TWI Ltd, Granta Park, Gt. Abington, Cambridge, UK CB21 6AL
Paper published in Aluminium surface science and technology conference, Beaune, France, 14-18 May 2006.
Abstract
Designers are constantly looking for new opportunities to improve product performance and aesthetics and materials joining technology plays a key role in the choices they are able to make.
This paper examines the challenges to effective joining of aluminium alloys both to itself and to other materials. It will look at a number of current joining processes and will critically examine both the benefits and limitations of the current solutions being used for joining.
Introduction
Fusion welding; joining by melting the two (or more) parts of a work-piece together, is a commonly used technique for achieving high quality joints but it is not without challenges.
Welding porosity arises through the difference of solubility of hydrogen, usually introduced to the weld through contamination (parent plate and/or consumables) in the molten state. High solubility of hydrogen, 20 times higher in the molten state, makes it almost impossible to produce fusion welds without porosity.
The significant difference between the melting points of the metal (~660°C for pure Al) and its surface oxide (2060°C for Al 2 O 3 ) means that oxide layer needs to be removed (or at least reduced) to allow effective melting of the metal and the dispersion of the oxide. Mechanical scraping of the surface can be beneficial but the oxide layer quickly reforms. Cathodic cleaning, a method which exploits the effect of connecting the arc welding electrode to the positive terminal of the power source, can be used to break up and disperse the oxide whilst also heating and melting the work-piece.
As with other metallic alloys certain Al alloy systems (not pure Al) are more prone than others to hot cracking during fusion welding. This is a function of how the alloy solidifies during welding with different alloying elements(silicon, copper, magnesium, lithium etc) which have different influences on an alloy's susceptibility to hot-cracking. In welding the tendency towards hot-cracking can often be alleviated by adding appropriate alloying additions via a filler material usually in the form of a wire consumable.
Loss of strength can occur as a consequence of fusion welding. The properties of the welded joint will be very dependent on the condition of the material (as cast, annealed, work hardened, solid solution hardened, precipitation hardened) before welding. The properties of the fusion or weld zone are going to be similar to an as-cast structure, although some modification can be made by selecting appropriate consumable materials, so if the parent is in a hardened state there will be considerable mismatch between the weld and the parent material. A similar loss of strength in hardened alloys occurs in the Heat Affected Zone (HAZ) this cannot be alleviated by adding alloying elements to the weld. Another practical consideration in fusion welding is that excessively high heat inputs (energy per unit length) can cause significant distortion to occur which, in severe cases, can adversely affect desired geometry and aesthetic appearance.
Arc welding
Arc welding is probably the best-known of the fusion welding processes and is in widespread use for joining aluminium throughout the world. Both the gas Tungsten Arc Welding (GTAW) and Gas Metal Arc Welding (GMAW) processes are suitable for welding aluminium alloys. To help break down and disperse the surface oxide GTAW is usually used with an Alternating Current (AC) but direct current (DC) GMAW being normally connected Electrode-Positive is already predisposed to this need. Probably the most significant development in arc welding, having a direct influence on aluminium joining, is the introduction of microprocessor controlled inverter-based power supplies. These are capable of producing very sophisticated pulsed output characteristics which can be used to control metal transfer, burn-off rate and heat-input characteristics. An example of this type of development is variable polarity GMAW, which is capable of high productivity (high deposition rate) with a lower equivalent heat-input compared with conventional GMAW. Figure 1 shows the effects on bead profile and parent metal melting by varying the relative polarity of the electrode and the parent plate. This ability is of particular interest when welding thin sheet material that might otherwise be severely distorted during welding using more conventional arc techniques. Other developments of the GMAW process of interest are the systems working in a controlled dip-transfer mode. These systems are capable of precise control of arc heating and metering of wire additions into the molten weld pool to give high quality low heat-input joints. A further extension of the application of such systems is their use in brazing which is creating considerable interest for hybrid metal joining, between, for example, aluminium and steel.
Laser and laser-arc hybrid welding techniques
High power density processes such as laser welding combine the ability to produce deeply penetrating welds with low overall heat input. Aluminium alloys however present particular challenges to the use of lasers for welding. Compared to other metals, such as steel, aluminium has a relatively high reflectivity (low absorption), at room temperature, to commonly used industrial laser radiation (1064-10,000nm). As a general rule, the longer the wavelength, the higher the reflectivity. This, coupled with the fact that thermal conductivity of aluminium is also relatively high, has in the past, made the initiation and stability of welding problematic. However, with the introduction of higher mean power (>3kW), high brightness Neodymium:Yttrium Aluminium Garnet (Nd:YAG) lasers (1064nm) practical laser welding of aluminium is now possible. Figure 2 shows a cross-section of a weld made in a 2 mm thick, 5000 series aluminium alloy using a 3kW Nd:YAG laser at a welding speed of 2.2m/min. The capabilities of lasers have further been improved by the introduction of Ytterbium fibre lasers that have very high mean power (>7kW) and very good beam quality, a characteristic which allows the laser to be focused to a very small spot size to produce very high power density at the material surface. This, together with the relative high overall energy efficiency and a small footprint of the fibre laser means that laser welding of aluminium is now not only feasible but also a viable option for many applications.
There is considerable interest from industry in further increasing welding performance by combining the high penetration capability of laser welding with the tolerance to misalignment and poor fit-up of an arc process. An example of a hybrid laser-arc process is the combination of either Nd:YAG or the Yb Fibre laser with the GMAW process. Figure 3 shows a typical arrangement for bringing the two processes together at the work-piece. This particular combination offers the added benefit of being able to add filler metal during welding to help control the weld properties. Figure 4 shows the cross-section of a weld made using the laser-arc hybrid process in a 2mm thick, 5000 series alloy at a welding speed of 4.5m/min. When compared to the weld shown in Figure 2, in the same material, it can be seen that the combination of the two processes greatly increases the volume of the material melted and increases the welding speed.
Resistance Spot Welding
Resistance spot welding is a popular process for joining thin sheet aluminium. The principle of operation involves the clamping of two or more overlapping sheets between electrodes and passing a high current between the electrodes. Resistive heating at the faying surfaces in the sheet aluminium causes a small nugget of molten material to be formed.
Process repeatability and weld quality is very dependent on the ability to maintain the geometry of the electrode tips so that appropriate clamping forces are present and good electrical contact always exists between the tip and the materials being welded. As a consequence of tool wear and metal pick-up frequent dressing or replacement of electrodes is often a requirement.
Mechanical Fasteners
Of considerable interest to the automotive industry is self-piercing riveting and press joining methods, which can be automated in a similar way to spot welding. Self-piercing riveting involves the use of a punch and die to drive a tubular rivet into the sheets to be joined. The rivet is expanded in the lower sheet, normally without piercing it, and forms a mechanical interlock, this is shown in cross-section in Figure 5.
Press joining (or clinching) as shown in Figure 6 involves a similar operation but without the rivets. A punch deforms the sheets into a die that is specially designed to permit interlocking of the sheets in a button formed on one side of the sheet. For vehicle body applications, the sheets are generally not pierced through in the making of the joint.
Comparative studies of commercially available self-piercing riveting, clinching and spot welding methods indicate that average peel and shear failure loads for the self-piercing rivets and spot welds in 1.6m thick 5182 aluminium alloy are comparable with the values for the self piercing riveting being slightly higher.
For press joints, peel and shear strengths of less than half that obtained with self-piercing rivets and spot welds were achieved.
The added advantage of the self-piercing rivets and press joint techniques is that they can be used for dissimilar material combinations such as aluminium/steel joints and on lightweight aluminium/plastic sandwich material; both of which are impossible to join using resistance spot welding.
The use of adhesives in the joint line is also increasingly attractive in combination with mechanical fastener systems. This combination avoids some of the problems of how to hold sheets in alignment during curing and provides the capability for sealing of joints. These hybrid joints are already being used in the production of aluminium alloy based vehicles in Europe.
Friction Stir Welding
Friction stir welding (FSW) is a significant development in the options available for joining aluminium alloys. FSW was invented in 1991 and has since grown steadily in application world-wide. Applications range from the purely-decorative to critical structural joints for aerospace applications.
The use of friction to generate heat for welding is not new but FSW is a particular variant of the friction processes, which is capable of providing unique opportunities over more traditional approaches. Figure 7 shows a schematic of the process principles. The process is based around the use of a rotating tool, typically consisting of a central pin with a surrounding shoulder that is initially pushed into the surface of a work-piece. The heat generated by the contact between the tool and work-piece causes the softening and under the tool forces the material begins to flow. The tool is then translated along the work-piece relative to joint line to produce a welded joint. Because the welded material has only been softened, not melted, degradation of many of the important properties of the joint - tensile strength, fatigue strength, toughness - can be reduced and can be very close to that of the parent material.
Modern friction stir welding equipment is available in various sizes with a range of capabilities. At one extreme such systems consist of a tool mounted on a large moving gantry with another, opposing tool, mounted on a moving carriage below. Such a system is shown in Figure 8. This system has been designed to be capable of welding aluminium alloys up to ~200mm thick in single pass. There is also increasing interest in the prospect of robotic FSW systems. These systems have been designed to be capable of carrying out welding on 3-dimensional fabrications, and with special tooling need access only from one side. These systems are likely to be optimised for thinner sheet metal components (<2.0mm) either for high speed welding or for friction stir spot welding (FSSW).
Bonding
Adhesive bonding is an important aluminium joining technique that can give excellent performance in service. For structural joints the materials to be joined require stringent surface preparation which typically would include initial cleaning and decreasing followed by etching and in the case of an aluminium alloy, anodising. Effective joining of metals to composite materials to create durable, high performance, lightweight hybrid structures is of interest to many sectors of industry.
A new method of preparing the surface of a metallic structure using a high speed scanning Electron Beam technique has recently been developed. The so-called Surfi-Sculpt TM technique is capable of produce very high aspect ratio protrusions in complex patterns on the surface of a variety of materials. Figure 9 shows the surface of an aluminium alloy sample that has been prepared using this technique. A typical area of say 25 x 25mm can be processed in less than ten seconds. The prepared surface protrusions can then be used as key for adhesive bonding or to attach composite materials. The results of initial tests show that joints produced in this way have improved strength and fail in a more predictable manner than without. Although it is too early to say whether this technology holds all the answers it does show considerable promise for the future.
Acknowledgements
Thanks to my colleagues at TWI for the hard work that has generated the information and pictures contained within this paper.
References
- Mathers Gene, The Welding of Aluminium and its Alloys. Woodhead Publishing Ltd and CRC Press 2002. ISBN 1 85573 567 9
- Lancaster J.F. Metallurgy of Welding. Sixth Edition. Woodhead Publishing Ltd. ISBN 1 85573 428 1