Friction Stir Processing

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FRICTION STIR PROCESSING FOR SUPERPLASTICITY Friction stir processing for superplasticity has the potential to expand the domain of superplastic FSP zone forming to several new enabling

concepts, such as selective Complete forming in corners and superplasticity, thick sheet edge following FSP superplasticity, and superplastic Conventional forging. SPF microstructure Rajiv S. Mishra* University of Missouri Incomplete forming in corners and edge Rolla, Missouri without FSP Fig. 1 — Influence of friction stir processing on enhanced formability of 5 mm uperplastic forming is a process in which thick 7475 Al sheet. Note that the FSP region fully forms, whereas the TMP region titanium or aluminum sheet or plate is does not. (Courtesy Murray Mahoney, Rockwell Scientific). heated to a temperature at which it be- comes soft enough to be formed into sharp Scurves and complex shapes without cracking. Its most attractive feature is the ability to create cost- effective unitized structures. While it is considered a critical technology for unitized structures, its application is limited to sheet metal forming of aero- space components for three primary reasons.

First, the starting material is limited to sheets that m m are less than 3 mm thick because of the thermo- (a) 200 m (b) 1 m mechanical processing needed to produce fine grain Fig. 2 — Microstructure of 25 FSP 7075 Al size, a prerequisite for superplastic properties. a commercial 2024 Al alloy, (a) (Mishra and Mahoney, 2000) Second, the superplastic forming rates are rather before FSP, and (b) after FSP. 20 slow, taking 20 to 200 minutes to form a compo- (c) Grain boundary misorien- nent. Third, the cost of starting materials with su- tation distribution of a FSP 15 perplastic properties is high. 7075 Al alloy shows predomi- nance of high- angle grain 10 The need for faster forming rates and lower cost boundaries. Backscattered SEM has resulted in several new research approaches in micrographs showing breakup 5 the last fifteen years. Some large coordinated ef- of constituent particles during Boundary distribution, % forts include the US-DOE USCAR and Free- FSP in 2024 Al, (d) before FSP, 10 20 30 40 50 60 70 domCAR programs, and the Japanese Innovation in and (e) after FSP. (c) Misorientation angle, degrees Superplasticity program. The automotive sector is exploring several applications for superplastic forming and this will grow with the demand for lighter, more fuel-efficient vehicles. In this brief review, recent efforts to apply friction stir processing (FSP) for enhanced superplasticity in commercial aluminum alloys is outlined. (The field of friction stir processing was overviewed in the October 2003 issue of Advanced Materials & (d) 200 mm (e) 1 mm Processes.) Also, examples are given to highlight op- portunities to expand the application of superplas- alloys: grain size and constituent particle size. Fric- ticity to new areas (Fig. 1). tion stirring produces very fine-grained microstructure in the stirred region. The grain refinement Microstructural refinement results from intense plastic deformation associated Two microstructural features are identified as with the movement of material from the front to critical for superplasticity in commercial aluminum the back of the rotating pin. It follows that a very *Member of ASM International fine-grained microstructure is developed along the ADVANCED MATERIALS & PROCESSES/FEBRUARY 2004 45 friction.qxd 1/17/04 6:40 PM Page 2

FSP path. The table summarizes the grain size data facturability. The overall ductility not only dictates for various commercial aluminum alloys after fric- the maximum strain possible during forming, but tion stir welding/processing. It should be noted also indirectly predicts the level of cavitation or that grain size of <10 mm is readily achieved. other failure mechanisms that may develop. In ad- Figures 2a and 2b show microstructure of a com- dition, the flow stresses should be kept quite low mercial 2024 Al alloy before and after processing. because gas pressure is most frequently applied to In addition, the grain boundary misorientation dis- form components. tribution of the friction-stirred zone shows pre- Figure 3 shows the variation of flow stress and dominantly high-angle grain boundaries in a FSP ductility with strain rate for a few commercial alu- 7075-Al alloy (Fig. 2c). Friction stirring has signifi- minum alloys. The graph shows that FSP leads to cant influence on the constituent particles as well. superplasticity at high strain rates, and that flow Figures 2d and 2e show the breakdown and ho- stresses near the optimum strain rate are lower than mogenization of constituent particles in a com- 10 MPa (1.5 ksi). Both are important because they mercial 2024 Al alloy. should help to lower the cost of superplastic forming.

Superplastic behavior Thinning and cavitation Three aspects of superplastic flow are very im- During superplastic flow, the effective cross sec- portant: tion of a material is changed by two factors: thin- • Optimum strain rate and temperature ning and cavitation. Thinning is quite predictable • Overall ductility and can be included in the design of the compo- • Flow stress nent. On the other hand, cavitation is an internal The optimum strain rate should be as high as defect and develops non-uniformly. Therefore, it possible to reduce the time of the forming cycle. is imperative that cavitation be avoided to achieve Also, a broader range of optimum strain rate and required post-forming properties. Designers temperature is beneficial. Any complex component should be aware of the critical strain for cavity nu- will have variations of strain rate with location, and cleation, and not just the overall superplastic duc- a larger processing window enables better manu- tility to failure. In commercial alu- 100 2000 7075 Al (3.8 mm) minum alloys, the FSP 2024 Al 7475 Al (3.0 mm) onset of cavitation is FSP 5083 Al 450°C 2024 Al (3.5 mm) FSP A356 linked to primary 530°C 1600 5083 Al (6.5 mm) FSP 7075 Al A356 (3.0 mm) constituent particles. FSP 7475 Al The conventional 1200 10 rolling-based thermo- mechanical processing 800 Ductility, % (TMP) route produces Flow stress, MPa 530°C 480°C a string of broken par- 400 ticles that not only 530°C lead to inhomo- 1 geneity (by clustering -4 -3 -2 -1 -4 -3 -2 -1 10 10 10 10 10 10 10 10 10 of cavities), but also Strain rate, per second Strain rate, per second (a) (b) anisotropy in super- Fig. 3 — The variation of (a) stress with strain rate, and (b) ductility with strain rate for a number of friction-stir plastic response. As processed aluminum alloys is shown in these graphs. mentioned earlier, FSP A summary of grain size in nugget zone of FSW/FSP commercial aluminum alloys Plate thickness, Rotation rate, Traverse speed, Material mm Tool geometry rpm mm/min Grain size, mm 1100Al 6.0 Cylindrical 400 60 4 2017Al-T6 3 Threaded, cylindrical 1250 60 9-10 2024Al 6.35 Threaded cylindrical 200-300 25.4 2.0-3.9 2095Al 1.6 — 1000 126-252 1.6 2519Al-T87 25.4 — 275 101.6 2-12 5054Al 6.0 — — — 6 5083Al 6.35 Threaded cylindrical 400 25.4 6.0 6061Al-T6 6.3 Cylindrical 300-1000 90-150 10 7010Al-T7651 6.35 — 180, 450 95 1.7, 6 7050Al-T651 6.35 — 350 15 1-4 7075Al-T651 6.35 Threaded cylindrical 350, 400 102, 152 3.8, 7.5 7475Al 6.35 — — — 2.2

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leads to refinement and homo- 100 geneous distribution of parti- TMP 7475 Al (Chen and Tan, 2001) cles. Figure 4 shows a compar- T = 480°C x 3 ison of cavitation in FSP and 10 = 1x10 /sec TMP 7XXX alloys. FSP significantly increases the critical strain value, and greatly re- 1 duces the extent of cavitation. FSP 3.8 mm - 7075Al 0.1 T = 480°C x = 1x102/sec Innovative concepts Cavity volume fraction, % FSP 7.5 mm - 7075Al T = 480°C Several innovative concepts x =1x102/sec have been developed to take 0.01 1 2 3 4 advantage of the superplas- True strain ticity induced by FSP. Among these are selective superplas- Fig. 4 — A comparison of cavitation during superplastic ticity, thick-sheet or plate deformation in commercial 7XXX alloys. Note that the forming, and superplastic FSP 7075 alloy shows lower cavitation and higher critical strain for onset of cavitation. forging. • Selective superplasticity: The idea of selective su- Fig. 5 — Il- perplasticity is based on its unique capability to lo- lustration of se- For more information: Rajiv S. Mishra, Director, Center lective friction cally modify the microstructure. Many components for Friction Stir Processing, Department of Metallurgical stir processing for have design features that require high formability in Engineering, University of Missouri, Rolla, MO 65409; the part in Figure a few limited areas. Figure 1 shows a component tel: 573/341-6361; e-mail: [email protected]; Web site: 1 to incorporate made from 5 mm thick 7475 Al sheet with a region www.umr.edu/~fricstir. (Several concepts in this paper are design flexibility of FSP. It can be clearly seen from the insets that the protected by US Patent Application 20020079351.) and cost advan- corners of FSP region form fully, whereas the TMP tages for thick side does not fully form. Acknowledgements sheet and plate The author gratefully acknowledges the support of the forming. In this particular example, note that only certain National Science Foundation through grants DMI- regions go through large deformation. Ideally, only 0085044 and DMI-0323725. Numerous discussions and those regions would be FSPed, as highlighted in collaborative research with Murray Mahoney over the Fig. 5. With this selective superplasticity approach, last four years are gratefully acknowledged. designers can adopt superplasticity for thick sheets and plates. Conceptually, a continuous- cast sheet or plate can be friction stir processed in local regions and superplastically formed for tremendous design flexibility and cost advantages. • Thick sheet or plate forming: As noted earlier, the current practice of superplastic forming is limited to sheet metal forming of <3 mm thicknesses. However, friction stir processing of plate up to 25 mm thick has been demonstrated, and over 65 mm thick aluminum alloy plates have been friction stir welded with special tooling, or by multiple passes from two sides. Conventional TMP does not in- troduce enough processing strain to impart fine grain size in 25 mm thick plate, or break down the constituent particles significantly from the cast stage. Conversely, FSP can induce fine grain size and break up the particles. This opens up the pos- sibility of thick-plate forming via FSP of selective regions. • Superplastic forging: The stock material of 25 to 50 mm thick plates can be FSPed to produce fine-grained stock material for forging. Lower flow stresses and finer grain size have the potential to produce forgings with improved properties. FSP can produce a microstructure conducive to high strain-rate superplasticity in commercial aluminum alloys. With local microstructural modi- fications, a three-step manufacturing process can be envisaged: Cast ( Friction Stir Process ( Superplastic forge/form. In addition, FSP com- bined with several innovative new concepts can give designers even more flexibility and cost advantages. ■ ADVANCED MATERIALS & PROCESSES/FEBRUARY 2004 Circle 23 or visit www.adinfo.cc 47