Physical Simulation of Friction Stir Welding and Processing of Nickel-Base Alloys Using Hot Torsion
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INTRODUCTION
FRICTION stir welding and processing are extremely complex thermo-mechanical processes and there is strong motivation to experimentally and computationally simulate the microstructure that evolves from these processes. There is little information on temperature, strain, strain rate, and process forces during the friction stir welding/processing (FSW/P) of Ni-base alloys. In this study, temperature information gathered from actual friction stir processing (FSP) runs was used in an attempt to simulate the actual FSP microstructure using hot torsion. The use of hot torsion has been used successfully to simulate FSW/P microstructures in both steel and titanium alloys.[1–7] Norton originally developed the technique for simulating FSW/P using hot torsion in the Gleeble and showed that actual FSW microstructures could be reproduced in iron and HSLA-65 using an appropriate selection of temperature, strain, and strain rate.[1,2] Sinfield et al.[4] further refined this original work and showed that the microstructure and hardness of FSW in HSLA-65 could be exactly reproduced by hot torsion. His studies also showed that the TMAZ could also be simulated using hot torsion. Failla et al. extended this work to stainless steels in order to study the effect of strain and strain rate during FSW/P on JAMES R. RULE, formerly with the Welding Engineering Program, Department of Materials Science and Engineering, The Ohio State University, 1248 Arthur E. Adams Dr., Columbus, OH 43221, is now Welding Engineer, with BP Inc., 501 Westlake Park Blvd., Houston, TX 77079. JOHN C. LIPPOLD, Professor, is with the Welding Engineering Program, Department of Materials Science and Engineering, The Ohio State University. Contact e-mail: lippold.1@ osu.edu. Manuscript submitted October 24, 2012. Article published online April 19, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite (fcc) at the friction stir welding temperatures. All these simulations used actual temperature data gathered from embedded thermocouples from FSW/P trials. A range of rotation (strain) and rotation rates (strain rates) are then used in an attempt to match the microstructure of the actual FSW/P trials. An example of this is shown in Figure 1 for Ti-6Al-4V where the matching of microstructure for processing both above and below the beta transus is nearly identical. For these experiments, a Gleeble 3800 machine equipped with a torsion unit has been used. The torsion simulation in the Gleeble uses an annular sample with a 1-inch (25 mm) gage length (Figure 2), which is resistively heated to a desired temperature and rotated in torsion at a particular number of revolutions at a given rotational speed (RPM), and then cooled at a programed rate.[1–4] Temperature is controlled and monitored by a thermocouple placed at the center of the gage section. Additionally, thermocouples are placed at the shoulder of the gage section such that the temperature gradient can be recorded and thermal control can be maintained should the center thermocouple detach during testin
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