Bulk eddy current measurements were made with Rohmann GmbH Elotest M2 with a probe diameter of approximately 1.3 cm, which was operated at a frequency of approximately 1 MHz. The values were recorded as % IACS, where the electrical conductivity of annealed Cu was referenced as 100% IACS at 20°C, and IACS refers to the International Annealed Copper Standard, which was established in 1913 (Ref. 47). Advertised accuracy of measurements was ± 0.1% IACS. Because the probe diameter was larger than the weld cross section, some air was picked up. Although this may have resulted in slightly lower values for % IACS, the comparative trend was considered valid. To evaluate the mechanical properties of only the SZ of the FSW, tensile specimens were designed with the gauge section entirely within the transverse section of the FSW SZ, as shown in Fig. 2A. The geometry was first machined, and then sliced using wire electrical discharge machining (EDM) into individual specimens 2.0 cm long × 0.64 cm wide × 0.03 cm thick, as shown in Fig. 2B and C, respectively. The tensile specimens were tested in uniaxial tension using a stepper-motor-driven miniature tensile tester with a 0.5-kN (100- lbf) load cell. All tests were run at ambient temperature at a constant crosshead velocity of 0.05 mm/min with a dataacquisition rate of 1 sample per s. The maximum load (Fmax) was divided by the initial specimen cross-sectional area (A) to calculate the engineering stress (σ). Yield strength (σYS) was defined using the 0.02% offset criteria (Ref. 48) and ultimate tensile strength (UTS) was defined using the maximum load carried by the specimen cross-sectional area. A JEOL 6500 F field emission, scanning electron microscope (FE-SEM) with an Oxford electron backscatter detector (EBSD) was used to obtain orientation image maps (OIM) of the SZ. Analysis was performed in 0.4-μm steps over 215- × 161-μm rectangular areas in the banded regions of the transverse sections. All OIM scans were obtained using an excitation condition of 20 kV with a working distance of 20 mm. EBSD/OIM was used to determine grain size based on a 5-deg misorientation angle. Transmission electron microscope (TEM) foils, 3 mm in diameter, were punched from the SZ region of the FSW specimens and were prepared for imaging using traditional techniques of mechanical thinning, two-sided dimpling, and ion milling to electron transparency. Initial images were obtained in a JEOL JEM- 100CX TEM with a tungsten filament operated at an accelerating voltage of 100 kV to obtain bright field image (BFI). Complementary higher-resolution BFIs and selected area diffraction (SAD) patterns Fig. 3 — Flow stress of 2219-T87 vs. temperature showing a precipitous drop at 0.5Tm before reaching an almost constant, linear plateau in the range of 0.7 to 0.9 Tm (Ref. 50). A B Fig. 5 — A — A low-magnification image of the base metal in which a few large overaged particles (200–500 nm) can be observed; B — the higher-magnification image shows the θ’ strengthening phases in the base metal matrix. were obtained using a JEOL 2010 200 KeV field emission (FE) TEM. A Rigaku Smartlab X-ray diffractometer (XRD) with Cu-k X-ray was used to identify the minor phases present in the aluminum matrix. A continuous scan was made at a rate of 0.035 deg/min over a 2-θ range of 18 to 55 deg. Fig. 4 — Phase diagram for the Al-Cu binary system (Ref. 51). The SZ temperature was taken to be that of the workpiece/weld tool interface or the shear zone. The shear zone temperature was calculated from the measured experimental torque values using an alternative heat index (Ref. 27). This numerical approach considered the power generated by rotating an axial symmetric WELDING JOURNAL 13-s WELDING RESEARCH Table 1 — Summary of FSW Conditions and Corresponding Shear Strain Rate RPM Tool Radius Shear Zone Shear (mm) Thickness Strain Rate (mm) (s-1) 150 6.35 0.13 5 × 104 200 6.35 0.13 6 × 104 300 6.35 0.13 9 × 104 Table 2 — Grain Size as Inferred from EBSD/OIM Specimen Grain Size (μm) AS RS 150 2.5 1.8 200 2.8 2.6 300 4.1 4.2
Welding Journal | January 2013
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