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Friction stir welding (FSW) is an innovative solid-state material joining method and it has been one of the most significant joining technology developments in the last two decades. It has evolved into a process focused on joining arc weldable (5xxx and 6xxx) and unweldable (2xxx and 7xxx) aluminum alloys to a point where it can be implemented by the aerospace and automotive industries for their joining needs.
A few of the important advantages of FSW over conventional joining techniques include improved joint properties and performance, low-deformation of the workpieces, a significant reduction in production costs and the freeing of skilled labor for use in other tasks. Compared to the conventional arc-welding of aluminum alloys, FSW produces a smaller heat affected zone, and it also allows the successful joining of aluminum alloys, steel, titanium, and dissimilar alloys with a stronger joint.
Despite the initial success of FSW, there are still many challenging problems that need to be overcome for its fully automated industrial application: the optimization of parameters, the detection of defects, and the control of the process. Extensive experimentation for joining a particular combination of materials helps in determining the process parameters for a particular weld setup. Effort has been concentrated on the modeling of the process in order to predict the thermo-mechanical conditions, to better understand the behavior of the workpiece and the conditions which result in successful weld formation and the lowering of residual stresses in the weldments. Process monitoring has been undertaken by capturing and processing the acoustic emission during welding for determining the quality of the weld and the status of the FSW tool (tool wear and tool breakage). Mechanical and microstructural characterization using tensile and peel tests, SEM micrographs and electron probe micro-analysis help in classifying the quality of the welds.
Originally, the FSW has been developed for joining high strength aluminum alloys and advanced aluminum alloys produced by powder metallurgy. The Boeing Company, Lockheed Martin, and NASA are the leading companies in the application of FSW. In principle, the FSW method can be applied to high melting temperature alloys such as nickel, steel, and titanium. There are a number of national and international research groups that have been working on the development of the friction stir welding of high melting temperature materials.
The first application for friction stir welding was the welding of long lengths of material in the aerospace, shipbuilding, and railway industries. Examples include large fuel tanks for space launch vehicles, cargo decks for high-speed ferries, and roofs for railway carriages.
In the last several years, the automotive industry has been aggressively studying the application of FSW in its environment. The drive to build more fuel-efficient vehicles has led to the increased use of aluminum in an effort to save on weight, which also improves recyclability when the vehicles are scrapped. In 2003, the Mazda Motor Corp., Japan, announced that they had developed a spot welding method based on FSW for manufacturing aluminum body assemblies. The technology has been applied for the rear doors and hood of Mazda's 2004 RX-8, a new four-door, four passenger sports car. Mazda reports that, unlike traditional resistance spot welding, the process produces no weld spatter, resulting in a significantly improved work environment.
A number of other companies in automotive industries have been introducing FSW in their production facilities. In the Advanced Materials & Processes, June 2004, Ford announced that their first application of FSW is in the welding of a multi-piece central aluminum tunnel in their new Ford GT. Based on Ford's statement, friction stir welding in comparison to the automated gas metal arc welding, improves the dimensional accuracy of the assembly and produces a 30% increase in joint strength.
In the last fifteen years, FSW has become a mature joining technology of aluminum alloys that is finding application in different industries such as aerospace, automotive, marine, etc. This simplified joining methodology, combined with the higher structural strength of the welds, increased reliability, and the reduced emissions are together estimated to have the potential to produce an annual economic benefit of more than $4.9 billion/year for the U.S. manufacturing industry. However, the application of this technology in joining high melting temperature materials has remained in development.
One new method that can be used to increase the bending limits of thick plates is friction-stir processing (FSP). The FSP method is a variation of traditional friction-stir welding (FSW), but in this case, it can be used to locally modify the properties in a part. The effect of processing the pretensile surface of a heat-treatable aluminum-alloy plate is to locally anneal and refine the grains in the material. This increases the surface ductility to a depth equivalent to the FSP penetration and significantly enhances its bending limit. This ductility increase can be attributed to both the refined and more homogeneous microstructure and to the annealed condition of the surface material. The separate contribution of each has not been determined.
However, in a separate research effort, attempts were made to create an annealed surface in an aluminum plate by rapid, localized induction heating. Frequencies were adjusted to provide shallow, skin-depth heating. Further, the goal was to minimize the loss of strength in the remainder of the plate, by rapid extraction of heat from the opposite side. This would have provided an annealed surface without the severe deformation associated with FSP, which leads to the recrystallization and the fine microstructure.
Unfortunately, due to the very high thermal conductivity of aluminum alloys, it was not possible to anneal a reasonable depth of material (at least 3-mm deep) without excessively overaging the remainder of the plate. Regardless of the rates of heating and cooling, a static thermal loading eventually resulted in a steady-state temperature profile, which caused softening to a much greater degree than was desired. Similar results were obtained using transient temperature profiles, i.e., induction power was cycled per a predetermined theoretical best-condition cycle. Based on these experiments, it does not appear that conventional heating approaches can selectively anneal thick aluminum plate without significantly reducing mechanical properties in the remainder of the plate.
The promise of using FSP as a method of tailoring material properties locally in a part leads us to develop a predictive model that will allow us to evaluate the effect of processing on bending performance. Most of the simulation work done thus far on FSW and processing has focused on the process itself. One of the first predictions of the thermal profile that occurs during FSW was based on the Rosenthal equation and on models that assumed that the tool had a moving heat source. These approaches simplified the problem by using an analytical model to calculate the heat input from friction and deformation.
Finite-element methods have been used to model heat transfer and to understand material flow, especially in steady-state conditions, using either an Eulerian approach or an Arbitrary Lagrangian Eulerian (ALE) approach, in which the material is modeled as a viscous fluid. This last approach provides information about material flow, both allowing for the evaluation of different tooling designs and permitting a temperature calculation based on material deformation.