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Effect of slurry composition on the microstructure and mechanical properties of SS316L open-cell foam
Materials Science & Engineering A 772 (2020) 138798
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Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Effect of slurry composition on the microstructure and mechanical properties of SS316L open-cell foam Teik Yi Lim a, Wei Zhai b, *, Xu Song c, Xiang Yu d, Tao Li a, Beng Wah Chua a, Fangsen Cui d a
Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research, Singapore Department of Mechanical Engineering, National University of Singapore, Singapore c Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Hong Kong, China d Institute of High Performance Computing, Agency for Science, Technology and Research, Singapore b
A R T I C L E I N F O
A B S T R A C T
Keywords: Metallic foam Template replication method Stainless steel Microstructure Stress/strain measurement
This experimental study systematically examines the effect of slurry composition on the microstructure and mechanical properties of open-cell foams, by altering the binder concentration and powder loading of SS316L slurries used in the template replication foam fabrication. The slurry and foam morphologies were observed by Scanning Electron Microscopy (SEM), whereas the rheological properties of the slurry were obtained using a rotational viscometer and parallel plate rheometer. Foam strut densities were measured by a gas pycnometer, which subsequently allowed for the calculation of foam porosity. The mechanical behaviours of the foams were attained through compression test. Results show that increasing both binder and powder contents lead to an increase in slurry viscosity, foam bulk density, green strut density and mechanical strength. A process window for foam fabrication was also proposed based on macroscopic foam morphology, rheological and mechanical properties, and corrosion behaviour of the resulting SS316L foams. Based on the proposed process window, the binder concentration is suggested to be within 10 vol% and 14 vol% and powder loading within Φ ¼ 0.75 and 0.80. This study demonstrates that the microstructure of foam produced by the template replication method can be well-controlled by optimising the composition of the slurry, which is able to attain a higher compression strength than that in the conventional range of Gibson and Ashby scaling law.
1. Introduction The exceptional properties of open-cell metallic foams have found them widespread uses not only in lightweight structural uses [1], but also functional applications such as electrochemical catalysts, vibration and acoustic control and even biomedical implants [2–5]. Various fabrication techniques have been developed since 1990s, and they may be classified into four different routes, namely casting, powder metal lurgy, metal vapour and metal ions. Foam fabrication by powder met allurgy is known to be more controllable than the others thanks to its simplicity and robustness, and it derives several well-known methods such as the space-holder method and template replication method. Of the two, the template replication method can produce foams having a porosity of up to 98% and does not require the application of external pressure [3], hence it becomes the focus of the current study. The template replication method typically involves the formulation of a metal slurry for the coating of a sacrificial polyurethane (PU) foam
template, which will then be debinded and sintered to obtain an opencell metallic foam and taking on the microstructure and morphology of the parent polymeric template [6–8]. The metal slurry commonly consists of the metal powder, a binder, dispersant and solvent. In some cases, a multi-binder system may also be employed in which a single-binder system is found to be impractical [3,5,7,9]. The binder serves as an adhesion promoter, as it increases the viscosity of the slurry and facilitates the adsorption of metal powder particles onto the poly meric template, thus increasing the resulting density and strength of the sintered metallic foam. Dispersant is added to the slurry to reduce sedimentation and agglomeration of powder particles, which may otherwise adversely affect the coating process and result in uneven distribution of densities and strength across different areas of the foam [10]. Studies have shown that apart from heat treatment, rheological properties of the slurry are important for the successful fabrication of open-cell metallic foams by template replication [5,7,9]. A good slurry
* Corresponding author. E-mail address: [email protected] (W. Zhai). https://doi.org/10.1016/j.msea.2019.138798 Received 22 August 2019; Received in revised form 19 November 2019; Accepted 8 December 2019 Available online 15 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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must be stable, meaning that it can deposit on the template as a uniform layer and is non-sedimentating [10]. Kennedy and Lin propose that increasing binder concentration will decrease the amount of sediment formed in the slurry over time and slow down the sedimentation phe nomenon [11]. Nonetheless, it is predicted that there exists a window for optimum binder concentration, in which the said concentration must be high enough to achieve adequate viscosity for the deposition of metal powder particles, yet below a certain ceiling. Once exceeded, it will result in slumping of the metallic foams during heat treatment. Another parameter that greatly affects the rheology of the slurry is the powder loading (Ф), which represents the ratio of metal powder and binder within the slurry: Φ ¼
VP VP þ VB
per inch) polyurethane (PU) foam from FoamPartner™ (FoamPartner Group, Switzerland), which acts as the sacrificial polymer template in the template replication process. The foam is placed into a beaker con taining the slurry, where it is repeatedly squeezed and allowed to expand to its original size to fully absorb the slurry. The repetitive pressing and squeezing motions should be short and fast to facilitate slurry uptake and infiltration. The soaked template must then be removed of excess slurry to prevent the formation of closed cells. The foam is removed from the slurry and repeatedly pressed between two sheets of absorbent tissue paper with a weight, until only faint slurry stains are visible on the tissue paper. The foam is then left to dry in atmosphere overnight. Smaller foam samples of approximately 1.5 cm � 1 cm x 2.5cm are then cut from the dried slurry-coated foam before being subjected to heat treatment. Thermal debinding and sintering processes were completed under nitrogen atmosphere using a Carbolite thermal furnace (Carbolite Ltd., UK), in which the foams were heated to and held at 300 � C, 400 � C and 600 � C for 30 min each, and then sintered at 1300 � C for 2 h before cooling down to 30 � C.
(1)
where VP and VB are the volumes of the metal powder and binder respectively [10]. Effects of the amount of the binder on the microstructure and me chanical properties have been carried out by Manonukul et al. and Ho et al. [10,12]. However, these studies made use of the template repli cation method to fabricate titanium and nickel alloy. Stainless steel foams are rarely studied comparing to more popular options such as aluminium and titanium except the work of Gauthier, Kennedy and Lin [11,13]. They have characterized the rheological properties of a SS316L slurry but did not further investigate the effect of slurry rheology on the mechanical properties of SS316L foam. From the fact that the binder affects not only the viscosity of the slurry but also the powder loading, we can expect that components of a slurry complement each other, and therefore the influences that one single component may bring is not straightforward but complicated. It is important to understand how each component affects the slurry in general and such parameters must be identified to effectively determine the optimal slurry composition, which can produce a slurry capable of fabricating foams with improved strength and functional performance. Therefore, it is essential for us to study the effects of different slurry components on the rheology of the slurry and their effects on the properties of the resultant foams. The aim of this study is to improve the mechanical strength of open-cell metallic foam for lightweight appli cations and increase the success rate of fabrication by optimising the slurry composition, especially by studying the effects of binder con centration and powder loading on the properties of the slurry and resultant foam.
2.2. Characterization 2.2.1. Slurry viscosity Viscosity and torque of the formulated slurries were measured using a Toki Sangyo TVB-15 rotational viscometer (Toki Sangyo Co., Ltd, Japan). Readings were obtained using the accompanying software (Viscoviewer) every 10 s over a set time of 1200 s. For slurries whose viscosities had exceeded the measurement range of the Toki Sangyo viscometer, an Anton Paar MCR 302 Rheometer (Anton Paar GmbH, Austria) was used. The slurry viscosities were measured over a shear rate of 0.001/s to 1000/s using parallel plates. Results were analysed using the Anton Paar RheoCompass software. 2.2.2. Powder morphology and foam microstructure The powder morphology, foam microstructure and pore morphology were observed using a JEOL JSM-IT300LV Tungsten Scanning Electron Microscope (SEM) (JEOL Ltd., Japan). Image analysis using the ImageJ software was performed to analyse and measure the average strut width of the foam. For each foam sample, more than 100 data points were recorded to minimize the effect of any random errors. 2.2.3. Foam density and porosity Strut density of the foam was measured using a Micromeritics Accupyc II 1340 Gas pycnometer (Micromeritics Instrument Corp., USA). The strut density was measured three times and the average value was taken to be the strut density. Bulk density was calculated by dividing the mass of the sintered foam sample over its volume. For each sample, at least three readings were taken for both mass and volume, and the average values were used for the calculation of the bulk density. The mass of the sintered foam was obtained using a mass balance whereas dimensions of the sintered foam were measured using Vernier callipers. Porosity of the foam can then be calculated using the formula � � Bulk density Porosity ¼ 1 100% Strut density
2. Experimental methods 2.1. Fabrication of SS316L open-cell foams SS316L slurries with varying binder concentrations and powder loadings were prepared using SS316L PF-5F powder (Atmix Corpora tion, Japan) (chemical composition shown in Table 1), stearic acid 95% (Sigma-Aldrich Co., USA), B-98 polyvinyl butyral (PVB) (Tape Casting Warehouse Inc., USA) and ethanol 99.9% as the metal powder, disper sant, binder and solvent respectively. Composition of the slurries that were prepared are listed below. To formulate the slurry, stearic acid is first added into ethanol and ball milled until the solution becomes clear, before adding SS316L powder and ball milling for 5 h. PVB is then added to the mixture and ball milled for another 19 h. The resulting slurry is used to coat a 5 cm � 5 cm x 3cm 60 ppi (pores
2.3. Mechanical testing Cylindrical foam samples with diameter of 10mm and height of 15mm were prepared by wire-cutting electrical discharge machining (EDM). The sample dimension and ratio obey the sample geometry re quirements stated in the ISO 13314 international standard for compression testing of porous and cellular materials [15]. Compression testing was carried out using an Instron 5982 universal tester at ambient temperature. A load cell of 1kN was used and the compression strain rate was set at 1 mm/min. Mould release wax was applied on the surface of
Table 1 Chemical composition (%) of the SS316L raw powder [14]. C
Si
Mn
Ni
Cr
Mo
Fe
�
� 1.00
� 2.00
12.0–15.0
16.0–18.0
2.0–3.0
Bal.
0.03
2
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the top and bottom compression plates to reduce any friction that may exist between the sample and the compression plate, which might otherwise result in inaccurate results. Three samples were tested for each solid loading and the compression test was set to end when the applied load reached 900 N. The test may also be interrupted once the foam has undergone densification. Fig. 1 shows the expected compressive behaviour of porous metallic foams. The stress-strain curve may be divided into three distinct regions, namely the elastic region (region I), plateau region (region II) and densification region (III). Compressive strength of the foam is denoted by σc and lies at the end of region I and the start of region II. It signifies the largest amount of stress that the foam may bear before plastic deformation and cell collapse begins. The compressive stress may be obtained by intersecting the stress-strain curve with a straight line that is parallel to the linear elastic region at the 0.2% strain offset [16]. If the linear elastic region does not transition into a plateau region with almost constant stress, or if fluctuations are present in region II, the first maximum compressive stress (FMCS) shall be used in place of σc ac cording ISO 13314. The plateau stress (σpl) is given by the arithmetical mean of stress values between a small strain interval, i.e. between 0.2 and 0.3 as stipulated by ISO 13314. The significance of σpl is that if the applied stress is maintained at this value for an extended amount of time, it will result in total cell collapse and densification once the densification strain, εd is met. εd is obtained by intersecting the trend lines fitted to the plateau and densification region, then reading off the strain value at the intersection point [15]. Fluctuations in stress values that may appear in region II are due to inhomogeneity of strut width, which will result in unequal load bearing between struts. Thinner struts may undergo premature failure, which will manifest as sudden de creases in stress values. As slurries with higher viscosity are more prone to difficult foam impregnation and uneven foam coating, the fluctua tions tend to appear in foams having higher solid loadings. In region III and above the densification stress (σd), the applied stress rises sharply as the struts have fully collapsed and the open-cell nature of the foam is lost.
Fig. 2. SS316L PF-5F raw powder at magnification of x3,000.
have shown that powder morphology influences the rheology of the metal slurry and spherical powder particles are more beneficial towards producing a stable slurry [7]. Thus, in such a case where the powder particles are not completely spherical, ball milling should be the preferred method of mixing during slurry formulation compared to other equipment with more vigorous mixing, so that anomalies in powder morphology are kept to a minimum. 3.2. Effect of dispersant The effect of stearic acid on the SS316L slurry is more evident at smaller magnifications (x100) as shown in the SEM images below. The slurry in Fig. 3(b) has a homogeneous appearance and seems welldispersed, whereas without the addition of stearic acid (Fig. 3(d)), dark streaks of binder can be seen within the heterogeneous slurry. At a larger magnification (x2,000), we can see in Fig. 3(c) that there are certain areas which are darker and have less powder particles. Compared to that of Fig. 3(a), the slurry without dispersant is less ho mogeneous as powder is not well-dispersed and binder coverage varies within different parts of the slurry. When dispersant is added, all powder particles within the slurry appear to be evenly coated by binder. This suggests that the addition of stearic acid is necessary for the powder to be well-dispersed within the slurry and that a more homogeneous and stable slurry can be obtained.
3. Results and discussion 3.1. Powder morphology SEM images of the SS316L raw powder (Fig. 2) reveal that the powder particles are largely spherical in shape, with a small proportion of irregularly-shaped particles. The mean particle size of spherical powder particles is 2.82 � 1.0μm, which is slightly deviated from the D50 value of 4.0 � 1.0μm provided by Atmix Corporation [14]. Studies
3.3. Foam morphology Table 3 shows the morphology of the foams having composition shown in Table 2 after sintering. In general, as binder concentration and powder loading decreased, foams were more likely to be irregular in shape and experiencing warping. Do note that 3.5 vol% PVB and 1 vol% PVB Ф ¼ 0.75 foams are excluded in Table 3 since they were fully burnt off after heat treatment, due to insufficient binder to hold SS316L par ticles together and withstand the sintering process. By visually inspecting and comparing foams with similar powder loading (Ф ¼ 0.75), the 10, 12 and 15 vol% PVB SS316L foams were able to maintain their structural integrity throughout the heat treatment process and experienced minimal to no slumping or warping. However, the 6.5 vol% PVB SS316L foam experienced severe slumping and shrinkage. A 3.5% difference in binder concentration (6.5 vol% PVB and 10 vol% PVB) had severely affected the foam integrity, but a 5% dif ference in the 10 vol% PVB and 15 vol% PVB foams had minimal effect on the structural morphology of the foams and no stark differences could be observed between them. This suggests that there should be a mini mum binder concentration required for foams to uphold their structural integrity upon sintering. As we also know that if too much binder is added, the resulting foam would undergo slumping, a maximum binder
Fig. 1. A typical stress-strain curve of SS316L open-cell foam under compression. 3
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Fig. 3. SS316L slurry with stearic acid at (a) x2,000 and (b) x100 magnification; and SS316L slurry without stearic acid at (c) x2,000 and (d) x100 magnification.
that the slurries behaved as Newtonian fluids and the viscosities measured for each PVB concentration were relatively constant even with change in spindle rotating speed (RPM). As predicted, the viscosity of the slurries increased with increasing PVB concentration as shown in Fig. 4(a). The effect of increasing viscosity is especially significant beyond 6.5 vol% PVB, with the largest increment being a 1.7-fold in crease from 12 vol% PVB to 15 vol% PVB. As binder concentration increases, the foams also have smoother strut surfaces and larger strut widths. The average strut widths of the foams are also shown in Fig. 4 (b). The trend suggests that more powder particles are available per unit volume with increasing binder concen tration, as an increased binder concentration can facilitate the adhesion of powder particles onto the template. Alongside strut width, it is shown that green strut density also in creases with binder concentration as shown in Fig. 4(c). When binder concentration is increased from 10 vol% PVB to 15 vol% PVB, the bulk density and green density increase by approximately 30%, whereas there is only a 1% difference in strut density. This shows that the in crease in adhesion promoter does indeed allow more powder particles to be deposited onto the template. In contrast, strut density is a processdependent property, i.e. it is a measure of densification of the sintered structure, and is thus directly affected by the heat treatment process and sintering parameters rather than the amount of material available for densification. Porosity as shown in Fig. 4(d) also decreases with increasing binder concentration, since increasing bulk density and strut width indirectly indicates that pores are decreasing in size, as more material has to be accommodated within the volume designated by the template dimensions. 10, 12 and 15 vol% PVB Ф ¼ 0.75 SS316L foams were selected to further undergo SEM analysis (Fig. 5). SEM micrographics revealed that the struts appeared rounder with increasing binder concentration, and it is due to an increase in suspension coating mass as mentioned by Quadbeck et al. [17]. The round pore morphology contributes to the mechanical strength of the foam as they can distribute applied stress uniformly. Geometrical discontinuities with small radii of curvature such as sharp points and corners act as stress concentration points which
Table 2 Binder and powder compositions of the SS316L slurries formulated. PVB concentration (vol%)
Powder loading (Ф)
PVB concentration (vol%)
Powder loading (Ф)
1 3.5 6.5 6.5 6.5 6.5 10 12
0.75 0.75 0.75 0.8 0.84 0.87 0.75 0.635
12 12 12 12 13 14 15 15
0.7 0.75 0.8 0.85 0.8 0.8 0.75 0.8
concentration should also exist. Therefore, we suggest that there exists a process window encompassing a range of binder concentrations for successfully fabricating foams by the slurry-coating method. The binder concentration of foams must lie within this window for the sintered products to be self-supported. On the other hand, when comparing foams with similar binder concentration, differences in foam morphology due to small changes in powder loading are more obvious. 12 vol% PVB slurries with powder loadings of 0.635, 0.7, 0.75, 0.8 and 0.85. Upon completion of sintering, only the 12 vol% PVB Ф ¼ 0.7, 0.75, 0.8 and 0.85 SS316L foams remained standing, whereas the Ф ¼ 0.635 foams had shrunk and warped. This observation further supports our hypothesis that a parameter window exists for each binder concentration, in which foams would be structurally strong if the powder loading lies within the win dow, provided the binder concentration does not change. 3.4. Effect of binder on slurry rheology and foam microstructure A batch of PVB slurries which contained only PVB and ethanol were prepared to study the effect of binder concentration on slurry viscosity. Slurries with binder concentrations of 1, 3.5, 6.5, 12 and 15 vol% PVB were prepared and loaded into the viscometer for testing. It was found 4
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Materials Science & Engineering A 772 (2020) 138798
Table 3 Photos of SS316L foams with varying binder concentration and powder loading. PVB Concentration (vol%) 6.5 Powder Loading (Ф)
0.87
0.85
-
10
12
13
14
15
-
-
-
-
-
-
-
-
-
-
-
-
-
-
0.84
-
0.8
-
-
0.75 0.7
0.635
-
-
-
-
-
-
-
-
-
-
Fig. 4. (a) Viscosity of PVB slurries as a function of PVB concentration, (b) strut width, (c) green and sintered strut density, bulk density, and (d) porosity of Ф ¼ 0.75 SS316L foams with varying PVB concentrations. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
may amplify any applied stress and result in crack initiation, propaga tion and ultimately failure [16]. Even though increasing the binder concentration seems beneficial towards foam properties, caution must be exercised because as viscosity
increases with PVB concentration, the probability of obtaining closed pores also increases. As the slurry becomes more viscous and more difficult to handle, it may result in incomplete removal of excess slurry during the coating step and the lead to the formation of clogged pores. 5
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Fig. 5. Microstructure of (a) 10, (b) 12, and (c) 15 vol% PVB Ф ¼ 0.75 SS316L foams. The red circles show the closed-cells within the foam. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. (a) Viscosity, (b) strut width, (c) strut and bulk density, and (d) porosity of 12 vol% PVB SS316L foams with varying powder loading. 6
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Fig. 7. Microstructure of 12 vol% PVB (a) Ф ¼ 0.7, (b) Ф ¼ 0.75, (c) Ф ¼ 0.8 and (d) Ф ¼ 0.85 foams at magnification of x50.
Note that clogged pores may also contribute to decreasing foam porosity.
rotating speed is increased from 0.6 rpm to 6 rpm. This suggests that the shear-thinning behaviour is more prominent and important in slurries with higher powder loading. When working with slurries with high powder loading, one should take advantage of this rheological property to achieve a higher slurry flow rate, as the viscosity of a slurry with high powder loading can be decreased to approach that of a slurry with lower powder loading just by increasing the shear rate. This also justifies the necessity for fast squeezing and pressing actions during foam coating, since such motions impart a higher shear rate to the slurry and will allow for more complete and easier slurry absorption, infiltration and subse quent removal from the foam.
3.5. Effect of SS316L powder on slurry rheology and foam morphology The 6.5 vol% PVB slurry was chosen to further study the effect of the addition of SS316L on slurry rheology, since it is predicted that the addition of powder into the slurry would increase the viscosity of the slurry drastically, and that a starting PVB slurry with lower viscosity would be required to fully visualize the effect of SS316L powder addi tion on the resulting slurry. The rheological properties stated below applies to all other SS316L slurries used in this study. The addition of SS316L powder to form 6.5 vol% PVB slurries with powder loading of 0.8, 0.84 and 0.87 increased the viscosity of the slurry as shown in Fig. 6. As powder loading increases, the viscosity for the slurries also increase when compared at the same spindle speed. The maximum viscosity that can be attained for each SS316L slurry are 1612 mPas, 31690 mPas and 69260 mPas for powder loading of 0.8, 0.84 and 0.87 respectively, which is a dramatic increase compared to the viscosity of the pure 6.5 vol% PVB slurry (27.74 mPas). Note that the viscosity of the slurry decreases with increasing spindle rotating speed. This shows that the slurry behaviour has changed from Newtonian fluid to a pseudoplastic non-Newtonian fluid upon addition of the SS316L powder. The pseudoplastic (shear-thinning) behaviour is particularly important during the slurry coating process because at high shear rates, the slurry will be able to flow with less resistance and the foam can take up the slurry more readily. At low shear rates, the slurry will be highly viscous and show adhesive properties, thus depositing more powder particles onto the foam template. As we can also see from Fig. 6(a) that for Ф ¼ 0.87, the viscosity ranges from approximately 70000 m Pas to slightly under 10000mPas over only a small increase in spindle rotating speed from 0.3 to 3 rpm, but there are no large changes in viscosity for Ф ¼ 0.8 even when the
Fig. 8. Green strut densities of sintered 12 vol% PVB and 6.5 vol% PVB SS316L foams. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 7
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Based on the SEM images in Fig. 7, a significant increase in strut thickness can be observed in the 12 vol% PVB Ф ¼ 0.85 compared to its Ф ¼ 0.7 counterpart. Further analysis using ImageJ confirmed that the 21.4% increase in powder loading resulted in a 22.6% strut width in crease, and the overall trend for the effect of SS316L powder on strut width is shown in Fig. 6(b). Like the effect of increasing binder con centration, struts also appear rounder as powder loading increases (Fig. 7). From this phenomenon, we can conclude that both binder concentration and powder loading contribute to the change in pore morphology and hence both components will affect the mechanical strength of the resulting foam. Increasing powder loading causes strut density and porosity to increase and decrease respectively as shown in Fig. 6(c) and (d), which is due to the higher amount of powder particles available for densification. As binder concentration and powder loading result in similar effects on the foam microstructure, the trends in green strut densities for SS316L foams coated with 12 vol% PVB and 6.5 vol% PVB SS316L slurries (Fig. 8) were studied. The 12 vol% PVB Ф ¼ 0.8 and 6.5 vol% PVB Ф ¼ 0.87 slurries were formulated to contain the same volume of SS316L powder. The green strut density of the 12 vol% PVB Ф ¼ 0.8 foam is higher than that of the 6.5 vol% PVB Ф ¼ 0.87 foam by 0.342 g/ cm3. This reinforces our statement previously made, in which a higher binder concentration promotes the adhesion of powder particles, resulting in a higher green strut density despite being given the same amount of SS316L powder. Although increasing the PVB concentration by 84.6% (from 6.5 vol% PVB to 12 vol% PVB) can result in a 0.3420 g/ cm3 increase in green strut density, merely increasing the powder loading by 3.6% (from 6.5 vol% PVB Ф ¼ 0.84 to 0.87) can result in a 0.3701 g/cm3 green strut density increase. From this, we conclude that the powder loading has a larger influence on the green strut density and mechanical properties of the foam.
denser structure. Note that the 15 vol% PVB Ф ¼ 0.8 foam was excluded from the trend as the sample had corroded after machining, making it unsuitable for comparison analysis as inaccurate results would be ob tained. The corrosion was suspected to have weakened the struts of the foam, resulting in poorer mechanical strength of the sample. If not for the corrosion, the sample is predicted to have mechanical strength similar to or higher than that of the 14 vol% PVB Ф ¼ 0.8 foam as seen in Fig. 9 (b). The increasing trends observed in Fig. 9(b) can be represented in the form of a full stress-strain curve in Fig. 9(a). The 13 vol% PVB curve can be predicted to lie in between the 12 and 14 vol% PVB curve. The 15 vol % PVB curve lies below the 12 vol% PVB curve due to corrosion which had weakened the sample in general. In Fig. 9(a), a stark difference between the 12 vol% PVB curve and the other two curves are evident in the plateau region. A typical stress-strain curve should have a smooth plateau region such as that seen in the 12 vol% PVB sample. The fluc tuations seen in the 14 and 15 vol% PVB sample can be attributed to the high viscosity of the slurry which had complicated slurry infiltration during foam coating, ultimately leading to unequal strut widths within the foam and uneven load bearing among struts. To investigate the effect of powder loading on mechanical strength, 12 vol% PVB Ф ¼ 0.75, 0.8 and 0.85 SS316L foams were also machined to obtain samples for mechanical testing. 12 vol% PVB Ф ¼ 0.635 and 0.7 foams were not tested as the size of these two foams that can be produced without extreme shrinkage or slumping is not large enough to meet the minimum sample size required for mechanical testing. The stress-strain compression curves in Fig. 10(a) are similar to those in Fig. 9(a). The curves shift upwards as the powder loading increases. There are fluctuations in compressive stress within the plateau region of the 12 vol% PVB Ф ¼ 0.85 sample, which can be attributed to the high viscosity of the slurry which complicated complete and thorough slurry infiltration during foam coating, resulting in uneven strut coating and unequal load bearing among struts. Compressive strength and plateau stress generally increase with increasing powder loading as shown in Fig. 10(b), similar to the trend observed in increasing binder content. Fluctuating compressive stress was also observed for the 12 vol% PVB Ф ¼ 0.85 sample in Fig. 10(a). Coupled with similar findings for the 14 and 15 vol% PVB Ф ¼ 0.8 samples, we can indeed suggest that foams with higher solids loading are more susceptible to stress fluctuations in the supposedly plateau region. When deformation modes of all the foams were inspected, it was found that in foam samples with a smooth plateau region, the crush bands were relatively planar and linear. Foams with fluctuating plateaus display either erratic-shaped crush bands or displaced struts. However, the locations where crush bands first form could not be precisely iden tified for either type of foam. Similar observations were made by Ho
3.6. Combined effect of binder and powder loading on mechanical strength in relation to the foam microstructure 12, 13, 14 and 15 vol% PVB Ф ¼ 0.8 SS316L foams were fabricated to investigate the effect of PVB concentration on mechanical strength as they possess adequate structural integrity to undergo machining and be cut into sample sizes suitable for mechanical testing. Representative stress-strain curves are selected and shown below to highlight a few irregularities in the mechanical properties of the foams tested, and ex planations for such anomalies will be provided. As binder concentration increases, compressive strength and plateau stress increase as shown in Fig. 9 (b). This is due to the increased strut width as powder content increases, resulting in increased mechanical strength as a higher compressive force is required to plastically deform a
Fig. 9. (a) Stress-strain curve for 12, 14 and 15 vol% PVB Ф ¼ 0.8 SS316L foams under compression at a strain rate of 1 mm/min; (b) Trends in compressive strength and plateau stress when binder concentration was increased for Ф ¼ 0.8 foam samples. 8
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Fig. 10. (a) Compressive behaviour of 12 vol% PVB Ф ¼ 0.75,0.8 and 0.85 SS316L foams under a compression strain rate of 1 mm/min; (b) Trends in compressive strength and plateau stress when powder loading was increased for 12 vol% PVB foam samples.
chanical properties of the SS316L foams, particularly plateau stress and compressive strength. The quantitative relationships as proposed by Gibson and Ashby are [2]. � �β ρ σ pl ¼ C2 σ y;s
ρs
� �γ
σ c ¼ C3 σy;s
ρ ρs
In which σ y;s is the yield strength of the strut material and is set to 205MPa [18]. ρρ is the relative density of the foam, in which ρ is the bulk s
density of the sintered foam whereas ρs is the density of the strut ma terial, taken to be 7.295 g/cm3 as measured using a gas pycnometer. C2, C3, β and γ are material constants which can be determined experimentally. σ By regressing σy;spl and σσy;sc against the relative densities on a double logarithmic plot in Fig. 12, the following values (refer to Table 4) were obtained for the material constants. The obtained shape factors C2 and C3 aligned well with the values proposed by Gibson and Ashby. On the other hand, the exponents were overestimated by the model, which may be due to the large distribution of values, seeing that the R2 values for the plots were only 0.6646 and 0.5251 respectively. However, it has also been stated the model is simplified and mathematically designed for open-cell foams having an equiaxed cell geometry and relatively homogeneous structure [19]. Previous studies have reported discrepancies in the exponents obtained due to such simplification of the mathematical model [12,20]. In our case, variations in cell geometries are present, and cell size distribution is relatively large, hence the deviations in values from those stated in literature. � �β However, as the value of the exponent decreases, the term ρρ and s � �γ ρ increases, thus if C2 and σy;s are held constant, σ pl increases ρ
Fig. 11. Deformation of (a) 12 vol% PVB Ф ¼ 0.8, (b) 15 vol% PVB Ф ¼ 0.8 and (c) 12 vol% PVB Ф ¼ 0.85 SS316L open-cell foams. Red lines indicate repre sentative crush bands that form as a result of compression. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
et al. in a study on Inconel foams [10]. In Fig. 11(a), besides the first crush band which appear at the top of the foam, a second crush band appeared at near the bottom part of the foam. In this particular sample, the foam first densified at the top, then the bottom, and was finally compressed towards the middle. The red circles in Fig. 11(b) and (c) indicate strut fragments that had broken off due to the compression. They were subsequently displaced from the foam body as compression was continuously applied perpendicular to the cross-section of the foam. For both foam samples, densification occurred from top to bottom of the foam, however the morphology of the crush band varied throughout the deformation process. Our findings are in good agreement with those brought forward by Ho et al., in which foams with higher solids loading deform in a more erratic and unpredictable manner [10]. The Gibson and Ashby model is employed here to predict the me
s
accordingly. As we have obtained exponent values lower than that stated in literature, this suggests that the foams produced in this experiment have higher strength than what can be conventionally produced. 3.7. Fabrication process window In Section 3.3, we suggested that by overlapping parameter windows for both binder concentration and powder loading, an optimum slurry composition may be identified to fabricate foams with higher-thanexpected mechanical strength. However, it is important to clarify that 9
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Fig. 12. Regression of (a) plateau stress and (b) compressive strength against relative density according to the scaling laws proposed by Gibson and Ashby.
concentration of 10 vol% PVB and below, as most but one of the foams fabricated had disintegrated or warped. The 6.5 vol% PVB Φ ¼ 0.87 foam had a regular structure after sintering, but it would not be practical to add an infinitely high amount of powder to obtain a self-supporting structure when the binder concentration can be increased and powder loading decreased. Take note that the powder loading is always less than 1. At 12 vol% PVB, the Φ ¼ 0.635 and Φ ¼ 0.70 foams were not strong enough to undergo compressive testing, hence naturally any Φ ¼ 0.70 foam with binder concentration less than 12 vol% PVB would not be ideal. Our upper limit is not defined by slumping due to excessive amount of binder, but instead the immensely high viscosity of the slurries which are likely to adversely influence fabrication. The viscosities of 12 vol% PVB Φ ¼ 0.85, 15 vol% PVB Φ ¼ 0.75, and 15 vol% PVB Φ ¼ 0.8 SS316L slurries were measured using an Anton Paar MCR 302 Rheometer as they had exceeded the measuring range for the Toki Sangyo TVB-15 rotational viscometer. Their standing viscos ities were all well above 10 kPa⋅s, as seen in Fig. 13. In particular, with reference to Fig. 6(a), the standing viscosity of 12 vol% PVB Φ ¼ 0.80 is within the range of 60 Pa⋅s–70 Pa⋅s; however when powder loading is merely increased by 0.05, the standing viscosity reaches 60 kPa⋅s. As mentioned several times throughout the main text, the difficulty in obtaining even slurry coating and probability of obtaining closed pores increases with increasing slurry viscosity. However in this case, even with fast squeezing and pressing of the foam during coating, it is expected that one is unable to sharply decrease the viscosity just by manually imparting shear, as the standing viscosity of the slurry is simply too high. Hence, these slurries may be deemed less ideal as they will be more difficult to process and easily result in clogged pores. In addition to high viscosity, the 15 vol% PVB Φ ¼ 0.8 foam had also corroded after machining, as mentioned in Section 3.6. In conclusion, the optimum slurry compositions is suggested to lie within the composition boundaries of
Table 4 Experimental and literature values of material constants obtained using the Gibson and Ashby model. Material constant
Experimental value
Literature value [2]
C2 β C3 γ
0.32 1.23 0.24 1.17
0.25–0.35 1.5–2.0 0.1–1.0 1.5
what would be deemed “optimum” will depend on the intended appli cation for the fabricated foam, and a single, “one-size-fits-all” compo sition may not be pinpointed. Hence, a process window encompassing a range of foam compositions that may produce self-supporting and me chanically adequate foams is proposed instead, which may hopefully save future researchers from the hassle of going through a large range of binder concentrations and powder loadings. With reference to Table 3, red borders represent samples that had warped, disintegrated, corroded, or had slurries with extremely high viscosities. Green borders represent “good samples” that remained structurally intact and regular (did not warp) after sintering. The plot reveals a large proportion of “good samples” to be within the ranges of 10 vol% PVB to 14 vol% PVB and Φ ¼ 0.75 to 0.80. These ranges can be said to define the process window of the SS316L foam fabrication pre sented in this study. The lower limit of this process window are foams with binder
(1) Binder concentration is suggested to be within 10 vol% PVB to 14 vol% PVB, and (2) Powder loading is suggested to be within Φ ¼ 0.75 to 0.80. Foams within this range are self-supporting, have relatively good mechanical properties and a lower possibility of obtaining closed pores, and are also easy to work with as the slurry viscosity will allow for easy and complete infiltration. Depending on the application, one may optimize their desired composition to be closer to the lower limit of the process window for a lighter foam, and a composition closer to the upper
Fig. 13. Viscosity of 12 vol% PVB Φ ¼ 0.85, 15 vol% PVB Φ ¼ 0.75 and 15 vol % PVB Φ ¼ 0.8 SS316L slurries measured using parallel plate rheometer. 10
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limit for foams which may require more strength.
Acknowledgement
4. Conclusion
The authors would like to acknowledge the A*STAR project U18–F007SR “Functionally Graded Metallic Foam and Micro-perforated Panel Composite: Experimental and Simulation Studies Towards Nextgenera tion Noise Barrier” for financial support.
The effects of binder concentration and metal powder loading on SS316L slurry rheology, SS316L open-cell foam morphology and its mechanical properties were studied. The addition of metal powder to a pure binder-solvent system will drastically increase the viscosity of the resulting slurry and alter the rheological properties of the slurry to become a non-Newtonian shear-thinning fluid. The shear-thinning behaviour is essential for the fabrication of porous foams by the tem plate replication method, as it allows slurry absorption and infiltration during the coating step, and deposition and adhesion of metal powder particles to the foam struts during drying and the subsequent heat treatment processes. Both binder and metal powder complement each other within the slurry, and increasing either of them can result in an increase in bulk density and mechanical strength of the resulting foam. Increasing the binder concentration allows more powder particles to be deposited onto the template as the binder helps the particles to adhere to the foam struts and increase the strut density. However, too much binder in the composition would result in a weak green body as all polymers within the foam will be burnt off during thermal debinding, causing extreme shrinkage or slumping of the structure. On the other hand, a higher powder loading is required for the foam structure to be selfsupporting and withstand the entire heat treatment, but too much powder in the composition would result in a very viscous slurry that increases the formation of clogged pores within the structure. Thus, it is evident that concentrations of both binder and powder influence the slurry rheology and foam properties greatly, and a balance between binder and powder concentration must be found. Nevertheless, by studying the trends in green strut densities and observing the structural integrity of sintered foams with varying binder and powder composi tions, it is found that the metal powder has a larger effect on the me chanical strength and successful fabrication of the foam. A process window that highlights a range of optimum slurry compositions was proposed. Based on this window, if the slurry has a binder concentration within 10 vol% to 14 vol% and powder loading within Φ ¼ 0.75 to 0.80, the resulting foam can be structurally self-supporting and mechanically adequate for various functional and even structural uses. For lightweight functional applications, a slurry formulation consisting of higher PVB concentration and less powder should be used, whereas more powder should be included in the composition for applications which require good mechanical strength and integrity or applications which require foams of larger sizes.
References [1] M. Gerber, R. Poss, A. Tillmann, G. Walther, B. Kieback, K. Wolf, F. H€ anel, Fabrication and properties of stainless steel foams for sandwich panels, J. Sandw. Struct. Mater. 14 (2012) 181–196, https://doi.org/10.1177/1099636212439878. [2] M.F. Ashby, A. Evans, N.A. Fleck, L.J. Gibson, J.W. Hutchinson, H.N. Wadley, Metal foams: a design guide, Mater. Des. 23 (2002) 119, https://doi.org/10.1016/ S0261-3069(01)00049-8. [3] S. Singh, N. Bhatnagar, A survey of fabrication and application of metallic foams (1925–2017), J. Porous Mater. 25 (2018) 537–554, https://doi.org/10.1007/ s10934-017-0467-1. [4] X. YU, W. ZHAI, X. SONG, F. CUI, Numerical and experimental study on the acoustic performance of Ni-based superalloy open cell foam, Procedia Eng. 214 (2018) 4–8, https://doi.org/10.1016/j.proeng.2017.08.186. [5] J.P. Li, C.A. Van Blitterswijk, K. De Groot, Factors having influence on the rheological properties of Ti6A14V slurry, J. Mater. Sci. Mater. Med. 15 (2004) 951–958, https://doi.org/10.1023/B:JMSM.0000042680.10087.15. [6] W. Zhai, X. Yu, X. Song, L.Y.L. Ang, F. Cui, H.P. Lee, T. Li, Microstructure-based experimental and numerical investigations on the sound absorption property of open-cell metallic foams manufactured by a template replication technique, Mater. Des. 137 (2018) 108–116, https://doi.org/10.1016/j.matdes.2017.10.016. [7] A. Manonukul, M. Tange, P. Srikudvien, N. Denmud, P. Wattanapornphan, Rheological properties of commercially pure titanium slurry for metallic foam production using replica impregnation method, Powder Technol. 266 (2014) 129–134, https://doi.org/10.1016/j.powtec.2014.06.030. [8] P. Quadbeck, K. Kümmel, R. Hauser, G. Standke, J. Adler, G. Stephani, Open cell metal foams – application-oriented structure and material selection, Proc. Cell Mat. (2010) 238–279. [9] H.I. Bakan, K. Korkmaz, Synthesis and properties of metal matrix composite foams based on austenitic stainless steels -titanium carbonitrides, Mater. Des. 83 (2015) 154–158, https://doi.org/10.1016/j.matdes.2015.06.016. [10] N.S.K. Ho, P. Li, S. Raghavan, T. Li, The effect of slurry composition on the microstructure and mechanical properties of open-cell Inconel foams manufactured by the slurry coating technique, Mater. Sci. Eng. A 687 (2017) 123–130, https:// doi.org/10.1016/j.msea.2017.01.038. [11] A.R. Kennedy, X. Lin, Preparation and characterisation of metal powder slurries for use as precursors for metal foams made by gel casting, Powder Metall. 54 (2010) 376–381, https://doi.org/10.1179/003258910x12707304454906. [12] A. Manonukul, P. Srikudvien, M. Tange, Microstructure and mechanical properties of commercially pure titanium foam with varied cell size fabricated by replica impregnation method, Mater. Sci. Eng. A 650 (2016) 432–437, https://doi.org/ 10.1016/j.msea.2015.10.074. [13] M. Gauthier, Structure and properties of open-cell 316L stainless steel foams produced by a powder metallurgy-based process, Porous Met. Met. Foam. Proc. 5th Int. Conf. Metfoam (2008) 149–152, 2007, http://www.scopus.com/inward/rec ord.url?eid¼2-s2.0-56549123632&partnerID¼40&md5¼784aef93f849641d9cf4 bff5b289707c. [14] Epson Atmix Corporation, MIM powder. http://www.atmix.co.jp/en/e_pow der_mimpowder.html, 2019. (Accessed 15 April 2019). [15] International Organization for Standardization, Mechanical Testing of Metals Ductility Testing - Compression Test for Porous and Cellular Metals, International Organization for Standardization, 2011. ISO 13314:2011. [16] J. William D. Callister, D.G. Rethwisch, Materials Science and Engineering: an Introduction, ninth ed., John Wiley & Sons, Inc., 2014. [17] P. Quadbeck, K. Kümmel, R. Hauser, G. Standke, J. Adler, G. Stephani, B. Kieback, Structural and material design of open-cell powder metallurgical foams, Adv. Eng. Mater. 13 (2011) 1024–1030, https://doi.org/10.1002/adem.201100023. [18] ASM Aerospace Specification Metals Inc, ASM material data sheet - AISI type 316L stainless steel, annealed bar, MatWeb (2019). http://asm.matweb.com/search/Spe cificMaterial.asp?bassnum¼MQ316Q. (Accessed 20 May 2019). [19] L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, Second, Cambridge University Press, 1997. [20] C. Park, S.R. Nutt, PM synthesis and properties of steel foams, Mater. Sci. Eng. A 288 (2000) 111–118, https://doi.org/10.1016/S0921-5093(00)00761-9.
Author contribution statement Teik Yi Lim: Conceptualization, Methodology, Validation, Investi gation, Writing - Original Draft. Wei Zhai: Conceptualization, Method ology, Resources, Writing - Review & Editing, Project administration. Xu Song: Writing - Review & Editing. Xiang Yu: Writing - Review & Editing. Tao Li: Resources. Beng Wah Chua: Funding acquisition, Supervision. Fangsen Cui: Funding acquisition, Supervision Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Effect of slurry composition on the microstructure and mechanical properties of SS316L open-cell foam Teik Yi Lim a, Wei Zhai b, *, Xu Song c, Xiang Yu d, Tao Li a, Beng Wah Chua a, Fangsen Cui d a
Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research, Singapore Department of Mechanical Engineering, National University of Singapore, Singapore c Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Hong Kong, China d Institute of High Performance Computing, Agency for Science, Technology and Research, Singapore b
A R T I C L E I N F O
A B S T R A C T
Keywords: Metallic foam Template replication method Stainless steel Microstructure Stress/strain measurement
This experimental study systematically examines the effect of slurry composition on the microstructure and mechanical properties of open-cell foams, by altering the binder concentration and powder loading of SS316L slurries used in the template replication foam fabrication. The slurry and foam morphologies were observed by Scanning Electron Microscopy (SEM), whereas the rheological properties of the slurry were obtained using a rotational viscometer and parallel plate rheometer. Foam strut densities were measured by a gas pycnometer, which subsequently allowed for the calculation of foam porosity. The mechanical behaviours of the foams were attained through compression test. Results show that increasing both binder and powder contents lead to an increase in slurry viscosity, foam bulk density, green strut density and mechanical strength. A process window for foam fabrication was also proposed based on macroscopic foam morphology, rheological and mechanical properties, and corrosion behaviour of the resulting SS316L foams. Based on the proposed process window, the binder concentration is suggested to be within 10 vol% and 14 vol% and powder loading within Φ ¼ 0.75 and 0.80. This study demonstrates that the microstructure of foam produced by the template replication method can be well-controlled by optimising the composition of the slurry, which is able to attain a higher compression strength than that in the conventional range of Gibson and Ashby scaling law.
1. Introduction The exceptional properties of open-cell metallic foams have found them widespread uses not only in lightweight structural uses [1], but also functional applications such as electrochemical catalysts, vibration and acoustic control and even biomedical implants [2–5]. Various fabrication techniques have been developed since 1990s, and they may be classified into four different routes, namely casting, powder metal lurgy, metal vapour and metal ions. Foam fabrication by powder met allurgy is known to be more controllable than the others thanks to its simplicity and robustness, and it derives several well-known methods such as the space-holder method and template replication method. Of the two, the template replication method can produce foams having a porosity of up to 98% and does not require the application of external pressure [3], hence it becomes the focus of the current study. The template replication method typically involves the formulation of a metal slurry for the coating of a sacrificial polyurethane (PU) foam
template, which will then be debinded and sintered to obtain an opencell metallic foam and taking on the microstructure and morphology of the parent polymeric template [6–8]. The metal slurry commonly consists of the metal powder, a binder, dispersant and solvent. In some cases, a multi-binder system may also be employed in which a single-binder system is found to be impractical [3,5,7,9]. The binder serves as an adhesion promoter, as it increases the viscosity of the slurry and facilitates the adsorption of metal powder particles onto the poly meric template, thus increasing the resulting density and strength of the sintered metallic foam. Dispersant is added to the slurry to reduce sedimentation and agglomeration of powder particles, which may otherwise adversely affect the coating process and result in uneven distribution of densities and strength across different areas of the foam [10]. Studies have shown that apart from heat treatment, rheological properties of the slurry are important for the successful fabrication of open-cell metallic foams by template replication [5,7,9]. A good slurry
* Corresponding author. E-mail address: [email protected] (W. Zhai). https://doi.org/10.1016/j.msea.2019.138798 Received 22 August 2019; Received in revised form 19 November 2019; Accepted 8 December 2019 Available online 15 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
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must be stable, meaning that it can deposit on the template as a uniform layer and is non-sedimentating [10]. Kennedy and Lin propose that increasing binder concentration will decrease the amount of sediment formed in the slurry over time and slow down the sedimentation phe nomenon [11]. Nonetheless, it is predicted that there exists a window for optimum binder concentration, in which the said concentration must be high enough to achieve adequate viscosity for the deposition of metal powder particles, yet below a certain ceiling. Once exceeded, it will result in slumping of the metallic foams during heat treatment. Another parameter that greatly affects the rheology of the slurry is the powder loading (Ф), which represents the ratio of metal powder and binder within the slurry: Φ ¼
VP VP þ VB
per inch) polyurethane (PU) foam from FoamPartner™ (FoamPartner Group, Switzerland), which acts as the sacrificial polymer template in the template replication process. The foam is placed into a beaker con taining the slurry, where it is repeatedly squeezed and allowed to expand to its original size to fully absorb the slurry. The repetitive pressing and squeezing motions should be short and fast to facilitate slurry uptake and infiltration. The soaked template must then be removed of excess slurry to prevent the formation of closed cells. The foam is removed from the slurry and repeatedly pressed between two sheets of absorbent tissue paper with a weight, until only faint slurry stains are visible on the tissue paper. The foam is then left to dry in atmosphere overnight. Smaller foam samples of approximately 1.5 cm � 1 cm x 2.5cm are then cut from the dried slurry-coated foam before being subjected to heat treatment. Thermal debinding and sintering processes were completed under nitrogen atmosphere using a Carbolite thermal furnace (Carbolite Ltd., UK), in which the foams were heated to and held at 300 � C, 400 � C and 600 � C for 30 min each, and then sintered at 1300 � C for 2 h before cooling down to 30 � C.
(1)
where VP and VB are the volumes of the metal powder and binder respectively [10]. Effects of the amount of the binder on the microstructure and me chanical properties have been carried out by Manonukul et al. and Ho et al. [10,12]. However, these studies made use of the template repli cation method to fabricate titanium and nickel alloy. Stainless steel foams are rarely studied comparing to more popular options such as aluminium and titanium except the work of Gauthier, Kennedy and Lin [11,13]. They have characterized the rheological properties of a SS316L slurry but did not further investigate the effect of slurry rheology on the mechanical properties of SS316L foam. From the fact that the binder affects not only the viscosity of the slurry but also the powder loading, we can expect that components of a slurry complement each other, and therefore the influences that one single component may bring is not straightforward but complicated. It is important to understand how each component affects the slurry in general and such parameters must be identified to effectively determine the optimal slurry composition, which can produce a slurry capable of fabricating foams with improved strength and functional performance. Therefore, it is essential for us to study the effects of different slurry components on the rheology of the slurry and their effects on the properties of the resultant foams. The aim of this study is to improve the mechanical strength of open-cell metallic foam for lightweight appli cations and increase the success rate of fabrication by optimising the slurry composition, especially by studying the effects of binder con centration and powder loading on the properties of the slurry and resultant foam.
2.2. Characterization 2.2.1. Slurry viscosity Viscosity and torque of the formulated slurries were measured using a Toki Sangyo TVB-15 rotational viscometer (Toki Sangyo Co., Ltd, Japan). Readings were obtained using the accompanying software (Viscoviewer) every 10 s over a set time of 1200 s. For slurries whose viscosities had exceeded the measurement range of the Toki Sangyo viscometer, an Anton Paar MCR 302 Rheometer (Anton Paar GmbH, Austria) was used. The slurry viscosities were measured over a shear rate of 0.001/s to 1000/s using parallel plates. Results were analysed using the Anton Paar RheoCompass software. 2.2.2. Powder morphology and foam microstructure The powder morphology, foam microstructure and pore morphology were observed using a JEOL JSM-IT300LV Tungsten Scanning Electron Microscope (SEM) (JEOL Ltd., Japan). Image analysis using the ImageJ software was performed to analyse and measure the average strut width of the foam. For each foam sample, more than 100 data points were recorded to minimize the effect of any random errors. 2.2.3. Foam density and porosity Strut density of the foam was measured using a Micromeritics Accupyc II 1340 Gas pycnometer (Micromeritics Instrument Corp., USA). The strut density was measured three times and the average value was taken to be the strut density. Bulk density was calculated by dividing the mass of the sintered foam sample over its volume. For each sample, at least three readings were taken for both mass and volume, and the average values were used for the calculation of the bulk density. The mass of the sintered foam was obtained using a mass balance whereas dimensions of the sintered foam were measured using Vernier callipers. Porosity of the foam can then be calculated using the formula � � Bulk density Porosity ¼ 1 100% Strut density
2. Experimental methods 2.1. Fabrication of SS316L open-cell foams SS316L slurries with varying binder concentrations and powder loadings were prepared using SS316L PF-5F powder (Atmix Corpora tion, Japan) (chemical composition shown in Table 1), stearic acid 95% (Sigma-Aldrich Co., USA), B-98 polyvinyl butyral (PVB) (Tape Casting Warehouse Inc., USA) and ethanol 99.9% as the metal powder, disper sant, binder and solvent respectively. Composition of the slurries that were prepared are listed below. To formulate the slurry, stearic acid is first added into ethanol and ball milled until the solution becomes clear, before adding SS316L powder and ball milling for 5 h. PVB is then added to the mixture and ball milled for another 19 h. The resulting slurry is used to coat a 5 cm � 5 cm x 3cm 60 ppi (pores
2.3. Mechanical testing Cylindrical foam samples with diameter of 10mm and height of 15mm were prepared by wire-cutting electrical discharge machining (EDM). The sample dimension and ratio obey the sample geometry re quirements stated in the ISO 13314 international standard for compression testing of porous and cellular materials [15]. Compression testing was carried out using an Instron 5982 universal tester at ambient temperature. A load cell of 1kN was used and the compression strain rate was set at 1 mm/min. Mould release wax was applied on the surface of
Table 1 Chemical composition (%) of the SS316L raw powder [14]. C
Si
Mn
Ni
Cr
Mo
Fe
�
� 1.00
� 2.00
12.0–15.0
16.0–18.0
2.0–3.0
Bal.
0.03
2
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the top and bottom compression plates to reduce any friction that may exist between the sample and the compression plate, which might otherwise result in inaccurate results. Three samples were tested for each solid loading and the compression test was set to end when the applied load reached 900 N. The test may also be interrupted once the foam has undergone densification. Fig. 1 shows the expected compressive behaviour of porous metallic foams. The stress-strain curve may be divided into three distinct regions, namely the elastic region (region I), plateau region (region II) and densification region (III). Compressive strength of the foam is denoted by σc and lies at the end of region I and the start of region II. It signifies the largest amount of stress that the foam may bear before plastic deformation and cell collapse begins. The compressive stress may be obtained by intersecting the stress-strain curve with a straight line that is parallel to the linear elastic region at the 0.2% strain offset [16]. If the linear elastic region does not transition into a plateau region with almost constant stress, or if fluctuations are present in region II, the first maximum compressive stress (FMCS) shall be used in place of σc ac cording ISO 13314. The plateau stress (σpl) is given by the arithmetical mean of stress values between a small strain interval, i.e. between 0.2 and 0.3 as stipulated by ISO 13314. The significance of σpl is that if the applied stress is maintained at this value for an extended amount of time, it will result in total cell collapse and densification once the densification strain, εd is met. εd is obtained by intersecting the trend lines fitted to the plateau and densification region, then reading off the strain value at the intersection point [15]. Fluctuations in stress values that may appear in region II are due to inhomogeneity of strut width, which will result in unequal load bearing between struts. Thinner struts may undergo premature failure, which will manifest as sudden de creases in stress values. As slurries with higher viscosity are more prone to difficult foam impregnation and uneven foam coating, the fluctua tions tend to appear in foams having higher solid loadings. In region III and above the densification stress (σd), the applied stress rises sharply as the struts have fully collapsed and the open-cell nature of the foam is lost.
Fig. 2. SS316L PF-5F raw powder at magnification of x3,000.
have shown that powder morphology influences the rheology of the metal slurry and spherical powder particles are more beneficial towards producing a stable slurry [7]. Thus, in such a case where the powder particles are not completely spherical, ball milling should be the preferred method of mixing during slurry formulation compared to other equipment with more vigorous mixing, so that anomalies in powder morphology are kept to a minimum. 3.2. Effect of dispersant The effect of stearic acid on the SS316L slurry is more evident at smaller magnifications (x100) as shown in the SEM images below. The slurry in Fig. 3(b) has a homogeneous appearance and seems welldispersed, whereas without the addition of stearic acid (Fig. 3(d)), dark streaks of binder can be seen within the heterogeneous slurry. At a larger magnification (x2,000), we can see in Fig. 3(c) that there are certain areas which are darker and have less powder particles. Compared to that of Fig. 3(a), the slurry without dispersant is less ho mogeneous as powder is not well-dispersed and binder coverage varies within different parts of the slurry. When dispersant is added, all powder particles within the slurry appear to be evenly coated by binder. This suggests that the addition of stearic acid is necessary for the powder to be well-dispersed within the slurry and that a more homogeneous and stable slurry can be obtained.
3. Results and discussion 3.1. Powder morphology SEM images of the SS316L raw powder (Fig. 2) reveal that the powder particles are largely spherical in shape, with a small proportion of irregularly-shaped particles. The mean particle size of spherical powder particles is 2.82 � 1.0μm, which is slightly deviated from the D50 value of 4.0 � 1.0μm provided by Atmix Corporation [14]. Studies
3.3. Foam morphology Table 3 shows the morphology of the foams having composition shown in Table 2 after sintering. In general, as binder concentration and powder loading decreased, foams were more likely to be irregular in shape and experiencing warping. Do note that 3.5 vol% PVB and 1 vol% PVB Ф ¼ 0.75 foams are excluded in Table 3 since they were fully burnt off after heat treatment, due to insufficient binder to hold SS316L par ticles together and withstand the sintering process. By visually inspecting and comparing foams with similar powder loading (Ф ¼ 0.75), the 10, 12 and 15 vol% PVB SS316L foams were able to maintain their structural integrity throughout the heat treatment process and experienced minimal to no slumping or warping. However, the 6.5 vol% PVB SS316L foam experienced severe slumping and shrinkage. A 3.5% difference in binder concentration (6.5 vol% PVB and 10 vol% PVB) had severely affected the foam integrity, but a 5% dif ference in the 10 vol% PVB and 15 vol% PVB foams had minimal effect on the structural morphology of the foams and no stark differences could be observed between them. This suggests that there should be a mini mum binder concentration required for foams to uphold their structural integrity upon sintering. As we also know that if too much binder is added, the resulting foam would undergo slumping, a maximum binder
Fig. 1. A typical stress-strain curve of SS316L open-cell foam under compression. 3
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Fig. 3. SS316L slurry with stearic acid at (a) x2,000 and (b) x100 magnification; and SS316L slurry without stearic acid at (c) x2,000 and (d) x100 magnification.
that the slurries behaved as Newtonian fluids and the viscosities measured for each PVB concentration were relatively constant even with change in spindle rotating speed (RPM). As predicted, the viscosity of the slurries increased with increasing PVB concentration as shown in Fig. 4(a). The effect of increasing viscosity is especially significant beyond 6.5 vol% PVB, with the largest increment being a 1.7-fold in crease from 12 vol% PVB to 15 vol% PVB. As binder concentration increases, the foams also have smoother strut surfaces and larger strut widths. The average strut widths of the foams are also shown in Fig. 4 (b). The trend suggests that more powder particles are available per unit volume with increasing binder concen tration, as an increased binder concentration can facilitate the adhesion of powder particles onto the template. Alongside strut width, it is shown that green strut density also in creases with binder concentration as shown in Fig. 4(c). When binder concentration is increased from 10 vol% PVB to 15 vol% PVB, the bulk density and green density increase by approximately 30%, whereas there is only a 1% difference in strut density. This shows that the in crease in adhesion promoter does indeed allow more powder particles to be deposited onto the template. In contrast, strut density is a processdependent property, i.e. it is a measure of densification of the sintered structure, and is thus directly affected by the heat treatment process and sintering parameters rather than the amount of material available for densification. Porosity as shown in Fig. 4(d) also decreases with increasing binder concentration, since increasing bulk density and strut width indirectly indicates that pores are decreasing in size, as more material has to be accommodated within the volume designated by the template dimensions. 10, 12 and 15 vol% PVB Ф ¼ 0.75 SS316L foams were selected to further undergo SEM analysis (Fig. 5). SEM micrographics revealed that the struts appeared rounder with increasing binder concentration, and it is due to an increase in suspension coating mass as mentioned by Quadbeck et al. [17]. The round pore morphology contributes to the mechanical strength of the foam as they can distribute applied stress uniformly. Geometrical discontinuities with small radii of curvature such as sharp points and corners act as stress concentration points which
Table 2 Binder and powder compositions of the SS316L slurries formulated. PVB concentration (vol%)
Powder loading (Ф)
PVB concentration (vol%)
Powder loading (Ф)
1 3.5 6.5 6.5 6.5 6.5 10 12
0.75 0.75 0.75 0.8 0.84 0.87 0.75 0.635
12 12 12 12 13 14 15 15
0.7 0.75 0.8 0.85 0.8 0.8 0.75 0.8
concentration should also exist. Therefore, we suggest that there exists a process window encompassing a range of binder concentrations for successfully fabricating foams by the slurry-coating method. The binder concentration of foams must lie within this window for the sintered products to be self-supported. On the other hand, when comparing foams with similar binder concentration, differences in foam morphology due to small changes in powder loading are more obvious. 12 vol% PVB slurries with powder loadings of 0.635, 0.7, 0.75, 0.8 and 0.85. Upon completion of sintering, only the 12 vol% PVB Ф ¼ 0.7, 0.75, 0.8 and 0.85 SS316L foams remained standing, whereas the Ф ¼ 0.635 foams had shrunk and warped. This observation further supports our hypothesis that a parameter window exists for each binder concentration, in which foams would be structurally strong if the powder loading lies within the win dow, provided the binder concentration does not change. 3.4. Effect of binder on slurry rheology and foam microstructure A batch of PVB slurries which contained only PVB and ethanol were prepared to study the effect of binder concentration on slurry viscosity. Slurries with binder concentrations of 1, 3.5, 6.5, 12 and 15 vol% PVB were prepared and loaded into the viscometer for testing. It was found 4
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Table 3 Photos of SS316L foams with varying binder concentration and powder loading. PVB Concentration (vol%) 6.5 Powder Loading (Ф)
0.87
0.85
-
10
12
13
14
15
-
-
-
-
-
-
-
-
-
-
-
-
-
-
0.84
-
0.8
-
-
0.75 0.7
0.635
-
-
-
-
-
-
-
-
-
-
Fig. 4. (a) Viscosity of PVB slurries as a function of PVB concentration, (b) strut width, (c) green and sintered strut density, bulk density, and (d) porosity of Ф ¼ 0.75 SS316L foams with varying PVB concentrations. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
may amplify any applied stress and result in crack initiation, propaga tion and ultimately failure [16]. Even though increasing the binder concentration seems beneficial towards foam properties, caution must be exercised because as viscosity
increases with PVB concentration, the probability of obtaining closed pores also increases. As the slurry becomes more viscous and more difficult to handle, it may result in incomplete removal of excess slurry during the coating step and the lead to the formation of clogged pores. 5
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Fig. 5. Microstructure of (a) 10, (b) 12, and (c) 15 vol% PVB Ф ¼ 0.75 SS316L foams. The red circles show the closed-cells within the foam. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. (a) Viscosity, (b) strut width, (c) strut and bulk density, and (d) porosity of 12 vol% PVB SS316L foams with varying powder loading. 6
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Fig. 7. Microstructure of 12 vol% PVB (a) Ф ¼ 0.7, (b) Ф ¼ 0.75, (c) Ф ¼ 0.8 and (d) Ф ¼ 0.85 foams at magnification of x50.
Note that clogged pores may also contribute to decreasing foam porosity.
rotating speed is increased from 0.6 rpm to 6 rpm. This suggests that the shear-thinning behaviour is more prominent and important in slurries with higher powder loading. When working with slurries with high powder loading, one should take advantage of this rheological property to achieve a higher slurry flow rate, as the viscosity of a slurry with high powder loading can be decreased to approach that of a slurry with lower powder loading just by increasing the shear rate. This also justifies the necessity for fast squeezing and pressing actions during foam coating, since such motions impart a higher shear rate to the slurry and will allow for more complete and easier slurry absorption, infiltration and subse quent removal from the foam.
3.5. Effect of SS316L powder on slurry rheology and foam morphology The 6.5 vol% PVB slurry was chosen to further study the effect of the addition of SS316L on slurry rheology, since it is predicted that the addition of powder into the slurry would increase the viscosity of the slurry drastically, and that a starting PVB slurry with lower viscosity would be required to fully visualize the effect of SS316L powder addi tion on the resulting slurry. The rheological properties stated below applies to all other SS316L slurries used in this study. The addition of SS316L powder to form 6.5 vol% PVB slurries with powder loading of 0.8, 0.84 and 0.87 increased the viscosity of the slurry as shown in Fig. 6. As powder loading increases, the viscosity for the slurries also increase when compared at the same spindle speed. The maximum viscosity that can be attained for each SS316L slurry are 1612 mPas, 31690 mPas and 69260 mPas for powder loading of 0.8, 0.84 and 0.87 respectively, which is a dramatic increase compared to the viscosity of the pure 6.5 vol% PVB slurry (27.74 mPas). Note that the viscosity of the slurry decreases with increasing spindle rotating speed. This shows that the slurry behaviour has changed from Newtonian fluid to a pseudoplastic non-Newtonian fluid upon addition of the SS316L powder. The pseudoplastic (shear-thinning) behaviour is particularly important during the slurry coating process because at high shear rates, the slurry will be able to flow with less resistance and the foam can take up the slurry more readily. At low shear rates, the slurry will be highly viscous and show adhesive properties, thus depositing more powder particles onto the foam template. As we can also see from Fig. 6(a) that for Ф ¼ 0.87, the viscosity ranges from approximately 70000 m Pas to slightly under 10000mPas over only a small increase in spindle rotating speed from 0.3 to 3 rpm, but there are no large changes in viscosity for Ф ¼ 0.8 even when the
Fig. 8. Green strut densities of sintered 12 vol% PVB and 6.5 vol% PVB SS316L foams. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 7
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Based on the SEM images in Fig. 7, a significant increase in strut thickness can be observed in the 12 vol% PVB Ф ¼ 0.85 compared to its Ф ¼ 0.7 counterpart. Further analysis using ImageJ confirmed that the 21.4% increase in powder loading resulted in a 22.6% strut width in crease, and the overall trend for the effect of SS316L powder on strut width is shown in Fig. 6(b). Like the effect of increasing binder con centration, struts also appear rounder as powder loading increases (Fig. 7). From this phenomenon, we can conclude that both binder concentration and powder loading contribute to the change in pore morphology and hence both components will affect the mechanical strength of the resulting foam. Increasing powder loading causes strut density and porosity to increase and decrease respectively as shown in Fig. 6(c) and (d), which is due to the higher amount of powder particles available for densification. As binder concentration and powder loading result in similar effects on the foam microstructure, the trends in green strut densities for SS316L foams coated with 12 vol% PVB and 6.5 vol% PVB SS316L slurries (Fig. 8) were studied. The 12 vol% PVB Ф ¼ 0.8 and 6.5 vol% PVB Ф ¼ 0.87 slurries were formulated to contain the same volume of SS316L powder. The green strut density of the 12 vol% PVB Ф ¼ 0.8 foam is higher than that of the 6.5 vol% PVB Ф ¼ 0.87 foam by 0.342 g/ cm3. This reinforces our statement previously made, in which a higher binder concentration promotes the adhesion of powder particles, resulting in a higher green strut density despite being given the same amount of SS316L powder. Although increasing the PVB concentration by 84.6% (from 6.5 vol% PVB to 12 vol% PVB) can result in a 0.3420 g/ cm3 increase in green strut density, merely increasing the powder loading by 3.6% (from 6.5 vol% PVB Ф ¼ 0.84 to 0.87) can result in a 0.3701 g/cm3 green strut density increase. From this, we conclude that the powder loading has a larger influence on the green strut density and mechanical properties of the foam.
denser structure. Note that the 15 vol% PVB Ф ¼ 0.8 foam was excluded from the trend as the sample had corroded after machining, making it unsuitable for comparison analysis as inaccurate results would be ob tained. The corrosion was suspected to have weakened the struts of the foam, resulting in poorer mechanical strength of the sample. If not for the corrosion, the sample is predicted to have mechanical strength similar to or higher than that of the 14 vol% PVB Ф ¼ 0.8 foam as seen in Fig. 9 (b). The increasing trends observed in Fig. 9(b) can be represented in the form of a full stress-strain curve in Fig. 9(a). The 13 vol% PVB curve can be predicted to lie in between the 12 and 14 vol% PVB curve. The 15 vol % PVB curve lies below the 12 vol% PVB curve due to corrosion which had weakened the sample in general. In Fig. 9(a), a stark difference between the 12 vol% PVB curve and the other two curves are evident in the plateau region. A typical stress-strain curve should have a smooth plateau region such as that seen in the 12 vol% PVB sample. The fluc tuations seen in the 14 and 15 vol% PVB sample can be attributed to the high viscosity of the slurry which had complicated slurry infiltration during foam coating, ultimately leading to unequal strut widths within the foam and uneven load bearing among struts. To investigate the effect of powder loading on mechanical strength, 12 vol% PVB Ф ¼ 0.75, 0.8 and 0.85 SS316L foams were also machined to obtain samples for mechanical testing. 12 vol% PVB Ф ¼ 0.635 and 0.7 foams were not tested as the size of these two foams that can be produced without extreme shrinkage or slumping is not large enough to meet the minimum sample size required for mechanical testing. The stress-strain compression curves in Fig. 10(a) are similar to those in Fig. 9(a). The curves shift upwards as the powder loading increases. There are fluctuations in compressive stress within the plateau region of the 12 vol% PVB Ф ¼ 0.85 sample, which can be attributed to the high viscosity of the slurry which complicated complete and thorough slurry infiltration during foam coating, resulting in uneven strut coating and unequal load bearing among struts. Compressive strength and plateau stress generally increase with increasing powder loading as shown in Fig. 10(b), similar to the trend observed in increasing binder content. Fluctuating compressive stress was also observed for the 12 vol% PVB Ф ¼ 0.85 sample in Fig. 10(a). Coupled with similar findings for the 14 and 15 vol% PVB Ф ¼ 0.8 samples, we can indeed suggest that foams with higher solids loading are more susceptible to stress fluctuations in the supposedly plateau region. When deformation modes of all the foams were inspected, it was found that in foam samples with a smooth plateau region, the crush bands were relatively planar and linear. Foams with fluctuating plateaus display either erratic-shaped crush bands or displaced struts. However, the locations where crush bands first form could not be precisely iden tified for either type of foam. Similar observations were made by Ho
3.6. Combined effect of binder and powder loading on mechanical strength in relation to the foam microstructure 12, 13, 14 and 15 vol% PVB Ф ¼ 0.8 SS316L foams were fabricated to investigate the effect of PVB concentration on mechanical strength as they possess adequate structural integrity to undergo machining and be cut into sample sizes suitable for mechanical testing. Representative stress-strain curves are selected and shown below to highlight a few irregularities in the mechanical properties of the foams tested, and ex planations for such anomalies will be provided. As binder concentration increases, compressive strength and plateau stress increase as shown in Fig. 9 (b). This is due to the increased strut width as powder content increases, resulting in increased mechanical strength as a higher compressive force is required to plastically deform a
Fig. 9. (a) Stress-strain curve for 12, 14 and 15 vol% PVB Ф ¼ 0.8 SS316L foams under compression at a strain rate of 1 mm/min; (b) Trends in compressive strength and plateau stress when binder concentration was increased for Ф ¼ 0.8 foam samples. 8
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Fig. 10. (a) Compressive behaviour of 12 vol% PVB Ф ¼ 0.75,0.8 and 0.85 SS316L foams under a compression strain rate of 1 mm/min; (b) Trends in compressive strength and plateau stress when powder loading was increased for 12 vol% PVB foam samples.
chanical properties of the SS316L foams, particularly plateau stress and compressive strength. The quantitative relationships as proposed by Gibson and Ashby are [2]. � �β ρ σ pl ¼ C2 σ y;s
ρs
� �γ
σ c ¼ C3 σy;s
ρ ρs
In which σ y;s is the yield strength of the strut material and is set to 205MPa [18]. ρρ is the relative density of the foam, in which ρ is the bulk s
density of the sintered foam whereas ρs is the density of the strut ma terial, taken to be 7.295 g/cm3 as measured using a gas pycnometer. C2, C3, β and γ are material constants which can be determined experimentally. σ By regressing σy;spl and σσy;sc against the relative densities on a double logarithmic plot in Fig. 12, the following values (refer to Table 4) were obtained for the material constants. The obtained shape factors C2 and C3 aligned well with the values proposed by Gibson and Ashby. On the other hand, the exponents were overestimated by the model, which may be due to the large distribution of values, seeing that the R2 values for the plots were only 0.6646 and 0.5251 respectively. However, it has also been stated the model is simplified and mathematically designed for open-cell foams having an equiaxed cell geometry and relatively homogeneous structure [19]. Previous studies have reported discrepancies in the exponents obtained due to such simplification of the mathematical model [12,20]. In our case, variations in cell geometries are present, and cell size distribution is relatively large, hence the deviations in values from those stated in literature. � �β However, as the value of the exponent decreases, the term ρρ and s � �γ ρ increases, thus if C2 and σy;s are held constant, σ pl increases ρ
Fig. 11. Deformation of (a) 12 vol% PVB Ф ¼ 0.8, (b) 15 vol% PVB Ф ¼ 0.8 and (c) 12 vol% PVB Ф ¼ 0.85 SS316L open-cell foams. Red lines indicate repre sentative crush bands that form as a result of compression. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
et al. in a study on Inconel foams [10]. In Fig. 11(a), besides the first crush band which appear at the top of the foam, a second crush band appeared at near the bottom part of the foam. In this particular sample, the foam first densified at the top, then the bottom, and was finally compressed towards the middle. The red circles in Fig. 11(b) and (c) indicate strut fragments that had broken off due to the compression. They were subsequently displaced from the foam body as compression was continuously applied perpendicular to the cross-section of the foam. For both foam samples, densification occurred from top to bottom of the foam, however the morphology of the crush band varied throughout the deformation process. Our findings are in good agreement with those brought forward by Ho et al., in which foams with higher solids loading deform in a more erratic and unpredictable manner [10]. The Gibson and Ashby model is employed here to predict the me
s
accordingly. As we have obtained exponent values lower than that stated in literature, this suggests that the foams produced in this experiment have higher strength than what can be conventionally produced. 3.7. Fabrication process window In Section 3.3, we suggested that by overlapping parameter windows for both binder concentration and powder loading, an optimum slurry composition may be identified to fabricate foams with higher-thanexpected mechanical strength. However, it is important to clarify that 9
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Fig. 12. Regression of (a) plateau stress and (b) compressive strength against relative density according to the scaling laws proposed by Gibson and Ashby.
concentration of 10 vol% PVB and below, as most but one of the foams fabricated had disintegrated or warped. The 6.5 vol% PVB Φ ¼ 0.87 foam had a regular structure after sintering, but it would not be practical to add an infinitely high amount of powder to obtain a self-supporting structure when the binder concentration can be increased and powder loading decreased. Take note that the powder loading is always less than 1. At 12 vol% PVB, the Φ ¼ 0.635 and Φ ¼ 0.70 foams were not strong enough to undergo compressive testing, hence naturally any Φ ¼ 0.70 foam with binder concentration less than 12 vol% PVB would not be ideal. Our upper limit is not defined by slumping due to excessive amount of binder, but instead the immensely high viscosity of the slurries which are likely to adversely influence fabrication. The viscosities of 12 vol% PVB Φ ¼ 0.85, 15 vol% PVB Φ ¼ 0.75, and 15 vol% PVB Φ ¼ 0.8 SS316L slurries were measured using an Anton Paar MCR 302 Rheometer as they had exceeded the measuring range for the Toki Sangyo TVB-15 rotational viscometer. Their standing viscos ities were all well above 10 kPa⋅s, as seen in Fig. 13. In particular, with reference to Fig. 6(a), the standing viscosity of 12 vol% PVB Φ ¼ 0.80 is within the range of 60 Pa⋅s–70 Pa⋅s; however when powder loading is merely increased by 0.05, the standing viscosity reaches 60 kPa⋅s. As mentioned several times throughout the main text, the difficulty in obtaining even slurry coating and probability of obtaining closed pores increases with increasing slurry viscosity. However in this case, even with fast squeezing and pressing of the foam during coating, it is expected that one is unable to sharply decrease the viscosity just by manually imparting shear, as the standing viscosity of the slurry is simply too high. Hence, these slurries may be deemed less ideal as they will be more difficult to process and easily result in clogged pores. In addition to high viscosity, the 15 vol% PVB Φ ¼ 0.8 foam had also corroded after machining, as mentioned in Section 3.6. In conclusion, the optimum slurry compositions is suggested to lie within the composition boundaries of
Table 4 Experimental and literature values of material constants obtained using the Gibson and Ashby model. Material constant
Experimental value
Literature value [2]
C2 β C3 γ
0.32 1.23 0.24 1.17
0.25–0.35 1.5–2.0 0.1–1.0 1.5
what would be deemed “optimum” will depend on the intended appli cation for the fabricated foam, and a single, “one-size-fits-all” compo sition may not be pinpointed. Hence, a process window encompassing a range of foam compositions that may produce self-supporting and me chanically adequate foams is proposed instead, which may hopefully save future researchers from the hassle of going through a large range of binder concentrations and powder loadings. With reference to Table 3, red borders represent samples that had warped, disintegrated, corroded, or had slurries with extremely high viscosities. Green borders represent “good samples” that remained structurally intact and regular (did not warp) after sintering. The plot reveals a large proportion of “good samples” to be within the ranges of 10 vol% PVB to 14 vol% PVB and Φ ¼ 0.75 to 0.80. These ranges can be said to define the process window of the SS316L foam fabrication pre sented in this study. The lower limit of this process window are foams with binder
(1) Binder concentration is suggested to be within 10 vol% PVB to 14 vol% PVB, and (2) Powder loading is suggested to be within Φ ¼ 0.75 to 0.80. Foams within this range are self-supporting, have relatively good mechanical properties and a lower possibility of obtaining closed pores, and are also easy to work with as the slurry viscosity will allow for easy and complete infiltration. Depending on the application, one may optimize their desired composition to be closer to the lower limit of the process window for a lighter foam, and a composition closer to the upper
Fig. 13. Viscosity of 12 vol% PVB Φ ¼ 0.85, 15 vol% PVB Φ ¼ 0.75 and 15 vol % PVB Φ ¼ 0.8 SS316L slurries measured using parallel plate rheometer. 10
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limit for foams which may require more strength.
Acknowledgement
4. Conclusion
The authors would like to acknowledge the A*STAR project U18–F007SR “Functionally Graded Metallic Foam and Micro-perforated Panel Composite: Experimental and Simulation Studies Towards Nextgenera tion Noise Barrier” for financial support.
The effects of binder concentration and metal powder loading on SS316L slurry rheology, SS316L open-cell foam morphology and its mechanical properties were studied. The addition of metal powder to a pure binder-solvent system will drastically increase the viscosity of the resulting slurry and alter the rheological properties of the slurry to become a non-Newtonian shear-thinning fluid. The shear-thinning behaviour is essential for the fabrication of porous foams by the tem plate replication method, as it allows slurry absorption and infiltration during the coating step, and deposition and adhesion of metal powder particles to the foam struts during drying and the subsequent heat treatment processes. Both binder and metal powder complement each other within the slurry, and increasing either of them can result in an increase in bulk density and mechanical strength of the resulting foam. Increasing the binder concentration allows more powder particles to be deposited onto the template as the binder helps the particles to adhere to the foam struts and increase the strut density. However, too much binder in the composition would result in a weak green body as all polymers within the foam will be burnt off during thermal debinding, causing extreme shrinkage or slumping of the structure. On the other hand, a higher powder loading is required for the foam structure to be selfsupporting and withstand the entire heat treatment, but too much powder in the composition would result in a very viscous slurry that increases the formation of clogged pores within the structure. Thus, it is evident that concentrations of both binder and powder influence the slurry rheology and foam properties greatly, and a balance between binder and powder concentration must be found. Nevertheless, by studying the trends in green strut densities and observing the structural integrity of sintered foams with varying binder and powder composi tions, it is found that the metal powder has a larger effect on the me chanical strength and successful fabrication of the foam. A process window that highlights a range of optimum slurry compositions was proposed. Based on this window, if the slurry has a binder concentration within 10 vol% to 14 vol% and powder loading within Φ ¼ 0.75 to 0.80, the resulting foam can be structurally self-supporting and mechanically adequate for various functional and even structural uses. For lightweight functional applications, a slurry formulation consisting of higher PVB concentration and less powder should be used, whereas more powder should be included in the composition for applications which require good mechanical strength and integrity or applications which require foams of larger sizes.
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Author contribution statement Teik Yi Lim: Conceptualization, Methodology, Validation, Investi gation, Writing - Original Draft. Wei Zhai: Conceptualization, Method ology, Resources, Writing - Review & Editing, Project administration. Xu Song: Writing - Review & Editing. Xiang Yu: Writing - Review & Editing. Tao Li: Resources. Beng Wah Chua: Funding acquisition, Supervision. Fangsen Cui: Funding acquisition, Supervision Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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