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For almost 100 years the ASM Handbook has captured the ongoing growth of applied knowledge covering the complete array of industrial materials and manufacturing processes. Within this archive, a major theme is the progressive development of materials having ever greater strength and heat resistance, primarily to meet performance objectives of transportation and defense applications. Unfortunately, the characteristics of such materials present challenges to the principal shaping processes of forming and material removal in reaching the complex shapes required. This, in turn, led to a parallel trend toward net shape to reduce or eliminate the negative aspects of material removal.

In more recent times, however, a completely different approach to shape making has evolved in which basic materials in liquid or particulate form, as well as filament or sheet form, are assembled point by point or layer by layer into the objective shape. Using the pinpoint accuracy of a laser, miniscule droplets from a printhead, or extrusion of material through a narrow nozzle, a variety of clever mechanisms have been devised to carry out these high-resolution building processes, now collectively known as additive manufacturing (AM), and frequently referred to as 3D printing. The current and rapidly expanding importance of AM merits its capture within this, ASM Handbook, Volume 24, Additive Manufacturing Processes. As a starting point, the first division presents an overview of the subject as well as deep insights into its historical development, authored by some of the key participants in that history as they trace the evolution of AM from its pre-computer roots to early commercialization of (largely) rapid prototyping machines, to modern serious tools for production of parts from all material classes.

A primary result of these newfound processes is the capability to produce shapes of greater complexity and with more refined geometric detail than can be obtained by conventional processes covered in previous ASM Handbook volumes. In fact, such capabilities enable designers and manufacturers to think beyond net shape and toward optimum shape – the placement of material only where it is needed to carry out the required transmission of stress, temperature, or electromagnetic fields. In addition, thermo-fluid management systems, such as heat exchangers and molding tools, can incorporate non-round and non-straight internal channels for enhanced efficiency. A further advantage of AM’s geometric flexibility is the combination of multiple parts into one component, eliminating assembly operations as well as individual part tooling and inventory. One highly publicized example involves the integration of some 20 parts into one fuel injection nozzle for aircraft turbine engines. To illustrate the advanced industrial development of AM, this component has been in mass production for more than a year at this writing.

Another exciting opportunity afforded by AM is modification of a material’s properties. The introduction of engineered porosity (i.e., printing material around void spaces) and lattice structures within the boundaries of the part effectively reduces its density, which is of great value in any transportation related application. The same approach can be used to spatially modify the localized density, strength, and thermal properties of a part, enabling functionally gradient materials to accommodate different needs in different locations of a part or component. In an advanced form, AM enables spatial variation of properties by building the parts with different materials point-to-point, or by varying process parameters to accomplish different microstructures within the same part.

This wide latitude in shape, structure, and compositional control has injected a spirit of excitement in the materials, design, and manufacturing communities. Materials science and engineering has a new field in which to apply the basic concepts of materials structure through advanced tools for material characterization. Likewise, designers now wander into a new world of possibilities opened by the seemingly limitless geometric flexibility of AM. Manufacturers can now consider a new array of development and production processes with potentially more efficient materials use, reduced time to market, and greater performance.

This Volume of the ASM Handbook series seeks to promote the excitement of AM by providing the latest knowledge in materials, processes, and applications. Following the history and introductory division, the complete suite of materials and processes for polymers and ceramics are detailed in the next two divisions. The fourth division describes the metal AM processes, but begins with in-depth description of the production and characterization of metal powders; such information has an outsized effect on success or failure of metal AM processes. The fifth division describes AM processing of a wide variety of materials, illustrating differences in characteristics of metal alloys produced by AM processes in contrast to conventional processes. The final division covers direct-write processes, taking advantage of AM processes to combine materials and devices for multifunctional engineering applications. Additional volumes are planned covering design and applications for additive manufacturing.

We wish to acknowledge the immense efforts by the article authors and division editors to bring this volume together. Considerable time is required to complete these assignments which, unfortunately, come at a time when the talents of the authors are in high demand within this rapidly expanding and dynamic industry as it evolves continuously to new levels of achievement.

Howard Kuhn, FASM
David L. Bourell, FASM
William Frazier, FASM
Mohsen Seifi

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