Conventional and bobbin friction stir welding of 12% chromium alloy ...

SPECIAL ISSUE ARTICLE

Conventional and bobbin friction stir welding of 12% chromium alloy steel using composite refractory tool materials W. M. Thomas*, C. S. Wiesner, D. J. Marks and D. G. Staines The use of matching tapers to provide a hot friction drive coupling for composite refractory tools for conventional and bobbin friction stir welding of 12% chromium alloy steel is described. The feasibility of using self-reacting (Bobbin) friction stir welding for steel has been demonstrated and the resulting microstructures have been evaluated. Keywords: Friction stir welding, Bobbin, Self-reacting stir welding, 12% chromium steel

Objectives

Introduction Friction stir technology is a continuous hot shear process involving a non-consumable rotating tool of harder material than the workpiece itself. The probe portion of the tool is entered into the workpiece creating a plasticised region around the immersed probe and the contacting part of the shoulder. There is a volumetric contribution to heat generation from adiabatic heating due to deformation within a third body region that surrounds the probe and part of the shoulder. The shoulder region of the tool provides an additional friction treatment to the workpiece top surface and prevents plasticised joint material from being expelled. Essentially, the shoulder and probe thermomechanically soften and then separate the material being processed by the passage of the probe through the material. The material flows around the probe and is then forge welded together at the trailing edge of the probe. This separation and welding together occur continuously by backfilling from the probe and compaction/containment from the shoulder. This transient separation/rewelding operation happens during and before the trailing edge of the shoulder moves away from the processed/weld track. The transient plasticised region immediately coalesces and forms a solid phase bond as the tool moves away. The friction stir welding (FSW) process was patented by TWI in December 1991, and this patent also included the first example of a self-reacting (bobbin) stir weld in aluminium.

Previous work on FSW of steel A growing number of papers cover the FSW of steel, but only limited work has been reported on bobbin stir welding of steel. The following investigation describes recent work at TWI on welding steel using both conventional and bobbin stir welding techniques. The basic principle of bobbin stir welding is shown in Fig. 1.

TWI Ltd, Granta Park, Great Abington, Cambridge CB21 6AL, UK *Corresponding author, email [email protected]

ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 1 December 2008; accepted 27 January 2009 DOI 10.1179/136217109X415893

To determine the feasibility of conventional and bobbin FSW of 8 mm and 12 mm thick 12%Cr steel and to establish the weld properties and microstructures produced. To determine the practicability and the effect of composite tools, comprising different shoulder and probe refractory materials, on welding performance.

Experimental Composite tool designs for conventional and bobbin FSW Composite tools comprising of two different refractory metal alloys were used. The material for the shoulder was selected to produce a smooth surface finish and the material for the probe was selected to have high strength and to achieve good coupling to the steel at the welding temperature. The tool was made using a ‘Morse taper’ principle, which is an ideal arrangement for securing the two materials under extremely hot welding conditions (Fig. 2). The matching tapered coupling allows for the secure and efficient combination of different refractory materials and provides a unique hot friction drive. The conical probe part compresses the tapered bore of the shoulder to produce hoop stresses within the shoulder. These hoop stresses can be accommodated easily by increasing the thickness and/or diameter of the shoulder. Frictional contact across the entire conical surface area of the interface between inner and outer matching tapers is sufficient to provide a large amount of torque transmission. Therefore, splines, bolts or keys and keyways and similar stress raisers are not required. Bobbin FSW has been shown to be effective for joining hollow extrusions and lap joints. Essentially, there are two types of bobbin, or self-reacting techniques: technique of a fixed gap, between the shoulders1–3 and one that allows the gap between the shoulders to adapt during the welding operation.4–9 Published investigations have confirmed the practicability of the self-reacting technique.1–9 The self-reacting principle of

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Conventional and bobbin FSW of 12%Cr alloy steel

a bobbin tool showing two shoulders separated by preset fixed length; b bobbin tool showing self-contained reactive forces 1 Salient features of self-reacting (bobbin) stir welding: 15workpiece; 25top shoulder; 35probe; 45bottom shoulder; 55reactive forces

all bobbin techniques means that the normal down force required by conventional FSW is essentially eliminated. The reactive forces within the weld are contained between the bobbin shoulders (Fig. 1b).

Material All welding trials were carried out on 12% chromium alloy steel supplied to DIN 1?4003. For conventional FSW, 12 mm thick and 1000 mm long plates were supplied and used in the hot rolled condition, which exhibited a microstructure that consists of tempered martensite, without any significant fraction of delta ferrite. For bobbin trials, 8 mm thick and 500 mm long test specimens were machined from 12 mm thick plate.

Welding procedure The steel workpiece plates were secured with work holding fixtures onto the machine traverse table. For conventional FSW, a pilot hole of smaller diameter than the probe was drilled between the abutting plates at the start of the weld seam. For bobbin stir welding, an interference slot with a width similar to the probe diameter was machined between abutting plates at the start of each butt weld, to allow most of the contact face from both shoulders to engage before the probe made full contact. Touchdown conditions were set to minimise the stress on the tool. Traversing was initiated after sufficient time to plasticise the workpiece material in contact with the shoulder and probe. The FSW operation was carried out at ambient temperature, and no auxiliary preheat or interpass heating of the workpiece was used. Friction stir welds were produced with the welding direction parallel to the rolling direction of the plate. To prevent the friction stir welded workpiece material adhering, an oxidised carbon steel anvil support plate was used. However, this was not successful in preventing carbon steel pick-up. Therefore, sacrificial, nominally 1 mm thick strips of similar material

a conventional FSW tool with single tapered coupling design; b bobbin stir welding tool showing friction tapered coupling designed for refractory material shoulders, refractory material probe and hot work tool steel body 2 Composite refractory tools: securing different refractory materials by tapered friction coupling design

composition (316L stainless steel) were placed under the weld region to protect the anvil and the root region of the weld from metal pick-up. Equal thickness plates were placed underneath the remaining workpiece to maintain flatness.

Weld and metallurgical assessment All welds were visually examined for surface roughness, the presence of surface breaking defects and side flash. A number of welds were tensile and bend tested, together with metallurgical examination. All sections were prepared in the direction looking towards the start of the weld, and for clarity, all macrographs are marked ‘advancing side’ and ‘retreating side’. Sections from the 12% chromium alloy were polished to a 1 mm finish and etched in an ethanol solution containing 2?5% picric acid and 2?5% hydrochloric acid.

Results Welding trials For conventional FSW, 1 m long test specimens were produced using composite tools (with a tapered probe and three flats) at rotational speed 584 rev min21, traverse rate 2?5 mm s21 and axial force of 32 kN. Bobbin stir welds were made using, initially parallel, and then tapered probes with three flats at rotational speed


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584 rev min21 and traverse rate 1?25 mm s21. No axial force was required.

Trials with composite tools Figure 3 shows both conventional FSW and bobbin tools that comprise two refractory materials, the probe manufactured from a refractory alloy with high tungsten content and the shoulders manufactured from refractory alloy with a low tungsten content. The shoulders and probe of the bobbin tool are attached to a hot work tool steel body.

Surface appearance The surface of the steel welds exhibited a uniform surface ripple (caused by the final sweep of the trailing edge of the rotating tool). The FSW weld was essentially smooth and flush with the surface (Fig. 4). No protective gas was used to shield the workpiece from atmospheric contamination during welding. However, a cooling blast of air was applied to the top and bottom shoulders of the bobbin tool.

Start and stop regions

a conventional FSW tool showing taper probe with three flats; b bobbin tool showing parallel probe with three flats (later improved versions were made with tapered probe and smaller bottom shoulder) 3 Typical composite refractory tools

Unless run-on and run-off plates are used, then start and stop regions may need to be discarded. Figure 5, however, shows that for conventional FSW, reasonable backfilling of a predrilled pilot hole can be achieved providing that the pilot hole was drilled slightly smaller than the probe. The use of a larger diameter pilot hole than the probe diameter leads to incomplete backfilling (Fig. 5b). For conventional FSW, a stop and restart can if necessary be accommodated anywhere along the seam. However, a disadvantage of the fixed gap bobbin technique is that to exit the tool needs either: (i) to complete an open ended joint (ii) to break out of the work piece

a conventional FSW; b top surface bobbin tool; c bottom surface bobbin tool; d detail of surface appearance 4 Typical surface appearance of friction stir welded 12%Cr alloy steel plate using composite tools


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a complete backfilling of pilot hole with hole smaller than probe diameter; b partial backfilling of pilot hole made with hole slightly larger than probe diameter 5 Macrosections of backﬁlled pilot holes

(iii) to reverse back the same way that it entered (a double welding operation).

Mechanical properties All tensile tests samples showed failure outside the weld region, at parent material strength levels (average UTS: 511?5 N mm22). Face, root and side bends achieved 180u.

Sacrificial anvil support plates and carbon pick-up Unlike bobbin stir welding, conventional FSW needs an anvil support plate of a higher melting point material than the workpiece material being welded. For convenience, oxidised sacrificial shims are used to prevent adhesion with the anvil. However, if these sacrificial shim plates are made from dissimilar steel such as carbon steel, then significant material pick-up can occur. Figure 6 shows carbon steel friction stirred into the 12%Cr steel. Such carbon steel contamination within the 12% chromium steel weld can have serious implications for localised corrosion resistance. Carbon steel contamination from the support plates can be avoided using a ceramic anvil material, compatible sacrificial shims plates or bobbin stir welding. A typical macrosection of a sound weld using a stainless steel (grade 316L) sacrificial shim plate is shown in Fig. 7.


a root region; b detail of root region 6 Carbon steel pick-up from anvil support shims

friction stir welds, although these may be further subdivided for certain materials. These regions are: (i) unaffected parent material (ii) material that has been affected by heat, but not mechanically deformed. This is defined as the heat affected zone (HAZ) (iii) material that has been affected by heat and mechanically deformed. This is defined as the thermomechanically affected zone (TMAZ). With respect to aluminium friction stir welds, the thermomechanically and dynamically recrystallised regions can normally be differentiated within the weld. The 12%Cr steel used in this work exhibits a crystallographic transformation on cooling, which obscures any evidence of thermomechanical recrystallisation. Therefore, in the context of this report, the TMAZ will also encompass the central weld region, which

Microstructural assessment of 12% chromium alloy steel friction stir welds Studies of a number of materials indicate that there are three primary microstructural regions to consider in

7 Conventional FSW butt weld in 12 mm thick 12%Cr alloy steel with shim plate


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8 Plasticity induced grain ﬂow at boundary between TMAZ and HAZ on advancing side

cannot be differentiated as a result of crystallographic transformation. Therefore, the microstructures of the steels studied in this work has been categorised in this way.

Conventional friction stir weld Microexamination of the TMAZ revealed a fairly equiaxed microstructure, indicating that recrystallisation had occurred. Recrystallisation within the TMAZ has also led to an increased ferrite grain size (of the order of 100–150 mm compared with the parent material). The untempered martensite content, which varied over the depth of the weld, was visually assessed to lie between 15 and 40% within the TMAZ. An abrupt change in structure was observed at the edge of the TMAZ where a plastically deformed zone


(Fig. 8) defined the TMAZ/HAZ boundary, separating the comparatively large grains observed in the centre of the weld from the finer grained structure associated with the HAZ. Immediately beside the TMAZ/HAZ boundary, a ‘transformed’ HAZ was observed, where untempered martensite had formed following transformation from austenite during cooling (visual assessment indicated ferrite levels up to y40%). Further, away from the TMAZ, beside the parent metal, a visibly different ‘untransformed’ HAZ was observed, characterised by the formation of carbides around the original tempered martensite. It was clear that this ‘untransformed’ HAZ had not been sufficiently hot to exceed the austenite transformation temperature, resulting in a similar, although visibly different microstructure to that of the parent steel, presumably resulting from some growth of carbides. The HAZ/parent steel boundary was defined as the point at which carbide precipitation and grain coarsening stops. The large delta ferrite grains and reduced martensite fraction present within the TMAZ would be expected to reduce the toughness exhibited by the welded joint (compared with the parent steel). Improvement in toughness could be achieved by the use of steel with a lower ferrite factor and possibly by modification of the welding parameters.

Microstructural assessment of 12% chromium alloy steel bobbin friction stir welds Figure 9a highlights the macroscopic appearance of an etched section taken through the bobbin weld. The main features observed were the TMAZ and HAZ present on either side of the weld. The presence of a distinct banded

a etched cross-section through friction stir bobbin weld in 12%Cr steel; b higher magnification view of fine grained ferrite banding; c higher magnification view of fully ferritic band; d representative microstructure on TMAZ/HAZ boundary on retreating side of weld 9 Bobbin stir weld


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zone can be observed at the right hand side of the image (the advancing side of the weld), where a much finer grained ferritic–martensitic structure could be observed (Fig. 9b). The presence of a band of fully ferritic material was also observed towards the centre of the weld (Fig. 9c). Microexamination of the sample revealed an apparent similarity in ferrite grain size (estimated to be about 100–200 mm diameter) and untempered martensite content (y20%) for both the top and bottom of the weld. This suggested that the level of heat input was similar for both sides of the weld. This would be expected for a bobbin weld, as the locations where the shoulders of the tool contact both top and bottom surfaces of the plate will generate heat, resulting in higher temperatures at the surfaces and coarsening of the delta ferrite grains. The TMAZ/HAZ region of the weld is illustrated in Fig. 9d, where a transition from the comparatively large grained ferrite and untempered martensite structure of the TMAZ to the fine grained of the parent material can be observed. Beside the TMAZ/HAZ boundary, the transition is characterised by a reduction in ferrite grain size to that of the parent material. The HAZ itself consisted of fine grained ferrite, of a similar grain size to the parent material; however, it appeared that the structure of the untempered martensite was better defined within the HAZ, illustrating that some transformation had occurred, although no grain growth was observed. This is a similar result to that observed previously for the ‘conventional’ friction stir weld of 12%Cr steel. The origin of the fine grained microstructural banding on the advancing side of the weld is not clear. There appear to be alternating bands of martensite and ferrite within this part of the sample; the untempered martensite volume fraction is much higher, around 50%, on this side of the joint, which is attributable to either differences in composition, deformation (and hence degree of recrystallisation) or temperature. Significant variation in composition seems unlikely. Given that it is present on the advancing side of the weld, it would seem likely that differences in deformation of the material during welding have resulted in differing degrees of recrystallisation, or that a temperature gradient existed across the weld. The differing phase balance on the two sides suggests that different peak temperatures were attained and hence supports the concept of a temperature gradient across the weld. The previous work conducted on a conventional friction stir weld highlighted a small fully ferritic band within the TMAZ. Therefore, it is possible that the geometry of the bobbin is altering both the material flow (a possible bifurcated flow), and heat input, in a manner that is conducive to the formation of this banded structure.

Discussion Friction stir weldability trials The work reported is encouraging as it demonstrates that FSW can be applied successfully to steel plate. FSW has produced sound single sided welds in 12 mm thick 12%Cr steel plate at a welding travel speed of 2?5 mm s21. Preliminary bobbin stir trials also show promise with 8 mm thick 12% chromium alloy steel plate welded at 1?25 mm s21.


Refractory metal FSW tools Initial trials on the FSW of steel were carried out at TWI Ltd in 1997. These were conducted with pure tungsten FSW tool. Because this material is brittle at room temperature, the tool was preheated by an oxyacetylene flame to about 400–500uC, and the 12%Cr steel was heated, mainly from close proximity when heating the tool. This preheat was designed to start the welding at a temperature above the ductile–brittle transition temperature of the tungsten. However, the ductile–brittle transition temperature of tungsten can be decreased below room temperature by the addition of other elements such as rhenium or lanthanum oxide. All welds made during the trial were carried out without any preheat. The investigation has demonstrated that different types of refractory materials are easily secured by a matching taper under extremely hot welding conditions while subject to high torsional loads. Used in combination, refractory materials can achieve smoother weld surfaces while still providing good coupling with extremely hot plasticised weld metal.

Self-reacting bobbin FSW Bobbin tools are similar to conventional FSW tools that are driven from one side in that the tool behaves as a rotating cantilever; nevertheless, they differ in certain aspects as follows. There are a number of features that make bobbin stir welding of steel attractive. Two shoulders provide sufficient heat generation from both sides of the workpiece without any heat loss through the anvil support plate. Containment of reactive forces within the tool itself means that compressive deformation (squashing) of the probe does not occur. The probe of a conventional FSW tool is subjected to multiaxial forces comprising torsion, bending and compression. The probe of a single sided bobbin tool is also subjected to multiaxial forces comprising of comparatively higher levels of torsion and bending, but only tensile forces rather than compressive forces are applied through the probe. The use of a tapered probe for both conventional and bobbin stir welding provides for a more uniformly stressed tool, which displaces substantially less material during welding than a cylindrical pin type probe. Furthermore, the use of a tapered probe for the bobbin tool enables a proportional reduction in the diameter of the lower shoulder of the bobbin tool. Reduction in the lower shoulder diameter results in lower frictional contact and resistance, therefore less torque and bending moment on the tool. The additional frictional contact provided by the lower shoulder and the absence of a backing anvil, which acts as a heat sink, means that the operating temperature will be higher than that of similar conventional welds. Tool design and process conditions will need to be adjusted to allow the welding travel speed to be increased benefiting from this additional heat generation. Both adaptive and fixed gap self-reacting techniques will need to be developed to meet the challenges of ferrous base materials. Bobbin welds essentially eliminate partial penetration, lack of penetration or root defects. Preliminary trials have also shown that lap welds produced by the bobbin technique have fewer problems with the adverse orientation of the notch at the edge of the weld.


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Conclusions


References

Based on the results and discussions given above, the following conclusions can be made: 1. Successful friction stir welds have been produced in 12 mm and 8 mm thick 12% chromium alloy steel. 2. The feasibility of using self-reacting (bobbin) FSW for 12% chromium alloy steel has been demonstrated and the resulting microstructures have been evaluated. 3. The use of a matching taper drive coupling to provide secure combinations of different refractory materials for composite tools has been demonstrated for conventional and self-reacting FSW tools.

Acknowledgement The authors are grateful for the support and contributions provided by Dr P. Woollin, Dr I. M. Norris and Mr C. E. D. Rowe (Cedar Metals Ltd).

1. W. M. Thomas, E. D. Nicholas, J. C. Needham, M. G. Murch, P. Temple-Smith and C. J. Dawes: ‘Improvements relating to friction welding’, European Patent Specification 0615480B1. 2. K. J. Colligan and J. R. Pickens: in ‘Friction stir welding and processing III’, (ed. K. V. Jata et al.), 161–170; 2005, Warrendale, PA, TMS. 3. M. Skinner and S. R. L. Edwards: Mater. Sci. Forum, 2003, 426– 432, 2849–2854. 4. F. Marie, D. Allehaux and B. Esmiller: Proc. 5th Int. Symp. on ‘Friction stir welding’, Metz, France, September 2004, TWI, Session 01, process developments, paper 1. 5. G. Sylva and R. Edwards: Proc. 5th Int. Symp. on ‘Friction stir welding’, Metz, France, September 2004, TWI, Session 01, process developments, paper 2. 6. D. Otsuka and Y. Sakai: Proc. 7th Int. Symp. on ‘Friction stir welding’, Awaji Island, Japan, May 2008, TWI, Session 8A, process control/3, paper 1. 7. F. Marie, B. Guerin, D. Deloison and D. Aliaga: Proc. 7th Int. Symp. on ‘Friction stir welding’, Awaji Island, Japan, May 2008, TWI, Session 8A, process control/3, paper 2.


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