A study on porosity and mechanical properties of the open aluminum metal foam through Spark Plasma Sintering SPD Technique

doi:10.21203/rs.3.rs-4629275/v1

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A study on porosity and mechanical properties of the open aluminum metal foam through Spark Plasma Sintering SPD Technique

https://doi.org/10.21203/rs.3.rs-4629275/v1

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Aluminum metal foam, distinguished by its lightweight nature, exceptional strength-to-weight ratio, and distinctive cellular structure, has emerged as a highly promising material with extensive applications across diverse industries. Various fabrication techniques, including the Sintering and Dissolution Process (SDP) and powder metallurgy, have been explored to produce metal foams. However, challenges persist in achieving uniform pore sizes and distributions, especially at temperatures surpassing the base material's melting point. To overcome this hurdle, simultaneous loading and heating during fabrication have been proposed, with Spark Plasma Sintering (SPS) emerging as a viable solution. This study delves into the manufacturing of aluminum metal foam utilizing NaCl space holders via SPS, investigating variations in space holder volume to analyze pore morphology and porosity. Additionally, the mechanical properties of the resulting foams are examined, providing valuable insights into the potential of SPS for crafting aluminum metal open foams with tailored properties. Porosity analysis, conducted through X-ray micro CT, reveals porosity ranging from 55–70% in metal foams with NaCl space holder volume fractions of 60–80%. Notably, a maximum energy absorption capacity of 23 MJ/mm³ is achieved in a metal foam with 57% porosity.

Closed metal foams

aluminum

porosity

plateau stress

energy absorption density

Aluminum metal foam represents a fascinating intersection of science, engineering, and materials innovation, offering a myriad of promising applications across various industries. Characterized by its lightweight nature, remarkable strength-to-weight ratio, and unique cellular structure, aluminum metal foam has garnered significant attention for its potential to revolutionize conventional materials in fields ranging from automotive and aerospace to biomedical and construction [1, 2].

There are various techniques for the fabrication of metal foams such as Direct gas blowing, sintering with blowing agents, investment casting, sintering and dissolution process (SDP), and powder metallurgy [1, 2]. SPD is the latest technique for the fabrication of metal foam where the metal powder is mixed with space holder followed by the fabrication of green compact specimens through the powder metallurgy route. Then the green compacted specimens are subjected to sintering for a given time at a constant value of temperature. Thereafter the sintered components are placed in the flowing water or stirred in the water which dissolved the space-holder. It left pores in the sintered components. The pore size is dependent on the space-holders size. Literature [3–6] applied SPD techniques and produces metals foams by using space holder NaCl, K₂CO₂ carbonate in electric furnace assisted sintering followed by the dissolution of space-holder in water. Authors produced the metal foam after melting the base material in the furnace which led to non-uniform size of pores. In addition, process temperature above the melting point of the base material results in the melting of it which expel the space-holder from their place. Because the density of the space-holder is less than the foam materials (i.e. Metal powder). This causing heterogeneous size of the pores in the metal foam when the proper ratio of metal/space-holder, and the temperature are not maintained.

The above problem can be tackled through the simultaneous action of the load and the heat on the die during the fabrication of metal foam. Spark plasma sintering (SPS) is one such technique. Researchers [7–10] have applied SDP rout in the SPS process to fabricate a metal foam of Al and Cu powder. They used 70% (i.e. Volume %) NaCl and mixed with the 30% ( i.e. Volume ) mixture powder of Al and Cu. Later they varied the percentage of Cu in the Al powder from 0–10%. Al₂Cu intermetallic has been found in the foam which improved plateau stress during the compression test. However, SPS is expensive and limited to the size and shape of the metal foams. However, detailed qualitative information about the pores were not discussed.

In the present authors have produced aluminum metal foam by using NaCl space holder through SPS. In the study, the volume of the space holder has been changed and studied the morphology pores and volume of porosity. Later mechanical properties of the fabricated foams have been studied.

Authors have fabricated the aluminum foams by using SPS. The volume fraction of NaCl space holder has been changed from 40%, 50%, 70% and 80% in the mixture NaCl and Al powder. The cuboidal shape of NaCl powder of an average size of 300 µm was mixed in the aluminum powders of size in the range of 15–26 µm. Later mixed powder of Al-NaCl was allowed to be compacted by using spark plasma sintering machine (SPS 625, Fuji Electronic Industrial Co. Ltd., Japan) in a graphite die. The diameter of the die was 12 mm. Later sintering of the specimens has been done at a temperature and pressure of 580°C and 70 MPa for a holding time of 20 min. Thereafter, sintered samples were kept in the flowing water for 24 hr at room temperature to remove the NaCl space holder from the specimens. Sintered samples were cylindrical in shape which has a diameter and height of 10 mm and 12 mm, respectively.

X-ray micro computed tomography (XMCT) scans were carried out on cylindrical samples of size diameter 10 mm and length 12 mm by using a X-ray computed tomography (GE-Phoenix model: V/TOME/XS). The three-dimensional imaging of the XMCT scans was generated by using a voxel size of 10 µm. The generated three-dimensional XMCT images were analyzed by using a analyzer tool (VG-STUDIO-MAX 2.20). The pores were separated from the aluminum matrix by applying a threshold of size 0.05. The process parameters for XMCT are voltage 150 kV, current 150 mA, internal time 500 ms, focal distance from object 40 mm, number of projections 1000 and voxel size of 10µm. During XMCT, an ROI CT filter has been used to reduce the ring effect. Also, the histogram has been maintained between background and material by using an external filter (0.5 mm Cu). During the test, beam hardening and ‘agc’ corrections have also been used for better imaging of pores and to solve the problems related to sample fixation. Later SEM (Ziess, EVO 18) has been used to take micrographs of samples. Compression tests on samples were carried out on Universal testing machine (UTM, Instron 8862) at a cross head speed of 0.25 mm/min. Later the pore size and shapes were studied by using scanning electron-microcopy (ZEISS, EVO 18).

The manufacture metal foams were characterized by analyzing the porosity and mechanical properties. The details are discussed in the following sections.

3.1 Porosity

The porosity of the sintered samples has been studied by using XMCT. Figure 1. Depicts the XMCT macro-graphs of the samples for the different ratios of NaCl and Cu powders. It can be seen that sintering followed by leaching of the sample in the flowing water yields the formation of the porous structure.

Regions shown in different colors reflect different volumes of the pores, and the dark color indicates cell walls of aluminum material. The density of the pores was found to be dependent on the volume fraction, and the size of the NaCl spacer. Referring to Fig. 1(a), non-uniform size and shapes of pores have been observed for the sample where the volume fraction of NaCl was 40%. In addition, the pores were not connected. The volume of pores in the sample was 120 mm³ which was 12.7% of the initial size of the green compacted sample (i.e. 942 mm³). However, as the volume of NaCl spacer increased from 40 to 80%, the density of pores increased, and interconnected pores observed. The almost homogenous porous structure was found on the samples which constituted NaCl volume fraction greater than 50%, as can be seen in Fig. 1(b) and (c). The volume fraction and the porosity of the samples have been depicted in Table 1. Results have shown that a sample that has an 80% volume fraction of NaCl powder has the highest value of porosity (i.e. 76%) as seen in Fig. 1(d). The reason behind the formation of the interconnected pores and the highest porosity is an increased volume fraction of the space-holder. Sintering at a lower volume of NaCl leads to the formation of a larger number of cell walls of the aluminum pores. The density of the aluminum cells was highest at a lower volume fraction of space-holder. Aluminum cells isolated the NaCl powders in the sintered samples. As a result, NaCl powder could not be exposed to the water during the leaching process. However, at an increased volume fraction of NaCl above 50%, the density aluminum cell wall was reduced and it leads to the better dissolution of the NaCl spacer into the water during leaching. Thus improper dissolution of space-holder occurred in the water medium at a lower volume fraction of NaCl powder which left cavity less pores. Figure 2 shows the connected and isolated pores in the fabricated metals foam

Table 1

Volume of porosity on the samples prepared at different volume fraction of NaCl.
Sample Id	Ratio of NaCl and Al powder (NaCl:Al)	Volume of pore (mm³)	Volume fraction (%)	Remark on Dissolution of NaCl
S1	40:60	120	12.7	Poor
S2	50:50	378	40.12	Better
S3	70:30	543	57.64	Better
S4	80:20	720	76.19	Better

Further to verify the porous structure, SEM images of the sintered samples have taken and shown in Fig. 3. Black color shows the presence of pore and white shows the aluminum substrate. Referring to Fig. 3a, aluminum cells are isolated to each other. At an increased volume fraction of the NaCl, interconnected pores were observed in the foam. However, non-uniform shape and size of pores were observed in the foam. This could be due to the change in the size and shape of the NaCl powders under the effect of heat and pressure of SPS process. In addition to that, pores have the same shape as the shape of the space-holder. This indicates that the shape and size of the pores are determined by the space holder. In addition, the better porous structure can be obtained when the volume fraction of the space holder is above 60%. Figure 3e shows the pores and cells and corresponding EDS results shows the distribution of Al, Na and Cl (Fig. 3f)

3.2 Mechanical properties

Compression tests were conducted on the fabricated samples to investigate both their plateau stress and energy absorption capacity. The results, depicting compressive stress versus strain, are illustrated in Fig. 4(a), revealing variations in the response of the metal foams under compression testing. Sample S1 (60:40, Al:NaCl) exhibited a linear response initially, transitioning into a plateau phase, indicative of pore collapse during compression. As the volume of the space holder NaCl increased, the initial linear curve diminished while the plateau regions expanded. Within this study, the compressive stress within the 20–50% strain interval was identified as the plateau region. The average stress within this plateau region was subsequently calculated and discussed.

The plateau stress of all samples have been studied and depicted in Fig. 4(a). It can be seen that the plateau stress reduced from 123 (i.e. S1) to 8 MPa (i.e. S4) as the volume fraction of NaCl space holder increased from 40 to 80%. The area within the plateau regions has been calculated as energy absorption density (E) and given in Eq. (1).

\(E={\int }_{0}^{0.5}\sigma d\epsilon\)

Eq. (1)

The energy absorption density (E) of all the samples has been calculated and is presented in Fig. 4(c). It is noteworthy that S3 samples exhibit the highest values of energy absorption capacity compared to the others. Furthermore, as indicated in Fig. 4(d), S3 also demonstrates the highest modulus value (K). This can be attributed to the uniform pore size and increased porosity in the samples with 70% NaCl content. However, when the volume of NaCl is increased to 80%, it significantly reduces the plateau stress by a factor of 9 compared to S3.

Spark plasma sintering has been successfully employed for the manufacturing of the aluminum open foam by using the sintering and dissolution process technique. The work can be concluded as follows:

The porosity of the foams is dependent on the volume fraction of the space-holder.

The shape and size of the pores are dependent on the geometry of the space-holder. In addition, interconnected open pores can be obtained when the volume fraction of the space holder is above 60%.

Maximum plateau stress was found at lowest volumetric fraction of NaCl space-holder.

Volumetric fraction of 70% of NaCl provide highest amount of energy absorption capacity. Beyond 80% of NaCl

Author contribution statement

Raju Prasad Mahto: Writing – review & editing, Visualization, Conceptualization, Alok Bhadauria: Writing – review & editing, Visualization, Conceptualization, Din Bandhu: review & editing

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fundings

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Acknowledgement

Authors acknowledge the central research facility at IIT Kharagpur for providing sample characterization facilities. Additionally, authors express appreciation to the Manipal Institute of Technology at the MAHE Bengaluru Campus for providing the necessary facilities.

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You are reading this latest preprint version

A study on porosity and mechanical properties of the open aluminum metal foam through Spark Plasma Sintering SPD Technique

Status:

Version 1

Abstract

Figures

1. Introduction

2. Experimental Set Up

3. Results and Discussions

3.1 Porosity

3.2 Mechanical properties

4. Conclusions

Declarations

Author contribution statement

Declaration of Competing Interest

Fundings

Acknowledgement

References

Status:

Version 1