Friction Stir Processing for Enhanced Low Temperature Formability Friction Stir Processing for Enhanced Low Temperature Formability A volume in the Friction Stir Welding and Processing Book Series Christopher B. Smith Wolf Robotics (formerly of Friction Stir Link) Rajiv S. Mishra University of North Texas AMSTERDAM(cid:129)BOSTON(cid:129)HEIDELBERG(cid:129)LONDON NEWYORK(cid:129)OXFORD(cid:129)PARIS(cid:129)SANDIEGO SANFRANCISCO(cid:129)SINGAPORE(cid:129)SYDNEY(cid:129)TOKYO Butterworth-HeinemannisanimprintofElsevier Butterworth-HeinemannisanimprintofElsevier 225WymanStreet,Waltham,MA02451,USA TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UK Firstedition2014 Copyrightr2014ElsevierInc.Allrightsreserved Nopartofthispublicationmaybereproducedortransmittedinanyformorbyanymeans, electronicormechanical,includingphotocopying,recording,oranyinformationstorageand retrievalsystem,withoutpermissioninwritingfromthepublisher.Detailsonhowtoseek permission,furtherinformationaboutthePublisher’spermissionspoliciesandourarrangement withorganizationssuchastheCopyrightClearanceCenterandtheCopyrightLicensingAgency, canbefoundatourwebsite:www.elsevier.com/permissions Thisbookandtheindividualcontributionscontainedinitareprotectedundercopyrightbythe Publisher(otherthanasmaybenotedherein). Notices Knowledgeandbestpracticeinthisfieldareconstantlychanging.Asnewresearchand experiencebroadenourunderstanding,changesinresearchmethods,professionalpractices, ormedicaltreatmentmaybecomenecessary. Practitionersandresearchersmustalwaysrelyontheirownexperienceandknowledgein evaluatingandusinganyinformation,methods,compounds,orexperimentsdescribedherein. Inusingsuchinformationormethodstheyshouldbemindfuloftheirownsafetyandthesafety ofothers,includingpartiesforwhomtheyhaveaprofessionalresponsibility. Tothefullestextentofthelaw,neitherthePublishernortheauthors,contributors,oreditors, assumeanyliabilityforanyinjuryand/ordamagetopersonsorpropertyasamatterofproducts liability,negligenceorotherwise,orfromanyuseoroperationofanymethods,products, instructions,orideascontainedinthematerialherein. BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress ISBN:978-0-12-420113-2 ForinformationonallButterworth-Heinemann visitourwebsiteatstore.elsevier.com ACKNOWLEDGMENTS This project was funded by the U.S. Navy under contract N00014-08-C- 0089. The prime contractors for this work were Friction Stir Link, Inc. (FSL) and the Missouri University of Science and Technology (MS&T). The U.S. Navy’s Naval Surface Warfare Center (cid:1) Carderock Division(cid:1) providedguidanceandtechnicaloversight.Allofthewelding and processing was performed by Friction Stir Link, Inc. with material property testing primarily being completed by MS&T. Other material property testing was performed by FSL, the South Dakota School of Mines and Technology (SDSMT), and a limited amount at material propertytestingsuppliers. The authors would like to sincerely thank all who have provided contributions that made this short book possible. First of all, thank you to the U.S. Navy for providing the monetary resources that have allowed a very comprehensive, expansive, and likely the largest data set to date, comparing friction stir processing, friction stir welding, gas metal arc welding, and base material. However, this would not have been possible without the foresight of Bruce Halverson and Scott Craw at Marinette Marine, as well as Maria Posada and Johnnie DeLoach at the U.S. Navy, with their ability to envision the benefits of the use of friction stir processing and friction stir welding. With their support and technical guidance, this project and the data pre- sented were created by a large team that includes colleagues at FSL, MS&T, and SDSMT. Major contributions to this work included, but were not limited to Adam O’brien, Lee Cerveny, and Jerry Opichka at FSL, as well as Arun Mohan, Jianqing Su, Gaurav Argade, and Kumar Kandasamy at MS&T. Furthermore, significant contributions and guidance were provided by Murray Mahoney (retired and for- merly of Rockwell Scientific) as well as Professor Mike Miles from Brigham Young University. Next, significant contributions were also provided by the team SDSMT for their structural testing contribution for which the authors would specifically like to thank Damon Fick and Christian Widener for their contributions. Providing additional guidance, the authors would also like to acknowledge the contribution viii Acknowledgments of Cathy Wong, Kirsten Green, and Nat Nappi of the U.S. Navy for their technical guidance on the testing methods and approaches that would provide the most valuable comparison data with the traditional approach. Finally, the authors would like to specifically recognize John F. Hinrichs of Friction Stir Link who was a great champion of this work and always encouraged advancement of welding and its related technologies and whose contributions to the welding commu- nity will be missed. This was truly a team effort. Thanks again to all...your efforts have all been very much appreciated! PREFACE This is the second volume in the recently launched short book series on friction stir welding and processing. As highlighted in the preface of the first book, the intention of this book series is to serve engineers and researchers engaged in advanced and innovative manufacturing techniques. Friction stir processing was started as a generic microstruc- tural modification technique almost 15 years back. In that period, fric- tion stir processing-related research has shown wide promise as a versatile microstructural modification technique and solid-state manufacturing technology. Yet, broader implementations have been sorely missing. Disruptive technologies face greater barrier to implemen- tation as designers do not have these in their traditional design tool box! Part of the inhibition is due to lack of maturity and availability of large data set. This book has primarily a technological flavor. It contains significant volume of datageneratedasapart of technologyapprovaldocumentfor theUSNavy.Theintentionbehindthisbookistoshareanexamplethat can boost the confidence with engineers and designers as they consider friction stir processing as a viable technology for advanced manufactur- ing. This short book series on friction stir welding and processing will includebooksthatadvanceboththescienceandtechnology. Rajiv S. Mishra University of North Texas March 8, 2014 11 CHAPTER Concept of Friction Stir Processing for Enhanced Formability 1.1 BACKGROUND Since its invention in 1991 and then first production implementation in 1995, friction stir welding (FSW) has experienced a continual increase in use throughout the world (Thomas et al., 1991; Dawes and Thomas, 1995). Its increase in implementation has occurred because of FSW’s benefits over traditional fusion welding processes, such as improved static strength, improved fatigue properties, less sensitivity to disturbances (e.g., contamination), and significantly less distortion. The benefits of FSW are arguably the most prominent when comparing with fusion welding of aluminum. As such, most of the implementation of FSW has occurred in the aluminum fabrication industry, especially for applications that have been specifically designed for FSW. The FSW process is fairly simple in concept and is illustrated in Figure 1.1 (Mishra and Ma, 2005). The FSW process first involves a machine initiating rotation of a friction stir tool. The friction stir tool has a probe and a shoulder, both with specially designed features. The FSW machine then plunges the rotating friction stir tool into the work- piece, creating heat locally via friction and plastic deformation of the material, softening the material to be welded. Once the probe is completely plunged into the workpiece and the shoulder contacts the face surface, the FSW machine initiates the traverse of the friction stir tool along the weld path or joint. While the FSW machine traverses the tool along the path, the rotation of the tool is maintained, with geometric features on the shoulder and probe displacing and mixing (i.e., stirring) material along the weld joint. Then, when the friction stir tool reaches the end of the path, the friction stir tool is retracted from the joint and finally rotation of the friction stir tool is ceased. During the initial years of research and implementation, it was observed that FSW would locally modify the material properties in and around the weld area. As FSW is an autogenous process FrictionStirProcessingforEnhancedLowTemperatureFormability.DOI:http://dx.doi.org/10.1016/B978-0-12-420113-2.00001-5 ©2014ElsevierInc.Allrightsreserved. 2 FrictionStirProcessingforEnhancedLowTemperatureFormability Downward force Welding direction Tool rotation Shoulder Friction stir welded region Pin Nugget Retreating side Advancing side Figure1.1Schematicillustrationoffrictionstirwelding. Downward force Processing direction Tool rotation Leading edge of Trailing edge of shoulder shoulder Shoulder Friction stir Pin processed region Nugget Retreating side Advancing side Figure1.2SchematicillustrationofFSP. (no filler material), the area of the weld has chemistry identical to the base material. With these two considerations, a variant of the FSW process was developed and is referred to as friction stir processing (FSP) (Mishra et al., 1999; Mishra and Mahoney, 2001). The FSP basically involves the same concept as FSW, but it is generally per- formed on base material, without a weld joint and is shown in Figure 1.2. FSP has been demonstrated to be capable of locally modifying various material properties, including but not limited to ductility/elongation, fatigue properties, static mechanical properties, corrosion properties, hardness, etc. (Mishra and Mahoney, 2007). The ability to modify material properties is material dependent. As such, FSP can be used to locally improve material properties to enhance ConceptofFrictionStirProcessingforEnhancedFormability 3 product capability or to enable different fabrication methods. While there are numerous potential applications, a few such applications include: 1. Enhancing the ductility of various aluminum alloys, enabling form- ing of components versus the traditional approach of fabrication (welding) of a number of detail components. 2. Improving hardness of various steel alloys. FSP can be used to displace the traditional hardness improvement processes such as heat treating, which are energy intensive. In addition, heat treat- ment processes tend to be global in nature, whereas FSP can be used to customize surface properties based on location. 3. Elimination of porosity and improvement of mechanical properties in cast materials. Specific to enhanced formability, potential applications exist in three broad areas: 1. FSP of sheet or plate to enhance or enable superplastic forming (high temperature forming). FSP has been demonstrated to be able to impart superplastic behavior (elongations .200%) in several aluminum alloys. Potential applications in this area would tend to allow replacement of a multicomponent assembly with a formed sheet. Sheet thickness in these applications would tend to be below 6mm although FSP conceptually opens possibilities of higher thick- nesses. Research has indicated that the most viable applications would have volumes in the several hundred to several thousand per year, though recent reductions in cost through automation of super- plastic forming has allowed superplastic forming and its variants to be commercially viable in higher production volume application. In addition, components with higher part counts using traditional fabrication approaches would tend to have better commercial via- bility. An example of an application is door structures for various industries. One such example is a ship door shown in Figure 1.3. 2. FSP of thin plate, followed by room temperature bending. Potential applications in this area could include roll forming or press brake forming of angle, C-channels, and other shapes. These shapes would otherwise be extruded or fabricated (fusion welded) from multiple sections of thin plate. With marine grade Al alloys (typi- cally 5XXX alloys) or with low volumes, extrusions tend to be 4 FrictionStirProcessingforEnhancedLowTemperatureFormability Figure1.3Anexampleofashipdoor. costly making this alternative approach very beneficial in marine fabrication and other applications where 5XXX alloys are used. Plate thicknesses in these applications would tend to be between 6 (1/4v) and 13mm (1/2v). These applications could additionally be characterized as requiring only a single FSP pass. Applications for such a process would be structural components used in multiple industries ranging from marine, truck trailer, rail car, etc. An exam- ple of a section of a structural component (an angle) FSP and then formed is shown in Figure 1.4. 3. FSP of thick plate, followed by room temperature bending. In these applications, fusion welding of multiple sections of thick plate could be replaced by FSP and then a forming operation. Potential applica- tions in this area could include major marine structural members or aluminum armored vehicles. The alternative is especially attractive in armor applications, as ballistic properties would not be affected nega- tively, unlike the traditional fusion welding approach. Furthermore, significant cost reductions could be realized. Plate thicknesses in these