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Solid‐state AM is a class of processes that use friction and diffusion‐based mechanisms to produce mechanical bonding without melting. Solid‐state processes like ultrasonic welding and friction‐stir welding have established applications [71, 72]. They can also be converted into AM processes by integrating the weld head onto a three or five‐axis robot coupled with a CNC mill to make a hybrid AM system. Three such processes are Ultrasonic Additive Manufacturing (UAM), Friction Stir Additive Manufacturing [73], and Cold Spray Additive Manufacturing [74]. FAM and CSAM are relatively new technologies with few research papers in literature exploring hybrid functional components. Yin, Yan, et al. [75] showcased the use of cold spray as a hybrid AM process applied to L‐PBF components. UAM, on the other hand, is a well‐established and often ignored AM technology that can make unique functional parts. The rest of this section is hence dedicated to functional components made by UAM.

Ultrasonic Additive Manufacturing (UAM) is a solid‐state joining manufacturing process that is commonly used in conjunction with a CNC mill to make functional metal components [76]. UAM offers advantages in material properties as compared to traditional metal joining and forming technologies. It uses normal force coupled with low frequency mechanical ultrasonic vibrations to create a solid‐state weld between a thin foil and an existing substrate. Ultrasonic welding is an established industrial bonding process for plastics and soft metals. UAM is a hybrid AM process which is essentially layer‐by‐layer ultrasonic welding combined with CNC machining after each layer, hence providing freeform fabrication capability [77]. Given the right bonding parameters, completely solid‐state metallurgically bonded welds can be fabricated. The quality of UAM components depends on several process parameters, including normal force, vibration amplitude, and speed of bonding along with geometrical and environmental factors. For many years, the material systems available to be bonded by the UAM process were limited to softer Aluminum alloys (Al 3003) due to the high energy requirement for other engineering materials. The Fabrisonic UAM systems overcome this hurdle by using a high‐power transducer and load cell, which makes it feasible to build components from Copper, Nickel, and Iron‐based alloy systems.

UAM involves high‐speed relative motion between foils to be joined. Several researchers have studied the mechanism of bonding in ultrasonic metal welding [89]. The bonding process is a function of the three input parameters; the force applied, the velocity of bonding, and the vibration amplitude. It is also dependent on surface roughness, temperature, base plate characteristics, part geometry, and build height, among other factors. The bonding process can be separated into (a) volumetric bonding and (b) surface bonding effects. Volumetric bonding effects include elastic and plastic deformation enhanced through reduced yield stress due to acoustic and thermal softening. Surface bonding effects include interfacial friction and shearing, which break up the oxide layers and bring more nascent metal‐to‐metal contact. Bond formation by ultrasonic welding requires two conditions to be fulfilled, (a) the generation of clean surfaces with no barrier layers at the atomic scale and (b) direct contact between these clean surfaces. Janaki Ram, Yang, and Stucker [90] suggested that the surface‐oxide layers are broken up by the vibrations and are displaced within the vicinity of the interface region. A schematic of the UAM process is shown in the Figure 1.5 with the weld head mounted on a three‐axis stage and can be switched with CNC tools.

The additive/subtractive nature and the room temperature processing capability of the process give rise to capabilities such as completely enclosed cooling channels, smart parts with embedded sensors, and composite materials that cannot be produced traditionally. Alloy systems that cannot be typically fusion welded together can be joined, making multi‐material components that do not suffer from common weld defects like embrittlement and solidification cracking. High power UAM can make fully dense bonds within several Al, Cu, Ni, and Fe alloys. However, UAM bonds are prone to delamination and must be treated as anisotropic composites with lower modulus along the build direction. It is also difficult to make high aspect ratio structures with UAM due to the inherent vibration during the process. However, these characteristics could be advantageous for several applications like heat transfer, impact toughness, dissimilar welding, and, most importantly, embedded sensors. An overview of functional UAM applications is given in Table 1.2. UAM is an often‐overlooked metal AM process, which has the unique capability to make functional components despite its limitations. Hence, the ongoing research on UAM is bound to find its way into high‐impact and niche applications.

Figure 1.5 Schematic of the UAM process. FBG's are high‐temperature optical fiber Bragg grating (FBG) strain sensors.

Source: A part of the figure is adapted in accordance with the Creative Commons license and is a copyright of Fabrisonic LLC [91]. © 2020 Fabrisonic LLC.

Table 1.2 Overview of functional UAM applications.

Materials	Type‐II	Function	References
Al/Steel joints		Al/Fe joints with applications in heat exchangers	[78]
Al/Ti	Strong Al/Ti joints which are difficult with traditional joining methods	[79]
Al/Fe/Ni/Ta/Cu	Demonstrated the multi‐material capability with a wide range of metals	[80]
Al/Cu	Interlayers allow the manufacture of traditionally unweldable alloys	[81]
	Type‐III	Metal matrix composites
Al/Carbon fiber		High impact toughness composite	[82]
Al/SiC/TiC	Fully embedded fibers can only be made by UAM	[83]
Al/Metpreg	Embedded Metpreg MMC makes lightweight and high strength composites	[84]
	Type‐IV	Integrated sensors
NiTi, FeGa		Shape memory alloys for sensing applications	[85]
Al6061 + Ta/Eu2O3	Fully enclosed neutron absorber components for Oakridge National Lab	[86]
Al matrix + optical fiber sensors	Embedded optical fiber strain sensors for high‐temperature sensing (up to 500 °C)	[87]
Embed electronics	Embedded surface mount resistor	[88]