Compressive Property and Nano-mechanical Behavior of Aluminium Cenosphere Composite Foam Developed by Powder Metallurgy Route

doi:10.21203/rs.3.rs-1964616/v1

Download PDF

Research Article

Compressive Property and Nano-mechanical Behavior of Aluminium Cenosphere Composite Foam Developed by Powder Metallurgy Route

https://doi.org/10.21203/rs.3.rs-1964616/v1

This work is licensed under a CC BY 4.0 License

Version 1

posted

You are reading this latest preprint version

In the present study, the mechanical properties in terms of nano and microhardness and compressive strength of aluminium cenosphere composite foam (CF) developed by powder metallurgy route was carried out. The CF was developed under a varied compaction pressure between 125 MPa and 375 MPa, and sintered by two step sintering (step 1: heating at 400 ºC for 1 hrs. and step 2: heating at 700ºC for 2 hrs.) with a cenosphere content varying from 5 vol.% to 50 vol.%. The effect of process parameters and cenosphere content on microstructure, porosity and its distribution, microhardness and nano mechanical properties was undertaken. Finally, the compressive behavior of the Al-cenosphere composite foam processed under varied compaction pressure and with varied cenosphere content was evaluated.

Aluminum

cenosphere

composite foam

microstructure

compressive property

Cenosphere as a space holder was successfully applied to develop aluminium and titanium based metallic foam [1–4]. Though metallic foam may be developed by melting and casting route; however, segregation of cenosphere and its non-uniform distribution in the matrix are main challenges which need to be addressed while developing Al cenosphere composite foam [5, 6]. In this regard, powder metallurgy is the unique route for developing cenosphere incorporated composite foam (CF) which can control the size, shape and distribution of cenosphere in the matrix precisely. Powder metallurgy was successfully applied for development of cenosphere incorporated Ti and Al composite foam [2, 3]. However, for the development of composite foam by powder metallurgy route, the process parameters (compaction pressure, sintering temperature and cenosphere vol.%) need to be controlled to achieve the desired properties in the material. Aluminium foam is considered to be an important class of material which has applications as break lining, brake rotors, engine blocks in automotive sectors as well as marine, aerospace, and deface sectors [1, 7–9]. In the past, several studies were undertaken to develop Al and Ti based cenosphere composite foam by powder metallurgy route [2, 3]. Mondal et al. [3] reported that there is damage of cenosphere in the matrix providing porosity in the foam at an applied pressure higher than 75 MPa. Guo et al.[1] compacted aluminium flyash cenosphere composite (5 to 20 wt.%) at 414 MPa, and reported the breakage of flyash and its uniform dispersion in the matrix. They also reported that the broken cenosphere particles adversely affected the density of the foam. Vogiatzis et al. [2] compacted aluminium cenosphere (20 to 40 vol.%) composite foam at pressure ranging from 200 MPa to 300 MPa and reported the influence of applied compaction pressure and vol.% of the cenosphere on the mechanical properties of the foam. However, an extensive studies concerning the effect of percentage cenosphere and compaction pressure on the mechanical properties were not undertaken. Jha et al. [10] reported that increased addition of cenosphere reduced the inter-particles distance of the foam and influenced its properties. However, the effect of cenosphere on the local change in properties was not undertaken.

The present study aims at understating the effect of compaction pressure (125 MPa − 375 MPa) and cenosphere content (5 vol.% to 50 vol.%) on the microstructure, physical properties, nano-mechanical property and compressive strength of aluminium cenosphere composite foam developed by powder metallurgy route.

2.1 Raw Materials and Sample fabrication

In the present study, commercially available aluminium powder (98% purity, particle size + 45 µm − 60 µm, make: Loba Chemie Pvt. Ltd., India) with the composition of (Si-0.674%, P-0.016%, Ti-0.01%, S-0.034%, Fe-0.07%, Mn-0.057%, Cu-0.02%, Cr-0.015%, Zn-0.02% and balance is Al), and cenosphere powder (+ 20 µm -120 µm, Cenosphere India Ltd., Kolkata, India) containing (Al₂O₃-23.24%, SiO₂-66.60%, Fe₂O₃-3.72%, K₂O-2.87%, TiO₂-1.36%, CaO-0.81%, MgO-0.66%, P₂O₅-0.25%, Na₂O-0.15%) were dried at 100 ºC for 1 hr. (for removal of residual gases and moisture) prior to fabrication of Al cenosphere composite foam. Al powders used in the present study were irregular in shape with average particle size ranging from 10 µm to 90 µm, (measured by detailed analysis of the microstructures of the as received powder captured in FEGSEM). Cenosphere particles were spherical in shape with average size ranging from 20 µm to 120 µm (measured microstructures of the captured in FESEM using an image analysis software). In addition, the microstructure of the cenosphere showed a porous shell wall with thickness 1 to 10% of total diameter of the cenosphere. Al and cenosphere (with the vol.% varying from 5 vol.% to 50 vol.%) were mixed in a slow speed tumbler at 100 rpm for 4 hrs. and subsequently subjected to compaction using 2 wt.% Polyvinyl alcohol (PVA) as binder with the applied pressure of 125, 250, and 375 MPa for 2 minutes to a shape of cylinder with the dimension of 10 mm diameter and 15 mm length. Sintering was conducted in a tubular furnace initially at 400 ºC for 1 hr. and then 700 ºC for 2 hrs. in argon atmosphere to avoid thermal shock, cracks, and retention of sample integrity. The schematic of the experimental steps followed in the present study is presented in Fig. 1. Followed by sintering, the density, relative density, and porosity of the CF has been measured extensively as a function of compaction pressure and cenosphere content.

2.2. Microcomputer tomography (µ-CT)

The size, shape, and distribution of porosities or voids and its interconnectivity were measured using microcomputer tomography (Model: Phoenix v|tome|xs µ-CT scanning system) analysis. For scanning the CF, the X-ray energy source of 100 mA, and 75 kV was selected, and a total of 1000 2D transmission images were captured in one complete rotation of the foam samples. Phoenix Datox Software, and VGStudio Max 2.0 softwaree were utilized for reconstructing the 3D tomogram, and to analysis and visualize the 3D rendered images, respectively. From the micro CT image, pore size, distribution and the sphericity of pores were analyzed extensively.

2.3 Microstructural Examination

The microstructure of the sintered pellets was observed using a Field emission gun Scanning Electron Microscope (FEGSEM) (Model: Zeiss SMT AG). The composition of the composite from using Energy-Dispersive X-Ray Spectroscopy (EDS) analysis attached to the FESEM.

2.4 Nanoindentation of foam

To understand the mechanical properties in the localized region (near the cenosphere, on the cenosphere, matrix, and at the interface) nano-indentation test was conducted using a nano indenter (Model: Tri Tec SA Nano indenter, Make: Anton Paar, Switzerland) at an applied maximum load of 100 mN, linear loading/unloading rate of 200 mN/min and 10 sec duel time. Indentation was applied using a Berkovich indenter with a tip radius of 0.1 µm, Poisson's ratio (υ) of 0.2, and tip elastic modulus of 865 GPa. Following indentation, the results were analysed using Indentation 7.3.10 software. For obtaining the load displacement curve of the composite foam, an average of 10 readings were recorded. The nano hardness and young’s modulus were calculated from the load-displacement curve using the Oliver and Pharr method [11]. The equations applied for calculation of the hardness, reduced modulus and young's modulus from the nano indentation data are stated below.

$$H=\frac{{P}_{max}}{A}$$

$$\frac{1}{{E}_{r}}= \frac{1-{\vartheta }_{s}^{2}}{{E}_{s}}+ \frac{1- {\vartheta }_{i}^{2}}{{E}_{i}}$$

Where, H is the nano hardness in MPa, P_max is maximum applied load, A is projected area, E_r, E_s and E_i is reduced modulus, modulus of sample and modulus of the indenter in GPa, and υ_s and υ_i are the Poisson's ratio of sample and indenter, respectively.

2.5 Mechanical properties of foam

The mechanical properties of the composite foam were evaluated in term of micro hardness and compressive strength. The microhardness of the CF was measured using a Vickers micro-hardness tester (Model: UHL-VMHT 001, Germany) using 100 gf normal load, and 10 sec dwell time. The average hardness of 15 different locations was used to calculate the average microhardness of the CF. For evaluating the compressive deformation behavior, cylindrical samples with L/D ratio is 1:1.5, was subjected to standard Universal Testing Machine (UTM) (Model Name: H50KS, Tinius Olsen, United Kingdom) at a strain rate of 0.01/s at room temperature. During the test, load and displacement data were recorded which was converted to the stress and strain using standard methodology. The peak or yield stress (σ_y), plateau stress (σ_pl – average stress between 5–30% compressive strain [12]), energy absorption (E_ab), capacity (${\text{E}}_{ab}={\int }_{0}^{{\epsilon }_{d}}\sigma d\epsilon$), and densification strain (ε_d) of these foam samples were determined from stress-strain diagrams. Followed by compressive deformation, the fractography of failed foam was observed in Field emission gun scanning electron microscope to understand the defamation behavior of the composite foam.

3.1. Microstructure

Figure 2 shows the scanning electron micrographs of the top surface of Al cenosphere composite foam developed at different compaction pressure of (a-c) Al and Al cenosphere CF with (d-f) 10 vol.% cenosphere, (g-i) 30 vol.% cenosphere and (j-l) 50 vol.% cenosphere, where, (a, d, g, j) shows the micrographs of the samples processed at 125 MPa compaction pressure, (b, e, h, k) shows the micrographs of the samples processed at 250 MPa compaction pressure (c, f, i, l) shows the micrographs of the samples processed at 375 MPa compaction pressure. From Fig. 2 it may be noted that the microstructure of the sintered Al is pore free with very fine grains all through out. The grain size of which does not vary significantly with compaction pressure. In cenosphere dispersed Al up to 10 vol.% of cenosphere, 375 MPa compaction pressure, cenosphere is seen to be dispersed uniformly without any visible damage of the cenosphere particles. However, in 30 vol.% cenosphere dispersed Al there is partial damage and full damage of cenosphere particles at the compaction pressure of 250 MPa and 375 MPa, respectively. Finally, at 50 vol.% of cenosphere the particles were seem to be damaged even at 125 MPa compactions pressure and degree of damage increased with increase in compaction pressure. Further it may also be concluded that the porosities present in the matrix decreased with increase in compaction load. There is also presence of cenosphere cluster in 50 vol.% of cenosphere dispersed Al which is due to (a) enhanced vol.% of cenosphere and (b) crushed particles of cenosphere due to higher compaction load[3]. Increased damage of cenosphere with increase in vol.% of cenosphere is because of the fact that mostly cenosphere take part in load bearing resulting in crushing in cenosphere cell wall because of its brittle nature [10]. Figures 3 (a, b) shows the (a) Scanning electron micrograph of Al cenosphere composite foam of 40 vol.% and (b) EDS spectra of the sample. From Fig. 3 (a) it may be noted that microstructure of sintered Al foam consists of presence of cenosphere in undamaged (arrow head) and crushed form. The EDS spectrum (cf. Figure 3b) shows the presence of aluminium oxide and silicon oxide in the microstructure of the foam, which confirms the presence of mullite (3Al₂O₃.2SiO₂) and sillimanite (Al₂SiO₅) phase into the matrix [2, 3]

3.2. Density and porosity

Figure 4 (a, b) shows the variation of (a) density, and (b) porosity of sintered Al, and Al cenosphere composite foam with 5 vol.% to 50 vol.% of cenosphere compacted at 125 MPa (Plot 1), 250 (plot 2), and 375 MPa (Plot 3). From 4 (a) it may be noted that with increase in vol.% of cenosphere from 5 to 30 vol.% the density of the foam decreases; however, with further addition of cenosphere from 40 to 50 vol.% the density of the composite foam increases. In addition, the compaction pressure also influences the density and porosity of the composite foam. With increase in compaction pressure from 125 to 375 MPa the density of the foam systematically increases and hence, porosity decreases. However, a systematic study of the effect of compaction pressure on porosity shows that with increase in compaction pressure form 125 MPa to 275 MPa the porosity decreases around 10 to 50%; however, the variation of porosity is minimum up to 6% from 275 MPa to 375 MPa. In addition, it may also be noted that with increase in cenosphere from 40 vol.% the density of the cenosphere increases significantly even at very low compaction pressure (125 MPa). The increment in density even at low compaction pressure at higher vol.% of cenosphere addition is due to fragmentation of cenosphere and its intermixing with Al causing reduction in volume fraction of space holder content in the matrix. As the vol.% of the cenosphere increases beyond 40 vol.% the percentage of broken cenosphere also increases. In the past, it was reported that cenosphere particles got damaged at a higher applied load and losing its integrity or core space [1–3, 13]. Since the cell wall of the cenosphere consists of the mullite phase possessing higher density, (3.2 g/cm³) as compared to as received cenosphere (0.7 g/cm³), breakage of cenosphere and its intermixing with the matrix increases the density of the composite especially, at high compaction pressure.

3.3 Microcomputer tomography (µ-CT)

Figure 5 (a-e) shows the 3-D µ-CT images of (a) sintered Al, and Al cenosphere composite foam with (c) 5 vol.%, (e) 10 vol.%, (g) 30 vol.% cenosphere and corresponding equivalent pore diameter of (b) sintered Al, and aluminium cenosphere composite foam with (d) 5 vol.%, (f) 10 vol.%, and (h) 30 vol.% cenosphere develop a compaction pressure of 375 MPa. From figure.5 (a, c, e, and g) it is clear that with increase in cenosphere from 5 to 30 vol.%, the porosity percent increases from 0.02% (for Al) to, 1.12% (for 5 vol.%), 1.90% (for 10 vol.%), and 4.62% (for 30 vol.%) of cenosphere CF. In addition, with increase in vol.% of cenosphere there are also signature of presence of broken particles and cluster cenosphere in CF. From the equivalent pore diameter distribution, it is clear that the presence of pores in the cenosphere is in the range of 50 µm; however, with the addition of cenosphere, most of the pores are in the range of of 50 to 150 µm. Table 1 summarizes the variation in porosity percentage, pore diameter, sphericity index, as a function of cenosphere vol.% and compaction pressure. From Table 1 it is observed that the average pore diameter increases from 81.59 µm to 86.17 µm, and the sphericity index decreases from 0.75 to 0.61 for 0 to 30 vol.% of the CF. The marginal increase in pore diameter and decrease in sphericity index confirms that with the higher compaction pressure and increased cenosphere vol.%, the particles of the cenosphere are broken. It also possibly occurs due to fragile nature of the cenosphere.

Figures 6 (a, c and e) show the 3D µ-CT images and (b, d, and f) show corresponding equivalent pore diameter for 50 vol.% cenosphere added composite foam developed by (a, b) 125 MPa, (c, d) 250 MPa and (e, f) 375 MPa compaction pressure. From the detailed analysis of volume faction of porosity using µ-CT it may be noted that the porosity percent decreases from 11.24 to 2.25%, with increased compaction pressure from 125 MPa to 375 MPa, which confirms an increased extent of sintering due to enhanced binding between the particles at a higher compaction pressure. The distribution of equivalent pore diameter shows with the increase in compaction pressure (from 125 MPa to 375 MPa), pore count decreases. It is observed that with 125, and 250 MPa compaction pressure the pores is in the lower range, i.e.,75 µm; however, compacting at using 375 MPa, shows the variation of average pore diameter in the range of 100 µm. The equivalent pore diameter of composite foam (cf. Table.1) initially decreases up to 275 MPa, and further increases from 84.11 µm to 94.07 µm, which confirms that application of higher compaction pressure causes loss of structural integrity of cenosphere. It occurs due to increased volume fraction of broken particles of the cenosphere which also increase the cenosphere clusters in the composite foam.

Figures 7 (a, b) shows the sphericity index with different equivalent pore diameter for the samples of (a) sintered Al and (b) Al composite foam with 5 vol.% cenosphere developed at 375 MPa compaction pressure. From Fig. 7 it may be noted that most of the pores in sintered Al have sphericity index from 0.6 to 0.8; however, the sphericity index decreases as the pore diameter increases after 125 µm. On the other hand, sphericity of the CF shows that the pore diameter up to 75 µm is having sphericity higher than 0.5; however, for CF with a larger pore diameter, the sphericity of CF decreases. From the above discussion, it could be concluded that the porosity decreases with compaction pressure; however, at a higher compaction pressure, spherity is lost due to loss of integrity of cenosphere.

Table 1 summarizes the sphericity of different samples as function of vol.% of cenosphere (at 375 MPa compaction pressure) and compaction pressure for (50 vol.% cenosphere). From Table 1 it may be noted that the sphericity of as-received Al at high compaction load is close to 1 (0.75) which signifies that porosities are present predominantly because of gas entrapment or incomplete binding between the particles. The sphericity marginally decreases with increase in vol.% of cenosphere i.e. 0.63 for 5 vol.% to 0.59 for 50 vol.% at applied compaction pressure at 375 MPa. The marginal decrease in the sphericity with increase in the vol.% of cenosphere in composite foam is attributed to presence of internal porosity due to inadequate sintering or breakage of cenosphere particles contributing to pore formation in the CF. For Al-50 vol.% composite foam the study of effect of compaction pressure on sphericity shows that with increase in compaction pressure from 125 to 375 MPa there is increase in sphericity from 0.53 to 0.59 which signifies pores are formed because of breakage of cenosphere and as increase in pressure increase the extent of damage of cenosphere the sphericity marginally increases.

Table.1 Effect of the varying percentage of cenosphere and compaction load into the microporosity, pore diameter and sphericity of pores.

Sample ID	Compaction pressure (MPa)	Porosity (%)	Average pore diameter (µm)	Sphericity
Al-0V	375	0.02	33.6	0.75
Al-5V	375	1.12	81.59	0.63
Al-10V	375	1.90	84.23	0.64
Al-30V	375	4.62	86.17	0.61
Al-50V	125	11.24	100.15	0.53
Al-50V	250	5.35	84.11	0.57
Al-50V	375	2.25	94.07	0.59

3.4 Mechanical properties of foam

3.4.1 Nanomechanical properties

A detailed study of the mechanical properties of the localized region was undertaken using Nanoindentation test carried out at an applied load of 100 mN. For the Nanoindentation test, 15 zones were selected to measure the average mechanical properties of 0, 5, 20, and 50 vol.% aluminium cenosphere CF. All tests have been performed on CF compacted by 275 MPa. Figure 8 (a-d) show the loading unloading behavior of foam as a function of depth, for as received Al (plot 1), and cenosphere dispersed CF with 5 vol.% cenosphere (plot 2), 20 vol.% cenosphere (plot 3), and 50 vol.% cenosphere (plot 4). Figures 8 (b-d) show (b) nano hardness, (c) Young’s modulus, and (d) energy absorption in the composite foam derived from the nano indentation plot calculated using Oliver and Pharr equation. From Fig. 8 (b-d) it may be noted that with increase in vol.% of cenosphere the nano hardness, and elastic modulus of the CF decreases from 502.81 MPa (for sintered Al) to 299.35 MPa (for 50 vol.%), and 64.06 GPa (for sintered Al) to 27.72 GPa (for 50 vol.% of cenosphere dispersed Al). However, the total energy absorption of foam increases from 95.914 nJ (for sintered Al) to 152.913 nJ (for 50 vol.% cenosphere dispersed of the CF). The higher hardness value observed in sintered Al is attributed to formation of defect free microstructure caused by perfect sintering and less vol.% of microporosities (0.2%). Decrease in average nano hardness with increase in cenosphere presence is attributed to presence of voids in the microstructure. The modulus of elasticity also follows the similar trend. In this regard it may be noted that after 20 vol.% of cenosphere shows marginally higher hardness and young modulus which is attributed to partial damage of cenosphere and its distribution in the matrix causing partially changing the microstructure from composite foam to composite hence an increased hardness. From Fig. 8d it may be noted that with increased in cenosphere there is increase in total energy absorption as compared to as received Al which confirmed that the composite foam is having a higher damping capacity than Al. The detailed nanoindentation results of the composite foam are summerised in Table 2.

Table.2 Nanomechanical properties of Composite foam through Nanoindentation route

Sample History	Nano Hardness (MPa)	Elastic Modulus (GPa)	Stiffness (mN/nm)	Elastic energy (nJ)	Plastic Energy (nJ)	Energy absorption (nJ)
Al-0V	502.81 ± 53.62	64.06 ± 2.81	1.09 ± 0.04	5.32 ± 0.31	90.59 ± 7.58	95.91 ± 7.38
Al-5V	356.69 ± 54.69	48.94 ± 4.53	1.01 ± 0.15	6.64 ± 1.06	114.82 ± 19.87	121.45 ± 18.95
Al-20V	272.19 ± 38.30	27.03 ± 7.91	0.64 ± 0.14	11.60 ± 3.41	139.45 ± 10.47	151.05 ± 13.57
Al-50V	299.35 ± 53.30	27.72 ± 4.93	0.33 ± 0.03	17.06 ± 3.01	135.85 ± 13.37	152.91 ± 16.36

To understand the effect of cenosphere particles dispersion on the mechanical properties of the composite foam nano indentation was carried out (b) between two cenosphere particles (c) matrix and (d) at the interface between cenosphere and matrix of cenosphere dispersed composite foam. The corresponding loading unloading behavior is shown in Fig. 9 (a). Table 3 summarizes the mechanical properties of the individual zone. From Table 3 it may be noted that the hardness and the modulus of the foam near the cenosphere particles increases; however, the interface between matrix and cenosphere shows the lowest hardness and the modulus of the CF. The lowest hardness of the foam at the interface possibly occurs due to the presence of interfacial defects.

Table.3 Nanomechanical properties of different zone of the composite foam through Nanoindentation route.

Sample Location

Nano Hardness

(MPa)

Elastic Modulus (GPa)

Stiffness

(mN/nm)

Elastic energy

(nJ)

Plastic

Energy

(nJ)

Energy absorption

(nJ)

Near cenosphere particle

259.47

19.93

0.45

12.51

157.67

170.18

Matrix of CF

179.37

12.28

0.33

24.12

194.37

218.48

Interface

153.49

15.81

0.46

13.59

189.54

203.14

From the above-mentioned discussion, it may be concluded that with increase in the cenosphere percentage, the hardness increases because of decrease in the gap between the cenosphere particles; however, to a larger vol.% of cenosphere above 30 vol.%, there are a large nos. of interfacial defects reducing overall mechanical properties of the foam.

3.4.2 Hardness of foam

Figure 10 shows the average microhardness of the top surface of sintered Al and Al cenosphere composite foam (5 vol.% to 50 vol.%) developed at different compaction pressure of 125, 250, and 375 MPa. From Fig. 10 it may be noted that there is systematic increase in hardness with increase in compaction pressure for both Al and Al cenosphere CF. The systematic increase in hardness with increase in compaction pressure for both as received Al and Al cenosphere composite foam is attributed to increased density of the foam and hence, reduced internal porosity with increase in compaction pressure. From the influence of cenosphere percentage it may be noted that there is increase in hardness due to addition of cenosphere for all the samples as compared to as-received Al under the present set of applied compaction pressure. However, up to 20 vol.% cenosphere added composite foam, there is increase in hardness with increase in cenosphere percentage; however, with increase in cenosphere percentage further to 30 vol.% there is a marginal decrease in hardness at 125 MPa applied pressure while in 250 and 375 MPa applied pressure the enhancement in hardness continues. Increase in hardness with increase in cenosphere percentage is attributed to increased percentage of cenosphere shell in the matrix. However, at a higher percentage of cenosphere (30 vol.%) there is marginal decrease in hardness at 125 MPa applied pressure possibly due to breakage of cenosphere and pore formation in the matrix. However, at higher pressure and for the higher vol.% of cenosphere added Al, possibly there is crushing of cenosphere and its intermixing with the matrix causing formation of mullite/ sillimanite dispersive, presence of oxides, aluminium silicate dispersed metal matrix composite whose hardness is higher than cenosphere. Hence, up to 20 vol.% of cenosphere the enhancement in the hardness of composite foam is because of presence of cenosphere, a marginal decrease in 30 vol.% cenosphere dispersed composite is because of presence of partial porosity due to breakage of cenosphere increasing the cenosphere content further (for 40 and 50 vol.% of cenosphere) especially at higher compaction pressure there is partial breakage, fragmentation of cenosphere cell and its dispersion in the matrix causing partial dispersion strengthening of the Al matrix along with the foam [14].

3.4.3 Compressive properties of foam

Figures 11 (a-d) show the (a) stress stain curve of Al and Al cenosphere composite foam (5 vol.% to 50 vol.% of cenosphere) and the corresponding (b) yield strength, (c) plateau strength, and (d) absorbed energy derived from the stress strain diagram. From Fig. 11 (a) it may be noted that the stress strain diagram of as received Al shows two distant regions i.e. elastic deformation and plastic deformation of the matrix. On the other hand, in the cenosphere dispersed composite foam, there are three zones, i.e. elastic deformation corresponding to the region showing elastic behavior up to the point of max yield stress or peak stress beyond which there is breakage of cenosphere and formation of cracks in the matrix, (b) plateau deformation where, the stress drops due to continuous fracture of the cenosphere and deformation in the matrix, and (c) densification zone where the stress of the CF increases at constant strain. The offset of 0.2% of strain is used to determine the value of yield strength of the foam sample. The yield strength (σ_y), plateau strength (σ_pl), and energy absorption capacity (E_ab) of foam derived from the stress-strain curve, are summarized in Figs. 11 (b –d). From Figs. 11b-d it is observed that with increase in compaction pressure from 125 MPa to 375 MPa, the strength of the foam (σ_y, σ_pl, and E_ab) increases in both sintered aluminium and the CF. The addition of the cenosphere, increases the strength of the foam, even at low density due to the presence of a hard cell wall of the cenosphere in the matrix [15, 16]. The addition of the cenosphere increases the σ_y, σ_pl and E_ab of the CF up to 20 vol.%. However, further addition (up to 40 vol.%) of the cenosphere decreases the strength of the foam. The 50 vol.% CF shows higher σ_y as compared to 40 vol.% due to increased density, and relatively less porosity in the foam. The lower strength of the CF when developed at 125 MPa compaction pressure as compared to 250 and 375 MPa compaction pressure confirms that at low compaction pressure bonding between particles is not adequate to occur increased mechanical properties of CF. The post-yield deformation occurs in cenosphere reinforced composite foam due to gradual buckling and bending of the cenosphere cell wall causing the formation of the plateau region [2]. It may further be noted that there is presence of serration in the stress strain diagram of CF which is due to continuous fracture of the cell wall of cenosphere particles. Similar behavior has also been observed in cenosphere dispersed Ti and Al cenosphere dispersed composite foam [2, 13]. The continued application of compressive force causes fragmented cenosphere particles causing the filling up of the cavities and densification of the foam. Besides, the general densification of foam also occurs in this plateau reason. After the plateau reason, the gradual increment in stress for constant strain is observed, due to complete densification of the foam. From the energy absorption capacity of the foam it is revealed that energy absorption capacity increases with an increase in compaction pressure and addition of the cenosphere up to 20 vol.%, following which it decreases at 30 vol.% cenosphere, at 40 vol.% of cenosphere it decreases further when 125 MPa compaction pressure (which shows this compaction pressure with higher vol.% of cenosphere dispersion reduces the bonding and between particles and shows imperfect sintering due to higher cenosphere content), and further increment in applied compaction pressure (250 and 375 MPa) increases significantly (due to densification of the foam), for 50 vol% CF the energy absorption decreases further, which shows the early failure of composite foam. The enhanced energy absorption capacity of the foam is due to presence hard cell wall of the cenosphere [17], which enhanced the load-carrying capacity of the foam. In concluding mark, it could further be stated that up to 20 vol.% of cenosphere dispersed CF shows the high energy absorption capacity; however, higher vol.% of cenosphere (30, 40, and 50 vol.%) dispersed CF shows lower energy absorption capacity of the CF due to premature failure of the foam.

Table.4. Hardness and compressive mechanical properties of the composite foam

Composition	Compaction pressure (MPa)	Hardness (HV)	Peak Stress (σ_y) (MPa)	Plateau stress (MPa)	Energy absorption (MPa)	Densification strain (ε_d)
Al-0V	125	30 ± 2	27.5 ± 1.16	33.55 ± 1.46	2304.73 ± 110.46	44.03
	250	36 ± 3	80.5 ± 4.83	106.39 ± 6.18	4552.43 ± 265.23	40.83
	375	37 ± 3	88.7 ± 5.22	110.30 ± 6.59	7556.38 ± 453.38	42.27
Al-5V	125	33 ± 3	31.06 ± 1.35	52.13 ± 2.59	7449.77 ± 362.55	52.82
	250	38 ± 5	72.92 ± 4.37	91.80 ± 5.48	9891.93 ± 581.62	50.59
	375	40 ± 7	87.4 ± 5.22	117.80 ± 7.06	13489.19 ± 809.35	50.44
Al-10V	125	38 ± 6	37.6 ± 1.78	53.09 ± 2.61	10713.81 ± 498.59	37.97
	250	40 ± 4	67.9 ± 4.09	99.69 ± 5.89	13287.20 ± 765.33	45.25
	375	44 ± 5	88.2 ± 5.27	118.85 ± 7.10	10936.98 ± 656.21	41.62
Al-20V	125	43 ± 5	40.6 ± 2.13	51.74 ± 2.55	11835.63 ± 583.82	48.34
	250	46 ± 5	77.3 ± 4.73	88.43 ± 5.30	12382.78 ± 742.63	48.67
	375	53 ± 8	84.2 ± 5.08	100.21 ± 6.0	13639.45 ± 832.29	57.32
Al-30V	125	42 ± 7	38.8 ± 1.99	54.13 ± 2.70	2138.70 ± 101.23	38.13
	250	46 ± 9	66.7 ± 4.06	60.66 ± 3.62	1779.34 ± 106.76	32.79
	375	51 ± 11	77.6 ± 4.68	73.80 ± 4.39	3798.68 ± 212.56	41.61
Al-40V	125	40 ± 6	20.67 ± 1.05	15.24 ± 0.77	514.17 ± 25.89	41.47
	250	49 ± 9	40.76 ± 2.45	44.25 ± 2.67	7534.80 ± 452.13	54.92
	375	56 ± 12	55.42 ± 3.42	63.17 ± 3.80	9885.37 ± 575.19	59.25
Al-50V	125	42 ± 11	42.6 ± 2.43	40.04 ± 2.01	1562.09 ± 72.13	41.38
	250	54 ± 17	75 ± 2.65	68.24 ± 4.14	3861.30 ± 231.52	49.98
	375	66 ± 14	100.3 ± 6.21	83.73 ± 5.02	6001.39 ± 358.18	52.67

3.4.4 Fractography of foam

Figures 12 (a-i) show the fractured surface of (a-c) as received sintered Al compacted at 250 MPa pressure, (d-f) 20 vol.% cenosphere dispersed CF compacted at 250 MPa, and (g-i) 50 vol.% cenosphere depressed CF compacted at (g) 125 MPa, (h) 250 MPa, and (i) 375 MPa. Where, (b, c) are the magnified image of sintered aluminium, and (e, f) are the magnified image of 20 vol.% cenosphere dispersed CF. Fracture surface of Al shows the presence of micro dimples (arrow head 1), microcracks (arrow head 2), and microporosities (arrow head 3). Hence the fractography of as received Al shows ductile mode of fracture. The 20 vol.% Al cenosphere CF shows the presence of big cracks in the fractured surface, the high magnification image shows the presence of broken particles of cenosphere and presence of dimple in the matrix (c.f. Figure 12b) the high magnification view of the same (c.f. Figure 12c) shows the presence of fracture at the interface between the cenosphere and matrix, from where it may be concluded that cracks initiate between the cenosphere and matrix during compressive loading. The fractography shows similar behaviour as that of the stress-strain curve. It shows that, at starting, the stress increases with strain due to the cenosphere addition; however, it decreases later on due to resistance of applied load by the larger size of cenosphere particles. These particles first resisted the applied load and collapsed to some degree, which further reduces the stress or formation of the plateau zone. The presence of broken particles of the cenosphere and the presence of the small particles of the cenosphere, confirms that the hard cell wall of the cenosphere hinders the crack formation, initiated from the matrix. The fractography of 50 vol.% Al cenosphere CF also shows presence of crack formation and collapse of cenosphere, in addition to micro cracks at the interface of the cenosphere at low applied load. On the other hand, at higher applied load there is signature of complete collapse of cenosphere cell and formation of voids in addition to its presence in the matrix.

In the present study, the aluminium cenosphere composite foam has been developed by powder metallurgy route using cenosphere as space holders from 0 to 50 vol.%, and compaction pressure from 125 to 375 MPa. The effect of cenosphere and applied compaction pressure on microstructure, density, microporosity, nanomechanical properties, hardness, and compressive mechanical properties of composite foam was studied. From the detailed study, the following conclusions may be drawn:

The microstructure of aluminium cenosphere composite foam shows that the cenosphere space holder is distributed uniformly throughout the matrix, and the cenosphere space holder formed closed porosities in the aluminium matrix.

The density of the foam decreases with increased addition of the cenosphere; however, the applied compaction pressure increases the density of the foam.

The µ-CT study shows that the microporosity of foam increases with the addition of cenosphere from 0.02% (for 0 vol.%) to 11.24% (for 50 vol.%); however, it decreases with the application of compaction pressure from 11.24% (for 50 vol.% at 125 MPa) to 2.25% (for 50 vol.% at 375 MPa).

The Nanoindentation results show that the mechanical properties of foam depend on the distribution of cenosphere particles, the presence of porosity, and the presence of interfaces in the CF. The interface between the cenosphere and matrix shows the lowest mechanical properties due to the presence of some degree of gaps.

The hardness of the foam increases from 30 HV (for 0 vol.%) to 66 HV (for 50 vol.%) with the addition of the cenosphere. The increment in the hardness of the CF observed because of the presence of hard cell wall of the cenosphere.

The strength of foam depends on the density of the CF. The yield strength of foam increases with the applied compaction pressure, and the addition of cenosphere up to 20 vol.%, further addition of cenosphere decreases the strength of the foam. However, 50 vol.% CF (applied compaction pressure is 250 and 375 MPa) shows highest yield strength due to increased density of the CF and dispersion of ceramic shell in the matrix.

The fractography shows that aluminium shows ductile fracture; however, the mixed type of fracture is observed in composite foam, which depends on the vol.% of the cenosphere. The higher vol.% CF shows a brittle fracture of the CF.

Acknowledgement

The partial financial grant from the Ministry of Human Resource Development (MHRD), N Delhi to AS for the said work is gratefully acknowledged. Partial financial support from Alexander von Humboldt Fellowship (Friedrich Wilhelm Bessel Award) to JDM is gratefully acknowledged. Characterization facilities extended by Central Research Facility, IIT Kharagpur is also gratefully acknowledged.

Credit authorship contribution statement

Conceptualization: A. S., D. P. M. and J. D. M., First draft of the paper: A. S. and J. D. M., Final reviewing: J. D. M.

Conflict of interest: On behalf of all authors, the corresponding author states that there is no conflict of interest.

Data Availability Statement

The authors declare that the data supporting the findings of this study are available within the article.

R.Q. Guo, P.K. Rohatgi, D. Nath, Preparation of aluminium-fly ash particulate composite by powder metallurgy technique, J. Mater. Sci. 32 (1997) 3971–3974. https://doi.org/10.1023/A:1018625118090.
C.A. Vogiatzis, A. Tsouknidas, D.T. Kountouras, S. Skolianos, Aluminum-ceramic cenospheres syntactic foams produced by powder metallurgy route, Mater. Des. 85 (2015) 444–454. https://doi.org/10.1016/j.matdes.2015.06.154.
D.P. Mondal, J. Datta Majumder, N. Jha, A. Badkul, S. Das, A. Patel, G. Gupta, Titanium-cenosphere syntactic foam made through powder metallurgy route, Mater. Des. 34 (2012) 82–89. https://doi.org/10.1016/j.matdes.2011.07.055.
A. Kumar Shukla, J. Dutta Majumdar, Studies on microstructure and mechanical properties of aluminium foam prepared by Spray Forming Route, Procedia Manuf. 35 (2019) 861–865. https://doi.org/10.1016/j.promfg.2019.06.032.
P.K. Rohatgi, D. Weiss, N. Gupta, Applications of fly ash in synthesizing low-cost MMCs for automotive and other applications, Jom. 58 (2006) 71–76. https://doi.org/10.1007/s11837-006-0232-4.
V.R. Kumar, C.R.P. Rao, Poornachandra, R. Suresh, Corrosion and Wear Studies on LM6 Grade Aluminum-Cenosphere Composite-An Experimental Approach, Mater. Today Proc. 5 (2018) 11667–11677. https://doi.org/10.1016/j.matpr.2018.02.136.
I.B. Singh, D.P. Mondal, M. Singh, A. Bhadkul, N. Jha, Corrosion behaviour of AA2014 aluminium alloy-cenospheres syntactic foam in 3.5% NaCl solution, Indian J. Chem. Technol. 21 (2014) 168–175.
P.K. Rohatgi, N. Gupta, B.F. Schultz, D.D. Luong, The Synthesis, compressive properties, and applications of metal matrix Syntactic foams, Jom. 63 (2011) 36–42. https://doi.org/https://doi.org/10.1007/s11837-011-0026-1.
S. Sahu, M.Z. Ansari, D.P. Mondal, C. Cho, Quasi-static compressive behaviour of aluminium cenosphere syntactic foams, Mater. Sci. Technol. (United Kingdom). 35 (2019) 856–864. https://doi.org/10.1080/02670836.2019.1593670.
N. Jha, A. Badkul, D.P. Mondal, S. Das, M. Singh, Sliding wear behaviour of aluminum syntactic foam: A comparison with Al10 wt% SiC composites, Tribol. Int. 44 (2011) 220–231. https://doi.org/10.1016/j.triboint.2010.10.004.
W.C. Oliver, G.M. Pharr, An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7 (1992) 1564–1583. https://doi.org/10.1557/JMR.1992.1564.
V.K. Jeenager, V. Pancholi, B.S.S. Daniel, Influence of cell wall microstructure on the energy absorption capability of aluminium foam, Mater. Des. 56 (2014) 454–459. https://doi.org/10.1016/j.matdes.2013.08.109.
N. Jha, D.P. Mondal, M.D. Goel, J.D. Majumdar, S. Das, O.P. Modi, Titanium cenosphere syntactic foam with coarser cenosphere fabricated by powder metallurgy at lower compaction load, Trans. Nonferrous Met. Soc. China (English Ed. 24 (2014) 89–99. https://doi.org/10.1016/S1003-6326(14)63032-6.
D.P. Mondal, J.D. Majumdar, M.D. Goel, G. Gupta, Characteristics and wear behavior of cenosphere dispersed titanium matrix composite developed by powder metallurgy route, Trans. Nonferrous Met. Soc. China (English Ed. 24 (2014) 1379–1386. https://doi.org/10.1016/S1003-6326(14)63202-7.
D.P. Mondal, S. Das, N. Ramakrishnan, K. Uday Bhasker, Cenosphere filled aluminum syntactic foam made through stir-casting technique, Compos. Part A Appl. Sci. Manuf. 40 (2009) 279–288. https://doi.org/10.1016/j.compositesa.2008.12.006.
M.D. Goel, M. Peroni, G. Solomos, D.P. Mondal, V.A. Matsagar, A.K. Gupta, M. Larcher, S. Marburg, Dynamic compression behavior of cenosphere aluminum alloy syntactic foam, Mater. Des. 42 (2012) 418–423. https://doi.org/10.1016/j.matdes.2012.06.013.
K. V. Mahendra, K. Radhakrishna, Fabrication of Al-4.5% Cu alloy with fly ash metal matrix composites and its characterization, Mater. Sci. Pol. 25 (2007) 57–68.

No competing interests reported.

PMCompreiveGraphical.tif

Download PDF

Version 1

posted

You are reading this latest preprint version

Compressive Property and Nano-mechanical Behavior of Aluminium Cenosphere Composite Foam Developed by Powder Metallurgy Route

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental

2.1 Raw Materials and Sample fabrication

2.2. Microcomputer tomography (µ-CT)

2.3 Microstructural Examination

2.4 Nanoindentation of foam

2.5 Mechanical properties of foam

3. Result And Discussion

3.1. Microstructure

3.2. Density and porosity

3.3 Microcomputer tomography (µ-CT)

3.4 Mechanical properties of foam

3.4.1 Nanomechanical properties

3.4.2 Hardness of foam

3.4.3 Compressive properties of foam

3.4.4 Fractography of foam

4. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1