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Ovalbumin as foaming agent for Ti6Al4V foams produced by gelcasting
Accepted Manuscript Ovalbumin as foaming agent for Ti6Al4V foams produced by gelcasting Lisa Biasetto, Elisângela Guzi de Moraes, Paolo Colombo, Franco Bonollo PII:
S0925-8388(16)31941-7
DOI:
10.1016/j.jallcom.2016.06.218
Reference:
JALCOM 38087
To appear in:
Journal of Alloys and Compounds
Received Date: 12 February 2016 Revised Date:
15 June 2016
Accepted Date: 21 June 2016
Please cite this article as: L. Biasetto, E.G. de Moraes, P. Colombo, F. Bonollo, Ovalbumin as foaming agent for Ti6Al4V foams produced by gelcasting, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.06.218. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title: Ovalbumin as foaming agent for Ti6Al4V foams produced by gelcasting Authors: Lisa Biasetto,1, † Elisângela Guzi de Moraes,2 Paolo Colombo2,3 and Franco Bonollo1 Authors’ affiliations:
Stradella San Nicola 3, 36100 Vicenza, Italy 2
Dipartimento di Ingegneria Industriale, Università di Padova, Via Marzolo 9, 35131
Padova, Italy 3
Department of Materials Science and Engineering, Pennsylvania State University,
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University Park, PA 16801, United States
†
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Dipartimento di Tecnica e Gestione dei Sistemi Industriali, Università di Padova,
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Corresponding author at: Dipartimento di Tecnica e Gestione dei Sistemi Industriali,
Università di Padova, Stradella San Nicola 3, 36100 Vicenza, Italy. Phone: (+39) 044
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499.8747;
Abstract
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e-mail: [email protected]
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Open cell Ti6Al4V metallic foams were fabricated by gelcasting and subsequent sintering. Foaming was conducted in an aqueous environment by using a protein and a surfactant as gelling agent and liquid foam stabilizer, respectively. The effect of different stirring velocities (700 rpm and 1500 rpm) on porosity and pore size distribution was investigated. The shaped foams were pressureless sintered at 1000°C, 1200°C and 1400°C in Argon. The sintering cycle was stopped at intermediate levels in order to study the compositional and morphological evolution of the foams. Ti6Al4V metallic foams with an average cell size of 499~885 µm and total porosity
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ACCEPTED MANUSCRIPT of 71~91 vol% were obtained. The compression strength, for samples sintered at 1400°C, ranged from 24.4± 6.8 MPa up to 79.1± 6.5 MPa.
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Keywords: Metallic Foams; Ti6Al4V; gelcasting, biopolymers.
1. Introduction
The development of titanium and titanium alloys foams for orthopedic applications
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has become a field of rising interest: the reduction of stiffness, associated to a reduction of density is very attractive in the biomaterials field because of the
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reduction of stress shielding between implant and bone, and the increase of specific surface area available for osteoblasts conduction and activity.
Because of its high melting point (Tm=1600-1660°C) and its high gas reactivity (O2 and N2) at the liquid state, titanium and its alloys are mainly processed using the
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Powder Metallurgy (PM) route, especially when porous structures want to be produced. Recent literature reports on different techniques developed to produce titanium and titanium alloys foams: in [1] titanium foams were produced via the
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infiltration of a polyurethane foam with a slurry containing gas atomized spherical
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titanium particles, in [2] Ti6Al4V for biomedical applications were prepared using ammonium bicarbonate as space holder. In [3,4] an overview of the different techniques developed so far to produce titanium foams is reported and include: sintering of uniform and non-uniform powders preform using a mild pressure, use of a gaseus blowing agent, use of processes based on expansion of pressurized bubbles. The different techniques give a wide range of porosity that span from 40% to 95% so as the compression strength may vary form few to some tens of MPa. For comparison, cancellous bone is characterized by compression strength varying form 3
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continuing bone development, and larger than 100 µm [2].
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pores are requested to be interconnected to maintain the vascular system required for
In order to fill these requirements, in this research work, open cell Ti6Al4V metallic foams were fabricated by powder metallurgy and the gelcasting route.
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Conventional gelcasting, a near-net-shape technology, is a well-established colloidal processing method, previously applied to ceramics [6] and rarely to metals powders
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[7], that combines foaming, setting and in situ formation of a percolating network of particles into powder suspensions. This technique is suitable to produce complexshaped porous components with good mechanical properties [8]. The control of pore size and connectivity is possible through the variation of the expansion of the foams
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before setting [9]. The main disadvantages of the gelcasting approach are the large amount of liquid required to obtain a slurry of appropriate viscosity, the levels of shrinkage involved during drying of the bodies, the toxicity of the monomers (acrylate
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polymers) originally used and the necessity to atmosphere control [6].
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To the best of our knowledge, the gelcasting technique, relying on the temperatureinduced gelling of various biopolymers, such as globular proteins [8], has only been very rarely used for the production of metallic foams. In contrast, the pioneering work of Tuck & Evans[12] and Lyckfeldt et al.[10], explored different concentrations of ovalbumin (an environmental-friendly setting agent) in Al2O3 and Si3N4 suspensions, in order to produce porous or dense ceramic materials[9]. In addition, Dhara&Bhargava [11] used ovalbumin from freshly extracted egg white to produce both Al2O3 foams and Aluminum foams with
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ACCEPTED MANUSCRIPT porosities up to 95 vol.% and 45 vol.%, respectively. In [5], titanium foams were produced via gelcasting using a water-based powder suspension, where agar agar was used as gelling agent and Tergitol™ as foaming agent. The foams possessed a porosity ranging from 70 to 80 vol.%.
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In this work, a protein and a surfactant were used as gelling agent and liquid foam stabilizer, respectively. The effect of suspension mixing rate and sintering
temperature on the foam final composition and properties were investigated. The
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Ti6Al4V foams were characterized by a dual mode (macrometer and micrometer
2. Experimental procedure
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range) pores structure.
Ti6Al4V spherical gas atomized powders (TLS Technik GmbH & Co. Spezialpulver
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KG, Bitterfeld, Germany), grain size d50 = 45 µm and purity ~99.7%, were used as raw material in this study. 1 wt% relative to the powders of Polyethylenimine (PEI, Mw = ~750,000 g/mol, solution 50 wt% in H2O, Sigma-Aldrich, Italy) was used as an
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electrosteric dispersion agent to stabilize the suspensions. 5 wt% of Ovalbumin – Egg white albumen (Mw = ~45,000 g/mol, AppliChem Gmbh, Darmstadt, Germany), was
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used as gelling agent and 0.1 vol% of Tergitol™ TMN 10 (Mw = ~683 g/mol, SigmaAldrich, Italy), was used as nonionic surfactant and foam stabilizer. 0.5 wt% of watersoluble methylcellulose (MC, Methocel™, A4M, Dow Chemical Company) solution was used to prepare Ti6Al4V slurries with 35 vol% of solids and containing PEI as cationic polyelectrolyte. MC was used as thickening agent. 0.5 wt% of MC solutions were prepared in a flask containing deionized water (Millipore, electrical resistivity > 18 MΩ·cm-1) heated up
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ACCEPTED MANUSCRIPT to 90°C followed by addition of MC and subsequent magnetic stirring for 120 min. Afterward, the solution was cooled to room temperature in continuous stirring [12]. The Ti6Al4V powder was added stepwise to the MC solutions containing 1 wt% of PEI as cationic polyelectrolyte, upon continuous stirring with a laboratory mixer
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(Yellow line OST Basic, IKA-Werke Gmbh & Co. KG, Staufen, Germany) at 2000
rpm for 20 min at room temperature. The dominating driving force for PEI adsorption is most likely the electrostatic attraction between the positively charged polymer and
aliquots of HNO3 (65% concentrated).
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the negatively charged Ti6Al4V surfaces at pH = 6, which was set by adding small
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Afterwards, the slurry with a 35 vol% concentration was aerated with a double shear mixer during 2 minutes followed by addition of 5 wt% of ovalbumin (egg white, based on the metallic powder content) and subsequently the suspension was vigorously stirred at 700 rpm or 1500 rpm for 3 minutes in order to achieve foaming.
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0.1 vol% of Tergitol™ TMN 10 was added in order to stabilize the wet foam. Then, the foams were poured in a Teflon mold and thermal gelling occurred in a dryer at 80°C for 2 hours because of the cross-linking of amino acids (cistein), followed by
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drying at ambient air for approximately 24h [13]. Sintering was conducted in two steps: a first ramp up to 550°C (2 h; 0.5°C/min heating rate), to decompose the
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organic phase using high vacuum (P=10-4 Pa) as atmosphere; a second ramp up to 1000°C, 1200°C or 1400°C (2 h, 2°C/min heating rate) under 99.99 % Argon flow. Different processing conditions were tested, such as varying mixing rate and sintering at different temperatures, with the aim of determining their effect on the compositional and morphological evolutions of the foams. In Table 1, an overview of the prepared samples is reported.
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INSERT TABLE 1 The crystalline phases were determined on ground samples by X-ray diffraction
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(Bruker D8 Advance diffractometer, Karlsruhe, Germany), using CuKα radiation λ= 1.542 Å, at 40 kV and 40 mA. The 2θ range was varied from 10° to 80°with a step size of 0.05° and a step time of 2 s. Phase identification was performed using the
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Match software package (Crystal Impact GbR, Bonn, Germany) supported by ICDD
PDF-2 Powder Diffraction File. The bulk density of metallic powders was measured
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by gas (helium) pycnometry (AccuPyc 1330, Micromeritics, Norcross, GA), and the total porosity was calculated from the weight-to-volume ratio of the samples. The microstructure of the Ti6Al4V metallic foams was characterized by a field emission gun scanning electron microscope (FEG-SEM Quanta 200, Eindhoven, The
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Netherland), equipped with an energy-dispersive X-ray spectroscopy (EDS-EDAX) detector. The average cell and cell window sizes were obtained from FEG-SEM images using the linear intercept method according to ASTM E112-12 (diagonal
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opposite directions), using an image analysis program (Axio Vision LE)[14]. When
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the cells are spherical and uniformly distributed, according to ASTM D3576-98, the relationship between the average measured chord length t and the average sphere diameter D is: D = 1.623·t. The mechanical behavior of the Ti6Al4V metallic foams was determined by uniaxial compressive strength tests performed using a hydraulic mechanical testing machine (1121 UTM, Instron, Norwood, MA, USA), according to ISO13314:2011 standard, where the sigma yield was kept at the maximum of the linear elastic region. The cross-head speed was 1.0 mm/min and the compressive load cell was 10 kN.
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ACCEPTED MANUSCRIPT Specimens with a nominal size of 10x10x10 mm3, cut from larger bodies, were tested for each sample. Each data point represents the average value of five individual tests.
3. Results and discussion
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The X-ray diffraction patterns of the sintered Ti6Al4V metallic foams are reported in Figure 1. After the heat treatment at 1000°C, almost stoichiometric titanium carbide (TiC) and hypo-stoichiometric titanium oxide (Ti6O) were present in the sample
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(Ti64_1). The presence of Ti6O seems to increase with increasing sintering
temperature (Ti64_2, 1200°C and Ti64_3, 1400°C), as it can be observed in Figure 1,
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where the diffraction pattern for Ti64_3 is characterized by a broadening of the peak at 2θ=35.6°, 38.4° and 39.9°. The presence of both carbides and oxides within the Ti6Al4V structure can be attributed to the presence of residual oxygen and carbon deriving from the organic compounds used for gelling and foaming. The formation of
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rhombohedral Ti6O can be associated to oxygen diffusion within the hexagonal α-Ti structure [15, 16]. The presence of oxygen in correspondence of α-Ti is likely to give
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a solid solution where oxygen diffuses within its reticular hexagonal structure. INSERT FIGURE 1
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The presence of β-Ti was not clearly detected by XRD investigations. EDS maps are reported in Figure 2, where samples Ti64_1, Ti64_2 and Ti64_3 are compared. It is possible to observe the effect of sintering temperature on the sintering degree, that increased with increasing temperature, as expected. For sintering at 1000°C (Ti64_1) the presence of a well distributed vanadium rich phase is present within the grains. At the neck’s junction between the spherical particles, a slight increase of carbon can be observed. For sintering at 1200°C (Ti64_2) the vanadium rich phase becomes more concentrated and is characterized by a larger size. At the
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ACCEPTED MANUSCRIPT neck’s junction, the carbon element is now organized in roundish particles where Al and V signal intensity decreases, thus confirming the presence of TiC, as reported in the XRD analyses. The sintering at 1 400°C gives well sintered powders, where the typical polyhedron shape can be observed; at the junction between the two particles
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reported as an example, it is possible to observe the presence of a carbon rich phase, thus confirming the presence of TiC. In addition, the vanadium rich beta-phase coalesced at the grain boundary.
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INSERT FIGURE 2
The morphology of the samples sintered at different temperatures and processed at a
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mixing rate of 700 rpm (3a, 3b, 3c) and 1500 rpm (3e, 3f, 3g) can be observed in Figure 3. The temperature effect results in increasing sintering degree; while at 1000°C and 1200°C the spherical shape of particles is still detectable, at 1400°C particles appear to be well joined to each other.
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INSERT FIGURE 3
The high stability of the wet foams, achieved by thermal gelling of the liquid phase
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(~80°C, for 2 hours), allowed for a narrow distribution in the average cell and
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window size. The average cell size and cell window size, the total porosity and density values (see later) of the sintered foams are reported in Table 2. The foams morphology consisted of open, interconnected cells, possessing a size inversely proportional to the mixing rate, as a result of the stresses applied to the bubble surface through a series of breakup steps of larger bubbles during shearing. According to the Taylor’s model, the deformation of the dispersed phase only takes place when the shear stress (ηc ) surpasses the interfacial stress (σ /R0), where ηc, is the viscosity of the continuous phase; γ , is the shear rate and σ is the interfacial
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ACCEPTED MANUSCRIPT tension [17]. As a consequence of the increasing of the mechanical shearing (up to 1500 rpm), the cell sizes decreased, and we can observe that it influences the porosity by the means of the increasing of the cell wall thickness [18]. The interplay between the setting of the foam by gel formation and the time allowed
controls the cell size and cell size distribution [14].
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for the destabilization phenomena of the liquid foam (Ostwald ripening and drainage)
The foams are characterized by a bimodal pore size distribution: the large pores
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(d>100 µm, cells) that are the result of the foaming process (air bubbles surrounded
by the Ti64 particles) and smaller pores (d<100 µm, cell windows) distributed on the
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cell walls, that can be attributed to the contact points among the bubbles formed in the liquid. In addition, the further smaller porosity (d< 10 µm) can be attributed to the fact that Ti64 powders are not in strict contact to each other but are just covering the surface of the bubbles, so that they present a contact point that tends to extend upon
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sintering (as observed in Figure 2) but which is not capable of filling the entire empty space among particles. The final result is, consequently, a highly interconnected
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structure with hierarchical porosity.
Samples sintered at 1000°C and 1200°C are characterized by poor mechanical properties deriving from the uncomplete sintering and the high porosity percentage. Typical curves are reported as example in Figure 4, where it can be observed how the sintering temperature greatly affect the mechanical response of samples processed at 1500 rpm (Ti64_4 and Ti64_5), where the sigma yield increases from 2±0.7 to 15.1±1.6 while for samples processed at 700 rpm the values vary from almost 0, when sintered at 1000°C to 2.1±0.5 MPa. It should also be noted the absence of the
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ACCEPTED MANUSCRIPT densification region in samples sintered at low temperature (Fig.4a): the samples started to damage in correspondence of the plateau, where debris were ejected from the sample. In Figure 4b, representative stress-strain curves of the two set of samples (700 rpm
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and 1500 rpm) sintered at 1400 °C are reported. The curves present the typical shape for metallic foams [19] characterized by three regions: quasi elastic, plateau and densification.
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INSERT FIGURE 4
The quasi elastic region is concluded with a maximum of the curve corresponding to
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the sigma yield [19]. The values of sigma yield increases with decreasing cell size. The presence of microcellular porosity positively affect the mechanical response of cellular solids [20].
The behavior of the foams, sintered at 1400°C, in the plateau region is intermediate
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between pure metallic foams (flat and constant plateau, ductile foam) and composite metallic foams where the plateau exhibits waviness (brittle foam). The foams developed in this work are characterized by the presence of in-situ formed ceramic
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particles (mainly TiC) at the grain boundaries that are responsible of the brittle
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component of the mechanical behavior [21]. A crack normal to the load direction appeared after the yield strength, as can be observed in Figure 4b, following what described in the literature [19]. In table 3 an overview of the main mechanical properties of the foams sintered at 1400°C is reported. The plateau stress is more than doubled from 700 rpm to 1500 rpm thus confirming the effect of reduction of cells size. On the other end, the densification strain only slightly varies (from 74% to 80%) changing stirring velocity and consequently cells size. This behavior could be attributed to the fact that the plateau stress (corresponding to plastic deformation of
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evaluation.
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INSERT TABLE 3
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densification strain is not affected by the microstructure of the foam is still under
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In Figure 5, the compression yield strength and porosity of the foams are reported as a function of the sintering temperature. The total porosity decreased with increasing
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heating temperature for samples processed at both 700 rpm (from 87% to 72%) and 1500 rpm (from 90% to 81%), due to increased sintering. On the other hand, the compression yield strength increased with increasing sintering temperature and with
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decreasing amount of porosity. The maximum compression yield strength of 79.1±
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6.5 was observed for the samples processed at 1500 rpm and sintered at 1400°C (e.g. the samples with the highest relative density). The effect of porosity or density on the mechanical properties of foams is a well known behavior, reported in the literature [23], that is here confirmed. The compression yield strength values of the prepared foams sintered at 1400°C were compared to the properties of titanium or titanium alloys foams reported in [2, 3] as function of total porosity (P%). It can be observed that the Ti6Al4V developed in this
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ACCEPTED MANUSCRIPT work by the gelcasting route follow the trend of foams produced by the space holder and replica technique. INSERT FIGURE 6 Further experiments are now under way in order to study the effect of structure and
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composition on the functional and structural properties of the produced foams.
4. Conclusions
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Gelcasting using biopolymers, such as ovalbumin, enabled to manufacture highly interconnected Ti6Al4V metallic foams with total porosity ranging from ~71 to
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91vol%, as well as a wide range of cell and cell-window sizes. Different sintering temperatures were used in order to study the phase evolution (α-Ti and β-Ti) as well as the reaction products between the metallic alloy and the residues of the organic gelling agents. TiC formed at the particles’ boundary starting from 1200°C. The
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vanadium rich beta-phase was uniformly distributed within the powders after sintering at 1000°C, while the V-rich phase tended to coalesce and to segregate at the grain boundary (for sintering at T=1400°C) with increasing sintering temperature.
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Changing the mixing rate (700 rpm and 1500 rpm) led to samples possessing different
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pore volumes and strength. The method proposed in this work extends the range of applicability of gelcasting technique also to titanium alloy powders and can be potentially extended to other metallic powders such as aluminum. The use of gelcasting allows for the casting of titanium complex shapes with reduced secondary operations and this may represent an important added value for the production at the industrial scale of titanium based complex components, such as orthopedic implants.
5. Acknowledgments: this work was developed within the Project 3Poli3, financed
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ACCEPTED MANUSCRIPT by Cassa di Risparmio di Verona and Fondazione Studi Universitari di Vicenza, Italy. Authors are grateful to dr. Saramaria Carturan from LNL-INFN for her kind assistance during heat treatments.
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6. References
[1] K. Korkmaz, The effect of Micro-arc Oxidation treatment on the microstructure and properties of open cell Ti6Al4V alloy foams, Surf. & Coat. Tech. 272 (2015):
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72–78.
[2] M. E. Dizlek, M. Guden, U. Turkan, A. Tasdemirci, Processing and compression
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testing of Ti6Al4V foams for biomedical applications, J Mater Sci. 44.6 (2009): 1512–19.
[3] D.C. Dunand, Processing of titanium foams, Adv Eng Mater. 6.6 (2004): 369–76. [4] R. Singh, P.D. Lee, R.J Dashwood, T.C. Lindley, Titanium foams for biomedical applications: a review, Mater Sci Tech. 25.3–4 (2010): 127–136. [5] M.D.M. Innocentini, R.K. Faleiros, Jr. R. Pisani, I. Thijs, J. Luyten, S. Mullens
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Permeability of porous gelcast scaffolds for bone tissue Engineering, J Porous Mater. 17.5 (2010): 615–27.
[6] P. Sepulveda, Gelcasting foams for porous ceramics, Am Ceram Soc Bull. 76.10 (1997): 61–65.
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[7] K.A. Erk, D.C. Dunand, K.R. Shull, Titanium with controllable pore fractions by thermoreversible gelcasting of TiH2, Acta Mater. 56.18 (2008): 5147–57.
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[8] M.I. Nieto, I. Santacruz, R. Moreno, Shaping of dense advanced ceramics and coatings by gelation of polysaccharides, Adv Eng Mater. 16.6 (2014): 637–54.
[9] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, Processing routes to macroporous ceramics – A review, J Am Ceram Soc. 89.6 (2006): 1771–89.
[10] O. Lyckfeldt, J. Brandt, S. Lesca, Protein forming—a novel shaping technique for ceramics, J Eur Ceram Soc. 20.14 (2000): 2551–59. [11] S. Dhara, P. Bhargava. A simple direct casting route to ceramic foams, J Am Ceram Soc. 86.10 (2003): 1645- 50. [12] C. Tuck, J.R.G. Evans, Porous ceramics prepared from aqueous foams. J Mater Sci Lett.18.13 (1999): 1003–05. 13
ACCEPTED MANUSCRIPT [13] F.S. Ortega, P. Sepulveda, M.D.M. Innocentini, V.C. Pandolfelli. Surfactants: a necessity for producing porous ceramics, Am Ceram Soc Bull. 80.4 (2001): 37–42. [14] E.G. Moraes, P. Colombo, Silicon nitride foams from emulsions, Mat Lett. 128 (2014): 128–31. [15] F.J.C. Braga, R.F.C. Marques, E.A. Filho, A. C. Guastaldi, Surface modification
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of Ti dental implants by Nd:YVO4 laser irradiation, Appl Surf Sci. 253.23 (2007): 9203–08.
[16] A. Jostsons, P.G. McDougall, Fault structures in Ti2O, Phys Stat Sol. 29.2 (1968): 873–89.
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[17] U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Tailoring the microstructure of particle-stabilized wet foams, Langmuir. 23.3 (2007): 1025–32.
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[18] S. Meille, M. Lombardi, J. Chevalier, L. Montanaro, Mechanical properties of porous ceramics in compression: On the transition between elastic, brittle, and cellular behavior, J Eur Ceram Soc. 32.15 (2012): 3959–67. [19] Hans-Peter Degischer, Brigitte Kriszt, Handbook of Cellular Metals, WileyVCH, 2002, ISBN 3527303391, 9783527303397
[20] P. Colombo, E. Bernardo and L.Biasetto, Novel Microcellular Ceramics from a
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Silicone Resin, J Am Ceram Soc. 87.1 (2004): 152–54. [21] I. Duarte, JMF Ferreira. Composite and nanocomposite metal foams (review). Materials 9(2). 2016, DOI: 10.3390/ma9020079.
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1997.
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[22] L.J. Gibson, M. F. Ashby, Cellular solids, Cambridge University Press, 2nd Ed,
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ACCEPTED MANUSCRIPT Table 1. Overview of the prepared samples
Mixing Rate
Tsint
Sample [rpm]
[°C]
700
1000
Ti64_2
700
1200
Ti64_3
700
1400
Ti64_4
1500
1000
Ti64_5
1500
Ti64_6
1500
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Ti64_1
1200
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Mixing Rate
Tsint
Sample
Cell size
Window size
Porosity
[g · cm-3]
(d50)
(d50)
[vol%]
[µm]
[µm]
[°C]
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[rpm]
Density
700
1000
0.39 ± 0.01
733 ± 79
124 ± 71
91.1 ± 0.6
Ti64_2
700
1200
0.42 ± 0.01
633 ± 80
79 ± 48
90.4 ± 0.4
Ti64_3
700
1400
0.79 ± 0.08
732 ±130
140 ± 80
81.3 ± 0.5
Ti64_4
1500
1000
0.51 ± 0.02
92 ± 45
88.3 ± 0.8
Ti64_5
1500
1200
Ti64_6
1500
1400
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579 ± 61
0.51 ± 0.01
586 ± 65
84 ± 39
88.4 ± 0.8
1.25 ± 0.1
532 ±145
119 ± 92
70.7 ± 0.9
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Ti64_1
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Yield Stress [MPa]
24.4± 6.8
79.1± 6.5 MPa
Plateau Stress20-40 [MPa]
23.3±1.2
61.3±3.5
Densification Strain [%]
76±2
81±1
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700 rpm (Ti64_3)
ACCEPTED MANUSCRIPT Figure 1. XRD patterns for samples heat treated at 1000, 1200 and 1400°C. Figure 2. EDS Maps of polished section of samples Ti64_1, Ti64_2 and Ti64_3, from left to right. Figure 3. Morphology of foams processed at 700 rpm (up) and 1500 rpm (down) and sintered using high vacuum and argon at 1000°C (a and d), 1200°C (b and e) and 1400°C (c and f). Figure 4. Stress-strain curves for samples processed at 700 rpm and 1500 rpm and sintered at 1000°C and 1200°C (a)
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and 1400°C (b).
Figure 5. Amount of porosity and compression strength as function of the sintering temperature for samples processed at 700 rpm and 1500 rpm.
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Figure 6. Compression stress yield of titanium and titanium alloy foams found in the literature and produced in this
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1. Ti6Al4V foams were produced by the combination of gelcasting and powder metallurgy 2. Foams possessing a porosity higher than 70% were produced 3. Morphology and composition were varied by processing conditions (mixing rate and sintering tenmperature) 4. Compression strength ranged form 24.4± 6.8 MPa up to 79.1± 6.5 MPa, depending on processing conditions.
S0925-8388(16)31941-7
DOI:
10.1016/j.jallcom.2016.06.218
Reference:
JALCOM 38087
To appear in:
Journal of Alloys and Compounds
Received Date: 12 February 2016 Revised Date:
15 June 2016
Accepted Date: 21 June 2016
Please cite this article as: L. Biasetto, E.G. de Moraes, P. Colombo, F. Bonollo, Ovalbumin as foaming agent for Ti6Al4V foams produced by gelcasting, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.06.218. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Title: Ovalbumin as foaming agent for Ti6Al4V foams produced by gelcasting Authors: Lisa Biasetto,1, † Elisângela Guzi de Moraes,2 Paolo Colombo2,3 and Franco Bonollo1 Authors’ affiliations:
Stradella San Nicola 3, 36100 Vicenza, Italy 2
Dipartimento di Ingegneria Industriale, Università di Padova, Via Marzolo 9, 35131
Padova, Italy 3
Department of Materials Science and Engineering, Pennsylvania State University,
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University Park, PA 16801, United States
†
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Dipartimento di Tecnica e Gestione dei Sistemi Industriali, Università di Padova,
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Corresponding author at: Dipartimento di Tecnica e Gestione dei Sistemi Industriali,
Università di Padova, Stradella San Nicola 3, 36100 Vicenza, Italy. Phone: (+39) 044
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499.8747;
Abstract
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e-mail: [email protected]
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Open cell Ti6Al4V metallic foams were fabricated by gelcasting and subsequent sintering. Foaming was conducted in an aqueous environment by using a protein and a surfactant as gelling agent and liquid foam stabilizer, respectively. The effect of different stirring velocities (700 rpm and 1500 rpm) on porosity and pore size distribution was investigated. The shaped foams were pressureless sintered at 1000°C, 1200°C and 1400°C in Argon. The sintering cycle was stopped at intermediate levels in order to study the compositional and morphological evolution of the foams. Ti6Al4V metallic foams with an average cell size of 499~885 µm and total porosity
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ACCEPTED MANUSCRIPT of 71~91 vol% were obtained. The compression strength, for samples sintered at 1400°C, ranged from 24.4± 6.8 MPa up to 79.1± 6.5 MPa.
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Keywords: Metallic Foams; Ti6Al4V; gelcasting, biopolymers.
1. Introduction
The development of titanium and titanium alloys foams for orthopedic applications
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has become a field of rising interest: the reduction of stiffness, associated to a reduction of density is very attractive in the biomaterials field because of the
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reduction of stress shielding between implant and bone, and the increase of specific surface area available for osteoblasts conduction and activity.
Because of its high melting point (Tm=1600-1660°C) and its high gas reactivity (O2 and N2) at the liquid state, titanium and its alloys are mainly processed using the
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Powder Metallurgy (PM) route, especially when porous structures want to be produced. Recent literature reports on different techniques developed to produce titanium and titanium alloys foams: in [1] titanium foams were produced via the
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infiltration of a polyurethane foam with a slurry containing gas atomized spherical
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titanium particles, in [2] Ti6Al4V for biomedical applications were prepared using ammonium bicarbonate as space holder. In [3,4] an overview of the different techniques developed so far to produce titanium foams is reported and include: sintering of uniform and non-uniform powders preform using a mild pressure, use of a gaseus blowing agent, use of processes based on expansion of pressurized bubbles. The different techniques give a wide range of porosity that span from 40% to 95% so as the compression strength may vary form few to some tens of MPa. For comparison, cancellous bone is characterized by compression strength varying form 3
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ACCEPTED MANUSCRIPT to 20 MPa and Young Modulus varying from 10 to 40 GPa. Besides the mechanical properties, there are other macrostructural requirements to allow cell seeding and migration throughout the scaffolds, such as high porosity level and pore size [5]. The
continuing bone development, and larger than 100 µm [2].
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pores are requested to be interconnected to maintain the vascular system required for
In order to fill these requirements, in this research work, open cell Ti6Al4V metallic foams were fabricated by powder metallurgy and the gelcasting route.
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Conventional gelcasting, a near-net-shape technology, is a well-established colloidal processing method, previously applied to ceramics [6] and rarely to metals powders
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[7], that combines foaming, setting and in situ formation of a percolating network of particles into powder suspensions. This technique is suitable to produce complexshaped porous components with good mechanical properties [8]. The control of pore size and connectivity is possible through the variation of the expansion of the foams
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before setting [9]. The main disadvantages of the gelcasting approach are the large amount of liquid required to obtain a slurry of appropriate viscosity, the levels of shrinkage involved during drying of the bodies, the toxicity of the monomers (acrylate
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polymers) originally used and the necessity to atmosphere control [6].
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To the best of our knowledge, the gelcasting technique, relying on the temperatureinduced gelling of various biopolymers, such as globular proteins [8], has only been very rarely used for the production of metallic foams. In contrast, the pioneering work of Tuck & Evans[12] and Lyckfeldt et al.[10], explored different concentrations of ovalbumin (an environmental-friendly setting agent) in Al2O3 and Si3N4 suspensions, in order to produce porous or dense ceramic materials[9]. In addition, Dhara&Bhargava [11] used ovalbumin from freshly extracted egg white to produce both Al2O3 foams and Aluminum foams with
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ACCEPTED MANUSCRIPT porosities up to 95 vol.% and 45 vol.%, respectively. In [5], titanium foams were produced via gelcasting using a water-based powder suspension, where agar agar was used as gelling agent and Tergitol™ as foaming agent. The foams possessed a porosity ranging from 70 to 80 vol.%.
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In this work, a protein and a surfactant were used as gelling agent and liquid foam stabilizer, respectively. The effect of suspension mixing rate and sintering
temperature on the foam final composition and properties were investigated. The
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Ti6Al4V foams were characterized by a dual mode (macrometer and micrometer
2. Experimental procedure
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range) pores structure.
Ti6Al4V spherical gas atomized powders (TLS Technik GmbH & Co. Spezialpulver
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KG, Bitterfeld, Germany), grain size d50 = 45 µm and purity ~99.7%, were used as raw material in this study. 1 wt% relative to the powders of Polyethylenimine (PEI, Mw = ~750,000 g/mol, solution 50 wt% in H2O, Sigma-Aldrich, Italy) was used as an
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electrosteric dispersion agent to stabilize the suspensions. 5 wt% of Ovalbumin – Egg white albumen (Mw = ~45,000 g/mol, AppliChem Gmbh, Darmstadt, Germany), was
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used as gelling agent and 0.1 vol% of Tergitol™ TMN 10 (Mw = ~683 g/mol, SigmaAldrich, Italy), was used as nonionic surfactant and foam stabilizer. 0.5 wt% of watersoluble methylcellulose (MC, Methocel™, A4M, Dow Chemical Company) solution was used to prepare Ti6Al4V slurries with 35 vol% of solids and containing PEI as cationic polyelectrolyte. MC was used as thickening agent. 0.5 wt% of MC solutions were prepared in a flask containing deionized water (Millipore, electrical resistivity > 18 MΩ·cm-1) heated up
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ACCEPTED MANUSCRIPT to 90°C followed by addition of MC and subsequent magnetic stirring for 120 min. Afterward, the solution was cooled to room temperature in continuous stirring [12]. The Ti6Al4V powder was added stepwise to the MC solutions containing 1 wt% of PEI as cationic polyelectrolyte, upon continuous stirring with a laboratory mixer
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(Yellow line OST Basic, IKA-Werke Gmbh & Co. KG, Staufen, Germany) at 2000
rpm for 20 min at room temperature. The dominating driving force for PEI adsorption is most likely the electrostatic attraction between the positively charged polymer and
aliquots of HNO3 (65% concentrated).
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the negatively charged Ti6Al4V surfaces at pH = 6, which was set by adding small
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Afterwards, the slurry with a 35 vol% concentration was aerated with a double shear mixer during 2 minutes followed by addition of 5 wt% of ovalbumin (egg white, based on the metallic powder content) and subsequently the suspension was vigorously stirred at 700 rpm or 1500 rpm for 3 minutes in order to achieve foaming.
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0.1 vol% of Tergitol™ TMN 10 was added in order to stabilize the wet foam. Then, the foams were poured in a Teflon mold and thermal gelling occurred in a dryer at 80°C for 2 hours because of the cross-linking of amino acids (cistein), followed by
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drying at ambient air for approximately 24h [13]. Sintering was conducted in two steps: a first ramp up to 550°C (2 h; 0.5°C/min heating rate), to decompose the
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organic phase using high vacuum (P=10-4 Pa) as atmosphere; a second ramp up to 1000°C, 1200°C or 1400°C (2 h, 2°C/min heating rate) under 99.99 % Argon flow. Different processing conditions were tested, such as varying mixing rate and sintering at different temperatures, with the aim of determining their effect on the compositional and morphological evolutions of the foams. In Table 1, an overview of the prepared samples is reported.
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INSERT TABLE 1 The crystalline phases were determined on ground samples by X-ray diffraction
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(Bruker D8 Advance diffractometer, Karlsruhe, Germany), using CuKα radiation λ= 1.542 Å, at 40 kV and 40 mA. The 2θ range was varied from 10° to 80°with a step size of 0.05° and a step time of 2 s. Phase identification was performed using the
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Match software package (Crystal Impact GbR, Bonn, Germany) supported by ICDD
PDF-2 Powder Diffraction File. The bulk density of metallic powders was measured
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by gas (helium) pycnometry (AccuPyc 1330, Micromeritics, Norcross, GA), and the total porosity was calculated from the weight-to-volume ratio of the samples. The microstructure of the Ti6Al4V metallic foams was characterized by a field emission gun scanning electron microscope (FEG-SEM Quanta 200, Eindhoven, The
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Netherland), equipped with an energy-dispersive X-ray spectroscopy (EDS-EDAX) detector. The average cell and cell window sizes were obtained from FEG-SEM images using the linear intercept method according to ASTM E112-12 (diagonal
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opposite directions), using an image analysis program (Axio Vision LE)[14]. When
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the cells are spherical and uniformly distributed, according to ASTM D3576-98, the relationship between the average measured chord length t and the average sphere diameter D is: D = 1.623·t. The mechanical behavior of the Ti6Al4V metallic foams was determined by uniaxial compressive strength tests performed using a hydraulic mechanical testing machine (1121 UTM, Instron, Norwood, MA, USA), according to ISO13314:2011 standard, where the sigma yield was kept at the maximum of the linear elastic region. The cross-head speed was 1.0 mm/min and the compressive load cell was 10 kN.
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ACCEPTED MANUSCRIPT Specimens with a nominal size of 10x10x10 mm3, cut from larger bodies, were tested for each sample. Each data point represents the average value of five individual tests.
3. Results and discussion
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The X-ray diffraction patterns of the sintered Ti6Al4V metallic foams are reported in Figure 1. After the heat treatment at 1000°C, almost stoichiometric titanium carbide (TiC) and hypo-stoichiometric titanium oxide (Ti6O) were present in the sample
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(Ti64_1). The presence of Ti6O seems to increase with increasing sintering
temperature (Ti64_2, 1200°C and Ti64_3, 1400°C), as it can be observed in Figure 1,
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where the diffraction pattern for Ti64_3 is characterized by a broadening of the peak at 2θ=35.6°, 38.4° and 39.9°. The presence of both carbides and oxides within the Ti6Al4V structure can be attributed to the presence of residual oxygen and carbon deriving from the organic compounds used for gelling and foaming. The formation of
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rhombohedral Ti6O can be associated to oxygen diffusion within the hexagonal α-Ti structure [15, 16]. The presence of oxygen in correspondence of α-Ti is likely to give
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a solid solution where oxygen diffuses within its reticular hexagonal structure. INSERT FIGURE 1
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The presence of β-Ti was not clearly detected by XRD investigations. EDS maps are reported in Figure 2, where samples Ti64_1, Ti64_2 and Ti64_3 are compared. It is possible to observe the effect of sintering temperature on the sintering degree, that increased with increasing temperature, as expected. For sintering at 1000°C (Ti64_1) the presence of a well distributed vanadium rich phase is present within the grains. At the neck’s junction between the spherical particles, a slight increase of carbon can be observed. For sintering at 1200°C (Ti64_2) the vanadium rich phase becomes more concentrated and is characterized by a larger size. At the
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ACCEPTED MANUSCRIPT neck’s junction, the carbon element is now organized in roundish particles where Al and V signal intensity decreases, thus confirming the presence of TiC, as reported in the XRD analyses. The sintering at 1 400°C gives well sintered powders, where the typical polyhedron shape can be observed; at the junction between the two particles
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reported as an example, it is possible to observe the presence of a carbon rich phase, thus confirming the presence of TiC. In addition, the vanadium rich beta-phase coalesced at the grain boundary.
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INSERT FIGURE 2
The morphology of the samples sintered at different temperatures and processed at a
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mixing rate of 700 rpm (3a, 3b, 3c) and 1500 rpm (3e, 3f, 3g) can be observed in Figure 3. The temperature effect results in increasing sintering degree; while at 1000°C and 1200°C the spherical shape of particles is still detectable, at 1400°C particles appear to be well joined to each other.
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INSERT FIGURE 3
The high stability of the wet foams, achieved by thermal gelling of the liquid phase
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(~80°C, for 2 hours), allowed for a narrow distribution in the average cell and
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window size. The average cell size and cell window size, the total porosity and density values (see later) of the sintered foams are reported in Table 2. The foams morphology consisted of open, interconnected cells, possessing a size inversely proportional to the mixing rate, as a result of the stresses applied to the bubble surface through a series of breakup steps of larger bubbles during shearing. According to the Taylor’s model, the deformation of the dispersed phase only takes place when the shear stress (ηc ) surpasses the interfacial stress (σ /R0), where ηc, is the viscosity of the continuous phase; γ , is the shear rate and σ is the interfacial
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ACCEPTED MANUSCRIPT tension [17]. As a consequence of the increasing of the mechanical shearing (up to 1500 rpm), the cell sizes decreased, and we can observe that it influences the porosity by the means of the increasing of the cell wall thickness [18]. The interplay between the setting of the foam by gel formation and the time allowed
controls the cell size and cell size distribution [14].
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for the destabilization phenomena of the liquid foam (Ostwald ripening and drainage)
The foams are characterized by a bimodal pore size distribution: the large pores
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(d>100 µm, cells) that are the result of the foaming process (air bubbles surrounded
by the Ti64 particles) and smaller pores (d<100 µm, cell windows) distributed on the
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cell walls, that can be attributed to the contact points among the bubbles formed in the liquid. In addition, the further smaller porosity (d< 10 µm) can be attributed to the fact that Ti64 powders are not in strict contact to each other but are just covering the surface of the bubbles, so that they present a contact point that tends to extend upon
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sintering (as observed in Figure 2) but which is not capable of filling the entire empty space among particles. The final result is, consequently, a highly interconnected
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structure with hierarchical porosity.
Samples sintered at 1000°C and 1200°C are characterized by poor mechanical properties deriving from the uncomplete sintering and the high porosity percentage. Typical curves are reported as example in Figure 4, where it can be observed how the sintering temperature greatly affect the mechanical response of samples processed at 1500 rpm (Ti64_4 and Ti64_5), where the sigma yield increases from 2±0.7 to 15.1±1.6 while for samples processed at 700 rpm the values vary from almost 0, when sintered at 1000°C to 2.1±0.5 MPa. It should also be noted the absence of the
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ACCEPTED MANUSCRIPT densification region in samples sintered at low temperature (Fig.4a): the samples started to damage in correspondence of the plateau, where debris were ejected from the sample. In Figure 4b, representative stress-strain curves of the two set of samples (700 rpm
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and 1500 rpm) sintered at 1400 °C are reported. The curves present the typical shape for metallic foams [19] characterized by three regions: quasi elastic, plateau and densification.
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INSERT FIGURE 4
The quasi elastic region is concluded with a maximum of the curve corresponding to
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the sigma yield [19]. The values of sigma yield increases with decreasing cell size. The presence of microcellular porosity positively affect the mechanical response of cellular solids [20].
The behavior of the foams, sintered at 1400°C, in the plateau region is intermediate
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between pure metallic foams (flat and constant plateau, ductile foam) and composite metallic foams where the plateau exhibits waviness (brittle foam). The foams developed in this work are characterized by the presence of in-situ formed ceramic
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particles (mainly TiC) at the grain boundaries that are responsible of the brittle
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component of the mechanical behavior [21]. A crack normal to the load direction appeared after the yield strength, as can be observed in Figure 4b, following what described in the literature [19]. In table 3 an overview of the main mechanical properties of the foams sintered at 1400°C is reported. The plateau stress is more than doubled from 700 rpm to 1500 rpm thus confirming the effect of reduction of cells size. On the other end, the densification strain only slightly varies (from 74% to 80%) changing stirring velocity and consequently cells size. This behavior could be attributed to the fact that the plateau stress (corresponding to plastic deformation of
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ACCEPTED MANUSCRIPT the cells) should be associated to the moment exerted by the applied load, proportional to the length of the cell [22] and consequently decreasing with the decrease of cell size. The long plateau for the foams developed in this work make them good candidates for energy absorption applications. The reason why the
evaluation.
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INSERT TABLE 3
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densification strain is not affected by the microstructure of the foam is still under
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In Figure 5, the compression yield strength and porosity of the foams are reported as a function of the sintering temperature. The total porosity decreased with increasing
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heating temperature for samples processed at both 700 rpm (from 87% to 72%) and 1500 rpm (from 90% to 81%), due to increased sintering. On the other hand, the compression yield strength increased with increasing sintering temperature and with
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decreasing amount of porosity. The maximum compression yield strength of 79.1±
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6.5 was observed for the samples processed at 1500 rpm and sintered at 1400°C (e.g. the samples with the highest relative density). The effect of porosity or density on the mechanical properties of foams is a well known behavior, reported in the literature [23], that is here confirmed. The compression yield strength values of the prepared foams sintered at 1400°C were compared to the properties of titanium or titanium alloys foams reported in [2, 3] as function of total porosity (P%). It can be observed that the Ti6Al4V developed in this
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ACCEPTED MANUSCRIPT work by the gelcasting route follow the trend of foams produced by the space holder and replica technique. INSERT FIGURE 6 Further experiments are now under way in order to study the effect of structure and
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composition on the functional and structural properties of the produced foams.
4. Conclusions
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Gelcasting using biopolymers, such as ovalbumin, enabled to manufacture highly interconnected Ti6Al4V metallic foams with total porosity ranging from ~71 to
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91vol%, as well as a wide range of cell and cell-window sizes. Different sintering temperatures were used in order to study the phase evolution (α-Ti and β-Ti) as well as the reaction products between the metallic alloy and the residues of the organic gelling agents. TiC formed at the particles’ boundary starting from 1200°C. The
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vanadium rich beta-phase was uniformly distributed within the powders after sintering at 1000°C, while the V-rich phase tended to coalesce and to segregate at the grain boundary (for sintering at T=1400°C) with increasing sintering temperature.
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Changing the mixing rate (700 rpm and 1500 rpm) led to samples possessing different
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pore volumes and strength. The method proposed in this work extends the range of applicability of gelcasting technique also to titanium alloy powders and can be potentially extended to other metallic powders such as aluminum. The use of gelcasting allows for the casting of titanium complex shapes with reduced secondary operations and this may represent an important added value for the production at the industrial scale of titanium based complex components, such as orthopedic implants.
5. Acknowledgments: this work was developed within the Project 3Poli3, financed
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ACCEPTED MANUSCRIPT by Cassa di Risparmio di Verona and Fondazione Studi Universitari di Vicenza, Italy. Authors are grateful to dr. Saramaria Carturan from LNL-INFN for her kind assistance during heat treatments.
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6. References
[1] K. Korkmaz, The effect of Micro-arc Oxidation treatment on the microstructure and properties of open cell Ti6Al4V alloy foams, Surf. & Coat. Tech. 272 (2015):
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72–78.
[2] M. E. Dizlek, M. Guden, U. Turkan, A. Tasdemirci, Processing and compression
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testing of Ti6Al4V foams for biomedical applications, J Mater Sci. 44.6 (2009): 1512–19.
[3] D.C. Dunand, Processing of titanium foams, Adv Eng Mater. 6.6 (2004): 369–76. [4] R. Singh, P.D. Lee, R.J Dashwood, T.C. Lindley, Titanium foams for biomedical applications: a review, Mater Sci Tech. 25.3–4 (2010): 127–136. [5] M.D.M. Innocentini, R.K. Faleiros, Jr. R. Pisani, I. Thijs, J. Luyten, S. Mullens
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Permeability of porous gelcast scaffolds for bone tissue Engineering, J Porous Mater. 17.5 (2010): 615–27.
[6] P. Sepulveda, Gelcasting foams for porous ceramics, Am Ceram Soc Bull. 76.10 (1997): 61–65.
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[7] K.A. Erk, D.C. Dunand, K.R. Shull, Titanium with controllable pore fractions by thermoreversible gelcasting of TiH2, Acta Mater. 56.18 (2008): 5147–57.
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[8] M.I. Nieto, I. Santacruz, R. Moreno, Shaping of dense advanced ceramics and coatings by gelation of polysaccharides, Adv Eng Mater. 16.6 (2014): 637–54.
[9] A.R. Studart, U.T. Gonzenbach, E. Tervoort, L.J. Gauckler, Processing routes to macroporous ceramics – A review, J Am Ceram Soc. 89.6 (2006): 1771–89.
[10] O. Lyckfeldt, J. Brandt, S. Lesca, Protein forming—a novel shaping technique for ceramics, J Eur Ceram Soc. 20.14 (2000): 2551–59. [11] S. Dhara, P. Bhargava. A simple direct casting route to ceramic foams, J Am Ceram Soc. 86.10 (2003): 1645- 50. [12] C. Tuck, J.R.G. Evans, Porous ceramics prepared from aqueous foams. J Mater Sci Lett.18.13 (1999): 1003–05. 13
ACCEPTED MANUSCRIPT [13] F.S. Ortega, P. Sepulveda, M.D.M. Innocentini, V.C. Pandolfelli. Surfactants: a necessity for producing porous ceramics, Am Ceram Soc Bull. 80.4 (2001): 37–42. [14] E.G. Moraes, P. Colombo, Silicon nitride foams from emulsions, Mat Lett. 128 (2014): 128–31. [15] F.J.C. Braga, R.F.C. Marques, E.A. Filho, A. C. Guastaldi, Surface modification
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of Ti dental implants by Nd:YVO4 laser irradiation, Appl Surf Sci. 253.23 (2007): 9203–08.
[16] A. Jostsons, P.G. McDougall, Fault structures in Ti2O, Phys Stat Sol. 29.2 (1968): 873–89.
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[17] U.T. Gonzenbach, A.R. Studart, E. Tervoort, L.J. Gauckler, Tailoring the microstructure of particle-stabilized wet foams, Langmuir. 23.3 (2007): 1025–32.
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[18] S. Meille, M. Lombardi, J. Chevalier, L. Montanaro, Mechanical properties of porous ceramics in compression: On the transition between elastic, brittle, and cellular behavior, J Eur Ceram Soc. 32.15 (2012): 3959–67. [19] Hans-Peter Degischer, Brigitte Kriszt, Handbook of Cellular Metals, WileyVCH, 2002, ISBN 3527303391, 9783527303397
[20] P. Colombo, E. Bernardo and L.Biasetto, Novel Microcellular Ceramics from a
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Silicone Resin, J Am Ceram Soc. 87.1 (2004): 152–54. [21] I. Duarte, JMF Ferreira. Composite and nanocomposite metal foams (review). Materials 9(2). 2016, DOI: 10.3390/ma9020079.
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1997.
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[22] L.J. Gibson, M. F. Ashby, Cellular solids, Cambridge University Press, 2nd Ed,
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ACCEPTED MANUSCRIPT Table 1. Overview of the prepared samples
Mixing Rate
Tsint
Sample [rpm]
[°C]
700
1000
Ti64_2
700
1200
Ti64_3
700
1400
Ti64_4
1500
1000
Ti64_5
1500
Ti64_6
1500
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Ti64_1
1200
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1400
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Mixing Rate
Tsint
Sample
Cell size
Window size
Porosity
[g · cm-3]
(d50)
(d50)
[vol%]
[µm]
[µm]
[°C]
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[rpm]
Density
700
1000
0.39 ± 0.01
733 ± 79
124 ± 71
91.1 ± 0.6
Ti64_2
700
1200
0.42 ± 0.01
633 ± 80
79 ± 48
90.4 ± 0.4
Ti64_3
700
1400
0.79 ± 0.08
732 ±130
140 ± 80
81.3 ± 0.5
Ti64_4
1500
1000
0.51 ± 0.02
92 ± 45
88.3 ± 0.8
Ti64_5
1500
1200
Ti64_6
1500
1400
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579 ± 61
0.51 ± 0.01
586 ± 65
84 ± 39
88.4 ± 0.8
1.25 ± 0.1
532 ±145
119 ± 92
70.7 ± 0.9
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Ti64_1
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Yield Stress [MPa]
24.4± 6.8
79.1± 6.5 MPa
Plateau Stress20-40 [MPa]
23.3±1.2
61.3±3.5
Densification Strain [%]
76±2
81±1
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700 rpm (Ti64_3)
ACCEPTED MANUSCRIPT Figure 1. XRD patterns for samples heat treated at 1000, 1200 and 1400°C. Figure 2. EDS Maps of polished section of samples Ti64_1, Ti64_2 and Ti64_3, from left to right. Figure 3. Morphology of foams processed at 700 rpm (up) and 1500 rpm (down) and sintered using high vacuum and argon at 1000°C (a and d), 1200°C (b and e) and 1400°C (c and f). Figure 4. Stress-strain curves for samples processed at 700 rpm and 1500 rpm and sintered at 1000°C and 1200°C (a)
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and 1400°C (b).
Figure 5. Amount of porosity and compression strength as function of the sintering temperature for samples processed at 700 rpm and 1500 rpm.
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Figure 6. Compression stress yield of titanium and titanium alloy foams found in the literature and produced in this
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1. Ti6Al4V foams were produced by the combination of gelcasting and powder metallurgy 2. Foams possessing a porosity higher than 70% were produced 3. Morphology and composition were varied by processing conditions (mixing rate and sintering tenmperature) 4. Compression strength ranged form 24.4± 6.8 MPa up to 79.1± 6.5 MPa, depending on processing conditions.