v115n10a2 Friction processing as an alternative joining ... · was invented and patented by TWI (World Centre for Materials Joining Technology) in the UK in the early 1990s. A description

IntroductionAssessing the potential for using alternativejoining and repair techniques for specificapplications in a given industrial sector isinformed by an appreciation of the current sizeand potential growth in that sector. Powergeneration by nuclear fission has been out offavour over the past few years, due to severalhigh-profile nuclear incidents, e.g. theFukushima Daiichi nuclear disaster, as well asconcerns over disposal of nuclear waste andspent fuel rods. However, due to rapidincreases in the demand for electricity, coupled

with many fossil-fuel fired plants reachingend-of-life, there is renewed interest ininstalling additional nuclear capacity.Proposing friction processing as an alternative(and relatively untried) joining technology forthe nuclear industry might be viewed aspotentially perilous because of stringentrequirements for validation of weld and repairprocedures. The intention in this paper is tointroduce some modern developments in thefriction processing arena and to outline thepotential that these processes hold formanufacturing of new components and formaintenance and life extension of ageingnuclear power plant.

The World Nuclear Association (WNA,2015) states that as at April 2015, there were437 nuclear reactors in operation, with another65 under construction and a further 481 eitherin planning or proposed. Clearly, there issignificant potential for the application offriction processing techniques in this powersector. The age of reactors varies, from first-generation reactors developed in the 1950s tothe current generation III reactors introducedin 1996 in Japan, with generation IV reactorsstill under development with a proposedintroduction date of 2020. The main reactortypes currently operational are pressurizedwater reactors (PWRs) – 273 units, boiling-water reactors (BWRs) – 81 units, pressurizedheavy-water reactors (CANDU-PHWR) – 48units, gas-cooled reactors (AGR and Magnox)– 15 units, light-water graphite reactors(RBMK and EGP) –15 units, and fast neutronbreeder reactors (FBR) – 2 units (WNA, 2015).

The majority of reactors currently inoperation are relatively elderly in terms of theirdesign life and would require decommissioning

Friction processing as an alternativejoining technology for the nuclearindustry by D.G. Hattingh*‡, L. von Wielligh*, W. Thomas† and M.N. James‡

SynopsisThe process of joining materials by friction is based on generating the heatnecessary to create a solid-state mechanical bond between two fayingsurfaces to be joined. In simple terms, the components to be joined aresubjected to frictional heating between rubbing surfaces, causing anincrease in interface temperature and leading to localized softening ofinterface material, creating what is described as a ’third body’ plasticizedlayer. This plasticized zone reduces the energy input rate from frictionalheating and hence prevents macroscopic melting. The plasticized layer canno longer transmit sufficient stress as it effectively behaves as a lubricant(Boldyrev and Voinov, 1980; Godet, 1984; Singer, 1998; Suery, Blandin,and Dendievel, 1994). The potential for this solid-state frictional joiningprocess to create high-performance joints between, for example, dissimilarmaterials with limited detrimental metallurgical impact, and reduceddefect population and residual stress level, has had a very significantimpact on fabrication and repair in industrial sectors such as transport.This paper presents a brief overview of the advances made within thefamily of friction processing technologies that could potentially beexploited in the nuclear industry as alternative joining and repairtechniques to fusion welding.

Modern friction processing technologies can be placed into two maincategories: those that make use of a consumable tool to achieve theintended repair or joint (friction stud and friction hydro-pillar processing)and those making use of a non-consumable tool (friction stir welding).The most mature friction joining technology is friction rotary welding,where a joint is formed between original parent materials only. A newaddition in this category is linear friction welding, which opens thepotential for joining complex near-net-shape geometries by frictionheating. The continuous innovation in friction processing over the last 25years has led to the development of a number of unique processes andapplications, highlighting the adaptability of friction processes forspecialized applications for high-value engineering components.

Keywordsfriction processing, joining, leak sealing, weld repair technology.

* Nelson Mandela Metropolitan University.† Co-Tropic Ltd., TWI.‡ University of Plymouth.© The Southern African Institute of Mining and

Metallurgy, 2015. ISSN 2225-6253. Paper receivedAug. 2015.

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http://dx.doi.org/10.17159/2411-9717/2015/v115n10a2

Friction processing as an alternative joining technology for the nuclear industry

in the short term if life extension methodologies are notadopted. The high capital cost and extensive lead timeinvolved in bringing new nuclear plants online make thewhole idea of accurately determining the remaining life ofexisting plants very topical.

The graphs in Figure 1 and Figure 2 were populated fromdata published by the WNA (2015) in 2015 and give anindication of the current operational plants, plants underconstruction, and proposed plants. Considering Figure 1,which shows the percentage contribution from nuclearfission, over the decade 2003 to 2013, to the electric powerneeds of both the ‘BRICS’ nations and of developed countries,it is clear that most of these countries made very little capitalinvestment in additional nuclear generation capacity, withmost countries showing a moderate decline in nuclear contri-bution. With the exception of France, and to a lesser extentBrazil, most countries currently obtain less than 10% of theirelectricity needs from nuclear. The future for nuclear powerlooks much more promising when one considers the potentialadditional capacity from the current build programme, andthe planned and proposed additions to the nuclear fleet overthe next 20 years (Figure 2).

Therefore, taking an overview of the installed, planned,and proposed future nuclear capacity, and factoring indevelopments in respect of the safer, more standardized andcost-effective generation IV reactors, there is considerableopportunity to plan for the implementation of alternativejoining and repair technologies in the nuclear industry.

The basic principles of generating electricity from nuclearfission are very similar to those used in fossil-fired thermalpower plants, if viewed from the secondary circuit (steam

generation) point onwards. Steam generated by controllednuclear fission is used to drive turbines, which are intercon-nected with electrical generators. A number of the mainmaterials used in the construction of nuclear power plants,including Zircaloy, can be joined or repaired by friction-basedprocesses. The joining of thin-wall sections intended for usein fuel rod application by rotary friction processes is currentlybeing studied at the Nelson Mandela Metropolitan University(NMMU) Work is also being done on Cr-Mo-V alloy steelsand various grades of stainless steel.

Friction processing technologiesOne of the simplest ways to warm up your hands is to rubthem together. This action generates heat in proportion to therapidity of the rubbing motion, the pressure applied on therubbing surfaces, and the duration. Similar concepts are usedto provide the primary thermal energy source in frictionprocessing (FP), allowing lower temperature solid-state jointsand repair processes to be applied to a wide variety ofmaterials (particularly the magnesium and aluminium lightalloys) used in manufacturing industry (Thomas andDuncan, 2010). In summary, friction processing comprises aset of solid-state joining techniques that are carried out bygenerating heat at a contact interface. This softens thematerial and allows plastic flow to occur which, incombination with an applied pressure, can be used to createstrong, low-defect, and low tensile residual stress bonds(Maalekian , 2007; Gibson, 1997; ASM International, 1993;Serva-Tech Systems, n.d. (a); Thomas and Dolby, 2001).

There are various types of friction processes, asillustrated in Figure 3.

The main friction processing joining technologiesconsidered in this paper are rotary friction welding (RFW),friction stir welding (FSW), friction hydro-pillar processing(FHPP, which can employ either tapered or parallel sidedconfigurations), and linear friction welding (LFW). LFW is a

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Figure 2 – Nuclear power generation landscape expressed in MWe(WNA, 2015)

Figure 1 – Nuclear power share for the period 2003 to 2013 (WNA, 2015)

Figure 3 – Friction process technologies (TWI) (Thomas and Dolby,2002)

relatively new and versatile process that permits the joiningof irregular shapes, allowing for near-net-shape fabrication.In this range of friction technologies, there are autogenousprocesses that form a joint by using parent material only(RFW and LFW), i.e. without the addition of any filler metal,processes that make use of a consumable tool (FHPP), andprocesses that use non-consumable tools (FSW).

There are four basic classifications for relative motion infriction welding (BSI Group, 2000; Serva-Tech Systems, n.d.(b)), namely: rotary, angular oscillation, linear reciprocation,and orbital motion. Figure 4 illustrates these variousmotions.

Fabrication by frictional heating is a well-establishedtechnology. The first use of friction knowledge to process andshape materials dates back to 1891, when Bevington (1891)used heat generated by friction to form and join tubes.

Rotary friction welding (RFW) RFW is a well-established ‘workhorse’ friction joiningtechnology that has been widely used for over a century innumerous applications ranging across the automotive,construction, aerospace, and medical sectors. Two alternativeRFW techniques, known as continuous drive and inertiafriction welding, are currently in use; the latter is the mostwidely adopted process. In both techniques joints are madeby placing a rotating component in contact with a stationarycomponent while applying a load perpendicularly to thecontact interface. Once the interface reaches the appropriatewelding temperature to plastically displace and fuse thematerials by forging, rotation is stopped and the weld isallowed to solidify while maintaining the forging force(Thomas and Duncan, 2010; Thomas, Nicholas, and Kallee,2001) as illustrated in Figure 5. The differences between thetwo methods are mainly in process control, with continuous

drive welding being done with a constant rotational speedthat may be varied at different stages of the weld cycle, whileduring inertia welding the process begins at a relatively highrotation speed, which gradually reduces to zero (Thomas andDuncan, 2010).

Friction hydro-pillar processing (FHPP)Like most of the modern friction technologies, FHPP (Figure 6)was invented and patented by TWI (World Centre for MaterialsJoining Technology) in the UK in the early 1990s. Adescription of the process, as given by Thomas and Nicholas(1992), states that friction hydro-pillar bonding is achieved byrotating a consumable tool coaxially in a hole while under load.The frictional heat generated results in the production of apillar of continuous plasticized layers until the hole is filled.The plasticized material consists of a series of shear layers orinterfaces which solidifies under the influence of the appliedforce. One of the main original observations was that theplasticized material advances more rapidly than the axial feedrate of the consumable tool, which results in the rising of thefrictional interface along the consumable tool to form thedynamically recrystallized deposit material. During the processa good degree of plasticization must be maintained at theinterface, allowing hydrostatic forces to be transmitted axiallyand radially to the inside of the hole in order to achieve a goodmetallurgical bond (Thomas and Nicholas, 1992).

A large body of reported work from various laboratorieshas shown that good mechanical integrity can be achieved inFHPP welds, with metallographic examination indicating afine-grained microstructure in the welded zone (TWI Bulletin,1997). The original geometry proposed for the FHPP processwas based on a horizontal cylindrical arrangement, withspecific clearance between the tool and the sides of the hole.However, in more recent applications tapered holes andconsumables are becoming more common, in which the angleof the two tapers is slightly different (Thomas and Temple-Smith, 1996; Bulbring et al., 2012; Meyer, 2001).Wedderburn (2013) reported that the tapered arrangementallows for an additional reactive force to develop horizontallyfrom the sidewall in addition to the vertical hydrodynamicforce, which can be utilized for heat generation when makingthe joint, and that this is beneficial for materials with poorextrusion properties.


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Figure 5 – Schematic of the basic rotary friction welding process(Pinheiro, 2008)

Figure 4 – Relative motion classification in friction welding(Wedderburn, 2013)

Figure 6 – Schematic representation of the friction hydro-pillar process(Thomas, 1997)


A number of research institutes have claimed that goodquality FHPP welds can be made in steel and certain non-ferrous materials using a parallel hole geometry. In thiscontext a ‘good quality’ weld is characterized by high impact,tensile, and bend properties (TWI Bulletin, 1997). Thissentiment is supported by Wedderburn (2013) Bullbring, etal., (2012) who reported that good mechanical propertieswere achieved with a taper configuration on most thick-wallsteam pipe materials evaluated. Where welds were madeusing parallel tool and holes, periodic changes in themicrostructure were observed, where regions within thedeposit material were not fully transformed. This effect wasmore noticeable when a high tool rotation speed andconsumable displacement (upset) rate were used.Microstructural variations were also exacerbated when thehole depth to hole diameter exceeded a ratio of about 3.5:1(Meyer, 2003).

There is limited published work on the influence ofprocess parameters on FHPP welds. From the work by Meyer(2003), Wedderburn (2013), and Bulbring et al., (2012) onthe use of FHPP for high-strength low-alloy steel, it isapparent that hole geometry is more critical than tool shape.Meyer (2003) also noted that the influence on heatgeneration and bond quality is similar to that observed inconventional friction welding, although the rotational speedrequired for a good FHPP weld is significantly lower than inconventional friction welding. The forging force, which has amajor influence on weld properties in conventional frictionwelding, appears to have little or no influence on most FHPPwelds and its effect is limited to the upper area near thesurface region (Wedderburn, 2013; Bullbring, et al., 2012;Meyer, 2003).

Friction stir welding (FSW)FSW must be considered as one of the leading innovations insolid-state joining in the last 50 years. Invented and patentedby TWI (1991), FSW is a joining technique that allows bothlow and high melting point metals, including aluminium,lead, magnesium, steel, titanium, zirconium, and copper, tobe continuously welded with a non-consumable tool(Nicholas, 1998; Reynolds, Seidel, and Simonsen, 1999;Dawes, 2000). The wear rate and cost of refractory tooling isa limiting factor in welding steel alloys. One of the keybenefits of this solid-state friction welding technique is the

capability of welding dissimilar metals as well as joiningcertain metals that are difficult to weld by fusion processes,e.g. 2xxx series aluminium alloys. The process can best bedescribed as a continuous ‘hot-shear’ process involving anon-consumable tool that is responsible for mixing andforging the metals across the joint line (Delany et al., 2005).Selection of the tool geometry and material are importantconsiderations in achieving a sound joint. The mostimportant aspects of tool design are the pin and shouldergeometry, as these are responsible for forming the joint.

The basic principle of the FSW process involves plunginga rotating tool between abutting faces of the work piece andthen traversing it along the joint line (Figure 7). Rotarymotion of the tool generates frictional heat at the contactsurfaces, softening the material and creating a plasticisedzone around the tool pin and below the shoulder.

Apart from generating heat, the shoulder also containsthe plasticized material in the joint zone, which provides aforging action that assists with the formation of a defect-freesolid-state joint. In the nuclear industry, FSW has beenapplied to the manufacture of copper canisters for encapsu-lating nuclear waste (Andersson and Andrews, 1999) as wellas for leak sealing or material reprocessing techniques usedin the repair of surface-breaking or near-surface defects (VonWielligh, 2012).

Linear friction welding (LFW)(LFW is a joining technology that is ideal for welding non-symmetrical components. This solid-state joining processgenerates frictional heat through the linear forced interactionbetween a stationary surface and a reciprocating surface(Nentwig, 1995; Nicholas, 2003). The process requires alarge force applied perpendicular to the weld interface and, asthere is no containment of the hot plasticized material, acontinuous layer of flash is expelled during the rapid linearmovement. Since this softened ’third-body’ material is notcontained, surface oxides and other impurities are ejectedtogether with the plasticized ‘flash’ (Bhamji et al., 2011).

Unfortunately, LFW requires a major capital investmentdue to the complexity and size of the welding platforms. Thehigh process force must be contained within the platform

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Figure 8 – Schematic diagram of the linear friction welding process(Bhamji et al., 2011)

Figure 7 – Principle of friction stir welding (Thomas, Nicholas, andKallee, 2001)

structure, while achieving the required level of displacementand process control necessary to ensure joint integrity andgeometrical accuracy means that tight specifications are seton all system components and control algorithms. Hence LFWis considered feasible for the production of high-valueengineering components, e.g. joining of aero-enginecompressor blades to compressor disks (Wanjara and Jahazi,2005). LFW applications are predicated on the suitability ofthe process for joining materials with good high-temperatureproperties, low thermal conductivity, and acceptablecompressive yield and shear strength properties (Wanjaraand Jahazi, 2005). Low thermal conductivity helps confineheat to the interface region, while appropriate high-temperature properties (high yield strength and meltingpoint) allow a high level of frictional heat generation to occurbefore plastic collapse. Materials with good high-temperaturemechanical properties and low thermal conductivities, e.g.titanium, zirconium, and nickel alloys, are particularlysuitable for LFW applications.

Applications of friction technologies in the nuclearindustryA number of studies have shown that friction welding can beused to make sound joints and cost-effective repairs in thenuclear industry. These include the work done in a Swedishprogramme that evaluated FSW for encapsulating nuclear fuelwaste (Andersson and Andrews, 1999), while TWIdemonstrated that thermocouple probes can be fitted to boilerheader domes forged from 316 austenitic stainless steel forthe Hartlepool and Heysham nuclear power stations (TWIGlobal, 2014). Hy-Ten, an independent rebar and accessorysupplier, has gained approval for supplying friction weldedrebar couplers to be used in concrete construction by thenuclear industry (Maguire, 2012). Locally, the NMMU hasdeveloped two technologies with potential applications in thenuclear industry; one relating to leak sealing by FSW thatemploys a low-force partial-penetration methodology, and theother the registered Weldcore® process, which employs FHPPprinciples to core and repair sites used for creep assessmentas part of remaining life evaluation.

Nuclear waste storage encapsulation using FSWThis project formed part of a Swedish programme thatevaluated new ways of encapsulating nuclear fuel wastebefore storing it underground. The capsules weremanufactured from copper and consisted of a copper cylinderwith a base and lid (Andersson and Andrews, 1999). Thecapsule demand was estimated at 200 per year. Importantconsiderations in fabrication included inspection of the weldsand guarantees on achieving defect-free joints that wouldprevent water entering the canister during long-term storageunderground (Swedish Nuclear Fuel and Waste ManagementCo. (SKB), 2001). Typical canister dimensions were diameter1050 mm, length 4830 mm, and a total weight of 27 t. Tosatisfy the design requirements for environmental andchemical resistance, a copper alloy equivalent to EN 133/63with a 50 mm wall thickness was selected for the canisters.To improve creep resistance of the copper, 50 ppm ofphosphorus was specified in the alloy (Swedish Nuclear Fueland Waste Management Co. (2001). The project developed anelectron-beam welding solution for sealing the capsules and

also considered the use of FSW. The main consideration inselecting FSW as a potential alternative process was thesolid-state nature of the joining process, together with theability to make high-quality welds with a fully automatedplatform and standardized tool technology.

The investigation of FSW as an alternative joiningprocess started by considering the welding of 10 mm thickcopper plate strips about 0.5 m in length. This study wasused to find a suitable tool material as well as evaluatedifferent tool geometries. The only reference study at the timeinvolved FSW of thick-section aluminium plate with meltingtemperatures (659°C) well below that of copper (1083°C).However, copper tends to soften at lower a temperature,which allows the use of FSW on thick copper sections(Swedish Nuclear Fuel and Waste Management Co., 2001).This study led to the specification of process forces andpower requirements for welding 50 mm copper plate. Thiswas done by progressively increasing the plate thickness to50 mm while checking that the weld microstructure remainedacceptable for the identified application. The weld zoneshowed a fine equiaxed grain structure in the weld nuggetregion. Based on the success of these initial trials, SKBproceeded with the development of a FSW platform to weldlids to short (2 m) sections of 50 mm thick tubes. The tubewas placed in a vertical position with the welding head andtool in a horizontal position while the welding head rotatedaround the tube. For each weld, specific weld parameterswere recorded, providing a complete record of all the essentialvariables and their control during the welding process. Theseresults, in conjunction with non-destructive testing, form animportant resource for interpreting weld parameters and data.

Once adequate circumferential friction stir-welded jointswere achieved, attention was focused on eliminating the exithole left at the end of the weld run. This project has clearlydemonstrated the feasibility of fabricating capsules by FSW toenclose nuclear waste for long-term storage.

Leak sealing by FSWNuclear facilities situated close to the ocean could experiencematerial degradation, especially of stainless steel vessels andpipework, arising from environmentally-induced stresscorrosion cracking (SCC). This prompted an investigation intoidentifying a suitable refurbishment technique that wouldprovide a cost-effective and permanent leak-sealing/SCCrepair technique. Implementation of the repair process wasrequired to have no influence on the operation of the plant,and also to fulfil the stringent nuclear safety regulations. TheNMMU was therefore requested to investigate the feasibilityof using FSW as a potential leak-sealing technique for 304Lstainless steel. This study focused on establishing atechnology procedure for the identified application, whichentailed reviewing the process, tool material, and toolgeometry designs to develop a solution suitable for high-temperature FSW of unsupported, partial penetration leaksealing of 304L stainless steel with water backing.

The approach taken in the investigation was initially toestablish a theoretical processing parameter window frompublished literature. Subsequently, a suitable tool alloy andtool profile were identified before preliminary experimentalwork led to a low-force processing window for 304L stainlesssteel. Various experiments were carried out to determine theeffect of process parameters on weld quality and the


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processing forces required to achieve high quality. Thegoverning factor was found to be the downwards forgingforce required to achieve a fully consolidated, partialpenetration weld that would seal a leak in stainless steelsheet resulting from, for example, SCC. Finally, a completeweld procedure specification was proposed and evaluated bywelding on a mock-up of the real application with waterseeping through the simulated crack during the weldingprocess. In principle, this is a similar concept to FSW underwater, which has been reported by Ambroziak and Gul(2007).

Such applications require refractory tool materials, andW-25wt%Re was selected for the leak-sealing application asit provides good resistance under conditions of unevensurfaces, high vibratory loads, and plate deflection (VonWielligh, 2012). Additional benefits include the ability to re-profile the tool using standard machining processes, henceincreasing tool life. Process development focused onestablishing a low-force processing window for the 304L

stainless steel that minimized plate deflection during weldingin an unsupported condition. The investigation led to thedevelopment of a new dual-control tool plunge strategy, inwhich the operator controls the maximum downwards Z-forceduring the plunge stage while simultaneously controlling theplunge rate and depth (see Figure 9). Plunge rate is, however,also a function of the Z-force feedback, which could be seenas a significant advantage because the maximum platedeflection at the start of the weld can therefore be limited(Von Wielligh, 2012).

The minimum Z-force required for repeatable weldconsolidation using tools with a 14 mm, 12 mm, or 10 mmdiameter shoulder was found to be 14.25 kN, 10.5 kN, andand 9 kN respectively (Von Wielligh, 2012). This investi-gation showed that water cooling and water seepage throughthe simulated SCC had only a minor effect on weld qualityand that the dominant factor influencing weld quality wasplate deflection. A full preliminary welding procedure specifi-cation was developed for both the 14 mm and 12 mmshoulder diameter tools, but only the procedure for the 14mm tool was deemed robust enough for industrialapplication.

Un-etched images of typical weld cross-sections obtainedduring the repair trials of SCC in stainless steel are shown inFigure 10. This figure shows the defect population observedat the start, middle, and end positions in two welds, A and B,made using different weld parameters while water wasseeping through the SCC, using two different sets of processparameters.

On the etched specimen in the macrograph in Figure 11,the lighter etching material clearly shows a well-defined flowpattern in the weld cross-section. This material was found tobe tungsten, a by-product of tool wear that has diffused intothe weld. The darker line extending beyond the flaw intendedto simulate a stress corrosion crack at the bottom of weldcross-section is termed a ‘lazy S’ defect in FSW and is notpart of the simulated crack. It is generally ascribed to the

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Figure 10 – The defect population found in weld cross-sections along the weld length representing the start, middle, and end of welds for two differentwelding parameter sets (Von Wielligh, 2012)

Figure 9 – Difference in the Z-force feedback for different controlstrategies (Von Wielligh, 2012)

presence of oxidation and corrosion by-products of the wirecutting process, and serves as a good indication of howactual corrosion products are likely to be distributed acrossthe weld.

Vickers microhardness tests did not show any significantchange in the hardness of the weld area or heat-affected zonecompared with the parent material. This is to be expected, asaustenitic stainless steels are not hardenable by heattreatment (Von Wielligh, 2012).

The feasibility of applying this FSW process to crackrepair in industry was tested using a mock-up of theindustrial application of a tank containing a stress-corrosioncrack. A simulated crack was successfully welded in anunsupported 304L stainless steel plate surface in theannealed condition with water backing up to a depth of 2 mm.

The weld repair process required making a number ofoverlapping friction stir welds, with a step distance betweenthem equal to the pin diameter. The step direction wastowards the retreating side of the weld, because flashformation arising from plate deflection made it impractical tostep towards the advancing side.

The typical weld repair pattern is shown in Figure 13. Thefirst weld, W1, is carried out from left to right with aclockwise spindle rotation. The retreating side of the weld isthus located at the bottom. The second weld, W2, is carriedout from right to left with an anticlockwise spindle rotationsuch that the retreating side remains at the bottom of theweld. This process of alternating welds is continued until thecracked area has been covered.

Figure 13 shows an example of such a weld repair withfive overlapping welds, covering an area of 120 mm × 20mm. In this figure, all the loosely attached flash has beenremoved.

This investigation has shown that leak sealing of stress-corrosion cracks in an unsupported tank surface is possibleby using overlapping, partial penetration, friction stirwelding. The investigation also highlighted the fact that dueto material thinning during the FSW process, the total areathan can be repaired has to be limited, based on allowableplate thinning. A reduction in the original plate thicknessoccurs with each successive overlapping weld. The decreasein plate thickness is likely to be a function of plate thickness;in other words, dependent on constraint. Furthermore, FSWtool exit holes can be partially eliminated by using tools witha retractable pin. In summary, FSW is a potential repairtechnique for SCC, although for unsupported welding carefulconsideration must be given to the influence of platethickness, surface condition of the area to be processed andits size, and deflection during welding, as well as thepermanent deformation (thinning) induced by the process.FSW is also likely to alter the metallurgical and mechanicalproperties in the processed area, and this aspect also needsconsideration. Associated friction processes such as frictionsurface dressing or friction cladding could also be consideredfor such repair procedures if it is accepted that the mainpurpose of the process is not to recover structural integrity,but rather top-seal seepage of the liquid inside the tankthrough stress corrosion cracks. Friction cladding is anadditive process, which has the potential to reduce platedeflection and permanent deformation imposed by the frictionprocess. Figure 14 shows the heat generation in the toolwhile traversing along a repair zone with water leakingthrough the simulated crack. The installation of thedeveloped platform is shown in Figure 15.



Figure 12 – The welding pattern of five consecutive welds used tofriction stir process an area of 120 × 20 mm (Von Wielligh, 2012).(Welds W1, W3, and W5 were welded from left to right and welds W2and W4 from right to left)

Figure 11 – Macrograph of a typical friction stir processed SCC, etchedwith Marbles reagent [30]

Figure 14 – FSW in process as water leaks through the simulated crack(Von Wielligh, 2012)

Figure 13 – The visual appearance of a friction stir processed tanksurface area with the loosely connected flash removed on theadvancing side of the weld (Von Wielligh, 2012)


Weldcore® sample removal and repair technologyOngoing assessment of the remaining life of high-temperature and -pressure (HTP) components in nuclearpower stations is of paramount importance in ensuring theirsafe and cost-effective operation. Although failure can havesevere consequences, economic considerations are requiringoperators to extend the operating lives of plants currently inoperation. To ensure safe operation, more direct experimentalassessments have to be made of remaining life tocomplement and underpin prediction models. Neutronirradiation embrittlement is one of the concerns for thenuclear industry, as this could limit the service life of reactorpressure vessels. Improved understanding of the underlyingcauses of embrittlement and better calibration of predictionmodels will provide both regulators and power plantoperators with improved estimates of vessel remaining life(Odette and Lucas, 2001). The NMMU, in collaboration withEskom, has developed the patented Weldcore® process,which is a sampling and repair process that is currently usedin fossil fuel power stations to assist in sample removal forcreep damage assessment.

The process involves removal of a cylindrical

metallurgical sample and hole preparation for repair in foursteps, as illustrated in Figure 16. The hole is then repairedusing friction taper hydro-pillar processing (FTHPP) asillustrated in Figure 17.

Current Weldcore® applications are primarily in a coringand repair procedure for local metallurgical sample removalsites. The advantages of the process are that the metallurgicalcore can extend much closer to the centre of a thick-walledpressure pipe, and that the coring and repair can beaccomplished during plant operation. The core hole may beeither tapered or vertical-sided, depending upon the thicknessof the material to be welded and the depth of the hole. Fordeeper repair welds, a cylindrical configuration for the hole

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Figure 15 – Set-up for the repair process feasibility investigation, showing the FSW platform attached to a curved stainless steel water-filled test coupon(Von Wielligh, 2012)

Figure 16 – Illustration of four Weldcore® coring stages (Hattingh et al.,2012)

Figure 17 – Schematic of the FTHPP process with macro appearance ofa cross-section of a weld (Hattingh et al., 2012)

and rotating tool is recommended to ensure that the processforce and torque are limited. The consumable rotating tool istypically made of the same alloy as the base metal, althoughit is possible to achieve a matched, over-matched, or under-matched weld zone. On completion of the weld, the remainingportion of the consumable rotating tool can be cut off andground flush with the metal surface. Since the ’weld’ does notproduce a liquid weld puddle, the orientation of the weld isnot affected by gravity, hence making the process position-independent.

The main factors making the Weldcore® processattractive to the nuclear fraternity relate to platform size(which is compact and hence suitable for on-site work) anda high degree of process automation. The Weldcore®

platforms were initially developed for high-temperaturesteam pipes used in power stations. However, the success ofthe platform has led to further developments for applicationsthat include turbine disc and reducer sections.

Figure 18 shows the modular arrangement andadaptability of the welding platforms that were developed atthe NMMU for specific applications of the Weldcore® processin the power generation industry.

The in-situ application on steam pipes required thedesign and development of a portable reconfigurable platformthat allowed operation at various locations throughout apower station. The modular arrangement of the weldingplatform, involving a frame, spindle, and drive motor,facilitates ease of transport, positioning, and mounting.Electrical and hydraulic power and control connections areoperated via a remote control unit.

The process and portable platform allow for the in-situremoval of a representative sample of metal from a structure(pipes, turbine rotors, etc.) that can be used for accuratemetallurgical assessment of the remaining life of thestructure. It provides superior data to other current methodsas the excised core is a far more representative sample of thethrough-thickness than other techniques, with a shorter

acquisition downtime. The process has been tested on steampipes and turbine components in the power generationindustry and is in the final phases of being tested onstructures on a petrochemical plant. The main benefit of theWeldcore® process is a reduction in the risk of failure andhence better management of safe life in high-valueengineering components.

DiscussionThe introduction of a new or alternative joining and repairprocesses into the nuclear industry entails a number ofchallenges from the engineering and regulatory points ofview. Nuclear power, for obvious reasons, is a tightlyregulated industry, but the authors propose that byintroducing improved life assessment standardization, therisk and complexity of safely operating these types offacilities could be further reduced. Cost-effective conditionmonitoring is an implicit part of life extension for ageingplant, and of the through-life cost and performanceoptimization of new installations.

Friction processing presents a viable alternativemethodology for some aspects of condition monitoring andrepair of thermal power plant, with significant advantagesaccruing from its solid-state nature (which opens the door towelding of dissimilar metals). A solid-state joining processlimits microstructural phase transformations, while lowerprocess temperatures result in a significant reduction inthermal stresses and associated distortion compared withconventional fusion welding processes. The lower thermalgradient results in a narrow heat-affected zone with a fineequiaxed weld nugget and limited grain growth in thethermomechanically transformed region. Thesemicrostructures are normally associated with good strengthand toughness. As illustrated in the applications presented inthis paper, friction processing lends itself to a high degree ofautomation and process control, giving repeatable welds andallowing sophisticated monitoring of weld quality. Friction



Figure 18 – Developmental progression of Weldcore® equipment (Hattingh et al., 2012)


welding offers the power generation industry a clean,reconfigurable welding process suitable for complex andbespoke applications and which can greatly reduce processtime and cost of condition monitoring and repair.

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v115n10a2 Friction processing as an alternative joining ... · was invented and patented by TWI (World Centre for Materials Joining Technology) in the UK in the early 1990s. A description