Published in Welding and Metal Fabrication, 2000, Vol. 68, No. 7, July/August, pp12-14 by DMG World Media UK Ltd - http://www.dmgworldmedia.com
With the use of welding techniques increasing in prominence in the aerospace industry, Richard Freeman reviews the current welding specifications and reports on the forthcoming publication of a new standard.
Despite the relatively low usage of welding techniques in aircraft manufacture, compared with other joining processes, welding and the aerospace industry have been linked since the second quarter of the 20 th Century. One of the first recorded attempts to weld aircraft was by Fokker, fabricating aircraft with welded tubular fuselages. However the early aircraft builders quickly moved to riveting as the prime joining method for aluminium alloys, and many of the early examples of welding were crude. The advancement of welding techniques, specifically Gas Tungsten Arc Welding (GTAW), was certainly helped by the 'space race' between the USA and former USSR, which pushed the design of welded aluminium structures to new levels. Despite the tremendous progress made by the welding community in dramatically improving traditional processes and the development of new and improved techniques (electron beam, plasma, friction and laser), welding still remains a process which is not at the forefront of aircraft design and stress analysis engineers' minds.
In the early days, qualification of the welder, and the welding process, was carried out by means of visual, metallurgical, and tensile testing. The advent of radiography for weld inspection in 1926, enabled previously hidden defects to be found, and weld acceptance criteria was established in the 1950's using evaluation of the stress analysis techniques around at the time. It is interesting to note that the much of the acceptance criteria currently in use in the industry still dates back to the 1950's, despite the significant progress made by the analysis community in finite element and modelling programmes.
Pioneering specifications in the US
One of the countries who led the way in pioneering welding specifications was the USA and it aligned its aircraft industry to the welding certification standard MIL-STD-1595 'Qualification of Aircraft, Missile and Aerospace Fusion Welders' in 1977. This was later changed to MIL-STD-1595A in 1982, relaxing much of the previous alignment with the pressure vessel code. The industry consolidated the aircraft welding standards for aluminium, magnesium and steels into MIL-STD-2219 'Fusion Welding for Aerospace Applications' in 1988. This specification did not satisfy the industry, and two attempts to reach a consensus for Revision A failed between 1988 and 1991. The American Welding Society (AWS) initiated the action to create a commercial aerospace welding specification in 1993, and following the US Department of Defence decision to cancel many of the Military Standards (MIL-STD) in 1994, this group began work on a new specification (AWS D17.1).
Getting to grips with the new specification
The group of aircraft and aerospace welding engineers working on the specification is vast and varied. Representatives come from the original equipment sector (commercial and military aircraft, space vehicles and propulsion systems), the US armed forces, a number of FAA repair houses, a commercial airline and a number of consultants (including TWI and EWI - Edison Welding Institute). In the time that this group was formed, it was announced that MIL-STD-1595A and MIL-STD-2219 would be cancelled and would be adopted by the SAE (Society of Automotive Engineers) as AMS (Aerospace Material Standards), until the publication of the AWS D17.1 document. This specification follows the structure of MIL-STD-2219, with the D17 Committee split into sub-committees to write the design, performance, procedure, fabrication and inspection sections. However, important additional sections in this new specification include ground support equipment and weld repair ( Figs 1 and 2
Fig.2. Friction plug welding repair of VPPA welds in an aluminium alloy (A12 195) space shuttle super lightweight tank dome assembly (Photos: Courtesy of Lockheed Martin Space Systems, Michoud Operations)
At the recent Committee meeting held at the AWS Show in Chicago, the D17.1 document was released for publication, pending some minor editorial corrections, and should become an official document in the third quarter of 2000. The Committee is scheduled to meet every 6 months to consider planned revisions for the document, although the time period between revisions has yet to be formalised. A number of topics received considerable attention during the production of the document, and will be continuously reviewed with a view to providing updated and impartial information for future revisions. Two of those topics, titanium discolouration and acceptance criteria for aluminium alloys, are very topical in the industry at present. Despite the tremendous effort by the US aerospace industry to produce this document, the irony is that most companies will re-write the document as a company specific standard, as this relates more closely to their product range and is easier to control and revise.
Two other AWS D17 sub-committees are also involved in producing new specifications in resistance spot welding (D17.2) and friction stir welding (D17.3). The latter process, invented by TWI in 1991, is being used in the production of components for space by Boeing, is under development at Lockheed Martin ( Fig 3) and will feature in the manufacture of parts for the new Airbus aircraft (A3XX).
Fig.3. Two examples of friction stir welding (Photos: Courtesy of Lockheed Martin Space Systems, Michoud Operations)
Vienna agreement initiates European changes
The situation with European aerospace specifications is somewhat confused, and there appears to be a deep mistrust of the US Military Standards and their derivatives. As in North America, company specific specifications dominate, although this is complicated by the existence of national standards in many of the European countries. The situation is changing, however, as a result of the Vienna agreement between the CEN (Comité Européen De Normalisation) and ISO (International Standards Organisation) standards bodies. A new work item at either CEN or ISO should assess the presence and suitability of any relevant existing standard or work item produced by the other body, and if the pre-existing standard is relevant, it must be adopted by the other organisation. TC121 is the Technical Committee in CEN responsible for welding related standards, in particular BS EN (EuroNorm) 287 (Approval testing of welders for fusion welding) and BS EN 288 (Specification and approval of welding procedures for metallic materials). Parts of ISO 15614 Weld Procedure Testing and ISO 9606 Welder Qualification Testing, have been harmonised to produce, for example, the EN ISO 9606-4 Approval Testing of Welders for Fusion Welding of Nickel Alloys, which has superseded EN 287 Part 4 in some situations. Despite the recent changes the situation is still horribly complex however, and any new standards will no doubt be converted into company specific documents the moment they become public.
Following the example of extensive collaboration in the military aircraft sector in Europe with Eurofighter, and in the USA/Europe with the Joint Strike Fighter, the aerospace industry is also learning to work together on standardisation. Efforts are being made to bring standards bodies together in Europe, although the language barrier is a constant obstacle. The Chief Executive of TWI, Bevan Braithwaite, is the current President of the IIW (International Institute of Welding), and is also trying to bring the national welding institutes across the world together to harmonise their activities more efficiently.
The author would like to thank the following people. Mark Sapp (Materials Engineer) of the NAVAIR TEAM at the Naval Aviation Depot at Cherry Point, North Carolina, USA provided the background information on the history of US aerospace standards. Mark is an active member of the AWS D17 Committee, and is also custodian of an excellent web site on the history of welding ( http://home.newbernnc.com/~sapp_m/wh_001.htm ). Julian Timothy (Principal Welding Engineer) of ABB Alsthom Power in Lincoln, UK provided some information on the current status of European standards in the industry. Julian sits on Working Group 9 of the CEN TC 121 Committee, to advise particularly on the welding of nickel based alloys.
Dr. Richard Freeman is Deputy Business Development Manager for the Aerospace Industry Sector at TWI, based in Cambridge, UK. Prior to joining TWI he spent six years working for Lucas Aerospace Actuation Systems (now TRW) as a Principal Materials Engineer engaged in metallurgical and process support. He was a CAA approved signatory for the use of a range of engineering materials and processes in new designs. He is also an active member of the AWS D17 Committee.