A B foil in VHP-UAM samples. Therefore, with careful consideration between ultrasonic power and bulk foil hardness, it may be possible to use the power level in addition to other process parameters as a means to achieve the desired homogeneity or range of the bulk hardness or bulk material properties of VHP-UAM samples. Microstructure and Properties Correlation In order to understand the hardening and softening behavior of Al3003- H18 foils in VHP-UAM samples, it was important to analyze the linkage between processing and material properties by looking at the thermomechanical cycles in VHP-UAM. During VHP-UAM, the level of cyclic deformation increased with an increased number of welding passes or number of layers deposited on top. The amount of shear deformation felt at the interface also rose with increased vibration amplitude. Thus, the amount of plastic deformation and plastic heating at the interface was larger at a higher vibration amplitude. This was confirmed by higher thickness reduction of Al3003-H18 foil at a higher normal force and a higher vibration amplitude in samples TB- 10-28-8000 and SL-80- 34-5340. Figure 9 displays the schematic of the stored energy change during typical thermo-mechanical processing of Al3003. It is known that after rolling or cold working, the material possesses higher stored energy in the form of a larger volume of dislocations, grain boundaries, and smaller grain sizes, while upon hot working the material achieves an intermediate energy state (Ref. 21). During UAM and VHP-UAM, onset of dynamic recrystallization and dynamic recovery took place in the Al3003-H18 microstructure. Depending on whether the relative amount of additional stored energy through plastic deformation or the released energy via dynamic recovery and dynamic recrystallization, the final energy in the as-processed UAM or VHP-UAM condition may be larger or smaller than the initial energy state of as-received Al3003-H18 foil. Since UAM has less power than VHP-UAM and thus less energy during processing, the temperature rise and plastic deformation may not be high enough to produce extensive volume of dynamic recrystallization, the final stored energy of UAM was expected to be higher than that of VHP-UAM. Thus, the hardening could be referred to as the increase in stored energy in lower power UAM and the softening was referred to as the decrease in stored energy in higher power VHP-UAM as shown in Fig. 9B. The lower hardness results in this work supported this phenomena that WELDING RESEARCH when large vibration amplitude or larger power was used, the microstructure released the additional energy through dynamic recrystallization, dynamic recovery, and grain growth resulting in the final microstructure, which was different from the one processed with lower vibration amplitude or lower power. The effect of processing parameters on the change in microstructure is the subject of the ongoing research of the authors (Ref. 22). Figure 10 illustrates the microstructure of selected TB-10-28-5340 VHP-UAM samples in as-processed and heat-treated (343ºC for 2 h) conditions (Ref. 22). The onset of dynamic recrystallization can be clearly seen in the interface regions of the as-processed sample, especially at the interface of layer 8, where equiaxed grains are present and the color of the grains are brighter (grains are clearly defined and can be seen as white or brighter grains in image quality map). It was also noted that the bulk microstructure was mainly very small and elongated grains in the asprocessed condition turned into much larger grains after heat treatment at 343ºC for 2 h, which was similar to the UAM microstructures found by Schick (Ref. 23). This confirmed that upon heat treatment, the microstructure in the bulk region of Al3003-H18 VHP-UAM build processed at 28-μm vibration amplitude underwent static recrystallization and grain growth resulting in final bulk hardness of 40 VHN similar to the hardness of heat-treated original foil. Variation of Hardness with Positions and Layers The reason why each VHP-UAM samples had different trends of decreasing, increasing, or consistent bulk hardness values from bottom layers to top layers was mainly due to the bulk foil undergoing hardening behavior and softening behavior at the same time. In VHP-UAM samples, the heating during the thermo-mechanical phase softened the foil via dynamic recovery, but the normal compressive force gave additional hardening of the bulk foil via warm or hot working. JUNE 2016 / WELDING JOURNAL 191-s Fig. 10 — Microstructures variations in the top three layers of Al3003H18 VHPUAM samples. A — Asprocessed TB10285340; B — heattreated 343°C, 2 h, TB10285340. The images are obtained from an image quality map using the electron backscattered diffraction technique.
Welding Journal | June 2016
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