Friction Joining for Stainless Steel and Aluminium Uses

Presented at 'Productivity beyond 2000': IIW Asian Pacific Welding Congress, Auckland, New Zealand, February 1996

Author Details

Wayne Thomas is a Principal Research Engineer in the Friction and Forge Processes Group at TWI. He gained his MPhil from Brunel University, Materials Technology and is the author of a number of technical papers and has been responsible for the innovation and introduction of a number of emergent technologies.

Dave Nicholas is Head of the Friction & Forge Processes Group which is concerned with research on, and development and application of friction welding (rotary, linear, stud and surfacing), flash welding, MIAB and MIAF welding, and explosive welding. He joined TWI in 1967, after obtaining his degree in metallurgy, working for the first year with resistance welding then in friction welding where he led the Section for almost 20 years.

Abstract

This paper will focus on some of the opportunities presented by three friction technologies for stainless steel, aluminium and stainless steel to aluminium which are receiving widespread interest.

Friction Hydro Pillar Processing (FHPP)

FHPP is a material processing and joining technology still in the development stage, but with potentially much to offer for a number of industries. This new method of joining shows promise for thick plate stainless steel and thick plate aluminium storage spheres, and live bus bars, etc.

Friction Stir Welding (FSW)

FSW is a continuous non-consumable process that produces solid-phase, low distortion, good appearance welds at relatively low cost. The technique involves a rotating tool consisting of a probe usually of harder material than the workpiece itself.

Friction Plunge Welding

Friction plunge welding joins dissimilar materials. A close relative of other friction techniques, friction plunge welding involves immersing a relatively hard material in a relatively soft material.

1. Introduction

Materials joining is a key enabling technology which impacts competitiveness and reliability in almost all manufacturing sectors. Virtually all manufactured products are made from components which must have been joined. Continuous improvements and effective application of emerging technology for joining are essential for manufacturers to remain competitive. This paper will describe the latest developments being carried out at TWI and focus on some of the opportunities presented by three novel friction technologies for joining stainless steel, aluminium, and stainless steel to aluminium.

The recorded use of frictional heat for solid-phase joining techniques dates back over a hundred years. The friction welding process, however, to a large extent has been restricted to round, square, or rectangular bars. More recently, TWI has been working on new techniques which now allow solid-phase friction welding to be applied to sheet and plate material as a viable option for plate fabrication in a range of materials. Friction welding has now gained a prominent position world-wide as a joining process for many industries as diverse as sub-sea and aerospace.

2. Friction hydro pillar processing

2.1. Description of process

Friction hydro pillar processing (FHPP) is a comparatively recent solid-phase welding technique ^[1,2,3]. Invented at TWI, this technique is the focus of considerable R&D interest because of its potential in fabrication and manufacturing where it offers a number of novel production routes. The FHPP technique is still under development, but already shows promise for joining and repairing thick plate in ferrous and non-ferrous materials. Conventional fusion welding of thick section fabrications involves lengthy processing sequences and with some process large volumes of consumable material. In contrast, use of the FHPP welding technique should provide a reduction in joint preparation and weld filler metal, which will lead to significant cost savings.

 

The FHPP technique involves rotating a consumable rod co-axially in a circular hole, under an applied load to continuously generate a plasticised layer. The layer consists of an almost infinite series of adiabatic shear surfaces ^[4]. The main features of the process are illustrated in Fig. 1. During FHPP the consumable is fully plasticised at the frictional interface across the bore of the hole. This interface travels through the thickness of the workpiece. The plasticised material develops at a rate faster than the feed rate of the consumable rod. This means that the frictional rubbing surface rises along the consumable to form the dynamically recrystallised deposit material. The plasticised material at the rotational interface is maintained in a sufficiently viscous condition for hydrostatic forces to be transmitted, both axially and radially, to the bore of a parallel sided hole enabling a metallurgical bond to be achieved. Since this material is being forced hydrostatically into the surrounding bore, the diameter of the deposit material is nominally greater than the feedstock material.

2.2. FHPP - parallel holes

Very good quality FHPP welds have been produced, using a parallel hole geometry, in steel and certain non-ferrous materials, and these have been characterised by good impact, tensile, and bend results.

Sudden changes in microstructure are sometimes observed, and are considered to be the result of an almost instantaneous shift in the rotational frictional interface. The rotating consumable is believed to seize at its current frictional interface (rubbing surface) and then shear at a location some distance further along the consumable, creating a new frictional interface. This effect is shown in Fig. 2. If sufficiently large, the area between these periodic interfaces shows little sign of extensive plastic deformation, even though the bond is good. (The bond between these regions of partially transformed consumable material is comparable with that of a normal friction weld). Presumably, if the hydro pillar processing action forces sufficient softer material between the rotating consumable and the side of the hole, (i.e. back extrusion of plasticised material) the area of frictional contact will increase. This additional area under frictional contact will lead to much higher torsional resistance, which can cause the area of weakest shear strength to move to a point further along the rotating consumable. This effect is particularly noticeable in parallel hole welds where a comparatively high rotation speed and a high consumable displacement (burnoff) rate were used.

 

In ideal conditions the distance moved by the rotational frictional interface should be very small, giving an almost continuous movement of a series of shear interfaces along the weld, typically as shown in Fig. 3.

 

2.3. FHPP - taper holes

Tapered holes and consumables with a correspondingly more acute taper can be used, as shown in Fig. 4. The use of tapered holes together with tapered consumables enables a reactive force as well as hydrodynamic force to be exploited in making the joint. This variant of the technique enables FHPP welds to be made in materials regarded as difficult to extrude or flow at forging temperature. The geometry is such that a nominally uniform gap is maintained between the changing cross-section of the consumable and the changing hole diameter during the welding operation. If the angle of the taper is too obtuse, the deposited pillar material will not climb ^[5,6]. The angle of the taper will to some extent be material-dependant, i.e. the relative ease with which the material can be conventionally extruded may be a feature worth noting. Current indications are that included angles of less than 30° for certain copper alloys and angles of less than 25° for aluminium alloys would be preferred.

The use of tapered holes and correspondingly more acute tapered consumables reduces the tendency for large shift in the location of the rotational interface to occur. The taper pillar welding technique allows comparatively higher rotational speeds and higher consumable displacement rates to be used than are possible with parallel holes and parallel consumables.

As well as providing a reactive support, tapered holes and tapered consumables provide an increasing cross-sectional area of correspondingly increased strength. In addition, the diverging gap above the interface tends to prevent the back extrusion of plasticised material, again reducing the tendency for large changes in frictional interface location.

Figure 5a & b shows the result of taper welding 6068 to 6082 aluminium alloy material.

 

2.4. Process characteristics

The FHPP weld cycle would benefit from a comparatively high consumable rotational speed for a short touch-down conditioning period, followed by a lower rotational speed for the main pillaring period, and finally by an even lower consumable rotational speed for the end part of the weld cycle. This applies to parallel and taper geometries.The higher rotational speeds for the initial part of the weld cycle would quickly raise the temperature of the comparatively cold substrate and consumable. A lower consumable rotational speed at the end part of the weld cycle would generate a thicker plasticised layer which, in turn, would fill any depression which can occur with a single speed weld cycle.

FHPP is an asymmetric process; depending on the elasticity of the material and the length of consumable stick out, the consumable may torsionally twist along its unsupported length. However, for consumables with short stick out, torsional twisting can be regarded as insignificant. With respect to material deposition, the friction rotational interface preferentially travels towards the relatively smaller mass consumable bar by a continuous helical shear ^[6]. Heat flow and thermal conduction causes the rubbing surface to be partially spherical in shape, as shown in Fig. 5a.

Figure 6 illustrates the FHPP technique being used to butt weld steel plates.

Work at TWI has shown that good mechanical integrity can be achieved: Figure 7a shows typical hammer bend tests and Fig. 7b shows typical tensile test, with failure away from the interface and heat affected zone (HAZ). Impact tests have demonstrated that a significant improvement in toughness properties can also be achieved, e.g. low carbon steel consumable material before FHPP gave impact properties of 31 Joules kV while the same material after FHPP gave deposit core impact properties of 115 Joules kV.

 

Metallographic examination has shown that the FHPP deposit material is hot-worked with very fine grained microstructure.

The process advantages can be summarised as follows:

• deep penetration narrow gap technique
• low cost (bar stock) consumables
• environmentally friendly process
• repair technique
• rapid - 100mm deep holes can be filled in less than 20 seconds
• suitable for magnetically hostile environment

Although the process is still at the development stage, it is not unreasonable to consider it for:

• One-sided rivetting of plates
• Repair of localised lamellar tearing and cracks
• Mechanical locking device
• Repair of wrongly positioned, drilled holes

 

3. Friction stir welding

3.1. Description of process

Friction stir welding (FSW) is a new welding technique invented and developed at TWI ^[7]. This technique has already made an impact on the aluminium producer and aluminium user industries and offers great potential for sectors such as ship building, automotive and aerospace. Friction stir welding is a technique which allows alloys based on aluminium, lead, copper and some thermoplastics to be butt welded continuously with a non-consumable tool. The technique brings the benefits of solid-phase friction welding to material forms e.g. plate, previously regarded as unsuitable^[8-12].

 

Friction stir welding is a continuous hot-shear process involving a non-consumable, rotating probe of harder material than the substrate itself. The basic principle of the process is shown in Fig. 8. Essentially, a portion of a specially shaped rotating tool is entered between the abutting faces of the workpiece (i.e. the joint). The tool's rotary motion generates frictional heat which creates a plasticised region (a local active zone) around the immersed portion of the tool, the contacting surface of the shouldered region on the tool and the workpiece top surface. The shouldered region provides additional friction treatment to the workpiece as well as preventing plasticised material from being expelled. The tool is then steadily moved along the joint line, with the plasticised zone cooling behind the tool to form a solid-phase joint as the tool moves forward.

Figure 9a shows an aluminium alloy test plate. Figure 9b show a detail of the weld surface appearance, with part circular ripples. These ripples are caused by the final sweep of the trailing edge of the shouldered region of the rotating tool and point toward the start of the weld.

 

Successful trials have been carried out on wrought aluminium, many wrought aluminium alloys and lead. Preliminary trials with copper have also shown promise. Test samples have been subjected to mechanical tensile and hammer-bend testing with good mechanical properties being achieved, Fig. 10 shows hammer bends and tensile tests. In addition, good fatigue properties have been achieved ^[12]. The metallographic examination of FSW specimens revealed the presence of a fine-grain microstructure.

 

3.2. Process characteristics

The leading edge of the rotating tool provides frictional heat and subsequent thermal softening in front of its specially shaped probe (a preheat effect). The greater the area of shouldered region of the rotating tool making contact with the joint surface the greater the frictional heat available. Increasing the diameter of the shouldered region, however, has practical limitations and tends to produce side flash on the weld surface. When welding the more difficult and thicker materials the flow of plasticised material around the probe becomes an important consideration. If comparatively large diameter probes are necessary, much larger volumes of material will be displaced, techniques which enhance the flow of plasticised material around the probe become essential. Comparatively large volume probes create can large voids.

The use of a profiled probe similar to a screw thread or helix has resulted in improved friction stir welding quality ^[13]. Research work is being carried out at TWI to evaluate a range of probes which are designed to improve the flow of plasticised material around and through the probe itself, as illustrated in Fig. 11.

 

The rationale for the above tool geometry can best be understood from the following statements:

• That the material being deformed around the probe can perhaps be treated in much the same way as a solid being extrude by friction ^[14].
• That the natural dynamic orbit (eccentricity) of the rotating tool allows plasticised material to pass around the probe. For every rotary machine there must be, to a greater or lesser extent, an inherent dynamic orbit about whichthe theoretical centre moves (no machine ever built can maintain true concentricity).
• ( That orbiting type (probes with very sight bias off-centre) and paddle type probes may allow material to flow more easily around the probe because the region of frictional effect is greater than its displacement volume.
• That the probe must pass through plasticised material - conversely that the plasticised material must pass around the probe. In the case of a whisk type tool this would additionally allow some of the plasticised material to passthrough the probe. Whisk type tools displace less material than solid tools of similar diameter.

Some of the advantages of the FSW process are summarised as follows:

• non-consumable - key hole technique
• solid-phase
• continuous - essentially unlimited length
• good surface appearance
• low distortion
• joint can be produced from one side
• simple to use
• 5G position
• no post-weld dressing is needed
• no loss of alloying elements, since no fusion takes place

 

4. Friction plunge welding

4.1. Description of Process

For structural fabrication of aluminium alloys, the friction plunge technique (aspects of which were invented and developed at TWI) may prove useful for certain dissimilar materials. Preliminary studies show joints with improved electrical, thermal, and mechanical properties are possible with friction plunge welding.

Friction plunge welding involves immersing a relatively hard material in a relatively soft material ^[14,15]. Relative motion between the workpiece components generates friction heat which creates a localised plasticised zone in the softer material. Since the harder material is pre-shaped to form re-entrant features (a dovetail effect) and is not consumed or deformed, the plasticised material flows into these re-entrant regions. After motion is stopped, the plasticised layer coalesces to form a mechanical interlock and depending on the materials being joined a metallurgical bond may also be formed.

The principle of friction plunge welding is shown in Fig. 12. A removable constraint or containment shoulder forces the relatively soft plasticised material back on to the relatively hard shaped component. This technique provides a degree of mechanical interlock which should help reduce the risk of catastrophic failure in service, especially at elevated temperature were dissimilar materials are susceptible to the formation of brittle intermetallic compounds. Figure 13 shows the typical re-entrant features of the friction plunge welding technique.

 

Friction plunge welding trials at TWI laboratories confirm that excellent mechanical integrity can be achieved. Metallurgical examination has shown that the joint interface region is free from defects and that the immersed, relatively hard material is surrounded by a recrystallised zone of the relatively soft material.

Attaching steel anchorages and end fittings to aluminium structures are some of potential applications being considered.

5. General comments

Although the new technologies described above are in different stages of development, there is no doubt that the use of these emergent friction processes will open up new markets and new opportunities. There is an expanding portfolio of friction techniques able to tackle a wide range of applications.

TWI has filed for patents on the above developments.

6. References 

1	Thomas W M, Nicholas E, D, Dolby R E, Dawes C J, Jones S B and Lilly R H	'Friction Plug Extrusion' International patent, PCT/GB92/01540,21.8.92.
2	Thomas W M & Nicholas E D	'Friction Hydro Pillar Processing' TWI, Connect, June 1992.
3	Thomas W M & Nicholas E D	'On Trial - A New Thick Plate Joining Technique' TWI Connect, April 1993.
4	Tegart McGregor W J	'Elements of Mechanical Metallurgy' Pub. Collier-Macmillan Ltd, pp 3 & 217.
5	Fukakusa K and Satoh T	'Travelling Phenomena of Rotational Plane During Friction Welding - Application of Friction Hardfacing' - International Symposium Resistance Welding and Related Welding Processes - 10-12 July 1986, Osaka.
6	Hasui Atsushi	'Effect of the Relative Difference of Bar Diameter on the Friction Welding Phenomena- Study on the Friction Welding of Different Diameter Bars' (report 1) Keio University, IIW doc Comm 11-679-81.
7	Thomas W M et al	'Friction Stir Butt Welding' International Patent Application No PCT/GB92 Patent Application No 9125978.8, 6th December 1991.
8	Thomas W M, Nicholas E D & Murch M G	'Friction Stir Welding?' TWI Connect, March 1993.
9	Thomas et al	'Friction Stir Welding' Metalworking Technology Europe 1994, Sterline Publications, pp 143-145.
10	Dawes C J	'An Introduction to Friction Stir Welding and its Development' Welding and Metal Fabrication, January 1995.
11	Midling O T, Morley E J & Kluken A O	'Joining of Aluminium Constructions by Friction Stir Welding' Hydro Aluminium, R & D Centre, 4265 Havik, Norway.
12	Midling O T	'Material Flow Behaviour and Microstructural Integrity of Friction Stir Butt Weldments' Proc. of the 4th International Conference on Aluminium Alloys, Atlanta, GA, USA, 11-16 September 1994.
13		Journal - 'Welding and Metal Fabrication', June 1995, pp 214.
14	Thomas W M	'Recycling of Non-Ferrous Powder and Machine Swarf by Friction Extrusion - An Introduction', International Conference - Recycling of Materials, Amsterdam, 19-21 October 1994.
15	Thomas W M, Nicholas E D & Needham J C	'Friction Joining' European Patent Specification 94303363.9, 20th May 1993.
16	Thomas W M & Nicholas E D	'Friction Takes the Plunge' TWI Connect, September 1993.