Electron beam melting is a similar process to selective laser melting. Electron beam melting differs from selective laser melting in that it is performed in a high vacuum. Due to the higher energy density of the electron beam compared to a standard laser, full melt conditions are achieved in the material faster than under laser melt technology. As a result, electron beam melting produces finished parts at a higher rate than similar laser-based systems.
In an electron beam additive manufacturing system, the electron beam is generated by applying a 60kV potential and focusing electron movement into a collimated beam using a magnetic lensing system. The accelerated electrons are then steered into the desired scan pattern using an additional electromagnetic lens stage that is controlled by the toolpath code created from the CAD model in use.
After electron beam formation, the part being worked is painted with the electron beam to form melt pools. These pools allow for complete liquid formation of the powder. After solidification, the beam generator is raised in the z direction and fresh powder is raked onto the new top layer of the part. It is worth noting that due to the high vacuum, cooling of electron beam melted parts takes a significantly longer time.
The powder subject to EBM processing is typically preheated. This allows for aggregation of the powder through a light sintering process. Preheating is intended to reduce thermal gradients during melting which allows for a faster melt cycle with fewer induced thermal stresses. However, preheating has some impact on downstream processes that may be undesirable. Metallurgical bonds are formed in the powder during preheating which may cause the powder to undergo uneven phase changes during the melting cycle.
High vacuum is maintained in the build chamber for several reasons. As electrons are lightweight charged particles, collisions with gasses will result in electron beam diffusion. As molten metal pools are being formed, splatter may occur. Maintaining the build chamber in a vacuum prevents ignition of reactive materials which may be part of the workpiece. Additionally, outgassing of chemically contaminated parts is promoted in a high vacuum state. This results in a more homogenous material with fewer contaminates after cooling.
Electron beam melting has shown particular promise in the production of nickel-based super-alloy precision aerospace components. These components are items such as jet engine compressor blading, engine nozzles, and space vehicle applications. This class of component requires low density, high strength components with excellent corrosion resistance in high temperature conditions. Titanium aluminides have been demonstrated to be ideal candidates for these components. Electron beam melting has proven to meet required specifications for microcrystalline structure formation while still being capable of producing highly complex parts.
Electron beam melting technology has been adapted to create high strength permanent rare earth magnets. Nd2Fe14B magnets have been found to be particularly well suited to production using additive manufacturing. To create these magnetics, the desired alloy is mixed and provided to the electron beam system as a powder. Because of the layer-by-layer production method, each layer beyond the first will naturally align magnetic domains during the liquid phase of the material. Current research in this area is on producing rare earth magnet powders of a higher magnetic flux density.
Electron beam melting technology allows for high quality material properties that are typically free of voids and relatively strong compared to other systems. EBM typically produces relatively tight tolerances and high quality finishes as well. Despite this, electron beam melting still maintains a relatively fast manufacturing cycle.
Unfortunately, electron beam melting is constrained by the requirement to maintain a high vacuum in the build chamber. This adds to the cost of the system while also increasing the maintenance frequency. Electron beam melting has a relatively high power consumption which increases manufacturing costs. An added risk of electron beam melting is the spontaneous formation of gamma ray radiation in the build chamber. This requires a tightly controlled vacuum chamber to protect personnel from radiation exposure.
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