Advanced joining techniques for the 21st century

Paper presented at 2004 International forum on welding technologies in aviation and space industries 11-14 Nov.2004, Beijing, China.

Abstract: A review of some of the Advanced Joining Techniques being developed to meet the challenges of industry's needs in the 21^st century. Advances in friction stir welding, laser processing, reduced pressure electron beam welding and metals to composites joining are discussed.

Introduction

This paper reviews some of the advanced joining techniques that have emerged recently from work at TWI and other organisations. TWI is the world's premier centre for materials joining technology, with some 500 staff in a number of locations within the UK and other parts of the world. Its headquarters are located in Cambridge, UK. TWI is a private, non-profit distributing company that provides R&Dmp;D, consultancy, training, technology transfer and advice for its2000+ Industrial Members worldwide. The aerospace industry sector, including airframe, aero engine and components suppliers now represents over 10% of TWI's Industrial Membership.

In this paper, four technologies will be reviewed and their relevance and benefits to the aerospace industry discussed. These technologies are:

1. Friction Stir Welding
2. Laser Processing
3. Reduced Pressure EB Welding
4. Metal to Composite Joining

1. Friction stir welding

Friction stir welding has developed quickly, since its invention in 1991 at TWI. It is a patented process ^[1,2] , and over 114 organisations have been granted licences to use the process. It is widely used in a number of different industries, but particularly in aerospace for the joining of aluminium alloys.

Friction stir welding uses a non-consumable rotating tool, Fig.1, which moves along the joint between two components to produce high-quality butt or lap welds. The FSW tool generally has a profiled pin and a shoulder with a larger diameter than that of the pin. The pin length is similar to the required weld depth. The pin is traversed along the joint line while the shoulder is in intimate contact with the top surface of the workpiece to avoid expelling softened material and provide consolidation.

1.1 FSW Tools

FSW tools are manufactured from a wear resistant material with good static and dynamic properties at elevated temperature. Current state-of-the-art tools permit up to 1000m of weld to be produced in 5mm thick aluminium extrusions without changing the tool. The design of the friction stir welding tools is the heart of this remarkable and still relatively new welding process.

Multi-helix tools, such as the MX Triflute ^TM Fig.2, have an odd number of relatively steeply angled flutes and incorporate a coarse helical ridge around the flutes' lands. ^[3] These reduce the tool volume further and therefore aid the material flow, the break-up and the dispersion of surface oxides. The tool shoulder profiles under investigation are designed to provide better coupling between the tool shoulder and the workpiece.

1.2 Weld quality

The weld nugget strength in the as-welded condition can be in excess of that in the heat affected zone. In the case of annealed materials, tensile tests usually fail in the un-affected material well away from the weld and heat affected zone. The weld properties of fully hardened (cold worked or heat treated) aluminium alloys can be improved by controlling the thermal cycle. For optimum properties in some alloys, a heat treatment after welding is the best choice, although this will not always be a practical solution for many applications.

Typical tensile properties of friction stir welded 7000 series aluminium alloys have been reported to reach 95% of those of the parent material. ^[4] Fatigue testing has shown that FSW joints produce substantially better results, with lower scatter, than other joining techniques. ^[5]

1.3 FSW Applications for Aerospace

Boeing has applied FSW to the Delta II rockets, and the first of these was launched successfully in August 1999. The Mars Odyssey spacecraft lifted off on a Delta II rocket in 2001, which demonstrated the strength and quality of longitudinal friction stir welded joints on all three cylindrical tank components. FSW technology for the Delta tanks increases the weld strength by 30 to 50% and achieves 60% cost saving, while reducing the manufacturing time from 23days to 6 days.

Four FSW machines have been installed at Lockheed Martin's facility in New Orleans for external fuel tanks of NASA Space Shuttles, Fig.4. The external tank measures 47m by 8.4m dia. and is the structural backbone of the Shuttle, absorbing most of the six million lbs of thrust at launch. FSW Benefits (over VPPA) have been reported to be an increase in UTS of up to 22%, a significant decrease in weld variability and a reduction in barrel production time from 47 hrs to 19 hrs.

The FSW process 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 see a high potential for joining aluminium alloys by FSW for skin-to-skin fuselage connections. They presented data that demonstrate that the mechanical and technological properties of these welds approach the properties of the parent material ^[6] . This could lead to the reduction of cost and weight through improved joint quality and the possibility of new design.

Boeing has demonstrated curvilinear FSW of a complex aircraft landing gear door. They have also successfully demonstrated FSW of sandwich assemblies by welding thin T-joints for a fighter aircraft fairing, which has been flight tested. ^[7]

Eclipse Aviation Corporation of Albuquerque, New Mexico, has decided to use FSW to replace traditional riveting and bonding processes ^[5] . This is likely to be the first application of FSW in high-volume aviation applications with the potential to dramatically lower assembly time and cost, Figs.5&6.

Eclipse Aviation Corporation announced in June 2002 that the FAA (US Federal Aviation Administration) has approved the FSW specification created for use in the assembly of the Eclipse 500 jet. FAA approval of this process specification, in conjunction with the receipt of the type certificate, will allow Eclipse Aviation to build production aircraft using FSW. The FSW process specification details the procedural requirements, quality assurance provisions, standards for tooling and material preparation necessary for the use of FSW in the assembly of aircraft.

The process is under development to replace riveting in the floor of the Lockheed Martin C130J and Boeing C17 military transport aircraft. It has been reported that the use of FSW in the C-130 cargo floor structure can reduce unit production costs by >25% while improving reliability, corrosion resistance and structural performance.

2. Laser welding

Fabrication or manufacture with lasers offers several potential benefits:

• Increased productivity - more rapid part manufacture at reduced cost.
• Improved quality - minimal distortion and reduced levels of rework.
• Enhanced performance - longer lifetimes or greater resistance to corrosion or high temperature. Also, laser repair technologies can give rise to significant life extensions of otherwise exhausted components.
• Novel fabrication routes and components - where alternative technologies simply are not available.

This section will describe some new developments and opportunities for manufacturing with lasers in three broad themes, Process Enhancement, Novel Manufacturing and New Laser Sources, and then will report some recent work in applying lasers to join aerospace aluminium alloys.

2.1 Process Enhancements - Hybrid Laser-Arc Welding

Due to the narrow deep penetration weld produced, laser welding offers several advantages over other welding processes, namely, high joining rates, low consumable costs, high reproducibility, and low levels of distortion, leading to greater precision in assembly and reduced rectification work.

Laser welding, however, is a demanding process and very tight control of a number of parameters such as fit-up and steel composition is required in order to produce welds of satisfactory quality and performance.

An enhanced range of applicability is available through hybrid laser-arc welding, in which the two welding processes are coupled in a single process zone, as shown in Fig.7. ^[8]

Options available include combining either a CO₂ , Nd:YAG, diode or fibre laser with an arc welding process such as gas metal arc welding, gas tungsten arc welding or plasma arc welding, or even combining two different laser sources.

Compared with the use of laser power alone, hybrid laser arc welding offers:

• Increased travel speed (x2) or increased penetration (x1.3).
• Improved tolerance to fit-up gap.
• Ability to add filler material to improve weld metal microstructure, joint quality and joint properties.
• Potentially improved energy coupling.
• Increased heat input and reduced hardness.

There are some drawbacks that include increased complexity, the need to define additional welding parameters and the requirement to establish new process parameters.

Nevertheless, hybrid laser-arc welding is now a production process in both the automotive and shipbuilding industries and has been shown to be a candidate process for girth welding gas transmission pipelines.

2.2 Laser direct metal deposition

For high value applications such as aero engine components it is generally economically attractive to repair worn or locally damaged parts rather than replace them. A candidate repair technology is laser direct metal deposition in which a laser is used to melt powder onto a substrate material. In contrast to approaches in which the powder is placed on the substrate, the powder is fed via a nozzle into the laser spot on the substrate. This results in the formation of a molten pool that, on cooling, leaves a solid deposit of the powder material on the substrate.

Laser deposition has two main advantages over other powder deposition techniques. Firstly, the operation is a low heat input process that reduces the likelihood of liquation cracking. Secondly, the use of very small spot sizes enables highly accurate and reproducible deposits to be made. Adaptive control systems have been developed that monitor the characteristics of the molten pool and adjust the process as necessary to maintain deposit quality. Fig.8 shows a multi-layer deposit in an aero engine alloy.

In addition to repair, direct laser metal deposition is suitable for original part build. By varying powder composition as a part is built up, a functionally graded component can be developed with particular performance characteristics directly related to position. The process can also be used for rapid prototyping.

For the process to gain widespread acceptability, however, development is required to increase deposition rates, deposition efficiency and to establish appropriate processing parameters for high accuracy and quality.

2.3 New laser sources

A wide range of lasers is available for materials processing, including CO₂ lasers, Nd:YAG lasers, direct diode lasers, excimer lasers and copper vapour lasers. Current developments of existing sources include improved beam quality, higher power levels, increased wall plug efficiency etc.

Recently, low power fibre lasers, which have been used in communications for many years, have been developed to the stage where power levels suitable for welding and cutting are available. The lasing medium is a Yb:YAG fibre and the wavelength of light emitted is 1070nm, very close to the Nd:YAG wavelength. Manufacturers claim that fibre lasers have better beam quality than Nd:YAG lasers of a similar power, enabling greater stand-off distances when welding. Additionally, wall plug efficiency is estimated to be between 20 and 30%, significantly greater than Nd:YAG or CO₂ sources; thus relatively low input power and reduced cooling is required. Fibre lasers are also smaller than Nd:YAG or CO₂ lasers of similar power.

TWI has acquired a 7kW fibre laser and commenced a number of research programmes into the advantages and disadvantages of fibre lasers.

2.4 Laser welding of aluminium alloys

Laser welding of aluminium alloys is challenging, with the most frequently encountered imperfections being porosity, solidification cracking and poor weld bead geometry. Recent work has shown that porosity can be reduced significantly and that good weld profiles can be produced when welding fully penetrating, square edge butt welds in 3.2mm thickness 2024 aluminium using 3kW Nd:YAG laser power ^[9] . The study concluded that by controlling the process conditions, it was possible to achieve a level of weld metal porosity lower than that defined in BS EN 13919-1:1997 or AWS D17.1:2001. The main findings were:

• A focus position on or 1mm below the material surface helps achieve full penetration welds with good weld profiles.
• A high-purity, low dew-point research grade helium shielding gas, delivered through a moisture and condensation-free shielding gas delivery system, produces less weld metal porosity compared with industrial grade gas.
• Removing the porous oxide layer prior to welding contributes to reducing the weld metal porosity. The elapsed time between material preparation and subsequent welding needs to be as short as possible to avoid atmospheric moisture pick-up.
• A further reduction in weld metal porosity can be achieved by cleaning the filler wire.
• The use of a twin-spot laser energy profile with a 0.27mm spot separation and a 50/50 energy distribution between two 0.45mm diameter spots helps eliminate coarse porosity in 3.2mm thick 2024 aluminium, but has less of an effect on pores smaller than 0.4-0.5mm diameter.

Such procedures have been used to fabricate stiffened panels in 2024 Al alloy containing T-butt welds shown in Fig.9.

3. Reduced pressure electron beam welding

Electron Beam welding (EBW) is a mature welding process that offers many advantages in terms of weld productivity, avoidance of distortion and minimal metallurgical disturbance. However, the necessity to weld in a high vacuum atmosphere has restricted the application of the process to components and structures that can be entirely contained within a vacuum chamber.

The past 10 years have seen the emergence of advanced high power EB welding technology which permits welding at significantly higher working pressure i.e.~1 mbar, some one thousand times higher pressure than conventionally used. This has been made possible by the development of an electron beam generator using a differentially pumped transfer column, Fig.10. This generator can produce an electron beam with the workpiece at near to atmospheric pressure; to date optimum welding performance has been achieved at pressures up to 1mbar. This in turn permits the use of a local vacuum arrangement whilst avoiding the need for a sophisticated sealing and pumping arrangement.

With this system it was shown to be possible to establish a welding vacuum ('Reduced Pressure') using a simple mechanical vacuum pump and crude seals. ^[10] The development of this process variant represents a step change as it permits the use of local sealing and pumping and potentially obviates the need for large chambers to weld big components. This also creates the opportunity of taking the welding equipment to the structure and applying the welding process 'on-site'.

Furthermore, this pressure regime the system was particularly tolerant to many of the variations that previously had hindered the adoption of EB welding for large structures such as:

• Component cleanliness - Welding at high vacuum requires that the both the immediate joint area and the entire assembly are relatively clean otherwise outgassing can occur. When working at 'Reduced Pressure', greatly increased tolerance to component cleanliness is observed and only the immediate joint area needs to be cleaned.
• Gun stand-off distance - The Reduced Pressure Beam is essentially parallel and has no well-defined focus position. In consequence, the gun to work distance can vary by more than 400mm without detriment to weld quality. In contrast, high vacuum EB welding requires that the gun to work distance is controlled to +/- 2mm to maintain an appropriate beam focus position.
• Pumping time - Operation at 1 mbar pressure, in contrast to 10^-3 mbar significantly reduces pumping time for a given volume. Even for large volumes, pumping times are measured in minutes as opposed to hours.
• Gun discharging - The combination of the beam transfer column and helium overpressure stage eliminates the risk of metal vapour or positive ions entering the electron gun and causing breakdown and interruption of the welding process. This is a serious consideration when welding high value, critical parts using EBW.
• Beam characteristics - The increased attenuation of the beam caused by the scattering effect of the Reduced Pressure atmosphere results in a 'softer' beam which results in wider weld profiles offering greater tolerance to joint gaps and better weld termination behaviour.

3.1 Joint preparation and weld geometry

RPEB Welding is performed in a single pass without a filler metal addition and thus simple square edge joints are used. This enables fabrication design to be simplified with significant savings in materials and machining, plus radically reduced and controllable distortion. The absence of welding consumables and very low heat input of RPEB welding reduce the cost and post-welding correction substantially.

3.2 Non destructive testing

Extensive experience has shown that weld defects are rare with RPEB because of the process reliability and accuracy of weld placement due to the use of real-time seam tracking. RPEB uniquely uses the welding beam itself to track the joint with great accuracy and compensate for machining inaccuracies and thermal distortion during welding. Nevertheless, should any flaws occur, X-ray inspection combined with phased array ultrasonics have proved successful in detection and sizing of planar weld flaws of less than 2mm height.

3.3 Application to aerospace

The RPEB process variant has been demonstrated to be particularly effective in welding heavy section steels, copper and aluminium alloys without any detrimental effects caused by the welding atmosphere. Potential applications include the production of large, thick section aluminium blanks of varying section thickness for subsequent finishing by machining.

It has been shown that more reactive metals such as titanium and its alloys can be processed successfully and initial trials have shown great promise. Research has commenced at TWI to examine whether the same potential benefits can be realised for titanium alloys and to establish the limitation, if any, of welding at this pressure.

4. Metal to composite joining

In order to fully exploit composite materials, they need to be easily incorporated into structures made from many different materials at the exact position required. Properties of the joint should be known, consistent and reproducible, easily inspected and retained over a loading history similar to that expected in service.

There are currently three commonly used techniques for this: adhesive bonding, mechanical fastening and a combination of bonding and fastening.

Bonded joints can be designed such that the adhesive sustains loads greater than the strength of the parent material ^[11] and do not exhibit failure due to fatigue loading. ^[12] However, despite the detailed design and manufacturing knowledge that exists, in-service experience, which shows that some bonds provide excellent service whereas others do not, has led to lack of confidence in adhesive bonding technology.

The use of mechanically fastened joints in metals is well understood. In these materials, yielding reduces the stress concentrations around the holes. However, in composite materials there is very little relief from the stress concentrations around the holes. The brittle nature of composite materials also means that there is virtually no capability for redistribution of load if one fastener in an assembly fails.

In many cases, in order to address the limitations of both joining techniques, hybrid joints containing both adhesives and mechanical fasteners are used. Such an approach defeats the main objectives of using the composite material i.e. size, weight and cost savings. The well-understood properties of metallic aerospace alloys become attractive again and limit the positive benefits of composite systems.

To overcome this problem, a proprietary material surface treatment technique, Surfi-Sculpt ^TM , and a joining process, Comeld ^TM , has been developed recently at TWI. ^[13] It offers the potential for joints to be made between fibre reinforced plastic (FRP) composite materials and metals with enhanced performance. It is expected that this joining technique will allow joint design to be revolutionised by overcoming some of the problems associated with adhesive bonding and mechanical fastening of composites to metals.

One of the joint geometries chosen for initial studies is shown in Fig.11.

Examples of the surface created using Surfi-Sculpt ^TM are shown in Figs 12 and 13. This surface treatment can be used to produce metal protrusions (known as proggles) in a variety of shapes and sizes on the material surface.

Control specimens were made with a simple grit blasted surface preparation.

For both the control and Comeld ^TM specimens, the joint was manufactured by lay-up of preforms of 400g/m² plain woven E-glass fabric (Carr Reinforcements) onto the metal specimens, followed by vacuum infusion of polyester resin (Crystic 489PA) into the glass fabric.

4.1 Testing and results

The Comeld ^TM and control specimens were loaded in tension at the rate of 1mm/minute until failure. Representative load-displacement curves are shown in Fig.14.

Fig.15 shows damage mechanisms that occurred before and during failure of the specimens.

Fig 15. Damage mechanisms in Comeld TM specimens

The load displacement curves, shown in Fig.14, demonstrate that the Comeld ^TM joint tested had a significantly greater load carrying capability than the corresponding control joint. The area under the load-displacement curves corresponds to the energy absorbed during failure of the specimen. The energy absorbed by the Comeld ^TM joint was more than double that absorbed by the control joint.

In the control specimens, failure occurred at the bond-line. These failures were very sudden and there was very little damage to the metal or composite substrates.

In the Comeld ^TM specimens, damage was visible before failure as whitening of the composite caused by matrix cracking. There was deformation of the metal proggles before failure of the composite and the Comeld ^TM specimens eventually failed due to shear failure of the composite just outside the proggle area. The damage in the metal and composite described above contributed to the additional area under the graph and therefore extra energy absorbed during failure when compared to the control specimens. Similar work on other material combinations has also demonstrated improved mechanical performance of Comeld ^TM joints over control joints. ^[14]

It can be concluded that the application of a new surface treatment technique, called Surfi-Sculpt ^TM , to a new joining system, called Comeld ^TM , improves the mechanical performance of joints between composite materials and metals in the following respects:

• Comeld ^TM joints had a higher load carrying capability than control joints.
• Comeld ^TM joints absorbed more than double the energy as the control joints before failure.
• Comeld ^TM joints failure via a more progressive failure mode than the control joints.

TWI is continuing development of both Surfi-Sculpt ^TM and Comeld ^TM .

5. Conclusions

Four advanced joining techniques have been reviewed and their applications in the aerospace industry discussed. Advanced joining techniques hold the key to lighter, safer and more affordable aerospace structures.

References

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