In the video below, Lisa Harouni gives an excellent introduction to additive manufacturing.
For processes that are powder based, there are two ways to apply the powder during the build process: powder bed fusion AM and powder-based directed energy deposition AM. There are two energy sources that are being applied for the AM of metals: laser and electron beam. The choice of technology to directly fabricate parts depends on the application and the material. Generally speaking, the electron beam systems tend to produce parts with less residual stress than the laser-based systems because the electron beam powder bed is operated at relatively high temperature. On the other hand, the rapid solidification microstructures that develop in the laser-based systems can be advantageous for properties. The laser-based powder injection systems are multi-material capable, which gives them the unique capability of changing the composition of the material being deposited "on the fly." The number of companies that manufacture metal powder bed based systems and powder injection systems are growing.
Metals produced using AM have structure, properties, and performance that can differ from their cast and wrought counterparts. These include: density, residual stresses, mechanical behavior, nonequilibrium microstructure, crystallographic texture:
- Density. While it is challenging to reproducibly obtain AM materials that are 100% of the reference density, AM methods can yield metal densities in excess of 99% of the reference density. Some materials are reported to be fabricated at full density and some are reported with a spread of densities (e.g., 99.2 ‑ 99.5%). Density is influenced by development of pores or entrapment of un-melted powders during processing. Hot isostatic pressing is occasionally used to improve as-fabricated densities.
- Residual stresses. Residual stresses can be “very high” in metal parts produced using laser-based AM methods. (Rangaswamy et al.: 2005; Wang et al.: 2008) Mitigation and optimization strategies are required and can include changing the substrate temperature, scan direction, or the application of post-deposition processing, perhaps even including in-situ shock wave processing. (Morgan et al.: 2001) Residual stress issues can be significantly reduced or even eliminated by using electron-beam-based AM systems.
- Mechanical behavior. Generally, because of the refined microstructure of metals produced using AM, an increase in strength and decrease in ductility is expected compared with conventional wrought alloys. Differences in fracture toughness and behavior under dynamic conditions are unknown.
- Nonequilibrium microstructures. Materials produced using AM methods can experience very high cooling rates (~103-108 K/s). (Espana et al.: 2011) At these cooling rates, several effects can be realized, depending on the material, including “suppression of diffusion-controlled solid-state phase transformations, formation of supersaturated solutions and nonequilibrium phases, formation of extremely fine, refined microstructures with little elemental segregation, and formation of very fine second-phase particles such as inclusions and carbides.” (Espana et al.: 2011) In some cases, these are desirable effects but must be considered on a case-by-case basis.
- Crystallographic texture. Because of the rapid cooling rates and directional solidification, significant crystallographic texture can be expected in metals made using AM processes. The texture and its effects can be somewhat controlled by varying the scan direction during deposition. (Thijs et al.: 2010)