Electron beam melting
( Selective ) electron beam melting ( (Selective) Electron Beam Melting , (S) EBM ) or electron beam sintering is an additive manufacturing method for layerwise production of metallic components from the powder bed. The process is counted as powder bed-based melting and is closely related to selective laser beam melting . Because of the commonality of the energy source, electron beam melting is also technologically related to the other methods of electron beam material processing .
Electron beam melting was registered for a patent in 1993 by Ralf Larsson in Sweden. After the patent was granted in 1997, he founded Arcam AB to sell the process commercially. Arcam AB is the largest supplier of EBM systems and the owner of the EBM brand .
Procedure
With the help of an electron beam as an energy source, a metal powder is melted in a targeted manner, which means that compact components with almost any geometry can be produced directly from the design data.
Based on a digital 3-D model, a layer of metal powder is alternately applied over the entire surface with a doctor blade and initially preheated over a large area using an electron beam and then locally melted. After cooling, the melt solidifies to form a solid layer of metal with an almost 100 percent structural density. The work table is then lowered by a layer thickness and the next layer of powder is applied to the previous one. These steps are repeated many times. After the actual manufacturing process, the loose powder is removed from the actual component using compressed air. In this way, the desired component is generated in layers. The process takes place in a vacuum to prevent a reaction with surrounding gases from taking place and thereby changing the material's material properties .
Plant engineering
The heart of the system is the vacuum chamber together with the electron beam gun . The vacuum chamber contains the powder reservoirs, the coating system for powder application and the work table on which the component is built. The electron beam cannon is installed on the vacuum chamber above the work table. In addition, there are other essential components, such as the high-voltage generator that supplies the electron beam cannon with electricity via a high-voltage cable, several vacuum pumps that vacuum the chamber and the cannon, the lifting unit that moves the work table up and down, the machine control and the operating unit for the plant operator.
Electron beam gun
The electron beam is generated, accelerated, shaped and deflected with the electron beam gun. The functional principle is based heavily on that of the Braun tube : The generation and guidance of the electron beam requires a high vacuum (in a sealed tube flask), which minimizes unwanted collisions of electrons on gas molecules and thus scattering and energy losses of the beam, as well as arcs and electrical flashovers between the cathode , Control electrode and anode.
The electron beam gun can be divided into two different sections according to its functions. In the upper section, the actual electron beam is generated and accelerated; in the lower section, it is shaped and deflected according to the requirements. The acceleration is achieved by electrostatic fields, whereas electromagnetic fields are used to control the beam shape and direction in the lower section.
Beam generation and acceleration
The beam generation begins at the cathode. If a high voltage is applied to this and it is also heated, electrons are emitted, which collect on the surface of the cathode and form an electron cloud. Opposite it is the anode, which forms the electrostatic opposite pole of the field and attracts the electrons. Although the electrons are accelerated towards the anode by a potential difference, the electron beam can pass it through a central hole in it, similar to a pinhole in optics.
The control electrode with cathode polarity is also arranged between the cathode and anode. By means of it and the applied voltage ( Wehnelt voltage ), the amount of electrons that can leave the electron cloud in the direction of the anode is controlled, and this determines the power of the electron beam. The combination of cathode, control electrode and anode is called a triode system. Alternative systems, which control the beam power directly via the cathode voltage instead of a control electrode, are accordingly called diode systems, but are significantly less precise and have not been widely used.
Beam focusing and deflection
Beyond the anode, changes are only mediated via electromagnetic fields and the resulting Lorentz force (according to Lenz's laws and three-finger rule ), whereby the speed of the electrons is no longer influenced:
The centering coil is located directly behind the anode , which counteracts beam scattering and ensures a beam with a controlled cross-section. This is a prerequisite for the further steps of beam shaping.
Downstream of the coil is the electromagnetic stigmator , which compensates for electrical and magnetic interference that would lead to an elliptical distortion of the electron beam. Its function is that a beam cross-section that is as circular as possible is always formed and the focus position is always at the same distance from the working plane.
The beam is focused by a subsequent toroidal coil, which, analogous to the function of a converging lens in optics, initially bundles the electron beam onto an (ideal) focal point, on the one hand to optimize the energy density in the cross-sectional area of the electron beam and, on the other hand, to regulate the energy can be worked with on the working level.
Finally, the electron beam passes two pairs of coils arranged perpendicular to one another, which can deflect the beam in either the X or Y direction. This controls the movement of the beam on the working plane in any direction and defines the points at which the powder material is melted.
Advantages and disadvantages
There are several advantages compared to traditional manufacturing processes such as casting , sintering or forging . These include:
- Great freedom of geometric design
- Shortening the time between development and market launch
- Higher material efficiency
- No costs for component-specific tools, molds, cores or the like
- Economic production of prototypes and / or small series
Compared to traditional manufacturing processes, the following disadvantages arise:
- Relatively high initial investment
- Relatively slow production of components
- The relatively small overall volume of the device limits the maximum possible dimensions of the component
- No economical large-scale production
In contrast to other additive manufacturing processes such as selective laser beam melting, the EBM process achieves a structural density of almost 100%. This results in components with material properties comparable to those produced by classic manufacturing processes.
Web links
- EBM on the Arcam company website
- Selective electron beam melting on the homepage of the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research IFAM
Individual evidence
- ^ Association of German Engineers eV (Ed.): VDI 3405, Additive Manufacturing Process Basics, Terms, Process Descriptions . Beuth Verlag GmbH, December 2014.
- ↑ Hagemann, Florian., Zäh, Michael, 1963-: Economic production with rapid technologies: User guide for the selection of suitable processes . Hanser, Munich 2006, ISBN 3-446-22854-3 .
- ↑ Gebhardt, Andreas .: Generative manufacturing processes: rapid prototyping - rapid tooling - rapid manufacturing . 3. Edition. Hanser, Munich 2007, ISBN 978-3-446-22666-1 .
- ^ Securities sales prospectus of SLM Solutions Group AG, April 25, 2014, page 146.
- ↑ a b c Wohlers Associates, Inc .: Wohlers report 2006: rapid prototyping & manufacturing state of the industry, annual worldwide progress report . Wohlers Associates, Fort Collins, Colo. 2006, ISBN 0-9754429-2-9 .
- ↑ Keese, Keese, Keese, Keese, Keese: Electron beam welding. Retrieved March 5, 2019 .
- ↑ a b c d e f g Lutzmann, Stefan .: Contribution to the process control of electron beam melting . Utz, Herbert, Munich 2011, ISBN 978-3-8316-4070-6 .
- ↑ Stelzer, Ralph, Technische Universität Dresden, Design, Develop, Experience (EEE) 06.30-07.01 2016 Dresden: Design, Develop, Experience 2016 - Contributions to virtual product development and design technology Dresden, June 30th - July 1st, 2016 . Dresden 2016, ISBN 978-3-95908-062-0 .
- ↑ Prof. Dr.-Ing. Michael F. Zäh, Dipl.-Ing. Markus Kahnert: Using the electron beam for the selective sintering of metallic powders . paper-iwb, Augsburg.
- ↑ a b Electron Beam Melting (EBM process). In: 3D printers and more | threedom. Accessed March 5, 2019 (German).
- ↑ Electron Beam Melting Technology. In: 3Dnatives. July 11, 2016, accessed on March 5, 2019 (German).
- ↑ a b c d e Kahnert, Markus .: Scan strategies for improved process control in electron beam melting (EBM) . Utz, Munich 2015, ISBN 978-3-8316-4416-2 .
- ↑ Electron Beam Melting (EBM process). Retrieved February 9, 2019 .