SOLID FREEFORM FABRICATION

Qualification of Ti6Al4V ELI Alloy Produced by Laser PowderBed Fusion for Biomedical Applications

I. YADROITSEV,1,4 P. KRAKHMALEV,2 I. YADROITSAVA,1

and A. DU PLESSIS3

1.—Department of Mechanical and Mechatronic Engineering, Central University of Technology,Free State, Bloemfontein 9300, South Africa. 2.—Department of Engineering and Physics, Karl-stad University, 651 88 Karlstad, Sweden. 3.—CT Scanner Facility, University of Stellenbosch,Stellenbosch 7602, South Africa. 4.—e-mail: iyadroitsau@cut.ac.za

Rectangular Ti6Al4V extralow interstitials (ELI) samples were manufacturedby laser powder bed fusion (LPBF) in vertical and horizontal orientationsrelative to the build platform and subjected to various heat treatments. De-tailed analyses of porosity, microstructure, residual stress, tensile properties,fatigue, and fracture surfaces were performed based on x-ray micro-computedtomography, scanning electron microscopy, and x-ray diffraction methods. Thetypes of fracture and the tensile fracture mechanisms of the LPBF Ti6Al4VELI alloy were also studied. Detailed analysis of the microstructure and thecorresponding mechanical properties were compared against standard speci-fications for conventional Ti6Al4V alloy for use in surgical implant applica-tions. Conclusions regarding the mechanical properties and heat treatment ofLPBF Ti6Al4V ELI for biomedical applications are made.

INTRODUCTION

The mechanical properties, heat treatments, andprocessing of conventional Ti6Al4V are well docu-mented in standards for biomedical applications(ASTM F1108-14, F147208, F136-13, F620-11,ISO20160:2006) (Supplementary Fig. S1). Thechemical composition requirements for additivelymanufactured (AM) alloys coincide with those forconventional materials (Supplementary Table SI).For wide application in the medical industry, it iscrucial that laser powder bed fusion (LPBF)Ti6Al4V extralow interstitials (ELI) implants com-ply with international standards regarding theirmicrostructure and mechanical properties.

This investigation is dedicated to systematicanalysis of the defects, microstructure, andmechanical properties of LPBF Ti6Al4V ELI,making use of horizontal and vertical samples inas-built and heat-treated conditions. The impor-tance of heat treatment to achieve LPBF materialswith designed complex properties is alsoillustrated.

MATERIALS AND METHODS

Spherical argon-atomized Ti6Al4V ELI powderswith the chemical compositions and size distribu-tions indicated in Tables SI and SII were used.Vertical and horizontal rectangular blocks withdimensions of 10 mm 9 10 mm 9 60 mm were pro-duced directly on Ti6Al4V substrates using anEOSINT M280 system at volume rate of 5 mm3/sfor powder layer thickness of 30 lm with argon asprotective atmosphere and oxygen level in thechamber of 0.07–0.12%.

The first set of as-built blocks were cut off fromthe substrate for further tests without heat treat-ment. The second and third sets of specimensremained on the substrate and were heat treatedin Ar atmosphere at 650�C for stress relief, then cutoff. The third set was additionally annealed in avacuum furnace at 950�C (Supplementary Fig. S2).

Round specimens with threaded ends weremachined from the rectangular blocks according toASTM E8M standard (gauge length four times thediameter). Tensile tests were performed using an

JOM

DOI: 10.1007/s11837-017-2655-5� 2017 The Minerals, Metals & Materials Society

Instron 1342 servohydraulic testing machine withclip-on extensometer of 12.5 mm under strain rateof 0.5 mm/min and 1.5 mm/min after removal of theextensometer.

To investigate defects and porosity, the sampleswere subjected to x-ray micro-computed tomogra-phy (microCT) with resolution of 10 lm.1

RESULTS AND DISCUSSION

Porosity

MicroCT scans showed that the vertical andhorizontal as-built LPBF samples had very lowlevels of porosity (0.0004% and 0.0018%, respec-tively). The maximum detected pore size was205 lm (Supplementary Table SIII). The pores werefairly randomly distributed (SupplementaryFigs. S3, S4).

Microstructure

In LPBF of Ti6Al4V, the material solidifies athigh cooling rates (about 106 K/s), resulting information of metastable structures of a¢ hexagonalmartensite with pronounced prior b-texturization inthe building direction (Supplementary Fig. S5).Prior b-phase showed columnar growth with highlypronounced h100i texture, whereas the texture ofthe hexagonal a¢ phase was quite weak.2 Under thepresent conditions, decomposition of nonequilibriumphases did not occur in the as-built and stress-relieved samples. X-ray diffraction (XRD) andtransmission electron microscopy (TEM) analysisconfirmed needle-like structure without any b pre-cipitations. The thermomechanical history of LPBFTi6Al4V also resulted in the presence of 101�2

� �-

type twins in the as-built and stress-relieved sam-ples. This type of twins in hexagonal close-packed(hcp) materials is often reported as tensile twins,and such twinning could be a possible mechanismfor accommodation of thermal stresses duringmanufacturing.3

The columnar boundaries of prior b-phase wereobserved in as-built specimens, and all specimensafter heat treatments. In the annealed specimens,they were also clearly visible, although the marten-sitic needle-like microstructure was decomposed(Supplementary Fig. S6). Decomposition of a¢-martensite is accompanied by nucleation of fine aprecipitates at martensite plate boundaries, enrich-ment of the surroundings with b-stabilizers, andfinally, formation of equilibrium a + b phase mix-ture.4 Nuclei of equiaxial a-phase were found inglobular phase enriched by a-stabilizing Al, beingunevenly distributed in the microstructure. In theheat-treated specimens, no continuous alpha net-work at prior b-grain boundaries was found. Thistype of microstructure corresponds exactly to stan-dards for biomedical applications required forannealed Ti6Al4V (ISO 20160, 2006).

Tensile Properties

Tensile tests were performed to investigate theinfluence of heat treatment on the mechanicalcharacteristics of the specimens manufactured inhorizontal and vertical directions. The modulus ofelasticity did not vary significantly among thespecimens and lay in the range of 110–119 GPa.The dispersion of data points in the tensile dataaround the mean value was more pronouncedamong the horizontal samples (Fig. 1). The as-builtsamples exhibited higher ultimate tensile strength(UTS) compared with the stress-relieved andannealed specimens. Higher strength and lowerductility of as-built samples can be explained bypresence of residual stresses and fine martensiticstructure.5 High strength characteristics are typicalfor as-built LPBF Ti6Al4V.2,4,6 After stress-reliefheat treatment, the UTS decreased by 6–8% fromthe initial, as-built level. The yield strength (YS)after stress-relieving heat treatment did not changein comparison with as-built samples. Annealing ledto significantly improved ductility of LPBF Ti6Al4V.Elongation at break was increased up to 20%, andreduction of area reached values above 46%. Thesechanges in properties resulted from changes in themicrostructure, which became close to conventional.

Fatigue Properties

To investigate the influence of surface roughness,heat treatment, and microstructure on the fatigueperformance, three-point bending fatigue tests werecarried out at room temperature. The specimen wasoriented so that the top or side surface was sub-jected to the highest tensile stresses, while the otherthree surfaces were machined and ground(Table SIV). As a reference, one series of as-builthorizontal specimens was tested after machiningand grinding of all surfaces (Rz � 5 lm). Due to thelimited numbers of specimens available, fatiguelimit tests were not done; instead, the fatigue life ofmaterials was estimated at one maximum stressvalue of 700 MPa.

The highest roughness values were measured onthe side surface (Rz � 120–135 lm), and, accord-ingly, these specimens demonstrated shorter life tofailure. Higher roughness implies deeper valleys onthe surface, which can be interpreted as notches.Stress concentration in notches leads to acceleratedcrack nucleation. The top surface (Rz � 30–60 lm)demonstrated slightly better performance, whilemachined specimens (Rz � 5 lm) demonstratedthe best performance and the highest number ofcycles to failure (N). Residual stresses contributedsubstantially to fatigue life (Fig. 2). Stress reliefresulted in an increase in the number of cycles tofailure in vertical specimens. In the horizontalspecimens, the effect was opposite, and specimensafter stress relief showed lower number of cycles tofailure. This unexpected result is difficult to explain.

Yadroitsev, Krakhmalev, Yadroitsava, and Du Plessis

Such a decrease in the number of cycles to failurecould be a result of the complex distribution ofstresses in the sample.5 Nevertheless, this effectrequires further investigation with more statisti-cally relevant numbers of samples at differentmaximum stress values.

The crack propagation rate was investigatedbased on electron microscopy observations of stria-tions on fracture surfaces (Fig. 2). Unlike previousreports,7,8 the crack penetration behavior was foundto depend on specimen orientation. The influence ofresidual stresses on the crack propagation behavior,according to the data obtained in this study, wasnot great. It was observed that parameters formaterials manufactured with the same orientationin as-built and stress-relieved conditions variedinsignificantly.

Specimens manufactured horizontally but testedwith the highest stresses at the side surface showedthe steepest slope of the linear part of the da/dNcurve compared with horizontal samples tested at

the top surface, or vertical specimens. These differ-ences in crack propagation rate can be explainedbased on the interaction of the crack withmicrostructural features and interfaces in the LPBFmaterial. For specimens manufactured horizontallyand tested at the top surface, the layers have crackarrester orientation (Supplementary Table SIV).Interaction of a crack with internal interfaces cantherefore result in crack deviation, thus decreasingthe crack propagation rate. In horizontal specimenstested at the side surface, layers have crack dividerorientation, therefore resisting crack growth moreefficiently. The lowest crack propagation rate wasobserved for vertical specimens, for which interlayerinterfaces were in-plane with the crack growthdirection.

This behavior can be explained by assuming thatcolumn boundaries of prior b-phase also resist crackpropagation. In the horizontal specimens, the resis-tance of these boundaries is not very high, as thecrack grows along boundaries. Therefore, interlayer

Fig. 1. Tensile mechanical properties of Ti6Al4V ELI samples.

Fig. 2. Fatigue crack propagation rate and cycles to failure of LPBF Ti6Al4V ELI.

Qualification of Ti6Al4V ELI Alloy Produced by Laser Powder Bed Fusion for BiomedicalApplications

boundaries are more important in specimens testedwith this orientation. In vertical specimens, thecrack penetrates perpendicularly to the boundariesof columnar prior b-grains. In these specimens, them value is the lowest; i.e., the crack propagationrate is also low. These specimens showed the lowestnumber of cycles to failure, and the observed crackbefore the final failure occurred was shorter com-pared with the horizontally manufactured speci-mens. Thus, prior b-grain boundaries seem to beefficient in terms of resistance to fatigue crackpropagation, but the material showed the lowestfatigue life.

Fracture Surface Analysis

The fracture surfaces after tensile testing ofspecimens in as-built, stress-relieved, and annealedconditions showed cup-and-cone shapes and obviousnecking. In the annealed samples, necking wasmore pronounced. Necking is commonly associatedwith ductile fracture. The central area, commonlydescribed as a region of fibrous fracture, was clearlydistinguished, apart from the shear lips on theperiphery. The surface in the fibrous zone wasirregular (Supplementary Fig. S7). SEM analysisrevealed formation of dimples, suggesting dimpledrupture fracture in the fibrous zone. Althoughdimpled rupture was dominant, quasi-cleavagefacets were observed on both as-built and stress-relieved samples. Analysis at high magnificationrevealed martensite needles on the quasi-cleavagesurfaces of the as-built and stress-relieved samples,confirming that these regions formed when thecrack propagated along a martensite colony. Whenthe crack reached a primary b-grain boundary orfusion boundary, the growth direction changed. Theinterface therefore acted as a crack deflector, pre-venting quick failure. This conclusion agrees withresults presented in Ref. 9. In annealed samples,features of brittle fracture such as quasi-cleavagesurfaces were not found. The dimples from the innerfracture zone were more pronounced, with largersize and shape elongated in the direction of the load.

Cup-and-cone morphology is typical for fracturecontrolled by pore coalescence mechanisms. Theprocess of pore formation and coalescence wasinvestigated by means of micro-computed tomogra-phy. The as-built and stress-relieved specimenswere prestrained to 1.57–9.44%, and the pore sizeswere investigated. MicroCT scans showed porecoalescence under loading in the prestrained sam-ples; these pores looked like agglomerates of severalpores (Fig. 3a). Some pores were interconnected viaquite thin channels, possibly representing the ini-tial stage of formation of cracks or new pores. As thespecimen was strained, pore coarsening becameclearly pronounced and became dominant in thenecking area (Fig. 3b). The number of pores identi-fied by microCT scans significantly increased in thisregion. Analysis of the cumulative frequency

distributions of equivalent pore diameters in pre-strained, as-built, and stress-relieved samplesrevealed that pores became larger with increasingstrain. Most of the new pores were observed in theneck region, and they act as nucleation points forcracks, leading to final failure.3,10

Influence of Microstructure on Fracture Mode

To observe the influence of building direction, i.e.,microstructure and orientation of prior b-grains, onthe mechanical behavior of LPBF Ti6Al4V, longitu-dinal cross-sections of broken samples were inves-tigated. In the horizontal specimens, two cross-sections were made, one along and one across thebuilding direction. In all specimens, narrowing andelongation of prior b-grains was observed in theneck region.

In the horizontal specimens that were testedperpendicular to the building direction, a crack grewrather in intergranular mode, following the bound-aries of prior b-columns. Crack propagation wasobserved along the boundaries of prior b-columns andalso through the columnar grains, apparently paral-lel to the lamellae visible in the microstructure(Supplementary Fig. S8a, b). The presence of themixed fracture mode could explain the experimentalobservation of quasi-cleavage and dimpled fracturemodes simultaneously. In the vertical specimens,only intragranular fracture was observed in thecross-sections (Supplementary Fig. S8c). Grainboundaries represent weak points in the microstruc-ture where cracks can propagate more easily.2

Apparently, the presence of long prior b-grain bound-aries perpendicular to the loading direction could bethe reason for the lower ductility observed experi-mentally for the horizontal specimens.

After stress relief, there were no differences inmechanical properties among the vertical and hori-zontal samples.The mixedfracture modewasobservedfor horizontal specimens (Supplementary Fig. S9),similar to the case of as-built samples. Narrowing andelongation of priorb-grains in a stress-relieved verticalsample is presented in Fig. S9b.

Heat treatment at 950�C led to changes in themicrostructure and mechanical properties. Marten-sitic needle-like structure transformed to a-phaselamellae with some b-phase. Only very small(� 0.5 lm) globular grains of a-phase were foundafter stress relief.3 It is notable that, after anneal-ing, prior b-phase columns boundaries were stillrecognizable in the microstructure (SupplementaryFig. S6). After loading of heat-treated specimens, noindication of brittleness was observed at the frac-ture surface and grains were well plasticallydeformed (Supplementary Fig. S10). The results ofthe fracture surface analysis allow speculation thatcrack propagation occurred along a-lamellae or a–binterfaces. The influence of prior b-grain boundarieswas not as great as was observed for the horizontalas-built specimens.

Yadroitsev, Krakhmalev, Yadroitsava, and Du Plessis

Observation of cross-sections near the neck regionin as-built specimens and after heat treatmentsrevealed that nucleation and coalescence of poreswas the main mechanism governing the failureunder tension. Pores observed in the as-built andstress-relieved materials were elongated and mostlylocated between a¢-martensite needles or on colonyboundaries (Supplementary Figs. S8, S9). Stress

relief did not change the pore nucleation sites;therefore, it is possible to conclude that the porecoalescence was related to anisotropy in themicrostructure. In the annealed material, poresnucleated at a-lamella boundaries and grew undertension (Supplementary Fig. S10). These micro-scopy observations are in very good agreement withthe microCT investigations described above. It was

Fig. 3. Reconstruction of porosity in as-built sample deformed to 8.2% from initial length (a) and necking creation in stress-relieved sampledeformed from 3.55% to 9.44% from initial length (b). Defect analysis shows color-coded porosity distribution in transparent three-dimensional(3D) views. Inset shows colour-coded thickness variation, clearly indicating the necking region.

Qualification of Ti6Al4V ELI Alloy Produced by Laser Powder Bed Fusion for BiomedicalApplications

not the pores existing in the materials after man-ufacturing that resulted in the final failure; rather,new pores nucleated, grew, and coalesced to createthe final crack that led to failure.

CONCLUSION

Using LPBF, high density can be achieved bychoosing optimal process parameters and scanningstrategy. In the present study, for Ti6Al4V ELIsamples, the density measured by microCT wasabove 99.99% (for pores> 30 lm). The typical poresize in as-built material ranged from 30 lm to200 lm. MicroCT scans and cross-sectioning micro-scopic analysis revealed that the largest pores werepredominantly elongated in shape and could beconsidered as interlayer pores.

LPBF manufacturing leads to high residualstresses in as-built objects and unique microstruc-ture due to high cooling rates. Ti6Al4V ELI as-builtmicrostructure consisted of hexagonal a¢ martensiticneedles. No b precipitations were found in the as-built samples or after stress-relief heat treatment.Annealing resulted in decomposition of a¢ marten-site and transformed the microstructure to a + bphase mixture with small equiaxial grains of aphase randomly distributed in the volume.

As-built samples fabricated in vertical and hori-zontal directions showed high strength(� 1250 MPa) and relatively low ductility (� 10%)due to the presence of residual stresses and very finemartensitic structure. After stress-relief heat treat-ment, the UTS decreased by 6–8% while the YTSremained unchanged. Annealing led to a significantimprovement in the ductility; the elongation of breakincreased up to 20%. The modulus of elasticity variedinsignificantly, ranging from 110 GPa to 119 GPa.

Investigations on the development of the porosityin as-built and heat-treated samples using inter-rupted tensile tests revealed that nucleation andcoalescence of new pores, instead of growth ofexisting pores in the necking region, was the mainmechanism causing fracture.

The bending fatigue properties of horizontal spec-imens revealed a correlation between the number ofcycles to failure and the surface roughness. Speci-mens with rougher surface demonstrated decreasednumber of cycles to failure. The crack propagationrate was found to depend on the specimen orientationbut insignificantly on residual stresses.

Cup-and-cone fracture was observed in specimensin as-built, stress-relieved, and heat-treated condi-tions. A mixed fracture mode with features ofductile dimpled rapture and brittle quasi-cleavagesurfaces was found in as-built and stress-relievedsamples. In annealed condition, the fracture modewas ductile.

Analysis of the influence of the building directionon the fracture modes showed that, in horizontalspecimens, intergranular (along prior b-grainboundaries) and intragranular (along lamellae in acolony) fracture occurred. In vertical samples, onlyintragranular type of fracture was found. Bound-aries of prior b-grains initiating intergranular frac-ture could therefore be a reason for the decreasedductility observed in tension tests of horizontalspecimens.

LPBF Ti6Al4V ELI has a specific microstructureand mechanical properties, which are comparable oreven superior to those of conventional material.Thus, there is a clear need to accelerate thedevelopment of new standards to provide clearquality indicators for use of this type of materialin biomedical applications.

ACKNOWLEDGEMENTS

This work is based on research supported by theSouth African Research Chairs Initiative of theDepartment of Science and Technology and Na-tional Research Foundation of South Africa (GrantNo. 97994) and the Collaborative Program in Addi-tive Manufacturing (Contract No. CSIR-NLC-CPAM-15-MOA-CUT-01).

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The online version of this article (https://doi.org/10.1007/s11837-017-2655-5) contains supplemen-tary material, which is available to authorizedusers.

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Yadroitsev, Krakhmalev, Yadroitsava, and Du Plessis