Ma / Mishra Friction Stir Superplasticity for Unitized Structures

A volume in the Friction Stir Welding and Processing Book Series
1. Auflage 2014
ISBN: 978-0-12-420013-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 108 Seiten

Reihe: Friction Stir Welding and Processing

This book describes the fundamentals and potential applications of 'friction stir superplasticity for unitized structures'. Conventional superplastic forming of sheets is limited to the thickness of 3 mm because the fine grained starting material is produced by rolling. Friction stir superplasticity has grown rapidly in the last decade because of the effectiveness of microstructural refinement. The thickness of the material remains almost constant, and that allows for forming of thick sheets/plates, which was not possible before. The field has reached a point where designers have opportunities to expand the extent of unitized structures, which are structures in which the traditional primary part and any supporting structures are fabricated as a single unit. With advanced optimization and material considerations, this class of structures can be lighter weight and more efficient, making them less costly, as well as mechanically less complex, reducing areas of possible failure. - Discusses how friction stir processing allows selective microstructural refinement without thickness change - Demonstrates how higher thickness sheets and plates can be superplastically formed - Examples are presented for aluminum, magnesium and titanium alloys - Covers the production of low-cost unitized structures by selectively processing cast sheets/plates

Rajiv S. Mishra is a professor in the Department of Materials Science and Engineering, and Site Director, NSF IUCRC for Friction Stir Processing, at the University of North Texas. Dr. Mishra's publication record includes 255 papers. Out of these, 10 of his papers have more than 100 citations. He has many 'firsts' in the field of friction stir welding and processing. He co-authored the first review paper (2005), co-edited the first book on the subject (2007), edited/co-edited seven TMS symposium proceedings, and served as guest editor for Viewpoint Set in Scripta Materialia (2008). He also has three patents in this field. He published the first paper on friction stir processing (2000) as a microstructural modification tool.

Autoren/Hrsg.

Ma, Zongyi

Mishra, Rajiv S.

Leseproben

Chapter 3 High-Strain-Rate Superplasticity
High-strain-rate superplasticity (HSRS) refers to the superplasticity achieved at an optimum strain rate of =1×10-2 s-1. According to the prediction by the constitutive relationship for superplasticity, the optimum strain rate increased with decreasing the grain size. Based on the fine-grain size of 0.7–10 µm produced in aluminum alloys via friction stir processing (FSP), it is expected that the FSP fine-grained aluminum alloys would exhibit high-strain-rate superplastic behavior with the decrease in the grain size. Table 3.1 summarizes the superplastic data of a number of aluminum alloys prepared by FSP. It is indicated that HSRS has been achieved in several FSP aluminum alloys, such as 7075Al, 2024Al, Al–Mg–Zr, Al–Zn–Mg–Sc, and Al–Mg–Sc. Keywords
HSRS; FSP; grain boundary sliding; grain boundary sliding; grain boundary migration; HAGBs; friction stir processing High-strain-rate superplasticity (HSRS) refers to the superplasticity achieved at an optimum strain rate of =1×10-2 s-1 [34]. According to the prediction by the constitutive relationship for superplasticity, the optimum strain rate increased with decreasing the grain size. Based on the fine-grain size of 0.7–10 µm produced in aluminum alloys via friction stir processing (FSP), it is expected that the FSP fine-grained aluminum alloys would exhibit high-strain-rate superplastic behavior with the decrease in the grain size. Table 3.1 summarizes the superplastic data of a number of aluminum alloys prepared by FSP. It is indicated that HSRS has been achieved in several FSP aluminum alloys, such as 7075Al, 2024Al, Al–Mg–Zr, Al–Zn–Mg–Sc, and Al–Mg–Sc. Table 3.1 Summary of Superplastic Elongation Observed in a Number of Aluminum Alloys 7075 480 1×10-2 1250 2024 430 1×10-2 525 5083 530 3×10-3 590 A356 530 1×10-3 650 Al–4Mg–1Zr 525 1×10-1 1280 Al–Zn–Mg–Sc 310 3×10-2 1800 3.1 Superplastic Behavior
Figure 3.1(a) shows the variation of elongation with initial strain rate for as-rolled and as-FSP 7075Al alloys. As-rolled 7075Al alloy did not exhibit superplastic elongation at 480–490°C for initial strain rate of 1×10-3–1×10-1 s-1. FSP resulted in generation of significant superplasticity in 7075Al alloy. For the FSP 7.5 µm-7075Al, an optimum strain rate of 3×10-3 s-1 for maximum elongation was observed. A decrease in the grain size from 7.5 to 3.8 µm resulted in significantly enhanced superplasticity and a shift to higher optimum strain rate. Superplastic elongations >1250% were obtained in the strain rate range of 3×10-3–3×10-2 s-1. Even at a high initial strain rate of 1×10-1 s-1, an elongation of 750% was attained demonstrating HSRS.
Figure 3.1 Variation of elongation with (a) initial strain rate and (b) test temperature for as-rolled and FSP 7075Al alloys [24]. Figure 3.1(b) shows the effect of temperature on the superplastic ductility of the FSP 7075Al at initial strain rates of 3×10-3 and 1×10-2 s-1. Compared to the FSP 7.5 µm-7075Al, the FSP 3.8 µm-7075Al exhibited significantly enhanced superplasticity over a wide temperature range of 420–510°C, a shift to higher optimum superplastic strain rates, and a lower optimum superplastic deformation temperature. Further, even at the high temperature of 530°C, the FSP 7.5 µm and 3.8 µm-7075Al alloys still exhibited large superplastic elongations of 640% and 800%, respectively, for an initial strain rate of 1×10-2 s-1. With decreasing the grain size further, the optimum strain rate for maximum superplastic ductility continues to increase according to the constitutive relationship for superplasticity. Figure 3.2(a) shows the variation of elongation with initial strain rate for FSP fine-grained Al–4Mg–1Zr alloy with an average size of 1.5 µm. Optimum superplasticity was observed at a high strain rate of 1×10-1 s-1 (even 3×10-1 s-1 for 450°C) at temperatures ranging from 425°C to 525°C. Maximum superplastic elongation of 1280% was obtained at 525°C and 1×10-1 s-1. The optimum strain rate for the FSP Al–4Mg–1Zr is one order of magnitude larger than that for the FSP 3.8 µm-7075Al alloy. Furthermore, the FSP fine-grained Al–4Mg–1Zr alloy exhibited an excellent thermal stability due to the presence of the fine Al3Zr particles. Even at a higher temperature of 550°C, the FSP Al–4Mg–1Zr exhibited a superplastic elongation of 1210% (Figure 3.2(b)). Figure 3.3 shows the tested specimens of the FSP Al–4Mg–1Zr alloy deformed to failure at 1×10-1 s-1 for different temperatures. The specimens show neck-free elongation that is characteristic of superplastic flow.
Figure 3.2 Variation of elongation with (a) initial strain rate and (b) test temperature for FSP 1.5 µm Al–4Mg–1Zr [25].
Figure 3.3 Appearance of specimens before and after superplastic deformation at (a) 525°C and different strain rates and (b) 1×10-1 s-1 and different temperatures [25]. Similarly, FSP 2024Al alloy with an average grain size of 3.9 µm also exhibited HSRS. Figure 3.4(a) presents ductility data of the FSP 2024Al obtained at 430°C and 450°C for different strain rates of 1×10-3–1×10-1 s-1. A maximum ductility of ~525% was achieved for FSP 2024Al alloy at a strain rate of 1×10-2 s-1 and 430°C. As compared to the results of the FSP alloy, the ductility data of the parent alloy are quite marginal (~70–100%). Ductility values are plotted against temperature in Figure 3.4(b) for the optimum strain rate of 1×10-2 s-1. It shows that the ductility of FSP 2024Al alloy drops off significantly at and above 470°C and it is only ~41% at 490°C. This is attributed to abnormal grain growth in FSP 2024Al alloy, evidenced by unusual increase in flow stress [29]. On the other hand, the parent material shows modest ductility values (105–120%) all through the temperature range at a strain rate of 1×10-2 s-1.
Figure 3.4 Variation of elongation with (a) initial strain rate and (b) test temperature for FSP 3.9 µm-2024Al [29]. To highlight the HSRS characteristics of FSP aluminum alloys, the relative superplastic domains of 2024Al alloys prepared by different processes are illustrated in Figure 3.5 as strain rate vs. temperature. Each regime is outlined with a rectangular region to show the range of temperature and strain rate, where the alloy exhibits superplasticity. It is noted from Figure 3.5 that conventional thermo-mechanically processed (TMP) 2024Al alloys do not exhibit HSRS [35–37]. Superplasticity in aluminum alloys is typically manifested at ~475°C (0.8Tm) or higher temperatures. The optimum superplasticity temperature of 430°C (0.75Tm) for the FSP 2024Al is at least ~50°C lower than that for aluminum alloys prepared by the conventional techniques.
Figure 3.5 A temperature–strain rate map depicting the superplastic domains for 2024 Al alloys processed by different routes [29]. It is quite obvious from Figure 3.5 that HSRS can be achieved in the P/M 2024Al (modified composition) [38,39] at a very high strain rate (3 s-1) and higher temperature (500°C). This is attributed to its fine-grain microstructure stabilized by intermetallic particles. However, the material processed by powder metallurgy route is expensive and being a modified composition alloy, its knowledge database for other properties is also limited. Although superplastic behaviors of the ECAP 2024Al and the FSP 2024Al alloys are comparable [40,41], optimum superplasticity was achieved at a lower temperature in the ECAP 2024Al primarily due to finer grain size. However, it can be noted that the ECAP 2024Al was pressed for eight passes to achieve optimum superplastic microstructure. For FSP 2024Al, only one pass is needed to produce fine-grained microstructure. Figure 3.6 shows the effect of FSP on flow stress (at true strain of 0.1) of 7075Al alloys as a function of initial strain rate. FSP resulted in significantly reduced flow stress in 7075Al alloy, which decreased with decreasing grain size. Low flow stress is one of the characteristics of superplastic deformation. Furthermore, the...