EFFECT OF NANOMATERIAL ADDITION USING GMAW AND … · 2016. 7. 22. · 372 C. Vimalraj, P. Kah, B. Mvola and J. Martikainen elements was due to the TiO 2 nanoparticles that aided

370 C. Vimalraj, P. Kah, B. Mvola and J. Martikainen

© 2016 Advanced Study Center C]. Ltd.

Rev. Adv. Mater. Sci. 44 (2016) 370-382

Corresponding author: Paul Kah, e-mail: [email protected]

EFFECT OF NANOMATERIAL ADDITION USING GMAWAND GTAW PROCESSES

Cyril Vimalraj, Paul Kah, Belinga Mvola and Jukka Martikainen

Laboratory of Welding Technology, Lappeenranta University of Technology, P.O. Box 20, FI 53851,Lappeenranta, Finland

Received: July 10, 2015

Abstract. The need for industries to remain competitive in the welding business has created anecessity t] devel]p inn]vative pr]cesses that can exceed cust]mers’ demand. Significantimprovements in weld efficiency during the past decades still have their drawbacks, specificallyin weld strength properties. Recent innovative technologies have created the smallest possiblesolid material, known as nanomaterial, and its introduction in welding production has improvedthe weld strength properties and overcome unstable microstructures in the weld. This studycritically reviews the methods of introducing nanomaterial to the weldments and the characteris-tics of the welds produced by GMA (gas metal arc) and GTA (gas tungsten arc) welding pro-cesses. The study mainly focuses on changes in the microstructural formation and strengthproperties on the welded joint and also discusses the factors influencing such improvementsdue to the addition of nanomaterials. The literature review shows that the effect of nanomaterialaddition in the welding process modifies the physics of the joint region, thereby resulting insignificant improvement in the strength properties, with a stable microstructure in the weld. Ingeneral, the factors that have a major influence on joint strength are the dispersion, characteris-tics, quantity and selection of nanomaterials. The nanomaterials addition does not affect thefundamental properties and characteristics of the base metals and the filler metal. However, insome cases, the addition of nanomaterials leads to the deterioration of the joint properties byunstable microstructural formations. Research is still being conducted to achieve high weldproperties in various materials through different welding processes and on other factors thatinfluence joint strength.

1. INTRODUCTION

In the manufacturing field, various processes (suchas machining, drilling, riveting, forming, mouldingand welding) have been intensively developed togenerate quality products economically. Welding isone of the most important processes in manufac-turing, and welding processes have been signifi-cantly improved. For example, the construction ofcomplex structures requires joining performed bywelding. The strength of the whole complex struc-ture is determined by the strength of the joined re-gion. The welding process, however, reduces the

material properties while joining. As the joining oftwo materials involves some complex issues andproblems, various studies have been carried out toimprove the weld joint and reduce problems in join-ing materials. Recently, nanomaterials have beenintroduced in the welding process to improve thejoint properties. The addition of nanomaterials hasnot only fulfilled the demands set on welding by theindustries, but also revolutionised the materials.

Due to the nanomaterial size factor, however,there are issues to consider when introducingnanomaterial into welding [1]. Problems have beensolved by manufacturing welding electrodes and filler

371Effect of nanomaterial addition using GMAW and GTAW processes

metal, as well as coating on base metals with addednanomaterials (i.e. nanopowders and nanowires).The usage of nanomaterial-added materials improvesthe weld properties and reduces defect formations[1]. For instance, the welding of aluminium and itsalloys by the TIG process with multi-walled carbonnanotubes (MWCNTs) added in the filler materialproduced a joint with better mechanical propertiesand reduced defect formations compared to a weldformed with no added MWCNTs in the filler material[2]. Moreover, several studies show that addingnanomaterials in the filler metal improves the jointstrength of welding electrodes [3,4]. Therefore, bet-ter weld properties have been established by theaddition of nanoparticles to joints [5].

There are various nanomaterial classifications,but the mostly required nanomaterials in weldingproduction are nanoparticles and nano-carbon-aceous elements [6]. Metal oxide, ceramic carbides,rare element oxide and silver are some of thenanoparticles used in welding production.

Addition of nanomaterials to the joining materi-als has created a need to understand the effect ofnanomaterials on the weldments. This study reviewsthe literature about the ways of introducingnanomaterials to the joint in GMAW and GTAW pro-cesses, as well as the nanomaterial effect on theweld microstructural change, weld properties andweld appearance. Moreover, this study focuses onthe factors that lead to changes in the weld micro-structure, such as grain refinement, phase changein the microstructure, precipitation at the grainboundaries, as well as weld strength properties,such as improvement in hardness, tensile strength,impact strength, corrosion resistance strength andwear resistance. However, other effects and factorsin the weld metal formed with nanomaterial addedare yet to be examined.

2. NANOMATERIALS USED ASCOATING IN WELDINGELECTRODE IN GMAW PROCESS

Because of the increasing demand of high-strengthmetals in various applications, their joining requiresextraordinary material insertion to improve their char-acteristics. For welding sophisticated metals,nanomaterials improve the properties of the weldmetal [7]. As the GMAW process is the most com-monly used welding method in the industries andmanufacturing sites, the introduction ofnanomaterials into the welding process has beenrealised with welding electrodes. The nanomaterialshave been mixed in the flux of the welding elec-

trode, in which the core wire of the electrode has acoating with a nanomaterial-mixed flux. Thenanoparticle-coated electrodes are employed inwelding high-strength structural steels and otherspecial metals to achieve weld deposits with greaterstrength and better properties. For instance, tough-ness properties are a major issue with high-strengthstructural steels; improving this property in the struc-tural steel requires the microstructural control andnucleation of grain boundaries [8,9], which is pos-sible through the addition of nanoparticles into thejoint.

A previous study stated that the GMAW processwith titanium microparticles added in the weldingelectrodes exhibited weld metal formation with bet-ter properties. Grain refinement was attained by re-duced particle size due to the increased pinningeffect by the microparticles added [10]. The reasonfor selecting Ti particles instead of other elementsis because of their nucleation characteristics, grainrefinement, high melting point and stability, as wellas increases in the recovery element during solidifi-cation [11-13]. However, titanium oxide formation oraddition has a negative aspect, since non-metallicaddition in the weld metal reduces the corrosionresistance by pitting action [14]. The nanoparticlecoating in the welding electrode has various advan-tages over conventional electrodes, for example,stable transfer in the molten droplet consisting ofnucleation elements (i.e. Ti) [15,16]. This nucleationelement assists in the grain refinement and nucle-ation of the microstructure leading to property im-provement. As the nanoparticle coating on the elec-trode has been used for the stabilisation of moltendroplets, the addition of nanoparticles would pro-vide improvement in the properties of the weld metal.

3. EFFECT OF NANOPARTICLE-COATED ELECTRODE ON WELDMETAL CHEMICAL COMPOSITION

Weld metal composition is one of the factors thatdetermine the weld microstructure and propertiesby the alloying elements present in the weld. More-over, the weld composition is homogeneous by thedilution of the base metal; however, filler metal addi-tion modifies the chemical composition. For in-stance, carbon steels welded by the E11018M elec-trode coated with TiO

2 nanoparticles had a different

chemical composition compared to the weld metalformed without nanoparticles added in the electrode.Elements such as Mn, Ni and Mo have been recov-ered in the weld metal produced by the electrodecoated with TiO

2 nanoparticles. The presence of the


elements was due to the TiO2 nanoparticles that

aided in the reaction of the slag metal due to thesize of the particles. However, the Cr element in theweld composition formed by the electrodes was re-duced, which was due to the Ti reducing corrosionresistance [15].

The slag metal reaction due to the generation ofmore oxygen molecules (O

2) by the detachment of

the Ti elements from the TiO2 nanoparticles also

resulted in the disappearance of elements [17]. Thisoccurred on the weldments of low-alloyed steels bythe increased O

2 content resulting in a faster degra-

dation of the Mn, C, P, S and Si contents, due tooxide formations [18]. The degradation of elementsin the weld metal was also noted in the experimentby Fattahi et al. [19] on welding low-carbon steelsby multi-passing with the TiO

2 nanoparticle-coated

electrode. The weld metal formed showed a de-crease in the composition of elements, such as Si,Mn, and C, compared to the weld metal withoutnanoparticles. However, instead of forming Mn metaloxide, the Mn was combined with Ti forming com-plex inclusions, which had a significant effect onthe microstructure. The complex inclusion in theweld metal exhibited a composition of titanium ox-ide as the black core and manganese oxide as theouter layer around the core, shown in Fig. 1. Thiscomplex inclusion had a positive effect on the nucle-ation site for redistribution of elements [16].

This complex inclusion forms with high oxygencontent; however, in the case of low oxygen con-tent, Chen et al. [16] showed that welding low-al-loyed steel while melting formed a stable Ti

3O

5

phase, rather than a TiO2 phase. The TiO

2 phase

only forms with very high oxygen content, which isabove 0.0077% [20].

Hence, observations show that elements havebeen recovered in the steel weld metal bynanoparticle addition through slag metal reaction,depending on the oxygen content. However, slagmetal reaction also resulted in the degradation ofelement composition in the weld and complex in-clusions in welds. As the TiO

2 nanoparticles gener-

ate oxygen content in the weld pool through thedetachment of Ti, the amount of nanoparticles addedplays a vital role in the chemical composition of theweld metal.

4. EFFECT OF NANOPARTICLE-COATED ELECTRODE ON WELDMICROSTRUCTURE

In the steel microstructure, the ferrite microstruc-tural phase has a beneficial effect, since it improvesthe properties and stability of the steel weld joint.Achieving acicular ferrite microstructural phasesrequires the dispersion of nucleation sites that con-secutively depends on the thermodynamic proper-ties of liquid metal [21]. Moreover, there are otherfactors inducing the formation of acicular ferrite, suchas the grain size of austenite structure, alloying el-ements and the cooling rate of the weld metal [22-24]. Early research showed that without active in-clusion in the weld, the occurrence of acicular fer-rite formation is hardly possible [25]. Ti is one of theactivating elements in weld metal for the formationof the acicular ferrite phase compared to other me-tallic inclusions. Moreover, Ti aids in grain refine-ment for better weld metal formations [26,27].

As studies showed, the alloying element andgrain size influence acicular ferrite formation, andthe quantity of nanoparticles added in the weldingelectrode varies the alloying element and has a greatimpact in the microstructure of acicular phase for-mation. Fattahi et al. [18] showed that welding low-carbon steel with increasing the TiO

2 nanoparticle

content in the electrode decreased the ferrite grainboundary and improved the acicular andWidmanstatten ferrite contents. A low amount of TiO

2

nanoparticles (i.e. 0.006 wt.%) in the weld exhib-ited ferrite grain boundaries, a high amount ofWidmanstatten ferrite and acicular ferrite comparedto the weld metal without nanoparticles. The highWidmanstatten ferrite content was due to the cool-ing rate variation and low amount of complex inclu-sions [28,29]. A medium content of TiO

2

nanoparticles (i.e. 0.023 wt.%) in the weld resultedin higher acicular ferrite content and a small amountof ferrite grain boundaries. However, a high contentof TiO

2 nanoparticles (i.e. 0.027 wt.%) in the weld

Fig. 1. Mn complex inclusion in the weld metal oflow alloy steel using TiO

2 nanoparticles. Reprinted

with permission from C. Chen, H. Xue, H. Peng,L. Yan, L. Zhi and S. Wang // Journal ofNanomaterials 2014 (2014) 1. (c) 2014 DeepDyve,Inc.


exhibited more Widmanstatten ferrite and ferrite grainboundaries with an increase in grain growth com-pared to the weld without nanoparticles. This graingrowth resulted in coarse grains, which was due tothe increase in the reheated zone by arc constric-tion [18].

The phenomenon of increased acicular ferritecontent after a nominal amount of nanoparticlesadded has also been noted in the welding of carbonsteel with multi-pass on the weld metal by an elec-trode coated with TiO

2 nanoparticles [10,19]. Due

to the multiple passes, the weld metal showed acolumnar grain zone and reheated grain zone. Thereheated grain zone exhibited both coarse grainsand fine grains that had no effect until with variationin the TiO

2 nanoparticles in the welding electrode

[10,19], illustrated in Fig. 2. Fig. 2 shows that themicrostructure formation by the medium content ofTiO

2 nanoparticles in the weld metal produced more

fine grain structures compared to the low and highTiO

2 content nanoparticle weld metals. The main

reason for the fine grains is the oxide particle refine-ment, which leads to a very fine intergranular ferritegrains by nanoparticle addition [18].

The studies show variation in the grain size be-tween the steel weld metals formed with varyingcontents of TiO

2 nanoparticles in the electrodes,

which is shown in Fig. 2. The variation in grain sizewas due to the low quantity of complex inclusions,weld metal cooling rate, increase in nucleation sitesand improvement in arc stability by the TiO

2

nanoparticles added. Moreover, multiple passes inthe weld produced significant variation in the grainsize.

To identify the effects and factors of complexinclusion in the weld microstructure, Pal and Maity[15] welded carbon steel by TiO

2 nanoparticles

added in the welding electrode resulting in complexinclusion formation while solidification. This com-plex inclusion in the weld led to fine grains and more

Fig. 2. Weld beads microstructural formation of low carbon steels with (a) low, (b) medium, and (c) highTiO

2 content nanoparticles . Reprinted with permission from M. Fattahi, N. Nabhani, M. R. Vaezi and

E. Rahimi // Materials Science and Engineering: A 528 (2011) 27. (c) 2011 Elsevier B.V.

acicular ferrite structures compared to the weld metalformed without TiO

2 nanoparticles. Fine grains with

a significant content of acicular ferrite structures havebeen supported by a turbulent flow in the weld pool.Chen et al. [16] also found similar acicular ferriteformation when welding low-alloy steel with an elec-trode coated with TiO

2 nanoparticles. Moreover,

Chen et al. [16] investigated the complex inclusionf]rmati]ns and sh]wed that the particles ]f the Mn–Ti–O inclusi]ns in the weld metal led t] the refine-ment of the microstructure by detaining columnargrain growth. The complex inclusion exhibited theformation of Ti

2O

3, Ti

3O

5, and MnTi

2O

5 compounds

that aided in grain refinement. During the solidifica-tion of the weldment, the MnTi

2O

5 content increased

leading to a manganese-depleted zone in the weldpromoting nucleation and intergranular formation [26].Therefore, the complex inclusions formed with aturbulent flow by added nanoparticles improved ac-icular ferrite content, refinement of grains and inter-granular nucleation, which resulted in better proper-ties.

Apart from the complex inclusion and turbulentflow, the recovered element Mn precipitated on thegrain boundaries weakening the nucleation of bainiteformation, thereby resulting in the formation of ac-icular ferrite. This occurred only with hardened ele-ments (i.e. Vanadium, Ti, and Nitrogen) while cool-ing [30].

Hence, the studies clearly show that nanoparticleaddition through electrode coating in the GMAWprocess improve stable microstructural formationsin steel weld metal. Moreover, the nanoparticles aidin the recovery of elements, thereby forming com-plex inclusions, as well as increase the solidifica-tion time by the compounds. This nanoparticle ad-dition assisted grain refinement and acicular ferriteformation with the significant effect by the quantityof nanoparticles added.


5. EFFECT OF NANOPARTICLE-COATED ELECTRODE ON THEWELD METAL MECHANICALPROPERTIES

Previous sections showed that nanomaterial insertedinto the weld through a welding electrode resultedin better microstructural formation and refinementof grains, which could lead to improved weld proper-ties. Studies show that the refinement of grains couldincrease the hardness and tensile strength of thejoint [31,32]. Fine-grain formation throughnanoparticles could provide higher strength com-pared to the welded joints with no addednanoparticles.

5.1. Hardness

Hardness property is an essential characteristic inthe joint strength. If the hardness value in the weldregion increases above the nominal level correspond-ing to the base metal, the weld impact strength isreduced and leads to the occurrence of brittle frac-ture or crack propagation. However, if the hardnessvalue of the weld increases uniformly correspond-ing to the level of the base metal, the joint exhibitsbetter integrity and performance. Research showedthat the weld metal with increasing nanoparticleaddition by coating on the base metal provided es-calation in the hardness values [3,33]. This hard-ness improvement was also observed in the experi-ment by Fattahi et al. [19] on welding low-carbonsteel with the TiO

2 nano-coated electrode, which

was due to the grain refinement in the microstruc-

Fig. 3. Hardness in the low carbon weld metal with varying content of TiO2 nanoparticles. Replotted from M.

Fattahi, N. Nabhani, M. G. Hosseini, N. Arabian and E. Rahimi // Micron 45 (2013). (c) 2013 Elsevier B.V.

ture. Moreover, the hardness value increased by in-creasing the amount of TiO

2 nanoparticles in the

electrode, shown in Fig. 3. This hardness incrementwas due to the increase of acicular ferrite content inmicrostructural formation.

In some cases, welds with increasing contentsof nanoparticles gradually increased their hardnessvalue, but, after attaining a sustainable value, thehardness value started to decrease even though fur-ther increasing the amount of nanoparticles addedinto the weld. This effect was observed in weldinglow-carbon steel with TiO

2 nanoparticles showing

that the hardness increased with an increasing TiO2

nanoparticle content in the electrode coating anddecreased gradually. In addition, the hardness valuedecreased gradually from the columnar zone to thereheated zone, which may be due to the change inthe microstructure and the grain size. However, theweld formed with a medium content of TiO

2

nanoparticles exhibited the highest hardness com-pared to the other weld metals formed with low andhigh contents of TiO

2 nanoparticles [18]. The simi-

lar phenomenon was also observed in the experi-ment by Chen et al. [17] on low-carbon steel withnano-marble addition (i.e. CaCO

3) in the electrode.

This decrease in hardness in both studies could bedue to the size factor. The increment in the hard-ness value was due to the significant formation ofacicular ferrite in the steel weld metal [18].

Thus, studies show that the hardness of the weldformed with added nanoparticles through the elec-trode coating increases with the nanoparticle con-tent. This hardness increase is due to acicular fer-


rite in the microstructure. However, studies alsoshow that increasing the amount of nanoparticlesadded increases hardness to a sustainable level,after which it decreases gradually, which is due tothe nanomaterial size factor.

5.2. Tensile and impact properties

A previous study showed that TiO2 nanoparticles

added as an alloying element improved the tensilestrength in C–Mn steels [34]. The weld f]rmed withnanoparticles added through an electrode coatingalso improved the yield strength by the refinementof grains. Research on welding carbon steel withadded TiO

2 nanoparticles in the electrode showed

that the weld composition exhibited the recovery ofelements, which aided in the lowering of tempera-ture, resulting in grain refinement and improving ten-sile strength [15]. To identify the factors that im-proved the tensile strength, the experiment by Fattahiet al. [19] on welding low-carbon steel with a TiO

2

nanoparticle-coated electrode obtained welds withimproved tensile and yield strength by an increasedTiO

2 content in the electrode. The study concluded

that the improvement in the strength mainly resultedfrom the improved grain size due to the refinementand increased acicular ferrite content in the micro-structure.

The study by Fattahi et al. [18] showed that withthe medium content of TiO

2 nanoparticles in the elec-

trode exhibits more yield and ultimate tensilestrength compared to other weld metals formed withlow and high TiO

2 nanoparticle contents, shown in

Fig. 4. Tensile properties formed in the low carbon steel weld metals by varying TiO2 nanoparticles content

in electrode. Replotted from M. Fattahi, N. Nabhani, M. R. Vaezi and E. Rahimi // Materials Science andEngineering: A 528 (2011) 27. (c) 2011 Elsevier B.V.

Fig. 4. Earlier studies proved that the reduction ingrain size by grain refinement would assist in in-creasing the tensile strength [35]. Therefore, theincreased tensile strength was due to the reducedgrain size, a greater amount of acicular ferrite andcolumnar zone reduction in the weld [18]. The de-creased tensile properties in the weld metal formedby a high content of TiO

2 nanoparticles in the elec-

trode were due to the increased oxygen content,increased allotriomorphic ferrite grain boundary anddecreased columnar zone grain size.

Apart from the tensile strength, the impact tough-ness also increased with the content of nanoparticlesin the weld metal. This was stated in the study byFattahi et al. [19] showing that an improvement inthe impact strength increased with increased ac-icular ferrite content and grain refinement in the weldmetal. Previous research showed a relation betweentoughness and the dimple size, where the tough-ness improved with the decrease in the dimple sizes[36]. Moreover, the experiment by Pal and Maity[15] also agreed on the statement that toughnessincreased with an increased nanoparticle content.However, the reason for the improvement in the im-pact toughness was not only grain refinement, butalso the oxygen and nitrogen level which was alsoone reason that was manipulated by the Ti content.

However, previous studies showed that a highoxygen content in the weld metal negatively influ-ences the properties of the joint by reducing thetoughness of the joint [37,38]. The experiment byFattahi et al. [18] showed that the medium amountof TiO

2 nanoparticles in the electrode had higher


Fig. 5. Impact strength in the low carbon steel weld formed with varying content of TiO2 nanoparticles

electrode. Replotted from M. Fattahi, N. Nabhani, M. R. Vaezi and E. Rahimi // Materials Science andEngineering: A 528 (2011) 27. (c) 2011 Elsevier B.V.

toughness in the weld metal at -30 °C than the weldmetals formed by low and high TiO

2 nanoparticle

amounts in the electrode, which is shown in Fig. 5.So, the joint welded with the high TiO

2 nanoparticle

content in the electrode had an increase in oxygenwith an increase in the allotriomorphic ferrite con-tent, which resulted in decreased impact strength.The low TiO

2 nanoparticle content exhibited a high

Widmanstatten content in the weld, which deterio-rated the impact strength of the joint. Research re-ported that cracks can be produced and propagatedin the Widmanstatten ferrite, which is unfavourablefor impact strength [32]. Hence, the reduction inthe toughness strength in the weld metal was dueto the increase in the oxygen content, variation ingrain size, the amount of acicular ferrite, cleavagesin the microstructures and inclusion characteristics[18].

Therefore, the nanoparticles added through elec-trode coating improved the tensile and yield strength.Increasing the nanoparticle content increased thetensile and yield strength by grain refinement in theweld. However, a significant increase in thenanoparticle content resulted in the reduction of ten-sile strength due to the oxygen content and colum-nar grain in the weld. In addition, the impact strengthalso decreased in the weld by the effect of oxygenwith a significant increase in the nanoparticle con-tent. However, a nominal level of nanoparticles inthe weld increased the impact strength.

5.3. Corrosion resistance

The corrosion resistance property is an importantcharacteristic for welds, used in corrosive environ-

ments. It has been shown that the weld formed bynanoparticles added through coating on the basemetal improved the properties. For the case of elec-trode coating, the experiment by Fattahi et al. [10]on welding low-carbon steel with the addition of TiO

2

nanoparticles in the electrode decreased corrosionresistance at the grain boundaries and around theTi-based inclusions of the weld metal. This was at-tributed to the non-metallic inclusions into the weldmetal that act as a pitting agent for corrosion [39].The difference in the alloying element concentra-tion at the grain boundaries results in more corro-sion at the grain boundaries in the weld metal. Inaddition, increasing the TiO

2 nanoparticles in the

weld metal leads to an increase in the formation ofa manganese-depleted zone around the Ti-basedinclusion. This can result in the destruction of car-bon steels due to reduced corrosion resistance alongthe manganese-depleted zone, when the weld issubjected to environments containing Cl [40]. How-ever, the weld metal without added Ti nanoparticlesshowed the lowest corrosion rate [10]. Thus, TiO

2

addition is not suitable for improving the corrosionresistance property in low-carbon weld metal.

5.4. Wear resistance

Wear resistance also plays an important role in theintegrity of the joint, if the joint is subjected to afriction environment. Welding low-carbon steel withthe addition of nano-marble (i.e. CaCO

3) in the weld-

ing electrode showed an increase in the wear resis-tance with the increase of the nano-marble contentin the electrode, which was estimated by the weightloss, as shown in Table 1. However, the weld formed


with a nominal level of nano-marble exhibited higherwear resistance compared to the weld formed withthe highest content of nano-marble [17]. The rea-sons for the improvement in wear resistance areyet to be examined.

Thus, nanomaterial in the welding electrode coat-ing showed improvement in the properties. More-over, wear resistance reduced the weight lossthrough the increasing content of nanomaterial inthe welding electrode.

6. NANOMATERIALS USED ASFILLER MATERIALS IN GTAWPROCESS

The chemical composition of the filler metal is oneof the determining factors for the formation of themicrostructure and properties in the weld. The se-lection of filler metal is mostly done based on thecomposition of the base metals [31]. Studies haveshown that the small particles used in the filler elec-trode improve the weld strength. The uniform distri-bution of particles is also required in filler metal fab-rication. The filler material can be in various forms,such as powders, layers and wires. However, theselection of a filler should be adaptable to the weld-ing process. For the non-consumable electrode inarc welding processes (i.e. GTAW), the usage ofappropriate filler metal in the joint improves the mi-crostructure as well as the joint properties. As stud-ies maintained that the usage of small particles havean immense effect, the nanomaterials were addedas an alloying element when manufacturing fillermaterials. The filler metals fabricated withnanomaterials had a uniform dispersion within thefiller metal and to the weld formation, which alsoprovided improvement in the properties [41]. Theuniform dispersion ensured uniform cooling and mi-crostructural formations in the weld metal, which in

Table 1. Abrasion wear loss varies by the nano-marble content in the low carbon steel weld metal . Basedon data from B. Chen, F. Han, Y. Huang, K. Lu, Y. Liu and L. Li // Welding Journal 88 (2009) 5. (c) 2009American Welding Society.

Electrode with CaCO3 nano-marbles addition (%) Abrasion weight loss, (g)

in flux welded in low carbon steel

Electr]de 1 – 0% - Weld metal 1 0.021Electr]de 2 – 10% - Weld metal 2 0.018Electr]de 3 – 20% - Weld metal 3 0.008Electr]de 4 – 25% - Weld metal 4 0.005Electr]de 5 – 50% - Weld metal 5 0.011Electr]de 6 – 100% - Weld metal 6 0.016

turn would form better properties in the weld metal[42]. Some of the nanoparticles added in the fillermetals were metal oxides, noble metals and car-bide particles.

Research showed that the carbonaceous ele-ment used as a reinforcing element in Al alloys ex-hibited a remarkable improvement in the propertiesand the refinement of grains [43]. This carbonaceouselement could be used in filler metal for welding Almetals to improve the properties of the joint. Thecarbonaceous nanomaterials such as GNS andCNT, improve the properties by manipulating thegrain growth. The GNS is the structural basic ele-ment for the CNT. Hence, if the GNS inclusion im-proves the properties of the Al metal, then CNT wouldbe showing a better improvement of the properties.

Recently introduced in welding, composite fillerwires containing nanoparticles significantly improvethe properties of the joint. The composite filler met-als also reduce defect formations. Moreover, thecomposite filler metals formed with small-sized par-ticles (such as microparticles and nanoparticles)improve the properties of the joint [44]. The follow-ing section describes the effect of nanomaterialadded fillers in the GTAW welding process.

6.1. Effect of filler metal withnanomaterial on defects

Research has shown that the nanoparticles includedwhile alloying improves microstructural formation andreduces defects. Nanoparticle addition with uniformdispersion in the filler metal showed a reduction inthe defects of the weld metal. Fattahi et al. [2] ob-served defect formation, such as a lack of penetra-tion and coarse micro-porosity in the weld formedby filler metal without nanomaterial. The weld formedwith MWCNT filler metal exhibited reduced defectsand, yet, increased fine micro-porosity in the weld.


MWCNT filler metal also improved weld penetrationand the appearance of the weld seam by decreas-ing the fluidity of the liquid weld metal.

The accumulations of nanoparticles in the weldled to negative effects such as defect formationsand degradation of properties. This occurred in theexperiment of Fattahi et al. [45] showing that themicro-crack formations by the cluster of GNS par-ticle formation result in insufficient bonding with Al.Moreover, a large cluster of nanoparticle formationresulted in easy propagation of cracks. However, ahigher content of GNS in the weld metal with uni-form dispersion reduces discontinuity formation.Therefore, carbonaceous nanoparticles addedthrough filler metal reduced discontinuity formation;however, the accumulation of particles has to beconsidered to reduce the negative effects on thejoint [46,47]. Some of the factors influencing themicrostructure of Al by the addition of MWCNT weredispersion of MWCNT in Al, material deformationand interfacial reaction between Al and MWCNT.

6.2. Effects of filler metal withnanomaterials on microstructure

Welding of Al with the filler metal containing MWCNTresulted in a weld with improved microstructural sta-bility, thereby leading to improved joint strength [2].This was due to grain refinement by the pinning ef-fect at the grain boundaries [48]. Moreover, weldingAl with GNS in the filler metal showed variation inthe weld metal microstructure formed with and with-out GNS in the filler metal. The weld metal formedwithout added nanomaterial exhibited dendritic andequiaxed -Al grains and Al-Si eutectic phases;however, the weld formed with GNS addition in thefiller metal only showed equiaxed a-Al grains. Thismicrostructural change occurred by the addition ofGNS due to its heterogeneous nucleation site, the

Fig. 6. Weld microstructures of 6061 Al alloys formed (a) without, (b) by 2.5 wt.%, and (c) by 5 wt.% of TiCnanoparticles . Reprinted with permission from M. Fattahi, M. Mohammady, N. Sajjadi, M. Honarmand, Y.Fattahi and S. Akhavan // Journal of Materials Processing Technology 217 (2015). (c) 2015 Elsevier B.V.

pinning effect of GNS on the grain boundaries anddeceleration of grain growth. Moreover, increasingthe content of GNS in the filler metal resulted in fineequiaxed grains on the weld metal [45]. Carbon-aceous nanoparticles in the filler metal improved theweld microstructure through grain refinement andtransformed the microstructural phases into similarphases.

As carbonaceous nanoparticles showed grainrefinement in the Al weld metal, it is anticipated thatthe TiC nanoparticles would provide better refine-ment and improvement in properties. The reason forchoosing TiC nanoparticles instead of othernanoparticles for welding Al alloys is their low den-sity and the good wear resistance and weldabilityprovided. Research also showed the Al alloys withTiC nanoparticles as an alloying element in the al-loy improved the mechanical properties by fine grainrefinement [49]. Welding Al alloys with TiCnanoparticles as filler material improved the micro-structure with grain uniformity and reduced graingrowth. Grain refinements in the weld were improvedby the pinning effect of TiC nanoparticles on theboundaries and the nucleation of TiC nanoparticles.This pinning effect occurred by the decreased pri-mary -Al grain size by the increased volume frac-tion of TiC nanoparticles. Therefore, increasing theTiC nanoparticle content in the filler metal reducedthe grain size and improved grain refinement, whichis shown in Fig. 6. Nucleation site formation wasenhanced by the lattice mismatch between Al andTiC nanoparticles [41].

Therefore, both the carbonaceous nanoparticlesand ceramic nanoparticles in the filler metal resultedin welds with refined microstructures. In addition,both the weld formed with carbonaceous and ce-ramic nanoparticles led to grain refinement by thepinning and nucleation effect at the grain bound-


aries. Moreover, increasing the nanoparticle contentin the filler metal increased fine grain refinement.

6.3. Effects of filler metal withnanomaterials on mechanicalweld properties

The carbonaceous and ceramic nanoparticles in thefiller metal produced refinement in the microstruc-ture, which would improve hardness and tensilestrength. Research has shown that small grain sizeformed in the weld metal increases the hardnessand the tensile strength of the joint.

6.4.Hardness

Research showed that the decrease in the grainsize increases the hardness of the metal. The car-bonaceous nanoparticles (i.e. MWCNT) added inthe electrode with Al weld metal exhibited reducedgrain size, which resulted in increased hardness.Moreover, there was another factor that increasedhardness, namely, the formation of Al carbide (Al

4C

3)

influencing the rate of cooling in the weld metal [2].This was also reported in the experiment by Fattahiet al. [45] on welding Al alloys with GNS in the fillermetal. The reason for the increase in hardness wasthe same, but there was yet another reason, namely,the increase of dislocation density. Moreover, in-creasing the GNS content in the filler metal in-creased the microhardness in the weld metal, whichis shown in Fig. 7. However, the weld metal formedby the filler metal without added nanomaterialshowed hardness below the base metal hardness.This decrease was due to the increased tempera-ture above the recrystallisation of the metal [2]. The

Fig. 7. Microhardness of the 6061 Al alloy weld metal by varying GNS content in filler metal. Replotted fromM. Fattahi, A. R. Gholami, A. Eynalvandpour, E. Ahmadi, Y. Fattahi and S. Akhavan // Micron 64 (2014). (c)2014 Elsevier B.V.

reasons for the improvement of hardness were grainrefinement, increase of dislocation density and for-mation of aluminium carbide. Grain refinement wascaused by an increased volume fraction at the grainboundaries through nanomaterial addition. The dis-location density increased in the weld metal by adifference in the thermal expansion of Al and GNSor MWCNT. Lastly, the carbonaceous element ex-hibited Al carbide formation while microstructuralformation through easy bonding between elements[45].

When ceramic nanoparticles (TiC) were addedin the filler metal, the weld showed increased grainrefinement, thereby leading to improvement in thehardness of the Al weld joint. Moreover, weld hard-ness increased with an increase in the TiCnanoparticle content in the filler metal [41]. Two otherfactors increased the joint hardness: Firstly, thereduced grain size of the -Al enhanced the hard-ness by the decreased grain size. Another reasonfor increased hardness was the difference in thethermal coefficient of expansion between the Al andTiC nanoparticles, thereby producing thermal strains.Hardness, however, reduced at the HAZ, which wasdue to the grain growth and recrystallization alongthe joint [41].

Therefore, either carbonaceous or ceramicnanoparticles in the filler metal increased the hard-ness in the weld metal. Increasing the content ofnanoparticles improved the hardness properties. Thereasons for the increase in hardness are same forboth the nanoparticles:1. Reduction in grain size,2. Formation of aluminium carbide (Al

4C

3) and

3. Increased dislocation density (this occurred onlyin TiC and GNS).


6.5. Tensile property

For the case of tensile strength, the Al alloy jointformed with MWCNT added in the filler metal im-proved the ultimate tensile strength with reasonableductility compared to the other weld metal, as shownin Fig. 8, [2]. The increase in tensile strength wasdue to the thermal expansion difference between Aland MWCNT. In addition, the grain refinement anduniform dispersion of MWCNT were also importantfactors for the improvement of the tensile strength.

Moreover, the addition of GNS in the filler metalalso improved the ultimate tensile and yield strengthof the Al weld metal, due to the uniform distributionand unique properties of GNS. The uniform disper-sion of GNS restricts the dislocation motion, therebystrengthening the Al joint. Moreover, the improve-ment of tensile strength was also aided by grainrefinement, which occurred by the fine grains dis-rupting the dislocation movement. This disruptionled to an increasing dislocation density with uni-form grain boundaries that improved the rate of strainhardening [45].

Ceramic nanoparticles (i.e. TiC) added to the fillermetal produced increased ultimate tensile strengthand yield strength and, yet, reduced elongation. Thisimprovement was due to three reasons: Firstly, thethermal mismatch between the TiC nanoparticlesand Al could increase the dislocation density in theweld, which increases the tensile strength. Secondly,the decreased grain size by grain refinement im-proved the tensile strength. Thirdly, dislocation move-ment was obstructed by the fine TiC particles,thereby increasing the tensile strength of the joint.

Fig. 8. Stress vs Strain curve for the 1050 Al alloy weld metals with/without nanomaterial addition. Reprintedwith permission from M. Fattahi, N. Nabhani, E. Rashidkhani, Y. Fattahi, S. Akhavan and N. Arabian //Micron 54-55 (2013). (c) 2013 Elsevier B.V.

Moreover, the fracture surface showed ductile frac-ture by the dimples formed. The weld fractured with5 wt.% of TiC nanoparticles showed fine and uni-form dimples compared to the weld formed with 2.5wt.% of TiC nanoparticles and without nanoparticlesin the filler metal [41].

Hence, the weld tensile strength increased withthe filler metal consisting of carbonaceous (i.e.MWCNT and GNS) or ceramic nanoparticles (i.e.TiC); however, elongation decreased. The reasonsfor the improvement of the weld were1. Uniform dispersion of nanoparticles,2. Grain refinement and3. Properties of nanoparticles added in the fillermetal.

7. CONCLUSIONS

Installation and structures are designed with high-strength materials for increased stability and longlife. But during manufacturing, processes like form-ing, machining and welding reduce the materialstrength, thereby degrading the material properties.Specially, GTAW and GMAW processes degradethe strength of the material at the joining regionsdue to, for example, the heat input, dilution factorand filler metal addition. When added to the fillermetals and electrode coating, proper nanomaterialsin the joining region have compensated for theselosses and improved their material strength and prop-erties.

Overall, the introduction of nanomaterial to thejoint produces


- Reduction of weld defects, such as base metalpores and crack initiation occurring by non-uniformshrinkage with uniformly distributed nanoparticles.However, an inappropriate selection of nanoparticlesand joint design results in void formations in theweld.- Reduction of grain size in the microstructure. Whenadding the proper size and uniform distribution ofnanoparticles increases the pinning effect on grainboundary and grain nucleation during welding.- Stable microstructural formation. In order to im-prove particular properties, a specific phase in themicrostructure needs to be increased, which is ob-tained by appropriate nanoparticle addition and witha specific quantity of nanomaterial.- Improvement of joint mechanical properties. Hard-ness is improved through uniform nanoparticle dis-tribution leading to uniform heat input and dissipa-tion. However, nanoparticle accumulation results innon-uniform hardness. Tensile strength improvedthrough fine microstructural formation by the appro-priate selection of nanoparticles and their charac-teristics.- Improved corrosion resistance and wear resistance,which can be achieved by a suitable addition ofnanoparticles in accordance with the base mate-rial, as well as the fine microstructure by the uni-form dispersion and nominal quantity of nanomaterialadded.

These improvements have been influenced bythe main factors such as the uniform dispersion ofnanoparticles, selection of nanomaterials based ontheir characteristics and the quantity as well as thesize of the nanoparticles. Welding with nanomaterialadded has been mostly executed by nanopowdersand the nano-carbonaceous element. Metal oxides,ceramic carbides and rare element oxidenanopowders are mostly used in weld production.In the case of nano-carbonaceous elements, CNTand GNS have been used.

In welding, the effect of nanoparticles modifiesthe physics of the joining region, thereby resultingin significant improvement in the welding region witha stable microstructure and properties. However,nanomaterials have been significantly used in weldproduction to improve material strength. Researchis being conducted to achieve high joint integrity invarious metals using various welding processes suchas friction stir welding, GTAW, GMAW and laserbeam welding. More research is still being conductedon improving the joint efficiency in dissimilar weld-ing through nanoparticles added.

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