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] |
1.5 Hybrid AM Through Green Body Sintering
The indirect manufacture of metallic and ceramic components via AM employs a process chain by which a blended feedstock is used to build the component. This feedstock contains a powdered filler material and a matrix material that, during additive manufacture, gives the component its as‐built strength. This part is known as the green part and is the origin of a multistage process by which the binder matrix is removed. The powdered filler is sintered and densified to form the final component. The process chain can be enabled by a variety of AM technologies and with a variety of process chains for debinding, sintering, and densification. As such, this section deals with the three most common AM methods and the two most common sintering process chains.
1.5.1 Common AM Technologies for Green Body Manufacturing
The entry‐level technology by which a green body component is commonly manufactured is through filament extrusion [92]. There is a variety of feedstock for extrusion‐based AM systems that have been engineered for this purpose. For high throughput industrial application, binder jetting is the fastest build method of all industrially applicable manufacturing methods and is thus competitive in many cases to laser PBF. Since no melt pool and plasma is generated during manufacture, the general tolerances are superior to those of laser PBF. As the green body part shrinks up to 30% during sintering, tolerances are further enhanced. The final method for green body manufacture using AM is by employing vat photopolymerization, where the photopolymer acts as the matrix material and is blended with the powdered filler. This method has a very low throughput yet is still highly industrially relevant, given that vat photopolymerization exhibits a spatial resolution, one order of magnitude better than that of the former methods [93]. Whereas there
