is three times larger in VHP-UAM (9 kW maximum) than in UAM and is capable of producing larger normal force (15 kN maximum) and larger vibration amplitude (52 μm maximum). Two 4.5-kW transducers instead of one transducer in UAM are combined in a push-pull configuration to produce maximum power of 9 kW, as shown in Fig. 1A. Several researchers have been investigating the relationship between the process parameters and the bond quality of the UAM parts (Refs. 2, 10–14). Major process parameters studied include normal force, vibration amplitude, and weld speed. The metallic bond quality is measured in terms of linear weld density (the ratio of bonded area over the entire interface) and bond strength. It was found both linear weld density and strength increase with higher normal force and higher vibration amplitude, and decrease with higher weld speed (Refs. 13, 15). However, the weld speed should not be so low as to affect the productivity rate and cause localized melting or sticking of the foil material on the sonotrode surface. This leads to equipment downtime because the sonotrode needs to be cleaned and in some cases resurfaced (Ref. 10). While most researchers focus on improving the bond quality, there are limited works and knowledge regarding the change in bulk properties, such as hardness of the foils after UAM and VHP-UAM. Kong et al. (Ref. 16) was the first to report the change in microhardness at the weld interface of aluminum 6061 UAM foils when different levels of normal force, vibration amplitude, and weld speed were used. Their results showed increasing hardness in the foil processed with higher normal force, larger vibration amplitude, and lower weld speed (Ref. 16). It was also A B found that when lower normal force was applied, the hardness at the interfaces of UAM foils was smaller than the hardness of the original foil, while the interface hardness was higher when larger normal force was used. However, this work did not measure the change in hardness farther away from the interface, i.e., in the bulk region of the UAM foil. Li and Soar (Ref. 17) performed nanoindentation to obtain the hardness values across the foil thicknesses of UAM samples made from aluminum 3003-O foils. It was found that plastic deformation during UAM increased the hardness of aluminum 3003-O UAM foils compared to the original hardness in the as-received condition. Schick et al. (Ref. 2) also reported increased hardness in the bulk of Al3003-H18 foil due to UAM processing. Interestingly, the result was opposite in VHP-UAM, where Sriraman et al. (Ref. 4) reported decreased hardness in hard-temper copper C11000 foil after VHP-UAM. It was proposed that dynamic recrystallization and dynamic recovery as a result of tempera- WELDING RESEARCH 186-s WELDING JOURNAL / JUNE 2016, VOL. 95 Fig. 2 — Optical micrographs of asprocessed Al3003H18 VHPUAM samples along NDTD planes. Fig. 1 — Schematic diagram of very highpower ultrasonic additive manufacturing (VHPUAM). A — Schematic diagram of VHPUAM process (courtesy of Fabrisonic LLC); B — schematic diagram of VHPUAM build construction illustrating different layers and interfaces with Vicker hardness indent in the bulk of each layer. Table 1 — List of Sample ID with Number of Layers and Sets of Vibration Amplitude and Normal Force Used to Fabricate All Al3003H18 VHPUAM Samples Sample ID Number of Layers Vibration Amplitude (m) Normal Force (N) TB10285340 10 28 5340 TB10384000 10 38 4000 TB10388000 10 38 8000 SL66285340 66 28 5340 SL80344000 80 34 4000 SL80345340 80 34 5340
Welding Journal | June 2016
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