Friction stir welding of titanium alloys - A progress update

TWI Ltd, Cambridge, UK

Paper presented at 10th World Conference on Titanium, Hamburg, 13-18 July 2003

Abstract

This paper describes recent progress made on the application of the novel technique of Friction Stir Welding (FSW) to the joining of Ti alloys. FSW was invented by TWI in 1991 and has grown rapidly over the last decade into an important manufacturing process. FSW is now in industrial use worldwide, predominantly for the joining of Al alloys. Many users of the process have reported significant cost savings in manufacture, coupled with improvements in weld quality and component performance. Interest is now growing in the use of this novel technique for the joining of a range of other materials, including Ti alloys.

Although most Ti alloys are generally weldable, problems with workpiece distortion, and poor weld quality, can occur. In addition, some of the more sophisticated alloy grades are difficult to weld by conventional fusion techniques. FSW offers the possibility of a new, cost effective, method of producing high quality, low distortion, welds in titanium sheet and plate.

In this paper, the results of recent trials on FSW of Ti alloys are reported. Weld characteristics are described, as are the findings of preliminary investigations of weld microstructure, and mechanical properties. This paper provides an update on the state of the art with respect to the development of this important new welding process for the joining of Ti alloys.

1 Introduction: Friction Stir Welding

Friction stir welding is an important new non-fusion technique for joining sheet and plate material. ^[1] FSW was invented by TWI in 1991, and is a TWI licensed technology. ^[2] The basic form of the process uses a cylindrical (non-consumable) tool, consisting of a flat circular shoulder, with a smaller probe protruding from its centre. The tool is rotated and plunged into the joint line (between two rigidly clamped plates) so that the shoulder sits on the plate surface and the probe is buried in the workpiece, as shown in Fig.1.

Friction between the rotating tool and the plate material generates heat, and the high normal pressure from the tool causes a plasticised zone to form around the probe. The tool is then traversed, frictionally heating and plasticising new material as it moves along the joint line. As the tool traverses, the probe stirs the locally plasticised area and forms a solid-phase joint.

The development and take up of FSW over the last decade has been very rapid. Almost all current uses involve the joining of Al alloys, for applications including: airframes and aircraft components, ^[3,4,5] ship decking and structures, ^[6] rail carriages, ^[7] automotive components, ^[8] bridge components, ^[9] and space launch systems. ^[10,11] In addition to Al components, development of FSW has been reported for the joining of magnesium alloys ^[12] and copper, ^[13] and interest is growing in the FSW of steels ^[14] and titanium alloys. ^[15]

2 FSW of titanium alloys

Although the majority of common titanium alloys are generally weldable by conventional means, problems with workpiece distortion, and poor weld quality, can occur. In addition, some of the more advanced titanium alloys (such as Ti-6246 and Ti-17) can be difficult to weld by fusion processes. The development of FSW offers the possibility of a new, cost effective, method of producing high quality, low distortion, welds in Ti sheet and plate.

The first trials on FSW of Ti were carried out as early as 1995, as part of TWI's internal research programme. These initial welds were conducted on commercially pure (grade 2) titanium, and proved the potential of applying FSW to Ti alloys. A section from one of these initial trials is shown in Fig.2

The darker areas in this section show where the material has been heated to above the ß transus (around 900°C in this material). The lighter areas of the weld were found to be untransformed, but significantly refined.

The success of these early trials led to the formation of a TWI Group Sponsored Project (GSP) in 1996. Under this GSP a number of TWI member companies came together to jointly fund a research programme on FSW of titanium alloys. This GSP ran from 1996 through to 2002, and forms the basis of the majority of the information reported in this paper.

2.1 Friction Stir Welding of Ti-6Al-4V

The majority of the work during TWI's group sponsored project on the FSW of Ti alloys was carried out on Ti-6Al-4V plate of 6.35mm (¼ inch) thickness. Following the identification of a suitable tool material, an extensive programme of welding trials was carried out to develop effective tool designs and processing conditions for the FSW of 6.35mm thickness Ti-6Al-4V plate. This ultimately led to the production of fully-formed, high quality friction stir welds in Ti-6Al-4V, as shown in Fig.3 and 4.

2.2 Alternative workpiece thicknesses and alloys

In addition to the main body of work on the development of FSW for 6.35mm thickness Ti-6Al-4V, two additional studies were carried out on alternative applications as follows:

2.2.1 FSW of 3mm thickness Ti-6Al-4V

A small number of trials were conducted to assess the application of FSW to 3mm Ti-6Al-4V sheet. As in the 6.35mm thickness work, the FSW tool designs and welding conditions used were adjusted in order to achieve a good quality weld, as shown in Fig.5.

Using optimised welding conditions, very little distortion was produced in this case. Minimal workpiece distortion is a significant advantage of the FSW process, and it was encouraging to note that this characteristic extended to the joining of relatively thin Ti sheet.

2.2.2 FSW of 6mm thickness Ti-15V-3Al-3Cr-3Sn

A short study was also conducted on the application of FSW to Ti-15V-3Al-3Cr-3Sn of 6.7mm plate thickness. This beta phase Ti alloy was found to be significantly more formable than the alpha-beta Ti-6Al-4V. FSW of Ti-15V-3Al-3Cr-3Sn generated lower peak temperatures (approx. 800°C) than those observed in Ti-6Al-4V (approx. 1000-1200°C), and excellent weld surface quality was achieved, as shown in Fig.6.

2.3 Microstructure of Friction Stir Welded Ti-6Al-4V

Weld sections were taken throughout the research programme, and were evaluated by optical and scanning electron microscopy. These sections clearly showed the area heated and stirred by the friction stir welding tool, surrounded by a very narrow heat affected zone, which highlighted the low thermal conductivity of Ti. The weld root was observed in many cases to lie outside the hot deformed zone, and in early trials, voids in this area were relatively common. A typical example of this weld structure is shown in Fig.7.

A detailed microstructural characterisation was carried out on the three different zones identified in Fig.7, as reported below.

2.3.1 Microstructure of Zone A - Parent Material

The parent material was found to consist of a rolled microstructure of elongated gains of alpha (light) in a matrix of alpha and beta (dark), as shown in Fig.8.

2.3.2 Microstructure of zone B - deformed zone

In the deformed weld zone, the microstructure shows evidence of alpha-beta transformation, which is known to occur at around 990°C in Ti-6Al-4V. Significant grain growth appears to occur at this elevated temperature, producing large equiaxed beta grains in the weld centre. The beta phase reverts on cooling, and the resultant weld microstructure consists of large alpha grains with a smaller amount of retained beta, as shown in Fig.9. The extent of the grain growth in this region suggests that there is potential to reduce the heat input to this area of the weld.

2.3.3 Microstructure of zone C - partially transformed zone

The weld root zone microstructure in this case shows that only partial transformation has occurred in this region. Grain growth is also much less than that seen in the weld, leaving a fairly fine-grained structure with small areas of transformed beta, as shown in Fig.10. This partially transformed structure confirms that lower temperatures have been experienced in this area of the weld, which probably accounts for the voids observed in this region.

In addition to the microstructural examination, hardness tests were conducted across the centre line of this weld (in the fully transformed zone). The hardness was found to have increased (from about 305HV base) to around 340HV in the weld zone. This rise is consistent with an increased proportion of alpha phase in the transformed weld structure.

2.4 Tensile properties of Friction Stir Welded Ti-6Al-4V

Transverse tensile tests were carried out on selected good quality welds, produced during the later stages of the experimental programme. The results of these are reported in Table 1.

Table 1. Tensile properties of selected welds in 6.35mm thickness Ti-6Al-4V

Sample Number	Section Area (mm 2 )	Max. Load (kN)	Max. Stress (N/mm 2 )	Elongation %
Parent	-	-	1035	14
w5.7A	66.0	67.3	1020	8.5
w5.7B	65.7	64.8	986	8.5
w5.18A	60.2	64.3	1068	8
w5.18B	61.2	65.1	1064	7.5

The results of these preliminary tensile tests were encouraging, with strengths in excess of the parent material reported in some cases. An increase in strength in the weld zone was expected (due to the higher proportion of alpha phase in the transformed region) and these results confirm that high quality friction stir welds have been produced.

3 Characteristics of the FSW process in titanium alloys

The early trial shown in Fig.7 illustrates one of the main challenges in the FSW of Ti alloys: generation of sufficient heat at the weld root. It can be clearly seen that the root of this weld is not fully formed, and the partially transformed microstructure of the root zone (shown in Fig.10) confirms that insufficient heat has been generated in this area. The temperature reached during FSW is closely related to the rubbing velocity of the tool on the workpiece material. At the weld surface, the large shoulder diameter leads to a high rubbing velocity, and generation of sufficient heat is not a problem. However, at the weld root, the smaller pin of the tool has a much lower rubbing velocity, producing considerably less heat. Increasing the tool rotation speed can raise the temperature at the weld root, but this improvement is limited by the possibility of generating too much heat at the weld surface, leading to material over-softening and loss of weld containment.

3.1 Tool lifetime in Friction Stir Welding of titanium alloys

Preliminary investigations of tool lifetime in the FSW of 6.35mm Ti-6Al-4V were conducted. There are three main ways in which a FSW tool can degrade during use, as follows:

1. Tool fracture
2. Tool deformation
3. Tool wear

Large-scale tool fracture during FSW is usually an obvious event, brought about by the processing forces exceeding the strength limits of the tool pin. This form of failure has not been commonly observed in the FSW of titanium alloys. However, smaller scale tool fracture (loss of small parts of the tool pin) has been observed in Ti FSW, particularly under non-optimised welding conditions.

Tool deformation is probably the most important possible failure mechanism in the FSW of titanium alloys. The extreme processing conditions generated during FSW of Ti-6Al-4V can cause forging of the tools, particularly in the colder root area of the weld. Tool deformation can be controlled, by careful pin design, and by the use of appropriate welding conditions, but it cannot be entirely eliminated at this time. It is likely that in future further improvements will be made in the tool materials in order to address this issue. At the present time however, tools used for FSW of Ti alloys must be regularly inspected.

The third possible mechanism for tool degradation is wear, either by gradual abrasion of the tool surface, or by chemical dissolution of the tool material into the workpiece. Initial studies have shown that (under optimised welding conditions) neither of these mechanisms are of great concern in the FSW of titanium alloys.

In the limited tool lifetime experiments conducted to date, individual FSW tools have been used to produce up to 5m of weld in 6.35mm thick Ti-6Al-4V without noticeable degradation.

4 Summary

The current state of the art with respect to FSW of Ti alloys can be defined as follows:

• The feasibility of joining Ti alloys by Friction Stir Welding has been proven.
• Welds have been successfully made in CP Ti, Ti-6Al-4V, and Ti-15V-3Al-3Cr-3Sn.
• Weld tensile strengths similar to those of the parent alloy have been achieved in 6.35mm thickness Ti-6Al-4V.

Friction stir welding of titanium alloys is still an emerging technology, and the following limitations exist at this time:

• The FSW process has not yet been fully optimised for this application, and the joining of Ti alloys by FSW remains a challenging undertaking.
• Limited experience exists of the application of this technology to real components.
• The advanced tool technology and supporting systems required for FSW of Ti are currently relatively expensive.

TWI is currently in the process of conducting further work aimed at improving the quality and repeatability of FSW in Ti alloys. It is believed that the results of this work will be of significant benefit to the commercial application of this technology.

5 Acknowledgements

The author would like to thank the sponsors of GSP 5689: ESAB, The Boeing Company, and Snecma Moteurs, for their support, and for their permission to release the information contained herein. Acknowledgements are also due to Dr Philip Threadgill and Mrs Debbie Oglesby, for their assistance in the preparation of this paper.

6 References

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