Flying high .... with the aerospace industry
TWI Bulletin, March - April 2009
World leading progress in processes and materials
Richard Freeman joined TWI from industry in 1996, and is currently the Aerospace Business Manager and Chairman of the Aerospace Industry Panel. He is the UK advisor to the American Welding Society AWS D17.1 Committee on 'Fusion Welding for Aerospace Applications', and an Executive Committee member of the Titanium Information Group.
The aerospace industry sector, including airframe, aeroengine and components suppliers now represents 15% of TWI¹s industrial membership. Richard Freeman reviews TWI¹s increasing role in the aerospace industry,through the development of technologies for use in current and future aircraft.
TWI's aerospace business has grown significantly in the last decade, and most of the Original Equipment Manufacturers (OEMs) and several large Tier 1 and Tier 2 suppliers are now Industrial Members of TWI. 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 Industry 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 32nd meeting in March 2009. The six monthly Panel meetings are well attended by industry, as a result of being linked into the calendar with progress meetings from aerospace related Group Sponsored Projects, as part of 'aerospace week'.
In October 2008 TWI and EWI organised the 4th International Seminar on the Joining of Aerospace Materials, which was held in Toulouse with the support of Airbus. The event attracted 64 technologists from 11 countries, and finished with a tour of the A380 facility. Plans are already underway to hold the 5th International event in September/October 2010 in the USA.
Titanium joining developments
The civil aerospace market is significantly increasing demand for titanium, with Boeing and Airbus having signed agreements with a number of titanium suppliers to guarantee material supply over the next five to 10 years, for theB787 and A350XWB respectively.
Due to the cost of titanium, and the limited availability of some specialist alloys, commercial alternatives to make cheaper titanium are being developed in several countries. However, they are unlikely to help the aerospace industry in the short term, and that is why the use of additive manufacturing techniques to make near net shape parts is gaining prominence. TWI is at the forefront of this development at its facilities in Cambridge and Yorkshire.
Linear friction welding involves holding an oscillating component against a stationary block. Frictional heating is generated by linear motion and the two blocks are then forged together to produce a solid phase joint. It is a very repeatable, high quality method for joining titanium alloys and is an established production process used by Rolls-Royce and MTU to manufacture bladed discs (Fig.1). TWI is also working with The Boeing Company on the demonstration of the capability of linear friction welding in fabricating titanium alloy preforms to near net shape. The technique is regarded as a cost effective fabrication route to conventional production methods, and Boeing has applied for a patent in this area. A large programme of work has resulted in the saving of a considerable amount of titanium and a subsequent reduction in machining costs, and is likely to be adopted on future Boeing aircraft.
Fig.1. Bladed discs manufactured by MTU for the Eurofighter Typhoon
Friction stir welding (FSW) was invented by TWI in 1991, and is now licensed by over 180 companies worldwide. A cylindrical shouldered tool with a profiled probe is rotated and slowly plunged into the joint line between two pieces of sheet or plate material which are butted together.
The parts are clamped onto a backing bar in a manner that prevents the abutting faces from being forced apart. Frictional heat is generated between the wear resistant welding tool and the material of the workpieces. This heat causes the latter to soften without reaching the melting point and allows traversing of the tool along the weld line to produce a solid phase joint.
It is used for joining aluminium alloys in production for the lower fuselage joints in the Airbus A340-500 aircraft, and for the cargo barrier beams in the Boeing 747 and 777 freighter aircraft.
The technique has also been developed for low thermal conductivity materials such as titanium, in which the FSW probe rotates through a stationary shoulder component. This allows welding of titanium alloys with very little tool wear, and the appearance of the weld surface is very smooth (Fig.2). The stationary shoulder FSW of titanium is currently being reviewed by major airframe and engine OEMs.
Fig.2. Stationary shoulder FSW of titanium
In a development with The Boeing Company, TWI worked with Aeromet in the UK to develop a low cost titanium structure by the use of laser welding and subsequent hot forming. Laser welding offers a number of advantages over other welding processes, including low distortion and high joint completion rates. In addition, the smaller molten pool makes shielding and control of oxidation easier in atmosphere with local shielding.
By coupling hot forming with the laser welding process, simple net thickness plate and sheet structures can be welded and then hot formed to bend the structure to its finished complex shape and simultaneously stress relieve the structure. This provides a repeatable product in an inert condition. TWI developed a technique to Nd:YAG laser weld a titanium structure (Fig.3), meeting the Class A acceptance criteria of AWS D17.1 before Aeromet subsequently hot formed the structure meeting the dimensional requirements set by Boeing.
Fig.3. ND:YAG laser welding of titanium structure
TWI is also working with leading aerospace companies to look at high productivity welding of titanium alloys using a combination of arc and laser welding techniques. Plasma welding, which is regularly used in the welding of aircraft engine components, is being compared against TOPTIG (an automated development by Air Liquide in which the filler wire is added to the arc as opposed to the weld pool), Interpulse (a high frequency direct current electrode negative TIG process from VBC Group), a reduced spatter MIG welding process using a novel wire from Daido Steel in Japan and the Yb:YAG fibre laser system based at TWI's Yorkshire Technology Centre in Rotherham.
At TWI Wales in Port Talbot, the NDT group is working with leading aircraft engine manufacturers to develop phased array ultrasonic inspection of titanium billets. Phased array ultrasonic systems use multi-element probes, which are individually excited under computer control.
By exciting each piezo-composite element in a controlled manner a focused beam of ultrasound can be generated. This beam can be steered by use of the software. Linear and sectorial scans are possible, and, in conjunction with probe scanners, two and three dimensional views can be generated showing the sizes and locations of any flaws detected. Current production billet inspection systems are either 'conventional', using single transducers, or multizone (MZI),using a number of probes focused at different depths in the material.
The conventional systems achieve relatively low sensitivity, especially on large diameter billets. MZI achieves higher sensitivity than the conventional method, but previous studies have shown that large variations in response are exhibited. TWI has made significant progress in developing phased array inspection in this area to improve on sensitivity, reliability and repeatability, and further work is planned with this multi-partner consortium.
Joining of high temperature alloys
In the aircraft engine market, TWI has been working closely with a leading engine manufacturer to develop direct metal laser deposition (DMLD) for repair of blades and seal segments (Fig.4). In the DMLD process, a laser beam is used to form a melt pool on a metallic substrate, into which powder is fed. The powder melts to form a deposit that is fusion bonded to the substrate. Both the laser and nozzle from which the powder is delivered are manipulated using a CNC robot or gantry system. The process is also known as laser cladding, laser deposition, laser engineering net shape (LENS) and laser additive manufacture (LAM).
Fig.4. Seal segment repair using DMLD
Recent work conducted at TWI on FSW of 3mm and 4mm thick nickel based superalloys has demonstrated the feasibility of using relatively cheap, readily available, ceramic tool materials to join both solid solution and precipitation hardened Ni alloys. Ceramic FSW tools have been used to produce sound friction stir welds in both 3mm thick Inconel 625 and 4mm thick Inconel 718.
Initial results have been encouraging with welding speeds of up to 100mm/min possible. Based on this preliminary work, plans are in place to launch a Group Sponsored Project (GSP) in March 2009 to develop the technology further.Particular emphasis will be placed on optimising tool technology and process parameters for a number of thin section workpiece materials. Rigorous testing of weld quality will be performed on optimised welds for each test case.Performance of the tool, in particular its wear resistance, will be closely monitored.
Work is also being carried out on the arc welding of nickel based alloys in the TWI Core Research Programme, using many of the techniques used in the high productivity welding of titanium alloys projects. The Electron Beam Group has also pioneered the use of an electron beam deflection technique for successfully welding crack sensitive nickel alloys such as MAR-M-002.
Composites
The use of composites as structural components in aircraft is increasing due to good strength to weight ratios. However their limited surface properties prevent their use in applications where wear resistance, thermal management or electrical conductivity are required. To extend composite applications, coatings are required to provide protection and increase functionality of the surface. Thermal spraying offers the prospect of producing a wide range of coatings with increased functionality. Due to the low melting point of composite resins, and the high temperatures associated with thermal spraying processes, it is difficult to deposit well adhered coatings on composite substrates without damaging them.
The Surface Engineering group at TWI has successfully demonstrated the feasibility of coating composite materials, such as carbon fibre reinforced polymers (CRFP), using various thermal spraying processes (Fig.5). Maximum adhesion is achieved through careful selection of coating materials and control of the surface preparation. Bond strengths up to 11MPa can be achieved for coatings based on zinc and aluminium alloys, exceeding that of other coatings,such as paints, and approaching levels exhibited by arc sprayed aluminium on steel. TWI has also developed novel spraying techniques to deposit graded coatings incorporating functional top coats.
Fig.5. Metal sprayed coating on carbon fibre composite panel
Hard coatings, such as WC-Co-Cr, can be deposited on CFRP following application of a bond coat. Other functional layers may also be incorporated, for example to provide thermal insulation for protecting composites in high temperature applications. Initial tests have shown, that in four point bending, coatings remain attached even after failure of the underlying composite. Promising fatigue results have also been achieved, with coatings surviving0.5million cycles through significant deflection in four point bending.
TWI is working on coatings for specific applications such as lightning strike, as well as looking at applications on other carbon and glass based composites that incorporate thermoset systems, such as epoxy and phenolic resins.
The NDT Group at TWI Wales is working on the non-destructive inspection of composite materials using techniques such as computed tomography, laser shearography, infra-red thermography and ultrasonic area scanning. There has also been a development from the NDT Group in Cambridge on the inspection of adhesive joint specimens by two ultrasonic techniques using high-frequency shear waves for the characterisation and location of kissing bonds. While this work started on the inspection of adhesive bonds between aluminium plates, it is being developed for adhesively bonded composites in conjunction with leading aerospace organisations.
Supporting industry
In the UK, a £38 million three year programme called Integrated Wing brings together 21 leading UK organisations with the objective of integrating and validating the most promising combination of technologies related to development of wings, wing systems, landing gear and fuel systems. Led by Airbus, it involves companies such as Messier Dowty, Bombardier, Smiths, GKN Aerospace, Goodrich, Ultra Electronics and Eaton, alongside prominent UK research and technology organisations such as TWI and QinetiQ and several UK universities.
This programme is supported by the UK Technology Strategy Board, and TWI is working on two large joining projects from its Cambridge and Yorkshire facilities to support the project partnership.
TWI is also working with some of the 16 partners in the £103 million UK Government collaborative aviation research programme, led by Airbus and known as the Next Generation Composite Wing (NGCW). The NGCW programme will revolutionise technologies that will improve future wing design processes and help to maximise the eco-efficiency of future aircraft designs.
Conclusion
TWI continues to work with its worldwide customer base from its UK technology centres in Cambridge, Rotherham and Port Talbot, to provide innovative welding, joining and inspection developments in order to assist in developing long term, cost effective solutions for future aircraft.
