Friction stir joining of aluminium alloys

TWI Bulletin, November - December 1995

Chris Dawes joined TWI some 30 years ago and has wide experience in researching and developing welding technologies. He now works in Electron Beam and Friction Processes following 12 years in the Laser department. His present day interest in friction welding and surfacing dates back to the late sixties when he was closely involved in pioneering the first micro-friction welding machine.

Wayne Thomas is a Principal Research Engineer in Friction and Forge Processes Group. He joined TWI in 1983, having spent 23 years in heavy engineering and fabrication in the steel industry. He gained his MPhil from Brunel University, Materials Technology. He has been responsible for the innovation and introduction of a number of emergent technologies.

Friction stir welding is perhaps the most remarkable and potentially useful new welding technique that has been invented and developed this decade. As Chris Dawes and Wayne Thomas explain it has made it possible to weld, in a simple manner, a number of materials that were previously extremely difficult to weld reliably without voids, cracking or distortion.

The friction stir welding technique (invented, patented ^[1] and developed by TWI) is a derivative of conventional friction welding, which enables the advantages of solid phase welding to be applied to the fabrication of long butt ( Fig.1) and lap joints, with very little post weld distortion. Moreover, it is a simple to operate, very cost effective, machine tool technology offering many advantages.

Fig.1 A 2 metre long friction stir butt weld, made in 6.5mm thick 6082 aluminium alloy. The welding operation has produced very little distortion. The welding machine is to the left of the picture

The joining of aluminium alloys, especially those which are often difficult to weld, has been the initial target for developing and judging the performance of friction stir welding (FSW). Work to date has concentrated in single pass welds in material thicknesses from 1.6 to 12.7mm.

To accelerate the development and industrial use of the process, TWI is conducting a major Group Sponsored Project for an international group of seventeen companies. In the work to date, systematic welding trials have been conducted on various 2xxx (Al-Cu), 5xxx (Al-Mg) and 6xxx (Al-Mg-Si) series alloys. In each case a high level of weld quality and process repeatability has been observed. Trials are currently being conducted on a 7xxx (Al-Zn) series alloy.

This article, certain aspects of which are presented by kind permission of the group of sponsors, introduces the basic principle of FSW, pointing out the practical advantages and disadvantages, and most important of all, describes the exceptionally good metallurgical and mechanical properties, including fatigue, which can be achieved. (For the time being the sponsors, understandably, do not wish to release parameter and welding tool technology details).

Principle of operation

Fig.2 Friction stir welding concept

A friction stir weld, see Fig.2, is formed by plunging a rotating shouldered pin tool, with a pin length slightly less than the weld depth required, into the faying faces until the tool shoulder is in intimate contact with the work surface and then moving the work against the pin, or vice-versa. The rotating pin within the workpiece friction heats the metal and produces a plasticised tubular shaft of metal around the pin. As the pin is moved in the direction of welding the leading face of the pin, assisted by a special pin profile, forces plasticised material to the back of the pin whilst applying a substantial forging force to consolidate the weld metal.

Weld properties in aluminium alloys

Fig.3 A through thickness, transverse cross section, of a weld made in 6.4mm thick 6082 alloy

The transverse, through thickness, cross section of a butt weld made in 6.4mm thick, 6082 alloy, ( Fig.3), shows a typical solid-phase friction stir weld formation. The weld comprises a continuous consolidated nugget of forged material with a much refined grain size. The elliptical rings in the weld metal, ( Fig.3), are a product of the welding tool profile and forward movement per revolution in relation to the temperature gradient through the depth of the weld. At optimised welding conditions (which have proved to have a generous tolerance to variation) welds can be achieved, which are completely void and crack free.

The solid-phase weld formation produced by FSW provides three important metallurgical advantages when compared to fusion welds in aluminium alloys: first, joining in the solid-phase eliminates cracking often associated with fusion welding processes, e.g. liquation or solidification cracking; second, there is no loss of alloying elements through weld metal evaporation and the alloy composition is preserved; and finally, the crushing, stirring and forging action of the welding tool produces a weld metal with a finer grain structure than the parent metal. The latter generally enables the weld metal strength, in the as-welded condition, to be in excess of that in the HAZ. Such an example can be seen in Fig.4, which shows tensile tested specimens, made in alloy 2014A in the T6 condition, which have failed in the HAZ outside the weld metal. In the case of materials in the O condition, tensile failures can occur in the parent material well away from the weld and HAZ region, as shown in the case of alloy 5083 in Fig.5. Tensile test data for welds made in this alloy are given in Table 1.

Table 1: Tensile test data for welds made in 6mm thick alloy 5083 in the O condition (average of three samples taken over 400mm weld length).

The friction stir weld metal and HAZ in solution heat treated and artificially aged aluminium alloys ( e.g. 2xxx and 6xxx series) can be returned to a strength close to the fully heat treated parent metal, by using a post weld ageing heat treatment. Research in Norway has reported ^[2] achieving 90% of the T5 strength in 6060 alloy.

Bend tests with the weld root in tension are more searching in terms of weld metal strength and ductility than tensile tests. References to Fig.4, 5 and 6 shows that friction stir welds in alloys 2014A, 5083 and 6082 can easily withstand 180° bend tests without failure (the bend radii used were 5t, 2t and 3t respectively; t = sheet thickness).

Fig.4 180° bend (root in tension) and tensile tested welds made in alloy 2014, T6 condition. Tensile failure has occurred in the HAZ to one side of the weld metal 

Fig.5 180° bend (root in tension) and tensile tested welds made in alloy 5083, O condition. Tensile failure has occurred in the parent metal away from the HAZ and weld metal

Fig.6 180° bend (root in tension) tested welds made in alloy 6082, T6 condition, in thickness of 1.6, 6.4 and 12.7mm

The initial results of fatigue tests (conducted in tension) on friction stir welds made in 6mm thick 5083 (O condition) and 2014 (T6 condition) alloys, are quite exceptional in that they show little scatter and are far better thanhave been obtained to date from GTA and MIG processes. The fatigue performance of friction stir welds in alloy 5083, in the O condition, are comparable to that of the parent plate when tested using a stress ratio of 0:1. Despite thefact that the fatigue tested friction stir welds were single pass from one side, the results have substantially exceeded BS 8118 class 35, and the European design recommendation ECCS B3 for fusion welded joints. Fatigue endurance testresults in relation to the latter design recommendations are shown in Fig.7.

Fig.7 Fatigue endurance test results for friction stir butt welded and plain plate specimens

The advantages

Manufacturing cost savings

The welding operation is simple, energy efficient and eliminates the need for costly consumables:

• The simple push button electromechanical machine tool equipment is energy efficient - a single pass 12.5mm deep weld can be made in 6xxx alloy with a gross power of 3kW - requires very little maintenance and, apart from welding tools and electric power, relies on no other consumables.
• The welding process does not require filler wires and weld pool shielding gas.
• Special joint edge profiling is unnecessary.
• Oxide removal immediately prior to welding is unnecessary.
• The technique is ideally suited to automation.
• If necessary the welding operation can take place in all positions from down hand to overhead.

Weld qualities that provide opportunities for improved and new product designs

The low distortion repeatable quality solid-phase welds can improve existing products and lead to a number of new product designs previously not possible. For example, in the case of aluminium alloys:

• Welds can be made in alloys which cannot be fusion welded because of crack sensitivities.
• High joint strengths can be achieved in heat treatable alloys.
• No porosity.
• The solid-phase weld formation enables the retention of metallurgical properties in alloys, such as metal matrix composite materials, or those produced by rapid solidification processing techniques, where fusion welding can give rise to unfavourable metallurgical reactions.
• Dissimilar material conditions can be joined, i.e. castings to extrusions, castings to wrought products, etc.
• Many component shapes, such as: long, large cross section, one-off, box sections and spars, which normally would not be practical, or cost effective, to extrude or cast, can now be fabricated by FSW.
• Extruded lightweight panels, which are difficult to extrude in large sizes, or fusion weld together without distortion, can be friction stir butt welded together to form larger structures, such as the internal decking for ships, or chassis and platforms for rail and heavy road vehicles.
• Difficult to make hollow castings, such as manifolds, can be produced more easily by FSW together two simple solid castings, or possible extrusions.

Product quality assurance

The process is completely mechanical and therefore the welding operation and weld energy input is accurately controlled:

• Weld quality is determined by the stir welding tool profile and pre-set mechanical machine actions and consequently simple in-process monitoring can be used to terminate the welding operation if the machine actions were to deviate from the selected machine settings.
• Since the welding operation is mechanical the monitored output of the individual machine settings can be digitised and stored if necessary to provide a case history of each weld.
• The faying faces do not have to be close fitting prior to welding. Gaps can be tolerated, e.g. 0.2mm for 1.6mm sheet and 1.25mm for 12.7mm plate.

Safety

The welding operation only requires normal cutting machine (milling type) tool guards. The process is clean and does not produce any major safety hazards, such as welding fume or radiation.

The disadvantages

The single pass welding speeds in some sheet alloys are slower than for some mechanised arc welding techniques (although to date single pass welds 18mm deep have been made by FSW):

• The parts must be rigidly clamped against a backing bar, to prevent weld metal breakout, if full penetration welds are required (it may be possible to overcome this problem in the future if a bobbin tool concept under investigation can be perfected).
• At the end of each weld run a hole is left where the tool pin is withdrawn.
• Run-on/run-off plates are necessary where continuous welds are required from one edge of a plate to the other.
• Due to workpiece clamping and access requirements, applications where portable equipment could be used may be limited.
  

Future development

The development of FSW at TWI is to move ahead on two fronts: a third phase of the international Group Sponsored Project and via the TWI Core Research Programme. The former will continue research into aluminium alloys, and establish weld properties data to help enable acceptance of FSW by the classification societies: such as for shipbuilding and aerospace applications. The Core Research Programme is aimed at establishing tool materials with sufficient hot strength and wear resistance to enable welding of higher melting point materials, e.g. copper and titanium.

From a welding process standpoint, the future scope for FSW initially lies in the progress of the welding tool development. The tool technology is the heart of the FSW process. Tool shape determines the heating, plastic flow and forging pattern of the plasticised weld metal. Tool size determines the weld size, welding speed and tool strength. The tool material determines the rate of friction heating, tool strength and working temperature: the latter ultimately determining which materials can be friction stir welded. Each of these tool technology aspects will be studied to try to establish a combination, which will produce sound welds and the best tolerance to process parameters at the required working temperature.

The process research to date has concentrated on conventional butt and lap joints. The FSW technique, however, is suited to several other joint configurations, see Fig.8. The joint configurations illustrated offer further design opportunities for numerous industrial applications and consequently will be investigated.

Fig.8 Joint configurations suited to FSW:

a) Square butt;
b) Combined butt and lap;
c) Single lap;
d) Multiple lap;
e) 3 piece T butt;
f) 2 piece T butt;
g) Edge butt;
h) Possible extrusion design to
enable corner fillet weld to be made.

As FSW moves towards industrial use, the development of suitable industrial equipment will need to be accelerated. TWI has developed a moving head machine, capable of producing 2m long weld lengths. This machine supplements an original fixed head research machine, which has a moving table and produces a maximum weld length of 700mm. For specific industrial welding applications FSW heads for short welds could of course be fitted to robot arms, for long welds of several metres, multiple heads could be fitted to a gantry.

At this time two major machine/welding equipment manufacturers (both members of the Group Sponsored Project) are taking serious interest in FSW, with a view to providing the commercial equipment.

Potential applications

TWI is concentrating on drawing FSW to the attentions of industries that use, or wish to use, aluminium alloys, but experience fabrication difficulties when using existing welding techniques. Some such industries and product applications are presented in Table 2.

Table 2 Industries in which FSW could have a major application.

Overview

Friction stir welding is a remarkable new welding technique for joining aluminium alloys and has the potential for welding other higher melting point materials. Although still an infant welding process it has already been developed beyond a laboratory curiosity and has been proved as a potential practical welding technique. Several industrial companies are conducting pilot studies for using FSW in production.

Designers, welding and production engineers considering welded fabrications involving aluminium alloys are advised to consider this welding technique and also maintain a watching brief.

References

Acknowledgement

TWI would like to thank the Industrial Sponsors of Group Sponsored Project 5651, 'Development of the New Friction Stir Technique for Welding Aluminium', for giving permission to publish some of the information presented in this article.