Brigham Young University (BYU), under the direction of Dr. Tracy Nelson, has been involved in friction stir welding and processing (FSW&P) research since 1998. Initially, FSW research was funded by companies like Lockheed Martin Corp., Boeing, and Alcoa. These initial contracts aided BYU in establishing a research basis that has grown into an international hub for FSW activities.
Over the past seven years, BYU has made significant contributions in FSW&P. Areas of contribution include: Process Development, Microstructural Characterization, Modeling, and Machine design and controls. Presently, there are five faculty, fourteen graduate students, four undergraduate students, and two visiting scholars involved in the FSW activities at BYU. Our faculty and students have published over forty papers in international journals and proceedings, along with over ten patents pending.
Our objectives in the Friction Stir Research Lab (FSRL) at BYU are to: establish a better fundamental understanding of FSW&P, and improve the technology and broaden its application for our industrial and federal sponsors.
Each of the areas of competency in the FSRL is briefly described below.
Process Fundamentals and Development
Researchers at BYU have developed unique FSW&P tooling for joining both aluminum and steels. Over the past few year, research have developed new pin tool designs and process parameters which have shown to minimize, or eliminate, stress corrosion cracking in butt joints, and minimize the interface uplift observed in lap joints in high strength aluminum alloys.
With our unique capabilities in FS equipment, researchers in the FSRL are establishing the basis for understanding the relationship between process parameters, measurable process outputs and possible new control schemes. Our process development capabilities are unrivaled.
BYU has also developed methods for manufacturing built-up stiffened structures via FSW in both aluminum and steels.
Many have demonstrated capabilities in FSW flat panels and structures. BYU developed and patented a method for FSW&P of non-linear complex curvature components. Unlike the bobbin tool technology, the method developed at BYU is simple and inexpensive.
One of the most significant developments in the FSRL has been advances in tool materials for joining steels and other higher temperature materials. Polycrystalline cubic boron nitride (PCBN) tools have been successfully used to join low medium and high carbon steels, stainless steels (austenitic, superaustenitic, duplex and martensitic), and nickel base alloys. Currently, PCBN tool life is 80 m (260 feet) in carbon steel.
The FSRL has successfully developed and applied a novel FSW method that has been successfully applied to polymers. Recent results in polypropylene have exceeded the DVS requirement for joining plastics. Other successfully welded polymers include polyethylene, high density polyethylene, and polycarbonate.
Many of these technologies, and others, patented at BYU are available for licensing through the Technology Transfer office.
Researchers at BYU realized years ago that most FSW equipment being manufactured was insufficient. We recognized the need for a control system that could be modified as understanding of the process evolved. Similarly, FSW equipment was not designed by machine tool manufactures and lacked some of the essential elements that are standard on most machine tools. It was clear that the most feasible option was to design and build a FSW machine that had the desired capabilities.
BYU designed and retrofitted a heavy duty vertical milling machine capable of handling the forces necessary to be successful FSW a wide variety of materials. The machine is fully automated and is capable of controlling weld quality based on the depth of the tool, travel and rotational speed of the tool, the vertical force on the tool, and other important process parameters.
The control system and software were also developed here. As a result, researchers are able to change parameters at any time during the weld, add additional monitoring capabilities, and implement new control schemes quickly as our understanding of the process evolves.
The system also collects all essential process information. Loads and torques on all axes, as well as multiple thermal cycles in both the tool and weld are collected. In addition, tool life as a function of base material, tool design, tool material, total welding time, number of plunges, or linear travel distance are also captured. All data collected is stored in a searchable database.
A user friendly database was designed with research and development in mind, allowing researchers easy access and manipulation of data relevant to their studies. The database is fully searchable within any data or input field (date, program, tool, material, operator, etc.). Researchers can track essential process outputs (loads, torques temperature) as a function of process inputs and compare this with any data in the database.
BYU has published numerous papers that have described the evolution of microstructure associated with FSW&P. Most research has been in high strength low density aluminum alloys. Various means have been utilize to assess texture and texture gradients, recovery and recrystallization, dislocation densities and precipitate morphologies in the weld nugget, TMAZ and HAZ. As a result of these efforts, there is generally a better understanding of the microstructural evolution which has been useful in optimizing the process and validating numerical models.
The FSRL has at its disposal the most technologically advanced equipment to support on-going research and development in FSW&P. Some of the more important capabilities are listed below:
- The FSW-Process Development System. BYU has two Kearney & Trecker milling machines for process development. These machines appear to meet the needs for FSW of HTM better than any of the currently available FSW machines. Current process development systems at BYU are instrumented with a 3-axis dynamometer and variable speed spindle and drive motors. The FSRL is currently negotiating the procurement of a third machine expected to be delivered by January 2005.
- One MTS and two Instron tensile testers ranging in capacity from 100 kN for larger samples down to 450 N for micro tensile samples also allowing axial fatigue testing to be investigated.
- Three scanning electron microscopes including a Phillips XL30 S-FEG and a Phillips XL30 ESEM FEG. OIM™ and EDS capabilities are also available on these microscopes.
- Two transmission electron microscopes including a Technai F30 TEM and a Technai F20 Analytical STEM with EDS and PEELS capabilities.
- One microhardness tester capable of testing on both the Knoop and Vickers scales.
- Four automatic polishers allowing up to 16 samples to be polished simultaneously.
- One inverted stage metallograph and one stereoscope.
- Full precision machine shop including a water jet, two wire EDM machines, one EDM drill, and several CNC mills and lathes.
If you have any questions, please contact: Dr. Tracy W. Nelson