Friction Stir Superplasticity for Unitized Structures: A volume in the Friction Stir Welding and Processing Book Series

By Zongyi Ma and Rajiv S. Mishra

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

• Language: English
• Publisher: Elsevier Science
• Release date: May 27, 2014
• ISBN: 9780124200135

Book preview

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Chapter 1

Introduction

Superplasticity refers to the ability of materials to exhibit high uniform elongation when pulled in tension while maintaining a stable microstructure. Superplastic forming (SPF) of commercial aluminum alloys has been considered as one of the important fabrication methods for unitized components in automotive and aerospace industries. There is increasing interest in these industries to implement wider use of SPF of commercial aluminum alloys in fabricating complex parts.

Keywords

Superplasticity; SPF; thermo-mechanical processing; thermo-mechanical treatment; equal channel angular pressing; accumulative roll bonding; friction stir processing

Superplasticity refers to the ability of materials to exhibit high uniform elongation when pulled in tension while maintaining a stable microstructure. Superplastic forming (SPF) of commercial aluminum alloys has been considered as one of the important fabrication methods for unitized components in automotive and aerospace industries. There is increasing interest in these industries to implement wider use of SPF of commercial aluminum alloys in fabricating complex parts.

A prerequisite for achieving structural superplasticity is a fine-grain size, typically less than 15 μm. Conventionally, elaborate thermo-mechanical processing (TMP) is needed to obtain a fine microstructure conductive to superplastic deformation. For example, Paton et al. [1] developed a four-step TMP treatment to obtain grain size in the range of 8–14 μm in commercial 7075 and 7475 aluminum alloys. Jiang et al. [2,3] suggested an improved TMP approach for 7075 Al alloy that involved solution treatment, overaging, multiple pass warm rolling (200–220°C) with intermittent reheating and a recrystallization treatment. Clearly, TMP for fine-grain microstructures is complex and time-consuming and leads to increased material cost. Furthermore, the optimum superplastic strain rate of 1–10×10−4  s−1 obtained in these aluminum alloys [1–3] is too slow for superplastic forging/forming of components in the automotive