Additive manufacturing of a functionally graded material from Ti-6Al-4V to Invar: Experimental characterization and thermodynamic calculations

Accepted Manuscript Additive Manufacturing of a Functionally Graded Material from Ti-6Al-4V to Invar: Experimental Characterization and Thermodynamic Calculations

Lourdes D. Bobbio, Richard A. Otis, John Paul Borgonia, R. Peter Dillon, Andrew A. Shapiro, Zi-Kui Liu, Allison M. Beese PII:

S1359-6454(16)31018-7

DOI:

10.1016/j.actamat.2016.12.070

Reference:

AM 13458

To appear in:

Acta Materialia

Received Date:

17 December 2016

Accepted Date:

29 December 2016

Please cite this article as: Lourdes D. Bobbio, Richard A. Otis, John Paul Borgonia, R. Peter Dillon, Andrew A. Shapiro, Zi-Kui Liu, Allison M. Beese, Additive Manufacturing of a Functionally Graded Material from Ti-6Al-4V to Invar: Experimental Characterization and Thermodynamic Calculations, Acta Materialia (2016), doi: 10.1016/j.actamat.2016.12.070

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Additive Manufacturing of a Functionally Graded Material from Ti-6Al-4V to Invar: Experimental Characterization and Thermodynamic Calculations

Lourdes D. Bobbio1, Richard A. Otis1, John Paul Borgonia2, R. Peter Dillon2, Andrew A. Shapiro2, Zi-Kui Liu1, Allison M. Beese1* 1

Department of Materials Science & Engineering, Pennsylvania State University, University Park, PA, 16802, USA

2

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA * Corresponding author: [email protected]

Abstract In functionally graded materials (FGMs), the elemental composition, or structure, within a component varies gradually as a function of position, allowing for the gradual transition from one alloy to another, and the local tailoring of properties. One method for fabricating FGMs with varying elemental composition is through layer-by-layer directed energy deposition additive manufacturing. This work combines experimental characterization and computational analysis to investigate a material graded from Ti-6Al-4V to Invar 36 (64 wt.% Fe, 36 wt.% Ni). The microstructure, composition, phases, and microhardness were determined as a function of position within the FGM. During the fabrication process, detrimental phases associated with the compositional blending of the Ti-6Al-4V and Invar may form, leading to cracking in the final deposited part. Intermetallic phases (FeTi, Fe2Ti, Ni3Ti, and NiTi2) were experimentally identified to occur throughout the gradient region, and were considered as the reason that the

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FGM cracked during fabrication. CALPHAD (CALculation of PHase Diagrams) thermodynamic calculations were used concurrently to predict phases that would form during the manufacturing process and were compared to the experimental results. The experimentalcomputational approach described herein for characterizing FGMs can be used to improve the understanding and design of other FGMs.

1. Introduction Functionally graded materials (FGMs) are a class of material systems with properties that vary over one or more dimensions by progressively changing the chemistry or microstructure with position. FGMs are attractive because, by spatially varying the chemistry and/or microstructure, the local properties of components are also varied, resulting in components with properties (mechanical, thermal, optical, magnetic, etc.) that generally cannot be obtained using traditional metallurgy techniques. Techniques that have been used to create FGMs with chemistry variations include vapor deposition techniques (primarily used for functionally graded coatings) [1], ultrasonic welding [2], fusion welding [3], layer/disk re-melting [4], powder metallurgy [5], and centrifugal methods [6]. These methods work well for producing gradients over a length scales of less than 1 mm. However, additive manufacturing (AM) is a key enabling technology in the layer-by-layer fabrication of metal FGMs with chemistry variations over a larger length scale (on the order of a tens of mm). In directed energy deposition (DED) AM, powder or wire feedstock is fed directly into a melt pool generated by a heat source. When using powder feedstock, this melt pool is created in the substrate or layer below using a laser, and multiple hoppers feed the powders through the nozzles into the melt pool [1,7]. Thus, DED can

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be used to create FGMs by varying the relative fractions of two or more powders fed into the melt pool as a function of position. In the existing literature, a variety of FGMs with different constituent alloys have been studied, including Invar 36 (64 wt.% Fe, 36 wt.% Ni)/stainless steel [8], TiC/Ti [9], Ti-6Al4V/Inconel 718 [2], Ti-6Al-4V/γ-TiAl [10], and Fe/FeAl [4]. In one study, DED was used to fabricate an FGM from AISI type 304L stainless steel to Inconel 625 [11] and the chemistry, microstructure, microhardness, and primary and secondary phases and their compositions were examined. While the component was fabricated successfully with no macroscopic cracking, the authors established through experimental and computational analysis that the secondary phase, identified as transition metal carbides, formed in the gradient region, leading to micro-cracks [11]. Hofmann et al. [12] studied the elemental composition, mechanical properties, and phase compositions in two different FGMs: one graded from Ti-6Al-4V to pure V and another graded from 304L stainless steel to Invar 36. The Ti-6Al-4V to V FGM exhibited distinct changes in phases, hardness, and composition across the gradient; additionally, some of the trial FGMs cracked during fabrication as a result of brittle phase formation. Both of the terminal alloys in the 304L SS to Invar gradient have similar concentrations of Fe, and there was no cracking present in the gradient region of this FGM, despite the fact that the coefficient of thermal expansion (CTE) dropped from approximately 15 × 10-6 K-1 of 304L SS to 0.01 × 10-6 K-1 [13] of Invar around 10 mm into the gradient region. The authors successfully created a gradient in ferromagnetism, where the Invar side was ferromagnetic and the 304L SS side was paramagnetic. The authors postulated that FGMs could be optimized through the use of multi-

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component phase diagrams to avoid undesirable phases, such as the brittle phases seen in the Ti6Al-4V to V FGM [12]. In another study, a Ti-6Al-4V to 304L stainless steel FGM made by DED cracked during fabrication [14]. The authors found that brittle FeTi and Fe-V-Cr sigma phases formed when stainless steel was introduced into the build, which led to the cracking. Using this information and the Fe-V-Cr phase diagram generated by Thermo-Calc [15], the authors proposed alternate gradient paths based on their observations that could avoid the formation of sigma phase [14]. In the present work, a sample graded from Ti-6Al-4V to Invar 36 (referred to herein as Invar) was fabricated using DED. These two terminal alloys were chosen because of their disparate thermal and mechanical properties. Invar has a near-zero CTE of approximately 0.01 × 10-6 K-1 at room temperature [13], which is useful for applications in environments with large variations in temperature in order to avoid thermal shock and maintain dimensional accuracy, for example telescope technology and high performance precision optical mirror substrates [13]. On the other hand, Ti-6Al-4V has a much higher CTE of 8.5-10 × 10-6 K-1 at room temperature [16,17]. However, Ti-6Al-4V is corrosion resistant [18] and has a low density (4.43 g/cm3) and a high strength to weight ratio (246 kN-m/kg) [19] compared to Invar’s high density (8.05 g/cm3) and correspondingly lower strength to weight ratio (76 kN-m/kg) [20]. These two alloys, when joined directly through dissimilar welding, result in a joint in which intermetallics could be present and provide a point of weakness or failure [21]. In particular, the large difference in CTE between the two alloys could result in failure under significant temperature excursions as the joint would act as a location of stress concentration. However, because an FGM can provide a gradual change in elemental composition, the properties, including CTE, should change gradually across the gradient, resulting in a lower likelihood of component failure during

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temperature excursions. Because of this, the Ti-6Al-4V to Invar FGM has potential applications in extreme environment conditions, such as those found in aerospace or nuclear reactors. Studying this particular FGM system can provide insight into how experimental characterization and computational predictions can be used in concert to determine whether two well-known alloys can be successfully graded using a linear gradient path. If not, it will provide insight into whether an alternative, nonlinear pathway is needed, and can be designed, through which this FGM can be successfully fabricated [22,23]. While much of the existing literature has focused primarily on fabricating and characterizing FGMs, this study uses a combined experimental-computational approach to measure and predict phases that precipitate during fabrication, with the ultimate goal of designing the optimal gradient pathway for this material system [22,23]. In the present work, the local microstructure, elemental composition, mechanical properties, and phase composition of the FGM were experimentally characterized as a function of position and compared to the structure and properties of the constituent alloys. Due to the complex thermal and composition profiles of the AM processes, it is extremely challenging to simulate the phase formations during the FGM fabrication. However, as shown in previous publications in the literature [11,12] and in the present work, properly selected phase equilibrium calculations can provide critical information to predict the potency of phases formed in FGM parts.

2. Experimental The gradient sample was fabricated using a directed energy deposition system (RPM 557 Laser Deposition System), schematically shown in Figure 1. The square post samples, with a base area of 15 mm × 15 mm, were fabricated using a YAG laser, operated at a laser power of

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900W, and a scanning speed of 12.7 mm/s. The hatch angle was 90 degrees, meaning that for each layer, the scanning direction rotated 90 degrees. The layer height was 0.38 mm, and the hatch width was 0.58 mm. The fabrication process was conducted in an argon atmosphere. This deposition system has the ability to deposit up to four different powders during fabrication, and allows for the volumetric fraction of these powders to be altered by about 1% per deposited layer. For this FGM, 21 layers of pre-alloyed Ti-6Al-4V powder were deposited onto a Ti-6Al4V substrate. Next, a gradient region was deposited in which the volume percentage of Ti-6Al4V was decreased by 3 vol.% per layer and replaced by 3 vol.% of pre-alloyed Invar powder per layer for 32 layers until the gradual transition from Ti-6Al-4V to Invar was complete at layer 53. Finally, 22 layers of pure Invar were deposited, corresponding to layer 54 through 75. The final height of the post was approximately 28.5 mm. The post, shown in Figure 2a, was cut off of the baseplate and sectioned into two halves using wire electric discharge machining, during which the sample broke in half vertically. The two pieces of one half were mounted in epoxy, ground, and polished using standard metallographic techniques with a final polish using a 0.05 µm silica suspension for eight minutes. The sample was then etched using Kroll’s reagent (2% hydrofluoric acid and 3% nitric acid in water) [24] for four seconds. The microstructure, chemical composition, and mechanical properties of the FGM were characterized as a function of location. A scanning electron microscope (SEM FEI Quanta 200) was used to investigate grain morphology. Chemical composition and elemental segregation analysis were performed using energy dispersive x-ray spectroscopy (EDS) on the same system as the SEM with a silicon drift detector (Oxford X-act PentaFET Precision). The microhardness of the sample over the entire FGM was measured using a Vickers indenter (Leco V-100-C1) with

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a load of 300 g and a 15 second dwell time. In order to identify the phases present in the gradient zone, X-ray diffraction (XRD) patterns were collected using a Bragg-Brentano-type diffractometer (Panalytical Empyrean) with a Cu K-α X-ray source operated at 45 kV and 40 mA (λ = 1.54 Å). Additional phase analysis was performed using electron backscatter diffraction (EBSD, Oxford Nordlys Max2). In order to do small scale phase analysis, thin foils were extracted from one layer of the bulk sample using a focused ion beam (Quanta 200 3D Dual Beam FIB). A transmission electron microscope (TEM, FEI Technai G2 20 XTWIN and FEI Talos F200X) was used to acquire selected area diffraction patterns and to perform small scale elemental composition analysis.

3. Computational A computational study of the phases was performed using the CALculation of PHAse Diagrams (CALPHAD) technique to predict the equilibrium phase relations of the system to aid in secondary phase determination throughout the gradient. While AM is a non-equilibrium process, as the molten material undergoes rapid solidification followed by thermal cycles, performing phase equilibrium calculations at several temperatures, and spanning the entire gradient chemistry, can provide important insight on the phases present in the FGM as demonstrated in the literature [11,12]. The key issue is to select a temperature close to where the phase transformations of interest start to be kinetically frozen out [11]. A general alloy thermodynamic database covering the entire composition range from Ti6Al-4V to Invar 36 is not available because multi-component thermodynamic databases are usually optimized for a particular alloy system such as the TCFE8 database for steels and TTI3 for titanium alloys from ThermoCalc [15,25] or PanIron for steels and PanTitanium for titanium

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alloys from CompuTherm [26,27]. Due to the limitation of existing multi-component databases, the recently re-modeled ternary Fe-Ni-Ti subsystem [28] was used to delineate the phases in the middle of the FGM post. This is a reasonable approximation in light of the relatively low concentrations of Al and V in the composition range of the FGM post, and the appropriateness of this approximation is assessed in Section 5 (Discussion) below. This approach of using the re-modeled ternary Fe-Ni-Ti subsystem helps to mitigate some problems found with the commercial multi-component databases available for this system, shown in Figure 3. In particular there are differences in the phase equilibria computed from the commercial TCFE8 database and a recent assessment [28] of the Fe-Ni-Ti system, for example at 1100 K. The ordered B2 FeTi phase, while having continuous solid solubility from FeTi to B2 NiTi in the more recent phase diagram, does not appear at all in the Fe-Ti side in the ternary isothermal section calculated from the TCFE8 database because the database does not cover the high Ti content region. Instead the C14 Laves phase is shown to extend into the region.

4. Results 4.1. Computational Results As seen in Figure 2b, around layer 26 of the FGM, the layers appear to have overflowed the layers below. The calculated solidus and liquidus temperatures throughout the gradient region from pure Ti to Invar are given in Figure 4. These calculations showed that the solidus and liquidus curves drop by approximately 650 K at a volume fraction of Invar of approximately 0.1-0.2, which corresponds to layers 25-29. The CALPHAD model was used to determine the secondary phases that precipitated during fabrication as a function of position in the gradient. It predicted the formation of four

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secondary phases as shown in the equilibrium phase diagram at 1100 K in Figure 5. This temperature was selected since it falls below the solidus curve as well as the beta transus. The following phases were predicted to precipitate at equilibrium: FeTi (B2), Fe2Ti (C14, Laves), Ni3Ti (DO24), and NiTi2 (without structure class). It should be noted that all compounds have extensive solubility of the third element dissolved into them, and nomenclatures used here are just for convenience to differentiate various phases. Furthermore, the bcc phase and the ordered B2 phase are modeled with one Gibbs energy function, i.e., they are treated as one phase. The equilibrium phase diagram also showed the presence of the bcc Ti and fcc Invar primary phases plus a small region of hcp-Ti at the Ti corner. In this manuscript, the nomenclature of phases are as follows: hcp (Ti-rich solid solution of Fe, Ni, Ti), bcc/B2 (disorder Ti-rich solid solution of Fe, Ni, Ti and the ordered compound based on FeTi and NiTi with homogeneity ranges of Fe, Ni, Ti), C14 (based on Fe2Ti with homogeneity ranges of Fe, Ni, Ti), fcc (based on the Invar with homogeneity ranges of Fe, Ni, Ti), DO24 (based on Ni3Ti with homogeneity ranges of Fe, Ni, Ti), (Ni,Fe)Ti2 (based on NiTi2 with homogeneity ranges of Fe, Ni). These phase identifications are used interchangeably. At 1100K from pure Ti to Fe-36Ni (Invar), it starts with the hcp solution phase followed by the bcc solution phase (see Figure 5). The bcc phase exists in a single-phase region until 12 vol.% Invar, when NiTi2 begins to form. Near 24 vol.% Invar the bcc disappears, leaving only NiTi2 with Fe dissolved in it. Then ordered B2 begins to form, with all the NiTi2 disappearing at 36 vol.% Invar. A C14 Laves phase begins to displace B2 near 42 vol.% Invar, with the amount of B2 becomes zero by 55 vol.% Invar. Shortly thereafter, fcc and a small amount of Ni3Ti begin to form, with the C14 and Ni3Ti decreasing to zero by about 82 vol.% Invar, leaving only fcc.

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4.2. Experimental Results EDS line scans were used to determine the deposited chemistry as a function of position in comparison with the planned composition. Figure 6 shows the nominally planned and experimentally measured composition, in weight percent, of the constituent elements present in the FGM. Overall, the actual composition of the final part did not deviate significantly from the planned composition. However, there was a significant amount of fluctuation in the measured elemental composition as a function of position in the gradient region. These fluctuations were due to elemental segregation into separate phases, which were observed in the EDS maps and are discussed below. In order to have a complete picture of the evolution of the phases that formed, the SEM secondary electron images and the EDS phase maps must be examined in parallel. Initially, the microstructure of the pure Ti-6Al-4V region consisted of columnar prior-β grains with fine α laths as shown in Figure 7a-b, which is characteristic of additively manufactured Ti-6Al-4V (e.g., [24,29]). There was a distinct change in microstructure at the start of the gradient region, layer 22 (97 vol.% Ti-6Al-4V, 3 vol.% Invar), where Fe2Ti precipitates were found at the grain boundaries and within the grains, as seen in Figure 8a and Figure 9. As more Invar was added, the microstructure started to separate into distinct phases as shown in Figure 8b which corresponds to layer 26 (85 vol.% Ti-6Al-4V, 15 vol.% Invar), until there was a distinct segregation of two phases in layer 33 (64 vol.% Ti-6Al-4V, 36 vol.% Invar) as shown in Figure 8c. The EDS maps of layer 33 show that the two distinct phases correspond to a Fe-rich phase and a Ni/Al rich phase, and this local elemental segregation is what caused the fluctuations in the EDS line scans. By layer 47 (21 vol.% Ti-6Al-4V, 79 vol.% Invar), phase separation was no

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longer present, as shown in Figure 8f; however, small (average size of approximately 2.5 µm) Ti-rich particles were present within the primary Invar matrix. The pure Invar had a dendritic structure, as shown in Figure 7c-d. The Vickers microhardness of the sample as a function of position is given in Figure 10. The pure Ti-6Al-4V region of the sample had an average hardness 380 ± 2.7 HV, which is typical for Ti-6Al-4V deposited by AM [30,31], while the pure Invar region had an average hardness of 141 ± 2.7 HV, also typical of Invar deposited by AM [32]. The hardness in the gradient region did not follow a rule of mixtures decrease from that of Ti-6Al-4V to that of Invar. Instead, there were three regions of significantly higher hardness plateaus. The first high hardness regime, ‘hardness regime 1,’ occurred from 9.9 to 12.6 mm from the substrate, corresponding to 15-33 vol.% Invar, with an average hardness of 700 ± 9.3 HV. The second high hardness regime, ‘hardness regime 2,’ occurred from 12.6 mm to 14.5 mm, corresponding to 36-48 vol.% Invar, with an average hardness of 858 ± 18 HV. The final region of high hardness, ‘hardness regime 3,’ which occurred between 17.1 and 18.2 mm, corresponding to 7382 vol.% Invar, had an average hardness of 382 ± 5.8 HV, which similar to the initial hardness of the pure Ti-6Al-4V region, but higher than the pure Invar region. The sharp increases in hardness are due to the presence of secondary phases in the FGM, which are discussed below. XRD was used to experimentally determine if secondary phases were present. While XRD can indicate the presence of different phases, it has two main limitations. First, it cannot provide the exact location of these phases, since a line focus was used to span 1 mm of the height of the FGM for each measurement (with a penetration depth of approximately 0.1 mm). Since the average layer thickness was approximately 0.38 mm, two to four layers were included in each XRD pattern. Second, it cannot definitively identify the phases present. Representative

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XRD patterns are given in Figure 11, showing the hcp/bcc structure of pure Ti-6Al-4V and fcc structure of pure Invar at the two ends of the FGM. In the gradient region, five additional XRD peaks are present, which do not correspond to those of Ti-6Al-4V or Invar, thus indicating the presence of secondary phases. In layers 20-35 (hardness regimes 1 and 2), an additional peak is present at 41.5°, which could be attributed to NiTi2 or Ni3Ti. In layers 22-31 (hardness regime 1 only), an additional peak at 62° is present, which could be attributed to FeTi. In layers 33-45 (hardness regimes 2 and 3), an additional peak at 72.5° is present, which could be attributed to either Fe2Ti or NiTi2. In layers 33-47 (hardness regimes 2 and 3), an additional peak at 70.5° is present, which could be attributed to Fe2Ti. In layers 39-45 (hardness regime 3 only), an additional peak at 68° is present, which could be attributed to NiTi2. The high hardness regimes that were observed can be explained by the presence of secondary phases throughout the gradient region. In order to identify the local variations in phases, EBSD was used to generate phase maps from each of the three regimes. These maps are given in Figure 12, with the area percentages of the identified phases given in Table 1. Note that there is inherent uncertainty in EBSD phase identification of particles on the order of a few µm; therefore, while the phases in Figure 12 and Table 2 that correspond to under approximately 2 area % are reported, these may contain measurement errors. In layer 26 (15 vol.% Invar, Figure 12a), which corresponds to hardness regime 1, there are two distinct microstructural regions, consisting of three different phases identified by EBSD. In the region occupying the largest area (71% of the total image area), FeTi is present, and in the region occupying a smaller area (15% of the total image area), Fe2Ti and small amounts of Ti are present. FeTi and Fe2Ti were both predicted by thermodynamic calculations as shown above. Two different layers in hardness regime 2 were analyzed with EBSD. In layer 33 (36 vol.% Invar, Figure 12b), there are also two

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areas of segregation. FeTi (35% of the total image area) is present in the area that EDS maps indicate is rich in Ni and Al (Figure 8c). The fact that this region is rich in Ni and Al signifies that these elements dissolved into the FeTi lattice. The other area in hardness regime 2 is composed primarily of Fe2Ti (61% of the total image area), which corresponds to the Fe-rich regions in the EDS map of this layer (Figure 8c). While the presence of Fe2Ti and FeTi in this composition range are predicted by thermodynamic calculations, Ni3Ti is predicted to form in different composition ranges and may have precipitated during cooling. Specifically, since solubility decreases with decreasing temperature, different phases can precipitate out during cooling. Additionally, compositional variation within a layer undergoing solidification and cooling may lead to the precipitation of different phases than those with the elemental composition of the overall layer itself. Layer 35 (42 vol.% Invar, Figure 12c), which is still in hardness regime 2, has all the same phases present as layer 33 (Fe2Ti = 35%, Ni3Ti = 19%, Ti = 7%, FeTi = 8%, all of the total area), as well as the beginning of fcc Invar (10% of the total area); however, fcc Invar was not computationally predicted to appear yet at this point in the gradient. While Fe and Ti are present throughout layer 35, the Fe/Ti-rich regions of the EDS map (Figure 8d) correspond to the Fe2Ti phase and the Ni/Al-rich regions of the EDS map (Figure 8d) correspond to FeTi with Ni and Al dissolved, similar to layer 33. The majority phase in layer 45 (73 vol.% Invar, Figure 12d), which corresponds to hardness regime 3, was found to be fcc Invar (90% of the total area), which agrees with the EDS map (Figure 8e), as it is composed primarily of Fe and Ni. However, it is hypothesized that the reason this region had a higher hardness than pure Invar is because it contains small particles observed via EDS (Error! Reference source not found.e) and identified as Fe2Ti (4% of the

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total area), Ni3Ti (1% of the total area), Ti (hcp) (1% of the total area), and FeTi (3% of the total area). Ti (hcp) is the only phase not predicted by thermodynamic calculations to exist in the composition range, and again, may have formed during further cooling. The segregation present in the gradient, specifically around layer 33, was further analyzed in order to more precisely determine the composition of these phases. This layer was chosen for further analysis as the peak hardness occurs at this point, and it is also where the sample cracked macroscopically. The elemental analysis done using the high-resolution SuperX EDS capabilities of the TEM confirmed the results of the SEM EDS, but with more localized, small-scale (on the order of 500 nm) analysis points. The elemental maps are given in Figure 13. These maps indicate two distinct areas within the analyzed region: a Ni/Al-rich region and a Ferich region. Selected area diffraction patterns (SADPs) from both regions were acquired and indexed to identify the phases in these regions. The Ni/Al-rich region had inter-planar spacings of 4.8 1/nm and 8.4 1/nm with an angle of 74° between the planes, corresponding to the (110) and (2-11) planes of FeTi. The Fe-rich region had inter-planar spacings of 2.4 1/nm and 3.5 1/nm, with an angle of 63° between the two planes, corresponding to the (110) and (102) planes of Fe2Ti. These data, along with the EBSD phase maps, confirm that the Ni/Al-rich area is FeTi and the Fe-rich area is Fe2Ti.

5. Discussion The experimental and computational investigations of this study provide complementary information. In one case, the experiments guided the computation as the observed material overflow indicated that a computational model of the solidus/liquidus temperature was needed to determine whether or not a low melting point composition was encountered in the linear

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gradient, as well as to determine the range of temperatures that should be investigated in constructing equilibrium phase diagrams. The computational prediction for the low melting point is in good agreement with the experimental observations, indicating that the overflow of material was caused by the minima in the solidus/liquidus curves. During AM, as new layers are added on top of existing layers, the existing layers are subjected to rapid heating and cooling cycles. When depositing a monolithic alloy, the material melted below corresponds to one or a few layers [33]. However, since the laser power and speed were held constant throughout the deposition, and the solidus and liquidus drop drastically at 12-18 vol.% Invar, the reheating of layers 25-29 during deposition of layers 30 and above could raise the temperature enough in layers 25-29 to remelt them, resulting in the overflow of material. This analysis provides an explanation for the material overflow during fabrication in light of the stabilization of the liquid at significantly lower temperatures with addition of Invar to Ti-6Al-4V. Those results, in turn, pointed to the need for calculations of equilibrium phase diagrams at multiple temperatures. The process of additive manufacturing is inherently non-equilibrium due to first, the rapid solidification of the molten pool, and second, the repeated thermal cycles with the addition of subsequent layers, resulting in complex phase transformations in the FGM. It is certain that phase transformation kinetics are important in determining the phase fractions and morphologies; but from the point of view of FGM design, the detrimental phases must not exist as equilibrium phases at high temperatures where phase transformation kinetics are not a limiting factor for phase formations of interest. The fact that every phase that was experimentally observed was also computationally predicted in the present work and a previous publication [11], serves as evidence that the use of equilibrium phase diagrams, at the appropriate temperatures, is appropriate for the study of FGMs made by AM. Furthermore, the fact that no

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phases that contained V and only FeTi containing Al were experimentally identified confirms the fact that the thermodynamic calculations, based on the subsystem of Fe-Ni-Ti, were appropriate to predict the phases present in the FGM. The major discrepancy between the experimental results and thermodynamic calculations were differences in the relative amounts of phases and their respective locations, in terms of overall chemistry, which could be attributed to composition inhomogeneous and complex phase transformation kinetics and are expected. For example, thermodynamic predictions indicate that there should be nearly 100% of each of the secondary phases at different locations throughout the gradient. A comparison of the phases found experimentally and predicted computationally is given in Table 3. The major crack occurred in layer 33, which corresponded to 64 vol.% Ti-6Al-4V and 36 vol.% Invar. Both experimental characterization and computational predictions indicated that the phases present in this region were FeTi and Fe2Ti, which were strongly phase separated and intertwined. The coefficients of thermal expansion of FeTi and Fe2Ti at 300 K are 9.5 × 10-6 K-1 [34] and 8 × 10-6 K-1 [35]), respectively, and their room temperature elastic modulus values are also different with FeTi having an elastic modulus of 262 GPa and Fe2Ti having an elastic modulus of 241 GPa [36]. In regions with strongly segregated phases that are intertwined, in order to prevent cracking, the strains in the two phases must be compatible during cooling. The mismatch in thermal expansion and stiffness between FeTi and Fe2Ti is sufficient during cooling to induce stresses that exceed 4 GPa, and therefore fracture of the two individual phases [37–39]. Residual stresses are developed during rapid solidification and shrinking of the molten material, and subsequently during cooling of the material as the phases try to contract, but are constrained by strains in neighboring phases. Therefore, it is postulated that the residual stresses developed

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during the thermal cycles within this layer of strongly phase segregated FeTi and Fe2Ti intermetallics, due to mismatches in CTE and elastic modulus, led to the macroscopic cracking seen in the FGM at this layer. Conversely, the presence of an intermetallic with a soft metallic phase may not have resulted in macroscopic cracking, as the metallic phase could plastically deform to accommodate the strains imposed by the intermetallic phase during cooling. This points to the need for computational predictions to guide FGM design to avoid or incorporate particular phases.

6. Summary and Conclusion In the present paper, the experimental and computational investigation of an FGM from Ti-6Al4V to Invar fabricated by DED AM is presented. The local microstructure, mechanical properties, elemental composition, and phase composition were characterized. The primary findings of this study are as follows: 

The linearly graded Ti-6Al-4V to Invar FGM contained morphological defects that included material overflow and macroscopic cracking, which preclude the current linear gradient pathway from use in applications.



A low melting point composition, identified by CALPHAD-based thermodynamic calculations, resulting in a drop of approximately 650 K in both solidus and liquidus, was the reason for the material overflow around 12-18 vol.% Invar.



The cracking throughout the gradient was due to the formation of secondary phases in the gradient region, and in particular, the residual stresses developed during cooling due to mismatches in elastic modulus and CTE of disparate phases. These intermetallic phases, identified by a combination of EBSD, XRD, and CALPHAD-based thermodynamic

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calculations, were identified to be FeTi (B2), Fe2Ti (Laves), Ni3Ti (DO24), and NiTi2 (intermetallic). 

All of the experimentally identified secondary phases (FeTi, Fe2Ti, Ni3Ti, and NiTi2) were also predicted by CALPHAD-based thermodynamic calculations. However, the volume fraction and precise location of a computationally predicted phase within the gradient did not always match with its experimentally deduced fraction and location, partially due to the isothermal calculation conditions.

This work, which shows the complementary experimental/computational work needed to advance the development of FGMs, can be used to guide the design of a new FGM of Ti-6Al-4V to Invar made with a nonlinear gradient pathway to avoid deleterious phases.

Acknowledgments Part of this work was supported by a NASA Space Technology Research Fellowship under grant NNX14AL43H. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (NASA).

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Figures

Figure 1. Schematic of the directed energy deposition (DED) process for fabricating a component graded from powder A to powder B with an inset showing the delivery of the two different powders into the melt pool.

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Figure 2. Images of the FGM sample graded from Ti-6Al-4V to Invar. (a) Photograph of sample post still attached to the baseplate prior to cutting. (b) Optical macrograph of the two parts of the sample with numbers denoting the bottom of each layer. Layers 1-21 are Ti-6Al-4V, layers 2253 correspond to the gradient region, and layers 54-75 are Invar. 2

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Figure 3. Phase equilibria computed from the (a) TCFE8 database and (b) a recent assessment [28] of the Fe-Ni-Ti system at 1100 K. The ordered B2 FeTi phase, while having continuous solid solubility from FeTi to B2 NiTi in the recent diagram, does not appear at all in the TCFE8 diagram. Instead the C14 Laves phase is shown to extend into the region understood to actually contain B2.

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Figure 4. Solidus and liquidus temperatures as a function of composition along the vertical section from pure Ti to Invar (Fe-36Ni). As the weight fraction of Invar increases, the liquidus and solidus temperatures sharply drop almost 700 K, achieving minimums of roughly 1300 K at 18 vol.% and 12 vol.% Invar, respectively.

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Figure 5. Phase fractions (molar), as predicted by CALPHAD modeling, as a function of composition at 1100 K along the vertical section from pure Ti to Invar. The bcc phase exists in a single-phase region until 8 vol.% Invar, when NiTi2 begins to form. All the bcc is reacted, leaving only NiTi2 near 25 vol.% Invar. Then ordered B2 FeTi begins to form, with all the NiTi2 disappearing at 37 vol.% Invar. A C14 Laves phase begins to displace bcc near 42 vol.% Invar, with the bcc fully reacted away by 57 vol.% Invar. Shortly thereafter, fcc and a small amount of Ni3Ti begin to form, with all the C14 and Ni3Ti transformed by 94 vol.% Invar, leaving only fcc.

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Figure 6. Comparison of the nominally predicted composition (lines) and composition experimentally measured by EDS (symbols) in vol.% as a function of position from the baseplate.

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Figure 7. Optical and scanning electron microscope images of (a-b) the pure Ti-6Al-4V region of the sample and (c-d) the pure Invar region of the sample, where the build direction is vertical in all images. In (a), the columnar prior-β grains, which grow in the build direction, are visible, as are horizontal bands corresponding to subsequent build layers. In (b), the fine α-lath microstructure is visible. In (c), large columnar grains are visible. In (d), the dendritic structure is visible.

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Figure 8. Secondary electron SEM and EDS elemental maps for selected layers showing the microstructural evolution and the evolution of elemental segregation for each of the five constituent elements. Six layers are shown – the first layer of the gradient (layer 22), one at the first crack (layer 26), one in the middle of the gradient region and at the second crack (layer 33), and one towards the end of the gradient region (layer 47).

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Figure 9. EBSD phase map of Layer 22 (3 vol.% Invar, 97 vol.% Ti-6Al-4V), which shows the Fe2Ti precipitates both at the grain boundaries and within the grain itself.

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Figure 10. Vickers hardness (plotted as average +/- standard deviation) as a function of position from the bottom of the sample overlaid with the decreasing Ti-6Al-4V content and increasing Invar content from EDS line scans.

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Figure 11. X-ray diffraction patterns for various layers in the sample as a function of position, showing the peak locations for hcp/bcc Ti-6Al-4V, and fcc Invar. Extra peaks in the gradient region not corresponding to Ti-6Al-4V or Invar are circled. Fractions of Ti-6Al-4V and Invar are presented in vol.%.

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Figure 12. Electron backscatter diffraction (EBSD) phase maps within the gradient region. (a) In layer 26, FeTi is the majority phase with Fe2Ti present in the interdendritic regions. (b) Layer 33 is composed primarily of FeTi and Fe2Ti. (c) In layer 35, there is an increase in Fe2Ti and decrease of FeTi compared to layer 33, and the fcc Invar phase begins to appear. (d) In layer 45, fcc Invar is the majority phase with particles of FeTi, Fe2Ti, and Ni3Ti.

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Figure 13. TEM images of thin specimen extracted from layer 33. (a) A scanning transmission electron microscope (STEM) image of the entire foil showing the mass contrast between different phases. The black box indicates where the inset high-angle annular dark field (HAADF) image is from, which is the area from which EDS maps were taken. (b) Selected area diffraction pattern (SADP) of the darker region of the sample in (a), which was identified as FeTi. (c) SADP of the lighter region of the sample in (a), which was identified as Fe2Ti. (d) EDS maps of the inset in (a).

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Tables

Table 1. Crystallographic information of primary and secondary phases present in the gradient region of the Ti-6Al-4V to Invar FGM.

Phase

Crystal Structure

Space Group

Lattice Parameters

Ti (hcp)

Hexagonal

6/mm

a=b=2.9511, c=4.6843

FeTi (B2)

Cubic

Pm-3m

a=b=c=2.976

Fe2Ti (C14)

Hexagonal

P63/mmc

a=b=4.757, c=7.829

Ni3Ti (DO24)

Hexagonal

P63/mmc

a=b=5.109, c=8.299

NiTi2

Cubic

Fd-3m

a=b=c=11.3070

Invar (fcc)

Cubic

Fm-3m

a=b=c=3.59281

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Table 2. Area fraction of phases identified by EBSD. A dashed line indicates that the phase was excluded during EBSD analysis. The zero solution is the fraction of the selected area whose crystal structure could not be resolved by the software. Phase

Layer 3

Volume % Invar Phase Ti (hcp) FeTi (B2) Fe2Ti (C14) Ni3Ti (DO24) NiTi2 Invar (fcc) Zero Solution

0 77.9 0.4 0.1 0.0 21.6

Layer Layer Layer Layer 26 33 35 45 15 36 42 73 Area (vol.) % of Phase 1.1 7.25 1.3 71.4 34.8 7.65 3.4 14.6 60.8 43.3 3.7 0.5 19.23 1.4 0.1 0.0 0.2 0.1 1.0 0.5 10.2 89.9 11.3 3.87 12.2 3.4

2

Layer 49 85

Layer 75 100

0.1 0.1 1.1 0.2 0.0 90.1 8.3

0.0 0.0 0.0 0.0 3.12 95.8 1.1

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Table 3. Comparison between experimentally observed and computationally predicted phases. Layer % Invar HV

3 0 360 ±10

26 15 705 ± 6.9

33 36 870 ± 72

45 73 380 ± 26

75 100 130 ± 8.5

EBSD

Ti

Ti, FeTi, Fe2Ti

Ti, FeTi, Fe2Ti, Ni3Ti

Ti, FeTi, Fe2Ti, Ni3Ti, Invar

NiTi2, Invar

XRD – extra peaks

n/a

NiTi2, Ni3Ti, FeTi

Fe2Ti, NiTi2

Fe2Ti, NiTi2

n/a

Calculations

Ti

Ti, FeTi, NiTi2

Ti, FeTi, NiTi2

FeTi, Fe2Ti, Ni3Ti, Invar

Invar

3