1
Introduction and Overview

Hang Z. Yu1, Nihan Tuncer2, and Zhili Feng3

1Virginia Tech, 445 Old Turner Street, Blacksburg, VA, 24061, USA

2Desktop Metal Inc., 63 3rd Ave, Burlington, MA, 01803, USA

3Oak Ridge National Laboratory, 1 Bethel Valley Rd, Oak Ridge, TN, 37831, USA

Additive manufacturing generally denotes scalable fabrication (printing) of 3D components and structures for industrial production. Employing a layer-by-layer or voxel-by-voxel approach, additive manufacturing has started to shift the manufacturing paradigm and revolutionize the way components are produced. It not only offers unparalleled design freedom and efficiency for creating complex geometries, but also opens the door to the production of lighter, stronger, multifunctional, and multimaterial parts [1]. Its versatility knows almost no bounds; nearly all types of materials can be transformed into intricate 3D components through additive manufacturing, including polymers, ceramics, metals, composites, and even natural materials. With the vast global market of metal component production and the extensive use of metallic materials in diverse industrial sectors, there has been a surge in interest of metal additive manufacturing particularly over the past decade [2-4].

Metal additive manufacturing approaches can come in two key forms: fusion-based (i.e., beam-based) and solid-state (i.e., nonbeam-based) methods, both with their distinctive advantages. The former fundamentally relies on selective melting and rapid solidification to progressively build a structure, while the latter harnesses a high strain rate, extensive plastic deformation, or thermally induced atomic diffusion to metallurgically bond the material to build a structure. Fusion-based approaches, including powder bed fusion (e.g., selective laser melting [SLM] and e-beam melting [EBM]) and directed energy deposition (DED) (e.g., laser engineered net shaping [LENS] and wire arc additive manufacturing [WAAM]) have been the primary focus of industry and academia at the time of writing. This is not surprising, as much of the processes and equipment are based on similar fusion-based welding processes widely applied in the industry for decades. Similar to casting [5] and fusion welding [6], both of which are bulk-scale melting-solidification manufacturing processes, fusion-based additive manufacturing is challenged by porosity, residual stress, and hot cracking [7]. Compared to casting, the additive nature exacerbates these issues because of the small molten pool size, large thermal gradient, and rapid cooling rates. Additionally, epitaxial solidification leads to the natural formation of textured, columnar grain structures along the build direction, presenting a hurdle for microstructure and isotropy control [8]. These issues also limit melt-based methods to weldable alloys.

These critical issues stem from the melting and solidification nature of fusion-based additive manufacturing and can be avoided if melting is not present in the process. This motivates the development of a series of emerging nonbeam-based, solid-state processes for metal additive manufacturing - which is the focus of this book. The cutting-edge solid-state technologies explored in this book encompass cold spray additive manufacturing (CSAM), additive friction stir deposition (AFSD), ultrasonic additive manufacturing (UAM), and sintering-based processes like binder jetting additive manufacturing (BJAM) and material extrusion-enabled metal additive manufacturing (MEAM).

This relatively new field of manufacturing technologies is continuing to develop at a fast pace along with a growing wealth of research articles and white papers. The aim of this book is to present the principles and effects of the physical phenomena that each solid-state additive manufacturing method is built upon, as well as an in-depth picture of the process fundamentals, the resulting microstructures and properties, and the key industrial applications. Starting with an overview and historical perspective of metal additive manufacturing, this chapter proceeds to offer frameworks for categorizing solid-state additive manufacturing methods based on bonding mechanisms and relationship between building and consolidation. It then discusses the potential and limitations of nonbeam-based, solid-state metal additive manufacturing methods, which are implemented through deformation-based or sintering-based approaches. Furthermore, the chapter outlines the structure of the book, providing a glimpse of the topics of all the following chapters.

1.1 Overview and History of Metal Additive Manufacturing

Offering a "disruptive" concept that enables greater design freedom, rapid prototyping, and the production of complex geometries that were previously unachievable, metal additive manufacturing has enormous potential for enhancing performance such as strength and durability, weight and waste reduction, customization, as well as on-demand production and supply chain risk reduction. It has found applications in aerospace, space, automotive, defense, healthcare, and many other industries, driving innovation and reshaping the manufacturing landscape. Based on different material feeding and bonding mechanisms, metal additive manufacturing can be implemented by SLM, selective EBM, LENS, WAAM, CSAM, BJAM, UAM, and AFSD. Depending on the process, the feedstock can be in the form of powder, wire, sheet/foil, and solid bar. The first four technologies are based on melting and rapid solidification, and are thus termed "fusion-based" or "beam-based." The last four are based on solid-state processes without melting; they are the focus of this book.

Figure 1.1 A brief history of metal additive manufacturing development over the last 40 years.

As illustrated in Figure 1.1, the history of metal additive manufacturing dates to the 1980s when additive manufacturing, in general, was in its early stages. Similar to the case with other technologies, different terminologies were invented and used for different additive manufacturing processes as they developed. Selective laser sintering (SLS) was patented by Carl Deckard in 1986 [9], the first 3D printed parts were demonstrated by Manriquez-Frayre and Bourell in 1990 [10], and Electro Optical Systems (EOS) introduced its initial SLS machine in 1995. On the other hand, the first SLM patent was issued in 1995 by the Fraunhofer Institute Institut für Lasertechnik (ILT) in Germany, eventually leading to SLM Solutions Gesellschaft mit beschränkter Haftung (GmbH) in the early 2000s [11]. SLM or SLS falls under the category of powder bed fusion additive manufacturing.

Another significant technology within the powder bed fusion category is selective EBM, patented by Larson in 1993 [12]. In 2002, the first commercial EBM machine was launched by Arcam, which was later acquired by General Electric (GE) in 2016. Enabling the fabrication of complex geometries with high spatial resolution, powder bed fusion has emerged as one of the leading metal additive manufacturing technologies today.

LENS represents another important example that leverages high-energy laser beam for metal additive manufacturing [13]. LENS involves melting and fusing nozzle-delivered metal powder onto a substrate in a layer-by-layer fashion to create intricate 3D components. The technology was patented by Sandia National Laboratories in 1994 and later commercialized by Optomec in the early 2000s. LENS belongs to the category of DED, where the material is fed in powder form.

Another notable technology in this category is WAAM. The roots of WAAM can be traced back to the 1920s when Baker proposed using an electric arc and filler wires to deposit metal ornaments [14]. In the welding industry, arc welding, laser welding, and electron-beam welding are widely used for cladding of large-scale structures and rebuild of aircraft turbine rotor tips. They are early on primitive WAAM. In recent years, advancements in robotics, sensors, and control systems have propelled the progress of WAAM technology. Precise control of welding parameters and robotic movement has improved accuracy and repeatability, not to mention the high build rate and excellent scalability offered by WAAM.

Now let us briefly review the history of solid-state metal additive manufacturing processes, wherein the feedstock is not melted. Our first focus is on cold spray, a technology with a long history dating back to the early twentieth century. The modern "cold spray" phenomenon was discovered by Papyrin and Alkhimov in the 1980s [15, 16]. Subsequently, in 1994, the National Center for Manufacturing Sciences consortium, including companies like Ford Motor Company, GE Aircraft Engines, General Motors Corporation, the Naval Aviation Depot, and...