Airbus evaluates friction stir welding
Dr. Paul Colegrove earned his PhD at the University of Cambridge on the Modeling of Friction Stir Welding. Most recently he accepted a position as a lecturer in welding engineering at Cranfield University.
Dr. Paul Colegrove of Cranfield University reports on the benefits of Friction Stir Welding and the modeling conducted by Airbus. A custom user interface to a COMSOL Multiphysics model allows Airbus engineers to quickly determine a weld`s thermal properties and strength.
Patented in 1991, Friction Stir Welding (FSW) has since been used widely to create strong joints in aluminum alloys. The aircraft industry has started to adapt this technology, and now the largest manufacturers-including Airbus, an EADS Company-are studying how to cut manufacturing costs with it. First, though, they want mathematical modeling to help them fully understand the process before making massive investments in retooling their manufacturing lines.
As an aluminum-joining technology, FSW offers significant improvements over arc welding or riveted joints. The welds are stronger, lighter, and can be done more quickly. Especially useful in welding aluminum alloys that are otherwise difficult to join, the process results in low distortion, you can weld thick sections in one pass, and produce welds with excellent mechanical properties.
In the FSW process (Figure 1), a cylindrical tool made up of a shoulder and a threaded pin is spun and inserted into the joint between two pieces of metal. The rotating shoulder and the pin generate heat-but not enough to melt the metal. Instead, the softened, plasticized material forms a solid phase made up of a fine-grained material with no entrapped oxides or gas porosity. The crushing, stirring, and forging action produces a joint with a finer microstructure than the parent material. The process can even join dissimilar aluminum alloys, while FSW joints can have twice the strength of riveted joints.
Already in the air
Figure 1: In Friction Stir Welding, a shoulder sits on the metal surfaces and creates heat through friction, while a pin stirs the plasticized-heated materials to create the weld. (photo courtesy of Eclipse Aviation, Albuquerque, NM)
FSW is used in a number of industrial applications, including within the aerospace industry. In commercial airlines, however, its adoption has been slower because those manufacturers must be certain of a process´quality before making the enormous financial investments needed to implement it. Thus far, only one aircraft manufacturer, Eclipse Aviation of Albuquerque, New Mexico, uses FSW. This firm manufacturers the Eclipse 500, a 6-passenger Very Light Jet (VLJ). Its economic advantages are due in large part to FSW, which the firm reports as being energy efficient. One tool can typically be used for 1000 meters of joint length, there is no filler wire or gas shielding, no welder certification is required, and there is no grinding, brushing, or pickling necessary for mass production.
Aware of these advantages, the European manufacturer Airbus is investigating plans to introduce this technology into its manufacturing plants. Riveting is a slow, labor-intensive process, and replacing rivets with a continuous welded joint not only results in faster manufacturing at lower costs, but the distributed load results in a stronger joint. However, because large airliners experience higher stresses and shorter fatigue life, the technology must be brought online carefully. Many aspects of the tool and operating parameters can influence the amount of heat and the quality of the weld: the raw materials, tool diameter, tool geometry, rotational speed, weld travel speed, and down force. This process must be optimized for the materials used in commercial aircraft.
Thus Airbus asked several institutions to join it in a consortium to study FSW. During my time at Cambridge University I developed the heat-generation model presented in this article, while Nicholas Kamp, Joseph Robson, and Andrew Sullivan from Manchester University studied the microstructural aspects.
Closeup examination of a weld
One of the first results was a research project that created a mathematical model of FSW that allows Airbus engineers to look "inside" a weld to examine temperature distributions and changes in microstructures. To enable Airbus engineers to access the model easily, we created a GUI-driven simulation tool so they could look at a weld´s thermal properties and ultimate strength.
The COMSOL Multiphysics model couples a 3D thermal analysis, for calculating heat flow, to 2D axisymmetric swirl flow simulations, for calculating both the flow and heat generation (Figure 2). The thermal analysis calculates the 3D temperature field from the heat flux imposed at the tool surface. It captures the effect of the tool movement, the thermal boundary conditions, and the thermal properties of the material being welded. The model then extrudes the temperature distribution near the tool surface from the 3D boundary to the domain in the 2D model.
Figure 2: The multigeometric multiphysics model of Friction Stir Welding couples a 3D thermal analysis to 2D axisymmetric swirl flow.
That next part of the model, in turn, analyzes the rotational flow of material through a 2D cross section underneath the shoulder. As a final step, it calculates the overall heat flux from this section and sends it back to the 3D analysis.
The analyses use specific modeling interfaces available in the Heat Transfer and Chemical Engineering Modules as well as tools for coupling the variables from the 2D and 3D modeling domains. Use of the 2D modeling domain saved computational memory and time, while results deviated only 1% to 2% from full 3D models of the flow. The analyses are solved simultaneously to guarantee faster and better solution convergence.
The final output plot is the 3D thermal profile (Figure 5). Here an engineer can calculate various statistics such as the temperature of the welded material at the shoulder and tip of the pin as well as the power input (or heat generation).
Obvious choice for modeling
Looking at the model requirements, it´s easy to see why COMSOL Multiphysics was the obvious choice. Not only is it easy to use, but we were also able to link the 2D axisymmetric physics to 3D physics - something that would be extremely difficult to do with other packages. It was also easy to link in the microstructural-material aspects, which we developed in MATLAB® (MathWorks, Inc.).
The model couples a 3D thermal analysis to 2D axisymmetric swirl flow.
Airbus is funding a follow-on project that will allow us to refine the model, both in terms of thermal and microstructural analysis. COMSOL Multiphysics´ flexibility and ability to simplify complex physical processes will be an integral part of this work. Furthermore, FSW is a very complex process, and we still want to perform considerable experimentation and validation to make the model as accurate as possible.
Read the research paper at:
www.comsol.com/academic/papers/1614