Hence, the trend of increasing or decreasing depended on which effect was higher between hardening behavior through additional work hardening and softening through adiabatic heating, which induced the dynamic recovery process. Sriraman et al. reported peaks of transient temperature exist at the bottom interfaces, while another layer was deposited during VHP-UAM (Ref. 20). This means that although the peak temperatures at the interface may not be as high (in the order of 100°–150°C), and the duration of those peak temperatures may be relatively short (in the order of 50 ms), changes in hardness was possible as a result of changes in microstructure and reduction in foil thickness. When larger shear deformation occurred at the interface as a result of higher vibration amplitude, there was more accumulated dislocation density and higher stored energy in the foil. The higher stored energy together with increasing temperature during VHP-UAM process enhanced the driving force for dynamic recovery and/or dynamic recrystallization to occur, and thus resulted in decreased bulk foil hardness. It was also noted that the increased weld speed during metallic bonding above the 50th layer resulted in less heat dissipation time. This can also give rise in the degree of softening effect in the bottom layers of VHPUAM samples fabricated from the SL7200 machine. Sriraman et al. also reported the decrease in peak temperatures at the bonded interfaces at taller build height or when more layers were deposited due to lower ultrasonic energy transmitted into the sample (Ref. 20). Their finding agrees well with the current result, where the ultrasonic power decreases with increasing build height as less ultrasonic power was supplied by the VHP-UAM machine while metallic bonding an additional layer as the sample gets taller. This result also agreed with the observed phenomenon of a lack of bonding with an increase in specimen height, as demonstrated by Gilbert (Ref. 24). However, this hypothesis is yet to be proven. It is also possible that the vibration of the whole dynamic system during VHP-UAM, while the sonotrode is in contact with the sample, could generate different vibration modes that affect the actual power drawn. For example, the vibration modes and the stick or slip between sonotrode and top foil surface may play a crucial role in determining the actual amount of ultrasonic energy dissipated and going into the bonded interfaces (Refs. 25–27). The different vibration modes and magnitudes in the TB machine and the SL7200 machine may yield the answer to why the hardness and power correlations are unique for each VHP-UAM system and sample size. Conclusions Increases in vibration amplitude and normal force resulted in large deformation in the bulk of VHP-UAM samples, as seen in a large reduction in thickness of VHP-UAM samples processed at larger vibration amplitudes. The hardness in taller VHP-UAM samples showed a trend of lower hardness in bottom layers and higher hardness in top layers, which was largely due to the accumulative effects of thermo-mechanical cycles of plastic deformation and heating generated at each interface. After heat treatment at 343ºC for 2 h, all layers of Al3003-H18 foils in VHP-UAM samples reached the same final hardness (near 40 VHN) and were the same as the heat-treated original foil, implying the onset of recrystallization occurred. As the build got taller, less power was used to provide specific amplitude and thus less change in stored energy, microstructure, and hardness compared to the original as-received Al3003-H18 foil. 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Welding Journal | June 2016
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