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Microstructures and mechanical properties of electronbeam-rapid manufactured Ti–6Al–4V biomedical prototypescompared to wrought Ti–6Al–4V
L.E. Murra,⁎, E.V. Esquivela, S.A. Quinonesb, S.M. Gaytana, M.I. Lopeza, E.Y. Martineza,F. Medinac, D.H. Hernandeza, E. Martineza, J.L. Martineza, S.W. Stafforda, D.K. Browna,T. Hopped, W. Meyersd, U. Lindhee, R.B. Wickerc
aDepartment of Metallurgical and Materials Engineering, The University of Texas at El Paso, El Paso, TX 79968, USAbDepartment of Electrical and Computer Engineering and Keck Center for 3-D Innovation, The University of Texas at El Paso, El Paso,TX 79968, USAcDepartment of Mechanical Engineering and Keck Center for 3-D Innovation, The University of Texas at El Paso, El Paso, TX 79968, USAdStratasys, Eden Prairie, MN 55344, USAeArcam AB, Mölndal, Sweden
Article history:Received 12 January 2008Received in revised form 22May 2008Accepted 18 July 2008
This study represents an exploratory characterization and comparison of electron-beammelted (EBM) or rapid manufacturing (RM) of Ti–6Al–4V components (from nominal 30 μmdiameter powder) with wrought products. Acicular α and associated β microstructuresobserved by optical metallography and electron microscopy (SEM and TEM) are comparedalong with corresponding tensile test and hardness data; including the initial powderparticles where the Vickers microindentation hardness averaged 5.0 GPa in comparisonwith the fully dense, EB manufactured product with an average microindentation hardnessranging from 3.6 to 3.9 GPa. This compared with wrought products where the Vickersmicroindentation hardness averaged 4.0 GPa. Values of UTS for the EBM samples averaged1.18 GPa for elongations ranging from 16 to 25%. Biomaterials/biomedical applications ofEBM prototypes in direct prosthesis or implant manufacturing from CT or MRI data arediscussed in the context of this work, especially prospects for tailoring physical propertiesthrough EB control to achieve customized and optimized implant and prosthetic productsdirect from CT-scans.
Amongst the more popular metallic biomaterials – stainlesssteels, cobalt-based alloys (Co–Cr–Mo) and Ti and Ti-basedalloys (especially Ti–6Al–4V) – utilized for hard tissue replace-ments such as artificial knee joints, hip joints, and bone plates,Ti–6Al–4V is of particular interest as a consequence of itsexcellent biocompatibility, lightweight (a density of 4.4 g/cm3),
r).
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excellent balance of mechanical properties, and associatedcorrosion resistance and human allergic response [1]. Mostconventional Ti–6Al–4V implants are fabricated by precisionCAD-driven machining of cast or wrought material, especiallyTi–6Al–4V ELI (Extra Low Interstitial) which has lower speci-fied limits of Fe and especially interstial elements O and C incomparison with commercial Grade 5 Ti–6Al–4V. Ti–6Al–4Vforms a spontaneous oxide film upon exposure to oxygen or
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water, and this feature in fact accounts for its excellentcorrosion resistance in the body as a biomedical implant.
Recent advances in rapid manufacturing utilizing metal oralloy powders allow for the layer-by-layer building of complex,functional parts designed in a 3D CAD program or createdfrom a CT-scan or microscan. Rapid manufacturing (RM) orrapid prototyping (RP) has been referred to as a “renaissance inmanufacturing”, especially for economies associated with thediversity of high-technology industries [2]. RM eliminates theconventional design for manufacturing in fabricating custo-mized, geometrically complex components with gradedmaterials compositions and/or properties which change withrequirements that propagate across scales. In freeform RMprocesses, powder particles (usually polymer or ceramicmaterials) are used to assemble or build these complex systemgeometries through computer-controlled, self-assembly ofpowder layers by sintering or melting.
Electron beam manufacturing is a type of RM or freeformfabrication technology for the direct manufacturing of metalproducts from a powder precursor melted layer-by-layer withan electron beam in vacuum. The electron beammelting (EBM)machine reads in data from a digitally scanned, 3D model,sliced into individual layers, and lays down successive, 100 μmthick metal powder layers which are progressively meltedthrough the controlled EB scanning process to build theproduct model. Quality products require the development ofa set of optimized processing conditions or parameters whichassure uniformity and control of microstructure and asso-ciated mechanical properties and performance. Especiallypromising directions for EBM involve the direct fabrication ofcustom orthopedic implants and related biomedical applica-tions, particularly involving the production of Ti–6Al–4Vcomponents with selected structure–property features [3].An important aspect of these biomedical applications is theability to emulate or improve upon more conventional cast orwrought metal or alloy precursor products.
In this study, we have systematically examined the evolu-tion and variations of microstructures, along with theirassociated mechanical properties (hardness, yield strength,UTS, elongation), for simple EBM built Ti–6Al–4V biomedicalprototypes, utilizing optical metallography and scanning andtransmission electron microscopy. This has included theexamination of the initial Ti–6Al–4V powder precursor as well.In addition, we have compared these observed and measuredstructure–property relationships with structure (microstruc-ture) — property relationships in forged (wrought) Ti–6Al–4Vproducts. Finally, we have also examined variations in the EB-built productmicrostructures, and used these observations andthemeasurements of attendantmechanical behavior to discussprospects for controlling and optimizing processing conditionsin EBM of biomedical products built from Ti–6Al–4V powder.
2.1. Biomedical Implant Building: The Electron BeamMelting System
The electron beammelting (EBM) system utilized in this studywas the ARCAM EBM S400. This system, represented schema-
tically in Fig. 1, allows solid parts to be directly manufacturedfrom metal or alloy powder. In this study, we built simple testcylinders measuring 6.8 cm in length and 1.2 cm in diameterfromGrade 5 Ti–6Al–4V powder having a nominal compositionshown in Table 1. The system builds layers (~100 μm thick)from the bottom up by selectively scanning the focusedelectron beam to melt specific areas of the powder bed usinga 3D CADmodel, while powder is continuously added from thepowder cassettes (4) to the top as shown in Fig. 1. The rakeshown at (5) in Fig. 1 moves laterally between the two powdercassettes (4) to distribute even powder layers over the surfaceafter each build layer is complete. As layers are completed, thebuild table (7) moves down. The entire build occurs in vacuum.
The EBM system is an electron optical system essentiallyidentical to an EB welding unit or a scanning electron micro-scope (SEM) where an electron gun (1) generates a focused EB(2) which can be systematically scanned (by deflection coils(3)) across the building part; directed by the CAD design. Theelectron gun operates nominally at 60 kV and can develop anenergy density in excess of 102 kW/cm2. However, beamcurrent as well as scan rate and scan sequence variations
Fig. 2 –Schematic protocol for microscopic analysis andhardness testing of EBM cylindrical prototypes shown inFig. 1.
Table 1 – Nominal chemical compositions for Ti–6Al–4V(wt.%)
See ASTM designation F136-82 (ASTM, Philadelphia, PA, 1994), 19–20.
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allow for a wide range of build parameters. In addition, thebuild table (7) is normally beam heated (in this study to~750 °C) and the actual build parameters in this study heatedthe cylindrical build (6) to between 620 °C and 690 °C. The buildparameters in this study allowed for microstructure–propertyvariations from the top to the bottom of the cylindrical samplein order to demonstrate, in a simple fashion, the ability totailor the performance features; in fact to systematicallyinfluence the structure–property–processing–performanceparadigm characteristic of modern materials science andengineering.
2.2. EBM–Ti–6Al–4V Sample Analysis
The cylindrical Ti–6Al–4V samples built by EBM as describedabove were analyzed using a protocol illustrated schemati-cally in Fig. 2. This protocol involved the slicing of thecylinders roughly 1 cm from the top and bottom, and cuttingthe remaining section in half. The top section cut from thecylinder was then mounted and examined in the scanningelectron microscopy while a slice (~2 mm) from the matchingtop section cut was mounted, ground, and polished forobservation by optical (light) microscopy. Sections for opticalmetallographic observations near the 1 cm end cuts and thecenter of the cylinders were also systematically tested forhardness using Vickersmicroindentation aswell as a Rockwellhardness tester. A similar, ~1 mm adjacent slice was preparedfor observation in the transmission electron microscope bygrinding to ~0.2 mm thickness and punching 3 mm diameterdiscs which were further electropolished to create electrontransparent thin sections.
Corresponding or similar cylindrical builds were fabricatedin the EBM system (Fig. 1) which were either slightly largerthan the 1.2 cm diameter by 6.8 cm length. These were used tomachine tensile specimens. However, for the second series oftest cylinders (which we designate EMB-2) two tensile speci-mens were machined directly from the 1.2 cm×6.8 cm cylin-ders. Special grips were fabricated to accommodate thesetensile specimens, alongwith similarly prepared tensile speci-mens from the two comparative wrought material (Ti–6Al–4V)samples. The details of these analytical protocols and testprocedures will be described in detail below.
2.3. Ti–6Al–4V Wrought Samples
Two different wrought samples of Ti–6Al–4V with compo-sitions characterized by the Grade 5 composition ranges forTi–6Al–4V shown in Table 1 were utilized in this study forcomparison with the EBM builds. One sample, designatedW-1
was a section from a billet forged at 1040 °C which is above thebeta transus to produce a coarse plate-like α with some inter-granular β. The second sample, designatedW-2, was a sectioncut from a billet forged and solution treated 1 h at 950 °C, air-cooled and then annealed 2 h at 700 °C. This produced anequiaxed, α/β microstructure very different from the largeacicular α-plates for W-1. Note that all samples W-1 and W-2were taken from the same billets, respectively.
These two wrought samples were examined through es-sentially the same analytical protocol used for the EBM testcylinders described above. In addition, tensile samples similarto those prepared from the EBM cylinders were prepared fromboth W-1 and W-2 designated wrought materials.
2.4. Optical Metallography
Sample coupons (slices) from the EBM cylindrical specimensas well as representative coupons from the wrought materialswere mechanically ground and polished to 1200 grit and finalpolished with 0.3 μm diamond paste. After polishing andrinsing in acetone and ethanol, samples were prepared foroptical metallography by etching with a solution consisting of100 mL H2O, 2.5 mL HF, and 5 mL HNO3. The etched sampleswere observed in a Reichert MEF4 A/M optical metallograph.
The starting powder utilized in the EBM builds was placedin standardmounting epoxy and ground, polished, and etchedas described above. This allowed the microstructure of thepowder to be examined by optical metallography.
2.5. Electron Microscopy
Samples polished and etched as described above for opticalmetallography were also observed directly in a Hitachi S4800field-emission scanning electronmicroscope (FESEM) utilizingsecondary electron (SE) imaging at an accelerating voltageof 20 kV. The starting Ti–6Al–4V powder was placed on aconducting tape and observed in the FESEM as well.
Sectionswere cut from the various sample coupons or frommating surfaces and ground and polished to a thickness of~0.2 mm. Standard 3 mm transmission electron microscope(TEM) discs were punched from these thinned sections,dimpled on both sides and electropolished using a Struers
Fig. 3 –Histogram of Ti–6Al–4V starting powder particle size (diameter) distribution from a series of FESEM images.
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Tenupol-5 dual-jet unit using a solution consisting of 0.9 Lmethanol to which 50 mL H2SO4 was added. The electro-polishing solution was cooled to −10 °C and the electropolish-ing voltage varied between 15 and 25 V at 5 A to observe thecharacteristic polishing plateau. These voltage conditionsvaried for the different microstructures and crystallographic(α+β) mixtures in particular. The resulting electron transpar-ent thin sections were then examined in a Hitachi H-8000analytical TEM at 200 kV accelerating potential; utilizing agoniometer-tilt stage.
2.6. Hardness Testing
Samples prepared for optical metallography as describedabove were examined in a Shimadzu HMV-2000 microinden-tation (Vickers) hardness tester (25 gf (0.25 N) load at 10 s). Thepowder samples embedded in mounting epoxy and polishedand etched for optical metallographywere similarly examinedby Vickers microindentation hardness testing using the 25 gfload. A minimum of 10 indentations were made and theVickers microindentation hardness (HV) values averaged.
Following Vickers microindentation hardness measure-ments, sufficiently thick specimens were tested using theRockwell C-scale hardness (HRC) (150 kgf (1.5 kN) load). In
Fig. 4 –FESEM view of an EBM sample (EBM-1). TS representsthe top surface of the build.
some cases thicker coupons were tested to insure accuratehardness indentation readings (HRC).
2.7. Tensile Testing
Tensile specimens were machined from the experimentalmaterials and tested in an Instron 500 R tensile machine usingspecial grips at an engineering strain rate of 3×10−3 s−1 at
Fig. 5 –FESEM view of EBM-1 sample normal to top surface(TS) (a) and optical metallographic view of a cross-sectioncorresponding to a section ~1 cm from the top surface (TS) in(a). Arrow designates outer particle surface region.
Fig. 6 –Optical metallographic views showing acicular,α-plate (Widmanstätten) microstructures at ~1 cm from thetop (a) and ~1 cm from the bottom (b) of an EBM-1 sample.Magnification is the same and shown in (a).
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room temperature (20 °C). Following tensile testing to failure,the fracture surfaces were observed in the FESEM.
Fig. 7 –Examples of TEM bright-field images of dislocationsubstructures in the α-plates of the EBM-1 sample section inFig. 10(a). (a) shows the microstructure of the α-plates withthe β boundary zones (black). (b) is a magnified view of anα-plate region with dislocation substructure.
3. Results and Discussion
3.1. EBM and Wrought Ti–6Al–4V Structuresand Microstructures
From a series of FESEM images of the starting sphericalparticles particle diameters were measured and plotted in ahistogram showing particle size distribution which is repro-duced in Fig. 3. The bimodal size distribution in Fig. 3 is evidentin the sintering of smaller particles to larger particles. Themean (or average) overall particle size in Fig. 3 is 30 μm, whilethe average, large-particle diameter (mean of the large-particle distribution) is 60 μm. The small attached particlesaverage less than 10 μm diameter.
Fig. 4 shows an FESEM view of an EBM-1 series cylinderlooking down from the top surface (TS) of the build. Fig. 5shows an FESEM normal view (Fig. 5(a)) and an opticalmetallographic view (Fig. 5(b)) of a section through the top(~1 cm from the top) in Figs. 4 and 5(a) illustrating thehomogeneous, continuous fully dense and primarily acicular
α-plate, Widmanstätten-like microstructure extending fromthe surface particles through the cross-section views from thetop and bottom (~1 cm from the ends) portions of the EBM-1samples etched to be observed by optical metallography at thesame magnification shown in the marker in Fig. 6(a). It can benoted that the average acicular α-plate thickness is 3.2 μm inthe top region of the build (Fig. 6(a)) while the corresponding,average acicular α-plate (and lamellar-like) thickness is 1.6 μmin the bottom region of the build (Fig. 6(b)); a factor of 2difference in plate size or thickness. It should also be noted inFigs. 4 and 5 that the uniformity observed included continuousmelt of particles except of course at the outer surface. It wasrare to observe unmelted particles, but in a few cases porousregions were observed where particles were not melted.
Fig. 7 shows typical dislocation substructures in theacicular α-plates corresponding to the top section opticalmetallographic microstructures shown in Fig. 6(a). Thedislocation densities measured in Fig. 7(a) were slightlygreater than 1010 cm−2 while in Fig. 7(b) the dislocation densitywas estimated to be 5×109 cm−2, assuming a thickness of theelectron transparent section to be 0.4 μm [4] which was anaverage of several measurements using twin boundaryprojections. The maximum variance was a factor of 2, whichstill places the dislocation densities at ~1010/cm2. This
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dislocation density range (~109–1010 cm−2) represents a typicaldeformation range for metals.
The microstructures in the EBM-2 build samples weresimilar to those for the EBM-1 build samples shown typicallyin Fig. 6, but the variance of the α-plate microstructure fromthe top (~1 cm) to the bottom (~1 cm) of the build was slightlydifferent as shown in Fig. 8. The average α-plate thicknessin Fig. 8(a) was 2.1 μm in contrast to 1.4 μm in Fig. 8(b)representing the bottom of the EBM-2 cylinder. The finer α-microstructure near the bottom of the build in Fig. 8(b) wasalso more lamellar-like than acicular and the lamellae wereshorter than those in the bottom section of EMB-1 (Fig. 10(b)).
Figs. 5, 6, and 8 attest to the uniformity of microstructuresacross the EBM cylindrical builds as well as the variance inmicrostructures and associated mechanical properties whichcan be achieved with small variations in build parameters tobe discussed in more detail below.
The primarily acicular α-plate microstructures shown forthe EBM-1 and EBM-2 prototypes shown in Figs. 5–8 can becompared with the W-1 and W-2 wrought Ti–6Al–4V samplesshown typically in Fig. 9(a) and (b) respectively.Fig. 9(a) showsa primarily acicular α-platemicrostructure of theW-1wroughtsample while Fig. 9(b) shows an equiaxed α/β mixture andsome coarse, acicular alpha. These microstructures and otherphase structures have been described in some detail [5–7].
Fig. 8 –Optical metallographic images comparing acicularα-plates in the top (~1 cm) section of EBM-2 samples (a) andthe bottom section (~1 cm) (b). Magnification is the same andshown in (a).
Fig. 9 –Comparative optical metallographic images shown inW-1 (a) and W-2 (b) wrought Ti–6Al–4V microstructures.Magnification is the same and shown in (a).
Fig. 10 shows some examples of themicrostructural detailsin the W-1 samples (Fig. 10(a)), including α-plates and β inFig. 10(b) and dislocation substructures in Fig. 10(c), where thecorresponding dislocation density was ~2×109 cm−2 in con-trast to 5×109 cm−2 measured for the EBM-1 sample illustratedin Fig. 7(b) [4].
3.2. Mechanical Properties: Hardness and TensileTest Comparisons
The average microindentation hardness for a range ofparticles was 5.0 GPa (HV500). Correspondingly the averageVickers microindentation hardness for the top end of EBM-1specimens characterized by Fig. 6(a), and the bottom portioncharacterized by Fig. 6(b) was 3.6 GPa and 3.9 GPa, respectively.Similarly, the Vickers microindentation hardness averagescorresponding to the top and bottom regions of EBM-2 buildsrepresented by Fig. 8 were 3.6 and 4.6 GPa, respectively. Theoverall average Vickers microindentation hardness for EMB-1and EBM-2 samples was 3.8 and 4.1 GPa respectively.
The Rockwell C-scale hardness (HRC) averages for EMB-1and EBM-2 samples at the top and bottom sections, as wellas the average hardness values for overall sample hardnessare listed in Table 2 along with the corresponding Vickersmicroindentation hardness averages for comparison. Table 2also shows the nominal (Grade 5) wrought Ti–6Al–4V Rockwell
Fig. 10 –Wrought (W-1) Ti–6Al–4V microstructures. (a) Optical metallographic overview. (b) TEM image of acicular α with βboundaries corresponding to an area featured at the arrow in (a). (c) Magnified view of planar dislocation arrays in α-plate.
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C hardness (HRC) to be 37. Using this hardness as a basis, itis observed that the EBM samples are 8% (EMB-1) and 35%(EBM-2) harder. Correspondingly the wrought samples com-pared in this study are also correspondingly harder: 30% (W-1)and 41% (W-2) respectively.
It is worth noting that there is good agreement or corre-lation between the microstructures, particularly the averageα-plate dimensions and the hardness values listed in Table 2;both Vickers microindentation hardness and the HRC values.That is, formanymetal and alloy systems the yield strength aswell as the hardness is related to either grain size (or thereciprocal square root of grain diameter, D) in the classicalHall–Petch relationship, or more generally microstructuralstrengthening or hardening components such as dislocationdensity, ρ, and structural or microstructural partitioning di-mensions; including the grain or phase dimension:
YS ¼ rOþK=ffiffiffiffiffiD;
pð1Þ
the classical Hall–Petch relationship or the more specificrelationship:
YS ¼ rO þ K=ffiffiffiffiD
pþ KV
ffiffiffiq
p þ KWD�n; ð2Þ
where σO is the intrinsic or single-crystal yield strength, K, K',and Kq are material constants, Δ is a grain (or subgrain) parti-tioning dimension (such as intertwin spacing, martensiticphase dimension, etc., and n can vary from 0.5 to 1. If we let
YSiH=3 ð3Þ
and
H ¼ Ho þ K=ffiffiffiffiffiffiffiDa;
pð4Þ
where H is the measured Vickers microindentation hardnessand Δα is the average α-plate thickness, it can be observedthat the Vickers microindentation hardness values shown inTable 2 for the top and bottom of the experimental specimens
Table 2 –Mechanical properties of EBM and wrought Ti–6Al–4V
a HV (Vickers hardness) for 25 gf (0.25 N) load at 10 s dwell. 1 HV=0.01 GPa.b YS (0.2% engineering offset yield stress), UTS, and Elongation (%) were obtained from tensile testing at 20 °C at a strain rate of 3×10−3 s−1.c Measurements using line intercept in enlarged views of Figs. 11 and 12.
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designated EBM-1 and EMB-2 are consistent with the α-platedimensions or variations indimensions; e.g.Δα=3.2 μm(Fig. 6(a))corresponding toH=3.6 GPa (Table 2) versusΔα=1.6 μm(Fig. 6(b))corresponding toH=3.9 GPa (Table 2) in Eq. (4). This correspondsto ΔH from top to bottom (Table 2) of 8% for EBM-1. If we assumethat Ho≡K=1 (with appropriate units), (in Eq. (4)) then forΔα=3.2 μm at the top and Δα=1.6 μm at the bottom results in
Fig. 11 –EBM fracture samples and FESEM fracture surfaces. (a) EBshown in (a).
ΔH≅11%. In either case there isanapparentandsimilar variationin the hardness with microstructure.
By comparison, the EBM cylindrical builds as well as thecomparative wrought samples in this study also have highertensile strengths than the nominal Ti–6Al–4V Grade 5 nominalstrength wrought and cast material (Table 2). The elongationof the EBM-1 sample shown in Table 2 exhibits a 67% better
M-1 sample. (b) EBM-2 sample. Magnification is the same as
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elongation. Some cast Ti–6Al–4V products have exhibitedelongations of 5 to 6% at a UTS of 1 GPa. The yield stress(YS) and ultimate tensile strength (UTS) values are also inexcess of the nominal values shown in Table 2 for wroughtTi–6Al–4V. A comparison of the Rockwell C (HRC) hardnessvalues with the corresponding UTS values in terms of equiv-alent conversions (see: www.gordonengland.co.uk/hardness/hardness_conversion_lm.html) is in very reasonable agree-ment (a variance ranging from 3% for EBM-1 to 19% for W-1). Itis also of interest to note that the ratio of average specimenmicrohardness/yield stress (HV/YS) is in reasonable agree-ment with the general rule of thumb for metals and alloyswhere HV/YS=~3 (Eq. (3)). This would allow the yield and ulti-mate tensile stress to be approximated from digital (Vickers)microhardness testing of surfaces for Ti–6Al–4V.
The EBM materials properties in Table 2 are considerablybetter than powder metallurgy Ti–6Al–4V products recentlysummarized by Froes et al. [8] where even for HIPed productsthe UTS and elongation do not exceed the ASTM Grade 5values shown in Table 2.
Fig. 11 shows the necked and failed EBM tensile testspecimens (EBM-1 and EBM-2) and their associated fracturesurface features observed in the FESEM. Fig. 12 shows thecorresponding fracture surface features for the two wroughttensile specimens (W-1 and W-2) for comparison and withreference to Table 2. The fracture features in Figs. 11 and 12(a)
Fig. 12 –Wrought sample fracture surfaces observed in theFESEM. (a) W-1 (acicularαmicrostructure). (b) W-2 (equiaxedα/β). Magnification is the same as shown in (a).
illustrate the expected ductile-dimple behavior, especially forthe 25% elongation for EBM-1 (Fig. 11(a)). However, 12(b) showsmore complex fracture features for the equiaxed, mixed α/βgrain structures. It is especially revealing to compare Figs. 12and 9. Note also that the tensile data for the EBM samplesrepresents the average for the entire test cylinders; top andbottom.
On examining Figs. 11 and 12 a significant difference indimple diameter is apparent in equiaxed regions. The averagedimple diameters were measured using the line interceptmethod for both the wrought and EBM tensile samples. Thesevalues are listed in Table 2 and generally indicate amuch finerdimple diameter for the EBM samples as compared to thewrought specimens. The mean equiaxed dimple diameter forthe EBM samples was 4.4 μm, while the wrought materialaveraged 6.7 μm, or approximately 50% larger. Both materialsdemonstrated an increase in tensile ductility with increasingdimple size (Table 2), but inclusions were rarely found in theEBM samples. Noting that the average tensile ductility of theEBM samples was 58% higher with smaller dimple diameters,it appears that a model involving the fracture of inclusionparticles and initial growth of associatedmicrovoids, and theirsubsequent link-up by rupture of the intervening ligaments isdifferent for the EMB samples. This behavior warrants furtherstudy.
When the mechanical properties for the EBM samples inTable 2 are considered in terms of the corresponding micro-structures, especially those shown in Figs. 6 and 8, it isapparent that there is considerable potential for relativelyprecise control of both structures and properties during theEBM production of even complex parts; especially in the caseof Ti–6Al–4V. As we noted earlier in connection with themicrostructure–microhardness variations from the top to thebottom of the experimental builds (EBM-1 and EBM-2: e.g.Figs. 6 and 8 and Table 2), the corresponding HRC was variedfrom 40 to 50 with no systematic variation in the buildparameters. It should be possible to create graded mechanicalproperties, especially hardness so as to adjust the strengthand wear properties as necessary, perhaps within a few layerdimensions. This is especially pertinent where a largebiomechanical incompatibility may exist such as Ti–6Al–4Vfemoral stems in hip replacements where a high strength(UTS) as evident in Table 2 is so much larger than that of bone(b0.3 GPa) [9]. These features have also been described in arecent article by Krishna et al. [10] where functional porousimplants are presented. With EBM it may be possible toselectively grade the alloy strength, fatigue resistance, andelastic modulus for greater bone compatibility. In addition, itis apparent that adjusting the starting powder particle size orsize distributionmay also allow for layer dimension variationsif the beam energy and beam scan are appropriately varied,although there is at present only sparce evidence of eitherthe ability to grade biomedical implants built by rapid manu-facturing or the ability to vary layer building by changingpowder size. Tool-path-based porosity variations using laserprocessing have also been described [10]. Of course we havepresented experimental evidence for Ti–6Al–4V, but otherpowder alloy systems may allow for an even wider variancein structure–property–performance features. For example,Ortiz et al. [11] have demonstrated that thermomechanical
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treatment routines for Ti40Ta and Ti50Ta exhibit highstrength and superior corrosion resistance relative to Ti–6Al–4V. In fact, aged Ti50Ta achieved a tensile strength roughly70% higher than Ti–6Al–4V. Powders of Ti50Ta would nomin-ally melt at a temperature of 2425 °C, or 33% higher than Ti–6Al–4V (where TM=1825 °C). This could be accomplished usingEB manufacturing or laser RP.
The advantages of EBM are vested in the intrinsic featuresof an electron beam: easily controlled beam focus and energyas well as electrically controlled scan. This allows forvariations in powder layer building, including a range of liquidphase sintering to complete particle melting or layer melting.These build variations can be embedded in CAD design or inCT-scan designs.
4. Summary and Conclusions
Utilizing simple Ti–6Al–4V cylindrical specimens built by layermanufacturing from nominal 30 μm powder using an electronbeam, we have demonstrated the ability to produce partscomparable in strength (UTS) and elongation to the very bestwrought Ti–6Al–4V products. These variations are difficult ifnot impossible to achieve for wrought or cast products, andwith these potential mechanical property variations usingEBM technology, potentially superior medical implants areachievable. In addition, we have demonstrated the ability tovary the hardness from HRC 37 to HRC 42 within a dimen-sional range of ~4 cmwithout any systematic variations of thebuild parameters, and to produce a maximum hardness ofHRC 50. This suggests that graded properties such as hardnessin Ti–6Al–4V might range below HRC 37 and above HRC 50within an effective range of only a few tens of layers (~2–3 mm). In addition, reductions in powder size by ~30% to20 μmnominal size may allow layer dimensions to be reducedto b100 μm and further refine this property feature size.Correspondingly, elongations achieved by the EBM processranged from roughly 23% to 92% greater than the averageelongation for high-strength Ti–6Al–4V forgings. These fea-tures were supported by fractography examination in theFESEM and the variations in hardness were also consistentwith microstructure variations observed by both opticalmetallography and transmission electron microscopy.
To a large extent this study and review represents a modelfor representing the materials science and engineering para-
digm as it may be applied to layer manufacturing from metalor alloy powders; utilizing modern materials characterizationtools, including mechanical property testing, to study mate-rials structure (microstructure), properties, processing andperformance.
Acknowledgements
This research was supported by Mr. and Mrs. MacIntoshMurchison Chair Endowments at the University of Texas atEl Paso.
R E F E R E N C E S
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