1.2 AM Standard Definition and Classification
The American Society for Testing and Materials (ASTM) and International Standards Organization (ISO) have jointly established two committees to develop standards for the AM industry. The joint committees have proposed the definition of AM based on an active standard of ASTM ISO/ASTM52900, developed by Subcommittee F42.91, as
“Additive manufacturing (AM) is process of joining materials to make parts from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing and formative manufacturing methodologies.”
This standard also elaborates on the functionality of manufactured parts as
The functionality of a manufactured object is derived from the combination of the object's geometry and properties. In order to achieve this combination, a manufacturing process is made up of a series of operations and sub‐processes that bring the shape of the intended geometry to a material capable of possessing the desired properties. The shaping of materials into objects within a manufacturing process can be achieved by one, or combinations of three basic principles: Formative shaping, subtractive shaping and additive shaping.
The same standard categorizes AM into seven processes as:
Binder Jetting
Directed Energy Deposition
Material Extrusion
Material Jetting
Powder Bed Fusion
Sheet Lamination
VAT Photopolymerization
The same standard briefly defines each of these AM processes:
Binder jetting, an additive manufacturing process in which a liquid bonding agent is selectively deposited to join powder materials.
Directed energy deposition, an additive manufacturing process in which focused thermal energy is used to fuse materials by melting as they are being deposited. Note, “Focused thermal energy” means that an energy source (e.g. laser, electron beam, or plasma arc) is focused to melt the materials being deposited.
Material extrusion, an additive manufacturing process in which material is selectively dispensed through a nozzle or orifice.
Material jetting, an additive manufacturing process in which droplets of build material are selectively deposited. Note: Example materials include photopolymer and wax.
Powder bed fusion, an additive manufacturing process in which thermal energy selectively fuses regions of a powder bed.
Sheet lamination, an additive manufacturing process in which sheets of material are bonded to form a part.
Vat photopolymerization, an additive manufacturing process in which liquid photopolymer in a vat is selectively cured by light‐activated polymerization.
From these processes, almost all have been used for metal AM. However, industry has embraced the following processes for metal manufacturing more widely (note that the sequence shows the domination of each process in the market): (i) Powder Bed Fusion, (ii) Directed Energy Deposition, (iii) Binder Jetting, (iv) Material Extrusion, (v) Material Jetting, and (vi) Sheet Lamination. The first three are the most popular technologies for metal AM. In Chapter 2, these processes are explained in detail.
In all these processes, when it comes to metal AM, several main concepts are shared: (i) motion systems, (ii) energy or binding sources, and (iii) material delivery mechanisms. Chapter 3 will explain the main components used in the three major metal AM processes.
1.3 Why Metal Additive Manufacturing?
From the early days of AM, the technology has been evolved substantially. Advancement after advancement in AM is announced almost daily. While AM has been substantially changed from 30 years ago, it will be unrecognizable form the current status in 2030. But why such enthusiasm exists in industry and academia to try to understand metal AM and work hard to address its challenges and adopt it to their products? There are several main factors for this motivation:
On‐demand low‐cost rapid prototyping: One of the major applications of AM is the manufacture of functional prototypes. Such prototyping usually carries at a fraction of the cost compared with other conventional processes and at usually non‐disputable speeds. This rapid turnaround usually accelerates the design cycle (design, test, revision, and redesign). Products such as molds that would require more than 4–6 months to develop can be ready for operation in 2–3 months if being made by AM.
Simpler supply chain for effective low‐volume production: Low‐volume niche production usually requires more investment. Due to this issue, conventional manufacturers usually do not embrace low‐volume production; however, AM companies can level this niche. Many time‐consuming and expensive manufacturing techniques can be superseded by rapid and efficient metal AM for low‐volume manufacturing. However, for mass production, AM is still lagging behind conventional techniques such as casting and forging. One of the reasons of this feature is that AM usually needs a simpler supply chain with fewer players involved. Although AM's supply chain is still under development by industry, it is expected to see more and more low‐volume manufacturing by AM as the supply chain is reliably in place. Lowering the AM material costs will be another factor to foster AM adoption for low‐volume manufacturing when the technology moves toward series manufacturing eventually. Initial costs are usually lower for AM than conventional methods because of the minimum need for tools and jig/fixtures needed for assembly costs. In conventional manufacturing (e.g. casting), each part needs a unique mold. To compensate for the cost of tools for each identical part, the number of products should be very high. AM does not usually need any specialized tooling; therefore, there are essentially no initial costs (called fixed costs too). Due to this saving, it is possible to get to the breakeven point sooner and make profits even with lower volumes.
Geometric complexity may be free: AM enables the fabrication of complex shapes that cannot be produced by any other conventional manufacturing methods (Figure 1.3). The additive nature of AM offers an opportunity where geometric complexity may not come at a higher price. Unlike conventional methods, AM offers a platform for “design for use” rather than “design for manufacture.” Parts with complex or organic geometry optimized for performance may cost lower; however, attention must be given to the fact that not all complex parts and geometrical features are manufacturable by AM. Process constraints in metal AM (e.g., overhanging features) may cause issues in terms of residual stresses and defects, thus complexity may not come with full freedom!
Lightweighting: Manufacturers have been trying to fabricate both greener and more economical products. Lightweight components provide two goals: (i) the parts with reduced weight take less energy to move; thus, energy consumption drops, and (ii) less raw materials are used. Both reasons indicate that the production of lightweight components has a positive impact on costs, resources, and the environment. Resource prices are virtually going up worldwide; thus, reducing material consumption is vitally important for product development. AM is nicely linked with topology optimization, making it possible to design and manufacture high‐strength but lightweight structures, where conventional manufacturing processes fail to do so. Chapter 10 highlights how topology optimization and lattice structure design handshake with AM to make the fabrication of lightweight structures possible.