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      - Pre-1998 articles
        
        1997 articles
        
        Taking off - TWI and the aerospace industry
        
        Tomorrow's solutions today - new plastics joining methods
        
        An initial feasibility study of moving contact arc welding
        
        Laser welding of steel sheet - the carbon monoxide alternative
        
        Microsystems assembly and interconnection
        
        A novel approach to joining polypropylene - focused infrared welding
        
        Proving and improving electronics reliability
        
        Limit load analysis of a bolt with a chordal flaw
        
        The Internet comes of age - no longer a web of intrigue
        
        Information services speaking, can I help you?
        
        What really happens - current theories for welding plastics
        
        Improved heat treatment for superduplex thin walled tubes
        
        Stainless steel weld attack - electrochemical corrosion testing
        
        The flux-cored arc process for wet welding and cutting - an assessment
        
        Impact performance - design using fibre reinforced polymer composites
        
        What are TWI's training, examination and fabrication services? (Bulletin, January-February 1997)
        
        Friction stir welds in aluminium alloys - preliminary microstructural assessment
        
        The need for gas shielding - positive advantages for two friction processes

Taking off - TWI and the aerospace industry

TWI Bulletin, July/August 1997

Richard Freeman

Richard joined TWI from industry in 1996. He works for the Materials Department in a Business Development role, with particular emphasis on the development of the automotive and aerospace industrial sectors.

John Bord

John has spent many years closely involved in the development of TWI Industrial members worldwide, with specific responsibility for the aerospace sector. He joined TWI in 1962.

The aerospace industry sector, including the aircraft manufacturers and components suppliers, now represents almost 10% of TWI industrial membership. Richard Freeman and John Bord review TWI's ever increasing role in the aerospace industry.

The major aerospace companies in Europe and the USA, involved in primary and secondary flying controls, are members of TWI or EWI (Table 1) . By setting up an Aerospace Industry Team and Panel, TWI has significantly increased its ability to interface with customers on the most appropriate topics. As more demands are placed on the supply chain, so TWI can offer its expertise in the joining of a variety of metallic and non-metallic materials.

The aerospace industry team and panel

Industry sector teams and panels are established at TWI to work on industry specific activities, to promote the needs of their sector internally within TWI and globally. The aerospace team is made up of TWI staff who are in regular contact with the industry, to establish current and future requirements. They include experts in the full range of joining and related engineering processes relevant to the industry.

The current composition of the team serving the aerospace industry, which is chaired by Sue Dunkerton, includes specialists in:

non-metals/brazing/bonding	Sue Dunkerton
laser welding	Kevin Ayres
friction welding/electron beam welding	Phillip Threadgill
materials	Mike Gittos
adhesives	Greg Thomas
surface engineering/arc welding	David Harvey
non-destructive testing	Ian Munns
structural integrity	Mike Dawes
training	Gene Mathers
business development	Richard Freeman
membership development	John Bord

The Panel meets twice each year to respond to the needs of the aerospace industry. It consists of representatives of Industrial Member companies, who are invited to meet and discuss issues relating to their industry. The Panel is proving to be a valuable forum for discussion, covering many technical requirements (short, medium and long-term) of this industry sector. A similar event takes place at the Edison Welding Institute for the North American Industrial Members. Some recent topics discussed at TWI Panel meetings are shown below in Table 2:

Table 2: Recent presentations at the Aerospace Industry Panel

February 1995	new materials, technology transfer, academic research capabilities
September 1995	tailored blank welding, welding and cutting of titanium, technology transfer
February 1996	structural composites, thixoforming, metal matrix composites
September 1996	superplastic forming/diffusion bonding, fabrication versus casting
March 1997	rapid prototyping, thin sheet fracture toughness testing

Other support activities include structuring workshops and events relevant to the industry. The Panel recently organised a repair and maintenance workshop, which was well attended by airlines, original equipment manufacturers and repair and maintenance organisations. They were shown TWI's capability in this area, including fusion and solid state welding, thermal spraying and adhesive bonding.

In October 1997 an Aerospace Joining Workshop, organised by TWI, will be held at Rolls-Royce. This event is open to key contacts in the industry, and is further evidence of TWI's role in bringing together supply chain companies responsible for materials joining.

Successful projects completed for Industrial Members

The Aerospace Industry Team and Panel is proving to be a success in identifying projects for TWI's Core Research Programme that will benefit their industry in coming years. Despite this initiative, the majority of project work carried out by TWI for the aerospace sector is confidential, single client work. Competitive advantage is the primary reason for this trend, but some pre-competitive projects have recently been launched.

Below is a review of TWI's role in the aerospace sector. It does not identify intimate details of projects, or the companies involved in the work, but demonstrates TWI's increasing role with the aerospace supply chain, and the methods by which it can offer support in the future. Reference is also made to the Core Research Programme, which underpins work at TWI.

Arc welding

Many years of experience in the arc welding field have led to developments in the key processes (TIG, plasma, MIG etc), and most recently in control and monitoring (back and top-face methods) to raise productivity and quality levels. Developments for the aerospace industry relate to improvements in techniques to overcome metallurgical problems (crack sensitive alloys), to increase penetration capabilities (constricted arc methods) and to minimise distortion.

Laser welding

Nd:YAG laser welding of a variety of materials including titanium, nickel and aluminium alloys has been successful. The thickness of material varied from 0.7-2.5mm, and the primary benefits were consistency of penetration and low distortion. Welding of thicker titanium, aluminium and stainless steel alloys (up to 8mm) has also been carried out using CO ₂ lasers. Examples include combustion chamber components, airframe and aero-engine components and ancilliary equipment.

Fig. 1. Electron beam welded model wing. (Courtesy of British Aerospace Airbus)

Electron beam welding

The benefit of this joining technology is in increasing the range of alloys that can be welded to produce high integrity joints. The majority of projects have been conducted on nickel-based alloys and titanium-based intermetallics for airframe, engine and instrumentation components. Models have been constructed using electron beam welding, for use in wind tunnel experiments. This includes the manufacture of a wing, designed and built by British Aerospace Airbus, for testing by European partners involved in the Future Large Aircraft programme (Fig.1).

Fig.2 Friction stir welding rotary process

Friction welding

TWI is the world leader in development, application and industrialisation of the friction welding processes. The latest of these, friction stir welding (Fig.2), provides engineers with the opportunity to join previously reported unweldable aluminium alloys such as the 2000 and 7000 series (Fig.3). All the major airframe manufacturers are developing friction stir applications, leading to a whole range of development projects at TWI.

Fig.3 Friction stir welding of an aluminium alloy panel

Following on from the success of its aluminium alloy work, TWI has just started work on a project concerned with titanium alloys. The initial signs are very encouraging, leading to the confident prediction that major benefits for the aerospace industry will be identified very soon.

Linear friction welding is another good example of innovative research being successfully launched into industrial applications. The process has been used to attach turbine blades to discs (titanium and nickel based alloys) leading to substantial weight reductions.

Surface engineering

TWI has been prominent in development of sprayed thermal barrier coatings by the high velocity oxyfuel (HVOF) process. Based on yttria stabilised zirconia, the coatings are used in high temperature applications such as jet engines. TWI is also active in thermal spraying of Al2O3, Cr2C3, WC and Inconel 625 for a variety of customers.

A successful application of thermal spraying is demonstrated in a project for Marshalls Aerospace, the Cambridge aircraft refitting company. Badly worn titanium alloy flap and slat tracks, from Lockheed Tristar aircraft, were repaired by TIG welding and HVOF spraying of tungsten carbide/cobalt to give a wear resistant surface (Fig.4). TWI assisted Marshalls Aerospace with a turn round time of 24 hours.

Fig.4 Slat track repaired by TIG welding and HVOF spraying

One current Core Research Project is tailored towards the durability of adhesive joints in fatigue loaded environments. The results will benefit a number of suppliers in the industry who have not previously used this technology due to the lack of appropriate data. A recently completed Group Sponsored Project developed a finite element package known as Gluemaker. It compares the performance of different adhesives and substrate combinations at varying loads and temperatures. The major adhesives companies all participated so that a best practice package could be assembled using their products.

Quality assurance of adhesive joints is extremely important, and TWI was involved in a Eureka project to develop a software package called Quasiat (Quality Assurance In Adhesive Technology). This project enabled generation of a quality system, to assist industry when specifying adhesive bonding as the preferred joining medium.

Recently TWI's Centre for Adhesives Technology and the Microtechnology Centre combined resources to assist a customer in replacing lead-based solders with a conductive adhesive. Use of an adhesive allowed for lower curing temperatures, resulting in less effect on the substrate material and no change in operating performance.

Non-destructive testing (NDT)

NDT is an extremely important tool in the aerospace industry for original equipment inspection and life extension of ageing components. TWI has been at the forefront of new developments in this area. With the advent of composite materials, so the range of NDT techniques has developed to enable inspection to be carried out.

TWI was recently contracted to evaluate a composite platform that had been damaged during transit to a customer. Using ultrasonic examination, specialists were able to demonstrate that no significant disbond had occurred and the component was fit for service.

Infra-red thermography and micro-focus radiography have also been successfully used by TWI, to check for defects in jet engine applications. Use of eddy current technology to determine the contamination level of welded titanium structures has recently been launched as a Group Sponsored Project. This area of work, directly relevant to the aerospace industry, is aimed at reducing the need for in-chamber welding of titanium and improving the quality assurance procedures. It will lead to improved productivity levels, and increased confidence when welding titanium.

Structural integrity

TWI has long been associated with new developments in the field of mechanical testing. The development of fracture toughness and crack tip opening displacement testing originated from Abington in the 1960s. Researchers have recently been involved in ground breaking work in the area of determining resistance to stable crack extension in thin sheet aerospace material.

Abington became involved in this area following a DERA (Defence Evaluation Research Agency) concern about the lack of appropriate BSI test methods for thin sheet materials. The work has involved testing 1.6mm thickness sheet in the following aluminium alloys, 8090-T81, 2091-T8X and 2024-T3 (clad and unclad). In a project funded by the DTI, TWI worked with DERA in determining test methods for 200, 700 and 2000mm wide test specimens (Fig.5).

Fig.5 Thin sheet centre crack tension test

Using Abaqus finite element modelling, it was possible to obtain a better understanding of the profile of the growing crack and how to characterise its resistance to crack extension. Theoretical and experimental measurement of the crack mouth and crack tip opening displacements were within a few percent of each other, and it was found possible to predict the fracture behaviour of 2000mm wide specimens from tests on 200mm wide specimens. TWI is now working with the DERA to draft a British Standard using a 200mm wide specimen.

Materials

The Materials Department at TWI has extensive failure investigation experience, and is often asked to work with aerospace companies to determine the cause of joint failures. They regularly work with the process departments within TWI on projects concerned with the weldability of materials. They have been at the forefront in developing weldability tests to assess solidification cracking, HAZ and weld metal hydrogen cracking. TWI's experience has been used in the development of fusion welding techniques for a range of aluminium-lithium alloys, and a rapidly solidified aluminium alloy (8009).

There has been recent interest in the use of intermetallics in the industry, and TWI has experience in their joining requirements. Linear friction welding and electron beam diffusion bonding have been used to demonstrate the capability of joining titanium, nickel and aluminium aluminides for jet engine components. A Core Research Project in this area is also providing valuable knowledge including information regarding joining of intermetallic alloys (TiAl, Ti3Al, Ni3Al, Fe3Al) by TIG, MIG, electron beam, laser and friction welding processes.

Applicable Technologies

New technologies are continually researched and developed for industrial applications. Those listed below are envisaged to have a major impact in the future.

Activated-TIG (A-TIG) welding

This technique, developed at the Paton Institute in the Ukraine, produces a constricted arc and gives increased penetration when compared to conventional TIG welding using the same welding conditions (Figs 6-9). This is achieved by using a surface active flux, applied to the weld preparation. A major Group Sponsored Project, together with significant effort in the Core Research Programme, has successfully confirmed a number of performance benefits over conventional TIG welding. They include increased penetration and tolerance to material variation.

Fig.6 Conventional TIG arc in argon shielding gas

Fig.7 A-TIG arc in argon shielding gas

Fig.9 Weld bead profile in 6mm thick austenitic stainless steel with A-TIG welding

Qualified procedures have already been produced for use as a deep penetration repair technique, and work is now underway in the development of fluxes for welding of titanium and nickel alloys.

Microjoining processes

Housed in the Microtechnology Centre, this technology area offers a comprehensive array of electronic packaging and testing facilities. An adjacent laboratory has state-of-the-art microjoining and machining facilities including lasers, arc and resistance processes. Projects have been conducted on changing the design rules concerned with surface mount applications, to enable more productive microjoining techniques to be carried out. Areas targetted in the aerospace industry include sensors used in flying controls and engine management applications.

Composites

Use of composites as part of airframe structures has increased dramatically over the last ten years. This trend covers both civil and military applications with resulting weight and operational cost savings. Development work also included using composite materials for radar invisibility. TWI was active in the development of welding techniques for APC2 PEEK thermoplastic material in the 1980s.

More staff with thermoset composites experience have recently been recruited to work in the Centre for Adhesives Technology, providing expertise in materials selection, design and structural performance.

Non-destructive testing

Recent acquisition of the Lock-In thermography system has significantly enhanced TWI's capability in the area of NDT. This system will enable the detection and precise location of delaminations and inclusions in composite materials. The modulated heating source is combined with an ability to inspect relatively large areas, allowing the system to probe deeper below the surface than passive thermography equipment.

Collaborative Projects

TWI is programme managing a project funded by the EPSRC under the Innovative Manufacturing Initiative (IMI). The Cost Effective Welding of Aerospace Materials (CEMWAM) project currently involves the following organisations: TWI, UMIST, Cranfield University, University of Liverpool, University of Essex, British Aerospace, Short Brothers, Rolls-Royce and DERA. Its objectives are to:

develop fusion welding for airframe structures
establish the acceptability of welded aluminium alloys
demonstrate reliable welding of high temperature alloys
model power beam welding
establish monitoring and process control

Training

For the first time, TWI is offering training courses primarily for the aerospace industry. There are a mixture of courses, including:

introductory training based on the fundamental aspects of technology, for those entering the industry
advanced theory based courses for experienced engineers
hands-on skill courses for operators, supervisors and those wishing to appreciate the practical problems of materials and joining

In more detail, the courses offered include adhesive bonding, resistance welding, welding metallurgy and non-destructive testing. A course brochure is available from the Training and Examination Services Department.

Conclusion

Three separately commissioned reports have suggested that the expected passenger growth rate will be in the region of 5% per annum over the next 20 years. ^1-3 The worldwide forecast demand for new aircraft in this period has been estimated to be between 11000 and 16000. ^1,3 The strongest traffic growth will occur in the Asia-Pacific region, averaging 7% per annum, and will equal the North American traffic by 2015. ¹

TWI is well placed to help the aerospace industry, with experience in joining metallic and non-metallic materials by the full range of available and new techniques. The Aerospace Industry Team and Panel is proving a successful tool in identifying the needs of the original equipment manufacturers, suppliers and sub-contractors in the industry.

Further details on topics mentioned in this article can be obtained by contacting the authors, or any member of the Aerospace Industry Team.

References

1 'Boeing 1996 Current Market Outlook.' March 1996

2 'Market Outlook for Commercial Aero-Engines 1995-2014.' Rolls-Royce Report December 1995.

3 Potential World Market Demand for Passenger Aircraft 1996-2015.' DTI Report February 1996.

Developing titanium for industrial use is of particular interest to the aerospace sector - TWI is currently running two multi-client projects on titanium. To find out more please contact the project leader:
A new approach to monitoring the quality of titanium alloy welds - Mike Gittos: E-mail: Mike Gittos

Friction stir welding of titanium alloys - Philip Threadgill: E-mail: Philip Threadgill