The Center for Friction Stir Processing conducts research in four major areas related to friction stir processing.
1. Tool Design
The tools used for FSP have a great influence on the process. In the CFSP, we investigate how tools can be changed in order to better meet the needs of the process.
Novel Tool Designs
The Center has explored several novel tool designs. The Convex Scrolled Shoulder Step Spiral probe (CS4) tool was developed to allow a tool to run at zero tilt and to compensate for workpiece thickness variations in z-axis position control mode.
Special tools have been developed and explored for use in Friction Stir Spot Welding (FSSW), including CounterflowTM, V-flute, and WiperTM geometries. These tools have been useful in creating in-situ fasteners to connect aircraft skin to structural elements.
Tools have also been designed for use in FSSW magnesium alloy sheet for automotive applications.
Novel Tool Materials
For many alloys, especially those with high softening temperatures, the choice of materials for FSP is critical. The CFSP has compared various refractory alloy tool materials for FSP of titanium alloys.
Effect of Features on Tool Performance
The CFSP has done some of the most extensive work available on the effects of tool features on the FSP process. Given the variety of possible tools, the work is certainly not exhaustive. Nevertheless, the effects of probe features and CS4 geometric parameters on process forces have been explored as part of the Center research program.
2. Process Control
Significant expansion of process capabilities can be expected as the ability to control the process is improved. The Center is involved in ongoing projects to improve in-process control.
Temperature Control on FSP
As an elevated-temperature metalworking process, control of the temperature in the process zone is important in achieving desired final properties. Traditionally, temperature has been controlled primarily by selecting processing parameters that will lead to the approximately correct temperature. Work in the Center has developed improved methods of controlling the tool temperature through a cascade controller. This controller allows the operator to select a desired temperature that wil be automatically maintained.
Thermal Boundary Control in FSP
As a transient elevated-temperature process, thermal boundary conditions have a significant effect on the process zone. Work at the Center has explored how changing the boundary conditions can lead to improved process capabilities and better final properties.
Friction Stir Welding is much more repeatable than most traditional welding processes. In addition, most FSW equipment is capable of monitoring process forces. By careful analysis of the process forces, deliberately-created weld defects have been successfully identified, potentially eliminating the need for post-weld NDE. Work continues to explore the possibilities for adjusting process parameters in response to the in-process NDE signals to avoid the creation of weld discontinuities.
3. Process Fundamentals
Fundamental exploration of the interaction between processing parameters and the resulting material properties have been explored in steel, titanium, aluminum, and magnesium. The understanding of the effects of peak temperature, cooling rate, strain, and strain rate allows for the tailoring of the process to create the desired properties.
Thermal Control in FSP
The tool shoulder, tool features, and the backing plate material have significant effects on the stability of the process and the resulting properties. Careful, intelligent control of the thermal environment of the process has led to new applications and higher productivity and quality in the welding process.
Material Flow in FSP
A variety of tracer studies and numerical simulations have been used to explore the flow of the workpiece material in FSP. In addition to its importance in welding (to ensure a complete weld joint), this is very significant in FSP techniques that involve the creation and modification of particulate-reinforced metal-matrix composites.
4. Innovative Applications
In addition to traditional welding applications, a number of innovative applications of this technology have been developed at the CFSP.
Repair and Return to Service
Repair and refurbishment of aging aircraft is a significant opportunity for supporting the national infrastructure. Research work at the CFSP has focused on ways to use this solid-state processing technology to repair damage to aircraft structures.
It may be possible to replace rivets in aircraft structures by using FSP to form in-situ fasteners (ISF) by deforming the base material and creating new geometry and material properties at the fastening location. The CFSP has explored typical properties available in ISF as a function of processing parameters and tool geometry. Panels created with ISF have been compared with riveted panels and found to perform at least as well. Work continues in the potentially breakthrough technology.
Similarly, it may be possible to replace resistance spot welds in automotive apoplications through the use of Friction Stir Spot Welding (FSSW). This technology has been explored in steel, aluminum, and magnesium sheet materials.
Dissimilar Metal Bonding
Weight reduction is an essential component of the strategies for increasing motor vehicle fuel efficiency. One method of reducing weight is through the increased use of aluminum and magnesium alloys, as well as the use of higher-strength steels. Methods for bonding these different types of metals are essential to their integration in commercial vehicles.
FSW has shown promise in welding of dissimilar alloys. Work at the CFSP has explored FSW of different Al alloys, Al to Mg, and Al to steel.
Embedding ceramic particles into a metal matrix can lead to MMCs with significantly improved properties. FSW as a method of joining these particulate composites has been explored at the Center. FSP is also being explored as method of producing locally-reinforced MMCs with the improved properties tailored to the regions of greatest need.