Novel Techniques for Corner Joints Using Friction Stir Welding

¹TWI Technology Centre (Yorkshire)
²Centre for Doctoral training in Advanced Metallic Systems, Sheffield

Paper presented at TMS 2011 Annual Meeting & Exhibition. San Diego, CA., USA. 27 Feb. - 3 Mar. 2011

Friction stir welding, aluminium alloys, corner welding, novel FSW techniques

Abstract

Most commercial FSW applications use simple butt joint configurations and alternative designs such as T-sections and corner welds are very rarely considered. This paper presents the development of novel techniques which have demonstrated the ability to produce high quality internal corner welds using an adaption of Stationary Shoulder Friction Stir Welding. Further enhancements using a shaped shoulder have also allowed fillet radii to be added to internal corners and a consumable filler wire to provide the material for the fillet. The principles of these techniques are explained including the results of process development trials on a range of aluminium alloys including metallurgical examinations, mechanical property evaluations and how they are related to the thermal weld cycles. The development of these techniques has the potential to be applied to a range of new joint geometries and extend product design possibilities.

Introduction

Friction Stir Welding (FSW) is a relatively mature solid state joining technology that can be used to weld most aluminium alloys, including those that are difficult to weld using conventional fusion-based processes.^[1] This technology is of particular interest for transport applications, since welded st ructures are considered to offer cost and weight savings. Most commercial FSW applications use simple butt joint configurations and alternative designs such as T-sections, corner welds, box sections etc are very rarely considered.

In late 2004 - early 2005 TWI developed an innovative technique called Stationary Shoulder Friction Stir Welding (SSFSW) mainly for welding low heat conductivity materials such as titanium alloys where a more uniform heat input into the weld is beneficial. The key welding mechanism in SSFSW consists of a rotating pin located in a non-rotating shoulder component, which slides over the surface of the material during welding. Butt welding trials with the SSFSW system on 6mm thickness Ti-6Al-4V plate produced very smooth, almost polished, weld surfaces with no reduction in cross-section and a consistent and almost linear heat input throughout the weld section. ^[2]

The SSFSW technique offered the potential to join plates which are positioned in different angular planes such as T joints by using a shaped stationary shoulder. This technique may help overcome some of the issues of making T joints using conventional FSW tooling reported by Steel et al^[3] of eliminating joint interfaces and the formation of small cracks in the corners reported by V Osanic et al^[4] and also eliminating undercutting of the weld section. M Penalva^[5] presented work in 2010 on making unreinforced T-butt joints using the SSFSW technique; however small bulk void defects were reported.

Corner SSFSW Joints

The welding mechanism developed consisted of a rotating pin located in a non-rotating shoulder shaped to the internal corner of the plates to be welded. The shaped shoulder contains the stirred material and slides over the surface of the material during welding. Figure 1 shows a schematic of the technique applied to a T weld configuration.

Figure 1. Schematic of T Weld using Corner SSFSW

Initial trials fabricating sample T sections in 8mm thick wrought aluminium alloy 6082-T6 plates and also in a dissimilar alloy combination of AA7075-T6 and AA2014-T6 produced welds of excellent weld quality and components with low distortion as shown in Figure 2.

Figure 2. A T section fabricated by Corner SSFSW in 8mm thick AA7075-T6 and AA2014-T6: a) General appearance; and b) macrograph

The samples showed all the benefits of the SSFSW technique; very smooth weld surfaces, no reduction in cross-section and small heat affected zones however the sharp internal corner was considered detrimental to the joint properties, particularly with regards to joint fatigue susceptibility. The latest developments, particularly in shoulder design, have demonstrated the capability of minimising these stress raisers by forming a fillet radius between abutting plates.

Corner Fillet SSFSW Joints

First technique developed to achieve a fillet radius was using the trailing edge of the stationary shoulder shoe to shape the exiting weld material. An oversized leg was machined to incorporate two 45° chamfers. This additional material was stirred during the welding process and then extruded or forged into the fillet shape on exit from the shoulder. This technique was called Corner Fillet SSFSW. Figure 3 shows a schematic of the technique applied to an internal corner weld configuration.

Figure 3. Schematic of Corner Fillet SSFSW: a) showing a view into the weld; and b) away from the weld

Again, sample T components were produced in a range of aluminium alloy grades. An example of a dissimilar alloy combination of AA7075-T6 and AA2014-T6 T section is shown in Figure 4.

Figure 4. A T section fabricated by Corner Fillet SSFSW in 8mm thick AA7075-T6 and AA2014-T6: a) General appearance; and b) macrograph

Figure 5 shows two hardness maps taken from two T sections fabricated by Corner Fillet SSFSW in 8mm thick AA7075-T6 and AA2014-T6 made at different weld traverse rates and hence weld energy inputs. The weld sequence consisted of the first weld being made on the right hand side of the T leg followed by the left hand side. These plots show clearly the annealing effect of the second weld on the previous weld and also the larger heat affected zone and lower hardness in the higher energy input weld.

Figure 5. T sections fabricated by Corner Fillet SSFSW in 8mm thick AA7075-T6 and AA2014-T6: a) High heat input; and b) Low heat input

Although little optimisation of the technique has been undertaken the mechanical performance of these welds was good. Two simple tensile mechanical tests were performed; a pull test on the plate and a 'pull off' test on the leg as shown schematically in Figure 6.

Figure 6. Schematic drawing of mechanical testing of Corner Fillet SSFSW in 8mm thick AA7075-T6 and AA2014-T6: a) Pull test of base; and b) Pull off test on leg

The results of these initial tests are shown in Table 1. The heat treatable alloys in both test failed in the heat affected zones with the 5083-O failing in the parent material. The tensile properties of the joint in the heat treatable alloys are expected to be improved through optimisation of the process.

Table 1. Mechanical testing results of Corner Fillet SSFSW in various aluminium alloys

* Denotes failure in parent material

Corner AdStir fillet SSFSW joints

In the second technique, it has been demonstrated that filler wire can be added into the corner weld during the welding process. This technique, christened Corner AdStir Fillet SSFSW, offers the potential to produce filleted corner joints from flat wrought plates, the fillet material being provided by the filler wire similar to fusion welding processes however no joint weld preparation is required. Figure 7 shows a schematic of the technique.

Figure 7. Schematic of Corner AdStir Fillet SSFSW: a) showing a view into the weld; and b) away from the weld

The initial trials were performed on similar aluminium alloy T joint configuration using AA6082-T6 as both the component and filler wire material. These produced components of similar general appearance to previous T sections produce using Corner Fillet SSFSW. The weld macrostructure is shown in Figure 8a. As only limited filler wire was available for these trials AA6082-T6 was used again as the filler for making a similar aluminium alloy T Section in AA5083-O. Sound welds were again made; the macrostructure of this weld is shown in Figure 8b. This section clearly shows the extent of metal mixing in the joint.

Figure 8. T sections fabricated by Corner AdStir Fillet SSFSW in 8mm thick; a) AA6082-T6 base and leg with a AA6082-T6 filler; b) AA5083-O base and leg with a AA6082-T6 filler; c) AA5083-O base and leg with a AA5356-T6 filler; and d) AA2014-T6 base and 7075-T6 leg with a AA6082-T6 filler

This weld was again repeated using a TIG consumable welding rod manufactured by ESAB called OK Tigrod 5356. This filler wire is a widely used welding alloy and can be classified as a general-purpose type filler alloy and is typically chosen because of its relatively high shear strength. The resultant macrostructure of this weld is shown in Figure 8c.

Finally a dissimilar alloy combination AA7075-T6 and AA2014-T6 T section was fabricated with an AA6082-T6 filler. The resultant macrostructure of this three alloy weld is shown in Figure 8d.

All the four T Sections were mechanically tested. The results of these tests are shown in Table 2. Again the heat treatable alloys joints using the AA6082-T6 filler both test failed in the heat affected zone. A typical failure is shown in Figure 9a. The joints made in the AA5083-O failed in the parent material irrespective of the filler material used as shown in Figure 9b. The tensile properties of the joints made in the heat treatable alloys are expected to be improved through optimisation of the process.

Table 2. Mechanical testing results of Corner AdStir Fillet SSFSW in various aluminium alloys

* Denotes failure in parent material

Figure 9. Fracture positions on 8mm thick T sections fabricated by Corner AdStir Fillet SSFSW: a) AA6082-T6 base and leg with a AA6082-T6 filler; and b) AA5083-O base and leg with a AA6082-T6 filler

Finally to assess the ability of the AdStir technique to compensate for poor plate alignment a simple trial was performed in a T section where the leg plate was prepared to simulate an increasing gap between itself and the base plate of 0 to 1mm. These plates were welded for one side only using the Corner AdStir Fillet technique. The filler wire not only provided the material to form the filler but also the gap between the plates. The weld had a very good appearance and a sound macrosection as shown in Figure 10a) and b respectively.

Figure 10. T section fabricated by Corner AdStir Fillet SSFSW in 8mm thick AA6082-T6 plates with a similar filler wire with a varying gap between plates: a) General appearance; and b) Macrosection showing sound weld and gap in the plates at the back of weld

Summary

This article provides an overview of recent work at TWI on novel techniques for corner joints using stationary shoulder friction stir welding, a variant of the FSW process. These techniques may prove to be a major step forward for solid phase welding and also enabling welding of joint configurations deemed impossible by conventional FSW techniques.

In addition to the advantages of SSFSW, the newly developed corner welding techniques offer the potential to extend the range of applications where FSW can be applied and also offering some new unique attributes.

• Use of wrought plates instead of bespoke extrusions
• Structures fabricated where size and thickness ratios prohibit extrusion use
• Fabricated components with tailored properties by using dissimilar materials
• Addition of third body filler materials potentially offering enhanced performance
• AdStir may increase allowable tolerances on component set up

Work is continuing at TWI to further develop and assess these and other SSFSW techniques and it is believed these new process have great potential and may offer significant technical and economic advantages over conventional methods.

Current interests and activities are predominantly focused on the transport sectors; however it is believed that this work has relevance to many other industry sectors.

Patents

Nippon Light Metal (NLM) holds a granted Japanese patent relating to the Friction Stir Welding (FSW) of internal corner joints using a stationary shoulder, JP 4240579 B.

Acknowledgements

Research work in this area is being supported by TWI's Core Research Programmes, which are funded and overseen by TWI's industrial member companies.

References

1. P L Threadgill and M E Nunn (2003) 'A Review of Friction Stir Welding: Part 1 Process Overview', TWI Members Report MR760/2003
2. M J Russell and C Blignault (2006) 'Recent Developments in Friction Stir Welding of Ti Alloys', Proc. 6th Int. Symposium on Friction Stir Welding, Paper 14, Saint-Sauveur, Quebec, Canada
3. R J Steel, T W Nelson, C D Sorensen and S M Packer: 'Friction stir welding of steel T-joint configurations' (2005) Proc. Int. Conf. on 'Offshore and polar engineering', Seoul, Korea, ISOPE, 1-5
4. V Ocenásek, M Slámová, J F dos Santos and P Vilaça (2005) 'Microstructure and properties of friction stir welding aluminium alloys', Proc. METAL 2005-14th International 'Metallurgical &Materials Conference', Hradec nad Moravici, Czech Republic
5. Penalva M, Arregi B, Rivero A, Buffa G, Fratini L 2010 (2010) 'Development of corner fillet joints' 8th Int FSW Symp, Timmerdorfer Strand, Germany