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incorporates dynamically recrystallized grain structure inside the formed product [Hou W et al.,2020; Kuang B et al.,2015].
In this work, a bimetallic ring of Al/Cu is manufactured using friction stir assisted simultaneous joining and forming inside a closed die. The joining mechanism and evolution of the IMCS during the process are also studied using electron microscopy. The effect of the type and thickness of IMC on the electrical conductivity are briefly analyzed. An in-depth microstructural investigation has been made to evaluate dynamic recrystallization phenomena inside a forged part using electron backscattered microscopy. Later the microstructural results were correlated with hardness variation and resistivity inside the forged part
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of 200 rpm) and four different plunging speeds ranging from 4–10 mm/min (with an increase of 2 mm/min) to select the combination of the tool rotation speed and plunging speed in Table 4.2.
Other sets of parameters resulted in either softening of Mg core or excessive increase of forging force. The failed results of the preliminary experiment will not be further discussed in the present study since the failures were mostly due to insufficient or excessive energy input. With the selected combination of the tool rotation speed and plunging speed, two different dwell times were considered to see the effect of dwell time on diffusion, which is the core mechanism of solid-state joining between the Mg core and Al skin. Since the rotating FS-forging tool also served as an upper die during forging, the FS-forging became a closed die forging with a flash along the gap between the tool and the die cavity. After FS-forging, the top (2 mm) and bottom (3 mm) portions of a forged blank was machined (Fig. 4.2 (e)) to complete a bimetallic ring component with an approximate height of 3 mm (an outer diameter of 21 mm and an inner diameter of 8 mm).
After the FS-forging, cross-sections were prepared along the tool plunging direction (the height direction of a ring) for microstructural analysis. The cross-section of a forged blank was first examined by optical microscopy (OM, A1m Axio Imager, Carl Zeiss, Germany) to observe any macroscopic defects. Then, energy dispersive X-ray spectroscopy and electron backscatter diffraction (EBSD) analysis were conducted on the cross-sections of bimetallic rings. A field emission scanning electron microscope (FE-SEM, SU70, Hitachi, Japan) equipped with an EBSD system (Hikari EBSD detector with the TSL OIM 6.1 software, Hitachi, Japan) was used.
A standard metallographic grinding technique for polishing, combined with an ethanol-based diamond paste to avoid oxidation of the Mg core, was used. For the EBSD analysis, the cross sections of bimetallic rings were further polished using colloidal silica. The accelerating voltage
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of 15 kV and the working distance of 15 mm were used in the EBSD analysis. A critical
Figure 4.1 (a) Schematic of a Mg/Al bimetallic blank, (b) cylindrical FS-forging tool with a two- step shoulder and a die with a cylindrical cavity
Figure 4.2 (a)–(d) FS-forging stages, (e) a forged blank and a bimetallic ring
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Table 4.1 alloying elements of AZ31B and Al6061-T6 (wt.%)
Materials Mg Al Zn Ca Cr Mn Si Ti Cu Fe
Mg (AZ31B) Bal 2.5 0.60 0.04 0.20 0.10 - 0.05 0.005 Al 6061-T6 1.2 Bal 0.25 - 0.25 0.14 0.40 0.15 0.15 0.70
Table 4.2 Fs- forging parametersl Parameter
Sets
Tool rotational speed (rpm)
Tool plunging speed (mm/min)
Dwell Time (s)
1 1400 10 05
2 08
Figure 4.3 (a) A schematic of an Mg/Al bimetallic blank, (b) cylindrical FS-forging tool with a two-step shoulder and a die with a cylindrical cavity
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Table 4.3 Chemical composition of the Al and Cu Al 5052-
O (ISO AlMg2.5)
Al Mg Zn Cr Mn Si Cu Fe
Bal. 2.2 0.10 0.15 0.10 0.25 0.10 0.40
Pure Cu (C10100)
Cu Pb Fe P Ag As Zn
Bal. 0.005 0.005 0.0003 0.0025 0.002 0.005 Table 4.4 FS-forging process parameters
Tool RPM
Tool travel
speed Results observed
650
10/15/20
Formation of the IMC at tool Cu interface could be avoided, 550/15 produced the most
successful result 600
550
Figure 4.4 (a)–(d) FS-forging stages, (e) a forged blank and a bimetallic ring
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misorientation angle of 15° was taken for grain identification. The mechanical properties of the bimetallic rings were evaluated by microhardness measurement. A Vickers microhardness tester (Mitutoyo hardness testing machine, Mitutoyo, Japan) was used (a load of 0.49 N for 10 s).
The bimetallic blank of Al 5052-O and pure Cu (chemical compositions in Table7.1 and 7.2) were manufactured by a tightly fitting Cu cylinder inside the hollow Al tube and is used to forge and join simultaneously inside the closed die. The Al rods were machined into cylindrical tubes (the Al skin) with an inner diameter of 12 mm, a wall thickness of 4 mm, and a height of 12 mm. Then the Cu rods, which were machined into solid cylinders (the Cu core) with similar diameter and height of 12 mm, were tightly fitted into the Al skins as shown in Fig. 4.3 (a). The forging was carried out using a cylindrical two-step shoulder tool inside a cylindrical die, as exhibited in Figs. 7.1 (b & c). The friction stir forging was carried out by inserting the rotating tool and bimetallic blank inside the cavity of the die using a custom-made FSW machine (RM1A, Bond Technology, USA), as schematically shown by Fig. 4.4. (a-d). A two-step shouldered tool was plunged gradually inside the bimetallic tight fitted blank. The frictional heat generated softens the blank, and gradual tool plunging develops a favorable condition for the materials to be forged until the die fills and allows the diffusion between the Al-Cu to occur for joining. The tool rotation and plunging speed to carry out the process successfully are listed in Table 4.4. A separate study was conducted to finalize the parameters by varying the tool rotation from 1000- 800 rpm (with a decrease of 50 rpm) and different plunging speeds ranging from 20-10 mm/min (with a reduction of 5 mm/min). In most cases, the forged product demonstrated excessive thickening of the IMC, distortion of the copper, or damaged aluminum skin. Since Al/Cu are highly reactive, the IMC starts to form even at a very lower temperature of 300℃, thus dwell
time was avoided to restrict the further growth of the IMC. The forged product was polished
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using coarse MRE paper (100 μm grit) at the top and bottom to obtain a finished ring product.
The final bimetallic ring product was approximately 5mm in height and 24 mm in diameter.
The cross-section of the forged component was cut along the tool plunging direction for microstructural analysis. Further, the cross-section of the forged product was observed under optical microscopy (OM, A1m Axio Imager, Carl Zeiss, Germany). A field emission scanning electron microscope (FE-SEM, SU70, Hitachi, Japan) equipped with an EBSD system (Hikari EBSD detector with the TSL OIM 6.1 software, Hitachi, Japan) was used. A standard
metallographic grinding technique for polishing, combined with a glycol-based diamond paste, was used to generate a scratch-free surface. For the EBSD analysis, the cross-sections of bimetallic rings were further polished using colloidal silica. The accelerating voltage of 15 kV and the working distance of 15 mm were used in the EBSD analysis. A critical misorientation angle of 15° was taken for grain identification. The mechanical properties of the bimetallic rings were evaluated by microhardness measurement. Vickers microhardness tester (Mitutoyo
hardness testing machine, Mitutoyo, Japan) was used (a load of 0.49 N for 10 s) to measure the microhardness of the same cross-section. The resistivity across the interface of the forged cross section of Al/Cu was measured using the high-precision nano-voltmeter (Agilent 34420 A) with a resolution of 0.1μΩ