Characterization of Ti-6Al-4V open cellular foams fabricated by additive manufacturing using electron beam melting L.E. Murr 1,2, S.M. Gaytan 1,2, F. Medina 2, E. Martinez 1,2, J.L. Martinez 1,2, D.H. Hernandez 1,2, B.I. Machado 1,2, D.A. Ramirez 1,2, R.B. Wicker 2,3 Department of Metallurgical and Materials Engineering 1 The University of Texas at El Paso, El Paso, TX USA 2 W.M. Keck Center for 3D Innovation, The University of Texas at El Paso, El Paso, TX USA 3 Department of Mechanical Engineering, The University of Texas at El Paso, El Paso, TX USA

Abstract Ti-6Al-4V open cellular foams were fabricated by additive manufacturing using electron beam melting (EBM). Foam models were developed from CT-scans of aluminum open cellular foams and embedded in CAD for EBM. These foams were fabricated with solid cell structures as well as open (or hollow) cell structures and exhibit tailorable stiffness and strength. The strength in proportion to the measured microindentation hardness is as much as 40% higher for hollow or open cell (ligament) structures in contrast to solid, fully dense EBM fabricated components. Plots of relative stiffness versus relative density were in good agreement with the Gibson-Ashby model for open cellular materials. Stiffness or Young’s modulus values measured using a resonant frequency – damping analysis technique were found to vary inversely with porosity especially for solid ligament, open cellular structure foams. These foams exhibit the potential for novel biomedical, aeronautics, and automotive applications.

Experimental methods and analytical issues Figure 1 illustrates the important features for the EBM system which is essentially an electron optical device consisting of an electron gun (1), a focusing lens (2) and a beam scanning arrangement (3). The Ti-6Al- 4V (ELI) powder (insert in Fig. 1) having a nominal composition of 6.04% Al, 4.05% V and 0.13% O (balance Ti in weight percent) and an average spherical particle size of 30 µm[8], is gravity fed from the powder cassettes (4) onto the building component (6) and raked (5) as a layer (~100 µm thick). The electron beam pretreats the raked layer to ~650°C by scanning at ~15,000 mm/s at 30 mA beam current in eleven passes. This layer preheat is followed by a melt scan at 400 mm/s and a reduced beam current of 6 mA. The melt scan is driven by a 3D–CAD program which melts only selected layer areas according to a software/CAD created digital model, and the building component is fabricated by successively melted structural elements layer-by-layer. The build table ((7) in Fig. 1) moves down as the component builds.

Fig.1.EBM system schematic with SEM insert showing the precursor Ti-6Al-4V powder. The system components are represented as follows: (1) electron gun; (2) electron beam focusing lens; (3) electron beam deflection coils; (4) powder cassettes; (5) layer rake; (6) build or fabricating component; (7) build table (which is lowered with each layer addition). From references 7 & 8. Experimental methods and analytical issues

The elastic (Young’s) moduli for EBM-fabricated Ti-6Al-4V cellular foams were initially measured in this study using a resonant frequency and damping analyzer (RFDA) developed by IMCE, n.v. Genk, Belgium [9]. This non-destructive testing system is based on impulse-excitation using a small mechanical tapping device to create a mechanically induced vibration in the cellular foam sample [9]. This vibration consists of the sum of several resonant frequencies, f r, each of which dampens according to the energy absorption of the material to produce a measurement of the elastic modulus, E, in the form: E = ζmf r 2 where ζ is a geometrical (specimen) shape factor, m is the specimen mass, and f r is the prominent, resonant frequency [9]. This non-destructive testing measures the dynamic Young’s modulus in contrast to the static modulus for tensile or compression testing. This is often referred to as a stiffness-related number rather than the conventional Young’s modulus. Experimental methods and analytical issues

The cellular foams fabricated as test samples in this study were designed to satisfy the general metal foam requirements discussed by Ashby, et al. [1]: height/width >1.5, height >7 times the cell size or “pore” (channel) size. Fabricated test components were, after elastic modulus measurements, sliced in sections which were mounted, polished, and etched for optical metallographic examination and measurement of Vickers microindentation hardness (HV) using a 100 gf (~1N) load for a 10s dwell time. The etching of the mounted and polished sections involved a solution consisting of 100 mL H 2 0, 2.5 mL HF and 5 mL HNO 3. Representative sections cut from the cellular mesh samples were also observed in a Hitachi S-4800 field emission SEM operating at 20kV. Electron transparent specimens prepared from solid, full density (4.3 g/cm 3 ) test blocks and the test/simulation configurations illustrated in Fig.4, and described in detail by Murr, et al. [8], were observed in a Hitachi H-8000 TEM operating at 200kV accelerating potential, employing a goniometer – tilt stage. Experimental methods and analytical issues

The Ti-6Al-4V open cellular foam building by EBM was accomplished by creating 3D – digital models based upon CT-scanned aluminum alloy foams described previously [7]. These scanned foams were converted to bitmap files where the cellular foam build element can be altered in linear dimension and cellular structure size to create systematic porosity variations (or densities). Figure 2 illustrates 3D foam models which are created from these software systems to direct the layer-melt scan in the EBM (Fig. 1). Figure 2 utilizes the prime foam element (designated IX) to produce models for 2X (Fig. 2(a) and (b)) and 3X (Fig. 2(c) and (d)) foams. Foams were also built from 2.5X and 4X as well as implicit in Fig. 3. Figure 3 shows a CAD-model slice through a cellular foam build illustrating cell structure features implicit in the surface view in Fig. 2 (arrows in Fig. 2(c)). Experimental methods and analytical issues

Fig.2. Software (CAD) models for open cellular foams based upon micro-CT scans for aluminum alloy foams (a) 2X base foam or primary build unit (1X). (b) 2X test foam volume, (c) 3X base foam, (d) 3X test foam volume. In (b) the virtual volume is denoted by dotted lines. From Murr, et al. [7]. Experimental methods and analytical issues ab c

Fig.3.CAD model sections (2.3 cm x 2.3 cm) for the open cellular Ti-6Al-4V foams: 1X, 2X, 2.5X, 3X and 4X. (a) shows the least dense (or most porous) foam (4X) in contrast to the most dense (or least porous) foam (1X). 1X represents the base CAD element. (b) shows enlarged cell wall or ligament in 4X (a) having a solid (S) or open (O) structure. (c) shows the three intermediate cellular foam sections for solid cell structures (2X, 2.5X, 3X) at reduced magnification relative to (a). Experimental methods and analytical issues

Fig.4. Solidification rate test element surrogate. TEM 3mm discs can be punched from test strips having different thicknesses (t) and connectivity representing the foam cell dimensions (dotted circle). Experimental methods and analytical issues

Results and discussion Figures 5 and 6 illustrate the five different cellular foams fabricated by EBM utilizing CAD models illustrated typically in Fig. 2. Figure 5(a) shows the 3X, 2.5X, and 2X open cellular foams illustrated in cross- section models in Fig. (3(c) while Fig.6(a) shows the 1X, solid cell, base element foam and the 4X foams in both the solid cell (S) and open cell (O) wall structures (or connecting ligaments)

Fig.5. Fabricated, open cell structure foam test blocks. (a) from left: 3X, 2.5X,2X. (b) SEM view of cell structure in 2.5X foam sample showing layered surface features with sintered powder particles. Results and discussion

Fig.6.Comparison of 1X solid and 4X solid and open cell structure Ti-6Al-4V foams (a). (b) shows a solid cell structure 3X foam (left) and an open cell structure 4X foam (right) cross-section views. (c) shows enlarged, schematic views for cell ligaments or walls in (b) showing solid cell wall structure thickness designation (t S ) and open cell structure wall thickness designation (t o ) corresponding to the foam cross- sections in (b). Results and discussion

As implicit in Table1, the cell ligament or wall structures for both the solid cell structures and the open cell structures exhibit microindentation hardnesses which vary from roughly 29% to 37% greater than monolithic, bulk samples where the Vickers microindentation hardness is nominally 3.5 GPa. This in contrast to values ranging from 45 to 4.8 GPa for the thin ligaments or open cell walls. This results by rapid solidification facilitated by the small dimensions. It can be observed in Table 1 that the larger, solid ligament thickness exhibits consistently lower (~7%) microindentation hardness in contrast to the open cell wall thickness (Fig. 6(c)). Results and discussion

Table 1: Ti-6Al-4V Cellular Foam Properties * † Based upon the design (CAD) features noted in Figs. 2 and 3. *Pores/inch (ppi) † Based upon a fixed, fabricated virtual volume of cm 3 : for dimensions of 2.3cm x 2.3 cm x 3.6 cm. †† ρ o = 4.43 g/cm 3 **Note that the nominal, residual yield strength can be estimated as ~HV/3 where HV is the Vickers microindentation hardness. * † *E O = 110 GPa ** † The thickness for the solid cells is t S (ave.) while for the open cells it is t o (Fig.6(c)). t o will be essentially constant as noted since it represents the electron beam diameter as directed by the wafer support module discussed earlier. Results and discussion Foam CAD Designation* Pore Density (ppi)* Sample mass m(g) Density ρ (g/cm 3 ) † Relative Density (ρ/ρ O ) †† Porosity (%) Cell Wall or Ligament Thickness (mm)** † Cell (Wall) Hardness (GPa)** Resonant Frequency f r (kHz) Stiffness E (GPa) Relative Stiffness (E/E O )* † *x X –-Solid (S) 2X - Solid (S) 2.5X -Solid (S) 3X – Solid (S) 4X – Solid (S) X – Open (O) 2X – Open (O) 2.5X – Open (O) 3X – Open (O) 4X – Open (O)

Figure 7 compares a solid, monolithic build acicular (Widmanstätten), α-phase grain structure having a Vickers microindentation hardness of 3.6 GPa (Fig. 7(a)) with a 2.5X cellular foam with open cell structure exhibiting predominantly α´– martensite microstructure, having a Vickers microindentation hardness of 4.5 GPa. Correspondingly, Fig. 8 compares the solid cell wall structure 4X foam (Fig. 8(a)) with the open cell wall structure 4X foam (Fig. 8(b)); with microindentation hardness values of 4.6 GPa and 4.8 GPa, respectively. Figure 8(a) exhibits a finer acicular α-phase structure than Fig. 7(a) (both are at the same magnification). The microindentation hardness of 4.6 GPa for Fig. 8(a) in contrast to 3.6 GPa for Fig 7(a) is characteristic for microstructure strengthening where refined (or smaller dimension) microstructures imply higher strength and associated microindentation hardness, especially since the yield stress (σ Y ) for many metals and alloys is roughly a third of the Vickers microindentation hardness (HV): σ Y ≅ HV/3[10]. Figure 8(b) shows a mixture of α´-martensite and the hcp α-phase; primarily α´-martensite as in Fig. 7(b). Both microstructures in Figs. 7(b) and 8(b) are characteristic of thin wall structures in open cell ligaments (Fig.6(c)). The magnified microstructure image of the insert in Fig.8(a) also shows fine, particulated β-phase grains (arrow) which contribute to the residual hardening. Results and discussion

Fig.7.Comparative optical metallograph images for a solid (fully dense) monolithic component (a) and a 2.5X, open cell structure foam (b). The image in (b) represents a wall thickness region illustrated at t o in Fig. 6(c). The magnification is the same as shown in (a). Results and discussion

Fig.8.Comparison of optical metallographic views of microstructures in a solid cell structure 4X foam (a) and an open (or hollow) cell structure 4X foam (b). The corresponding microindentation hardnesses were 4.6 GPa in (a) and 4.8 GPa in (b). The magnification is the same as shown in (a). The insert in (a) is a twice magnified area showing refined, segments of β-phase (arrow). Results and discussion

Fig.9.Optical micrographs showing acicular α-phase microstructure for a solid, EBM fabricated Ti-6Al-4V cylinder (a) and a 1.1 mm thick strip in a test element as in Fig. 4 showing refined α and β-phase (dark dots). The hardness in (a) was 3.5 GPa in contrast to (b) where the hardness was 4.8 GPa. The magnification in (b) was the same as shown in (a). Results and discussion

Fig.10. TEM bright-field images corresponding to optical microscope images for corresponding microstructures in Fig. 9. (a) Solid monolith exhibiting 2 µm wide α- platelets with β-phase boundaries. Arrow denotes an α/α grain boundary. (b) 1.1mm test plate (Fig. 4) electropolished disc. The TEM image corresponds to Fig. 9(b) illustrating refined β (dark). The selected area electron diffraction (SAED) pattern insert in (a) shows the α-phase (hcp) (001) plane while the SAED pattern insert in (b) shows additional, multiple diffraction spots from the refine β-phase. Note the magnification of (a) and (b) is the same. Results and discussion

Fig.11. Enlarged TEM images showing α-phase and β- phase microstructures. (a) α-grains bounded primarily by simple grain boundaries as in Fig.10 (a) (arrow). Narrow β-phase boundaries are also shown. (b) β-phase boundary separating α-phase grains. Results and discussion

Figure 12 shows a log-log plot of the relative stiffness values versus the relative density values listed in Table 1. The straight-line fit to the solid (S) cell wall structure data points has a slope of 2.4, somewhat higher than the value of 2 fitted for a large number of open cell aluminum foam results as summarized by Gibson [11] and implicit in the Gibson-Ashby model for open cellular materials [2]: E/E o = (ρ/ρ o ) n (2) where n = 2 generally. However, experimentally it has been shown that n varies from about 1.8 to 2.2 [1]. In our earlier work involving Ti-6Al- 4V open cellular mesh structures, a value of n = 2.4 was also measured for a simple cubic mesh element array fabricated by EBM [7]. It can be noted in Fig. 12 that there is no similar straight-line fit for the open cell wall (O) foam structures unless the 2.5X (O) data point is ignored. The resulting, fitted line has a slope corresponding to n = 2.1. Results and discussion

Fig.12.Relative stiffness plotted against relative density for experimental solid cell wall (S) and open cell wall (O) Ti-6Al- 4V cellular foams. The solid line is fitted to the solid cell (S) foam data. Fig.13.Stiffness (Young’s modulus) versus porosity for solid cell wall (S) and open cell wall (O) Ti-6Al- 4V foams. The solid line is fitted to the solid cell (S) foam data. Results and discussion Fig.12.Fig.13.

The versatility of the EBM process to fabricate complex, multifunctional Ti-6Al-4V and other metal or alloy foam components is unprecedented. In fact open cellular Ti-6Al-4V foams produced in this work have not been fabricated by any other process. Figures 14 to 16 illustrate a few examples of complex foam structures which can be fabricated as monolithic components. Figure 14(a) to (c) shows several cylindrical foam models with varying porosity or density: higher density in the outer foam. Figure 14(d) illustrates a mesh element outer structure and a porous foam inner structure. These structures are illustrated more specifically in the cross-section model in Fig. 15(a) along with an EBM fabricated monolith – shown in Fig. 15(b). Figure 15 can serve as an example for next generation orthopaedic component fabrication where cortical bone stiffness and density are matched with the outer foam structure and the less dense inner foam is characteristic of trabecular bone [7]. The foam structures in Fig. 15(b) are also joined to a solid end form to create a complex monolithic product which cannot be fabricated by any known process. Results and discussion

Fig.14. Software/CAD models for the EBM fabrication of complex, varying open cellular foam components. (a) to (c) shows 3D, 3D half section, and end view, respectively. (d) shows a reticulated mesh element combined with an inner foam structure. Results and discussion

Fig.15. EBM CAD model half- section (a) and corresponding, fabricated monolithic foam component (b) which was cut to show the half section view. Results and discussion

Fig.16. Examples of Ti-6Al-4V open cellular foam (4X) prototypes fabricated by EBM. (a) Joined foam blocks (at arrows). (b) Large foam component prototype. Results and discussion a b

Summary and conclusions n nThe ability to fabricate stiff Ti-6Al-4V open cellular foams using EBM as demonstrated and described in this research program is a unique innovation not possible by other processing technologies. In fact, the stiffnesses or elastic moduli for these open cellular Ti-6Al-4V foams vary with density (especially relative stiffness versus relative density) consistent with the Gibson-Ashby foam model [2]. Moreover, the stiffness varies inversely with porosity or pore density consistent with literature values for a number of metal and alloy systems, especially aluminum [12, 13].

n nIn this study we have fabricated open cellular foams having both solid and open (or hollow) cell wall or ligaments structures. These complex structures have different cooling rates in contrast to bulk or solid monoliths of Ti-6Al-4V built by EBM which promote α´-martensite formation or granular β-phase which are often intermixed with refined, acicular (Widmanstätten) α-phase microstructure. This microstructure refinement in the cellular ligaments, and especially in the thin walls of hollow or open cell structures, promotes higher microindentation hardness and residual strength. Indeed the hardness increase for the foam cell structures in contrast to solid, fully dense monoliths, can be as high as 40%. Summary and conclusions

n nIt has been shown that complex foam structures can be fabricated with solid, fully dense monoliths by EBM. While this study has demonstrated innovative Ti-6Al-4V open cellular foams and complex, multifunctional component fabrication having a wide range of technology applications, the implications are that any metal or alloy system, or complex array which can be modeled in digital CAD, can be fabricated by additive manufacturing using EBM; with a suitable precursor powder. Summary and conclusions

Acknowledgements This research has been supported in part by Mr. and Mrs. MacIntosh Murchison Endowments at The University of Texas at El Paso. A portion of this research was also supported by Lockheed-Martin/ Aeronautics. Any expressed opinions, findings, conclusions or recommendations are the authors and not necessarily representative of those of Lockheed- Martin/Aeronautics. We are also grateful to Joris Bracke of Integrated Material Control Engineering, n.v. Genk, Belgium for measuring the stiffness values for the experimental foams.

References n n[1]M.F. Ashby, A. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N.G. Wadley, Metal Foams: A Design Guide, Butterworth- Heinemann, Boston, n n[2] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, Cambridge Univ. Press, New York, n n[3] H.P. Degischer, B. Kriszt (Eds.), Handbook of Cellular Metals, Wiley, Weinheim, n n[4] D.S. Schwartz, D.S. Shih, A.G. Evans, H.N.G. Wadley (Eds.), Porous and Cellular Materials for Structural Applications, MRS, Pittsburgh, n n[5] A. Ghosh, T. Sanders, D. Claar (Eds.), Processing and Properties of Lightweight Cellular Metals and Structures, TMS, Warrendale, PA, n n[6] D.C. Dunand, Adv. Engr. Mater, 6 (6) (2004) 369. n n[7] L.E. Murr, S.M. Gaytan, F. Medina, H. Lopez, E. Martinez, B.I. Machado, D.H. Hernandez, L. Martinez, M.I. Lopez, R.B. Wicker, J. Bracke, Phil. Trans. Roy, Soc. (London) (2009), in press.

n n[8] L.E. Murr, E.V. Esquivel, S.A. Quinones, S.M. Gaytan, M.I. Lopez, E.Y. Martinez, F. Medina, D.H. Hernandez, E. Martinez, J.L. Martinez, S.W. Stafford, D.K. Brown, T. Hoppe, W. Meyers, U. Lindhe, R.B. Wicker, Mater. Charact. 60 (2009) 96. n n[9] G. Roebben, B. Bollen, A. Brebels, O. Van Humbeeck, O. Van der Biest, Rev. Sci. Instrum, 68 (12) (1997) n n[10] L.E. Murr, Interfacial Phenomena in Metals and Alloys, Addison- Wesley, Reading, MA, 1975, p n n[11] L.J. Gibson, Ann. Rev. Mater. Sci. 30 (2000) 191. n n[12] N.G.D. Murray, D.C. Dunand, J. Mater. Res. 21 (2006) n n[13] K.A. Erk, D.C. Dunand, K.R. Shull, Acta Mater, 56 (2008) n n[14] L.E. Murr, S.A. Quinones, S.M. Gaytan, M.I. Lopez, A. Rodela, E.Y. Martinez, D.H. Hernandez, E. Martinez, F. Medina, R.B. Wicker, J. Mech. Behavior Biomed. Mater. 2 (2009) 20. References