Innovative Welding Methods to Prefabricate Aluminium Panels

Published in Speed at Sea, October 2004, p.23.

TWI Ltd has an amazing array of technologies applicable to aluminium ship and boat building. TWI, formerly known as The Welding Institute, has its headquarters near Cambridge, UK, and has a long association with the shipbuilding industry. TWI gives its Industrial Members access to world-class expertise and knowledge. It supports shipyards during the development, implementation and industrial application of welding procedures and associated technologies, e.g. through consultancy, highly confidential contract R&Dmp;D, training and qualification. Stephan Kallee, Sector Manager at TWI, focuses on providing Industrial Membership Services to the shipbuilding sector and informed Speed at Sea on the latest state of play of technologies as diverse as electron beam surface treatment, friction stir welding and laser welding of aluminium alloys and innovative methods of fire protection on aluminium ships, as shown below:

Electron beam surface treatment for aluminium to composite joints

TWI uses electron beams to reshape metal surfaces by growing protrusions that rise from the surface of the material. At the core of the process is the creation of the protrusions and holes. These are created by precise application of a high-intensity electron beam that is controlled by a programmable deflection system. As the electron beam is moved across the surface of a metal, it creates a small pool of molten metal in a track, or 'swipe'. As the beam moves along the swipe, the material heats rapidly at the point of interaction, but cools as the beam moves away. While the rising surface tension behind the beam pulls the hotter, molten, material back to the beginning of the swipe, the vapour pressure at the point of the beams action pushes the material backwards towards the beginning of the swipe. Material is displaced along the swipe in the direction opposite to the beam's travel. Such swipes may be repeated or overlapped several times, and each time more material is displaced from the bulk of the material to a common point. After several passes of the electron beam, a protrusion begins to grow and rises out of the swipe path. TWI has applied for a patent for this process and trademarked it as Surfi-Sculpt ^TM .

The height and width of each protrusion is defined by the electron beam size and the number and pattern of passes. While the physical limits of the process are being explored, protrusions in the order of 2mm high and 200 microns wide have been typical of those created within TWI's laboratories ( Fig.1). TWI has achieved success with Surfi-Sculpt ^TM in many metals, including aluminium, titanium and stainless steel. Bruce Dance, a Principal Research Metallurgist at TWI, said: 'In Surfi-Sculpt ^TM we have, for the first time, a tool that enables control and modification of materials properties at many different levels. Mechanical, electrical, magnetic, thermal, and other characteristics can be tailored precisely for specific applications, and this creates a multitude of exciting new possibilities'.

TWI is now launching a group sponsored project, to examine the use of Surfi-Sculpt ^TM for joining composites to metals - a process called Comeld ^TM ( Fig.2). 'The ability of Surfi-Sculpt(TM) to form strong and reliable joints between metals and composite materials is the most developed application of this process, and the area most likely to be exploited first,' said Colin Ribton, Manager of Electron Beam Development at TWI. The process will be used for producing aluminium to composite joints in prototypes representative for yacht building and naval mine countermeasures vessels.

Historically, composite to metal joints have presented significant design challenges if high levels of mechanical performance are to be achieved. This has meant that designers have been reticent to design structures incorporating joints, or have adopted highly conservative designs that increase weight and thus negate some of the benefit of using composite materials. Comeld ^TM joints, comprising a composite material and Surfi-Sculpt ^TM treated metal, have the potential to be used in highly stressed areas. Early indications are that they can absorb more than twice as much energy as conventional joints and that the joint can be engineered to fail in the manner, and at the position, of the designer's choosing.

Colin Ribton said: 'TWI is one of the world's leading developers of electron beam technology. Surfi-Sculpt is at the very frontier of manufacturing. We know it can radically improve the way products are made and perform, but we have only scratched the surface of the range of possible applications.'

Bruce Dance said: 'Although electron beam welding has been in use for many decades and under development at TWI since the 1960s, it is the development of the intensity and precision of the beam, combined with the development of the software used to programme and control the beam deflection that has made the development of Surfi-Sculpt possible.' TWI will adopt a licensing strategy to make Surfi-Sculpt ^TM technology available to end-users.

Friction Stir Welding commercially applied to prefabricate deck panels and bulk heads

Since the invention of friction stir welding at TWI in 1991, companies from all parts of the world have implemented the process in the fabrication of aluminium components and panels. Trendsetters were the Scandinavian aluminium extruders, which were in 1995 the first to apply the process commercially for the manufacture of hollow aluminium deep-freeze panels and for ship decks and bulkheads. Friction stir welded structures are now revolutionising the way in which high-speed ferry boats, hovercraft and cruise ships are built from prefabricated lightweight modules ( Fig.3&4).

The following organisations are known to have approved friction stir welding procedures or friction stir welded components according to their own guidelines: American Bureau of Shipping, Bureau Veritas, Det Norske Veritas, Germanischer Lloyd, Lloyd's Register, Registro Italiano Navale and TÜV Süddeutschland. Research and production FSW machines are commercially available from several machine manufacturers and include capability for welding up to 16m lengths. The latest machines and robots are being used for non-linear and three-dimensional joint lines. Several machines have multiple heads, which can be used simultaneously. The following comments on cost savings were published by users of the FSW process and speak for themselves:

• Ole Midling of Hydro Marine Aluminium reported that at shipyards using prefabricated FSW panels the 'improvement in the aluminium fabrication has resulted in 15% reduction in the man-hour per ton rate.'
• Stig Oma of Fjellstrand claimed 'a total fabrication cost saving of approximately 10% based on improved ship design, streamlined fabrication at the shipyard and by supply of prefabricated FSW panels and structures based on extruded profiles.' He said that using prefabricated FSW panels 'has enabled the yard to reduce the production period for a 60m long aluminium catamaran hull from 10 to 6 months, which means a 40% increase in production capacity and turn-over at the yard.'
• For 'Slipper', the US Army's new cargo interface pallet, 'FSW processing reduced the sandwich assembly cost, including raw materials, extruding, and welding, from 61% to only 19% of the total fabrication cost. The Air Force estimates the total cost savings attributed to FSW (for a projected buy of 140,000 Slippers) at $315 million.'

Current and future developments in TWI's Core Research Programme

TWI's Core Research Programme (CRP) consists of a series of applied research projects, which underpin TWI's highly confidential contract and consultancy services. Having a multi-million pound annual budget, this research programme aims to develop relevant knowledge and skills for transfer into industry to reduce manufacturing costs, encourage innovation, improve product quality and meet safety and reliability requirements. Industrial members of TWI can so gain and maintain competitive edge in the market place. Two projects in TWI's current core research programme focus on friction stir welding of aluminium alloys, as follows:

Friction based processes for repair are now being developed. Applications where these solid phase repair techniques would be beneficial will then be investigated in a CRP project on 'Friction based repair techniques including the development of portable friction stir welding'. In this project the technical requirements of portable equipment for making friction stir welds in thin aluminium sheets will be specified.

The relationship between spindle rotation speed, applied force, and material softening response in the generation of heat by friction is being investigated in a CRP project on 'Fundamentals of friction welding and friction processing of materials'. This project will produce guidelines for effective rubbing velocities for FSW in a range of commonly used workpiece materials. The project is to be extended in a later phase particularly to friction stir processing (FSP) of a number of alloys. It has already been demonstrated that the refined FSP microstructures of aluminium alloys provide better formability during super plastic forming than those of the parent material.

Reports on pre-competitive research in TWI's Core Research Programme

Industrial members of TWI have access to 20 CRP reports on friction stir welding so far. For instance, the microstructures of friction stir and arc welds have been characterised in 'A study of arc and friction stir welding of two aluminium alloys containing a low level scandium addition'. The researchers compared solidification crack susceptibility and determined the mechanical properties of TIG and friction stir welds in conventional and scandium containing alloys. In a study on 'Forces in friction stir welding of aluminium alloys' a commercially available dynamometer measured the horizontal and vertical forces and the torque generated during FSW operations. These data were used to evaluate the effects of friction stir welding parameters and tool geometry on the forces and torques generated during friction stir welding of selected aluminium alloys.

Macro and microstructural features of friction stir welds were examined in various materials, and a microstructural classification scheme for friction stir welds has been introduced as part of the CRP. The 'corrosion resistance of friction stir welds in aluminium alloys 2014A-T651 and 7075-T651' and 'Fracture toughness of friction stir welds in 2014A, 7075 and 5083 aluminium alloys' has been determined experimentally. Reports on 'Flaws in aluminium alloy friction stir welds', and more specifically 'The significance of root flaws in friction stir welds' are available to industrial members of TWI. The former report describes microstructurally the types of flaws in Al-Cu-Mn-Si-Mg aluminium alloy friction stir welds, when the welding conditions diverge from the established operating window. The latter report covers the effect of root flaws on the static and fatigue performance of friction stir welds made from one side.

Earlier reports covered the 'Tool developments for FSW of 6mm thick aluminium alloys', e.g. describing FSW tools capable of operating with zero tilt or FSW bobbin tools that can contain the weld metal about the tool pin and react the weld metal forging forces necessary for making sound welds. A prototype FSW tool for making lap joints that does not exhibit top sheet thinning or serious oxide related flaws in the weld nugget has also been developed.

New technology centres in Wales and Yorkshire

TWI has opened new laboratories in Wales and Yorkshire, where projects aligned to the regional emphasis on advanced manufacturing are carried out complementary to the activities at the headquarters in Cambridgeshire ( Fig.5).

In Port Talbot, TWI Technology Centre (Wales) focuses on the development and use of non-destructive testing technologies. Phased array ultrasonic systems are being developed there for assessing characteristic flaws in friction stir welds. These offer the possibility of performing inspections with ultrasonic beams of various angles and focal lengths using a single array of transducers. Software control over beam angle and focusing is achieved by application of precisely controlled delays to both the emission pulse and received signal for each element in an array of transducers.

In Sheffield, TWI Technology Centre (Yorkshire) focuses on laser and friction stir welding. Two new FSW machines are now being commissioned for this laboratory: Smart Technology Group Ltd has been contracted to supply a high-force machine that can supply 15t welding force ( Fig.6). It has a twin head configuration allowing simultaneous welding from both sides. Eleven CNC programmable axes achieve a true three-dimensional welding capability, to weld contoured and complex shapes. The machine also has a sophisticated data acquisition system for data logging, analysis and control. Transformation Technologies Inc. is building an FSW machine with a very accurate spindle to weld a range of steels and aluminium-based metal matrix composites at this new laboratory. The accuracy of the spindle increases the lifetime of the brittle tools that are used for FSW of these and high-melting temperature materials.

Laser welded sandwich panels with cores made from interlocking aluminium extrusions

Lightweight aluminium sandwich panels are extensively being used for yachts and high-speed ferryboats, where their stiffness and strength are of considerable importance. The honeycomb sandwich structures are usually bonded together using a high-performance adhesive. This is seen by many engineers to be the weakness in the construction, in particular when the sandwich structure is exposed to fire or water. Laser welded metal sandwich structures with extruded aluminium cores are now being developed for the industrial members of TWI, as Paul Burling, Principal Composites Engineer at TWI reports ( Fig.7&8).

In an experimental study at TWI, aluminium sandwich panels branded Ex-Struct ^TM were assembled from 2mm thick 5083 aluminium alloy sheets and 6060 aluminium alloy tubular extrusions. The design of the extrusion die was such that interlocking connections were created on the external circumference of the tubes. The diameter of the tubes was 30mm and the wall thickness 2mm. The clearance between a female and male connection was closely controlled. Steve Shi, Senior Project Leader in TWI's laser & sheet processes group, used laser spot welding and laser stake welding to demonstrate the feasibility of manufacturing sandwich panels and identify key factors affecting the weld quality and distortion. He used a 4047 (Al-13%Si) aluminium alloy filler wire with 1.2mm diameter, to reduce weld solidification cracking and to achieve a smooth top bead surface.

One demonstrator Ex-Struct ^TM sandwich panel was fabricated using Nd:YAG laser spot welding by locating the spots at the mechanical joint between the tubes ( Fig.9). Laser spot welding was conducted horizontally row by row from one side of the panel. The spot welds in each row were produced continuously by controlling the laser shutter open/close time at each joint. To avoid weld cracking, spot welding was conducted using a pulse pattern with a power ramp-down at the end of the pulse thus reducing the cooling rate. A controlled laser power ramp down could reduce the susceptibility to cracking of the weld. This sequence was then repeated for the other side of the panel.

Another demonstrator Ex-Struct ^TM sandwich panel was manufactured using Nd:YAG laser stake welding ( Fig.10). The degreased tubes were first assembled into a matrix on the bottom skin sheet. Welding was conducted linearly from one edge of the skin to the other in such a way that the weld passed through the mechanical joint between the tubes. This was repeated until the welding on one skin sheet was finished. This sequence was then repeated for the other side of the panel. Welds with a relatively smooth top bead and free of visible defects could be achieved at a speed of 2.6m/min using 3kW laser power and a wire feed rate of 3.0m/min.

Fire resistant partitions made from vermiculite particles in an inorganic binder

Barrikade ^® is a low density, fire resistant inorganic material developed by TWI, to improve the fire resistance of aluminium ships ( Fig.11&12). It consists of vermiculite particles and a blended silicate binder. It is non-combustible, and when heated produces negligible emission of toxic fumes. It is ideal for use as a core material in a range of applications such as bulkheads, firewalls and fire doors - since it is lightweight (typically 200-300kg/m ³ ) and offers thermal insulation. The shipbuilding industry is considering the use of Barrikade ^® in the manufacture of fire resistant walls, partitions and doors for high-speed ferries, cruise ships and for offshore accommodation modules. Low temperature insulation of freezer compartments of fishing boats, or heat-shielding of ammunition storage decks on military vessels are other potential applications of this new insulation material.

Trials have shown that control of processing variables enables the material to be produced in a range of thicknesses and densities and, once cured, Barrikade ^® can be easily cut, routed and shaped with hand held tools, similar to those used for woodworking. Furthermore, it has been shown that Barrikade ^® can adhere to a range of materials including wood, steel and aluminium, while a high temperature adhesive is available for applications requiring greater adhesion. Trials on a 20mm thick panel to evaluate its thermal properties have demonstrated that the temperature on one surface did not exceed 170°C after 70 minutes when the rear face had been exposed to a flame in excess of 1000°C. This suggests that the material has a potential to meet SOLAS A60 regulations. Further research work is necessary to optimise the production of Barrikade ^® panels and to determine suitable joining processes for industrial application. Temperature control during curing enables Barrikade ^® to be produced in densities of 150-350kg/m ³ , and thicknesses of 3-200mm. Thermal and mechanical properties can also be influenced.

For many years, TWI has been committed to provide services to the civil and military shipbuilding industry worldwide by offering novel solutions in joining technology for the broadest range of materials. With increasing international competition and the drive from high-speed ferryboat operators to reduce the weight of aluminium ships, there are strong pressures to both cut the whole lifecycle costs and to discover innovative but cost-effective manufacturing opportunities. Therefore, TWI's strategy is to add value to companies supplying or acquiring aluminium sheets, extrusions, castings or pre-fabricated panels and by supplying manufacturing solutions which reduce costs or add functionality. New ideas are contrived with an even greater emphasis on meeting current and future market drivers. As well as supporting developments in joining technology, TWI can now assist in materials development, coating technologies, distortion prediction, modelling and the performance and reliability of shipbuilding structures.

For further information on TWI's R&Dmp;D activities and industrial membership of TWI contact Stephan Kallee stephan.kallee@twi.co.uk Tel: +44 1223 899000