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Ultrasonic additive manufacturing (UAM) is a solid-state additive manufacturing technique employing principles of ultrasonic welding coupled with mechanized tape layering to fabricate fully functional parts. However, parts fabricated using UAM often exhibit a reduction in strength levels when loaded normal to the welding interfaces (Z-direction). In this work, the effect of post-weld heat treatments (PWHT) on Al-6061 builds fabricated using the UAM process was explored aiming to improve the mechanical strength of the UAM builds. Tensile testing with digital image correlation (DIC) coupled with metallography along with multi-scale structure characterization (SEM-EBSD) was used to investigate and rationalize the mechanical performance of the UAM builds. It was established that PWHTs may improve the Z-strength level by the factor of ~33.5 (from ~46 MPa to 177 MPa). The improvements in the strength level were primarily aided by material aging and grain growth across the bond interface.

Embedding fiber optics in metal components could enable new capabilities such as active monitoring of spatially distributed strain. Ultrasonic additive manufacturing is a suitable technique for embedding fiber optics because it allows fibers to be embedded in metals without melting and without the use of epoxy. However, for harsh environments that could have high temperatures or high radiation doses, traditional polymer-coated fibers cannot survive for extended periods of time. This work demonstrates successful embedding of commercially available copper-, nickel-, and aluminum-coated fibers into aluminum without any observable damage to the fiber. Copper-coated fibers embedded in copper show adequate light transmission, although residual strain could not be resolved. With further processing improvements, fibers embedded in copper or other hightemperature materials could enable even higher temperature operation. Optical transmission and spatially distributed strain were measured in the fibers embedded in aluminum. Measurements were taken after embedding and during heating to temperatures greater than 500 °C. Within the embedded region, both the copper-and aluminum-coated fibers showed strain that matched the expected strain in the surrounding aluminum matrix during heating. This suggests a strong interfacial bond strength that exceeds the maximum estimated fiber strain of 1.2% (871 MPa tensile stress). This demonstration of embedded fibers that can survive high temperatures and remain bonded to the metal matrix is the first step toward embedded fiber optic sensors for harsh environment applications.

Purpose This paper aims to comprehensively review ultrasonic additive manufacturing (UAM) process history, technology advancements, application areas and research areas. UAM, a hybrid 3D metal printing technology, uses ultrasonic energy to produce metallurgical bonds between layers of metal foils near room temperature. No melting occurs in the process – it is a solid-state 3D metal printing technology. Design/methodology/approach The paper is formatted chronologically to help readers better distinguish advancements and changes in the UAM process through the years. Contributions and advancements are summarized by academic or research institution following this chronological format. Findings This paper summarizes key physics of the process, characterization methods, mechanical properties, past and active research areas, process limitations and application areas. Originality/value This paper reviews the UAM process for the first time.

Fiber optic sensors have long been considered for use in structural health monitoring of components because of their small size, high accuracy, and their ability to perform spatially distributed strain measurements when embedded within a component or on its surface. The typical method for transferring strain from the component to the fiber is to use an epoxy, which may not survive extended exposure to high temperatures, thus necessitating a more elaborate technique to embed the fibers. This work represents a first step towards using additive manufacturing techniques to embed fiber optic sensors on stainless steel components for hightemperature strain monitoring, including quantification of the differential thermal strains that develop between the fiber and the steel substrate. Copper-coated optical fibers were embedded in nickel layers on top of a stainless steel substrate using an ultrasonic additive manufacturing technique. The embedded fibers showed minimal signal attenuation and clear compressive strain after embedding. Heating the embedded fibers in steps to temperatures of 300 °C-400 °C resulted in measured strains with values between the expected thermal strain in stainless steel and nickel. Finite element simulations confirm the measured strain values and show that the thermal strain depends on the thickness of the nickel layers deposited on top of the stainless steel substrate. While the fibers failed before reaching temperatures of 500 °C, it is suspected that these failures occurred due to a combination of (1) the lack of strain relief, (2) the hightemperature oxidation of the fiber's copper coating, and (3) improper sizing of the machined channel in which the fiber is placed prior to embedding. If proper coating selection and sizing of the channel can prevent the failures observed in this work, the next step would be to monitor strain during mechanical loading at high temperatures.