Additive Friction Stir Deposition

By Hang Z. Yu

Additive Friction Stir Deposition is a comprehensive summary of the state-of-the-art understanding on this emerging solid-state additive manufacturing technology. Sections cover additive friction stir deposition, encompassing advances in processing science, metallurgical science and innovative applications. The book presents a clear description of underlying physical phenomena, shows how the process determines the printing quality, covers resultant microstructure and properties in the as-printed state, highlights its key capabilities and limitations, and explores niche applications in repair, cladding and multi-material 3D printing.

Serving as an educational and research guide, this book aims to provide a holistic picture of additive friction stir deposition-based solid-state additive manufacturing as well as a thorough comparison to conventional beam-based metal additive manufacturing, such as powder bed fusion and directed energy deposition.

• Provides a clear process description of additive friction stir deposition and highlights key capabilities
• Summarizes the current research and application of additive friction stir deposition, including material flow, microstructure evolution, repair and dissimilar material cladding
• Discusses future applications and areas of research for this technology

• Language: English
• Publisher: Elsevier Science
• Release date: Jul 19, 2022
• ISBN: 9780128243954

Hang Z. Yu

Dr. Hang Z. Yu is an Associate Professor of Materials Science and Engineering at Virginia Tech. He received his bachelor’s degree in physics from Peking University in 2007 and his PhD degree in materials science and engineering from Massachusetts Institute of Technology in 2013. Prof. Yu is the recipient of DARPA (Defense Advanced Research Projects Agency) Young Faculty Award. At Virginia Tech, the primary interest of Prof. Yu’s research group lies in manufacturing science, with an emphasis on advanced materials processing and Industry 4.0. As a research pioneer of additive friction stir deposition, Prof. Yu is exploring multiple facets of the process, including integration of in situ monitoring and physics simulation for process control (temperature, force and torque, material flow, and distortion), design and synthesis of hybrid materials with innovative 3D internal structures, as well as use of the technology for structural repair, selective-area cladding, and materials recycling and upcycling.

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Chapter 1

Introduction

Abstract

This chapter provides an overview of the subject of this book, additive friction stir deposition. We begin by categorizing common additive manufacturing technologies and analyzing their feasibility for printing metals. We contrast solid-state additive technologies with conventional fusion-based approaches, such as powder bed fusion and directed energy deposition, while discussing their benefits and limitations. Within solid-state additive manufacturing technologies, we introduce the principle of additive friction stir deposition and compare its manufacturing capability with ultrasonic additive manufacturing and cold spray. A distinction is also drawn between additive friction stir deposition and friction stir additive manufacturing, which is a sheet lamination process. Finally, we discuss the organization of the book and provide a brief overview of the following chapters, which cover the process fundamentals, material flow, microstructure evolution, tooling, metal matrix composites, mechanical properties, niche applications, and future perspectives.

Keywords

Metal additive manufacturing; fusion-based additive manufacturing; solid-state additive manufacturing; powder bed fusion; directed energy deposition; additive friction stir deposition; cold spray; ultrasonic additive manufacturing; sintering-based metal additive manufacturing

The beginning is the most important part of the work.

― Plato

Shifting the manufacturing paradigm, additive manufacturing is the industrial production name for 3D printing that enables the creation of lighter, stronger, multifunctional, and multimaterial components in a layer-by-layer or voxel-by-voxel fashion. Almost all types of materials can be additively manufactured into 3D components, including polymers, ceramics, metals, composites, and even natural materials. Among these, metal additive manufacturing is of special industrial interest due to the widespread usage of metallic materials, including structural components, protective coatings, heat exchangers, and conducting wires. Like traditional metal manufacturing, which involves casting and forging, metal additive manufacturing can be implemented by fusion-based as well as solid-state approaches (Yu and Mishra, 2021). The former relies on melting and solidification, whereas the latter relies on plastic deformation or sintering. In this book, our focus is on an emerging solid-state metal additive process, additive friction stir deposition, which leverages high-temperature severe plastic deformation to enable location-specific metal deposition and printing.

This chapter introduces additive friction stir deposition and analyzes its place in the metal additive manufacturing spectrum. We begin with the classification of additive manufacturing technologies and the assessment of their suitability for printing metals. A distinction is drawn between the fusion-based and solid-state additive processes, wherein the benefits and limitations of each type are considered. Within the solid-state additive technologies, we introduce the principle of additive friction stir deposition and contrast its manufacturing capabilities with ultrasonic additive manufacturing and cold spray. We also compare additive friction stir deposition with another friction stir-derived technology, friction stir additive manufacturing, which is a sheet lamination process. Finally, we discuss the organization of the book and provide a brief overview of the chapters that follow—covering process fundamentals, material flow, microstructure evolution, tooling, metal matrix composites, mechanical properties, niche applications, and future perspectives.

1.1 Additive manufacturing for metals

As illustrated in Fig. 1.1, common additive manufacturing technologies can be categorized as follows (Gibson et al., 2015):

• Powder bed fusion, in which an energy beam is used to melt powder in a bed.

• Directed energy deposition, in which an energy beam and the feedstock are supplied simultaneously in the free space.

• Binder jetting, in which a polymeric binder is selectively deposited to glue powder together.

• Material extrusion or fused deposition modeling, in which a polymer is melted and extruded to the locations of interest.

• Material jetting, in which material (usually a polymer) is sprayed in droplet form.

• Vat photopolymerization or stereolithography, which leverages the interactions between UV light and photo-curable polymer.

• Sheet lamination, a cut-and-stack approach.

Figure 1.1 Categorization of additive technologies, including powder bed fusion, directed energy deposition, binder jetting, fused deposition modeling, material jetting, stereolithography, and sheet lamination. Reprinted with permission from Gibson, I., Rosen, D.W., Stucker, B., 2015. Additive Manufacturing Technologies: Rapid Prototyping to Direct Digital Manufacturing. Springer US.

For metal additive manufacturing, currently, the most popular approaches are powder bed fusion and directed energy deposition (Frazier, 2014). In powder bed fusion, high-quality metal particles are thinly laid out and subsequently selectively melted using a high-energy laser beam or electron beam. Upon cooling, a pattern of the solidified material is formed in the current layer (DebRoy et al., 2018). By repeating the powder deposition and selective melting and solidification in a layer-by-layer fashion, a specified 3D component can be built based on the G-code. In directed energy deposition, the component is built up in free space and the material (often powder or wire) and energy (laser or arc) are delivered simultaneously to the location of interest. The delivered material first melts and then solidifies upon cooling, forming metallurgical bonds with the preexisting material (Carroll et al., 2015). The key hardware is the powder feeding system, which can provide a flow of small quantities of powder with high precision.

Over the years, different terms have been used to refer to the same additive process. Selective laser melting (or sintering), direct metal laser sintering, and electron beam melting all refer to powder bed fusion. Laser metal deposition and laser engineered net shaping (i.e., LENS) are directed energy deposition processes. For laser-based powder bed fusion (Fig. 1.2), the key printer manufacturers include EOS, Renishaw, SLM Solutions, and Concept Laser (now part of GE Additive) (Gusarov et al., 2018). For electron beam-based powder bed fusion, Arcam AB (also part of GE Additive) has been dominating the manufacturing market. For directed energy deposition, Optomec is a notable industry leader and a pioneer of the LENS technology.

Figure 1.2 Example printers for powder bed fusion, with the manufacturers including EOS, Phenix Systems, TRUMPF, SLM Solutions, Concept Laser, and Renishaw. Reprinted with permission from Gusarov, A.V., Grigoriev, S.N., Volosova, M.A., Melnik, Y.A., Laskin, A., Kotoban, D.V., et al., 2018. On productivity of laser additive manufacturing. Journal of Materials Processing Technology 261, 213–232.

Although powder bed fusion and directed energy deposition differ in geometrical configuration and material feeding, they are based on the same liquid bonding mechanism, which critically relies on melting and solidification of the feedstock. These two categories are thus termed fusion-based or beam-based additive manufacturing (Yu et al., 2018). Like other fusion-based metal processes such as casting and fusion welding, control of porosity, residual stress, and hot cracking is challenging in fusion-based additive manufacturing. These problems are exacerbated by the small molten pool size, large thermal gradient, and rapid cooling rate rooted in the additive nature. Because textured, columnar grain structures naturally form along the build direction, microstructure control has also remained a persistent challenge in powder bed fusion and directed energy deposition (Basak and Das, 2016). Furthermore, melting requires higher energy consumption and more carbon dioxide emission, making these processes unfavorable from an environmental perspective.

Then, the question is: can we print metals using other additive technologies in Fig. 1.1 while avoiding melting? Binder jetting—the technology originally termed 3D printing—is a versatile process that selectively deposits binders to glue powders together, forming a green body that can be made of metals, ceramics, or composites (Gonzalez et al., 2016). This process involves the selective removal of binders followed by sintering. Fused deposition modeling, materials jetting, and stereolithography are widely used for printing polymers. The former is powerful for printing thermoplastics, whereas the latter two are suitable for photo-curable polymers. In principle, metal additive manufacturing can be implemented using these polymer printing processes by mixing metallic micro- or nano-particles with the polymers (Liu et al., 2020). As a result, a metal–polymer composite is first formed, followed by selective removal of the polymers (e.g., by burn-out) and sintering of the metal component. All the aforementioned processes may be termed sintering-based metal additive manufacturing, which refers to indirect approaches that initially form a metal-based composite followed by extensive postprocessing.

The last category in Fig. 1.1 is sheet lamination, which refers to hybrid processes involving both additive and subtractive procedures. In this category, there is a direct approach for metal additive manufacturing: ultrasonic additive manufacturing (Hehr and Norfolk, 2020), which leverages high-frequency sound waves for interfacial cleaning and bonding, followed by selective CNC (computer numerical control) milling of the bonded foils. Undoubtedly, ultrasonic additive manufacturing provides a low-temperature route for metal additive manufacturing; however, it is not a freeform process and cannot be used for location-specific deposition, reinforcement, and repair.

Given the limitations of fusion-based, sintering-based, as well as sheet lamination-based metal additive manufacturing, we have a strong motivation to search for or even develop new solid-state metal additive processes that go beyond the conventional list in Fig. 1.1.

1.2 Solid-state metal additive manufacturing

It is interesting to note the linkage between welding and metal additive manufacturing, both of which rely on material bonding. For example, in the category of directed energy deposition, laser engineered net shaping is an advanced version of laser welding, while wire arc additive manufacturing follows a similar principle of arc welding. The innovation in additive manufacturing lies in material feeding and print path control. Another example is ultrasonic additive manufacturing, which is simply an extension of ultrasonic welding to include CNC milling for shape control. These examples are summarized in Fig. 1.3 (Hehr and Dapino, 2017; Hu et al., 2018).

Figure 1.3 The linkage between welding and metal additive manufacturing, showing examples of (A) laser engineered net shaping, (B) wire arc additive manufacturing, and (C) ultrasonic additive manufacturing. Reprinted with permission from Hehr, A., Dapino, M.J., 2017. Dynamics of ultrasonic additive manufacturing. Ultrasonics 73, 49–66; Hu, Y., Ning, F., Cong, W., Li, Y., Wang, X., Wang, H., 2018. Ultrasonic vibration-assisted laser engineering net shaping of ZrO2-Al2O3 bulk parts: Effects on crack suppression, microstructure, and mechanical properties. Ceramics International 44, 2752–2760.

Solid-state welding or joining processes, therefore, may provide new insights into solid-state metal additive manufacturing; some existing welding processes may be advanced for additive manufacturing. Along that line, a notable option is friction stir welding (Fig. 1.4A; Gemme et al., 2010), in which a rotating nonconsumable tool is used to join two facing workpieces by leveraging the frictional heating at the tool-workpiece interface (Mishra and Ma, 2005). During welding, the material softens and undergoes severe plastic deformation due to compression and shear. The extensive material flow and rotation can lead to significant material mixing across the original boundaries, resulting in high-quality joining of the two workpieces. This welding process has shown great success in joining low-temperature materials, such as Al, Mg, and Cu (Mishra et al., 2014). The next question is: can we have a solid-state additive technology that follows the same bonding mechanism as friction stir welding—as long as we can add an appropriate material feeding mechanism and enable automatic deposition path control?

Figure 1.4 Processes based on the friction stir principle: (A) friction stir welding and (B) additive friction stir deposition. (A) is reprinted with permission from Gemme, F., Verreman, Y., Dubourg, L., Jahazi, M., 2010. Numerical analysis of the dwell phase in friction stir welding and comparison with experimental data. Materials Science and Engineering: A 527, 4152–4160.

The answer is yes. The corresponding technology is essentially the focus of this book, additive friction stir deposition, which integrates the friction stir principle with a robust material feeding mechanism to enable location-specific deposition. As shown in Fig. 1.4B, the machine's mechanical setup appears to be similar to that of fused deposition modeling: the feed material is supplied through a channel inside the print head before it is deposited onto the substrate. However, in fused deposition modeling, thermoplastic printing is implemented simply by heating and compression, whereas in additive friction stir deposition, the friction stir principle is employed. This is because the deformation of metallic feed material and formation of material–substrate bonding necessitate a much more efficient thermomechanical processing strategy. During deposition, the feed material—which is usually in the form of a metal rod—is stirred, severely deformed, and mixed with the substrate, resulting in metallurgical bond formation across the original interface. With strong interfacial bonding, the in-plane motion of the print head relative to the substrate results in a material pattern controlled by the G-code in each layer. Through layer-by-layer deposition, 3D metal components are naturally formed. In additive friction stir deposition, the extensive flow of the feed material and its mixing with the substrate guarantee good material quality even in the as-printed state.

At the time of writing, additive friction stir deposition, ultrasonic additive manufacturing, and cold spray represent the three major solid-state metal additive manufacturing technologies; the latter was previously considered a coating process but has gradually gained more attention from the additive manufacturing community (Yin et al., 2018). All of these processes, as illustrated in Fig. 1.5 (Yu and Mishra, 2021), involve direct printing of metals without the need for extensive postprocessing, such as polymer burn-out and metal densification (e.g., by sintering or hot isostatic pressing). Both additive friction stir deposition and cold spray are freeform processes that enable location-specific deposition, reinforcement, and repair, whereas ultrasonic additive manufacturing is a sheet lamination process requiring machining. Although deformation bonding is the prevalent bonding mechanism, cold spray and ultrasonic additive manufacturing only involve local plastic deformation at the layer interface or particle contact (Tuncer and Bose, 2020). In contrast, additive friction stir deposition involves global deformation in that all the material voxels in the feed-rod undergo severe plastic deformation at high temperatures.

Figure 1.5 Illustration of three major solid-state metal additive processes, additive friction stir deposition, ultrasonic additive manufacturing, and cold spray. The feed material and final grain structure are included. Reprinted with permission from Yu, H.Z., Mishra, R.S., 2021. Additive friction stir deposition: a deformation processing route to metal additive manufacturing. Materials Research Letters 9, 71–83.

Thanks to the material flow and stress-state (i.e., compression and shear), additive friction stir deposition is capable of producing a fully-dense as-printed material. There is no need for postprocess annealing to eliminate the porosity, which is necessary for cold spray. In addition, the global deformation nature leads to a uniform, equiaxed, fine microstructure due to dynamic recrystallization. More importantly, the resulting mechanical properties can be comparable to wrought or forged alloys. This is a unique advantage that distinguishes additive friction stir deposition from other metal additive processes. In a certain sense, we may consider additive friction stir deposition to be a forging-based metal additive manufacturing approach.

1.3 Additive friction stir deposition

Fig. 1.6A illustrates the cross-section of the printing process by additive friction stir deposition. A metal rod rapidly rotates as it passes through the rotating print head, generating frictional heat as it contacts the substrate. The feed-rod then heats up, yields, and extrudes to fill the space between the substrate and the rotating print head. The deposited material goes through several deformation steps, including initial uniaxial compression (Fig. 1.6B), compression and shear below the rotating feed-rod (Fig. 1.6C), and final shear-dominated deformation below the rotating print head (Fig. 1.6D) (Griffiths et al., 2021).

Figure 1.6 Illustration of the deformation steps in additive friction stir deposition. Reprinted with permission from Griffiths, R.J., Garcia, D., Song, J., Vasudevan, V.K., Steiner, M.A., Cai, W., et al., 2021. Solid-state additive manufacturing of aluminum and copper using additive friction stir deposition: Process-microstructure linkages. Materialia 15, 100967.

In additive friction stir deposition, the material is characterized by a macroscopic shape change from a rod to a thin disk by extrusion, followed by rotation and flow under the rotating print head. The print head regulates the layer thickness by imposing a vertical constraint. Its rotation also smears and shears the deposited material more and leads to good print quality with no porosity. Note that the surface layers of the substrate also get heated up and plastically deformed, mixing with the feed material and forming strong interfacial bonds.

Unlike friction stir welding, which produces steep strain gradients and significant microstructural differences between the stir zone, thermomechanically-affected zone, and heat-affected zone, additive friction stir deposition produces relatively uniform deformation with minor microstructural variability across the thickness or width. Additive friction stir deposition should be distinguished from another friction stir-derived technology, friction stir additive manufacturing (Palanivel et al., 2015), which is a sheet lamination process that involves stacking multiple metal layers (with a thickness of a few millimeters) and joining them by friction stir welding, followed by machining. Welding is done on the top layer of sheets using a custom-designed pin that penetrates them vertically, rotates, and traverses to create a joining line throughout the overlapping sheets. This stacking, welding, and machining process can be repeated allowing for the fabrication of large components in 3D. Although both are based on the friction stir principle for material bonding, additive friction stir deposition enables selective-area cladding, repair, and local feature buildup, which are all challenging to implement using friction stir additive manufacturing. Fig. 1.7 illustrates and compares the two processes (Yu,