Producing of titanium foam using titanium alloy (Al3Ti) by slurry
method.
S. Ahmad1, N.Muhamad2, A. Muchtar2, J. Sahari2, K. R. Jamaludin3, M. H. I. Ibrahim1, N.
H. Mohamad Nor4 and I. Murtadhahadi5.
1. Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn
Malaysia, Batu Pahat, Johor, Malaysia.
2. Faculty of Engineering, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor,
Malaysia.
3. College of Science and Technology, University Technology Malaysia City Campus,
54100, K.Lumpur, Malaysia.
4. Faculty of Engineering, MARA University of Technology, 40450 Shah Alam,
Malaysia.
5. Department of mechanical engineering Lhok seumawe state polytechnic , Acheh,
Indonesia
ABSTRACT
Titanium has an excellent mechanical properties, low density and bio-compatibility. Titanium
foams are attractive for structural and biomedical applications. Because of this properties and
applications we have tried to produce titanium foam using titanium alloy (TiAl6V4). In this paper,
the slurry method was selected to produce titanium foams, in which the sintering process was the
most critical part. Foams will be given to consider the properties of titanium foam after sintering
process. Titanium slurry was prepared by mixing titanium alloy powder, polyethylene glycol
(PEG), methylcellulose and water. Then, the polyurethane (PU) foam was impregnated in the
slurry and dried at room temperature. These were later sintered in a high temperature vacuum
furnace. The effects of sintering on the prepared titanium foams were quantified by measuring the
sintering time, soaking time and rate of heating. All this parameter was fixed to make sure the
result was consistent. The titanium foams were characterised using scanning electron microscopy
(SEM) and energy dispersive x-ray spectrometer (EDXRF). The range of pore size obtained is
between 388µm to 1.07mm, with strut size in the range of 59.4µm to 227µm. The elements found
in the material were titanium, aluminum, phosphorus and calcium. Lastly porosity, density and
compressive strength were tested for this sample.
Keywords; Titanium alloy, slurry method, polyurethane, sintering process, pore size and struts.
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1.0 INTRODUCTION.
Metal foam has many beneficial physical and mechanical properties, for example high
impact energy absorption, high stiffness, high strength to weight ratios, high gas permeability, and
high thermal conductivity. There are several applications of open-celled metal foam which is
widely used as lightweight constructional materials, architectural materials, impact absorbers,
silencers, filters, heat exchangers and implants.
The metal that was used to produce foam is aluminum, zinc, lead, stainless steel and
titanium. Aluminum was the most establish metal to produce foam [1]. In this study, titanium alloy
was used to produce titanium foam. Titanium and its alloy are excellent materials for lightweight
applications at elevated temperatures and are widely used in aeronautical applications. Titanium
foams have an additional potential for weight reduction and could even be suitable for functional
applications if the pore structure were open [2].
One of the most the most promising methods for manufacturing open porous titanium
materials is the sintering of compacted or extruded mixtures of powders and fillers that contain
removable space holder materials. For this study the method that been used are slurry method or
replication technique. This method can produce open celled and high porosity titanium foam [3-4].
The investigation was focused more on sintering process to make sure that titanium alloy wasn’t
oxide. The major limitation of titanium is its chemical reactivity with other materials at elevated
temperatures [5].
2.0 MATERIALS AND METHOD
Titanium alloy powder was purchased from TLS technik GmbH & Co, Germany. The particles
were spherical in shape and the diameters less than 20µm. Water soluble materials, Polyethylene
Glycol (PEG) and Carboxyl Methyl Cellulose (CMC) were used as binders. PEG and CMC are
water soluble materials. Polyurethane (PU) foam was used as scaffold. PU foam was cut to
cylindrical shape with 10 mm diameter and 20 mm height. This size was to fix with ASTM
standard for compressive strength testing.
2.1 PREPARATION OF SAMPLE
Initially, PEG and CMC were stirred in deionised water for one hour. Pure titanium powder was
subsequently added to the solution and stirred for two hours. Thickener was added to form the
slurry, while the stirring was continued for one hour. The function of the thickener was to improve
the slurry rheological property. The titanium slurry was used to impregnate polyurethane (PU)
foam. The PU foams were dipped into the slurry and the dipping and drying processes were
repeated until the struts of the foam were completely coated with titanium slurry. The excess
slurry was then removed by pressing the foam under a roller.
The samples were dried in the oven for 24 hours at 30oC. After the sample was completely
dried, the PU was removed from the matrix by heating it at 600oC for 60 minutes. Subsequently,
the samples were sintered at 1250oC with holding time of two hours. The rate for heating was
1oC/min. The sintering cycle is shown in Figure 1 whereas Figure 2 summarises the titanium
foam preparation process.
2
temp (oC)
2 hour
1250
1 hour
600
time (min)
Figure 1: Sintering cycle for the preparation of the titanium foams.
Polyurethane
Titanium slurry
Vacuum furnace
Sintered samples
Figure 2 : Schematic illustration of the production of the titanium foams
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2.2. SAMPLES CHARACTERISATION.
The software Mettler Toledo Stare System TGA 851e was used for the thermogravimetric
analysis (TGA). It is to measure the thermal stability and composition of the material. It measures
weight changes in the material as a function of temperature (or time) under controlled
atmosphere. TGA was used to study the pyrolysis process of the polyurethane foam and to
determine the burnt off temperature of the PU foam. A Philips XL30 Scanning Electron
Microscopy (SEM) was used for morphological characterization of the samples after sintering.
Furthermore, Energy Dispersive X-rays Fluorescence Spectrometer (EDXRF) Pan Analytical
Machine Model MINIPAL 4 was used to provide qualitative information on the elemental
composition of the samples after sintering process. Bruker model D8 advance X-ray Diffraction
(XRD) Analysis was used to characterize the chemical composition and structure of the samples.
XRD experiments were performed on both as received titanium alloy powder and the sintered
samples.
A liquid displacement method was used to measure the porosity and density of the samples.
In this study, the mechanical testing that be used is compressive strength. An Instron 4505
mechanical tester with 1.0 KN load cell was used for the compression mechanical test. The
crosshead speed was set at 1.00 mm/min, and the load was applied until the titanium foams were
cracked.
3.0 RESULT AND DISCUSSIONS.
The pyrolysis of the polymer is critical in the sintering of the ceramics especially for the polymer
sponge method [6]. Sufficient time should be given to allow polymer burnout before sintering. This
is to avoid cracks in the microstructure. In this research, the TGA was used to determine at which
temperature the polyurethane foam was completely burnt out. Figure 3 shows the weight change
of the polyurethane foam versus temperature. It shows that the polyurethane foam was
completely burnt out at 600 oC. Thus, to allow ample time for the complete burning before the
sintering started, the heating rate was set to 1 0C/min up to 600 oC with a dwell time of 1 hour.
Figure 3 : TGA analysis of the polyurethane foam.
Figure 4 shows the microstructure at different magnification of the titanium foam after sintering at
1250oC in the vacuum furnace. A microstructure with fully densified pore walls or strut is clearly
seen, indicating that the titanium alloy was fully sintered at 1250oC. The pore size was found to
be in the range of 1.07mm to 500µm. SEM result with 300X of magnification show that titanium
alloy was complete sintering because of alloying element in that sample; that was aluminum.
Melting point for aluminum was 660 oC [5] so all aluminum was melted in this sintering process.
Microspores also appear in this result and some ash from the PU foam. This ash was analyzed
using EDXRF. The strut also can be showed using SEM with 190X magnification, it shows in
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Figure 5. The size of the struts in range 59µm to 100 µm, same residual porosity was revealed in
the struts of the sintered body due to the pyrolysis of the polyurethane foam, which can reduce
mechanical strength of the structure.
Figure 4: SEM micrographs of titanium foam sintered in the vacuum furnace. The magnifications
for these samples are 30X (Left) and 300X (Right).
Figure 5: SEM micrographs of titanium foam sintered in the vacuum furnace. The magnifications
for these samples are 190X.
Elemental analyses of the sintered titanium alloy foam were performed using EDXRF. Figure 6
shows the graph for EDXRF. The element that was found is titanium, aluminum, calcium and
phosphorus. Titanium and aluminum was the element from the row material [7] and to proof this
statement; analysis of XRD done for raw material and sample after sintered. On the other hand,
the calcium and phosphorus elements came from the ash of polyurethane foam [8].
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Figure 6: EDXRF result for titanium foam sintered in the vacuum furnace.
400 300 3
After sinter
200 0
100 Before sinter
0
20
30 40 50
60
70
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80 2-Theta Figure 7: XRD result for titanium foam sintered in the vacuum furnace.
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90 In this study, the row material was tested with XRD to make sure the compound that is present
was same with sample after sintering process. Other wise, this result to make sure the sample
wasn’t oxide. If the sample was oxides, the different peak was appearing in this result for sintered
sample. XRD result for row material was following the pattern of PDF 2007:00-052-0859, it was
Aluminum Titanium (Al3Ti). The highest intensity value for 2 theta at 40.844. The peaks for
sample after sintering process was follow the paten and the highest intensity was same with row
material. Other than that, same low peak was appearing. It because same element likes calcium
and phosphorus was in that sample (showed in the EDXRF result).
Lastly the result for density, porosity and compressive strength was showing in the table 1. From
the literatures, the density was proportional with compressive strength but diverse proportional
with porosity if the sintering temperature increases. In this case we can’t see this trade because
we just sintered the sample at one temperature. Other wise, percentage of porosity was in theory
for our application. Our application is to produce bipolar plate for Polymer Electron Membrane
Fuel Cell (PEMFC). The strength stills not enough for this application [9].
Table 1: Result for density, porosity and compressive strength for titanium alloy foam.
Sintering temperature (oC)
Density (g/cm3)
Porosity (%)
Compressive strength (Mpa)
1250
0.88
72.76
14.85
4.0 CONCLUSIONS
Titanium foams have been produced by the slurry method without oxidation has occurred on
the samples. The titanium foam showed pore sizes ranging from 1.07mm to 500µm and the strut
was 59 to 300µm but these did not produce an interconnected structure. The porosity, density
and compressive strength was follow the literatures and must be extend the research on sintering
process and composition of the titanium alloy. The sintering process was found to play a critical
part in forming the titanium foams. For further research, the focus is to produce the stack for
PEMFC application.
5.0 REFERENCES.
[1]
Ashby, M. F., Evans, A. G., Hutchinson, J. W. and Fleck, N. A., “Metal Foams: A Design
Guide” University of Cambridge, Cambridge, UK, 1998.
[2] Degischer H.P and Kriszt B, “Handbook
Processing,Applications” Wiley-VCH, pp22- 23,2002.
of
Cellular
Metals:
Production,
[3] Toru S. “Production of High Porosity Metal Foams using Foamed Polysyrene Sphere”.
(Accessed April 18, 2007) [4] Derek L., Steven L., Jeffrey A. R., Andrew M., Donnamarie A., Zach S. and Bryan S.,
Proceeding MPIF 2008.
[5] Callister W.D. “Materials Science and Engineering an Introduction” John Wiley & Sons, UK,
pp 346-351, 2003.
[6] Hassna R. R. and Miqin Z. “preparation of porous hydroxyapatite scaffolds by combination of
the gel-casting and polimer spange methods” Journal biomaterials 24, pp 3293-3302, 2003.
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[7] Supplier TLS Technik GMbH & Co, Bitterfeld, Jermany.
[8] Jee C.S.Y., Ozguven N., Guo Z.X. and Evans J.R.G “Preparation of High Porosity Metal
Foams” Metallurgical and Materials Transactions B, 2000.
[9] Larminie J and Dicks A. “Fuel Cell Systems Explained” John Wiley & Sons, UK, pp 96, 2003.
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