A panel of experts...

TWI Bulletin, July - August 2003

Paul Burling obtained an HNC in Production Engineering with Merits and Advanced Mathematics from Cambridge College of Art and Technology. He has extensive experience in project management of large commercial projects world wide, which include in depth knowledge of composite material for military and commercial applications. This required comprehensive customer support throughout the life cycle of the projects. His technical expertise includes design, fabrication joining and costing composite materials; using novel production techniques in producing carbon laminate for manufacturing industry.

Steve Shi received his PhD on Engineering Materials from Sheffield University. He joined TWI in 2001 and has been working on laser material processing with particular interest in laser welding, hybrid laser-arc welding and joining of high strength materials. He has over 15 years research and industrial problem solving experience on metal materials and is currently working as a Senior Project Leader in Laser and Sheet Processes Group at TWI.

Laser and fabrications specialists at TWI combine their skills and experience to produce a super-strong lightweight panelling structure ideally suited to military applications.

Sandwich panels have been used extensively in many industrial sectors for applications where their stiffness, light weight and strength are of considerable importance. The honeycomb sandwich structures are usually bonded together using a high performance adhesive. It is this that is seen by many engineers to be the weakness in the construction, in particular when the sandwich structure is exposed to fire or water. As Steve Shi and Paul Burling report metal sandwich structures with corrugated cores and plate core are also available, manufactured by TIG spot welding, resistance welding, diffusion bonding, mechanical fastening and laser welding.

The development of high power laser welding, with its inherent ability to make single-sided deep penetration welds, has shown advantages over other methods for the manufacture of panel structures in terms of high welding speed and low distortion. Laser welded sandwich panels are now being used in shipbuilding. However, in some applications, there are potential design weaknesses in the bonded and welded panel structures due to the limitation of the core structure of the panel. TWI has developed a new Ex-Struct ^TM panel structure which is a modular aluminium sandwich panel that can be slotted together to create a larger structure. This new core structure has potential advantages over conventional structures in terms of versatility and performance. This work demonstrates the feasibility of manufacturing Ex-Struct ^TM sandwich panel using Nd:YAG laser stake welding.

Structure of Ex-Struct ^TM sandwich panels

The foundation of the modular board is an interlocking group of circular tubes or other shapes of extrusions. These tubes are made from an extruded length of material, the sectioned length of extrusion determining the height of the panel. The design of extrusion die is such that standard male and female connections are created on the external circumference of the tube for specific applications. They allow the tubes to be mechanically locked together in the X-Y plane using the male-female sliding joint ( Fig.1). To complete the sandwich, external skins need to be attached to the top and bottom of the tubular arrays by laser stake or spot welding.

The alternate male and female connections on the side of the finished panels can be used to slot the panels together to form large structures. The unique design and potential high performance of Ex-Struct ^TM panels lends itself to applications in a number of industrial sectors.

Fabrication of the Ex-Struct ^TM panel

For this preliminary work, the tube was produced by extrusion. The design of the extrusion die is such that standard male and female connections are created on the external circumference of the tube. The diameter of the tube was 30mm and the wall thickness 2mm. The extruded tube was cut to 150mm±0.1mm in length. The clearance between a female and male connection is closely controlled.

A 5083 aluminium alloy sheet (2.0mm thick) was used as the skin of the panel and a 6060 aluminium alloy for the tubes. A4047 (Al-13%Si) alloy filler wire (1.2mm diameter) was used to reduce the weld solidification cracking and to achieve a smooth top bead surface.

Two methods, ie Nd:YAG laser stake welding and Nd:YAG laser spot welding, were used to demonstrate the feasibility of manufacturing Ex-Struct ^TM panels and identify key factors affecting the weld quality and welding process.

Laser stake welding

A demonstrator panel (270x270mm) was manufactured using laser stake welding. The degreased tubes were first assembled into a matrix on the bottom skin sheet. Metal banding was used to hold the assembled tubes to prevent any horizontal movement. Heavy metal bars were applied on the top of the panel to exert a downward force to close the gap between the skin sheet and tubes. Welding was conducted linearly from one side to the other side of the panel in such a way that the linear 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 which are free of visible defects can be achieved at a speed of 2.6m/min using 3kW laser power and a wire feed rate of 3.0m/min. With the laser beam tilted 10 degrees from the vertical down and focused on the surface of the sheet. The penetration of the weld is about 4mm, as shown in Fig.2.

Laser spot welding

The Ex-Struct ^TM panel was fabricated using laser spot welding by locating the spot at the mechanical joint between the tubes, as illustrated in Fig.3.

Laser spot welding was conducted horizontally row by row from one side of the panel. The positions of welds in the first two rows were first recorded, and all the spot welds in each row were then produced continuously by controlling the laser shutter open/close time at each joint. Welds in the next two rows were then made using the same program with the starting position shifted 60mm (twice the diameter of the tube). This sequence was then repeated for the other side of the panel.

To avoid weld cracking, spot welding was conducted using the pulse pattern with a power ramp-down at the end of the pulse thus reducing the cooling rate and minimising weld cracking. A controlled laser power ramp down can reduce the susceptibility to cracking of the weld, as shown in Fig.4.

Spot welds with a concave top bead and free of visible cracks can be achieved at 3kW laser power with the laser beam focus point 2mm above surface of the skin sheet. The typical spot weld has a fusion zone diameter of about 6mm and a penetration of about 6mm. The deep penetration was intended to increase the fused areas between two tubes in the vertical direction increasing the resistance to shear.

Main factors affecting the weld quality and cost of the panel

Although these results indicated that laser welding can be used to manufacture Ex-Struct ^TM panel structures, there are a number of areas which need to be addressed in terms of cost and controllability.

Geometry and size of the extrusion

The main idea of the Ex-Struct ^TM panel is the specially designed tubes with external female and male connectors. The cost and performance of the panel was significantly affected by the size and geometry of the tube. The ideal geometry of the tube in the panel should be such that laser stake welding can easily be carried out by following the cross-sectional geometry of the tube so as to achieve the maximum weld length between the skin sheet and tubes.

If the size (diameter) of the tube is too small, large weld lengths are required which make the panel very expensive and susceptible to distortion. Therefore the size and shape of the extruded tube needs to be carefully considered. The cross-section of the tubes should be such that simple welding path patterns can be considered.

Variations in the tube length

One of the key factors affecting the quality of laser welds is the gap between the skin sheet and tube matrix, which is affected by variations in the tube length. Each individual tube in the panel matrix is cut from extrusions. Therefore, the cutting precision can have a critical effect on the weld quality. Any variations in the length of tubes will leave a gap between the skin sheet and the tube matrix because it cannot be corrected using the clamping system. This can lead to the formation of porosity and other weld defects.

Clearance between the female and male connectors of the tubes

A suitable clearance between the male and female connectors of the tube is necessary to ensure the easy assembly of a large number of tubes into a matrix (for example, more than 2000 tubes are needed for a 2m by 1m panel). A large clearance will lead to a poor fit-up and difficulty in positioning, which makes programming of the laser welding path more difficult. Too small a clearance could cause problems in the assembly of the tubes into a matrix.

Clamping system

Distortion is the major factor to be controlled. In particular the deflection, which results from the first weld, is not entirely removed when the second weld is carried out. Clamps need to apply force locally to close any gaps between the skin sheet and tubes. Results in this work indicated that clamping was crucial. For large panels, the following factors need to be considered for the clamping system:

• Rigid and efficient control of distortion.
• Enough access space for laser optics and other additional units and adapted control systems such as the wire feeder.
• Allowance for high volume automatic welding.

Weld quality control

Welds with a smooth top surface are required. With a large number of welds on each surface of the panel, consistent quality is critical, because any visible local defects will affect the performance of the whole panel.

Structure properties of the Ex-struct ^TM panels

The basic advantages of using sandwich structures over solid materials, such as improved stiffness and flexural strength, have been well documented. However, the Ex-Struct ^TM panel structure has a number of potential additional advantages over traditional sandwich structures, such as:

• Stiff in X-Y plane.
• Improved ability to absorb energy in the Z plane.
• Easily modified to specific requirements.
• Easy to repair using simple tools.
• Versatile to form large panel structures.
• Various densities can be produced by filling the core within or between the tubes.
• Reduced risk of delamination of a welded (not bonded) structure.
• Good durability.
• Load paths can be incorporated into panel construction.

One concern about the Ex-Struct ^TM panel was that the panel might be poor in shear as there were no significant connections between adjacent tubes. This could be a weakness in structural applications, particularly near supports and regions of direct loading. However, there are relatively few situations where sandwich panels require high shear loads over a small area. Most loads were uniformly distributed over a relatively large area, with the edges either clamped or simply supported. In addition, because the core cells are infinitely variable, the design could be modified to overcome most point loading situations. Shear transfer between tubes might be achieved in many ways including:

• Reducing tolerances and using a forced fit.
• Tapered tubes or connectors.
• Longitudinal welds between tubes
• Applying an angular twist.

With the unique design of the Ex-Struct ^TM panel structure, there are several features that can be changed within the panel to modify its characteristics, such as:

• Buckling performance could be enhanced by introducing gaps in the male connectors.
• A variety of different extrusion shapes could be used to reduce the density of the panels.
• More complex extrusion shapes could be used to produce small assemblies to minimise the number of the joints.
• Local strengthening could be introduced through solid extrusions or other means of core filling.
• The tubes could be filled with other materials including those which are energy absorbing, sound attenuating, fire resistant and heat insulating.
• Ex-Struct ^TM panel could be made from other materials, eg plastics, pultruded products or dissimilar material combinations.

In addition, no adhesives are used to construct Ex-Struct ^TM thus minimising the risk of delamination. The panel can easily be repaired on site by cutting away the damaged area and re-installing a new array and welding to adjacent skins. Cutting and shaping is also easy and can be performed using existing engineering equipment. Potential applications include ship panels such as decking, car decks and helidecks, bridge decks, railways and military vehicles.

Summary

It is possible to fabricate the Ex-Struct ^TM panel using either laser stake welding or laser spot welding. However, there are a number of areas which need to be addressed to reduce the cost and to increase the controlability for high volume productions.

The thickness and shape of the extrusion, clearance between the female and male connectors and variation in length are critical for laser welding in terms of weld quality, controlability and cost. The size and shape of the tube needs to be considered very carefully. The cross-section of the tubes should be such that simple welding path patterns can be considered.

Minimal clearance should be left to get a rigid interconnection between tubes as long as there is no problem in the assembly of the tubes.

The unique concept of Ex-Struct ^TM structure makes it very versatile. A variety of extrusion shapes could be used to achieve tailored thermal or mechanical properties across the whole panel structure or at local areas. More complex extrusion shapes could be used to produce small assemblies to minimise the number of joints. A large panel structure can be formed by slotting several small panels together using the alternative male and female connectors on the side of the finished panels.

Acknowledgements

The invaluable input provided by Geoff Booth, Paul Hilton, Graham Wylde, Christoph Gerritsen and Geert Verhaeghe is gratefully acknowledged.