Tushr Sonr ,Visvlingm Blsurmnin ,,Sudersnn Mlrvizhi ,Thiruvenktm Venkteswrn ,Dhenuvkond Sivkumr

a Centre for Materials Joining and Research(CEMAJOR),Department of Manufacturing Engineering,Annamalai University,Annamalai Nagar,608 002,Tamilnadu State,India

b Heat Treatment and Welding Metallurgy Division(HWMD),Materials and Metallurgy Group(MMG),Vikram Sarabhai Space Centre(VSSC),ISRO, Thiruvananthpuram,695 022,Kerala State,India

Keywords: GTCA welding Inconel 718 alloy Magnetically constricted arc traverse speed Tensile properties Microstructure Laves phase

ABSTRACT The magnetically constricted arc technique was implemented to mitigate the heat input related metallurgical problems in Gas Tungsten Arc Welding(GTAW)of Inconel 718 alloy particularly Nb segregation and subsequent laves phase evolution in fusion zone.This paper reports the direct effect of magnetically constricted arc traverse speed(MCATS)on bead pro file,tensile properties and microstructural evolution of Inconel 718 alloy sheets joined by Gas Tungsten Constricted Arc Welding(GTCAW)process.The mechanism amenable for the microstructural modi fication and corresponding in fluence on the tensile properties of joints is investigated both in qualitative and quantitative manner related to the mechanics of arc constriction and pulsing.It is correlated to the solidi fication conditions during welding.The relationship between MCATS and Arc Constriction Current(ACC)was derived.Its interaction effect on the magnetic arc constriction and joint performance was analysed.Results showed that the joints fabricated using CATS of 70 mm/min exhibited superior tensile properties(98.39%of base metal strength with 31.50%elongation).It is attributed to the grain re finement in fusion zone microstructure leading to the evolution of finer,discrete laves phase in interdendritic areas.

1.Introduction

Inconel 718 is a nickel-based high-performance superalloy typically used in aerospace sector for high temperature applications above 650°C due to its better mechanical performance and weldability among other grades of super alloys[1].The strengthening is attained by the precipitation of gamma prime[γ′-Ni3(Al,Ti)]and gamma double prime[γ′-Ni3Nb]precipitates[2].It is mainly employed in the elevated temperature systems of gas turbine and rocket engines such as turbine discs,blades,shafts and rocket combustor.The exceptional resistance of Inconel 718 alloy to strain age cracking is mainly because of its slow response to aging kinetics[3].

Gas Tungsten Arc(GTA)welding process is typically used for welding of Inconel 718 alloy sheets in manufacturing and service repair jobs of aeroengine components as it provides high quality,clean and precise welds.The process is shop friendly,economical and can be employed for onsite welding works.Despite this,welding of Inconel 718 alloy is mainly limited by the segregation of Nb and coarse interconnected laves phase evolution in weld metal microstructure owing to the high heat input in GTAWprocess[4,5].The laves phase is brittle intermetallic compound with topologically closed packed(TCP)structure and is detrimental to the mechanical properties of Inconel 718 alloy joints.It provides easy initiation and propagation of fracture[6].It leads to the premature failure of welded aeroengine components and increases the cost associated with component replacement.Also,it is vulnerable for HAZ micro fissuring and liquation cracking as a consequence of the liquation of NbCs and laves phase at grain boundaries[7].Moreover,the joining of metal sheets is more challenging to attain good weld without any defects,porosity and distortion[8].This problem can be minimized by pulsing of current and magnetic oscillation of arc in GTAW of Inconel 718 alloy which provides re finement of dendritic grains and laves phase to some extent in fusion zone[9,10].However,the tensile properties of joints are still inferior compared to the base metal.The high energy density electron beam(EB)and laser beam(LB)welding processes are effective in controlling the Nb segregation and laves phase evolution in Inconel 718 alloy welds which increases the mechanical performance of welded joints[11,12].However,the EBW and LBW joints shows high porosity and susceptibility to liquation cracking in HAZ due to the rapid cooling rate(10,000°C/s)[13,14].

The high heat input in GTAWprocess is mainly associated with low energy density due to the wider bell-shaped arc column.It causes the wastage of heat on outer flare and through the surface area of arc column to atmosphere.The large diameter of arc column at the bottom also reduces the energy density of the process due to the small number of electrons striking the joint seam per unit area.This results in high heat input requirement to achieve full joint penetration in conventional GTAW process.Thus,to resolve this problem,a recently emerged novel Gas Tungsten Constricted Arc(GTCA)welding process was employed to join Inconel 718 alloy sheets.It is the advanced con figuration of GTAW process,principally differentiated by magnetic constriction and high frequency accentuation of welding arc(up to 20 kHz).In GTCAwelding,the arc constriction current(known as Delta Current)is superimposed on main current to induce a magnetic field and constrict the arc.The magnetic constriction of arc minimizes the dissipation of heat input on outer flare and causes localized fusion of metal at the joint.Fig.1 shows the comparison between typical constricted arc in GTCAW and wider arc in conventional GTAWprocess[15].The magnetically constricted conical arc column enables the concentration of heat input over a narrow region.This results in large number of electrons striking the workpiece per unit area.The constricted arc column also minimizes the loss of heat to the atmosphere due to the comparatively smaller surface area of arc column.Thus,there is a requirement of low heat input to attain full penetration in GTCAW process compared to GTAW process.The GTCAW process was utilized to reduce the segregation of Nb and laves phase evolution in Inconel 718 alloy welds for improved joint performance to meet the requirements of next generation gas turbine engine.

Fig.1.Typical welding arc in a)GTCAWand b)GTAW process[15].

The reliability and performance of welded aeroengine components is highly dependent on the process parameters.The process parameters control the microstructural evolution,weld bead geometry and mechanical properties of welded joints.It governs the heat input and cooling rate.The segregation of Nb and laves phase evolution can also be minimized by the optimum selection of input parameters.Thus,it necessitates the optimization of process parameters.However,the optimization of process parameters is outrageous and time-intensive,So,there is a “one variable at a time” approach of design of experiments(DOE)to study the influence of process parameters on microstructure and performance of welded joints.The in fluence of MCATS on the quality of weld bead,microstructural evolution and performance of welded joints is important for the feasibility of the GTCAW process in aerospace applications.Also,it is essential to investigate the in fluence of MCATS on Nb segregation and laves phase evolution in fusion zone to achieve desirable microstructure and superior joint performance.

The published literature on welding of Inconel 718 alloy,till date,is mainly reported on GTAWprocess[16-20].Low heat input processes such as EBW[21-23]and LBW[24-26]were practiced to control the segregation of solute elements and subsequent laves phase evolution in Inconel 718 alloy welds.But these processes are costly and needs special environment for welding.The reported investigations on GTCA welding are mainly focussed on Steel[27,28]and Titanium alloys[29,30].However,there is a major research gap in the literature about the application of GTCAwelding process for joining Inconel 718 alloy sheets.Moreover,the available literature on GTCAW process are focussing on the effect of heat input,delta current and frequency on mechanical properties and microstructural characteristics of joints[31,32].The information available on the effect of MCATS is de ficient.So,the primary objective of this research work is to investigate the in fluence of MCATS on weld bead geometry,tensile properties and microstructural evolution of GTCA welded Inconel 718 alloy sheets used in aeroengine applications.

1.1.Experimental procedure

Rolled sheets of Inconel 718 alloy(2 mm thick),solutionized at 980°C,was chosen for the present investigation.The base metal elemental composition and mechanical properties are presented in Tables 1 and 2 separately.Autogenous butt welds were fabricated successfully using square butt joint design as displayed in Fig.2.The set-up of InterPulse GTCAW machine(Make:VBCie,UK;Model:InterPulse IE175i)is displayed in Fig.3.The pulsing characteristics of arc constriction current(ACC)is illustrated in Fig.4.The practical working limits(lower level and higher level)of MCATS was set after carrying out many welding trials.The range of MCATS was set considering the criteria such as appearance of weld surface,porosity,welding defects such as burn through,undercuts etc.Table 3 shows the welding parameters used to make the joints.Fig.5 shows the Inconel 718 alloy sheets joined by GTCAWprocess at different levels of MCATS.

Table 1 Base metal chemical composition(%by weight).

Table 2 Base metal mechanical properties.

Table 3 GTCAWparameters used to weld Inconel 718 alloy sheets

The transverse tensile specimens(smooth and notch type)were extracted from the welded sheets according to ASTM E8M − 05 standard.The dimensions of smooth and notch tensile specimens are displayed in Fig.6.Tensile testing was performed employing a servo-monitored tensile testing machine with a rated capacity of 50 kN running at 2 mm/min crosshead speed.The%elongation was calculated with 50 mm gauge length.The 0.2%yield strength was approximated using engineering stress-strain diagram.The fractured surface of the tensile specimens was studied using scanning electron microscope(SEM).The microhardness was measured across the mirror finished crosssectional area of welded specimens according to ASTM E384-17 standard employing Vickers Microhardness testing machine at an indentation load of 0.5 kg and 15 s dwell time.The microhardness distribution was recorded from weld centre to base metal region at an increment of 0.1 mm.The mean microhardness of FZ and HAZ was also reported.

Fig.2.Butt joint design used for welding sheets.

Fig.3.GTCA welding machine arrangement used to weld Inconel 718 alloy sheets.

Fig.4.ACC pulsing characteristics of GTCAWprocess[15].

Fig.5.Inconel 718 alloy joints made by GTCAWprocess.

Fig.6.Dimensions of smooth and notch tensile specimens.

The surface quality of the welds was examined visually.The cross-sectional area of the metallographic specimens was mirror finished and etched with Kalling’s reagent to record both macro and microstructure.The weld bead macrostructure was captured using a stereoscope.The optical microscope and scanning electron microscope(SEM)were employed to record the microstructure of different regions of welded joints at lower and higher magni fications.The weld bead characteristics,mean SDAS and volume fraction of laves phase(%)were approximated using ImageJ software.The qualitative and quantitative estimation of elemental composition of laves phase and dendrite core was performed using Energy Dispersive X-ray Microscopy(EDXM)technique.Nb mapping of fusion zone was executed to estimate the segregation of Nb resulted during solidi fication.

1.2.Calculation of heat input and cooling rate

Heat input(J/mm)calculation was done by applying the following general formula

whereIavg=average of Main Current and ACC(A),V=arc voltage(V),S=MCATS(mm/min),η=ef ficiency of the process(60%).Vishwakarma et al.[33]mentioned the typical relationship for the approximation of cooling rate in the weld area as given below.

whered2=secondary dendrite arm spacing(μm),v=cooling rate(°K/s),A=material constant andn=constant with a value between 0.25 and 0.5.Mehrabian et al.[34]expressed the values ofAandnto be 141 and 0.4,respectively for Inconel 718 alloy.Substituting these values in Eq.(2),an average cooling rate was calculated.

2.Results

2.1.Macrostructure

Fig.7 shows the in fluence of MCATS on bead pro file.All the welded joints were found to be defect-free including porosity and inclusions.The bath-tub shaped fusion zone pro file shows full penetration whereas the bowl-shaped top with narrower base indicates partial penetration.Full penetration was recorded in all the joints except the joints welded at higher level of 80 mm/min due to the insufficient heat input.The weld bead geometry is defective at lower and higher level of MCATS.The results showed excessive penetration with larger concavity on top weld bead surface and sagging bottom at lower level of 40 mm/min.However,it showed partial penetration up to 1.60 mm at higher level of 80 mm/min.

Table 4 shows the in fluence of MCATS on weld bead geometry.Increase in MCATS from 40 mm/min to 80 mm/min results in decrease in weld bead geometry.It showed 39.42%,56.62%and 46.77%decrease in bead width,fusion zone area and HAZ width respectively at MCATS of 80 mm/min compared to the 40 mm/min.The joints welded using 60 and 70 mm/min showed better weld bead geometry.

Table 4 Effect of MCATS on weld bead characteristics

Fig.7.Effect of MCATS on weld bead pro file.

2.2.Tensile properties

Fig.8 shows the photograph of fractured smooth and notch tensile specimens after tensile testing.The tensile specimens failed in fusion zone at all levels of MCATS.Such kind of tendency related to the failure of tensile specimens in fusion zone of Inconel 718 joints was reported by Ram et al.[12]and Reddy et al.[22].The failed specimens showed more deformation in weld region with reference to the base metal.Rao et al.[35]reported 30%localized deformation in fusion zone of constant current TIGwelds relative to 9%estimated over the gauge length of test specimen.Thus,the tensile strength of welded joints can be accounted as the tensile strength of fusion zone.This ascertains that the base metal region is moderately stronger than weld metal and tensile properties of joints are directed by fusion zone.

Fig.8.Fractured tensile specimens showing failure occurence in fusion zone.

The tensile properties of joints at increased levels of MCATS are presented in Table 5.The welded joints exhibit lower strength and ductility at all levels of MCATS as a result of divergence in microstructural features of base metal and weld metal.It is imputed to the solute segregation leading to the evolution of laves phase in interdendritic areas of fusion zone.The tensile properties increase with increase in MCATS from 40 mm/min to 70 mm/min and then decrease dramatically owing to the defect of incomplete penetration at 80 mm/min.It showed superior tensile properties when welded in the range of 60-70 mm/min.The joints welded at 40 mm/min showed 14.36%and 60.89%reduction in tensile strength and elongation relative to the base metal.However,the tensile strength and ductility of joints were signi ficantly improved at 70 mm/min.It showed only 1.61%and 17.89%reduction in tensile strength and ductility relative to the base metal.Thus,it exhibits 14.89%,16.08%and 111.97%increase in tensile strength,yield strength and elongation at 70 mm/min relative to 40 mm/min.The base metal and all the welded joints revealed notch sensitivity.Cai et al.[36]imputed the notch sensitivity of Inconel 718 alloy to the precipitation ofδphases.The joints welded in the range of 60-70 mm/min revealed higher notch tensile strength.It disclosed only 2.87-2.98%reduction in notch tensile strength of joints relative to base metal.The welded joints showed higher notch sensitivity at 40 mm/min.It showed 19.54%reduction in tensile strength in presence of notch relative to the base metal indicating the influence of laves phases on tensile behavior and plastic constraint of material at the notch.

Table 5 Effect of MCATS on tensile properties of welded joints

The as welded Inconel 718 GTAWjoints showed 21.70%and 50%reduction in tensile strength and ductility relative to the base metal in the investigation of Cortes et al.[37].Rodriguez et al.[38]showed that the tensile strength and elongation of GTA welded Inconel 718 joints were drastically reduced to 22.35%and 45.90%in comparison to the base metal.However,the joints fabricated at an optimum level of MCATS(70 mm/min)laid 20-30%improvement in tensile properties of joints over GTAWprocess.

Fig.9.SEM fractograph of smooth tensile specimens at different levels of MCATS.

2.3.Fracture surface

Fig.10.SEM fractograph of notch tensile specimens at different levels of MCATS.

The SEMfractographs of smooth and notch tensile specimens at increased levels of MCATS are presented in Figs.9 and 10.The base metal fracture surface showed completely dimpled regions and no preferential fracture path.However,it exhibited shallower dimples and coarse microvoids in presence of notch.The fractured surface of welded joints revealed a dendritic pattern and preferential fracture path.The laves particles were identi fied in interdendritic areas of fractured surfaces.The appearance of perfectly aligned tear ridges in the weld fracture surface points out that the failure occurred along the interdendritic areas due to the presence of the laves phase.It showed the presence of many quasi cleavage facets and dimples.

The notch tensile fracture surface showed comparatively larger cleavage facet regions and few dimples contributing to the notch brittleness.The presence of microcracks is due to the hard and brittle intermetallic laves phases which do not deform plastically.Many small microvoids were noticed in the fractured surface of joints welded in the range of 60-70 mm/min.Because the finer and distinct laves phase particles got pulled off absorbing strain energy during tensile rupture.The presence of equiaxed dimples in the tensile fracture surface indicates higher ductility at an optimum level of 60-70 mm/min.However,the tensile fracture surface at 40 mm/min showed many microcracks and much larger cleavage facet regions.The evolution of flat and irregular quasi cleavage facets in the weld fractured surface disclosed that the fracture initiated predominantly by the coarser thick film of laves phase owing to the excessive brittleness.The failure in fusion zone of welded joints is imputed to the presence of hard and brittle intermetallic laves phases in interdendritic regions.

2.4.Microhardness

Fig.11 shows the in fluence of MCATS on microhardness distribution of welded joints at mid-thickness region.It shows the microhardness gradient indicating the scale of uniformity of elemental distribution.The microhardness gradient decreases with an increase in MCATS.It is mainly associated with decrease in segregation and subsequent laves phase evolution in Inconel 718 alloy welds.The microhardness of fusion zone is recorded to be considerably less at some points from the weld centre.Lower microhardness values recorded in fusion zone mainly accounts for the tensile failure of joints in weld metal only.

Fig.11.Effect of MCATS on microhardness distribution of welded joints.

The microhardness of fusion zone(FZ)and heat affected zone(HAZ)are compiled in Table 6 and compared with the base metal.The microhardness of base metal is relatively higher than FZ and HAZ.The laves phase evolution in weld metal region expends the strengthening solute elements from the matrix and reduces the fusion zone hardness.The weld region microhardness followed similar trend of tensile strength at increased levels of MCATS.The microhardness of FZ and HAZ increases with increase in MCATS.The joints made using MCATS of 40 mm/min showed 13.70%and 17.80%decrease in hardness of FZ and HAZ compared with the base metal.However,it is decreased by 2.73%and 5.82%at an optimum level of 70 mm/min.Thus,the joints made using 70 mm/min manifest 12.70%and 14.58%increase in hardness of FZ and HAZ as compared to 40 mm/min.The dissolution and coarsening of niobium carbides during weld heating is associated with signi ficant grain growth in HAZ.It is mainly responsible for lower microhardness in HAZ particularly at 40 mm/min.

Table 6 Effect of MCATS on microhardness of various regions.

2.5.Microstructure

The optical and scanning electron micrographs of base metal solutionized at 980°C are shown in Fig.12.It shows niobium carbides across the grain boundaries.The twinning region appears as straight lines extending from the grains.Fig.13 shows the EDXM spectrum of niobium carbides representing the higher peaks of Nb and C.Figs.14 and 15 show the optical and scanning electron micrograph of the fusion zone at various levels of MCATS.As MCATS increases,the dendritic structure becomes finer due to the decrease in heat input and enhanced cooling rate.The joints welded at 40 mm/min showed coarser dendritic structure.However,it is much finer at 80 mm/min.Table 7 shows the in fluence of MCATS on microstructural characteristics of fusion zone.As MCATS increases heat input decreases and cooling rate increases in a proportional manner.This results in decrease in SDAS thereby re fining the fusion zone.Increase in MCATS from 40 mm/min to 80 mm/min increases the cooling rate from 667°K/s to 3738°K/s.This results in reduced growth of the dendrites and the average SDAS decreases from 10.46μm to 5.25μm.

The cooling rate and SDAS showed signi ficant in fluence on laves phase evolution in the fusion zone.Figs.16 and 17 show the optical and scanning electron micrograph of the laves phase at increased levels of MCATS.Increase in MCATS reduces the segregation of solute elements and laves phase evolution in fusion zone.The fusion zone of welded joints at 40 mm/min showed coarser and higher volume fraction of laves phase.It shows the evolution of thick continuous laves phase film decorating the dendrite core regions.However,it was comparatively much finer and lower in amount at 80 mm/min.Increase in MCATS above 50 mm/min showed the evolution of finer,discrete laves phase uniformly distributed in fusion zone.The mean size and volume fraction of laves phase at 40 mm/min is 9.73μm and 16.84%.However,the joints welded by using MCATS of 70 mm/min revealed 2.65μm and 7.22%mean size and volume content of laves phase in fusion zone respectively.Thus,it disclosed 72.76%and 57.12%reduction in mean size and volume content of laves phase in fusion zone at 70 mm/min compared to 40 mm/min contributing in superior tensile properties of joints.

Figs.18 and 19 show the optical and scanning electron micrograph of partially melted zone(PMZ)at increased levels of MCATS.The PMZ exhibited coarse columnar grains near the fusion zone interface.There is 40.16%reduction in the width of PMZ at 70 mm/min compared to 40 mm/min.The steep thermal gradient between the weld centre and base metal increases peak temperature immediately in the region surrounding the weld pool which is in between liquidus and solidus temperature range of the alloy.This gives rise to partial melting and increases the region of PMZ at 40 mm/min due to the high heat input.Figs.20 and 21 show the optical and scanning electron micrograph of heat affected zone(HAZ)at increased levels of MCATS.There is a signi ficant decrease in the width of HAZ at increased levels of MCATS.The joints welded at 40 mm/min showed severe grain growth and wider HAZ.The high heat input at 40 mm/min causes dissolution and coarsening of Nb carbides in HAZ.The width of HAZ reduces at increased levels of MCATS.It shows 32.25%reduction in width of HAZ at 70 mm/min relative to 40 mm/min.The joints welded above 50 mm/min showed the evolution of finer Nb carbides across the grain boundaries of HAZ which restricts the grain growth.Agilan et al.[39]found that the tendency for grain boundary liquation was more severe in coarse grains of HAZ than finer grains.However,liquation cracking was not observed in HAZ of GTCA welded Inconel 718 joints even at high heat input.

Fig.12.Microstructure of base metal at lower and higher magni fication:a)&b)by optical microscopy;c)&d)by scanning electron microscopy.

Fig.13.EDXM point scan analysis of niobium carbides(NbCs)in base metal.

2.6.EDXM analysis

The EDXM Nb mapping of fusion zone at increased levels of MCATS is displayed in Fig.22.The uniform distribution of niobium(Nb)was recorded in the wrought base metal.However,it was segregated in cast dendritic structure of the weld metal.Increase in MCATS results in decrease in segregation of Nb in fusion zone.It was observed to be much segregated at 40 mm/min.However,the joints welded in the range of 60-80 mm/min showed uniform distribution Nb in the fusion zone due to the reduced heat input during welding.The chemical composition of laves phase and dendrite core region at increased levels of MCATS are enumerated in Tables 8 and 9.Fig.23 shows the EDXMspectrum of laves phase and dendrite core at increased levels of MCATS.Laves phase is enriched with Mo,Ti,Si and Nb in interdendritic regions.The fusion zone showed the depletion of respective alloying elements in dendrite core regions.These findings con firm the results proclaimed by Odabasi et al.[40]and Ram et al.[41].The Nb content of laves phase decreases with an increase in MCATS.The Nb consumption is much higher about 41.36%in the laves phase at 40 mm/min resulting in lesser Nb content up to 1.38%in dendrite core.However,the joints made using MCATS of 80 mm/min showed 9.72%and 5.87%Nb content in laves phase and dendrite core respectively.The GTCA welding shows signi ficant improvements over GTAWprocess.Radhakrishna et al.[5]viewed 2%and 26%Nb content in dendritic core and interdendritic regions of constant current GTA welds.Ram et al.[9]disclosed that the Nb content of laves phase was reduced to 14.62%using current pulsing technique when compared with 19.54%in constant current GTAwelds.The Nb content in laves phase of EB and LB welds were reported to be 10%and 14%in the study of Reddy et al.[11]and Ram et al.[12].In comparison to the above processes,GTCAW revealed 10.67%-11.47%Nb content in laves phase of Inconel 718 alloy welds when welded in the range of 60-70 mm/min.However,the bene ficial effects of magnetic constriction of arc were not observed at 40 mm/min due to the high heat input resulting in more segregation of Nb and coarse laves phase evolution.

Fig.14.Optical micrograph of fusion zone(FZ)at various levels of MCATS.

Table 7 Effect of MCATS on microstructural features of fusion zone.

Fig.16.Optical micrograph of laves phase at various levels of MCATS.

Furthermore,the in fluence of welding processes on the partitioning tendency of solute elements is well reported by Odabasi et al.[40].Knorovsky et al.[42]estimated the tendency of alloying elements towards the laves phase evolution using partition coefficient(P).It is defined as the laves phase composition per unit nominal composition of base metal.Higher the partition coef ficient(P),more will be the partitioning tendency of particular alloying element for laves phase evolution during solidi fication.Cieslak et al.[43]quanti fied the degree and order of microsegregation during weld metal solidi fication by the distribution coef ficient(k).It defined as the dendrite core composition per unit base metal composition.The distribution coef ficient close to unity is desirable and indicates that the alloying element does not segregate strongly during solidi fication.The effect of MCATS on the partition(P)and distribution coef ficient(k)of alloying elements is enumerated in Tables 10 and 11.The alloying elements such as Nb,Mb,Ti,and Si have a greater tendency to segregate in interdendritic regions for the laves phase evolution.The partition coef ficient of solute(Nb,Mb,Ti and Si)decreases with increase in MCATS.The joints welded at 40 mm/min showed higher partition coef ficient of 5.41 and lesser value of distribution coef ficient as 0.18 for Nb,indicating more segregation tendency in laves phase evolution and corresponding depletion of Nb in dendrite core regions.The partition and distribution coef ficient of Nb calculated from the investigation of Ram et al.[9]was observed to be 3.84 and 0.27 for CC GTAWand 2.50 and 0.43 for PC GTAW welds respectively.The partitioning coef ficient(P)is much larger about 4.2 in the study of Radhakrishna et al.[5]for Inconel 718 GTAwelds.The partition coef ficient of Nb in EB and LB welds was calculated to be 2.02%and 2.78%respectively in the study of Reddy et al.[11]and Ram et al.[12].However,GTCA welds at MCATS of 70 mm/min showed much lower partition coef ficient(1.39)than GTAW,EBW and LBW processes.It disclosed that GTCA welding process reduces Nb segregation in Inconel 718 welds signi ficantly owing to the arc constriction and pulsing.

Table 8Effect of MCATS on elemental distribution in laves phase.

Table 9 Effect of MCATS on elemental distribution in dendrite core.

Table 10Effect of MCATS on partition coef ficient(P)of alloying elements.

Table 11 Effect of MCATS on distribution coef ficient(k)of alloying elements.

3.Discussion

3.1.Effect of MCATS on weld bead geometry

GTCAWis the modi fied con figuration of GTAWprocess in which the energy density of welding process is increased by implementing the technique of arc constriction.The magnetic field is induced around the welding arc by superimposing arc constriction current(ACC)on main current for compressing the arc.The basic principle of magnetic arc constriction and the arc wave form with rise time,fall time and agitation in GTCAWprocess is shown in Fig.24a)and b).Main current is kept higher than ACC for deeper penetration.ACC pulses with main current at a very high frequency up to 20 kHz in sawtooth shape waveform rather than square wave form as in Pulse current GTAW.Due to the arc constriction the heat input requirement for complete penetration decreases.Less heat is dissipated on the outermost flare.This allows for better heat management on welds whilst attaining full penetration.

Fig.17.SEM micrograph of laves phase at different levels of MCATS.

Fig.18.Optical micrograph of PMZ at different levels of MCATS.

Fig.19.SEM micrograph of PMZ at various levels of MCATS.

Fig.20.Optical micrograph of HAZ at various levels of MCATS.

The results disclosed an interaction effect prevailing between ACC and MCATS in GTCA welding process.Fig.25 illustrates the schematic diagram of interaction effect of MCATS and ACC on welding arc.As MCATS increases the duration of contact of welding arc with metal at the joint decreases.This also results in unstable welding arc and loss of stiffness at higher traverse speed owing to the disturbance of magnetic field required for arc constriction.Thus,it extends partial penetration at higher level of MCATS.It needs increased heat intensity and stiffer arc to increase penetration.This can be achieved by increasing ACC.Increase in ACC results in an increase in magnetic field which raises the constriction of arc.It leads to the increased energy density and provides stiffer arc required for deeper penetration.The stiffer arc effectively forces the molten metal on the advancing side of the molten pool whilst accomplishing full penetration.It is necessary for the keyhole effect of GTCAW process.Huang et al.[44]stated that the lower level of welding speed produces a stabilised welding arc which ensues severe interaction between the welding arc and molten weld pool.In GTCAWprocess,arc stability can be achieved at increased levels of MCATS by increasing the ACC.The bath-tub shaped weld bead pro file is caused by the high heat input extending more fusion of metal at the joint.However,the bowl-shaped weld bead with the narrower bottom is due to the lower heat input which causes less melting of metal at the joint seam.The insufficient heat input(388 J/mm)results in incomplete penetration at 80 mm/min.However,the higher heat input(621-776 J/mm)when welded in the range of 40-50 mm/min leads to excessive penetration with larger concavity at the top surface of the weld bead and sagging bottom.Hence the lower and higher levels of MCATS showed the defects of penetration.The MCATS in the range of 60-70 mm/min provides better weld bead pro file.

Fig.21.SEM micrograph of HAZ at increased levels of MCATS.

Fig.22.SEM EDXM Nb mapping of fusion zone at increased levels of MCATS.

3.2.Effect of MCATS on tensile properties

Fig.23.EDXM analysis of laves phase(a&b)and dendrite core(c&d)in fusion zone at MCATS of 40 mm/min and 70 mm/min.

The solidi fication conditions,elemental composition,and microstructural features of fusion zone extend signi ficant in fluence on tensile properties of joints.The tensile properties of joints are lower relative to the base metal.It is imputed to the evolution of laves phase in interdendritic areas of the weld microstructure.The hard and brittle laves phase does not show plastic deformation along with the ductile matrix during tensile loading.Laves phase evolution,aside from consuming the strengthening solute elements from matrix,diminishes the strength of the interface between matrix and laves phase which ensues easy crack initiation and provides a low energy fracture path for rapid crack propagation[9-11].The tensile properties are increased with increase in MCATS.It is imputed to the re finement of grains in fusion zone which leads to the evolution of finer discrete laves phase uniformly distributed in interdendritic areas.The evolution of finer,uniformly distributed hard and brittle laves particles in ductile austenitic matrix provides strengthening.Nb has less partition and high distribution coef ficient at MCATS of 70 mm/min as a result of the rapid solidi fication rate experienced by the weld pool.It results in less segregation tendency of Nb in fusion zone and reduced laves phase evolution.

Fig.24.a)Magnetic arc constriction b)Arc wave form(A=Rise Time;B=Fall time;C=Agitation in Gas Tungsten Constricted Arc welding[15].

Fig.25.In fluence of MCATS on magnetic arc constriction:a)Increase in MCATS(above 70 mm/min)at constant level of ACC;b)Increase in MCATS with increase in ACC.

The GTCAwelded Inconel 718 alloy joints welded at an optimum level of MCATS(70 mm/min)showed 20-30%improvement in joint performance compared to conventional GTAW process and comparable to EBWand LBWprocess.It showed 98.39%joint ef ficiency with 31.50%elongation.Rao et al.[35]reported 22%and 50%reduction in tensile strength and elongation of Inconel 718 GTAW joints relative to the base metal.Ram et al.[12]reported 1.02%and 33.34%reduction in tensile strength and elongation of Nd-YAG pulsed LBW Inconel 718 alloy joints as compared with the base metal.Reddy et al.[22]observed 7.34%and 20%reduction in tensile strength and elongation of EBWInconel 718 alloy joints.However,the GTCA welded joints fabricated at an optimum level of MCATS(70 mm/min)showed 1.61%and 17.18%reduction in tensile strength and elongation relative to the base metal.It refers to the reduced heat input by virtue of the increased energy density of welding arc accomplished through magnetic arc constriction and ACC pulsing in GTCA welding process.However,the advantages of magnetic arc constriction and pulsing were not observed at lower level of MCATS because of the high heat input.The fusion zone is much coarser at MCATS of 40 mm/min.The evolution of coarse dendritic grains in fusion zone results in more concentration of Mo,Nb,Si and Ti in interdendritic areas of the weld.It leads to the evolution of coarse thick interconnected laves phase film surrounding the dendritic core.The evolution of which abridges the coherency between solidi fication sub-grain boundaries(SSGBs)and degrades the tensile properties of joints.

The base metal indicated considerably higher notch brittleness.It is correlated to the presence of niobium carbides at the grain boundaries which provides the conditions of plastic constraint at the notch and results in reduced ductility during plastic deformation.The welded joints showed notch brittleness at all levels of MCATS.The morphology and volume content of laves phase present in interdendritic regions have considerable impact on the plastic constraint introduced at the notch.The coarser thick film of laves phase increases the degree of plastic constraint and stress concentration at the notch.This results in increased fracture stress and reduced plastic deformation at the notch.Such conditions favour high notch brittleness when welded at 40 mm/min.The reduction in strengthening due to the laves phase evolution can be also correlated to the reduced strain field.As laves phases are incoherent in nature,it does not impart strengthening.The coarsening of laves phase results in reduction in strain fields due to the consumption of solute atoms from the matrix.This causes reduced strain field interaction between the solute atoms and dislocations.Laves phase also acts as stress concentrating sites which aids in increasing the fracture stress and provides easy crack initiation and rapid crack propagation thereby reducing the tensile properties of joints.

3.3.Effect of MCATS on microhardness

The fusion zone hardness of Inconel 718 alloy mainly depends on the solidi fication conditions(heat input,weld cooling rate,and fluid flow)during welding which imparts signi ficant effect on the partitioning of alloying elements during solidi fication.The weld metal region showed the trend of variation in microhardness at all levels of MCATS.It is correlated to the partitioning of alloying elements in interdendritic regions which leads to the evolution of intermetallic laves phase.The laves phase consumes the strengthening elements such as Nb,Mb,Ti,and C leaving behind depleted dendritic core regions in fusion zone.Laves phase have a greater incoherency with the matrix and endows very little to enhance the hardness of weld region[45].The depletion of Nb from dendrite core results in lowest hardness in fusion zone which mainly accounts for the failure of joints in fusion zone only.The increase in hardness of fusion zone and HAZ at incremental levels of MCATS is attributed to the microstructural re finement of fusion zone and less grain growth in HAZ due to the reduced heat input and enhanced cooling rate.The evolution of finer dendritic structure reduces the depletion of strengthening elements from matrix in interdendritic region and raises the hardness of fusion zone.The presence of Nb carbides across the grain boundaries serves the purpose of reducing grain growth.The joints made using MCATS of 40 mm/min revealed dissolution and coarsening of Nb carbides in HAZ due to the high input associated with weld thermal cycle.It leads to severe grain growth in HAZ and reduces the hardness of HAZ considerably.However,the evolution of finer uniformly distributed Nb carbides across the grain boundaries tends to constrain the grain growth and have less detrimental effects on hardness of HAZ above MCATS of 50 mm/min.

3.4.Effect of MCATS on microstructure

The most important feature of MCATS is that it controls the heat input and governs the weld cooling rate.Thus,it shows signi ficant effect on the microstructural evolution of weld region.The mechanical properties of welds are in fluenced by changes in microstructure.Therefore,in terms of quality control and productivity,the selection and control of MCATS are important factors in arc welding.In Inconel 718 alloy,solidi fication commences with primary liquid directing toγreaction resulting in segregation of Mo,Ti,Nb,Si and C in interdendritic regions.The subsequent liquid→(γ+NbC)eutectic type reaction depreciates most of the carbon present in the molten metal until next eutectic reaction,liquid→(γ+Laves)occurs,thereby ending the solidi fication process[14].

An increase in MCATS lowers the heat input and enhances the cooling rate.Thus,the solidi fication of weld pool occurs at a faster rate,promoting grain re finement in fusion zone(FZ).Finer grain size tends to produce a stronger microstructure which is desirable.Another effect of increasing MCATS is that it reduces the Nb segregation and laves phase evolution in weld metal region.Inconel 718 alloy is alloyed with large number of alloying elements to increase the service performance at elevated temperature.It increases the solidi fication temperature range of Inconel 718 alloy up to 180°C.Thus,there is enough time for redistribution of alloying elements during solidi fication.Also,Nb has high partitioning coef ficient and lower diffusivity.This gives rise to heterogeneous distribution of Nb.The Nb partitions strongly to form NbC and laves phase.Due to the lower diffusivity,Nb segregates in the solute rich liquid rejected by the solidifying interface.Thus,the last to solidify solute rich liquid present in interdendritic regions of fusion zone is enriched with Nb.The segregation of Nb was minimized with increase in MCATS.It is correlated to the reduced heat input and enhanced cooling rate at increased levels of MCATS.The faster cooling rate experienced by the weld pool provides less time for redistribution of alloying elements.This reduces the partitioning tendency of Nb.The increased cooling rate also increases nucleation and imparts grain re finement in fusion zone.It leads to reduced interdendritic spacings which are privileged locations for solute segregation and laves phase formation.This is the main reason that the segregation of Nb and subsequent laves phase evolution is minimized at increased levels of MCATS.Gao et al.[46]reported that at higher heat input,there is more intermetallics formation which reduces the strength of joints.The GTCA welded Inconel 718 alloy joints were free from porosity,HAZ micro fissuring and solidi fication cracking related problems.It would extend the service life of welded aeroengine components and reduce the cost associated with component replacement.Magnetic constriction and pulsing of arc signi ficantly control the solidi fication conditions(heat input and cooling rate)during welding.However,the bene fits of magnetic constriction of arc and current pulsing were not observed at lower level of MCATS due to the high heat input.Sivaprasad et al.[10]reported similar consequences that the advantages of pulsating current in re finement of fusion zone could not be gained owing to the higher heat input employed in the welding process.The remelting of homogeneous nuclei advancing the solidliquid interface makes it dif ficult for the re finement of dendritic structure owing to the high temperature of the liquid.This provides enough time for solute redistribution and promotes high partitioning of solute elements in interdendritic liquid.Thus,it leads to the evolution of coarser thick film of laves phase decorating the interdendritic regions.

The segregation of Nb and laves phase evolution were minimized by 50-80%at optimum level of MACTS compared to conventional GTAWprocess.It would improve the response of welded joints to post weld heat treatment and precipitation of strengtheningγ’’[Ni3Nb]precipitates.Ram et al.[9]recorded 8%volume fraction of laves phase in pulsed current welds compared to 14.2%in constant current GTA welds of Inconel 718 alloy.Manikandan et al.[47]diminished the laves phase evolution in GTA Inconel 718 welds from 35.7%to 18.9%by employing the compound pulsating current technique.The volume content of laves phase in electron beam and laser beam welds of Inconel 718 alloy was reported to be 4%and 5.2%respectively in the study of Reddy et al.[11]and Ram et al.[12].However,the GTCA welded joints made using MCATS of 70 mm/min showed 7.22%volume fraction of laves phase in weld metal region resulting in improved tensile properties of the joints which is superior than GTAW and comparable to LBW and EBW processes.

3.5.Dendritic structure re finement

Segregation of solute elements and laves phase evolution during solidi fication of weld metal are substantially prompted by the welding process,technique,and input parameters that control the weld heat input and subsequent solidi fication rate.Magnetic constriction and pulsing of arc signi ficantly in fluence the solidi fication conditions during welding.Fig.26 shows the cycles of magnetic arc constriction and release in GTCAwelding process.ACC weaves periodically with main current and constricts the arc slowly in rise time.Once the rise time is completed,magnetic constriction is released quickly and the arc widens.Since the magnetic field is imposed on the arc and periodically reversed,circular internal motion is introduced in the weld pool around the electrode axis resulting in stirring of the molten pool.It reduces the segregation of solute elements during solidi fication.The convective force and liquid motion also increase which provides signi ficant undercooling in the molten weld pool.This results in increased nucleation rate and provides dendritic structure re finement.

Fig.26.Arc constriction behavior in GTCAWprocess[15].

In the GTCA welding process,ACC is pulsing at a very high frequency of 12 kHz.This causes the fluctuation of the arc column owing to the magnetic constriction and release of arc.It extends disruption in the weld thermal cycle and increases thermal oscillation in weld pool.The cyclic thermal oscillation due to the magnetic constriction and release of arc also causes remelting and breaking-off the growing dendrites.These fragmented dendrites offer sites for heterogeneous nucleation and re finement.The increased fluid flow in the molten pool due to the current pulsing aids in breaking off the dendrites and in carrying out the broken dendrites ahead of the solid-liquid interface,while the reduced thermal gradients aids in their survival[10,12].The cooling rate is raised as a consequence of magnetic constriction and pulsing of arc which provides grain re finement in fusion zone microstructure.The magnetic constriction and release of arc also facilitate the vibration of molten pool,giving rise to the breaking of dendrites.All these factors can have an important bearing on the evolution and re finement of fusion zone microstructure.

4.Conclusions

1.Magnetically Constricted Arc Traverse Speed(MCATS)showed pronounced effect on the weld bead pro file,tensile properties and evolution of microstructure in GTCA welded Inconel 718 alloy joints Hence,the optimum selection of MCATS is a crucial aspect in achieving the potential bene fits of GTCAWprocess.

2.The interaction effect prevails between MCATS and ACC.The ACC must be increased with increased levels of MCATS.

3.The joints welded using MCATS of 70 mm/min yielded superior tensile strength and elongation extending joint ef ficiency up to 98.39%in the 2 mm thick Inconel 718 alloy sheets.It is correlated to the grain re finement in fusion zone leading to the reduced segregation and subsequent laves phase evolution in interdendritic areas.

4.Increase in MCATS extends grain re finement in fusion zone and lowers the volume content of laves phase in weld metal which improves the tensile properties of joints.It mainly pertains to the increased cooling rate at increased levels of MCATS.

5.Heat input supplied to the weld pool showed predominant influence on the evolution of laves phase in fusion zone over the constriction and pulsing of arc.Thus,the bene ficial effects of arc constriction and pulsing were not perceived at MCATS of 40 mm/min leading to the signi ficant grain growth and evolution of thicker interconnected laves phase in interdendritic areas of fusion zone.

6.The Nb segregation is considerably reduced at increased levels of MCATS.The volume fraction of laves phase is decreased by 57.12%at MCATS of 70 mm/min as compared to 40 mm/min.Thereby contributing in increased tensile properties of joints.

Funding

This project work is funded by Indian Space Research Organization(ISRO)India.Project No.ISRO/RES/3/728/16-17.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to in fluence the work reported in this paper.

Acknowledgement

The authors record their gratitude to the Director,Vikram Sarabhai Space Centre(VSSC),ISRO,Trivandrum,Kerala for providing the financial support and base material to carry out this investigation rendered through ISRO RESPOND scheme(Project No.ISRO/RES/3/728/16-17).