A proposed diffusion-controlled wear mechanism of alloy steel frictionstir welding (FSW) tools used on an aluminum alloy

S.Yu. Tarasov a,b,n, V.E. Rubtsov b, E.A. Kolubaev a,b

a National Research Tomsk Polytechnic University, Russiab Institute of Strength Physics and Material Sciences SB RAS, Russia

a r t i c l e i n f o

Article history:Received 21 April 2014Received in revised form10 June 2014Accepted 13 June 2014Available online 23 June 2014

Keywords:Friction stir weldingWearSteelGrain boundary diffusionIntermetallic compoundEmbrittlement

a b s t r a c t

A study of diffusion wear mechanism in 1.2344 X40CrMoV5-1 steel FSW tool has been carried out fromthe standpoint of tribological layer generation and interaction with the tool's metal. It was shown thatduring FSW of AMg5M aluminum alloy the latter covers the tool's working surface and then iron/aluminum reaction diffusion is initiated under the conditions of high mechanical stress and temperature.Since diffusion by the former austenite grain boundaries is much faster than that of by volume anintermetallic compound is formed inside the tool's metal thus causing embrittlement and pulling out thetool's metal fragments.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

It could be found out from the relevant literature sourcesdevoted to studying the efficiency of different FSW tool shapesfor metal stirring in the weld that such an effect is to a great extentreduced by the aluminum alloy stuck to the FSW tool surface [1].The material flow in the weld under stirring has been a subject ofinvestigation in papers [2,3] and it was established that the FSWweld is formed by two interacting flows driven either by tool's pinor shoulder and therefore called “pin-driven” and “shoulder-driven” flows. The insufficient plasticizing may generate a flawin the zone where these two flows meet. Also, the so-called“onion-ring” structures are often found in the weld which arethe signs of insufficient intermixing of the pin-driven metal. Theeffect of FSW tool geometry in the form of pin/shoulder diameterratio was studied [4,5]. The conclusion to be made from thereviewed papers is that a steel FSW tool normally works beingcoated by a layer of aluminum alloy and this may be the reason forbad intermixing of pin-driven layers. Using the tribology approachand following [6] we may call this layer a tribological or mechani-cally mixed layer (MML) because similar layers are generated intribological experiments on ductile metals. It is interesting toanalyze the friction stir welding from the tribology standpoint

since structural changes induced by sliding in metals as well asMMLs are of very great interest in modern tribology.

These MMLs serve as a source of wear particles in sliding. Incase of FSW these wear particles detach from the from the tool'ssurface and then intermix with the weld metal thus serving asstress concentrators. It is known that MMLs are composed of veryfine grains and grain boundaries, therefore they can easily experi-ence transformations caused by diffusion in addition to the strain-induced structural ones.

Another aspect of the problem is that generation of thetribological layer on the tool's surface creates conditions fordiffusion of elements into the tool's metal and thus may causeits intense wear. This is especially acute problem in friction stirprocessing the high-melting point and composite materials [7,8].

However, wear of the steel FSW tools intended for aluminumalloys is not an unimportant problem since it interferes with thestirring efficiency and increases the probability of generatinga flaw.

The objective of this work is to reveal the structural degrada-tion factors and wear mechanism in FSW tools for weldingaluminum-magnesium alloy.

2. Materials and methods

Annealed aluminum alloy sheet 5 mm thickness sheets havebeen welded using a FSW machine developed and built at

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Wear

http://dx.doi.org/10.1016/j.wear.2014.06.0140043-1648/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author at: Institute of Strength Physics and Material Sciences SBRAS, Russia.

E-mail address: tsy@ispms.ru (S.Yu. Tarasov).

Wear 318 (2014) 130–134

Cheboksary plant “Sespel”. Chemical composition of the alloy isshown in Table 1, structurally it consists of aluminum/magnesiumsolid solution and inclusions basically being Mg2Si and Al6Mn. TheFSW tool had shoulder's diameter 19 mm and pin's diameter6 mm. Weld path length was 2000 m at 560 rpm, plunge force2600 kg, feed rate 500 mm/min. Chemical composition of the hotwork FSW tool steel is shown in Table 2.

SEM instrument Carl Zeiss EVO-50 attached with EDX equip-ment X-act (Oxford Instruments) has been used for detecting thechemical composition of the FSW tool and tribological layers. Themetallographic characterization has been carried out using opticalmicroscope on samples etched by a solution of 5 vol. % nitric acidin ethyl alcohol. Final polishing has been made with diamondpaste ASM 1/0 NOMG of grain size 0–1 μm.

3. Results

A macrophoto of the FSW tool working part shows that thetool's surface area is covered by a lustrous white tribologicaldeposits (Fig. 1). The peripheral spiral groove, which serves forforming a shoulder-driven metal flow [3] is almost fully filled by ametallic layer thus, breaking its proper interaction with the pin-driven flow.

It can be seen from optical image in Fig. 2 which is a crosssection area view of the worn FSW tool that these deposits are ofdifferent morphology and structure. Layers A and B look dark afteretching whereas layer C is bright. It is clear that all these depositshave been generated due to adhesion and mechanochemicaltransformation of severely deformed aluminum alloy on the steeltool's surface and therefore might be called tribological layers orMMLs. Layers A and C show a spiked pattern boundaries with thetool's metal which usually observable in case of diffusion of bothtypes such as volume and boundary (Fig. 3). Layer B looksinhomogeneous by its cross section and has flat type boundarywith the tool's metal. Layer D looks like severely deformed tool'smaterial with inner boundaries filled by some compound.

These layers differ from each other by their chemical composi-tion. The layer A chemical composition from EDX is shown in Fig. 4as distribution profiles of elements across its thickness and intothe tool's metal along the line shown in Fig. 3A. Provisionally threedifferent zones can be seen in Fig. 4 denoted as I, II and III.

Zone I is extremely inhomogeneous by its composition and infact composed of mechanochemically mixed layer where in addi-tion to the initial phases, both phase, structural and chemicaldeformation-induced transformations could occur. The opticalimages show that there are some light-gray zones which coincideby composition with the aluminum alloy (Fig. 4) and dark-grayinclusions enriched with magnesium, oxygen and silicon (Fig. 4).The distribution of aluminum in zone I is very inhomogeneous toobut its minimums correspond to maximums of magnesium, siliconand oxygen and vice versa.

Zone II demonstrates gradual increase in concentration of Featoms and simultaneous decrease in Al atomic concentrationFig. 4. This zone shows back scatter electron image contrast beingdifferent as compared to both steel and components and looks likea barrier layer on the surface of tool's material. Judging by theFe/Al at% ratio this layer corresponds to the reaction diffusion of Fein Al with the product FeAl3 intermetallic compound (IMC). TheIMC layer fully covers the steel surface and its spiked shapeindicates on the preferential diffusion by the former austenitegrain boundaries.

It is suggested that zone III corresponds to the tool's materialsubsurface which is enriched with carbon and depleted by irondue to diffusion of iron into aluminum and simultaneous forcingout the carbon atoms back to the tool by the migrating diffusionboundary (Figs. 3A and 4). However, the accuracy of the EDXcarbon detection is not high as well as the cross section area of thetool might be contaminated by extra carbon coming from adiamond paste used for polishing. In spite of being quite logicalthis suggestion has to be verified by further work.

Table 1Chemical composition of AMG5M alloy, wt%.

Al Mg Si Fe Zn Mn Ti Cu Be Impurities

91.9–94.68 4.8–5.8 o0.5 o0.5 o0.2 0.3–0.8 0.02–0.1 o0.1 0.0002–0.005 0.1

Table 2Chemical composition of FSW tool steel (1.2344 X40CrMoV5-1), wt%.

C Cr Mo V Mn Si P S Fe

0.35–0.42 4.8–5.5 1.20–1.50 0.85–1.15 0.25–0.5 0.8–1.2 0.03 0.02 Balance

Fig. 2. The cross section view of the FSW tool with different types of tribological layers.

Fig. 1. The FSW tool working part covered by tribological layer.

S.Yu. Tarasov et al. / Wear 318 (2014) 130–134 131

Similar inhomogeneous element distribution profiles areobserved in layer B (Fig. 3B) for aluminum, magnesium, siliconand oxygen (Fig. 5). We can see that the aluminum contentmaximums correspond to minimums of magnesium, silicon andoxygen within zone I. Also Zone III shows qualitatively theenrichment by carbon and depletion by iron. Zone II again consistsof thin continuous IMC layer formed only by volume diffusion.

Taking into account the above results we can conclude that zone Imaterial is composed of mechanochemically mixed componentsincluding particulates of aluminum alloy, magnesium silicide andoxides stuck to thin layer of FeAl3 intermetallics formed by reactiondiffusion of iron into aluminum during friction stir welding.

Fig. 6 shows the FSW tool particles detached from the basemetal and FeAl3 diffusion layer found both on the FSW tooland those particles. It is reasonable to suggest that there is a

Fig. 3. Different types of tribological layers on the FSW tool.

Fig. 4. The EDX profiles of elements in across the A layer.

Fig. 5. The EDX profiles of elements across the B layer.

S.Yu. Tarasov et al. / Wear 318 (2014) 130–134132

connection between the reaction grain boundary diffusion andpulling out the FSW tool fragments during the welding, i.e.diffusion wear. The rest of the C zone is the mechanochemicalmixture of aluminum, oxides and silicides.

Earlier stage of the diffusion wear can be observed in layer D(Fig. 7) where FeAl3 is formed hundreds of micrometers below thetool's surface by the grain boundary diffusion and thus cause thegrain boundary embrittlement with ensuing deformation and

detachment of the tool's fragments as shown in Fig. 6a. The voidsinside the intermetallic compound formations are formed accord-ing to the Kirkendall effect.

4. Discussion

During FSW the aluminum material is plasticized and heated.In addition to plasticizing, severe deformation in FSW generatesthe nanosized grains and all these factors serve to facilitateadhesion between the welded materials, reduce the efficiency ofstirring, produce the onion-like weld structures and increase theprobability of forming a defect.

From the tribological point of view a layer of material is deformedseverely by the tool and transferred into a nanosized grain state atelevated temperature. In so doing, there created all necessaryconditions for shear instability of a plasticized and nanocrystallinemetal layer [9] so that this layer may flow in quasi-viscous mannerbeing driven by the FSW pin over the underlying metal. In case ofstrong adhesion of this layer to the tool, there is poor intermixing inthe direction across the weld and this layer slides as a whole withoutbeing broken into fragments by the tool. Most part of sheardeformation in this case is concentrated at the interlayer boundary[10]. In our opinion this is the mechanism of so-called onion-likestructure generation in FSW.

Another aspect of this phenomenon is a mechanochemicalinteraction of severely deformed and plasticized metal both withenvironment and the tool material. The most intense oxidizing of atribological layer's metal on the surface of the tool may occur onwelding the sheets while this tool hangs in air between theoperations.

The rationale behind the FSW tool wear could be strongadhesion of plasticized and fine-grained aluminum alloy and thefollowing reaction diffusion of iron into aluminum. Let us dwell onthis suggestion.

The reaction-diffusion of iron into aluminum is a well-knownphenomenon that has been observed in many cases, for instance,when coating the iron-base materials by dipping them intoaluminum melt [11,12], in FSW of dissimilar materials such assteel and aluminum [13–15] or, for instance, in laser roll welding[16]. It is suggested in the latter paper that spike-shaped diffusionboundary is formed due to grain boundary diffusion and genera-tion of Kirkendall pores at these spikes. However, the volumediffusion is responsible for rounded shape of the spike tips (seeFig. 6, a). In our case, we can see that the reaction-diffusion is thepossible reason for FSW tool degradation by intergrain fracture,especially if taking into account the acceleration of diffusion bycontact stress and temperatures developed in the FSW [16].

The FSW tool wear by diffusion of elements from Ti-6Al-4Valloy into tungsten-rhenium FSW tool has been reported [17] tocause degradation and wear by subsurface fracture.

Also the FSW tool degradation mechanisms have been deter-mined for the FSW tools made of different materials such asА—99% W—1% La2O3, B—75% W—25% Re, C—70%W—20% Re—10%HfC in welding of a high speed steel [18]. Materials A, B, Cdegraded by plastic deformation and recrystallization, by twinningand fracture and by inter-crystallite fracture, respectively. Alsoadhesive wear was the case for all of them.

Intermetallic compounds FeAl3 and Fe2Al5 are rather brittlematerials and possess low strength which become even lowerwith temperature. In other words and without getting into detailsof diffusion front instability, we can say that the above citedpapers support our suggestion that the reaction-diffusion of ironinto aluminum in FSW may be the reason for wear by grainboundary embrittlement.

Fig. 6. The SEM image (a) and EDX composition (b) of intermetallic layer in the Clayer.

Fig. 7. The SEM image of the D layer with grain boundary diffusion and Kirkendallvoids shown by arrows.

S.Yu. Tarasov et al. / Wear 318 (2014) 130–134 133

5. Conclusion

1. During the FSW of AMg5M aluminum alloy the 1.2344X40CrMoV5-1 steel FSW tool is covered by tribologicalmechanically mixed layers. These layers are very inhomoge-neous by composition and found to consist of mixture ofaluminum alloy particles, oxides and silicides. In the vicinityof the tool's surface there is a layer of FeAl3 intermetalliccompound of both continuous and spiked morphologies.

2. Intensive volume and grain boundary diffusion of iron intoaluminum alloy tribological layers could be the reason forformation of brittle FeAl3 intermetallic compound along theformer austenite grain boundaries in the tool.

3. It is suggested that the fragments of the FSW tool material aredeformed and detached from the FSW tool by fracturing alongthe embrittled grain boundaries under the shear stress devel-oped on the surface of the tool during FSW.

Acknowledgment

This work has been carried out under Ministry of Education andScience of the Russian Federation Agreement no. 02.G25.31.0063made in accordance to RF Government Resolution No. 218 and withfinancial support of Program for Basic Scientific Research of the StateAcademy of Science on 2013-2020, RFBR Grants, Nos. 13-08-98088and 13-08-00324, respectively.

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