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Fabrication of Fe-FeAl Functionally Graded MaterialUsing the Wire-Arc Additive Manufacturing Process

CHEN SHEN, ZENGXI PAN, DOMINIC CUIURI, JON ROBERTS, and HUIJUN LI

A functionally gradient iron-aluminum wall structure with aluminum composition gradientfrom 0 at. pct to over 50 at. pct is fabricated using a wire-arc additive manufacturing (WAAM)system. The as-fabricated alloy is investigated using optical microstructure analysis, hardnesstesting, tensile testing, X-ray diffraction phase characterization, and electron-dispersivespectrometry. The comprehensive analysis of the experimental samples has shown that theWAAM system can be used for manufacturing iron aluminide functionally graded material withfull density, desired composition, and reasonable mechanical properties.

DOI: 10.1007/s11663-015-0509-5� The Minerals, Metals & Materials Society and ASM International 2015

I. INTRODUCTION

FUNCTIONALLY graded material (FGM) belongsto a class of advanced materials with properties thatprogressively vary over one or more dimensions. Since itwas applied in mid-1980 as the thermal barrier coatingfor hypersonic space plane projects,[1–5] FGM hasincreasingly attracted both research and commercialinterest due to its unique gradient and locally optimizedmaterial properties, which permits application in harshenvironments with high-temperature gradient, wear, andcorrosion. While the reproducibility of FGMs is impor-tant for its mass production in industrial applications,[6]

the deterministic gradient still remains difficult tocontrol and the dilution effect during fabrication hasnot been systematically explored.[7] In addition, thefabrication cost of the composition gradient material isvery high using current powder processing and fabrica-tion methods.[8]

Existing methods for FGM fabrication include layer/disk remelting,[9] chemical vapor deposition/infiltra-tion,[10] powder-based furnace remelting,[11] laser rapidprototyping,[12] and also weld arc deposition.[13] Forcertainmaterials, powder-based processes cannot directlyproduce functional parts with high structural integrity[14]

and often require expensive processing steps such as hotisostatic pressing to achieve the full density which isessential for highly loaded structural materials.[15]

Compared to powder-based processes, the arc weld-ing-based wire feed deposition method has significantlylower material supply cost, higher deposition rate, and

lower probability of oxide contamination.[16] Up to date,wire-arc-based additive manufacturing process has beenpreliminary investigated and applied to fabricate structureswith variousmaterials such as titanium[17,18] and aluminumalloys.[19] In this research, awire-arc additivemanufacturing(WAAM) system is utilized for in situ fabrication of ironaluminides.[20] A gas tungsten arc welding (GTAW) arcrather than gas metal arc welding (GMAW) arc is used asthe heat source, due to its higher arc stability when appliedto a wide range of ferrous, non-ferrous alloys, and theircombinations.[21] Compared to the GMAW process, theGTAWprocess generates negligible spatter and produces aquiescent melt pool, which is more desirable to produceconsistent material deposition with the desired chemicalcomposition. Pure iron and aluminum wires are fedseparately into themoltenweldingpool througha twin-wirefeeding system with a specific wire feed speed ratio in orderto control the material composition and achieve the desireddeposition rate.Asa result, this process is simultaneously anadditivemanufacturing (AM)process andan in situalloyingprocess.When considering a material that can be applied by

such as process, the Fe-Al intermetallic is attractive forapplications requiring high-damage resistance[22] due toits excellent oxidation and corrosion resistance, lowdensity, and low cost.[23–26] However, room temperaturebrittleness limits its application to industries. Althoughmany efforts have been made to improve room temper-ature ductility by additions of alloying elements andheat treatment, advancements have been limited.[27–29]

Therefore, it is proposed that a FGM combining Fe-Alintermetallics with Fe-based alloys such as steel wouldachieve high-corrosion resistance on the intermetallicside and high ductility on the opposite side, with asmooth transition of properties through the intermedi-ate material.According to the Fe-Al binary diagram (Figure 1),

two Fe-Al intermetallic phases, Fe3Al and FeAl, existfor Al content under 50 at pct. Both of these phases havedesirable mechanical properties and corrosion resistanceunder 773 K (500 �C).[30,31] To date, research on ironaluminides has focused mainly on mechanical properties

CHEN SHEN, Ph.D. Candidate, ZENGXI PAN, Senior Lecturer,DOMINIC CUIURI, Senior Research Fellow, JON ROBERTS,Undergraduate Student, and HUIJUN LI, Professor, are with theFaculty of Engineering and Information Sciences, School of Mechan-ical, Materials &Mechatronics, University of Wollongong, NorthfieldsAvenue, Wollongong, NSW 2522, Australia. Contact e-mail: [email protected]

Manuscript submitted June 4, 2015.Article published online November 23, 2015.

METALLURGICAL AND MATERIALS TRANSACTIONS B VOLUME 47B, FEBRUARY 2016—763

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such as room temperature ductility and high-tempera-ture creep resistance of the material with consistentcomposition, while few research results have beenpublished for the compositionally gradient material.Although compositionally gradient Fe-FeAl ingots havebeen produced by using layered powder metallurgy,these methods are still not able to fabricate samples withfull density and continuous phase and compositiontransitions.[22]

The purpose of the present study is to fabricate acompositionally graded Fe-FeAl FGM using theWAAM process with a designed composition gradient,and furthermore to investigate the material and mechan-ical properties of the buildup wall. Section II describesthe experimental setup for the WAAM process and thesample specifications. Section III presents the results forvarious mechanical and material tests. Section IV pro-vides a discussion of the test results, followed byconclusions in Section V.

II. EXPERIMENTAL SETUP

A. Wire-Arc Additive Manufacturing Process

A commercial GTAW inverter power source and amatching tungsten torch are used for the WAAMsystem. Two wire feeders are designed to feed both the0.9-mm diameter 99.5 pct purity annealed iron wire andthe 0.9-mm diameter 1080 grade aluminum wire into asingle welding pool to achieve in situ alloying. The wirefeeders have independent speed controls to achieveFe-FeAl FGM with varying composition ratio. Theangle between two wire feeding nozzles is approximately60 deg, and the angle between each nozzle and substratesurface is 30 deg to ensure the stability of melting poolduring double wire feeding, as shown in Figure 2. Thearc length is set to approximately 3.5 mm and a weldingcurrent of 140 A is used to produce a stable andconcentrated arc with sufficient welding heat input. Theinert gas shielding is achieved by pure argon. In addition

to the shielding provided by the GTAW torch itself, atrailing argon shield is applied with a flow rate of 9 L/min. The additional trailing shielding gas continues toflow during the manufacturing process and approxi-mately one extra minute after the arc is extinguished tominimize oxidation. 5.5 mm DH36 low-carbon ship-building steel was chosen as the substrate due to its goodweldability, to ensure the stability of the depositionprocess in the first few layers.The detailed parameters used for Fe-FeAl FGM

fabrication are listed in Table I. The Al compositiongradient of the buildup wall is designed to increasefrom 15 to 50 at. pct, with 5 at. pct increments afterevery four layers. A set of two 55 at. pct Al layers at thevery top of the deposit has been added to ensure that thefinal Al content reaches 50 at. pct, since theoretically thedilution from substrate can continuously influence theAl content in the buildup layers.All layers are deposited with an interpass temperature

of 673 K (400 �C) to avoid cracking defects that wouldotherwise result from the low-temperature brittleness ofFe-Al intermetallics.[33] The control of the interpasstemperature is achieved by clamping the substrate over aheating blanket placed in a thermal insulating box. Twopairs of thermal couples, one for the heating equipmentand one for the computer, are attached 20 mm from theadditive manufacturing position to measure the temper-ature. To achieve a stable molten pool and completemelting of the feed wires, the specific deposition energyis maintained at approximately 20 ± 1 kJ/g by adjustingthe wire feed speeds for both Fe and Al wire feeders, andmaintains the travel speed at 95 mm/min.

B. Material Tests and Characterizations

The sample cross sections and tensile specimens arecut from the buildup wall using the wire electricaldischarge machining (EDM) process, due to thelow-temperature brittleness of the Fe-Al intermetallicsprecluding other common cutting processes. The loca-tions of the samples within the deposited wall are shownin Figure 3(a). The etchant used for optical microstruc-ture inspection is 50 pct aqua regia with approximately3 seconds etching time. Vickers microhardness is mea-sured with a Struers DuraScan automatic hardnesstesting machine at both the vertical (for compositiongradient through the buildup wall) and horizontal (forcomposition uniformity in the layer sections) directionsacross the buildup wall sample using a 1 kg load and1-mm step size with 7 seconds indentation time for eachtesting point.The chemical composition of the buildup wall is

measured using a JEOL� JSM-6490LA scanning elec-tron microscopy (SEM) equipped with an energy-dis-persive X-ray spectrometer (EDS) operating at 20 kV.The composition testing points are located approxi-mately 50 lm laterally from the hardness testing posi-tions. The distribution of the hardness and compositiontesting lines are marked in Figure 3(b). The test alongthe vertical centerline is performed to measure thechemical composition gradient of the material. The testsalong the transverse lines (L1 to L6) are used to assessFig. 1—Fe-Al binary diagram.[32]

764—VOLUME 47B, FEBRUARY 2016 METALLURGICAL AND MATERIALS TRANSACTIONS B

the chemical composition homogeneity within the var-ious layers. Phase characterization is identified using aGBC� MMA X-ray diffractometer (XRD) with CuKaradiation (k = 1.5418 A); scanning angle (2h) is setfrom 20 to 100 deg with a 2 deg/min scanning speed. TheXRD tests are carried out on all the tensile specimens todetermine the phase change along the compositiongradient. The gage section of the tensile specimensalong the longitudinal direction is 10 mm 9 2 mm9 1.5 mm. The specimens are tested at room tempera-ture with a MTS370 load unit at a 5 9 10�2 s�1 strainrate. Subsequently, SEM images of the fracture surfacesare acquired for the fracture analysis.

III. EXPERIMENTAL RESULTS

A. Morphology

The macrostructure of the cross section in theas-fabricated buildup wall is shown in Figure 4. Thevariations of the grain morphologies can be observed, aswould be expected and in accordance with the designedcomposition gradient. Large columnar grains are pre-sent at the bottom and equiaxed grains in the uppersection, which are both iron-rich grains, and alu-minum-rich lump shaped grains are present at the verytop. The grains in the sample show epitaxial growthcaused by the layer-by-layer deposition process.

Fig. 2—Torch, wire feeder, and trailing shielding gas for the wire-arc additive manufacturing (WAAM) system.

Table I. Buildup Parameters

Al Content(at. pct)

Numberof Layers Current (A)

Travel Speed(mm/min)

Wire Feed Speed(mm/min)

Specific DepositionEnergy (kJ/g)Al Fe

15 4 140 95 254 900 20.9720 4 140 95 360 900 20.8525 4 140 95 481 900 20.4130 4 140 95 618 900 19.5935 4 140 95 776 900 19.8040 4 140 95 961 900 20.8345 4 140 95 786 600 20.9050 4 140 95 961 600 20.1655 2 140 95 1175 600 20.86


The microstructures with higher magnifications fromthe bottom section near the substrate to the top of thebuildup wall are shown in Figure 5. The bottom dilutionaffected region (Figure 5(a)) mainly contains carbon-in-duced acicular carbide precipitates, Fe3AlC0.5, whichnot only distribute in the grains but also in the grainboundaries.[34] As shown in Figure 5(b), the carbideprecipitates become less acicular in the upper layers inthe dilution affected region. The existence of the carbideprecipitates would form a precipitation strengthening tothe material and can be used to identify the dilutionaffected height, which is the main concern for compo-sition during the deposition.

Figure 5(c) shows the microstructure in the largecolumnar grains sized from 300 to 470 lm (measuredaccording to ASTM E112-10) in the middle section ofthe buildup wall. The acicular carbide precipitates havecompletely disappeared, which means that this section isnot influenced by dilution from the substrate. Incontrast to the large columnar grains in the bottomand middle section, the grains in the top section(Figure 5(d)) are equiaxed grains with the grain sizefrom 100 to 150 lm. The large difference in grain size isdue to the increased Al content and the phase changefrom DO3 structured Fe3Al to B2 structured FeAl.Figure 5(e) shows the grains near the boundary betweenFe-rich grains and aluminum-rich grains. In this region,while different from the previous equiaxed grains shownin Figure 5(d), the grain size does not significantlychange. The equiaxed grains near the boundary have anapparent stream-shaped pattern inside the grains whichindicates that the Al content almost reaches 50 at. pct inthese grains.[35]

The boundary between Fe-rich grains and alu-minum-rich grain is shown in Figure 5(f). A largeportion of fine oxide pits distributes along the bound-ary,[36] which makes the boundary very obvious in thelower magnification picture (Figure 5(h)). In the verytop region (Figure 5(g)), Al content reaches around50 at. pct. This is already the FeAl2 phase region in thebinary diagram. The microstructure mainly containssmall round grains in the grain boundaries and acicularwhite phase in the blocky large grains.

B. Phase Characterization and Chemical CompositionMeasurement

The results of XRD analysis for 11 tensile specimens(named S1 to S11 from the bottom to the top of thedeposited wall) and the top surface specimen (TS) areshown in Figure 6 for the investigation of phasevariation along the composition gradient. The tensilespecimens are used for XRD rather than the cross-sec-tional samples, because the results will be most accurate(or less ambiguous) when the structure is consistentthroughout the beam path for all measurement angles.The deposited structure and composition are inherentlymore homogeneous in the horizontal direction ofFigure 3(a), while the composition and phase constitu-tion are designed to change in the vertical direction.

Scanning the vertical cross-sectional samples will pro-duce pessimistic and unrepresentative results due to thebeam passing through material of changing composi-tion, depending on measurement angle.From Figure 6(a), the XRD result from the bottom

section of the buildup wall S1 (22.7 at. pct Al) showsthat it contains mainly the Fe3Al. In the twin peak, a-Fein (200) is observed which indicates this section ispartially dilution affected. In comparison to S1, the twinpeak has changed to the single (400) Fe3Al peak in theS2 (25.8 at. pct Al) pattern, and the appearance of theAl2O3 peaks implies a lack of inert gas protection inthese layers. The results from S3 to S6 are typical Fe3Aldiffraction patterns and match all four peaks in the03-065-4419 card. From S2 to S6, there is a decrease ofthe (400) Fe3Al peak with an increase of the (200) peak.This is due to the disappearance of the large columnarFe3Al grains during the increase of Al content.The peaks shown in S7 are characterized as the

B2-structured FeAl phase rather than Fe3Al phase asshown in the previous patterns, although the positionsof the peaks are fairly similar. Since the Al content in S7is 43.1 at. pct that is already in the FeAl region of thebinary diagram and the intensities of the peaks areapparently stronger than the ones in S6, this indicatesthe phase in this section has changed. The strong peaksin the rest of the patterns are characterized as the FeAlphase as well. Several Al2O3 peaks are detected in S9,S10, and S11, which are caused by the lack of inert gasprotection as in S2.The actual and designed composition gradient curves

along the centerline are shown in Figure 7(a). Theheight of each deposited layer (h) is estimated by theequation below.

h ¼ VFe þ VAl

vLw; ½1�

where, VFe is the volume of the melted Fe wire; VAl isthe volume of the melted Al wire; v is the travel speed;and Lw is the width of the deposited layer.Despite the Al content in the first few layers being

lower than the designed composition due to the influ-ence of dilution, the actual composition gradient is closeto the designed value. Rather than the designed stepcomposition curve, the actual composition shows asmooth transition due to the remelting process duringthe multi-layer deposition process.In addition, the chemical composition curves in the

transverse directions for different heights are drawn inFigure 7(b). In L1 and L2, the Al content on the leftside, where the iron wire was feed in, is lower than theright side, where the aluminum wire was fed in.However, in the middle sections of the buildup wall(L3, L4, L5), the Al content becomes more homoge-neous on both sides. In the very top section (L6), wherethe Al content has reached over 50 at. pct, the Alcontent becomes more inconsistent in the transversedirection as the remelting or reheating processes are notenough to homogenize the chemical compositions in thelast couple of layers.


C. Hardness and Tensile Tests

As shown in Figure 8, due to the phase variationinduced by the compositional gradient, the hardnessalong the vertical centerline of the buildup wall has alarge variation from 140 Hv, the average hardness ofDH36 substrate, to 650Hv near the top surface. It isevident that the increasing aluminum content producesan increase in hardness. In the region with an Al contentfrom 20 to 30 at. pct, there is a significant increase inhardness from about 250 Hv to over 350 Hv due to thevariation from large columnar-grained Fe3Al to equiax-ed-grained FeAl structure. In Figure 8, the slopes of the

hardness curve between 30 and 50 at. pct Al and over 50at. pct Al are very different. This implies another phasevariation from B2-structured FeAl to aluminum-richphases which have already been observed by themicrostructures shown in Figure 5(h).The tensile testing results shown in Table II indicate

the extremely low ductility of the buildup wall at roomtemperature due to its intermetallic microstructure. Thetensile measurement of specimens S9 and S10 (extractedfrom the top section) is not available, since thesespecimens fractured prior to loading. The relativelyhigh strength and elongation of S1 in comparison to S2are caused by the carbide precipitate at the bottomregion, as shown in Figures 5(a) and (b), which inducesprecipitation strengthening in the material and stops thefracture growth inside the grains. S5 has the bestmechanical properties among all specimens showingthe highest ultimate tensile strength (UTS), 0.2 pct offsetyield strength (0.2 pct YS), and elongation. At higher Alcontent, the mechanical properties of the specimens startto decrease rapidly. Specimen S11 shows almost noductility and low strength at the top surface.As shown in Figure 9, the fracture surfaces of the

tensile specimens exhibit mostly brittle transgranularlamellar fractures with stream patterns. No dimples areobserved throughout the sample. In S10 and S11,fracture surfaces without stream pattern are observedas the Al content has reached over 50 at. pct.

IV. DISCUSSIONS

Fe-Al intermetallic is appealing for the applicationsrequiring high-damage resistance due to its excellentoxidation and corrosion resistance property. Therefore,Fe-FeAl FGM combining Fe-Al intermetallic and steelis a promising material for applications which requirehigh-corrosion resistance on one side and adequatestrength on the opposite side such as cladding or coatingfor conventional engineering alloys.[37] For the buildupwall fabricated using the WAAM process with in situalloying, the experimental results show that the actualcomposition gradient is quite close to the designed value

Fig. 3—Sample preparations: (a) extraction positions; (b) hardness and composition testing lines on cross-sectional sample.

Fig. 4—The macrostructure of the cross section.


(Figure 7(a)), producing the expected high hardness andlow ductility values at room temperature. The bottomsection of the wall shows columnar grains with carbide

precipitate phases. The expected solution strengtheninginduced by the carbide precipitates is observed in thetensile testing results. The strength of S1 near the

Fig. 5—The microstructures in different sections (small black dots are etching pits): (a) dilution affected region near the boundary between build-up wall and substrate; (b) dilution affected region with less carbide precipitates; (c) large columnar grain in middle section; (d) equiaxed grains intop section; (e) equiaxed grains near the boundary between iron-rich grains and aluminum-rich grains; (f) the boundary between equiaxed grainsand lump shaped grains; (g) lump shaped grains near the top surface; (h) low magnification image of the boundary.


Fig. 6—XRD results: (a) S1 to S6; (b) S7 to the top surface.

Fig. 7—EDS results: (a) vertical centerline; (b) transverse lines.

Fig. 8—Hardness along the vertical centerline.

Table II. Tensile Testing Results at Room Temperature

TensileSpecimen

UTS(MPa)

0.2 pctYS (MPa)

Elongation(pct)

Al Content(at. pct)

S1 145.2 135.7 1.43 22.7S2 83.8 70.3 1.05 25.8S3 80 71.5 0.83 28.1S4 166.3 146.5 1.23 31.0S5 314.6 230.2 4.49 36.1S6 270.4 227.3 2.35 39.7S7 268.1 214.8 2.3 43.1S8 188.7 179.4 1.7 45.0S9 N/A N/A N/A 46.0S10 N/A N/A N/A 47.1S11 39.5 34.2 0.28 49.1


Fig. 9—Fracture surfaces of the tensile specimens.


substrate is much higher than S2 and S3 on thecenterline hardness curve, and this strengthening isoverborne by the increased aluminum hardening alongthe buildup. Also, the higher ductility shown in S1indicates the effects of the acicular phase Fe3AlC0.5,which reduces the crack sensitivity of the material.[38]

The lower Al content than designed in this section iscaused by the dilution from the DH36 steel substrate asthe carbide precipitates are observed, and both a-Fe andFe3Al phases are detected in S1 according to thediffraction peaks. The height of the dilution affectedarea can be reduced by decreasing the arcing current sothat less material of the substrate would go into thebuildup wall, since the penetration depth of the weldpool would be reduced. However, this would reducedeposition energy input and cause larger compositiondifferences in the transverse direction as shown in L1and L2 of Figure 7(b). In addition, the width of the weldpool would be reduced, resulting in a narrower walldeposit near the substrate. There are limitations to thechanges that can be made to the process parameters, andthese would be limited by the particular application.

The imbalanced Al content in the transverse directionhas moderated in the middle section as shown at L3 andL4 in Figure 7(b), particularly when the variation isevaluated as a fraction of the average content. Also, thematched curves of actual composition and designedcomposition (Figure 7(a)) imply the disappearance ofdilution influence. In this section, the shape and phase ofthe grains show evident variations with the increase ofAl content: from large columnar Fe3Al grains toequiaxed FeAl grains. This phase variation has alsobeen detected by the XRD patterns (from S6 to S7) inthis section. Although the fracture surfaces are still thelamellar transgranular fractures, S5 shows the highestUTS, 0.2 pct YS, and ductility among all specimens.This is due to the relatively smaller grain size than thelarge columnar Fe3Al grains in the lower near-substratezones, and also due to less Al content than the upperzones with increasingly pure FeAl. The Al content of S5is 36.1 at. pct, which is at the dashed boundary betweenFe3Al and FeAl of the binary diagram as shown inFigure 1.

In the top section of the buildup wall, the hardness ofthe material approaches 650 Hv and the tensile speci-mens show almost no ductility at room temperature. Incomparison to the specimens in the lower section, thefracture surfaces of S9 and S10 show less streampatterns and the fracture of S11 is completely ceramic-like brittle fractures without lamellar stream patterns.Besides the inferior mechanical properties, this sectionalso has an inhomogeneous chemical composition asshown at the L6 in Figure 7(b) due to the lack ofsubsequent remelting and reheating processes. This maybe corrected by applying heating-only weld passes afterthe last deposition layer, but further experimentation isneeded to confirm that there are no detrimental effectsfrom this additional processing step. In the diffractionpatterns of S9 and S10, Al2O3 peaks are detected whichimplies the lack of inert gas protection during thedeposition processes. This is due to the relatively poorershielding condition at the top of the buildup wall, where

the base plate and trailing shielding box form arelatively large gap and some air would possiblycontaminate the deposited material. The existence ofAl2O3 could be the reason for premature fracture of S9and S10 samples during the tensile tests at very lowloadings. Therefore, providing sufficient inert gas pro-tection is critical for the successful production of Fe-AlFGM with oxidation kept under a limited level.

V. CONCLUSIONS

This study has investigated the feasibility of fabricat-ing functionally gradient iron aluminide structures usingthe WAAM in situ alloying process. The experimentalresults demonstrated that the designed chemical com-position in the buildup wall can be accurately achievedby adjusting the ratio of the wire feed from iron andaluminum wires. The fabricated buildup wall contains acontinuous composition gradient in the vertical builddirection from 100 pct steel substrate to over 50 at. pctAl content. The chemical composition is generallyhomogeneous in the transverse direction, but is lessconsistent in both the dilution affected region at thebottom of the wall and in the final layers at the uppersurface. The mechanical properties throughout thedeposited wall have been measured, and the specimenshave shown values of room temperature strength andductility similar to those found in previous studies. Sincethe corrosion resistance property is critical for theapplication of this material, future study will focus onthe corrosion mechanism of the Fe-FeAl FGM.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financialsupport from China Scholarship Council (CSC),University of Wollongong (UOW), and Welding Tech-nology Institute of Australia (WTIA), and use of thefacilities within the UOW Electron Microscopy Center.

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