Systems can be classified by the energy source or the way the material is being joined, for example using a binder, laser, heated nozzle etc. Classification is also possible by the group of materials being processed, such as plastics, metals or ceramics. The feedstock state, with the most common ones being solid (powder, wire or sheet) or liquid, is also used to define the process.
Turbine manufacture with Inconel 718 for micro turbine applications
Almost every powder-bed based AM system uses a powder deposition method consisting of a coating mechanism to spread a powder layer onto a substrate plate and a powder reservoir.
Usually the layers have a thickness of 20 to 100 µm. Once the powder layer is distributed, a 2D slice is either bound together, known as 3D-Printing, or melted using an energy beam applied to the powder bed. In the second case the energy source is normally one high-power laser, but state-of-the-art systems can use two or more lasers with different power under inert gas atmosphere.
Direct process powder-bed systems are known as laser melting processes and are commercially available under different trade names such as Selective Laser Melting (SLM), Laser Cusing and Direct Metal Laser Sintering (DMLS). The only exception to this process principle is the Electron Beam melting (EBM) process, which uses an electron beam under full vacuum.
The melting process is repeated slice by slice, layer by layer, until the last layer is melted and the parts are complete. Then it is removed from the powder bed and post processed according to requirements.
Schematic diagram of the Selective Laser Melting (SLM) powder-bed process (Source VDI 3404)
Metal powder bed fusion machines are available today from Concept Laser GmbH, EOS GmbH, ReaLizer GmbH, Renishaw and SLM Solutions GmbH in Europe. These companies offer a variety of systems based on the similar selective laser melting principle, but giving their own processes different names. 3D-Systems, based in USA, also offers systems based on selective laser melting. The choice of the right machine is dependent on the requirements of the end user, with the type of laser unit, powder handling and build chamber being some of the main characteristics of the system to consider. The Arcam Q20 uses an electron beam to melt the metal powder (Courtesy Arcam AB) Arcam AB, based in Sweden, manufactures powder bed fusion systems that use an electron beam as the energy source for the melting process. A hybrid system that combines powder bed fusion with CNC milling is offered by the Japanese company Matsuura. Another system using a powder bed is the Höganäs Digital Metal process. Developed by fcubic, this system uses a precision inkjet to deposit a special ink on a 45 micron layer of metal powder. Another 45 micron layer of powder is applied and the printing step is repeated until the component is complete. The part is then sintered to reach final size and strength. One of the benefits of this system is that the build is undertaken at room temperature without the partial melting which occurs with laser or electron beam technology. In principle there is also no need for support structures during the build as it is supported by the powder bed.
The Arcam Q20 uses an electron beam to melt the metal powder (Courtesy Arcam AB)
Although powder-fed systems use the same feedstock, the way the material is added layer by layer differs notably. The powder flows through a nozzle being melted from a beam right on the surface of the treated part.
Schematic diagram of the laser cladding process (Courtesy Sulzer Ltd)
Powder-fed systems are also known as Laser Cladding, Directed Energy Deposition and Laser Metal Deposition. The process is highly precise and based on an automated deposition of a layer of material with a thickness varying between 0.1mm and several centimetres. The metallurgical bonding of the cladding material with the base material and the absence of undercutting are some features of this process. The process is dissimilar to other welding techniques in that a low heat input penetrates the substrate.
One development of this technology is the Laser Engineered Net Shaping (LENS) powder delivery system used by Optomec. This method allows adding material to an already existing part, which means it can be used to repair expensive metal components that may have been damaged, like chipped turbine blades and injection moulding tool inserts, offering a high flexibility in the clamping of the parts and the “coating” materials.
Companies offering systems based on the same principle are: BeAM from France, Trumpf from Germany and Sciaky from USA. An interesting approach of a hybrid system is the one offered by DMG Mori. The combination of the laser cladding principle with a 5-axis milling system opens new fields of applications in many industrial branches.