Friction Stir Welding and Processing IX: Dissimilar Applications
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Shaping and Forming Committee
Program Organizers: Yuri Hovanski, Brigham Young University; Rajiv Mishra, University of North Texas; Yutaka Sato, Tohoku University; Piyush Upadhyay, Pacific Northwest National Laboratory; David Yan, San José State University

Wednesday 2:00 PM
March 1, 2017
Room: 9
Location: San Diego Convention Ctr

Session Chair: Yuri Hovanski, Brigham Young University; Guntram Wagner, University of Chemnitz, Germany

2:00 PM  Invited
Joining Aluminum Alloys to High Strength Steels by Friction Spot Welding: Uceu Suhuddin1; Vanessa Fischer2; Jorge dos Santos1; 1Helmholtz-Zentrum Geesthacht; 2Federal University Rio Grande do Sul
    In order to reduce car body weight, many car producers try to increase the content of lightweight materials such as Al alloys and high strength steel in their products without sacrificing safety. Consequently joining dissimilar materials of Al to high strength steel is inevitable. One of the promising solutions in welding hybrid aluminum-steel combination is by solid-state friction-based joining process. In current presentation, the weldability of dissimilar joints of Al to high strength steel of dual phase steel (about 600 MPa ultimate tensile strength) and hot stamped boron steel (1500 MPa ultimate tensile strength) in lap joint configuration by friction spot welding (FSpW) also known as refill friction stir spot welding were studied. The results shows the mechanical properties of the joint with regards to lap shear strength exceed the minimum requirement according to American Welding Society standard for resistance spot welding for aerospace applications, AWS D17.2.

2:20 PM  Invited
Joining Dissimilar Material Using Friction Stir Scribe Technique: Piyush Upadhyay1; Yuri Hovanski1; Leo Fifield1; Blair Carlson2; Eric Boettcher3; Robert Ruokolainen4; Peter Busuttil5; 1Pacific Northwest National Laboratory; 2General Motors; 3Honda R & D Americas; 4FCA; 5 Kuka Systems North America, LLC.
    The ability to effectively join materials with vastly different melting points like Aluminum-Steel, Polymer composites - metals has been one of the road blocks in realizing multi-material components for light weighting efforts. Friction stir scribe (FSS) technique is a promising method that produces continuous overlap joint between materials with vastly different melting regimes and high temperature flow characteristics. FSS uses an offset cutting tool at the tip of the FSW pin to create an insitu mechanical interlock between material interfaces. With investments from Vehicle Technology office, US DOE and several automotive manufacturers and suppliers PNNL is developing the FSS process and has demonstrated viability of joining several material combinations. Details of welding trails, unique challenges and mitigation strategies in different material combinations will be discussed. Joint characterization including mechanical tests and joint performances will also be presented.

2:40 PM  
Influence of Stir Flow on Joint Quality during Friction Stir Lap Al-to-Cu Welding: Doddy Parningotan1; M. Tarrant2; Z.W. Chen1; A. Hilton1; T. Pasang1; 1Auckland University of Technology; 2National Aluminium Ltd
    Discontinuity free interface region of Al-to-Cu lap joints are required for high mechanical strength and low electrical resistance. While discontinuity free Al-to-Cu lap joints are difficult to produce using fusion welding, reviewing the literature, there is insufficient information on how such joints can be made by friction stir lap welding (FSLP). We will first briefly explain how material flow can cause voids, cavities or cracks during FSLW. Then, FSLW experiments are conducted using a tool design coupled with an appropriate positioning so that void-cavity free joints can be obtained. It has been found that speed conditions that cause insufficient downward flow also have resulted in insufficient formation of the interface layer thus in lack of joint. However, conditions that cause excessive downward flow also cause the intermetallic layer to crack. The balance of a mildly downward flow and a moderate growth of the interface layer without cracking will be demonstrated.

3:00 PM  
Process Force Reduction during Robotic Friction Stir Welding of Aluminium Alloys with Reduced Tool Aspect Ratios: Anna Regensburg1; René Schürer1; Michael Grätzel1; Michael Hasieber1; Jean Pierre Bergmann1; Jan Ansgar Gerken1; 1Technische Universität Ilmenau
    Friction Stir Welding has increasingly been gaining relevance for joining nonferrous metals, especially aluminum alloys. However, when considering FSW as an alternative for conventional processes, some characteristics of FSW like high process forces and clamping requirements still represent drawbacks regarding accessibility and work piece loading capacity. The tendency towards lightweight design and more complex components amplifies this challenge. Therefore, the objective of this investigation is to evaluate the scaling effects during FSW with reduced tool diameters at adjusted process parameters, in order to keep the shear stress conditions in the shear layer constant and hence counteract the compromised mechanical loadability of the scaled tools. The experiments are carried out for EN AW 5754 and 6060 sheets of different thicknesses and the potential of axial force reduction by reducing the material volume displaced by the pin and hence reduce the load on the workpiece and the required spindle torque are demonstrated.

3:20 PM  
Intermetallic Phase Formation at Al-steel Solid-state Joints – A Comparison between FSW and VFAW Processes: Genevieve Lee1; Kaleb Ponder1; Ali Nassiri1; Bert Liu1; Glenn Daehn1; Antonio Ramirez1; 1The Ohio State University
    In the pursuit of manufacturing lighter, more efficient vehicles, formation of brittle intermetallic compounds along Al-alloys to steel joint interfaces has proved a remarkable challenge, precluding its widespread use in the automotive industry. Intermetallic phases form rapidly along the Al-Fe interfaces, compromising the mechanical performance of these joints. Friction Stir Welding (FSW) parameters can be optimized for Al-steel joints, producing welds with minimal fractions or elimination of intermetallics. Alternatively, recently developed Vapor Foil Actuator Welding (VFAW)––which impacts an aluminum flyer plate onto a steel substrate––creates metallurgical bonding within microseconds. However, intermetallics are observed even on near-instantaneous welds. Dissimilar Al-steel FSW and VFAW joints have been extensively studied by Scanning Electron Microscopy (SEM) imaging, Transmission Kikuchi Diffraction (TKD) and Energy Dispersive X-ray Spectroscopy (XEDS) to clarify fundamental differences and further optimize both processes. The high-energy low-weld-time VFAW technique produces intermetallics along interfaces, while low-temperature long-weld-time FSW exhibits no observable intermetallics.

3:40 PM Break

4:00 PM  Invited
Avoiding Melting in Friction Stir Welds of Highly Dissimilar Melting Temperature Materials: Christian Widener1; Bharat Jasthi1; Todd Curtis1; MD. Shamsujjoha2; 1South Dakota School of Mines and Technology; 2University of Massachusetts, Amherst
    The main objective of this work is to discuss why melting is avoidable in friction stir welding even for highly dissimilar melting temperature materials by reviewing the mechanisms related to melting and their interaction with the heat generation and conduction phenomena occurring within the weld. Due to the welding temperature mismatch, it would be tempting to predict that welding dissimilar materials such as aluminum to steel would result in melting of the lower temperature alloy, however, this is generally not the case. In fact, successful welds have been created between these two materials with no signs of melting and at most intermetallic formation at the interface. In order to better understand this observation, a simple model is proposed for predicting and understanding where melting is more and less likely, along with some successful examples of various dissimilar metal combinations, including Al-steel, Cu-steel, and aluminum to polyphenylsulphone (PPSU).

4:20 PM  
Automated Optical Visualization of Materials Flow in Dissimilar Metal Friction Stir Welds: John Sosa1; Hamish Fraser1; Rajiv Mishra2; Satya Ganti3; Bryan Turner3; Brian Hayes3; Veeraraghavan Sundar3; 1The Ohio State University; 2University of North Texas; 3UES Inc.
    The emergence of 3D characterization tools has permitted significant advancement in the field of materials characterization. As an emerging solid-state joining process, friction stir welds feature multiple microstructural characteristics such as cracks, flow patterns, flash, and pores that can benefit from 3D analyses. Material flow in friction stir welds, especially with dissimilar welds, is a complex phenomenon. Typically visualization of material flow is achieved with embedded markers. In this study, Robo-Met.3D, an automated system for 3D materials characterization, was extended to characterize material flow in aluminum-steel joining. The optical microscopy data was analyzed using MIPAR, a software package capable of performing each stage of post-acquisition processing and analysis. The resulting visualizations provide insights into the friction stir joining process.

4:40 PM  
Realization of Ultrasound Enhanced Friction Stir Welded (USE-FSW) Al/Mg- and Al/Steel-Joints: Process and Robustness, Mechanical and Corrosive Properties: Marco Thomae1; Guntram Wagner1; Benjamin Strass2; Bernd Wolter2; Sigrid Benfer3; Wolfram Fuerbeth3; 1University of Chemnitz; 2Fraunhofer Institute for Nondestructive Testing IZFP Saarbrücken; 3DECHEMA-Forschungsinstitut
    As an innovative hybrid joining process USE-FSW was successful applied on Al/Mg-Joints and offers a beneficial impact on the resulting microstructure and mechanical properties. The always during the conventional FSW-process developing continuous Al3Mg2 and Al12Mg17 intermetallic layers can be avoided by USE-FSW as a result of the into the joining area transferred ultrasound power energy. In this case the intermetallic phases are spread over the complete Nugget area. Consequently the tensile strength increases 25 % and the fatigue strength rises 3.5 times. Based on this USE-FSW was investigated with regard to possible advantages for Al/Steel-Hybrid-Joints. Pre-investigations have shown significant differences in the microstructure of ultrasound enhanced and non-ultrasound enhanced EN AW-6061/DC04-Joints. While conventional FSW led to hooks and large particles of steel in the aluminum, the USE-FSW-Joints show remarkably smaller and more homogenous distributed steel particles in the Nugget. Beside mechanical investigations non-destructive and corrosive tests are carried out.

5:00 PM  
A Numerical Simulation for Dissimilar Aluminum Alloys Joined by Friction Stir Welding: Carter Hamilton1; Mateusz Kopyściański2; Aleksandra Węglowska3; Stanisław Dymek2; Adam Pietras3; 1Miami University; 2AGH University of Science and Technology; 3Institute of Welding
    Dissimilar aluminum alloy sheets of 2017A-T451 and 7075-T651 were friction stir welded. A numerical simulation was developed to visualize the material flow and temperature distribution and to correlate the microstructure with hardness behavior. Due to the flow of surface material into the workpiece thickness, the weld nugget is composed of alternating layers of 7075 and 2017A. These layers have unique temperature histories depending on the material’s initial position; therefore, they also have distinctive precipitate distributions. Supersaturated surface material flows into the process zone and forms a core in which GP zones re-precipitate upon cooling. Mid-plane and bottom-plane material flow toward the workpiece surface and encompass the surface material core. Within this region, the weld temperatures overage the equilibrium θ phase in 2017A, decreasing the hardness, and at the same time dissolve the equilibrium η/T phase in the 7075, leading to re-precipitation of GP zones upon cooling and a hardness recovery.