ture rise during VHP-UAM were the reasons why softening occurred in the bulk region of the original foil (Ref. 4). However, none of these works (Refs. 2, 4, 17) looked into the effect of UAM/VHP-UAM process parameters, i.e., vibration amplitude, normal force, and weld speed, on the amount of increase and decrease in the foil hardness after UAM and VHP-UAM. Hence, the objective of this study was to investigate the effect of processing parameters, namely vibration amplitude and normal force, on the bulk hardness of aluminum 3003-H18 foil in both the as-processed and heattreated conditions. A B C Experimental Procedures In this study, as-received Al3003- H18 foil with 25.4 mm width and 150-μm thickness was used to fabricate VHP-UAM samples on top of a 25.4-mm-thick Al3003-H18 substrate. The chemical composition in weight percentage of the foil is Al- 1Mn-0.7Fe-0.12Cu. The sonotrode is made of Ti-6Al-4V with surface roughness RA of 7 μm. Two different machines (the Test- Bed machine or TB machine and the commercial SonicLayer 7200 machine or SL7200 machine) were used to fabricate VHP-UAM samples. Table 1 lists the processing parameters used to fabricate Al3003-H18 VHP-UAM samples. WELDING RESEARCH Sample ID is designated for each VHP-UAM sample as Name of Machine (TB = Test Bed, SL = SonicLayer- 7200), number of layers, vibration amplitude (in μm), and normal force (in N). It is noted that VHP-UAM samples made from the TB machine contain 10 layers, while those made from the SL7200 machine have up to 80 layers (delamination occurred in sample SL- 66-28-5340 while attempting to bond 67th layer). The ultrasonic frequency was kept constant at 20 kHz. The welding speed was selected at 35.6 mm/s for all samples made from the TB machine and the first 50 layers in the SL7200 machine. Above the 50th layer, welding speed was adjusted to 42.7 mm/s. The baseline parameter was 28- m vibration amplitude and 5340 N normal force, as the combination of the two yielded good metallic bonding strength during preliminary peel testing (Ref. 18). It is noted that the normal forces of 4000, 5340, and 8000 N used in this study are more than twice the maximum capable normal force of 1500 N available in UAM machine. After VHP-UAM, a portion of the samples were used to prepare heattreated or annealed samples from the as-received foil and as-processed VHPUAM samples for comparison. These samples were heated to 343ºC for 2 h in an argon atmosphere followed by furnace cooling to room temperature. Samples were sectioned along the ND-TD plane using standard metallography techniques. An Olympus GX-51 microscope with a 12MP Olympus DP71 digital imaging system was used to capture optical images. Vicker microhardness was performed using a fully automated Leco AMH-43 system with the load of 10 g. Indentations were made 20 times along the middle region of each deposited foil layer at the spacing of 100–150 μm between each indentation to create a hardness map of each Al3003-H18 VHP-UAM sample. Results The optical micrographs of the selected Al3003-H18 VHP-UAM samples are displayed in Fig. 2. These images illustrate the macro view of the deposited layers from bottom to top showing the voids or unbonded regions distrib- JUNE 2016 / WELDING JOURNAL 187-s Fig. 3 — Plot of the average Vicker hardness measured at the middle of the bulk region of each deposited layer in three Al3003H18 VHPUAM samples fabricated from the TestBed machine compared to the average original Al3003H18 tape hardness. Fig. 4 — Hardness map obtained from Vicker hardness measurements at the middle of each layer in Al3003H18 VHPUAM samples fabricated using the SonicLayer 7200 machine. Table 2 — Average Reduction in Thickness (in percentage) of Al3003H18 VHPUAM Samples Sample Average Reduction in Thickness (%) TB10285340 2% TB10384000 2% TB10388000 10% SL66285340 4% SL80344000 5% SL80345340 6%
Welding Journal | June 2016
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