Innovation in Materials Joining Technologies (July 2006)

Authors: Sue Dunkerton ¹ and Jon Simpson ²

¹ TWI Ltd., Granta Park, Gt Abington, Cambridge, CB1 6AL, UK
² TWI Training & Certification (S.E.Asia) Sdn. Bhd., No 8 Jalan TSB 10, Sg. Buloh Industrial Park, 4700 Sg Buloh Selangor Darul Ehsan, Malaysia

Paper presented at IMTCE 2006, 17-20 July 2006, Kuala Lumpur, Malaysia.

Abstract

Today's companies must innovate to survive, particularly in an environment of increasing competition from low cost manufacturing countries. A global perspective is needed to explore new technological opportunities, and to have the networks and partners at an international level to exploit these opportunities to maximum benefit.

This paper addresses one organisation's strategy in developing new and exploitable materials joining technologies, with case studies, to demonstrate how success can be achieved at international level. Specific examples will include: investment in and development of world leading capabilities in lasers (fibre lasers and direct metal deposition) and electron beams (surface modification), innovation in friction stir welding, and extension of welding technologies into newer materials, such as textiles.

Application examples that span electronics, aerospace, shipping and clothing provide a route to spread technologies across sectors and an open approach to intellectual property ensures good licensing opportunities.

Introduction

Manufacturing is key to most developed and developing nations, having significant impact on employment, wealth creation, international standing and quality of life. Significant changes are underway with increased emphasis on knowledge based economies and value added manufacture, placing greater need on innovation in the technical and business processes necessary to remain competitive in the global market.

TWI addresses a broad spectrum of the manufacturing base with specific focus on joining and coating technologies - both key enabling technologies with potential for high value add. The company, as a research and technology organisation, has a long history of innovation and invention to serve manufacturing industry throughout the world. From the earliest cross-flow lasers for materials processing to the latest friction stir welding process, TWI has demonstrated innovation that has found major industrial application, leading in some cases to new businesses being established to exploit the technologies developed.

TWI's innovation strategy is to offer innovative solutions to customers and where the work is funded by an external party, the IP rights will mainly remain with that external party. Where work is funded internally, to develop a new process or other innovative improvement, TWI considers protection by way of patents. The intention is to have a small portfolio of patents that it licenses out to organisations worldwide. The ultimate intention is to have the technologies adopted in the industrial context, hence attractive licensing agreements are struck that recognise a win-win arrangement for all parties and freedom to operate as far as is possible.

One of the best-known innovations that TWI has patented in the last few years is Friction Stir Welding (FSW). This welding technique has revolutionised the welding of aluminium in particular and has been very influential in a number of markets, from aerospace to automotive. The licensing of TWI inventions, including FSW, generates over £1m income to TWI.

This paper will cover some of the latest items within the FSW process as well as introduce other recent innovations in laser, electron beam and new materials joining.

Friction stir welding

The early 1990s saw the invention of FSW1 and this process met such an industrial need that applications were using the technology within five years, and today there are more than 100 holders of a friction stir licence across the world. Most of these are industrial users of the technology.

FSW has a cylindrical, shouldered tool with a profiled probe which is rotated and slowly plunged into the joint line between two pieces of sheet or plate material, which are butted together, Fig.1. The parts have to be clamped onto a backing bar in a manner that prevents the abutting joint 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. The plasticised material is transferred from the leading edge of the tool to the trailing edge of the tool probe and is forged by the intimate contact of the tool shoulder and the pin profile. It leaves a solid phase bond between the two pieces.

Initially the process was proven on aluminium alloys up to about 25mm thickness but the rate of progress is so high that metal penetrations of >100m in a single pass can now be made in aluminium alloys. Aluminium remains the mostoften welded metal by the process because of its low strength at relatively low temperatures and good thermal conductivity, with many applications now throughout space, aerospace, marine, rail and automotive sectors.

As higher strength materials are evaluated, the process becomes more difficult predominantly with tool weal being an issue. However, development in tool design, tool material and improvements to the processing conditions are leading to signs that such materials will be weldable in the near future. To date, steel welding has been demonstrated at 25mm thickness and titanium alloy is being welded at 6mm thickness, Fig.2, with consistent performance, strengths comparable to parent material and with runs upto 5m already demonstrated.

A further modification to the FSW process, is friction stir spot welding. ^[2] This process utilises conventional FSW but with no translation of the welding tool, instead the tool is penetrated into the substrate and immediately retracted. In this way a series of spot welds can be created, Fig.3.

Joints have so far been demonstrated in aluminium and in steel, and the process is receiving considerable attention in the automotive and general transportation industries.

Electron beam processing

Welding

An electron beam (EB), as a power source for welding, is uniquely capable of deep penetration welding in a vacuum environment which has also enabled the process to produce very clean welds and surfaces. However, the vacuum requirement poses constraints that can sometimes limit its application particularly where larger structures are involved. These shortcomings of high vacuum (5 x 10 ^-3 mbar) EB welding inspired TWI, in the 1990s, to develop the Reduced Pressure EB variant which operates in the 0.1 to 10mbar regime and requires only mechanical pumps. This reduces pump-down time by typically a factor of three to five. In addition, in this pressure range, the use of relatively crude local seals is far more practical and many large-scale fabricators are currently taking a keen interest in this technology.

This has been a significant step forward, but the ultimate goal is to truly operate in a non-vacuum (NV) environment and work began on this back in the late 1970's. Early work elsewhere had demonstrated success in thin materials, but this does not utilise the inherent advantage of an EB so the TWI approach was to employ higher power levels in combination with slower welding speeds to allow good penetrations to be achieved. This met with some success and a particularly attractive feature was the rounded tip at the root of the welded zone (more attractive than that achieved with in-vacuum welding). However, there were significant drawbacks too: a very wide top bead/weld zone and voids and cracks in the remelted lower fusion zone, Fig.4.

Based on other welding work, it was felt that pulsing the beam current may lead to improvements in the weld quality, but this had not been done before at the voltages and power levels being proposed for thick section non-vacuumwelding. TWI therefore developed ^[3] both a high modulation frequency, locally coupled RF (radio frequency) pulsed grid supply and the control electronics to adjust pulse width, amplitude and modulation frequency; then applied these two, to a NVEB/Reduced Pressure welding head. Although early in the development cycle, this work looks very encouraging with initial welds showing a more typical electron beam weld section, Fig.5.

 

Surface Modification

Although EB processing is widely used for welding of metals, it is less well known for its surface treatment and texturing capability. In recent years, texturing has been developed to such a stage that it can be exploited commercially; the process being based on the electro-magnetic manipulation of the beam, at very high speed across a surface, to leave local spots of melted material having a dimpled texture. The beam locally melts and vaporises the surface, the vapour pressure then expelling material from a formed shallow hole, with the displaced material being manipulated to form specific features around the shallow hole, Fig.6. Such a finish increases surface area and greatly enhances adhesion onto the surface of coatings such a PVD (physical vapour deposition) or thermal spray.

Surfi-Sculpt ^TM is an extension of surface texturing and is a new and revolutionary materials processing technology ^[4,5] that enables controlled surface features to be developed on a range of materials, predominantly metal but with scope also for polymers and ceramics. Surfi-Sculpt features are developed by utilising a series of electro-magneticcoils to firstly focus the beam, then to deflect the beam around the material surface in a rapid and controlled manner at selected points on the surface. At each point a molten pool is formed, and the beam is then translated sideways which leads to material being moved from within the hole to region(s) around the hole as a result of vapour pressure and surface tension effects, Fig.7 and 8. By repeating this process at many local sites, protrusions of up to 2mm height and 0.2mm width are formed, each of which is accompanied by one or more corresponding intrusions or holes.

 

Unique patterns can be developed by the controlled manipulation of the beam, the variety of features possible seeming somewhat limitless currently, ^[6] Fig.9 and 10.

 

The process developed out of a belief that engineering of surfaces is important for many future applications plus the challenge to stretch electron beam technology for future, unknown needs. To date, the process is finding (or being considered for) applications as diverse as:

• enhancing joining of dissimilar materials for hybrid structures, for example:
  • joining polymer composites to metals (eg Comeld technology)
  • improving the adhesion of thermally sprayed coatings to metals
• controlling liquid or gas flow around a surface
• processing of pre-coated surfaces to provide local variations in surface properties
• sculpting of shape memory alloys

for industries as diverse as biomedical implants, wind turbines and jewellery!

Laser processing

Fibre delivered laser technology

Ytterbium (Yb) fibre lasers (not to be confused with fibre-delivered lasers where the fibre is merely an optical delivery mechanism) are solid state lasers in which an optical fibre doped with low levels of a rare earth element is the lasing medium. Ytterbium is generally used for the high power fibre lasers currently available for material processing and this emits a wavelength approximately the same as Nd:YAG lasers, ie between 1.060 and 1.085 micron. The laser beam excitation and manipulation, allows for a very compact laser source, which can also be delivered to the workpiece by a separate delivery fibre giving it much of the flexibility of the Nd:YAG lasers. ^[7]

Although at least 700W single-mode fibre laser modules are commercially available, the current manufacturing route of choice, to achieve output powers suitable for deep penetration keyhole welding of metals, is by combining the outputs from a series of lower power commercially available single-mode units (eg 200W) into a single fibre output. This enables high power capability, with TWI having one of the highest power units commercially available today (2006)at 7kW, Fig.11. Any reduction in beam quality by combining a series of modules, is negligible from a welding perspective - with the beam quality significantly better than the competing Nd:YAG technology.

Welding trials at TWI using the latest Yb fibre laser technology confirm that this new type of laser source can now be considered as an alternative to the CO ₂ or Nd:YAG laser for the welding of metal alloys, including steel, titanium and aluminium. A comparison of a fibre laser weld with a Nd:YAG laser weld, at the same power setting and spot size, demonstrates the faster speed that can be achieved with the fibre laser combined with a narrower weld and heat affected zone, Fig.12.

Recent developments in disc laser technology, also using Yb as the lasing medium and also claiming high wall plug efficiencies, have further complicated the choice for the industrial customer for laser sources. However, it is expected that the high beam quality solutions offered by these newer generation of fibre delivered lasers mean that they will become the laser of choice for many industrial welding applications and may further increase market share by development towards cutting and remote applications such as repair. Their compact design, easy set-up and minimal cooling requirement makes them an ideal laser source for on-site welding, for pipeline welding or shipbuilding.

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 deposition8 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, Fig.13.

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. Figure 14 shows a multi-layer deposit in an aeroengine 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.

Polymers and textiles welding

The use of lasers for welding plastics was demonstrated in the early 1970s. However, it was not until late in the 1990's that production applications started to be considered widely. Early CO ₂ lasers were first used to weld plastics (stake welds in a lap joint configuration) and thin polyolefin films (up to 0.1mm thick) have been welded with a CO ₂ laser at speeds in excess of 500m/min. However, the use of CO ₂ lasers for welding of plastics has not entered wide use and only recently have production examples of package sealing in the medical sector been made public. Only after the development of an alternative method of applying the laser energy using diode or Nd:YAG laser sources, has the laser found extended application for welding plastic components.

Nd:YAG and diode laser light (800-1100nm wavelength) will transmit through several millimetres of unpigmented polymer. The polymer can be designed to absorb and heat in these laser beams with the addition of an absorber. Transmission laser welding of thin and thick materials is therefore possible where a transmitting plastic overlays an absorbing plastic. This results in a method of welding plastics that does not mark the outer surfaces of the component. The melting is carried out only where it is required at the interface between pre-assembled parts. The absorbing material used in the process is typically carbon black. The first (published) part, mass produced using transmission laser welding was a keyless entry device for Mercedes in 1997. A further development in laser welding in 1998 was the invention of the Clearweld® process, which now allows infrared transparent plastics to be welded, thus further extending the range of possible applications.

Further developments are happening in the textiles industry, as countries are searching for routes to add value to their products and lose the high labour intensity of stitching particularly for advanced textiles. Laser welding is used to attach fabrics to rigid components such as microphones and work is ongoing to develop procedures and equipment for automated welding of garments.

For laser welding of textiles ^[9,10] , heating is carried out by passing an infrared laser beam through one of the fabrics to be absorbed by the lower fabric or a coating on the lower fabric. The fabrics are held under pressure at the time or just after the application of the laser by a sliding clamp or a roller. Melting is generated at the joint interface only, so the outer surfaces of the fabric are unaffected. Welding rates of at least 30m/min are possible depending on the fabrics being processed. The equipment is typically easily programmable for design or size changes. Variations exist using laser sources which heat the fabrics directly at a nip point as they are being rolled together.

The laser welding process consists of three stages which can be merged into one process if the absorber material is incorporated in advance of welding:

• Application of the laser absorber material system to the textile: the material system (given the name Clearweld®) may be applied in the form of a low-viscosity liquid, which dries rapidly to leave a very thin deposit on the surface of the textile. It may also be deposited on the surface of an interleaving film that is compatible with the textiles to be joined. A third option is to incorporate the absorber in a compatible interleaving film or into the textile fibres. In all cases the fabric can be prepared in advance of welding.
• Assembly of the seam and application of clamping pressure: the seam, including any interleaving film, is assembled and held in place such that the interface between the textiles can be irradiated through one of the textiles. The assembly also applies a clamping pressure to the joint during welding, without hindering access of the laser.
• Irradiation of the seam with a near infrared laser to melt the material where the absorber has been applied and create a permanent weld: a near infrared laser, such as a diode laser providing radiation at a wavelength of 940nm, is used to irradiate the seam. The absorber material system absorbs the laser radiation, concentrating the heat at the interface between the textiles. A thin film of polymer is melted in each textile and the application of clamping pressure brings these films into contact. The pressure is maintained as the films cool and solidify to produce a permanent weld.

Laser welding of textiles offers the benefits of easy automation potential, good aesthetics, high seam strength, leak tightness, cleanliness and speed. Demonstration garments have been prepared in polyester fabrics of two types; fleece and plain woven material, Fig.15.

Other applications include outdoor garments (nylon laminated with polytetrafluoroethylene, polyester laminated with bi-component polyurethane/PTFE membrane), automotive airbags (nylon 6,6), inflatable buildings and airships (PVC and polyurethane) and upholstery.

Summary

Innovation and the continued development of joining and surface technologies is key to enabling competitive advantage in future manufacturing. Such new technologies can, in some cases, completely revolutionise manufacturing sectors but even without this, improvements are always sought to meet increasing industry demand for reduced cost and weight, better environmental performance, improved reliability to name but a few.

Technologists need to ensure their technologies reach the market place as quickly as possible and working with the customer and developing mutually attractive intellectual property are essential to make this happen. TWI has demonstrated good examples of this and continues to innovate to ensure new ideas available to meet current and future market drivers.

References

1. Thomas W.M. et al. 1991. Friction stir butt welding. International Patent Application No PCT/GB92 Patent Application No.9125978.8, 6 December 1991.
2. Kallee S.W., Kell J.M., Thomas W.M. and Wiesner C.S. 2005. Development and implementation of innovative joining processes in the automotive industry. DVS Annual Welding Conference 'Große Schweißtechnische Tagung', Essen, Germany, 12-14 September 2005.
3. Sanderson A. 2005. High power non-vacuum EB welding. TWI Bulletin, May - June 2005.
4. Dance B.G.I. 2002. Modulated Surface Modification. International Patent Publication No. WO 2002/094497 A3.
5. Dance B.G.I and Kellar E.J.C. 2004. Workpiece Structure Modification. International Patent Publication No. WO 2004/028731 A1.
6. Buxton A.L. and Dance B.G.I. 2005. Surfi-Sculpt ^TM Revolutionary surface processing with an electron beam. 1st - 3rd Aug 2005 International Surface Engineering Congress & Exhibition St. Paul Minnesota, USA.
7. Verhaeghe G. and Hilton P. 2005. Battle of the Sources - Using a high-power Yb fibre laser for welding steel and aluminium. WLT Conference - Lasers in Manufacturing 2005, 13-16 June 2005, Munich, Germany.
8. http://www.twi.co.uk/content/dmd_lasertwi.html Laser deposition for repair, build and rapid prototyping 2005.
9. Hilton P., Jones I.A., Sallavanti R. 2000. Laser welding of fabrics using infrared absorbing dye. Conference, Joining of Advanced and Speciality Materials III, St Luois MO, pp136-141.
10. Jones I.A. 2005. Improving productivity and quality with laser seaming of fabrics. Tech Text Int, May 2005, pp35-38.

Acknowledgements

The authors wish to acknowledge the contribution of their colleagues in drawing this paper together: Mike Russell (FSW), Ian Jones and Marcus Warwick (laser welding of polymers), Allan Sanderson (non-vacuum electron beam welding),Colin Ribton and Bruce Dance (Surfi-Sculpt), David Howse (laser welding), Laura Barrett (IP).