Paper published as 'Brave new weld' in Aerospace Testing Technology International, issue 1, July 2002.
* Dr David McKeown was Marketing Manager at TWI Ltd at the time of publishing. He worked for 11 years on various aspects of welding aerospace materials before joining a welding equipment and consumable manufacturing company. He returned to TWI in 1995.
TWI Ltd is the largest independent research and technology organisation in Europe and it specialises in contract research, technology transfer and failure analysis in the field of welding, bonding and other joining technologies.
It is all very well to invent a new joining process and tell everyone how it could change their thinking but until joints have been tested and proven to be acceptable for an application, all is speculation. Friction stir welding isa process that has cleared this hurdle and is now in increasing use in the aerospace and other industry sectors.
Friction stir welding (FSW) was invented and patented by TWI Ltd of Cambridge, UK in 1991. The process uses a non-consumable rotating tool, which moves along the joint line between two abutting components. Many materials may now be friction stir welded [1] but the first commercial welds were in aluminium alloys and these remain the most important in the aerospace field. The rotating tool is made from a wear-resistant material and it is plunged into the material along the jointline then traversed to make a seam. The rotation generates heat that plasticises the material such that it welds together without actually melting forming a continuous weld seam between the components. The aerospace aluminium alloys, the 2000 and 7000 series, are generally considered unweldable because of solidification related problems. Since there is no fusion, such problems do not exist in FSW and the process can be used on just about all aluminium alloys, without the need for filler or shielding gas.
To be acceptable to an industry as safety critical as aerospace, a number of performance criteria must be determined for any joining process. Of major importance are mechanical properties, fatigue performance, defect occurrence and corrosion performance. TWI has been involved in determining these and more for interested parties. [2] An hydraulic burst test was also required on fuel tanks for rockets. FSW has passed all these tests with flying colours!
Weld Strength
Of foremost importance is the level of strength that can be achieved by any joining process. Fusion welding aluminium alloys often results in both the weld metal and the heat-affected zone (HAZ) being of lower strength than the parent material. Friction stir welded joints in annealed material can produce as-welded strength of the weld nugget in excess of parent plate properties due to the massive mechanical deformation occurring during production of the weld. Cross-weld tensile tests show this, failing in the unaffected material well away from the weld and HAZ.
The weld properties of heat-treatable aluminium alloys can be improved by controlling the thermal cycle, in particular by reducing the annealing and over-ageing effects in the HAZ, where lowest hardness and strength are found after welding. For optimum results, it would seem that post-weld heat treatment is the best choice, but it recognised this may not be possible for all applications. Threadgill [1] reviewed the literature on FSW showing the tensile strength of a wide range of aluminium alloys. FSW was consistently better than fusion welded equivalent specimens and often approached the parent material strength. [3]
Tensile strength of friction stir welded aluminium alloy specimens
| Material | Parent metal TS, MPa | Weld metal TS, MPa | Ratio TS (Weld): TS (Parent) |
|---|
| 2014-T6 |
435 |
350 |
0.80 |
| 7108-T79 |
370 |
320 |
0.86 |
| 7108-T79 post-weld naturally aged |
370 |
350 |
0.95 |
Fatigue tests have also been conducted on FSW specimens made from a wide range of aluminium alloys. The fatigue performance of FSW butt welds in 2000 and 7000 series alloys is comparable to that of the parent metal. Analysis of the body of available fatigue data on alloys considered 'weldable' by fusion processes shows that FSW welds perform at least as well, and in most cases very much better, than fusion welded equivalent samples. Scatter of results is noted to be particularly low with FSW samples.
Another large testing programme showed that FSW offers tremendous potential for low-cost joining of lightweight aluminium airframe structures for large civil aircraft such as the Airbus A380. Researchers at Airbus Deutschland have presented data showing that the mechanical and technological properties of FSW welds for skin-to-skin fuselage connections approach the properties of parent material. [4]
Weld Quality
The Boeing Company was faced with a problem when fabricating fuel tanks for their Delta rockets; fusion welding of 2014 and 2219 aluminium alloys gave, on average, a defect every 750 feet of welding. Whilst not high, this level of rejection required expensive repairs as tanks designed to hold liquid oxygen and liquid hydrogen have to be perfect. They therefore commissioned work at TWI to study the possibility of FSW for tank fabrication. Weld trials were very successful and an extensive range of tests was carried out including an hydraulic burst test on a model tank. The lack of defects in the welds made this a complete success and Boeing went into full-scale production of tanks by FSW. A Delta II containing FSW welds was first launched in August 1999. The Mars Odyssey, launched in April 2001, utilised the first pressurised FSW structures. Since its introduction to the Delta programme, 80,000 feet of FSW have been made without a single defect that required repair! The machinery investment was repaid by saving just one tank from repair, cycle times for production dropped by 80% [5] and the overall cost of welding fell massively from many dollars to cents per foot.
The Phantom Works of Boeing is pursuing FSW of curvilinear joint configurations and has demonstrated its feasibility on a complex aircraft landing gear door. The company has also flight tested FSW sandwich assemblies for a fighter aircraft fairing. [6]
Eclipse Aviation Corp has chosen FSW for the new low-cost business jet, Eclipse 500. Oliver Masefield, Vice President of Engineering for Eclipse is quoted as saying [7] 'It eliminates the need for thousands of rivets, resulting in reduced assembly costs, better quality joining, and stronger and lighter joints. Because this process is significantly faster than other structural jointing processes, we can drastically reduce the cycle time in production'. FSW will be used for cabin, aft fuselage, wings and engine mounts.
In order to achieve consistency of operation, the FSW process is highly automated with set-up being critical. It is therefore essential that sufficient data are known about the process so that the parameters can be accurately set. The trend is now towards on-line force and torque measurement for data monitoring and closed-loop control. Especially on robots and transportable machines, which may not be rigid enough to withstand deflection, load cells or more complex systems can be installed. TWI has experimented with a Kistler rotating dynamometer and a force measurement table, both of which use piezo crystals for measuring forces and torque. [8]
Corrosion performance
Aluminium alloys, and in particular aluminium weldments, can be prone to corrosion so there has been considerable interest in the performance of FSW samples. Li et al [9] found that FSW gave good results in stress corrosion tests on 2219-T87 and 2195-T8M4 materials. Four point bend tests showed no failures in any welds even when stresses to 90% of yield.
The 7000 series alloys are prone to pitting and stress corrosion under certain conditions and this may prove to be a limiting factor on their use. FSW alters the temper state of these age-hardenable alloys so care must be taken that testing has taken this into account when assessing likely performance. Work continues at a number of establishments, including TWI, in order to build the body of information on the performance of FSW in a variety of environments.
The Future
The aerospace industry now has a high performance, consistent process for joining airframe materials. The Boeing Company is committed to its use and is seeking its introduction into commercial aircraft. Airbus believes that FSW is an essential element in achieving the necessary weight saving to allow the A380 to meet payload expectations. Eclipse Aviation Corp offers the prospect of mass-produced business jets by replacing riveted design with FSW.
For rockets and space vehicles the Delta experience is only a beginning. Fokker Space has shown that FSW can be readily applied to lap joints in 7075-T7351 and is considering it for Ariane 5 motor thrust frames. Lockheed Martin isto introduce FSW to the production of the external fuel tanks for the Space Shuttle.
Several manufacturers offer off-the-shelf and custom-built FSW machines to meet this rising need and with work at TWI demonstrating the feasibility of friction stir being applied to such materials as titanium, copper and steel, the possibilities continue to expand.
Friction stir welding has come a long way in little over ten years; it clearly has a long future in front of it.
References
- Threadgill P L; 'Friction stir welding - the state of the art' TWI Industrial Members Report 678, May 1999.
- Kallee S W, Nicholas E D, Thomas W M: 'Industrialisation of friction stir welding for aerospace structures' Structures and Technologies - Challenges for Future Launchers Conf., Strasbourg, France, 11 - 14 Dec 2001.
- Dawes C J, Thomas W M: 'Friction stir joining of aluminium alloys'. Bulletin, Nov/Dec 1995.
- Lohwasser D: 'Application of friction stir welding for the aircraft industry' 2nd International Symposium on FSW, Gothenburg, 26-28 Jun 2000.
- Anon: 'Finding strength in new technology' Inside Delta, June 2001 ( www.boeing.com/defense-space/space/delta/id/inde0601.pdf)
- Talwar R, Bolser D, Lederich R and Baumann J: 'Friction stir welding of airframe structures' 2nd International Symposium on FSW, Gothenburg, 26-28 Jun 2000.
- Anon: 'Eclipse makes business jets affordable' Aerospace Engineering Apr 2002.
- Johnson R and Horrex N: 'Forces to be reckoned with - an examination of what acts where, and by how much, during the FSW process' Bulletin, Nov/Dec 2000.
- Li Z X, Arbegast W J, Hartley P J and Meletis E I: 'Microstructure characterisation and stress corrosion evaluation of friction stir welded Al-2195 and Al-2219 alloys'. Proc 5th Int Conf. on Trends in Welding Research, Pine Mountain GA, 1-5 Jun 1998.