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As a result, nearly the entire cross section of the gage of the tensile specimen corresponded to the SZ. In the quasi-static tensile tests using a conventional tensile testing machine, a displacement rate of 2.5 mm/min was applied to the specimen until fracture.
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Figure 3.5 (a) SEM of the region II in Fig. 5.3(a); (b)–(e) EDS mappings of the region II, and (f) point analysis of P1, as marked in (a)
Figure 3.6 (a) SEM of region III in Fig. 5.3(a); (b)–(c) EDS mappings of the region II, and (d) point analysis of P2, as marked in (a)
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Fig. 3.4 (c–e) exhibits the nearly homogeneous distribution of zinc, magnesium, and copper in the dark region, confirming the presence of the aluminum 7075 alloy. The results of the point analysis of P1 in Fig.3.5 (a) suggest that MgZn2 [Mahoney MW et al.,1998] and Al7Cu2Fe [Raghavan A et al.,1985] formed as precipitates with agglomeration in the SZ. In the bright region, the coarse precipitates contained both aluminum and silicon, as shown in the EDS area mapping in Fig.3. 6 (b) and (c). The results of the point analysis of P2 in Fig. 3.6 (a) suggest that the coarse precipitates may be Al2Si [Rana RS et al.,2012] (Fig. 3.6 (d)). However, since the solubility of silicon in aluminum is low, it is also possible that pure silicon precipitates formed in addition to Al2Si. The typical alloying elements of the aluminum 7075 alloy – zinc, magnesium, and copper – were rarely observed in the bright region. Therefore, it can be concluded that the dark region is the reinforcement (aluminum 7075 alloy)-enriched region (RER) and the bright region is the matrix (aluminum 1060 alloy)-enriched region (MER).
The base metal exhibited an elongated grain structure from the rolling process, as shown in the inverse pole figure (IPF) map in Fig. 3.7 (b) (region IV). On the contrary, the IPF maps of the onion ring structure (region V) and the MER (region VI) exhibited a nearly equiaxed grain structure (Fig. 3.7 (c) and (d)). Also, the grain size of the RER, as approximately marked by black dashed lines in Fig. 3.7 (c), is somewhat smaller than that of the MER, as listed in Table 3.3. For the AM-FSP specimen, the results of the EBSD analysis demon-strate that, because of FSP, the fraction of the high-angle grain boundary (HAB) significantly increased in the SZ than in the matrix metal (Table 3.3). The nearly equiaxed grain structure and the significant increase of the HAB fraction are evidence of dynamic recrystallization in the SZ during FSP. The precipitation behaviors in the dark and bright regions of the SZ also can be understood because
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Figure 3.7 (a) Cross section of the AM-FSP specimen; IPF, HAB & LAB maps of (b) the base metal, (c) the onion ring structure (region V), and (d) the MER (region VI)
Table 3.3 Fraction of HAB in matrix metal and the SZ Matrix metal
(1060 aluminum)
AM-FSP (SZ)
RER MER
Average grain size (μm) 22.89 10.68 3.42 1.38 6.15 2.96
HAB fraction (%) 53.1 86.9 83.7
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of increased grain boundaries and dislocation density during dynamic recrystallization, which can act as nucleation sites for precipitates [Suresh M et al.,2011].
The semi-elliptical intermixing of the RER and MER in the SZ of the AM-FSP specimen resulted in two distinct hardness profiles across the SZ due to the significantly different hardness values of these regions, as shown in Fig. 3.7. Even though the hardness of the MER was lower than that of the RER, the hardness of the MER was still clearly higher than that of the base metal and that of the FSP- only condition with the same FSP parameters. This can be explained by the fact that the MER was strengthened by the typical microstructural refinement of FSP, and by the precipitation of Al2Si. The difference of hardness between the MER and the FSP-only condition is mostly attributed to the different extent of precipitation and grain refinement in the SZ. For the FSP- only condition, the average grain size of the SZ (20 mm 8.36) was significantly larger than that of the MER. It is speculated that the AMed region affected the heat input/material mixing during FSP and consequently influenced the formation of precipitates and grain refinement [Hillert M et al.,1988] in the MER.
Given the distinct microstructures and hardness levels of the RER and MER in the SZ, the heterogeneous onion ring structure can be considered a composite structure with the RER as reinforcement in the MER matrix. However, it should be noted that application of rule of mixture to describe the enhanced mechanical properties of the SZ should be conducted with a care since a significant extent of microstructural change occurred during the process.
Due to the reinforcing effect of the RER and the strengthening of the MER by microstructural refinement, the mechanical properties of the AM-FSPed region significantly surpassed those of the base metal as confirmed by quasi-static tensile tests of specimens along the tool travel direction (Fig. 3.8). The engineering stress–strain curve of the specimen from the
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Figure 3.8 Microhardness profiles for the AM-FSP and FSP-only specimens
Figure 3.9 Stress strain curves for the AM-FSP specimen (along the tool travel direction) and the base metal
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Figure 3.10 An application of AM-FSP to locally reinforce sheet metal product for enhanced stiffness: (a) sheet metal forming, (b) sheet metal product with a groove along the perimeter, (c)
AM inside the groove, and (d) FSP along the groove-filled by AM to enhance the structural stiffness