Metal AM Processes and Materials
A variety of different technologies used in the metal additive manufacturing systems are available today. Systems may be categorized by the energy source or the manner in which the material is joined, e.g. by means of a binder, laser, heated nozzle, etc. It is also possible to classify the group of materials being processed, such as plastics, metals, or ceramics. The feedstock condition, the most common of which is solid (powder, wire, or sheet) or liquid, is often used to describe the process.
FIGURE 6.5 (a) Hybrid of inkjet printing and electrospinning; (b) Setup of wet-spinning
Almost every powder-bed AM system uses a powder deposition method comprised of a coating mechanism to spread a powder layer onto a substrate plate and a powder reservoir. Usually the layers are 20 to 100 pm thick. Once the powder layer has been distributed, a 2D slice, known as 3D printing, is bound together or melted using an energy beam applied to the powder bed. For the second example, the energy source is usually one high-powered laser, but the state-of-the-art systems can use two or more lasers of distinct power in an inert gas atmosphere.
Direct process powder-bed systems are known as laser melting processes and are available commercially under various 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 a full-vacuum electron beam. The melting cycle is repeated by slice, layer by layer, until the last layer is melted and the pieces are finished. It is then removed from the powder bed and processed as required (Figure 6.6).
Since the powder-fed systems are using the same feedstock, the manner in which the material is applied layer by layer varies considerably. The powder flows through the nozzle, which is melted from the beam right on the surface of the treated part (Figure 6.7)
Powder-fed systems are also known as laser cladding, direct energy deposition, and laser metal deposition. The process is highly efficient and is based on the automated deposition of a layer of material with a thickness of between 0.1 mm and a few
FIGURE 6.6 Schematic diagram of the selective laser melting (SLM) powder-bed process.
centimeters. Some features of this process are the metallurgical bonding of the cladding material with the base material and the lack of undercutting. The process is different from other welding techniques in that a low heat input penetrates the substrate.
One of the advancements in this technology is the laser engineered net shaping (LENS) powder delivery system used by Optomec. This technique allows the addition of material to an existing part, which means that it can be used to repair expensive metal components that may have been damaged, such as chipped turbine blades and injection molding tool inserts, offering a high degree of flexibility in the clamping of the parts and the “coating” materials.
The companies that offer systems based on the same principle are: BeAM from France, Trumpf from Germany, and Sciaky from the United States. An interesting approach to the hybrid system is the one offered by DMG Mori. The combination of the laser cladding theory and the 5-axis milling method opens up new fields of application in many industrial branches.