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Herein, the role of thermal softening in ultrasonic powder compaction is assessed by comparing the densification behaviors of nominally pure Cu and a thermally stable CuTa alloy. These materials have similar thermal properties, but pure Cu softens at much lower temperatures than does the CuTa alloy. Using a specialized ultrasonic powder compaction setup, in situ measurements of relative density, sonotrode power consumption, and temperature are collected, which together provide a time‐dependent geometric hardening parameter that reflects the structure of the compact. The geometric hardening data for the pure Cu powder reveal three distinct stages of densification: an initial particle rearrangement stage; a jamming transition where strong junctions develop between particles; and a final stage characterized by compatible plastic deformation. By contrast, the geometric hardening data for the thermally stable CuTa powder show that it remains a weak fluidized granular medium, despite experiencing higher normal pressures, oscillation amplitudes, and temperatures. The contrasting behaviors of the Cu and the CuTa powders suggest that a thermally activated jamming transition drives interparticle junction growth and densification in ultrasonic powder compaction.

Ultrasonic additive manufacturing has been used to fabricate laminated composites of commercially pure aluminum and a nanocrystalline nickel–cobalt (nc-NiCo) alloy. The nc-NiCo alloy would not weld to itself but readily welded to aluminum. Thus, by alternating between foils of nc-NiCo and Al, we achieved multi-material laminates with strong interlayer bonding. Electron microscopy showed that the nanoscale grain structure of the nc-NiCo was preserved during deposition and that the nc-NiCo/Al weld interface was decorated with comminuted surface oxides as well as Al–Ni–Co intermetallics. These findings are considered in light of process models of junction growth, interdiffusion, and grain growth, which together reveal how the different pressure- and temperature dependences of these phenomena give rise to a range of processing conditions that maximize bonding while minimizing coarsening and intermetallic formation. This analysis quantitatively demonstrates that using a soft, low melting point interlayer material decouples junction growth at the weld interface from grain growth in the nc-NiCo, expanding the range of optimal processing conditions. Graphical abstract

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Nanocrystalline alloys have high hardness values [1], excellent wear resistance [2], and other unique mechanical properties that make them ideal for structural applications [3]. However, because these materials have highly non-equilibrium microstructures that tend to coarsen at elevated temperatures [4], and because most welding processes involve a large thermal excursion, it is challenging to retain the structure of nanocrystalline materials when they are being joined. This inability to weld nanocrystalline metals without affecting their structure is a major obstacle that has prevented their widespread use.In order to overcome the challenges with welding nanocrystalline materials, we have explored joining nanostructured foils using a low-temperature, solid-phase welding process termed ultrasonic welding. Unlike traditional fusion-based welding processes which join workpieces by melting them, ultrasonic welding bonds workpieces using only plastic deformation. During an ultrasonic weld, there is some frictional heating at the weld interface. However a typical weld lasts less than 1s and the maximum temperature in the weld zone is normally less than 50% of the melting temperature of feedstock. Thus, it is expected that ultrasonic welding can preserve the grain structure of nanocrystalline feedstock.In this study we used ultrasonic welding to join Cu foils uniformly coated with a nanocrystalline Ag alloy (Ag-5W, at%). Next, we used a combination of 30kV transmission Kikuchi Diffraction (tKD), 300kV scanning transmission electron microscopy (STEM) secondary electron (SE), bright field (BF), annular dark field (ADF) imaging modes and X-ray mapping to compare the structure of the as-received material with that of the welded material. By correlating these electron microscopy techniques, we clarified the effects of ultrasonic welding on the grain size distribution, the crystallographic texture, and the solute distribution. Figure 1A is a STEM SE micrograph of the as-received Ag foil. Stereological measurements on the asreceived foils gave a mean circular equivalent grain size of 18 nm. Further, EDS measurements showed that the W and O were distributed uniformly throughout the Ag. The STEM SE and ADF micrographs, Fig. 1B and Fig. 1C respectively, of the welded material shown revealed dramatic effects of the welding process on foil nanostructure. In contrast with the equiaxed grains seen in the as-received material, grains in the welded material are clearly elongated, with their major axes roughly parallel to the sonotrode oscillation direction. Also evident in Figure 1B are spherical particles with a mean diameter of 90 nm. X-ray maps of these particles, shown in Figure 2, determined the presence of W and O, and once quantified, indicated they were WO2 dispersoids. Since these particles were not present in the asreceived material, they must have precipitated during the welding process. The same FIB lamella was used for tKD and STEM/EDS and is pictured in Figure 3A. The inverse pole figure (IPF) of Figure 3B shows relativ...

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