Impact Welding and Impulse Shape Calibration of Nickel and Titanium Alloys Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Bhuvi Swarna Lalitha Nirudhoddi Graduate Program in Materials Science and Engineering The Ohio State University 2019 Dissertation Committee Dr. Glenn S. Daehn, Advisor Dr. Xun Liu Dr. Stephen R. Niezgoda 1 Copyrighted by Bhuvi Swarna Lalitha Nirudhoddi 2019 2 Abstract High-temperature metallic materials such as nickel-based and titanium alloys are attractive as skin structures for aerospace vehicles. They can allow significant performance improvement and mass reduction in aircraft. However, there are substantial challenges in welding and forming them affordably for service. This project examines the use of impulse-based methods, as enabled by the vaporizing foil actuator method, for the impact welding and precise shaping of alloys Ni - 718, Ni - 625, Ni - 230, and Ti 6242. The mechanical properties and weld microstructure of four similar and Six dissimilar VFA spot welding combinations are presented and analyzed. Microhardness measurements showed the absence of a heat affected zone (HAZ). The dissimilar Ni - Ni joints and Ni - Ti joints exhibited high loads to failure in lap-shear tests and show great potential for applications involving transition joints, repair welding, medical devices, and more. The VFA method is cheap, safe, fast, durable, and marks the advancement in the solid-state joining of dissimilar nickel and titanium systems. Nickel alloys typically exhibit low springback during quasi-static forming processes. However, the large amounts of strain hardening that occurs during these operations often requires a second annealing stress relief operation. Titanium alloys are commonly known to exhibit high springback levels due to the high strength to stiffness ratios of titanium alloys. Sheet metals components are usually shaped by hot or ii superplastic forming. This process is expensive and has long lead-times. This work examines an athermal process to relax or remove the residual stresses and elastic strains in sheet metals. All the materials explored, especially titanium showed significant improvements in shape conformance when processed through the VFA method. Recent shock-based calibration studies have provided some insight into the previously unconfirmed mechanism of springback relief. The driving hypothesis for this physical phenomenon is that modest shock waves plastically relieve elastic residual stresses and result in target shape conformance. To better understand this mechanism, VFA based shock processing experiments were used to change the curvature of pre- strained materials to a fully flat shape. It is speculated that the change in shape is a consequence of elastic stress relief caused by the propagation of planar shockwaves. A mechanics and shock physics based theory for shockwave interaction with residual stresses in a pre-strained sample is proposed. The possible pressures involved in this process were calculated from shock breakout velocity profiles captured by a photon doppler velocimetry system. The preliminary pressures are estimated to have satisfied the plastic yield criterion for this loading condition. iii Dedication This dissertation is dedicated to Lord Sri Krishna, my beloved grandparents, parents, sister, brother-in-law, and nephew. I am eternally grateful for their boundless love, encouragement, and sacrifice. Sarvam Sri Krishnarpanam Astu iv Acknowledgments I would like to thank my advisor, Dr. Glenn Daehn, for his kindness, guidance, and generosity throughout my graduate career at Ohio State. I am very grateful for his patient explanations of complex phenomena and his undiminishing belief in my capabilities of understanding them. My family and I are forever in debt to his generosity and encouragement. I am thankful for the mentorship of Dr. Anupam Vivek and Geoff Taber through my time at the Impulse Manufacturing Lab (IML). Their knowledge, expertise, and perspective knows no bounds and has been instrumental in pushing me to think beyond the confines of the known. I would also like to thank my committee members Dr. Stephen Niezgoda and Dr. Xun Liu for the support, patience, and expertise offered through this process. I am very grateful to my past and present IML colleagues, particularly Brian Ufferman, Angshuman Kapil, and Yu Mao. They have been crucial to my journey of navigating the world of materials science. I would also like to acknowledge Dr. Steve Hansen for his detailed analysis and meticulous documentation of capturing shockwave breakouts. His efforts provided the groundwork for moving the technique forward. The microscopy and characterization analysis presented in this work would not have been v possible without the help of Jianxiong Li, Taylor Dittrich, Claire Cary, Keely Shorter, Chris Wagner, and Wayne Papageorge. My dreams of pursuing a higher education would not have been possible without the compassion of Dr. Tom Shih from Purdue University, who told me to always look for the opportunity in defeat. In these past ten years, I have been fortunate enough to find great friendship, support, and community, no matter where I lived in the US. I am especially grateful to Dhruv Garg, Vindhya Tumati, Kevin Vuong, Marlina Triesjayanti, Jing-Wei Lee, my fellow WEGC sisters, and the kind devotees of ISKCON Columbus for their encouragement and kinship throughout my time here. Finally, I would like to thank my family. It is impossible to put into words the amount of love and gratitude I feel towards them. No matter how many oceans and continents have parted us, I have always felt their love and support. It has been a blessing and a privilege to have been born to such humble parents and to have grown up with such an amazing sister. Thank you for believing in the little girl who dreamt of voyaging the cosmos. vi Vita 2013 B.Sc., Aeronautical and Purdue University Astronautical Engineering 2018 M.Sc., Materials Science and The Ohio State University Engineering 2015 - present Graduate Research Associate Department of Materials Science and Engineering, The Ohio State University Publications B. P. Thurston, A. Vivek, B. S. L. Nirudhoddi, and G. S. Daehn, “Vaporizing foil actuator welding,” MRS Bulletin, vol. 44, no. 8, pp. 637–642, 2019. P. Stechmann, David & Lim, Dasheng & Rotella, Saverio & Menon, Shankar & Nirudhoddi, Bhuvi. (2014). Design and Analysis of a High-Performance Hydrogen Peroxide Thrust Chamber Assembly. 50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference 2014. 10.2514/6.2014-3500. Fields of Study Major Field: Materials Science and Engineering vii Table of Contents Abstract ............................................................................................................................... ii Dedication .......................................................................................................................... iv Acknowledgments............................................................................................................... v Vita .................................................................................................................................... vii List of Tables ...................................................................................................................... x List of Figures .................................................................................................................... xi Chapter 1. Introduction ....................................................................................................... 1 Chapter 2. Impact Welding of Nickel and Titanium Alloys .............................................. 5 2.1 Background and Motivation ..................................................................................... 5 2.2 Literature Review...................................................................................................... 7 Similar Welding .......................................................................................................... 7 Dissimilar Welding ................................................................................................... 17 2.3 Materials and Methods ............................................................................................ 22 Materials ................................................................................................................... 22 Experimental Methods .............................................................................................. 24 Mechanical Testing and Characterization ................................................................. 31 2.4 Results and Discussion ........................................................................................... 37 Similar Patch Welds .................................................................................................. 39 Dissimilar Spot Welds: Nickel – Nickel ................................................................... 53 Dissimilar Spot Welds: Nickel – Titanium ............................................................... 62 2.5 Summary and Conclusions ..................................................................................... 77 Chapter 3. Impulse Based Shape Calibration using the Vaporizing Foil Actuator Method ........................................................................................................................................... 83