METAL ADDITIVE MANUFACTURING Machinery's Handbook, 31st Edition
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Metal Additive Manufacturing Additive manufacturing (AM), also known as 3D printing, is a group of processes in which materials are joined selectively to transform information in computer-aided design (CAD) files into three-dimensional (3D) parts. The term “additive” is used to distinguish these processes from “subtractive” (machining, grinding, and chemical or electrical ero- sion) or forming (casting, rolling, and forging). ASTM Committee F-42 was organized in 2009 to develop standards for AM. One of the committee’s first actions was to define standard terminology, embodied in ASTM Stan - dard F2792 (withdrawn); the current standard is ISO/ASTM 52900:2015. (For more on AM standards, see Standards for Additive Manufacturing on page 1565.) This standard groups AM processes into seven categories (listed here alphabetically): Binder Jetting: A liquid bonding agent is selectively deposited to join powdered mate- rial, layer by layer, until a 3D shape is formed. The binder is deposited as fine droplets through the jets, or orifices, of an ink-jet printer onto a layer of powder. Directed Energy Deposition: This process fuses material, which may be in the form of metal powder or wire, by melting as it is deposited into the focal point of a directed thermal energy source (laser, electron beam, or plasma arc). Material Extrusion: In this AM process, material is selectively dispensed through a nozzle or orifice. A wide range of extruded materials may be used, from plastics to con - crete to food. Material Jetting: Through controlled jetting, droplets of material are selectively depos- ited to build a form. Materials, which may incorporate photopolymer and wax, are depos- ited through the orifices of an inkjet printer. Powder Bed Fusion: This AM process uses thermal energy to selectively fuse areas of a powder bed. The energy source—a laser or electron beam—focuses to fuse each layer of powder that builds up the part. Sheet Lamination: In this process, sheets of material are stacked and bonded across their planar interfaces to form an object. Vat Photopolymerization: A specialized liquid photopolymer in a vat is selectively cured by light-activated polymerization. The light source may be a continuously moving laser or a digital array of simultaneous light projections. Reviewing this list, it can be seen that the material feed stock may be a liquid, powder, wire or filament, or sheet. In all cases, AM machines selectively convert material in one of these simple forms into a complex, 3D part, generally by adding layer upon layer. Additive Manufacturing Workflow.— Additive manufacturing processes, involving metals or other materials, follow the same basic workflow, as summarized in Table 8. A three-dimensional (3D) model is created using CAD software. (For more on this topic, see CAD/CAM on page 1390.) This model is converted into a surface model made up of interfacing triangles, resulting in an STL (Standard Tessellation Language) file. The build file is then sliced into two-dimensional (2D) layers. Each layer profile is used to control the AM machine’s motions, which add material in that profile and build the 3D model ( Table 8). Typically, the layer thickness is 0.004 in. (0.102 mm), but it can range from 0.0007 to 0.010 in. (0.0178 to 0.254 mm). AM processes have been adapted for commercial use under various trade names and are used to produce parts in metals, polymers, ceramics, and various composites. For example, the section Additive Manufacturing Plastics on page 611 describes processes used for plastics: stereolithography (SLA), which is defined by ASTM as vat polymeriza - tion ; selective laser sintering (SLS), which is an example of powder bed fusion ; fused deposition modeling (FDM), which is an example of extrusion ; binder jetting (BJ); and material jetting (MJ). This section discusses AM processes used to produce metal parts: powder bed fusion (PBF), including selective laser melting (SLM) and electron beam melting (EBM), binder jetting (BJ), and powder- and wire-fed directed energy deposition (DED).
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