|
|
|
The Joining Process That's "Stirring" Things Up for Aluminum Applications
12-27-02
You would have had to be in a cave in Afghanistan to have missed the excitement being generated by the friction stir welding process over the past decade. Considering that the process is being touted as being the key enabler in the cost-effective use of aluminum in a number of high performance applications, this excitement may be well justified. This article will provide a general description of the process, some of its advantages and weaknesses, and a sampling of some application areas, and closes with a look at the areas of future development.
Friction stir welding (FSW) is one member of a family of friction-based processes that have been under development, and was patented in 1991 by the Welding Institute (TWI) in the UK. It grew from work on the friction surfacing process, and is considered essentially a three dimensional version of friction surfacing. The process creates frictional heat between the workpieces to be joined, lowering the flow strength of the material, and then uses specially designed tools to create a stirring action that creates a solid state joint. Because the metal being joined is not melted but remains in the solid state, a number of alloys not considered weldable by conventional fusion welding processes can be joined by friction stir welding. While the process has been applied to copper, titanium, metal matrix composites, and magnesium, the majority of the work and application potential has been with aluminum alloys in the thickness range of 1.2 mm to 75 mm in thickness.
The FSW process creates a number of advantages over conventional fusion welding processes, including:
- Ability to weld "unweldable" alloys
- No defects such as blowholes and cracks
- Reduced deformation and shrinkage after joining
- Improved joint properties
- No spatter, fumes, or other environmental concerns
- Overhead welding is possible
Some current limitations include:
- Need for backup material to oppose the high forces imposed during the FSW process
- Tighter gap tolerances required (typically 10% of material thickness maximum)
- Not possible to construct three dimensional surfaces
- Welding on different thickness materials difficult
- Equipment and tooling tailored to the process are just becoming commercially available
- Limitation on joint configurations that can be effectively joined
The FSW process is being used or considered for use in a number of application areas. The first commercial application was in the manufacture of hollow aluminum panels for deep freezing of fish on fishing boats, utilizing FSW of extrusions. This approach has been used in other ship applications, including construction of stiffened panels for fast ferries. Specifically, in 1996, 40 meter high speed catamarans built in Norway utilized FSW panels for portions of the decks, superstructure, bulkheads, and floor. The ability to utilize smaller extrusions joined together into modular, prefabricated components for assembly has greatly changed the nature of aluminum shipbuilding. One development has even involved the evaluation of a prototype portable FSW machine for use at the construction site.
Use of FSW in the aerospace industry has also gained significant attention. A Delta II rocket with a FSW Interstage Module was successfully launched in August 1999. The joining of aluminum-lithium alloy 2195 by FSW in the External Tank application, pioneered by the Marshall Space Flight Center, enables significant weight savings in this application. Additional space components as well as airframe applications are under development. One example is development work at BAE for fabricating prototype wing and fuselage skins for large aircraft, among them the future Airbus A3XX.
Automotive applications of FSW aluminum are also of interest. Fabrication of suspension components and wheels has been discussed. The production of tailor-welded aluminum blanks for subsequent forming has been studied, with supporting technology developments in the areas of tooling and processes to weld different thickness materials. Tower Automotive has developed combinations of tools and control strategies to enable 5-axis robotic FSW processes, and improved FSW travel speeds into the range of 1 m/min on 4 mm thick 6061-T6 material. Evaluation of FSW as an assembly joining process for automotive structures is also underway.
FSW is also being utilized and considered for further deployment in the railcar industry as well as for LNG tanks.
By no means is it thought that the development of the FSW process is complete. Development in the areas of process and tooling are considered to be key at this time. Tooling designs that enable joining of thicker materials, require less power from the equipment, and provide desired weld structures are being explored. Larger and custom-made FSW machines are being made commercially available and installed. Process developments, such as the robotic capability and portable systems as discussed above, are receiving attention.
Furthermore, more fundamental studies of the process to better understand the mechanisms involved are underway. These include efforts to understand the microstructural development in the weld, adjoining thermomechanically-affected zone, and heat-affected zone. Exploratory studies are being carried out to expand the range of materials that can be welded. Development of process models is considered important as well.
It is remarkable that a process development patented only 10 years ago has made such an impact, testament to the need for such a breakthrough. What is equally interesting is the way in which the availability of this process has changed the thinking of product developers and manufacturers alike, allowing them to consider the design and construction of aluminum structures in very new and different ways that can only bode well for expanded applications.
Article provided courtesy of The Aluminum Association - www.aluminum.org
|
|
|
