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Microstructure and compressive deformation behavior of SS foam made through evaporation of urea as space holder
Accepted Manuscript Microstructure and Compressive Deformation Behavior of SS Foam Made Through Evaporation of Urea as Space Holder
Hemant Jain, Gaurav Gupta, Rajeev Kumar, D.P. Mondal PII:
S0254-0584(18)30998-2
DOI:
10.1016/j.matchemphys.2018.11.040
Reference:
MAC 21124
To appear in:
Materials Chemistry and Physics
Received Date:
30 September 2018
Accepted Date:
15 November 2018
Please cite this article as: Hemant Jain, Gaurav Gupta, Rajeev Kumar, D.P. Mondal, Microstructure and Compressive Deformation Behavior of SS Foam Made Through Evaporation of Urea as Space Holder, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.11.040
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Microstructure and Compressive Deformation Behavior of SS Foam Made Through Evaporation of Urea as Space Holder Hemant Jain a,b, Gaurav Gupta b, Rajeev Kumar b, D.P Mondal b* aAcademy of Scientific and Innovation Research (AcSIR) bCSIR- Advanced Materials and Processes Research Institute, Bhopal-462026, India
Graphical Abstract
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Microstructure and Compressive Deformation Behavior of SS Foam Made Through Evaporation of Urea as Space Holder Hemant Jain a, b, Gaurav Gupta b, Rajeev Kumar b, D.P. Mondala, b aAcademy of Scientific and Innovation Research (AcSIR) bCSIR- Advanced Materials and Processes Research Institute, Bhopal-462026, India Abstract Open cell 316L stainless steel foam (SSF) of varying porosities have been developed through powder metallurgy route using evaporative spherical urea particles (UP) as a space holder. Stainless steel powders (SSP) were cold compacted under 500 MPa pressure and sintered at 1200ºC for 1 hrs in a high vacuum atmosphere (10-4 mbar). Detailed Energy dispersive X-ray spectroscopy (EDS) analysis and X-ray diffraction pattern (XRD) conformed that no residue of space holder (urea) in sintered samples. The compressive deformation behavior of sintered foam samples with varying relative densities (ρrd) was conducted at 0.01s-1 strain rate. The yield strength, elastic modulus (Ef), plastic modulus, average plateau stress (σpl) and energy absorption (Eab) of the foam increases with increases in the relative density and these follows power law relationship with relative density. On the other hand, densification strain (ɛd) decreases with increases relative density. This has been discussed with the deformation mechanism of the stainless steel foam (SSF). Deformation of SSF is associated with cell wall (CW) bending, CW collapse due to bulking followed by shearing and facture of cell wall layer by layer. Keywords: Stainless steel foam, spherical urea particles, sintering, relative density, compression deformation, energy absorption. *Corresponding author. Dr. D. P. Mondal Telephone number: +91-755-2418952, Fax: +91-755-2457042, Email: [email protected], [email protected]
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1. Introduction SSF possesses a unique combination of lightweight, high strength, high energy absorption capacity and having superior physical, chemical and mechanical properties [1].These are being explored in different fields such as general engineering and biomedical applications [2]. SSF can be a substitute for solid stainless steel in terms of valuable lightweight products such as sandwich panels, high temperature oil/gas filters, heat exchangers, high temperature catalyst substrate etc [3]. A lot of research work has been done in Titanium [4] and Nickel foam [5]. Due to the high cost of these above foams, the stainless steel foam is now becoming an attractive candidate for high temperature engineering and biomedical application. They also possess excellent corrosion, oxidation resistances and good mechanical properties [6, 7]. SSF can be synthesized by different techniques such as impregnation [8], slurry foaming [9], selective laser melting [10], hot isostatic pressing [11], space holder technique [12]. Among of these methods, the space holder technique is a very simple and cost effective method that can give open cell structures with desirable cell size, shape, and porosity. Two types of space holder can be used for making open cell foam: one that evaporative materials such as ammonium bicarbonate [12], tropical starch [13], saccharose [14] magnesium [15, 16]. Another leachable space holder such as NaCl [16], carbamide [17] and potassium bromide [18] have been used. Many researchers synthesized stainless steel and other metal foam using urea granules as space holder and applied leachable technique before sintering [19, 20]. Bekoz and Oktay [21] prepared steel foam with porosities ranging from 49.2% to 71.0% using two different shapes of carbamide granules (spherical and irregular) using water leaching technique. The yield strength was reported of foam between 20 to 92 MPa, young’s modulus was ranging from 0.71 to 2.19 GPa. Mizaei and Paydar [22] attempted to make porous 316L SSF with porosity of 71.5 % using carbamide leaching process. Gulsoy and German [23] produced SSF using similar process. Matlu and Oktay [24] has made highly porous 17-PH SSF using similar process as mentioned above. UP are organic substance and highly soluble in water and cheaper as compared to other space holder. But sometime leachable process introduced distortion in shape of cells, cell walls (CWs) and increases the pore size, and also generated micro porosities in the CW. All above mention problems led to decrease in 2
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mechanical properties. Smorygo et al.[25] used coarse carbamide particles as space holder for synthesizing porous titanium foam structure using water leachable process. It was found that pores of foam with higher porosities got distorted and CW broken due to carbamide leach out. In the evaporative method, UP easily decomposed and don’t react with any metal and also removed completely without generating any harmful substances. Joshi et al. [26] used fine urea particles as space holder and adopted evaporative technique. In their work plateau region was not observed due to finer pore size. Furthermore, no study has been carried out to make 316L SSF using coarse urea as space holder materials through the evaporative technique. In this study, open cell 316L SSF with wide ranges of porosities were prepared using evaporative technique. Further, the microstructure, mechanical properties like yield stress, Ef, σpl, ɛd and Eab were examined in detail. 2. Experimental Detail 2.1 Raw materials Austenitic stainless steel (316L) powder (SSP) supplied by Alfa Aesar (Germany) with 99.9% purity was used as a parent material. The morphological characterization of 316L (SSP was carried out using scanning electron microscopy (SEM), (JEOL; Model: 5600). The powders were also examined for particles size distribution using laser diffraction particles analyzer (Horiba. Model no LA-950V2). The size distribution of SSP powder and microstructure view of SSP powder is shown in Fig. 1(a) and (b). From Fig. 1(a), it can be inferred that most of the particles are below 20 µm and the average particles size is 6 ± 0.2 µm. From Fig. 1(b), it can be seen that the particles are irregular in shape. The spherical UP are supplied by India potash Ltd. The size distribution of UP is shown in Fig. 1(c) and the photography of spherical urea particles is shown in Fig. 1(d). From Fig. 1(d), it’s seen that the diameter is between 1.5 to 2.5 mm. The average diameter of UP is 2 ± 0.5 mm. Spherical urea was selected as space holder because it gets easily vaporized at a low temperature (around 150ºC) without any reaction with parent metal. Water based Polyvinyl alcohol (PVA) solution (5 wt. % PVA and 95wt. % distilled water) was used as organic binder to provide green strength.
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2.2 Materials Synthesis SSP was mixed with coarser UP in different volume fractions (0.2, 0.3, 0.4, and 0.5). PVA solution was added in the above mixture. The mixture was mixed thoroughly in turbula mixer for 1 hr to ensure uniform mixing and distribution of particles. The mixed powder were cold compacted at compaction pressure ranging between 400-700 MPa using hydraulic press of 40 ton capacity. The pressure applied for approximately 30 seconds. The 500 MPa pressure is selected as optimum pressure. At pressure higher than 500 MPa, spherical UP distorted. Further, the cylindrical samples of 20 mm diameters and 15 ± 0.4 mm height were prepared. The die was made up of hardened die steel. An assumption was followed that the porosity produced only due to the particles of space holder. Therefore, varying amount of SSP and UP were used to get different intended porosities. The amount of different powders for each porosity level was calculated using following relation [29]:
𝑾𝑺𝑺𝑷 𝑾𝑼𝑷
=
𝝆𝑺𝑺𝑷(𝟏 ‒ 𝑽𝑼𝑷 ) (𝝆𝑼𝑷 .𝑽𝑼𝑷)
(1) Where WSSP represents weight of SSP, WUP denotes weight of UP used as a SH, ρSSP and ρUP represents the density of SSP and UP respectively, VUP is the volume fraction of UP (i.e. required porosity) in SSF. Green pellets were pre-heated at 200ºC for 4 hours to evaporate urea particles. During evaporation of UP, a large amount of nitrogen gas is created and a fraction of nitrogen may be absorbed by SSF samples. However, the extent of nitrogen dissolution in SSF is in ppm level. It has been found that the nitrogen addition to austenitic stainless steels enhances their hardness and strengths. Nitrogen is ppm level in austenitic stainless steels is an effective element not only in solid solution strengthening but also in grain size strengthening. As the steel remained in austenitic form and nitrogen absorbed in ppm level, the ductility of austenitic steels not get affected. The weight loss from samples at every stage was measured in order to confirm the complete removal of space holder particles. These preheated samples then heated at 400ºC for 1 hour to carry out the removal of organic binder. Finally, these compacts were sintered in vacuum (~10-5mbar) at 1200ºC for 1 hr while heating rate is maintained at 5ºC/min in order to avoid thermal stress. The process flow diagram for making sintered SSF is shown in Fig. 2(a). The photographic image of sintered SSF samples is shown in Fig. 2(b), 4
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where the porosities between 50-80% are mentioned above the samples. It is also mentioned here that none of the sintered samples were distorted during sintering. These sintered samples were characterized in term of the ρrd, microstructure and mechanical properties. The samples were also characterized through X-ray diffraction (XRD) and Energy dispersive X-ray spectroscopy (EDS), compressive deformation behavior. The ρrd of SSF sample was calculated with respect to theoretical density of stainless steel. The actual foam density was measured from the measurement of volume and weight of foam samples. 2.3. Microstructure Characterization The sintered SSF samples were polished using standard metallographic polishing technique and etched using picric acid based chemical reagent (3 gram picric acid, 25 ml ethanol and 5 ml HCl). The polished and etched SSF samples were washed in acetone using ultrasonic cleaner. Microstructural characteristics of the sintered SSF samples like cell size, CW thickness, and chemical composition were examined using Field Emission Scanning Electron Microscopy (FESEM), (Model: Nova NanoSem 430) and EDS facility (Model: IE synergy 250). The phase formation was studied through the X-ray diffraction (Model: Bruker, Model: D8 with CuKα radiation). The diffraction patterns were recorded at a scan rate of 2º/mint. 2.4. Mechanical property characterization Compression test of sintered SSF samples was carried out using Universal Testing machine (UTM; Model: Instron-8801) at room temperature and at strain rate of 0.01 s-1 and compressive stress-strain curves were recorded during the tests. For each experimental condition, at least three samples were tested. All these recorded data was used for the plot of the compressive stress-strain diagram. Furthermore, yield stress, σpl, Ef, plastic modulus, ɛd and Eab were determined from these stress-strain curves. Vickers micro hardness measurements were conducted on polished and etched foam samples before/after compressive deformation using a LEICA micro hardness tester (model: VMHT 30 A) using a load of 100 gf. The indentations were put at the center of the CW, for each case, at least 30 indentations were made to get the average values.
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3. Result and Discussion 3.1. Microstructure and phase evolution It is clearly observed from the microstructure that, SSP particles are irregular in shape and their average size was 6 ± 1.2µm as shown in Fig. 1(a). On the other hand, UP are spherical in shape and their average size was 2 mm ± 0.5 mm as shown in Fig. 1(b). Due to larger difference between space holder and matrix particle (SSP) (ratio~250-350), the SSP may form a coating over the surface of space holder. After evaporation of space holder followed by sintering, skeleton of SS is observed Fig. 2 (b). The cell size distribution can be related to size and distribution of UP. The cell size distribution of ρr.d= 0.27 is shown in Fig. 3. From this figure, it may be noted that the cell size is ranging from 1.2 to 2.4 mm whereas UP is 1.5 to 2.5 mm. The cell size is slightly less than the size of UP due to the shrinkage phenomena during sintering. The cell size and CW thickness is reported in Table 1. It can be observed that ρrd is decreasing, the cell size is increasing. However, SSF having ρrd=0.19, the slightly decreases in cell size was observed. When the UP content was increased, the resultant ρrd and CW thickness decreases. In higher ρrd foam, due to presence of more SSP, that is responsible for more shrinkage during sintering. Volume fraction ratio between matrix to space holder is reducing from 1:1 in 50% to 1:4 in 80% space holder content. Therefore the average CW thickness is reducing from 0.66 to 0.27 mm. UP being organic in nature act as a lubricant during the compaction of metal powder. Therefore, at higher urea content, powder mixture became more compressible. This resulted into higher green density and thus higher degree of sintered density. This observation can also be seen in Table 1, where the ρrd in case of 80% space holder is nearly 19%; whereas in case of 50% space holder is 43%. This indicates that the obtained density is ~14% less than the targeted density in case of lower content (50% UP). Whereas in case of higher content (80% UP), this obtained density is 5% less than that of sintered density. The higher compressibility of mixed powder in 1:4 ratios in case of higher space holder content enhances the contacts between matrix particles. This enhances sinterability and effect of this factor is responsible for attaining target density (19% against target of 20%) and reduction in cell size (Table 1). The microstructure of SSF samples with different ρrd are shown in Fig. 4(a-d). It may be noted that the cells (marked ‘C’) are 6
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interconnected, cell walls are the marked ‘CW’. The sintering temperature is lower than the melting point of SS. Therefore solid state diffusion take place during sintering process and cells size are fully open with interconnected pores with small amount of micro porosities in CWs. It was observed in Fig. 4 (c-d) that when higher percentage of UP was used, evaporation of UP sometimes resulted into CW breakage due to small wall thickness and higher rate of volatiles of UP. In the CW region Fig. 5(a), the localized fusion at SSP-SSP interface (marked ‘black arrow’) and micro porosity (marked ‘white arrow’) can be seen at higher magnification in Fig. 5(b). The EDS analysis of magnified view of CW region Fig. 5(c) is shown in Fig. 5(d). It is clear from the EDS that that no oxide is formed and no urea residue is left after sintering. The magnified micrograph in Fig. 5(c) is clearly showing densified metal particles without noticeable pores and cracks. The XRD of the SSP particles and SSF sintered sample is shown in Fig. 6. The XRD pattern of these samples is noted to be almost similar to that of as-received SSP particles, indicting no significant change in chemistry and phase constituent during the sintering of foam. 3.2. Mechanical Properties The compressive stress-stain curve of the SSF samples with the varying ρrd is shown in Fig. 7(a). It is clearly observed from this figure that the compressive stress-strain curves have distinct three regions: (i) elastic region. (ii) linear plateau region, (iii) densified region. The linear plateau region shows that the compressive stress increases slightly with increase in compressive strain. From Fig. 7(a), the yield stress notes were calculated & plotted with ρrd in Fig. 7(b), it can be connected the yield stress increases with ρrd. The change in cross section area is almost negligible during compaction, up to 0.45 compressive strains in the plateau region as well as plastic region. In the densified region, the cross section area increases with the reduction in height and the flow stress increases rapidly with slight increase in strain as similar to solid material. The plateau region in higher ρrd may be thinner. It is indirectly demonstrating that the pores are getting compacted during deformation and hence the diameter of the sample is not increasing proportionately with extent of cell deformation. The steeper slope in plateau region may also be due to CW, hardening because of austenitic to martensitics transformation and interaction of microhardness. In the plateau region, the increment in cross section area is nominal. Even after 50% of deformation crossectional area increased only by 15% in case of ρrd of 0.43. In case of ρrd of 0.2, the crossectional area increases only 5% 7
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decreases with decrease in ρrd. This indicated that Ef, as well as plastic modulus of the foam, decreases with increase in porosity. In an elastic region, CW bending occurred. Further, during compression, the CW tends to crack and fracture. The CW collapse due to buckling followed by shearing and fracturing of CW layer by layer. In the third region, the SSF behave like a solid material and stress increases due to strain hardening. The diameter after final compaction in each case is shown in Table 2. The above finding is also confirmed through Table 2. From the table, it can also be observed that with increasing the compressive strain, the diameter of foam samples increased. In case of foam having ρrd = 0.19, diameter is almost unchanged till 0.5 strain and further diameter changed drastically. It can be concluded from the observation that foam with ρrd = 0.19 has plateau region till 0.5 strains. Similarly, other foams with higher ρrd have lower plateau region defined by linear line after elastic region. The σpl was calculated from this linear line and the strain under this linear line is measured as ɛd. In order to understand the effect of ρrd on the plastic modulus (slope of the plateau region in stress-strain curve), it is plotted with respect to ρrd as show in Fig. 7(c). The plastic modulus is increasing with increases in ρrd. It is due to more strain hardening as results of higher CW thickness or more materials in cell. At higher ρrd more materials are subjected to deformation. Additionally, the CW are thinner at lower ρrd, therefore for better understanding, the normalized plastic modulus (ratio of plastic modulus and yield stress) is calculated and plotted as a function of ρrd. The normalized plastic modulus increases with in decreases ρrd as shown in Fig. 7(d). Eab was calculated from the area under the curve up to densification. Yield stress, plastic modulus, σpl and Eab increases with increase in ρrd but ɛd decrease with ρrd. All the parameters are reported in Table 3. 3.3. Effect of relative density 3.3.1. Plateau stress (σpl) The σpl is a function of ρrd for SSF is shown in Fig. 8(a). It is evident from this curve that σpl with ρrd follows a power law relationship.
()
𝛔𝐩𝐥 = 𝟑𝟖𝟖.𝟏
𝛒𝐟
𝟏.𝟔𝟔
(2)
𝛒𝐝
Where ρf and ρd are the density of SSF samples and dense SS respectively. The value of coefficient (C) and exponent (n) are estimated by curve fitting. The ‘C’ and ‘n’ calculated to be 8
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388.1 and 1.66 respectively. The strength of dense stainless steel reported to be 490 MPa [27]. Micro hardness values use a proportionally constant. Hence, equation no (2) can be written as: 𝛔𝐩𝐥 = 𝟎.𝟖𝟎𝛔𝐝
() 𝛒𝐟
𝟏.𝟔𝟔
(3)
𝛒𝐝
Where σpl and σd are the strength of SSF samples and dense SS respectively. Here, the constant and exponents values are noted to be 0.80 and 1.66 respectively. The empirically calculated proportionality constant ‘C’ and the exponent ‘n’ obtained from the theses values. The above relations were compared with the reported values as shown in Table 4. It is noted that the value of coefficient ‘C’ and exponent ‘n’ obtained in the present studies are in good agreement of SSF with varying ρrd. 3.3.2. Elastic modulus (Ef) The Ef is a function of ρrd for SSF is shown in Fig. 8(b). It is evident from this regime that (Ef) increases with ρrd as shown bellow:
()
𝐄𝐟 = 𝟗𝟖.𝟒𝟔
𝛒𝐟
𝟏.𝟖
(4)
𝛒𝐝
The Ef of dense stainless steel is reported to be 193 GPa [10]. Hence, equation no (4) can be written as:
()
𝐄𝐟 = 𝟎.𝟓𝟏𝐄𝐝
𝛒𝐟
𝟏.𝟖
(5)
𝛒𝐝
Here, the constant and exponents values are noted to be 0.51 and 1.7 respectively. The empirically calculated proportionality constant ‘C’ and the exponent ‘n’ obtained study were compared with the reported values Table 4. It is noted that these values of are in good agreement with reported values.
3.3.3. Densification strain (ɛd) The εd for the SSF samples is plotted as a function of ρrd as shown in Fig. 9. The εd in the stainless steel foam is found to be decreased linearly with increases in ρrd. Considering all the investigated material together as the following relation is obtained for densification as a function of ρrd. 9
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𝛆𝐝 = 𝟎.𝟓𝟎𝟎 - 𝟎.𝟒𝟐𝟕
If
() 𝜌𝑓
𝜌𝑑
() 𝛒𝐟
(6)
𝛒𝐝
becomes 1 i.e for dense material, εd = 0.07. This indicated that dense materials will have
almost no densification strain but when
() 𝜌𝑓
𝜌𝑑
is zero, εd is determined to be 0.5. Practically this
is not feasible. This is be because of the fact that the εd is difficult to determine accessibly. This is influenced by cell wall deformation, cell work hardening and less extent of cell closure deformation. A large fraction of cell remained undensified. These could be visible through the microstructure examination of deformed sample. Empirically calculated proportionality constant ‘Cd’ and the exponent ‘nd’ obtained from the theses values the above the relations were compared with the reported values as shown in Table 4. The empirical linear equation expresses the εd as a function of ρrd with reasonably excellent accuracy. During the densification, cells are coarser (cells left due to evaporate the UP). It was further observed that in the densified SSF, even the coarser cells are not completely compacted. Thus, the densification of such SSF starts relatively at lower strain. These facts become more prominent with increase in ρrd. 3.3.4. Energy absorption (Eab) The Eab of SSF samples as a function of ρrd is shown in Fig. 10(a). The Eab of SSF is calculated from the stress-strain curve up to 0.45% deformation by using standard methodology [28]. 𝐄𝐚𝐛 =
𝛆𝐝
∫𝟎 𝛔𝐝𝛆
(7)
Where, εd = densification strain, σ = stress, ε = strain. It is observed from the figure that the Eab also follows the power law relationship with ρrd. These relations clearly demonstrate that the Eab increase with increase in ρrd. This is primarily due to the fact that Eab is product of σpl and εd (area under the stress-strain curve). The plateau stress follows the power law relation with ρrd, thus the Eab also follows the similar type of relation. Comparison of UP used as a water leachable and evaporative process with base materials, space holder particles size, sintering temperature time and atmosphere, porosity, σpl/Ef, yield stress and Eab of open cell SSF reported in literature are given in Table 5 3.3.5. Energy absorption efficiency 10
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The energy absorption efficiency of SSF samples as a function of densification strain at different ρrd is shown in Fig. 10(b). The energy absorption efficiency “η” is calculated using the following equation [29, 30]:
𝜺
∫𝝈 (𝜺)𝒅𝜺 𝜼=
𝟎
(8)
𝝈𝒎𝒂𝒙𝜺
The terms σmaxε is the ideal Eab, where σmax is the maximum stress and ε is the specific strain. Fig. 10(b) shows the energy absorption efficiencies of SSF samples at various strains. It clearly shows that higher energy absorption efficiencies of SSF at the strain from 0.1 to 0.3 for all ρrd. It is noted that energy absorption efficiency increases with decrease in ρrd. The energy absorption efficiency depends on the behavior of compressive stress-strain behavior. The plastic modulus of SSF increases with increases in ρrd. This causes slope to increase in stress at higher ρrd. The plateau region is flatter at lower ρrd and hence the energy absorption efficiency of these samples is higher. As the energy absorption efficiency is higher in case of lower ρrd, it is prefers to use low density foams (highly porous foams) for shock energy or crash energy absorption application. High density foam may be applicable for blast resistance and structural applications.
3.4. Deformation mechanism
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The deformed 316L SFF samples is shown in Fig. 11(a). The micrograph of deformed samples (SSF of ρrd 0.43) at transverse section shown in Fig. 11(b-d) and for ρrd of 0.19 in Fig. 11(e-f). When the sample of higher ρrd starts to compacting, the CWs starts deforming plastically and finally get sheared, (marked white arrow) fracture (marked ‘F’) and compacted Fig. 11(b). Micro shear bands are also formed (marked black arrow). The cycles of cell walls (CWs) collapse, cell sinking were repeated and continue deformation layer by layer till the densification is completed. It is also noted that the CW get bended and also CWs gets sheared and cracked which are leading to CW crushing. Thus matrix is subject to more plastic deformation and subjected more strain hardening which is reflected in post deformation micro hardness measurement in Table 3. After densification micro porosities are remained in the CW Fig. 11(c). Additionally, the coarser CWs are subjected to less deformation and carry higher load. Because of theses, the work hardening of CWs less. The work hardening of the CWs is due to microstructure and austenitic to martensitics transformation. The austenitic to martensitics transformation is visible at higher magnification; micro shear band and martensitics needles are visible in Fig. 11(d). After densification regions, foams sample deforms towards densified materials and in this region, stress increases with strain. It may be noted clearly that the foams have not been fully densified even after the strain at which stress increases with strain. It is further noted that the foam with lower ρrd densified more and less porosity after densification is noted Fig. 11 (e-f). Thus also signified that at lower density CWs gets more effectively deformed. The shear band formation thus formed in higher deformation is clearly visible in Fig. 11(e) (marked white arrow). At lower ρrd more cracks at the lateral surface are observed due to shearing of CWs.
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4. Conclusion Following conclusion cab ne drawn from this study:
Spherical UP can be used as an excellent evaporative space holder for making open cell stainless steel foams with a desired cell size and ρrd.
The cell size and density may be varied by changing the ratio of SSP: UP in stainless steelurea mixture. ρrd and of CW thickness of SSF varies with variation of UP content.
Solid state diffusion take place during sintering process and cell size are fully open with uniform interconnected pores with small amount of micro porosities in CW.
The yield strength, plastic modulus, σpl, Ef and Eab of SSF is following power law relationship with ρrd. However, the ɛd following linear relationship with ρrd.
The σpl, Ef and ɛd have empirically correlation with ρrd. The coefficient and exponent in the present study are good agreement with reported values of SSF.
Deformation of SSF is associated with CW bending, CW collapse due to bulking followed by shearing and facture of CW layer by layer. Finally, SSF relatively more uniform compacted. Thus the SSF could be excellent candidate for replacing dense SS for implant, structural and engineering applications, especially for uniform energy absorption.
Acknowledgments The authors are thankful to Director, CSIR-AMPRI, Bhopal for giving his permission to publish this work. The author Hemant Jain is thankful to CSIR-India for providing Senior Research Fellowship (SRF).
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[4] Jian X, Hao C, Guibao Q, Yang Y, Xuewei L. Investigation on relationship between porosity and spacer content of titanium foams. Materials & Design. 2015;88:132-7. [5] Fan S-f, Zhang T, Yu K, Fang H-j, Xiong H-q, Dai Y-l, et al. Compressive properties and energy absorption characteristics of open-cell nickel foams. Transactions of Nonferrous Metals Society of China. 2017;27:117-24. [6] Kurgan N. Effects of sintering atmosphere on microstructure and mechanical property of sintered powder metallurgy 316L stainless steel. Materials & Design (1980-2015). 2013;52:995-8. [7] Zhou X-Y, Li J, Long B, Huo D-W. The oxidation resistance performance of stainless steel foam with 3D open-celled network structure at high temperature. Materials Science and Engineering: A. 2006;435-436:40-5. [8] Wang H, Zhou XY, Long B. Fabrication of Stainless Steel Foams Using Polymeric Sponge Impregnation Technology. Advanced Materials Research. 2014;1035:219-24. [9] Mad Rosip NI, Ahmad S, Jamaludin KR, Mat Noor F. Morphological Analysis of SS316L Foam Produced by Using Slurry Method. Advanced Materials Research. 2015;1087:68-72. [10] Yan C, Hao L, Hussein A, Young P, Raymont D. Advanced lightweight 316L stainless steel cellular lattice structures fabricated via selective laser melting. Materials & Design. 2014;55:533-41. [11] Essa K, Jamshidi P, Zou J, Attallah MM, Hassanin H. Porosity control in 316L stainless steel using cold and hot isostatic pressing. Materials & Design. 2018;138:21-9. [12] Mondal DP, Jain H, Das S, Jha AK. Stainless steel foams made through powder metallurgy route using NH4HCO3 as space holder. Materials & Design. 2015;88:430-7. [13] Mansourighasri A, Muhamad N, Sulong AB. Processing titanium foams using tapioca starch as a space holder. Journal of Materials Processing Technology. 2012;212:83-9. [14] Jakubowicz J, Adamek G, Pałka K, Andrzejewski D. Micro-CT analysis and mechanical properties of Ti spherical and polyhedral void composites made with saccharose as a space holder material. Materials Characterization. 2015;100:13-20. [15] Aydoğmuş T, Bor ET, Bor Ş. Phase Transformation Behavior of Porous TiNi Alloys Produced by Powder Metallurgy Using Magnesium as a Space Holder. Metallurgical and Materials Transactions A. 2011;42:2547-55.
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[16] Aida SF, Hijrah MN, Amirah AH, Zuhailawati H, Anasyida AS. Effect of NaCl as a Space Holder in Producing Open Cell A356 Aluminium Foam by Gravity Die Casting Process. Procedia Chemistry. 2016;19:234-40. [17] Bafti H, Habibolahzadeh A. Compressive properties of aluminum foam produced by powder-Carbamide spacer route. Materials & Design (1980-2015). 2013;52:404-11. [18] Mat Noor F, Zain MIM, Jamaludin KR, Hussin R, Kamdi Z, Ismail A, et al. Potassium Bromide as Space Holder for Titanium Foam Preparation. Applied Mechanics and Materials. 2014;465-466:922-6. [19] Mahammad Rafter MF, Ahmad S, Ibrahim R. The Effect of Different Composition of Stainless Steel (SS316L) Foam via Space Holder Method. Advanced Materials Research. 2016;1133:310-3. [20] Abdullah Z, Ahmad S, Ramli M. The Impact of Composition and Sintering Temperature for Stainless Steel Foams (SS316L) Fabricated by Space Holder Method with Urea as Space Holder. Materials Science Forum. 2017;888:413-7. [21] Bekoz N, Oktay E. Effects of carbamide shape and content on processing and properties of steel foams. Journal of Materials Processing Technology. 2012;212:2109-16. [22] Mirzaei M, Paydar MH. A novel process for manufacturing porous 316L stainless steel with uniform pore distribution. Materials & Design. 2017;121:442-9. [23] Gülsoy HÖ, German RM. Sintered foams from precipitation hardened stainless steel powder. Powder Metallurgy. 2008;51:350-3. [24] Mutlu I, Oktay E. Characterization of 17-4 PH stainless steel foam for biomedical applications in simulated body fluid and artificial saliva environments. Materials Science and Engineering: C. 2013;33:1125-31. [25] Smorygo O, Marukovich A, Mikutski V, Gokhale AA, Reddy GJ, Kumar JV. Highporosity titanium foams by powder coated space holder compaction method. Materials Letters. 2012;83:17-9. [26] Shailendra Joshi GKG, Mohit Sharma,Amit Telang, Taru Mahra. Synthesis & Characterization of Stainless Steel foam via Powder Metallurgy Taking Acicular Urea as Space Holder. Material Science Research India. 2015;12 43-9
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[27] Li ZN, Wei FA, La PQ, Ma FL. Enhanced Mechanical Properties of 316L Stainless Steel Prepared by Aluminothermic Reaction Subjected to Multiple Warm Rolling. Metals and Materials International. 2018;24:633-43. [28] Bafti H, Habibolahzadeh A. Production of aluminum foam by spherical carbamide space holder technique-processing parameters. Materials & Design. 2010;31:4122-9. [29] Xie B, Fan YZ, Mu TZ, Deng B. Fabrication and energy absorption properties of titanium foam with CaCl2 as a space holder. Materials Science and Engineering: A. 2017;708:419-23. [30] Mukherjee M, Ramamurty U, Garcia-Moreno F, Banhart J. The effect of cooling rate on the structure and properties of closed-cell aluminium foams. Acta Materialia. 2010;58:5031-42. [31] Kato K, Yamamoto A, Ochiai S, Wada M, Daigo Y, Kita K, et al. Cytocompatibility and mechanical properties of novel porous 316L stainless steel. Materials Science and Engineering: C. 2013;33:2736-43. [32] Oktay IMaE. Production and characterisation of Cr-Si-Ni-Mo steel foams. Indian Journal of Engineering & Materials Sciences. June 2011;Volume. 18:227-32. [33] Park C, Nutt SR. Strain rate sensitivity and defects in steel foam. Materials Science and Engineering: A. 2002;323:358-66. [34] Salimon A, Bréchet Y, Ashby MF, Greer AL. Potential applications for steel and titanium metal foams. Journal of Materials Science. 2005;40:5793-9. [35] Andrews E, Sanders W, Gibson LJ. Compressive and tensile behaviour of aluminum foams. Materials Science and Engineering: A. 1999;270:113-24. [36] Esen Z, Bor Ş. Processing of titanium foams using magnesium spacer particles. Scripta Materialia. 2007;56:341-4. [37] Niu W, Bai C, Qiu G, Wang Q. Processing and properties of porous titanium using space holder technique. Materials Science and Engineering: A. 2009;506:148-51. [38] Mondal DP, Goel MD, Bagde N, Jha N, Sahu S, Barnwal AK. Closed cell ZA27–SiC foam made through stir-casting technique. Materials & Design. 2014;57:315-24. [39] Aktay L, Kröplin B-H, Toksoy AK, Güden M. Finite element and coupled finite element/smooth particle hydrodynamics modeling of the quasi-static crushing of empty and foam-filled single, bitubular and constraint hexagonal-and square-packed aluminum tubes. Materials & Design. 2008;29:952-62.
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[40] Rajendran R, Moorthi A, Basu S. Numerical simulation of drop weight impact behaviour of closed cell aluminium foam. Materials & Design. 2009;30:2823-30. [41] Mutlu I, Oktay E. Biocompatibility of 17-4 PH stainless steel foam for implant applications. Bio-medical materials and engineering. 2011;21:223-33. [42] Bakan HI. A novel water leaching and sintering process for manufacturing highly porous stainless steel. Scripta Materialia. 2006;55:203-6. [43] ABDULLAH Z, AHMAD S, RAFTER MFM, SADIKIN A, RAHMAN MNA, ISMAIL A. The Effect of Different Urea Composition on Production of Porous Stainless Steel type 316L through Powder Metallurgy Technique.
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List of figures Fig. 1(a) Size distribution of SSP, (b) microstructure of SSP particles, (c) size distribution of UP and (d) spherical UP. Fig.2 (a) Process flow diagram for making sintered SSF and (b) Sintered SSF samples; top view and Front View. Fig. 3 Cell Size distribution of SSF with ρrd of 0.27. Fig. 4 Microstructure of SSF samples with different ρrd (a) ρrd=0.43, (b) ρrd =0.36, (c) ρrd =0.27 and (d) ρrd =0.19. Fig. 5(a) Microstructure of SSF of ρrd=0.19, (b) microstructure of CW of SSF, (c) microstructure of SSF at higher magnification and (d) EXD analysis of SSF of ρrd=0.19. Fig. 6 XRD pattern of SSP-particles and sintered SSF sample. Fig. 7(a) Compressive stress-strain curve of SSF with different ρrd, (b) yield stress, (c) plastic modulus (slope of plateau region) and (d) normalized plastic modulus (Plastic modulus/yield strength) as a function of ρrd. Fig. 8(a) Average plateau stress (σpl) and (b) elastic modulus (Ef) as a function ρrd. Fig. 9 Densification strain (ɛd) as a function of ρrd. Fig.10 (a) Energy absorption (Eab) as a function of ρrd and (b) Energy absorption efficiency vs densification strain at different ρrd. Fig.
11(a) Deformed SSF at different ρrd, (b-d) microstructure of deformed samples after densification with ρrd of 0.43 and (e-f) ρrd of 0.19.
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Fig. 1(a) Size distribution of SSP, (b) microstructure of SSP particles, (c) size distribution of UP and (d) spherical UP.
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Fig.2 (a) Process flow diagram for making sintered SSF and (b) Sintered SSF samples; top view and Front View.
Fig. 3 Cell Size distribution of SSF with ρrd = of 0.27.
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Fig.4 Microstructure of SSF samples with different ρrd (a) ρrd=0.43, (b) ρrd =0.36, (c) ρrd =0.27 and (d) ρrd =0.19.
Fig. 5(a) Microstructure of SSF of ρrd=0.19, (b) microstructure of CW of SSF, (c) microstructure of SSF at higher magnification and (d) EXD analysis of SSF of ρrd=0.19. 4
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Fig. 6 XRD pattern of SSP-particles and sintered SSF sample.
Fig. 7(a) Compressive stress-strain curve of SSF with different ρrd, (b) yield stress, (c) plastic modulus (slope of plateau region) and (d) normalized plastic modulus (Plastic modulus/yield strength) as a function of ρrd.
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Fig. 8(a) Average plateau stress (σpl) and (b) elastic modulus (Ef) as a function ρrd.
Fig. 9 Densification strain (ɛd) as a function of ρrd.
Fig.10 (a) Energy absorption (Eab) as a function of ρrd and (b) Energy absorption efficiency vs densification strain at different ρrd. 6
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Fig.
11(a) Deformed SSF at different ρrd, (b-d) microstructure of deformed samples after densification with ρrd of 0.43 and (e-f) ρrd of 0.19.
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Highlights
Urea can be used as excellent evaporative space holder for making SSF.
Cell sizes are almost related to the size of space holder.
The mechanical properties of SSF follow standard relationship.
Deformation behavior of SSF is strong as a function of relative density.
This technique is very simple, economical and highly reproducible, which may facilitate the commercialization of such metal foam.
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List of tables Table 1 Space holder volume %, target porosity, actual ρrd, cell size, CW thickness, micro hardness and microhardness deformed samples. Table 2 Intermediate diameter and final diameters. Table 3 Average plateau stress (σpl), elastic modulus (Ef), yield Stress, plastic modulus, energy absorption (Eab) and densification strain (ɛd) of the SSF samples (at strain rate = 0.01s-1). Table 4. Values of empirical constants reported by various researchers and the obtained in the present study for different empirical relations. Table 5. Comparison of the reported literature on open cell steel foam using urea (carbamide) as a leachable and evaporative process.
. Table 1 Space holder volume %, target porosity, actual ρrd, cell size, CW thickness, micro hardness and microhardness deformed samples. Space Target holder relative (vol. %) density
Actual relative density (ρrd)
Cell size (mm)
Cell wall Thickness (CW) (mm)
Micro hardness (kgf/mm2)
Micro hardness deformed samples (kgf/mm2)
1.57± 0.32
0.60 ± 0.16
235 ± 3
309 ± 3
50
0.50
0.44 ± 0.01
60
0.40
0.36 ± 0.01 1.78 ± 0.27
0.40 ± 0.06
231 ± 4
302 ± 2
70
0.30
0.26 ±0.01
1.89 ± 0.28
0.39 ± 0.06
228 ± 3
281 ± 3
80
0.20
0.19 ± 0.01 1.66 ± 0.31
0.27 ± 0.06
220 ± 3
277 ± 3
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Table 2 Intermediate diameter and final diameters.
Relative Total density compressive strain (ρrd)
Initial diameter (mm)
Final diameter (mm)
0.43
0.5
19.58 ± 0.09
22.25 ± 0.09
0.35
0.6
19.37 ± 0.1
23.42 ± 0.09
0.27
0.7
19.62 ± 0.1
24.45 ± 0.09
0.19
0.2 0.5 0.8
19.56 ± 0.1
19.91 ± 0.1 20.94 ± 0.1 24.94 ± 0.09
Table 3 Average plateau stress (σpl), elastic modulus (Ef), yield Stress, plastic modulus, energy absorption (Eab) and densification strain (ɛd) of the SSF samples. (at strain rate = 0.01s-1). R.D (ρrd)
0.43 0.36 0.27 0.19
Average Plateau (σpl) (MPa)
Elastic Modulus (Ef) (GPa)
Yield stress (MPa)
99.83 ± 0.9
22.65 ± 0.5 15.19 ± 0.3
45.3 ± 0.32
63.09 ± 1.1 41.7 ± 1.3 22.54 ± 1.3
9.18 ± 0.5 4.86 ± 0.5
28.33 ± 0.31 12.04 ± 0.35 5.4 ± 0.34
Plastic modulus (MPa)
Normalize d plastic modulus 7.23 ± 0.32
327.92 ± 0.2 8.43 ± 0.31 238.9 ± 0.2 154.54 ± 0.3 76.93 ± 0.2
12.83 ± 0.32 14.24 ± 0.32
Energy Absorpti on Eab (MJ/m3) 32.15 ± 0.25
Densification Strain (ɛd)
24.16 ± 0.23 16.19 ± 0.25 9.744 ± 0.26
0.35 ± 0.03
0.32 ± 0.03
0.38 ± 0.05 0.43 ± 0.05
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Table 4 Values of empirical constants reported by various researchers and the obtained in the present study for different empirical relations. Relation
𝝈𝒑𝒍 = 𝑪𝝈𝒅
“C’’
“n”
References
0.3
1.5
[31]
0.37
1.73
[32]
0.3
1.5
[33]
0.65
1.5
[34]
0.80
1.66
Present study
0.98
2
[35]
1.598
4.72
[36]
0.193
1.43
[37]
0.28
1.13
[12]
0.51
1.8
Present study
𝛒𝐟
0.85
0.82
[38]
𝛒𝐝
1.206
1.6
[39]
1
1.4
[40]
0.50
0.42
Present study
() 𝝆𝒇
𝝆𝒅
()
𝐄𝐟 = 𝐂𝐄𝐝
𝒏
𝝆𝒇
𝐧
𝝆𝒅
𝐄𝐝 = 𝐂 ‒ 𝐧
()
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Table 5 Comparison of the reported literature on open cell steel foam using urea (carbamide) as
Evaporative Process
Water Leachable Process
a leachable and evaporative process. Base materials
Urea (carbamide) average size (µm)
Sintering temperature, time and sintering atm.
Porosity (%)
Young’s / elastic modulus (GPa)
Yield stress (MPa)
Energy absorption (MJ/m3)
Ref.
17.4 PH stainless steel
710-1000
1260ºC -40 min Hydrogen atm.
82-40
1.14
40
------
[41]
17.4 PH stainless steel
750-1000
1350 ºC -60 min Hydrogen atm.
60-40
46-55
------
-----
[23]
316L stainless steel 316L stainless steel
750-1000
70
0.27
------
--------
[42]
60-50
-------
-------
-------
[20]
316L stainless steel 316L stainless steel
2000
1370 ºC -120 min Vacuum atm. 1000 ºC 1100 ºC 1200 ºC Argon atm. 1100 ºC -60 min Vacuum atm. 1100 ºC -120 min Argon atm.
60-40
------
-------
------
[26]
60-40
------
-------
-------
[43]
81-57
4.8-22.6
5.4 - 45.3
9.7- 32.2
Present study
316L stainless steel
--------
200 µm
2000-2600
1200 ºC -60 min Vacuum atm.
Hemant Jain, Gaurav Gupta, Rajeev Kumar, D.P. Mondal PII:
S0254-0584(18)30998-2
DOI:
10.1016/j.matchemphys.2018.11.040
Reference:
MAC 21124
To appear in:
Materials Chemistry and Physics
Received Date:
30 September 2018
Accepted Date:
15 November 2018
Please cite this article as: Hemant Jain, Gaurav Gupta, Rajeev Kumar, D.P. Mondal, Microstructure and Compressive Deformation Behavior of SS Foam Made Through Evaporation of Urea as Space Holder, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.11.040
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Microstructure and Compressive Deformation Behavior of SS Foam Made Through Evaporation of Urea as Space Holder Hemant Jain a,b, Gaurav Gupta b, Rajeev Kumar b, D.P Mondal b* aAcademy of Scientific and Innovation Research (AcSIR) bCSIR- Advanced Materials and Processes Research Institute, Bhopal-462026, India
Graphical Abstract
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Microstructure and Compressive Deformation Behavior of SS Foam Made Through Evaporation of Urea as Space Holder Hemant Jain a, b, Gaurav Gupta b, Rajeev Kumar b, D.P. Mondala, b aAcademy of Scientific and Innovation Research (AcSIR) bCSIR- Advanced Materials and Processes Research Institute, Bhopal-462026, India Abstract Open cell 316L stainless steel foam (SSF) of varying porosities have been developed through powder metallurgy route using evaporative spherical urea particles (UP) as a space holder. Stainless steel powders (SSP) were cold compacted under 500 MPa pressure and sintered at 1200ºC for 1 hrs in a high vacuum atmosphere (10-4 mbar). Detailed Energy dispersive X-ray spectroscopy (EDS) analysis and X-ray diffraction pattern (XRD) conformed that no residue of space holder (urea) in sintered samples. The compressive deformation behavior of sintered foam samples with varying relative densities (ρrd) was conducted at 0.01s-1 strain rate. The yield strength, elastic modulus (Ef), plastic modulus, average plateau stress (σpl) and energy absorption (Eab) of the foam increases with increases in the relative density and these follows power law relationship with relative density. On the other hand, densification strain (ɛd) decreases with increases relative density. This has been discussed with the deformation mechanism of the stainless steel foam (SSF). Deformation of SSF is associated with cell wall (CW) bending, CW collapse due to bulking followed by shearing and facture of cell wall layer by layer. Keywords: Stainless steel foam, spherical urea particles, sintering, relative density, compression deformation, energy absorption. *Corresponding author. Dr. D. P. Mondal Telephone number: +91-755-2418952, Fax: +91-755-2457042, Email: [email protected], [email protected]
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1. Introduction SSF possesses a unique combination of lightweight, high strength, high energy absorption capacity and having superior physical, chemical and mechanical properties [1].These are being explored in different fields such as general engineering and biomedical applications [2]. SSF can be a substitute for solid stainless steel in terms of valuable lightweight products such as sandwich panels, high temperature oil/gas filters, heat exchangers, high temperature catalyst substrate etc [3]. A lot of research work has been done in Titanium [4] and Nickel foam [5]. Due to the high cost of these above foams, the stainless steel foam is now becoming an attractive candidate for high temperature engineering and biomedical application. They also possess excellent corrosion, oxidation resistances and good mechanical properties [6, 7]. SSF can be synthesized by different techniques such as impregnation [8], slurry foaming [9], selective laser melting [10], hot isostatic pressing [11], space holder technique [12]. Among of these methods, the space holder technique is a very simple and cost effective method that can give open cell structures with desirable cell size, shape, and porosity. Two types of space holder can be used for making open cell foam: one that evaporative materials such as ammonium bicarbonate [12], tropical starch [13], saccharose [14] magnesium [15, 16]. Another leachable space holder such as NaCl [16], carbamide [17] and potassium bromide [18] have been used. Many researchers synthesized stainless steel and other metal foam using urea granules as space holder and applied leachable technique before sintering [19, 20]. Bekoz and Oktay [21] prepared steel foam with porosities ranging from 49.2% to 71.0% using two different shapes of carbamide granules (spherical and irregular) using water leaching technique. The yield strength was reported of foam between 20 to 92 MPa, young’s modulus was ranging from 0.71 to 2.19 GPa. Mizaei and Paydar [22] attempted to make porous 316L SSF with porosity of 71.5 % using carbamide leaching process. Gulsoy and German [23] produced SSF using similar process. Matlu and Oktay [24] has made highly porous 17-PH SSF using similar process as mentioned above. UP are organic substance and highly soluble in water and cheaper as compared to other space holder. But sometime leachable process introduced distortion in shape of cells, cell walls (CWs) and increases the pore size, and also generated micro porosities in the CW. All above mention problems led to decrease in 2
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mechanical properties. Smorygo et al.[25] used coarse carbamide particles as space holder for synthesizing porous titanium foam structure using water leachable process. It was found that pores of foam with higher porosities got distorted and CW broken due to carbamide leach out. In the evaporative method, UP easily decomposed and don’t react with any metal and also removed completely without generating any harmful substances. Joshi et al. [26] used fine urea particles as space holder and adopted evaporative technique. In their work plateau region was not observed due to finer pore size. Furthermore, no study has been carried out to make 316L SSF using coarse urea as space holder materials through the evaporative technique. In this study, open cell 316L SSF with wide ranges of porosities were prepared using evaporative technique. Further, the microstructure, mechanical properties like yield stress, Ef, σpl, ɛd and Eab were examined in detail. 2. Experimental Detail 2.1 Raw materials Austenitic stainless steel (316L) powder (SSP) supplied by Alfa Aesar (Germany) with 99.9% purity was used as a parent material. The morphological characterization of 316L (SSP was carried out using scanning electron microscopy (SEM), (JEOL; Model: 5600). The powders were also examined for particles size distribution using laser diffraction particles analyzer (Horiba. Model no LA-950V2). The size distribution of SSP powder and microstructure view of SSP powder is shown in Fig. 1(a) and (b). From Fig. 1(a), it can be inferred that most of the particles are below 20 µm and the average particles size is 6 ± 0.2 µm. From Fig. 1(b), it can be seen that the particles are irregular in shape. The spherical UP are supplied by India potash Ltd. The size distribution of UP is shown in Fig. 1(c) and the photography of spherical urea particles is shown in Fig. 1(d). From Fig. 1(d), it’s seen that the diameter is between 1.5 to 2.5 mm. The average diameter of UP is 2 ± 0.5 mm. Spherical urea was selected as space holder because it gets easily vaporized at a low temperature (around 150ºC) without any reaction with parent metal. Water based Polyvinyl alcohol (PVA) solution (5 wt. % PVA and 95wt. % distilled water) was used as organic binder to provide green strength.
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2.2 Materials Synthesis SSP was mixed with coarser UP in different volume fractions (0.2, 0.3, 0.4, and 0.5). PVA solution was added in the above mixture. The mixture was mixed thoroughly in turbula mixer for 1 hr to ensure uniform mixing and distribution of particles. The mixed powder were cold compacted at compaction pressure ranging between 400-700 MPa using hydraulic press of 40 ton capacity. The pressure applied for approximately 30 seconds. The 500 MPa pressure is selected as optimum pressure. At pressure higher than 500 MPa, spherical UP distorted. Further, the cylindrical samples of 20 mm diameters and 15 ± 0.4 mm height were prepared. The die was made up of hardened die steel. An assumption was followed that the porosity produced only due to the particles of space holder. Therefore, varying amount of SSP and UP were used to get different intended porosities. The amount of different powders for each porosity level was calculated using following relation [29]:
𝑾𝑺𝑺𝑷 𝑾𝑼𝑷
=
𝝆𝑺𝑺𝑷(𝟏 ‒ 𝑽𝑼𝑷 ) (𝝆𝑼𝑷 .𝑽𝑼𝑷)
(1) Where WSSP represents weight of SSP, WUP denotes weight of UP used as a SH, ρSSP and ρUP represents the density of SSP and UP respectively, VUP is the volume fraction of UP (i.e. required porosity) in SSF. Green pellets were pre-heated at 200ºC for 4 hours to evaporate urea particles. During evaporation of UP, a large amount of nitrogen gas is created and a fraction of nitrogen may be absorbed by SSF samples. However, the extent of nitrogen dissolution in SSF is in ppm level. It has been found that the nitrogen addition to austenitic stainless steels enhances their hardness and strengths. Nitrogen is ppm level in austenitic stainless steels is an effective element not only in solid solution strengthening but also in grain size strengthening. As the steel remained in austenitic form and nitrogen absorbed in ppm level, the ductility of austenitic steels not get affected. The weight loss from samples at every stage was measured in order to confirm the complete removal of space holder particles. These preheated samples then heated at 400ºC for 1 hour to carry out the removal of organic binder. Finally, these compacts were sintered in vacuum (~10-5mbar) at 1200ºC for 1 hr while heating rate is maintained at 5ºC/min in order to avoid thermal stress. The process flow diagram for making sintered SSF is shown in Fig. 2(a). The photographic image of sintered SSF samples is shown in Fig. 2(b), 4
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where the porosities between 50-80% are mentioned above the samples. It is also mentioned here that none of the sintered samples were distorted during sintering. These sintered samples were characterized in term of the ρrd, microstructure and mechanical properties. The samples were also characterized through X-ray diffraction (XRD) and Energy dispersive X-ray spectroscopy (EDS), compressive deformation behavior. The ρrd of SSF sample was calculated with respect to theoretical density of stainless steel. The actual foam density was measured from the measurement of volume and weight of foam samples. 2.3. Microstructure Characterization The sintered SSF samples were polished using standard metallographic polishing technique and etched using picric acid based chemical reagent (3 gram picric acid, 25 ml ethanol and 5 ml HCl). The polished and etched SSF samples were washed in acetone using ultrasonic cleaner. Microstructural characteristics of the sintered SSF samples like cell size, CW thickness, and chemical composition were examined using Field Emission Scanning Electron Microscopy (FESEM), (Model: Nova NanoSem 430) and EDS facility (Model: IE synergy 250). The phase formation was studied through the X-ray diffraction (Model: Bruker, Model: D8 with CuKα radiation). The diffraction patterns were recorded at a scan rate of 2º/mint. 2.4. Mechanical property characterization Compression test of sintered SSF samples was carried out using Universal Testing machine (UTM; Model: Instron-8801) at room temperature and at strain rate of 0.01 s-1 and compressive stress-strain curves were recorded during the tests. For each experimental condition, at least three samples were tested. All these recorded data was used for the plot of the compressive stress-strain diagram. Furthermore, yield stress, σpl, Ef, plastic modulus, ɛd and Eab were determined from these stress-strain curves. Vickers micro hardness measurements were conducted on polished and etched foam samples before/after compressive deformation using a LEICA micro hardness tester (model: VMHT 30 A) using a load of 100 gf. The indentations were put at the center of the CW, for each case, at least 30 indentations were made to get the average values.
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3. Result and Discussion 3.1. Microstructure and phase evolution It is clearly observed from the microstructure that, SSP particles are irregular in shape and their average size was 6 ± 1.2µm as shown in Fig. 1(a). On the other hand, UP are spherical in shape and their average size was 2 mm ± 0.5 mm as shown in Fig. 1(b). Due to larger difference between space holder and matrix particle (SSP) (ratio~250-350), the SSP may form a coating over the surface of space holder. After evaporation of space holder followed by sintering, skeleton of SS is observed Fig. 2 (b). The cell size distribution can be related to size and distribution of UP. The cell size distribution of ρr.d= 0.27 is shown in Fig. 3. From this figure, it may be noted that the cell size is ranging from 1.2 to 2.4 mm whereas UP is 1.5 to 2.5 mm. The cell size is slightly less than the size of UP due to the shrinkage phenomena during sintering. The cell size and CW thickness is reported in Table 1. It can be observed that ρrd is decreasing, the cell size is increasing. However, SSF having ρrd=0.19, the slightly decreases in cell size was observed. When the UP content was increased, the resultant ρrd and CW thickness decreases. In higher ρrd foam, due to presence of more SSP, that is responsible for more shrinkage during sintering. Volume fraction ratio between matrix to space holder is reducing from 1:1 in 50% to 1:4 in 80% space holder content. Therefore the average CW thickness is reducing from 0.66 to 0.27 mm. UP being organic in nature act as a lubricant during the compaction of metal powder. Therefore, at higher urea content, powder mixture became more compressible. This resulted into higher green density and thus higher degree of sintered density. This observation can also be seen in Table 1, where the ρrd in case of 80% space holder is nearly 19%; whereas in case of 50% space holder is 43%. This indicates that the obtained density is ~14% less than the targeted density in case of lower content (50% UP). Whereas in case of higher content (80% UP), this obtained density is 5% less than that of sintered density. The higher compressibility of mixed powder in 1:4 ratios in case of higher space holder content enhances the contacts between matrix particles. This enhances sinterability and effect of this factor is responsible for attaining target density (19% against target of 20%) and reduction in cell size (Table 1). The microstructure of SSF samples with different ρrd are shown in Fig. 4(a-d). It may be noted that the cells (marked ‘C’) are 6
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interconnected, cell walls are the marked ‘CW’. The sintering temperature is lower than the melting point of SS. Therefore solid state diffusion take place during sintering process and cells size are fully open with interconnected pores with small amount of micro porosities in CWs. It was observed in Fig. 4 (c-d) that when higher percentage of UP was used, evaporation of UP sometimes resulted into CW breakage due to small wall thickness and higher rate of volatiles of UP. In the CW region Fig. 5(a), the localized fusion at SSP-SSP interface (marked ‘black arrow’) and micro porosity (marked ‘white arrow’) can be seen at higher magnification in Fig. 5(b). The EDS analysis of magnified view of CW region Fig. 5(c) is shown in Fig. 5(d). It is clear from the EDS that that no oxide is formed and no urea residue is left after sintering. The magnified micrograph in Fig. 5(c) is clearly showing densified metal particles without noticeable pores and cracks. The XRD of the SSP particles and SSF sintered sample is shown in Fig. 6. The XRD pattern of these samples is noted to be almost similar to that of as-received SSP particles, indicting no significant change in chemistry and phase constituent during the sintering of foam. 3.2. Mechanical Properties The compressive stress-stain curve of the SSF samples with the varying ρrd is shown in Fig. 7(a). It is clearly observed from this figure that the compressive stress-strain curves have distinct three regions: (i) elastic region. (ii) linear plateau region, (iii) densified region. The linear plateau region shows that the compressive stress increases slightly with increase in compressive strain. From Fig. 7(a), the yield stress notes were calculated & plotted with ρrd in Fig. 7(b), it can be connected the yield stress increases with ρrd. The change in cross section area is almost negligible during compaction, up to 0.45 compressive strains in the plateau region as well as plastic region. In the densified region, the cross section area increases with the reduction in height and the flow stress increases rapidly with slight increase in strain as similar to solid material. The plateau region in higher ρrd may be thinner. It is indirectly demonstrating that the pores are getting compacted during deformation and hence the diameter of the sample is not increasing proportionately with extent of cell deformation. The steeper slope in plateau region may also be due to CW, hardening because of austenitic to martensitics transformation and interaction of microhardness. In the plateau region, the increment in cross section area is nominal. Even after 50% of deformation crossectional area increased only by 15% in case of ρrd of 0.43. In case of ρrd of 0.2, the crossectional area increases only 5% 7
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decreases with decrease in ρrd. This indicated that Ef, as well as plastic modulus of the foam, decreases with increase in porosity. In an elastic region, CW bending occurred. Further, during compression, the CW tends to crack and fracture. The CW collapse due to buckling followed by shearing and fracturing of CW layer by layer. In the third region, the SSF behave like a solid material and stress increases due to strain hardening. The diameter after final compaction in each case is shown in Table 2. The above finding is also confirmed through Table 2. From the table, it can also be observed that with increasing the compressive strain, the diameter of foam samples increased. In case of foam having ρrd = 0.19, diameter is almost unchanged till 0.5 strain and further diameter changed drastically. It can be concluded from the observation that foam with ρrd = 0.19 has plateau region till 0.5 strains. Similarly, other foams with higher ρrd have lower plateau region defined by linear line after elastic region. The σpl was calculated from this linear line and the strain under this linear line is measured as ɛd. In order to understand the effect of ρrd on the plastic modulus (slope of the plateau region in stress-strain curve), it is plotted with respect to ρrd as show in Fig. 7(c). The plastic modulus is increasing with increases in ρrd. It is due to more strain hardening as results of higher CW thickness or more materials in cell. At higher ρrd more materials are subjected to deformation. Additionally, the CW are thinner at lower ρrd, therefore for better understanding, the normalized plastic modulus (ratio of plastic modulus and yield stress) is calculated and plotted as a function of ρrd. The normalized plastic modulus increases with in decreases ρrd as shown in Fig. 7(d). Eab was calculated from the area under the curve up to densification. Yield stress, plastic modulus, σpl and Eab increases with increase in ρrd but ɛd decrease with ρrd. All the parameters are reported in Table 3. 3.3. Effect of relative density 3.3.1. Plateau stress (σpl) The σpl is a function of ρrd for SSF is shown in Fig. 8(a). It is evident from this curve that σpl with ρrd follows a power law relationship.
()
𝛔𝐩𝐥 = 𝟑𝟖𝟖.𝟏
𝛒𝐟
𝟏.𝟔𝟔
(2)
𝛒𝐝
Where ρf and ρd are the density of SSF samples and dense SS respectively. The value of coefficient (C) and exponent (n) are estimated by curve fitting. The ‘C’ and ‘n’ calculated to be 8
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388.1 and 1.66 respectively. The strength of dense stainless steel reported to be 490 MPa [27]. Micro hardness values use a proportionally constant. Hence, equation no (2) can be written as: 𝛔𝐩𝐥 = 𝟎.𝟖𝟎𝛔𝐝
() 𝛒𝐟
𝟏.𝟔𝟔
(3)
𝛒𝐝
Where σpl and σd are the strength of SSF samples and dense SS respectively. Here, the constant and exponents values are noted to be 0.80 and 1.66 respectively. The empirically calculated proportionality constant ‘C’ and the exponent ‘n’ obtained from the theses values. The above relations were compared with the reported values as shown in Table 4. It is noted that the value of coefficient ‘C’ and exponent ‘n’ obtained in the present studies are in good agreement of SSF with varying ρrd. 3.3.2. Elastic modulus (Ef) The Ef is a function of ρrd for SSF is shown in Fig. 8(b). It is evident from this regime that (Ef) increases with ρrd as shown bellow:
()
𝐄𝐟 = 𝟗𝟖.𝟒𝟔
𝛒𝐟
𝟏.𝟖
(4)
𝛒𝐝
The Ef of dense stainless steel is reported to be 193 GPa [10]. Hence, equation no (4) can be written as:
()
𝐄𝐟 = 𝟎.𝟓𝟏𝐄𝐝
𝛒𝐟
𝟏.𝟖
(5)
𝛒𝐝
Here, the constant and exponents values are noted to be 0.51 and 1.7 respectively. The empirically calculated proportionality constant ‘C’ and the exponent ‘n’ obtained study were compared with the reported values Table 4. It is noted that these values of are in good agreement with reported values.
3.3.3. Densification strain (ɛd) The εd for the SSF samples is plotted as a function of ρrd as shown in Fig. 9. The εd in the stainless steel foam is found to be decreased linearly with increases in ρrd. Considering all the investigated material together as the following relation is obtained for densification as a function of ρrd. 9
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𝛆𝐝 = 𝟎.𝟓𝟎𝟎 - 𝟎.𝟒𝟐𝟕
If
() 𝜌𝑓
𝜌𝑑
() 𝛒𝐟
(6)
𝛒𝐝
becomes 1 i.e for dense material, εd = 0.07. This indicated that dense materials will have
almost no densification strain but when
() 𝜌𝑓
𝜌𝑑
is zero, εd is determined to be 0.5. Practically this
is not feasible. This is be because of the fact that the εd is difficult to determine accessibly. This is influenced by cell wall deformation, cell work hardening and less extent of cell closure deformation. A large fraction of cell remained undensified. These could be visible through the microstructure examination of deformed sample. Empirically calculated proportionality constant ‘Cd’ and the exponent ‘nd’ obtained from the theses values the above the relations were compared with the reported values as shown in Table 4. The empirical linear equation expresses the εd as a function of ρrd with reasonably excellent accuracy. During the densification, cells are coarser (cells left due to evaporate the UP). It was further observed that in the densified SSF, even the coarser cells are not completely compacted. Thus, the densification of such SSF starts relatively at lower strain. These facts become more prominent with increase in ρrd. 3.3.4. Energy absorption (Eab) The Eab of SSF samples as a function of ρrd is shown in Fig. 10(a). The Eab of SSF is calculated from the stress-strain curve up to 0.45% deformation by using standard methodology [28]. 𝐄𝐚𝐛 =
𝛆𝐝
∫𝟎 𝛔𝐝𝛆
(7)
Where, εd = densification strain, σ = stress, ε = strain. It is observed from the figure that the Eab also follows the power law relationship with ρrd. These relations clearly demonstrate that the Eab increase with increase in ρrd. This is primarily due to the fact that Eab is product of σpl and εd (area under the stress-strain curve). The plateau stress follows the power law relation with ρrd, thus the Eab also follows the similar type of relation. Comparison of UP used as a water leachable and evaporative process with base materials, space holder particles size, sintering temperature time and atmosphere, porosity, σpl/Ef, yield stress and Eab of open cell SSF reported in literature are given in Table 5 3.3.5. Energy absorption efficiency 10
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The energy absorption efficiency of SSF samples as a function of densification strain at different ρrd is shown in Fig. 10(b). The energy absorption efficiency “η” is calculated using the following equation [29, 30]:
𝜺
∫𝝈 (𝜺)𝒅𝜺 𝜼=
𝟎
(8)
𝝈𝒎𝒂𝒙𝜺
The terms σmaxε is the ideal Eab, where σmax is the maximum stress and ε is the specific strain. Fig. 10(b) shows the energy absorption efficiencies of SSF samples at various strains. It clearly shows that higher energy absorption efficiencies of SSF at the strain from 0.1 to 0.3 for all ρrd. It is noted that energy absorption efficiency increases with decrease in ρrd. The energy absorption efficiency depends on the behavior of compressive stress-strain behavior. The plastic modulus of SSF increases with increases in ρrd. This causes slope to increase in stress at higher ρrd. The plateau region is flatter at lower ρrd and hence the energy absorption efficiency of these samples is higher. As the energy absorption efficiency is higher in case of lower ρrd, it is prefers to use low density foams (highly porous foams) for shock energy or crash energy absorption application. High density foam may be applicable for blast resistance and structural applications.
3.4. Deformation mechanism
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The deformed 316L SFF samples is shown in Fig. 11(a). The micrograph of deformed samples (SSF of ρrd 0.43) at transverse section shown in Fig. 11(b-d) and for ρrd of 0.19 in Fig. 11(e-f). When the sample of higher ρrd starts to compacting, the CWs starts deforming plastically and finally get sheared, (marked white arrow) fracture (marked ‘F’) and compacted Fig. 11(b). Micro shear bands are also formed (marked black arrow). The cycles of cell walls (CWs) collapse, cell sinking were repeated and continue deformation layer by layer till the densification is completed. It is also noted that the CW get bended and also CWs gets sheared and cracked which are leading to CW crushing. Thus matrix is subject to more plastic deformation and subjected more strain hardening which is reflected in post deformation micro hardness measurement in Table 3. After densification micro porosities are remained in the CW Fig. 11(c). Additionally, the coarser CWs are subjected to less deformation and carry higher load. Because of theses, the work hardening of CWs less. The work hardening of the CWs is due to microstructure and austenitic to martensitics transformation. The austenitic to martensitics transformation is visible at higher magnification; micro shear band and martensitics needles are visible in Fig. 11(d). After densification regions, foams sample deforms towards densified materials and in this region, stress increases with strain. It may be noted clearly that the foams have not been fully densified even after the strain at which stress increases with strain. It is further noted that the foam with lower ρrd densified more and less porosity after densification is noted Fig. 11 (e-f). Thus also signified that at lower density CWs gets more effectively deformed. The shear band formation thus formed in higher deformation is clearly visible in Fig. 11(e) (marked white arrow). At lower ρrd more cracks at the lateral surface are observed due to shearing of CWs.
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4. Conclusion Following conclusion cab ne drawn from this study:
Spherical UP can be used as an excellent evaporative space holder for making open cell stainless steel foams with a desired cell size and ρrd.
The cell size and density may be varied by changing the ratio of SSP: UP in stainless steelurea mixture. ρrd and of CW thickness of SSF varies with variation of UP content.
Solid state diffusion take place during sintering process and cell size are fully open with uniform interconnected pores with small amount of micro porosities in CW.
The yield strength, plastic modulus, σpl, Ef and Eab of SSF is following power law relationship with ρrd. However, the ɛd following linear relationship with ρrd.
The σpl, Ef and ɛd have empirically correlation with ρrd. The coefficient and exponent in the present study are good agreement with reported values of SSF.
Deformation of SSF is associated with CW bending, CW collapse due to bulking followed by shearing and facture of CW layer by layer. Finally, SSF relatively more uniform compacted. Thus the SSF could be excellent candidate for replacing dense SS for implant, structural and engineering applications, especially for uniform energy absorption.
Acknowledgments The authors are thankful to Director, CSIR-AMPRI, Bhopal for giving his permission to publish this work. The author Hemant Jain is thankful to CSIR-India for providing Senior Research Fellowship (SRF).
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List of figures Fig. 1(a) Size distribution of SSP, (b) microstructure of SSP particles, (c) size distribution of UP and (d) spherical UP. Fig.2 (a) Process flow diagram for making sintered SSF and (b) Sintered SSF samples; top view and Front View. Fig. 3 Cell Size distribution of SSF with ρrd of 0.27. Fig. 4 Microstructure of SSF samples with different ρrd (a) ρrd=0.43, (b) ρrd =0.36, (c) ρrd =0.27 and (d) ρrd =0.19. Fig. 5(a) Microstructure of SSF of ρrd=0.19, (b) microstructure of CW of SSF, (c) microstructure of SSF at higher magnification and (d) EXD analysis of SSF of ρrd=0.19. Fig. 6 XRD pattern of SSP-particles and sintered SSF sample. Fig. 7(a) Compressive stress-strain curve of SSF with different ρrd, (b) yield stress, (c) plastic modulus (slope of plateau region) and (d) normalized plastic modulus (Plastic modulus/yield strength) as a function of ρrd. Fig. 8(a) Average plateau stress (σpl) and (b) elastic modulus (Ef) as a function ρrd. Fig. 9 Densification strain (ɛd) as a function of ρrd. Fig.10 (a) Energy absorption (Eab) as a function of ρrd and (b) Energy absorption efficiency vs densification strain at different ρrd. Fig.
11(a) Deformed SSF at different ρrd, (b-d) microstructure of deformed samples after densification with ρrd of 0.43 and (e-f) ρrd of 0.19.
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Fig. 1(a) Size distribution of SSP, (b) microstructure of SSP particles, (c) size distribution of UP and (d) spherical UP.
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Fig.2 (a) Process flow diagram for making sintered SSF and (b) Sintered SSF samples; top view and Front View.
Fig. 3 Cell Size distribution of SSF with ρrd = of 0.27.
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Fig.4 Microstructure of SSF samples with different ρrd (a) ρrd=0.43, (b) ρrd =0.36, (c) ρrd =0.27 and (d) ρrd =0.19.
Fig. 5(a) Microstructure of SSF of ρrd=0.19, (b) microstructure of CW of SSF, (c) microstructure of SSF at higher magnification and (d) EXD analysis of SSF of ρrd=0.19. 4
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Fig. 6 XRD pattern of SSP-particles and sintered SSF sample.
Fig. 7(a) Compressive stress-strain curve of SSF with different ρrd, (b) yield stress, (c) plastic modulus (slope of plateau region) and (d) normalized plastic modulus (Plastic modulus/yield strength) as a function of ρrd.
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Fig. 8(a) Average plateau stress (σpl) and (b) elastic modulus (Ef) as a function ρrd.
Fig. 9 Densification strain (ɛd) as a function of ρrd.
Fig.10 (a) Energy absorption (Eab) as a function of ρrd and (b) Energy absorption efficiency vs densification strain at different ρrd. 6
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Fig.
11(a) Deformed SSF at different ρrd, (b-d) microstructure of deformed samples after densification with ρrd of 0.43 and (e-f) ρrd of 0.19.
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Highlights
Urea can be used as excellent evaporative space holder for making SSF.
Cell sizes are almost related to the size of space holder.
The mechanical properties of SSF follow standard relationship.
Deformation behavior of SSF is strong as a function of relative density.
This technique is very simple, economical and highly reproducible, which may facilitate the commercialization of such metal foam.
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List of tables Table 1 Space holder volume %, target porosity, actual ρrd, cell size, CW thickness, micro hardness and microhardness deformed samples. Table 2 Intermediate diameter and final diameters. Table 3 Average plateau stress (σpl), elastic modulus (Ef), yield Stress, plastic modulus, energy absorption (Eab) and densification strain (ɛd) of the SSF samples (at strain rate = 0.01s-1). Table 4. Values of empirical constants reported by various researchers and the obtained in the present study for different empirical relations. Table 5. Comparison of the reported literature on open cell steel foam using urea (carbamide) as a leachable and evaporative process.
. Table 1 Space holder volume %, target porosity, actual ρrd, cell size, CW thickness, micro hardness and microhardness deformed samples. Space Target holder relative (vol. %) density
Actual relative density (ρrd)
Cell size (mm)
Cell wall Thickness (CW) (mm)
Micro hardness (kgf/mm2)
Micro hardness deformed samples (kgf/mm2)
1.57± 0.32
0.60 ± 0.16
235 ± 3
309 ± 3
50
0.50
0.44 ± 0.01
60
0.40
0.36 ± 0.01 1.78 ± 0.27
0.40 ± 0.06
231 ± 4
302 ± 2
70
0.30
0.26 ±0.01
1.89 ± 0.28
0.39 ± 0.06
228 ± 3
281 ± 3
80
0.20
0.19 ± 0.01 1.66 ± 0.31
0.27 ± 0.06
220 ± 3
277 ± 3
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Table 2 Intermediate diameter and final diameters.
Relative Total density compressive strain (ρrd)
Initial diameter (mm)
Final diameter (mm)
0.43
0.5
19.58 ± 0.09
22.25 ± 0.09
0.35
0.6
19.37 ± 0.1
23.42 ± 0.09
0.27
0.7
19.62 ± 0.1
24.45 ± 0.09
0.19
0.2 0.5 0.8
19.56 ± 0.1
19.91 ± 0.1 20.94 ± 0.1 24.94 ± 0.09
Table 3 Average plateau stress (σpl), elastic modulus (Ef), yield Stress, plastic modulus, energy absorption (Eab) and densification strain (ɛd) of the SSF samples. (at strain rate = 0.01s-1). R.D (ρrd)
0.43 0.36 0.27 0.19
Average Plateau (σpl) (MPa)
Elastic Modulus (Ef) (GPa)
Yield stress (MPa)
99.83 ± 0.9
22.65 ± 0.5 15.19 ± 0.3
45.3 ± 0.32
63.09 ± 1.1 41.7 ± 1.3 22.54 ± 1.3
9.18 ± 0.5 4.86 ± 0.5
28.33 ± 0.31 12.04 ± 0.35 5.4 ± 0.34
Plastic modulus (MPa)
Normalize d plastic modulus 7.23 ± 0.32
327.92 ± 0.2 8.43 ± 0.31 238.9 ± 0.2 154.54 ± 0.3 76.93 ± 0.2
12.83 ± 0.32 14.24 ± 0.32
Energy Absorpti on Eab (MJ/m3) 32.15 ± 0.25
Densification Strain (ɛd)
24.16 ± 0.23 16.19 ± 0.25 9.744 ± 0.26
0.35 ± 0.03
0.32 ± 0.03
0.38 ± 0.05 0.43 ± 0.05
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Table 4 Values of empirical constants reported by various researchers and the obtained in the present study for different empirical relations. Relation
𝝈𝒑𝒍 = 𝑪𝝈𝒅
“C’’
“n”
References
0.3
1.5
[31]
0.37
1.73
[32]
0.3
1.5
[33]
0.65
1.5
[34]
0.80
1.66
Present study
0.98
2
[35]
1.598
4.72
[36]
0.193
1.43
[37]
0.28
1.13
[12]
0.51
1.8
Present study
𝛒𝐟
0.85
0.82
[38]
𝛒𝐝
1.206
1.6
[39]
1
1.4
[40]
0.50
0.42
Present study
() 𝝆𝒇
𝝆𝒅
()
𝐄𝐟 = 𝐂𝐄𝐝
𝒏
𝝆𝒇
𝐧
𝝆𝒅
𝐄𝐝 = 𝐂 ‒ 𝐧
()
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Table 5 Comparison of the reported literature on open cell steel foam using urea (carbamide) as
Evaporative Process
Water Leachable Process
a leachable and evaporative process. Base materials
Urea (carbamide) average size (µm)
Sintering temperature, time and sintering atm.
Porosity (%)
Young’s / elastic modulus (GPa)
Yield stress (MPa)
Energy absorption (MJ/m3)
Ref.
17.4 PH stainless steel
710-1000
1260ºC -40 min Hydrogen atm.
82-40
1.14
40
------
[41]
17.4 PH stainless steel
750-1000
1350 ºC -60 min Hydrogen atm.
60-40
46-55
------
-----
[23]
316L stainless steel 316L stainless steel
750-1000
70
0.27
------
--------
[42]
60-50
-------
-------
-------
[20]
316L stainless steel 316L stainless steel
2000
1370 ºC -120 min Vacuum atm. 1000 ºC 1100 ºC 1200 ºC Argon atm. 1100 ºC -60 min Vacuum atm. 1100 ºC -120 min Argon atm.
60-40
------
-------
------
[26]
60-40
------
-------
-------
[43]
81-57
4.8-22.6
5.4 - 45.3
9.7- 32.2
Present study
316L stainless steel
--------
200 µm
2000-2600
1200 ºC -60 min Vacuum atm.