
- Larry Greenemeier (November 9, 2012). "NASA Plans for 3-D Printing Rocket Engine Parts Could Boost Larger Manufacturing Trend". Scientific American. Retrieved November 13, 2012.
- S. Das and J. J. Beaman , “ Direct selective laser sintering of metals,” U.S. patent US6676892B2 (2004). Google Scholar
- W. Meiners , K. D. Wissenbach , and A. D. Gasser , “ Shaped body especially prototype or replacement part production,” U.S. patent DE19649849C1 (1998). Google Scholar
- ASTM F2792-10 Standard Terminology for Additive Manufacturing Technologies
- Abe, F., Costa Santos, E., Kitamura, Y., Osakada, K., Shiomi, M. 2003. Influence of forming conditions on the titanium model in rapid prototyping with the selective laser melting process. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 217 (1), pp. 119–126.
- Gibson, I. Rosen, D.W. and Stucker, B. (2010) Additive Manufacturing Technologies: Rapid Prototypingto Direct Digital Manufacturing. New York, Hiedelberg, Dordrecht, London: Springer. ISBN
- Wohlers, T. Wohlers Report 2010: Additive Manufacturing State of the Industry: Annual World Wide Progress Report. Fort Collins: Wohlers Associates.
- "How Selective Laser Melting Works". THRE3D.com. Retrieved 11 February 2014.