Flying high - with the aerospace industry....

TWI Bulletin, September - October 2003

Welding, cutting, surfacing, adhesives, fasteners...the aero sector embraces countless joining techniques. So does TWI...

Richard Freeman joined TWI from industry in 1996. He is Deputy Business Development Manager for the Aerospace Group, and is the UK advisor to the American Welding Society AWS D17.1 Committee on 'Fusion welding for aerospace applications'.

The aerospace industry sector, including airframe, aeroengine and components suppliers now represents over 10% of TWI Industrial Membership. Richard Freeman reviews TWI's increasing role in the aerospace industry, through the development of technologies for use in current and future aircraft.

Most of the major companies involved in the design and manufacture of civil and military aircraft, space vehicles and missile systems are current Industrial Members of TWI or EWI ( Table 1).

Table 1 Current Industrial Members in the aerospace industry sector

TWI Aerospace industry panel

In order to understand and support industry, TWI set up a number of Industry Panels in 1992. Each Panel covers a different manufacturing sector, combining representatives from industrial Member companies and appropriate TWI specialists. The Aerospace Panel has proved to be the most popular, providing an ideal opportunity for cross-referencing ideas and industry needs, and is about to hold its 22 ^nd meeting in September 2003. Table 2 gives a list of the presentations made at Industry Panels in recent years.

Table 2 Recent presentations at the Aerospace Industry Panel

TWI also has an active Aerospace Industry Team with a role to establish current and future requirements in order to support the growing membership in this sector. The team is made up of TWI staff with specialist knowledge of arc, laser, electron beam and friction welding, thermally sprayed coatings, plastics, composites, adhesives and non-destructive testing technology.

New developments for the aerospace industry

Friction welding

Friction stir welding has developed quickly, since its invention in 1991 at TWI, and now has almost 100 licensees. It is widely used in a number of different industries, but particularly in aerospace for the joining of aluminium alloys, with the first Boeing Delta II rocket with a friction stir welded Interstage module launched in 1999 and one with three friction stir welded tanks launched in 2001. The process is in serious development to replace riveting in the floor of the Lockheed Martin C130J and Boeing C17 military transport aircraft. It is also approved by the Federal Aviation Administration (FAA) to build up integrally stiffened skin panels by lap welding stringers and frames to pocketed skins for the Eclipse 500 six seat business jet ( Fig.1). TWI has worked closely with customers in all cases to assist with the development of the process into production environments. A Group Sponsored project to develop the FSW process and tools required to lap weld thin aluminium sheet has just started with four aerospace customers. In the early 1980s TWI demonstrated the viability of the linear friction welding technique for metals using modified equipment, and then designed an electro-mechanical machine with linear reciprocating mechanism in the mid 1980s. Two similar machines are now located at aircraft manufacturers in Europe. Linear friction welding has found industrial application in aircraft engine manufacture, where it has proved to be an ideal process for joining turbine blades to discs (blisks) ( Fig.2). The process has been used successfully to join a range of materials including steel, intermetallic materials, aluminium, nickel and titanium alloys. One of the current issues under investigation is the joining of single crystal nickel alloys to polycrystalline discs.

Laser welding

Laser welding of aluminium alloys is challenging, with the most frequently encountered imperfections being porosity, solidification cracking and poor weld bead geometry. TWI has recently investigated the use of 'dual focus' of 'twin spot' laser technology, when welding 2024 aluminium alloy in an effort to reduce the formation of large pores which can be formed by an unstable keyhole and can trap gases. One of the conclusions was that coarse porosity (over 0.6mm in diameter) was eliminated using twin spot focused energy distribution ( Figs 3 & 4). The twin spot process has also been applied to the welding of nickel based alloys for the aero engine industry, and encouraging results have been seen when welding IN625 to IN718.

Arc welding

Manual TIG welding is one of the most widely used welding processes in the fabrication of aircraft engines, and TWI is conducting work in its Core Research Programme to look at the welding of Waspalloy/U720Li polycrystalline and CMSX-10 single crystal blade materials, which will be fully reported in 2004.

Work has been carried out for the Cannon Muskegon Corporation in the USA, on the weld repair of surface defects in modified IN939 (known as CM939 Weldable ^TM ) alloy castings. Manual TIG welding trials were conducted using a welding system with stabilised low current, required for the low heat input welding procedures needed for the welding of nickel superalloys. Single and double layer welds were deposited onto modified IN939 using two different filler materials, Alloy 625 and Nimonic C-263, with two different welding currents. No HAZ cracking was observed in any of the weld repair samples made with either single or double layer repairs. TWI is also part of the large European project entitled 'Manufacturing & modelling of fabricated structural components' (MMFSC), which involves 19 partners from five EU countries and includes the main European based aircraft engine manufacturers. One of the work packages, managed by TWI, is concerned with Low Stress No Distortion (LSND) welding. Automated trials were carried out with liquid CO ₂ sprayed on to the back face of bead on plate welds to reduce distortion ( Fig.5). Thermal imaging was also used to study the temperature distributions on the component, and the level of distortion was visibly reduced using the LSND technique ( Fig.6).

Electron beam welding

Manipulating the beam so that it effectively becomes a multiple heat source can also allow a radical change in the application of the welding process. The application of multiple heat sources to the welding process allows the solidification rate of the weld metal to be controlled, as well as some of the stresses around the solidifying and cooling weld metal. At its simplest, the special welding procedure can consist of a simple travelling preheat zone ahead of the weldpool. At its most complex, a large change in the stresses around the weld can also be achieved, and welds with microstructures not normally found in welds of low distortion are made.

EB deflection during welding can also have a very beneficial effect on the top and bottom bead appearance of the weld, for example in such a case as the assembly of stator vanes of aeroengines ( Fig.7). Two of the key problems when welding stator vanes have been identified as weld cratering and weld spatter. One method that could improve matters is the use of the programmable deflection system to improve the cosmetic appearance of the welds by making a simultaneous cosmetic pass. The integral cosmetic pass is achieved by deflecting the EB periodically from the weldpool to a raster zone located behind the weldpool.

Electro spark deposition (ESD)

ESD is a micro-weld surfacing process used for localised coating and build up repair. Most of the development work has involved the application of coatings applied manually with a hand held applicator. TWI has recently adapted an ESD system for mechanised operation, to improve coating quality whilst increasing deposition rate ( Fig.8 Mechanised ESD on gas turbine blade aerofoil). An example of the process used to repair a single crystal blade is shown in Fig.9-11 (courtesy of Advanced Surfaces & Processes Inc). TWI has been active in the field of High Velocity Oxy Fuel (HVOF) coatings for many years, particularly in the deposition of thermal barrier coatings for aircraft engine components. TWI is now working with a number of aerospace companies on the use of tungsten carbide-cobalt coatings to replace electroplated hard chromium coatings, as further reductions in the airborne and waterborne limits for hexavalent Cr are being proposed in the USA and Europe.

TWI is also becoming involved in cold gas dynamic spraying (CGDS) or cold spray, for the deposition of high quality, high-density coatings and the rapid fabrication of near net shape components in metallic materials. The process is under investigation in the current Core Research Programme.

TWI is working closely with Keronite Ltd, a recent tenant on the Granta Park site, to develop the Keronite ^® process further for industry. Keronite is a plasma electrolytic surface treatment for aluminium, magnesium and titanium alloys. It transforms the metal surface to a hard, dense and adherent metal oxide ceramic layer ( Fig.12). It is applied at temperatures below 100°C and has no impact on the microstructure of the substrate material. A uniform coating thickness is applied over geometrically complex and restricted access surfaces. Preliminary trials indicate that Keronite coated aluminium alloys have a surface micro-hardness of up to 2000Hv, with wear and corrosion resistance superior to that of hard anodising or hard chrome plating.

Non-destructive testing (NDT)

The NDT area at TWI has grown rapidly over the last four years, and TWI is involved in a number of large European projects to support the aerospace industry. The ROBAIR project with an overall budget of £1.25 million is developing a robotic system for the inspection of aircraft wings and fuselage, with an end user panel of the Royal Air Force, British Airways and BAE Systems. In QUALISTIR, an NDT technique based on phased array ultrasonics has been developed to detect and characterise any flaws in friction stir welds. The recently started NANOSCAN project is concentrating on NDT techniques for the detection of defects in composite materials, with a number of European partners. In addition the group has recently launched Group Sponsored Projects in the area of digital radiography (to replace the need to process and store traditional film radiographs), and the further development of phased array ultrasonics for the detection of flaws in friction stir welds. Both of these projects include major companies from the aerospace industry.

Composites

A considerable amount of work on joining issues concerned with both thermoplastic and thermoset composite materials has been carried out in recent years at TWI within the Polymers Group. A current CRP project looking at the assessment of flaws and damage on the performance of adhesively bonded composites will be reported in 2004, and concentrated on a combination of testing and modelling of composite joints with artificial defects introduced. New methods of joining composites to dissimilar materials are being investigated at present, and results should be available in 2004.

Supporting UK industry

TWI is heavily involved in a large UK programme funded by the Department of Trade & Industry and the Ministry of Defence called Design, Manufacture and Performance of Stiffened Structures (DEWMIPS). This involves a considerable number of work packages on arc, laser and electron beam welding of aluminium and titanium alloys.

TWI is also investing in South Yorkshire, and opened a facility known as TWI Technology Centre in Orgreave in late 2002. This has been assisted by £2.8 million of support from the European Regional Development Fund through the Objective 1 Programme for South Yorkshire. Yorkshire Forward, the Regional Development Agency for Yorkshire and the Humber will also provide support. TWI will have state of the art friction stir welding and laser welding equipment in the facility in early 2004, and will be increasing the workforce to 39 over the next three years to support local industry in many sectors including aerospace.

A further regional initiative in South Wales to set up an NDT centre to assist local businesses is being discussed with the Welsh Development Agency, with plans to begin full operation in 2004.

Offset projects

Due to the amount of technology transfer carried out by TWI, it has been asked by European and American aerospace and defence customers to assist them in meeting their offset obligations in a number of countries, generated as a result of their sale of military equipment. Projects in countries such as Finland, Singapore and Taiwan have been successfully completed on subjects such as the design and fabrication of components using titanium, friction stir welding, high speed machining, adhesive bonding of thermoplastic composites and the development of lead free solder paste.

The latter project, completed in Taiwan, is of particular significance to industries in which electronic assemblies are currently manufactured using tin/lead solder. As a result of the EU Restriction of Hazardous Substances (RoHS) directive, it is proposed that a number of materials including lead, mercury, cadmium and hexavalent chromium are phased out in electrical and electronic products by January 2007. As Taiwan exports a vast proportion of its products into the EU, this project was critical to the development of a lead free solder compound for future manufacture.

Conclusion

TWI is well placed to help the aerospace industry with its expertise in the joining of metallic and non-metallic materials, using a wide variety of processes. TWI maintains a strict confidentiality policy, and there is a significant amount of work carried out for clients that cuts across many of these technologies, and others not mentioned here, that cannot be discussed in this article.

The constant search for innovative joining methods, coupled with the drive to take new developments into production, will ensure that TWI supports this industry for many years to come. Further details can be obtained directly from the author or by e-mailing aerospace@twi.co.uk