Friction Stir Welding and Processing IX: High Temperature Applications II
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

Tuesday 8:30 AM
February 28, 2017
Room: 9
Location: San Diego Convention Ctr

Session Chair: Murray Mahoney, Retired from Rockwell Scientific; Hidetoshi Fujii, Osaka University

8:30 AM Introductory Comments

8:35 AM  Invited
Evaluation of Ausformed H13 Tool Steel for FSW Tools: Murray Mahoney1; John Baumann2; Anthony Reynolds3; 1Retired from Rockwell Scientific; 2Boeing; 3University of South Carolina
    The operating environment for FSW tool pins include rotating, bending fatigue with a high mean stress, plus torsional and compressive loads all at a moderately elevated temperature (~482°C). The majority of FSW tool designs include features to facilitate material flow during FSW creating stress concentrations on the pin surface. In current commercial practice, the combination of these conditions results in a high rate of tool failure in welding high strength aluminum alloys for weld penetrations >12 mm. In prior studies, Ausformed H13 tool steel showed promise as a new tool material compared to MP159. However, ausforming requires thermomechanical processing and the method initially used was rolling. The yield following rolling was low due to distortion. Thus, equal channel angle extrusion was investigated as a more efficient approach to impart the required warm work. Results of these studies include notched tensile properties and notched rotating bending fatigue results, both at 482°C.

8:55 AM  Invited
Development of Friction Stir Processing for Repair of Nuclear Dry Cask Storage System Canisters: Kenneth Ross1; Ben Sutton2; Glenn Grant1; Gary Cannell3; Greg Frederick2; Robert Couch2; 1Pacific Northwest National Laboratory; 2Electric Power Research Institute; 3FLUOR
    The Nuclear Regulatory Commission has identified chloride-induced stress corrosion cracking (CISCC) of austenitic stainless steel dry cask storage system (DCSS) as an area of great concern. Friction Stir Processing (FSP) was used to repair laboratory- generated SCC cracks in representative stainless steel 304 coupons. Results of this study show the FSW is a viable method for repair and mitigation CISCC cracks. This paper highlights lessons learned and developed techniques relative to FSP development for crack repair in thick section stainless steel 304. These include: development of process parameters, welding at extremely low spindle speed, use of weld power and temperature control and optimization of these controls. NDE and destructive analysis are also presented to demonstrate effectiveness of the developed methods for SCC crack repair.

9:15 AM  Invited
Friction-Stir-Processing Microstructure Improvement Related to Fatigue-strength and Charpy-absorbed-Energy Increase of TIG-welded SS400 Steels: Kazuhiro Ito1; Tatsuya Okuda1; Hiroki Izumi1; Makoto Takahashi1; Kazuyuki Kohama1; Hajime Yamamoto1; Hidetoshi Fujii1; 1Osaka University
    To improve fatigue strength of high-tensile-steel welds, friction stir processing (FSP) is an effective grain-refinement technique for the topmost layer of the welds. In this study, fatigue-strength and Charpy-absorbed-energy increase was found in the tungsten inert gas (TIG)-welded SS400 plates. Fatigue life exhibited more than 5.0x106 cycles to failure, which is more than 2000% increase in comparison with those of the TIG specimens, at an applied stress amplitude of 270 MPa. Its Charpy absorbed energies exhibited about 110 J in the temperature range between -110°C and 40°C. FSP produced microstructure consisting of ultrafine eqiaxial ferrite grains beneath the TIG-welded surface. A cross-sectional TEM image indicates that an about 2 nm-thick thin layer of cementite was warped round the ultrafine ferrite grains in a certain FSP condition with low heat input. This can be attributed the related increase in fatigue strength and Charpy absorbed energy.

9:35 AM  Invited
Performance of Tungsten-based Alloy Tool Developed for Friction Stir Welding of Austenitic Stainless Steel: Yutaka Sato1; Ayuri Tsuji2; Tomohiro Takida2; Akihiko Ikegaya2; Akinori Shibata3; Hiroshi Ishizuka3; Hideki Moriguchi3; Shinichi Susukida1; Hiroyuki Kokawa1; 1Tohoku University; 2Allied Material; 3Nippon ITF
    Tool damage is critical during friction stir welding (FSW) of steels. As the cost-effective tool material, tungsten-based alloys containing hard particles, exhibiting high strength, ductility, and thermal conductivity at high temperatures, were newly developed for FSW of austenitic stainless steel in this study. The tungsten-based alloy tools containing various hard-particles were employed in FSW of type 304 austenitic stainless steel, and the wear behavior was examined. The tungsten-based alloy tools coated by a ceramic exhibited the good performance during FSW, but the tool wear could not be completely prevented. Degree of the tool wear depended on the high-temperature strength, i.e., it decreased with increasing high-temperature strength. The tool wear was significantly suppressed in the tungsten-based alloy with the higher high-temperature strength, and the tool shape was hardly changed even at the weld length of 20 m on the tool with 1.7mm long probe.

9:55 AM Break

10:10 AM  
Microstructure and Mechanical Properties of Beta-type Ti-15V-3Cr-3Al-3Sn Alloy Joints Fabricated by Friction Stir Welding: Huihong Liu1; Hidetoshi Fujii1; 1Joining and Welding Research Institute, Osaka University, Japan
    Ti-15V-3Cr-3Al-3Sn (Ti-15-3), as a commercial beta-type titanium alloy, has been used for aerospace applications because of its excellent mechanical properties, cold formability, and aging strengthening response. Conventional fusion welding is known to cause several issues such as the formation of columnar grains, high residual stress, large deformation, and macro segregation which deteriorate the quality of the weld joints. Friction stir welding (FSW) is an innovative joining technique enabling solid-state joining thereby facilitating the avoidance or minimization of fusion-welding-associated problems. In this study, FSW with various processing conditions was performed on the Ti-15-3 alloy. Microstructural evolution and mechanical properties of weld joints were systematically investigated in order to optimize the welding conditions to obtain a sound joint. Additionally, since previous studies mainly focused on the FSW of alpha- and (alpha+beta)-type titanium alloys rather than the beta-type alloys, this study also provides a complement understanding of the FSW of titanium alloys.

10:30 AM  Invited
Effect of Hydrogenation on Superplastic Behavior of Nugget in Friction Stir Welded Ti-6Al-4V Joints: Z.Y. Ma1; L.H. Wu1; B.L. Xiao1; 1Institute of Metal Research, Chinese Academy of Sciences
    The superplastic behavior of the nuggets in friction stir welded joints of Ti-6Al-4V alloy with or without hydrogenation was investigated at temperatures of 700-925oC and at strain rates of 1×10-4~3×10-2 s-1. The nugget without hydrogenation consisted of an ultrafine lamellar microstructure 178 nm in thickness and 7.9 in aspect ratio. A maximum elongation of 740% was achieved at 925oC and 1×10-3 s-1, which were mainly attributed to the globularization of the lamellae during annealing and superplastic deformation. The hydrogenated nugget also consisted of ultrafine lamellar microstructure within lots of martensites and hydrides, which showed a much lower optimum temperature and flow stress compared to the un-hydrogenated nugget. The enhanced superplastic deformation abilities of the nugget with hydrogenation were likely attributed to the hydrogen-induced effect. The enhancing mechanism was discussed in detail.

10:50 AM  
Investigation of Process Parameters for Friction Stir Processing (FSP) of Ti-6Al-4V Alloy: Sandip Chougule1; Digvijay Sheed1; Rajkumar Singh1; Nithyanand Prabhu2; Bhagwati Kashyap2; Kaushal Jha3; 1Bharat Forge ltd.; 2Indian Institute of Technology, Bomaby; 3Bhabha Atomic Research Centre, Mumbai
    Friction stir processing of the Ti-6Al-4V alloy was carried out. Various process parameters (tool traverse speed and tool rotation speed) were studied for successful FSP of Ti-6Al-4V. The process parameters were identified using macrostructure observation on the surface of processed plate and microstructure evolution in the stir zone (SZ) of the FSP specimen. The effect of tool traverse speed and tool rotation speed on microstructure evolution in the SZ, thermo-mechanical affected zone and heat affected zone were studied. The microstructure transformation from initial elongated α structure to prior β grains with α layer grain boundary consisting mixture of acicular α’ and very fine lamellar α/β colonies was observed at SZ for all the parameters except for the tool rotation speed of 600 rpm and traverse speed of 60 mm/min and 100 mm/min. Bands of DRX α and transformed β structure observed at SZ of 60 mm/min and 100 mm/min parameters.