Mechanical Properties and Deformation Behavior of Open-Cell Type Aluminum Foams with Structural Conditions and Alloy Composition

Abstract

Open-cell type aluminum foams with various structural conditions and alloy compositions were manufactured using the replication casting process. The porosity of the foams ranged from 55% to 62%, with pore sizes of 0.7~1.0 mm, 1.0~2.0 mm, and 2.8~3.4 mm. The alloys employed included commercial A356 and A383, as well as Al-6Mg-(0, 2, 4, 6)Si alloys. Compression tests were conducted under various conditions of the foams, and the results were comparatively analyzed based on the detailed structural conditions and alloy compositions. It was observed that for the same alloy composition and equivalent porosity, a reduction in pore size led to an increased number of cell walls, enhancing energy dispersion and resulting in higher compressive yield strength and energy absorption. Under the same pore size, a decrease in porosity increased the relative density and cell wall thickness, leading to improved compressive yield strength and energy absorption. Furthermore, compressive evaluation based on alloy composition revealed the influence of the inherent mechanical properties of the material on the mechanical properties of open-cell type aluminum foams. Specifically, it was confirmed that alloys with higher ductility exhibited elastic behavior of the internal cells under external stress, significantly influencing the energy absorption of foams.

1. Introduction

Porous aluminum can be classified into closed-cell and open-cell types based on the internal pore structure. In particular, open-cell type aluminum foam has interconnected internal pores, allowing fluids to flow through the material [1,2]. This characteristic makes it advantageous for use in applications such as filters, heat exchangers, catalysts, etc. [3,4,5]. Moreover, due to the structural features of open-cell type aluminum foams, they exhibit high strength and impact absorption capability relative to their low density, enabling their application in various fields [3,4,6].

Open-cell type aluminum foams can exhibit various characteristics depending on the structural conditions of the internal pores, such as pore size and porosity. For example, an internal pore structure that allows smooth fluid flow can maximize the heat dissipation efficiency of a product, while a lightweight structure capable of absorbing substantial amounts of impact energy can be designed for applications in the automotive and aerospace sectors [3,4,6,7,8]. The key advantage of open-cell type aluminum foams is their potential to combine multiple characteristics as needed for specific products [9]. Recently, research has been conducted to utilize this advantage by combining thermal and mechanical properties for application in the battery cooling systems of eco-friendly vehicles [10]. However, it is important to note that these diverse characteristics often involve trade-offs. Structural features designed to enhance mechanical properties such as strength and impact energy absorption can be detrimental from a thermal performance perspective. For instance, a higher porosity in open-cell type aluminum foam generally allows for better fluid flow within the structure, enhancing heat dissipation efficiency. Nevertheless, this advantage comes with the drawback of reduced mechanical strength due to the decrease in relative density [7,11,12,13]. These aspects should not be overlooked during the design process. Therefore, it is essential to establish a comprehensive database and understand the mechanisms based on the various structural conditions of open-cell type aluminum foams. As a result, it will be possible to manufacture the desired product that best meets specific requirements.

In the case of the mechanical properties of open-cell type aluminum foams, even with the same porosity, the thickness of the cell walls can vary based on the pore size, influencing the mechanical properties. Additionally, even if the pore size is similar, variations in the distribution and number of internal pores can result in differences in porosity and relative density, significantly affecting the mechanical properties of the foam. Many researchers have recently conducted studies on the mechanical properties of open-cell type metallic foams under various structural conditions. Mohsen Hajizadeh et al. fabricated specimens with different structural conditions of open-cell type aluminum foams and performed compression tests to evaluate their energy absorption capacity [11]. However, the simultaneous variation of pore size and porosity limited the ability to understand the influence of each individual condition on the mechanical properties. Rajeev Kumar et al. investigated the compressive deformation behavior of open-cell type aluminum foams with relatively high porosity (approximately 78~92%) and assessed the mechanical properties through the relative density and compressive strength [12]. They confirmed that higher relative density leads to increased compressive strength, but there was still a limitation in controlling single variables, as pore size varied with porosity. In the research by Sriram Sathaiah et al., mechanical properties were evaluated based on structural conditions of open-cell type metallic foams, but they did not control for variables by distinguishing between pore size and porosity conditions [13]. Karol Ćwieka and Jakub Skibiński reported on the relationship between structural conditions and elastic properties by distinguishing between pore size and relative density of open-cell type metallic foams [14]. Similarly, Martin Reder et al. investigated the mechanical properties of open-cell type metallic foams by distinguishing structural conditions such as pore size and porosity [15]. However, both studies focused on simulation data based on the geometric shapes of open-cell type metallic foams, indicating the need for validation through experimental data.

Open-cell type aluminum foams, characterized by their interconnected internal pores, can be manufactured using various methods, including conventional techniques such as sintering, investment casting, and vapor deposition. Recently, there has been active research on designing open-cell type aluminum foams using 3D printing techniques [16,17,18,19,20,21,22]. However, these manufacturing methods have several drawbacks, such as complex processes, long production times, and high costs. Despite their excellent properties and the ability to implement various characteristics based on their unique structural features, the application of open-cell type aluminum foams is limited by these issues. Therefore, to facilitate the practical use and application of open-cell type aluminum foams across a wide range of products, it is necessary to develop simplified and cost-effective manufacturing methods.

In this study, Na₂CO₃, a water-soluble salt, was utilized as a space holder to fabricate open-cell type aluminum foams through the replication casting process, which is a relatively simple method. This manufacturing method allows the production of open-cell type aluminum foams with various pore sizes and porosities depending on the process parameters [7]. The aim was to establish a comprehensive database of the mechanical properties of open-cell type aluminum foams under different structural conditions. To control the mechanical properties influenced by the diversity of pore structures within the open-cell type aluminum foams, the study focused on analyzing the changes in properties resulting from single influencing factors by controlling the pore size and porosity conditions within the foams. The influence of structural conditions on the mechanical properties of the foams was evaluated by observing changes in the properties as the internal pore size varied under similar porosity levels or as the porosity and relative density varied with a constant pore size. This investigation was conducted through compression tests using the open-cell type aluminum foams manufactured under each condition. The stress–strain curves obtained from the compression tests were used to quantify the compressive yield strength and energy absorption as a function of strain, and the results were systematically categorized and compared based on the conditions of each specimen. Additionally, the mechanical properties changes induced by the deformation behavior of internal pores were investigated through the variations observed in the stress–strain curves and the actual specimen images corresponding to different strain levels during the compression tests. Moreover, open-cell type aluminum foams with different alloy compositions (A356, A383, Al-6Mg) were manufactured to evaluate the mechanical properties under identical structural conditions of pore size and porosity. This approach allowed us to determine how the inherent characteristics of the alloy material influence the properties of open-cell type aluminum foams.

2. Experimental Procedure

2.1. Manufacturing of Open-Cell Type Aluminum Foams

The open-cell type aluminum foams used in this study were manufactured through the replication casting process. This process involved filling the empty spaces of stacked space holders with molten aluminum alloy. The space holder was composed of Na₂CO₃ with a purity of 99.8% (DUKSAN science, Seoul, Republic of Korea). The alloys used for manufacturing the open-cell type aluminum foams were A356 and A383 commercial aluminum alloys, as well as Al-6Mg alloys with varying Si content (0, 2, 4, and 6 wt.%). The compositions of these alloys are shown in

Table 1. Chemical composition of aluminum alloys in this work.

Alloy	Chemical Composition (wt.%)
	Si	Fe	Cu	Mn	Mg	Zn	Ti	Al
A356	7.08	0.20	0.08	0.05	0.29	0.02	0.12	bal.
A383	10.92	0.87	1.98	0.23	0.22	0.71	0.08	bal.
Al-6Mg	-	0.04	-	-	5.83	-	-	bal.
Al-6Mg-2Si	1.92	0.04	-	-	5.99	-	-	bal.
Al-6Mg-4Si	4.03	0.05	-	-	5.84	-	-	bal.
Al-6Mg-6Si	5.98	0.04	-	-	5.91	-	-	bal.

. The size of the internal pores of the foams is determined by the size of the space holder used. In this study, three sizes of space holders were utilized (0.7~1.0 mm, 1.0~2.0 mm, 2.8~3.4 mm), and they were manufactured through a crushing process, which resulted in an angular shape. To fabricate open-cell type aluminum foams, space holders of the desired size were stacked inside a chamber before injecting the aluminum alloy melts. Furthermore, the porosity of the open-cell type aluminum foams can vary depending on the stacking conditions of the space holder. The space holder in this process was divided into 150 g portions and stacked twice or divided into 60 g portions and stacked five times (the total amount of the space holder per charge: 300 g), creating two different stacking conditions. It was expected that a higher number of stacking layers would result in a denser arrangement of the space holder, thereby increasing the porosity. Additionally, to establish reproducible manufacturing conditions in this process, the same pressure must be applied when stacking the space holder. For this purpose, a ramming bar was prepared, and the compaction operation was performed with a pressure of 0.013 MPa for each ramming. Subsequently, 1 kg of aluminum alloy melts was poured into the chamber at 730 $℃$, and 2 bar of Ar gas was pressurized to ensure that the aluminum alloy melts could fill the spaces between the space holders. To prevent the solidification of the melts before it was fully filled, the space holders were preheated to 600 $℃$. Afterward, the space holders were dissolved in water, resulting in open-cell type aluminum foams with interconnected internal pores.

Figure 1a shows an image of A356 alloy open-cell type aluminum foam immediately after manufacturing under the condition that the space holder size was 2.8~3.4 mm and stacked five times (five rammings). To verify the internal pore distribution and uniformity at different positions, the sample was divided into five sections, each measuring 61 × 61 × 10 mm³, as shown in Figure 1a,b. The cross-sectional images of the open-cell type aluminum foam at different positions before and after dissolution are presented in Figure 2, and the density of the samples at different positions was found to be nearly identical. Figure 3 illustrates the open-cell type aluminum foams manufactured using A356 alloy under various manufacturing conditions. Since angular-shaped space holders were used, the pore shape of the foams replicated the shape of the space holder, and differences in pore size were observed depending on the size of the space holder.

2.2. Porosity Measurement of Open-Cell Type Aluminum Foam

For open-cell type aluminum foams composed of numerous interconnected pores, the structural characteristics can vary depending on the volume of pores per unit volume of the foam. This is typically expressed as the relative density ε or porosity θ of the foams. In this study, to measure the porosity of the foams based on different manufacturing parameters, the foams were processed using electrical discharge machining to a size of 47.5 × 47.5 × 42 mm³. To measure the relative density of each foam, non-porous specimens of the same size were prepared. Using the densities of the prepared open-cell type aluminum foams ${\rho }_f$ and the non-porous specimens ${\rho }_b$ the relative density of the open-cell type aluminum foams for each condition was measured using Equation (1).

$$\epsilon ={\displaystyle \frac{{\rho }_f}{{\rho }_b}}$$

(1)

The relative density of open-cell type aluminum foams increases as the volume of the aluminum alloy within the foams increases per unit volume. Conversely, this indicates a decrease in the volume of pores within the foams. The pore volume per unit volume of the foam can be expressed as porosity. Using the relative densities measured under various conditions through Equation (1), the porosity of the foams under each condition can be calculated using Equation (2).

$$\theta =\left(1-{\displaystyle \frac{{\rho }_f}{{\rho }_b}}\right)\times 100$$

(2)

2.3. Cell Wall Thickness Measurement of Open-Cell Type Aluminum Foam

The cell wall thickness of open-cell type aluminum foams can vary with pore size and porosity, significantly influencing the mechanical properties of the foams. In this study, the open-cell type aluminum foams were manufactured using angular-shaped space holders, resulting in irregular polyhedral internal pore shapes, as shown in Figure 3. Due to this irregularity, mathematical calculations to determine cell wall thickness are limited. Therefore, as illustrated in Figure 4, the cell wall thickness was measured by recording the thickness along seven lines (five horizontal and two diagonal) on the upper surface of the foams to determine the distribution and average cell wall thickness (using the Image Pro Plus V6.0 program).

2.4. Compression Test of Open-Cell Type Aluminum Foam

The purpose of this study was to understand the effect of structural characteristics on the mechanical properties of open-cell type aluminum foams by varying the pore size and porosity. Additionally, the study aimed to investigate the influence of different alloy compositions on the mechanical properties of these foams.

Figure 5 shows images of compression test specimens for open-cell type aluminum foams made from A356 and A383 aluminum alloys with different space holder sizes and two rammings. The test specimens were machined into rectangular shapes measuring 12.7 × 12.7 × 25.4 mm³, using electrical discharge machining to minimize sample damage due to the porous structure. The mechanical properties of the foams were evaluated through compression tests on these specimens, assessing the effect of structural conditions. Furthermore, the influence of alloy composition on the mechanical properties of open-cell type aluminum foams was analyzed using A356 and A383 aluminum alloys, as well as Al-6Mg-(0, 2, 4, 6)Si alloys. The compression tests were performed using a universal testing machine (DTU-900MHN, Daekyung Tech Co., Inchen, Republic of Korea) with a crosshead speed of 0.05 mm/s and a strain rate of 0.002 s⁻¹. At least five specimens were evaluated for each condition.

Figure 6 shows the stress–strain curves for open-cell type aluminum foams manufactured using A356 and A383 aluminum alloys with 55% porosity (two rammings). In all conditions, during the compression tests, the initial region exhibited an elastic region (i) with very low strain values where the foam behaved elastically under the applied compression load. This was followed by the plateau region (ii), where the internal pores began to deform plastically, causing the stress to either increase slowly or oscillate. Finally, the densification region (iii) appeared, where most of the pores collapsed, leading to a rapid increase in stress. This behavior can be observed in the images from the compression tests of open-cell type aluminum foams, as shown in Figure 7. After the elastic region (i), the deformation of the pores begins to appear, eventually leading to the initiation of pore collapse. This deformation behavior was consistent across specimens under the same conditions. To compare the mechanical properties for each condition, the compressive yield strength was measured using the 0.2% offset method. A comparative analysis was then performed based on the structural characteristics and alloy composition of the foams.

3. Results and Discussion

3.1. Porosity

Table 2. Relative density, porosity, and average cell wall thickness of open-cell type aluminum foams according to manufacturing conditions.

Alloy	Ramming Count	Space Holder Size (mm)	Relative $\mathbf{Density} \mathit{\epsilon }$	$\mathbf{Porosity} \mathit{\theta }$ (%)	Average Cell Wall Thickness (mm)
A356	Two	0.7~1.0	0.452 ± 0.006	54.8 ± 0.6	0.26 ± 0.01
		1.0~2.0	0.447 ± 0.008	55.3 ± 0.8	1.01 ± 0.05
		2.8~3.4	0.441 ± 0.006	55.9 ± 0.6	2.03 ± 0.07
	Five	0.7~1.0	0.388 ± 0.006	61.2 ± 0.6	0.22 ± 0.01
		1.0~2.0	0.381 ± 0.005	61.9 ± 0.5	0.88 ± 0.04
		2.8~3.4	0.373 ± 0.006	62.7 ± 0.6	1.76 ± 0.06
A383	Two	0.7~1.0	0.457 ± 0.005	54.3 ± 0.5	0.28 ± 0.02
		1.0~2.0	0.446 ± 0.004	55.4 ± 0.4	0.99 ± 0.04
		2.8~3.4	0.443 ± 0.006	55.7 ± 0.6	2.11 ± 0.08
	Five	0.7~1.0	0.386 ± 0.006	61.4 ± 0.6	0.23 ± 0.01
		1.0~2.0	0.381 ± 0.005	61.9 ± 0.5	0.85 ± 0.03
		2.8~3.4	0.372 ± 0.004	62.8 ± 0.4	1.78 ± 0.07
Al-6Mg	Two	1.0~2.0	0.449 ± 0.004	55.1 ± 0.4	1.01 ± 0.05
Al-6Mg-2Si		1.0~2.0	0.452 ± 0.005	54.8 ± 0.5	1.03 ± 0.04
Al-6Mg-4Si		1.0~2.0	0.447 ± 0.007	55.3 ± 0.7	0.99 ± 0.04
Al-6Mg-6Si		1.0~2.0	0.451 ± 0.005	54.9 ± 0.5	1.03 ± 0.05

shows the relative densities and porosities of the open-cell type aluminum foams manufactured by the replication casting process, measured using Equations (1) and (2), classified by the foam manufacturing conditions (space holder sizes, ramming counts, and alloy types). For A356 open-cell type aluminum foams with two rammings, the relative density is approximately 0.45, and the porosity is about 55%. Although there is a slight variation in space holder size, the porosity is generally at a similar level. With five rammings, the relative density of A356 open-cell type aluminum foams is about 0.38, and the porosity is approximately 62%. Similarly, under the five rammings condition, the porosity difference due to pore size is minimal and measured at a comparable level. For open-cell type aluminum foams manufactured using A383 aluminum alloy with two rammings, the relative density is 0.45, and the porosity is approximately 55%, while with five rammings, the relative density is 0.38, and the porosity is about 62%. These results show that the porosity of the foam can be controlled by the ramming count conditions of the space holder, and the space holder size determines the pore size of the foam. Additionally, for Al-6Mg-(0, 2, 4, 6)Si alloys open-cell type aluminum foams manufactured using space holder of 1.0~2.0 mm size, regardless of the Si content, the porosity is approximately 55% under the two rammings condition, which is comparable to the porosity of the foams manufactured under the same conditions using A356 and A383 aluminum alloys. Therefore, it was confirmed that the structural conditions of open-cell type aluminum foams can be effectively controlled through the replication casting process irrespective of alloy type.

3.2. Cell Wall Thickness

The results of measuring the cell wall thickness of the foams according to structural conditions and alloy types are presented in Table 2 and Figure 8. For A356 foams with two rammings, where the porosity is approximately 55%, the average cell wall thickness increased to 0.26 mm, 1.01 mm, and 2.03 mm as the pore size increased from 0.7~1.0 mm to 1.0~2.0 mm, and 2.8~3.4 mm, respectively. Even with the same porosity level, the cell wall thickness varies significantly with pore size, increasing as the pore size increases. This occurs because, during the manufacturing process, even if the density of the space holder per unit volume remains the same, larger space holders create wider gaps, which are filled by the aluminum alloy melts, resulting in thicker cell walls. This trend is also observed in A356 foams with a porosity of approximately 62% (five rammings). As the pore size increases from 0.7~1.0 mm, 1.0~2.0 mm, to 2.8~3.4 mm, the average cell wall thickness significantly increases from 0.22 mm to 0.88 mm and 1.76 mm, respectively.

To confirm the effect of porosity on cell wall thickness for a constant pore size, the average cell wall thickness of A356 foams with a pore size of 2.8~3.4 mm were compared. As the porosity increased from 55.9% (two rammings) to 62.7% (five rammings), the average cell wall thickness decreased from 2.03 mm to 1.76 mm. This is because, despite using space holders of the same size, the volume of space holders per unit volume increases, resulting in a decrease in the volume of aluminum alloy melts filling the space. Consequently, the porosity increases, and the cell wall thickness decreases. This trend is consistently observed when using space holders of different sizes, as well.

In Section 3.1, it was observed that the replication casting process produced open-cell type aluminum foams with similar porosity levels under the same manufacturing conditions, regardless of the alloy composition. As shown in Table 2 and Figure 8, the average cell wall thickness also remains consistent across different alloy types under identical manufacturing conditions. This indicates that the replication casting process in this study can control the structural characteristics of open-cell type aluminum foams, including pore size, porosity, and cell wall thickness, through manufacturing process parameters.

3.3. Mechanical Properties: Compression Test

3.3.1. Effect of Pore Size

To analyze the effect of pore size on the mechanical properties of open-cell type aluminum foams, it is essential to eliminate the potential influence of other variables, such as porosity and alloy type. As shown in Figure 9, the compressive yield strength results were organized according to changes in pore size under identical porosity and alloy conditions. The graph indicates that for A356 open-cell type aluminum foams with 55% porosity, the compressive yield strength increases to 0.78 MPa, 1.03 MPa, and 1.26 MPa as the pore size decreases from 2.8~3.4 mm, 1.0~2.0 mm, to 0.7~1.0 mm. Similarly, for A383 foams with 55% porosity, the compressive yield strength increases from 0.95 MPa, 1.61 MPa, to 2.40 MPa as the pore size decreases from 2.8~3.4 mm, 1.0~2.0 mm, to 0.7~1.0 mm. This trend is also observed when the porosity is 62%, where the compressive yield strength increases for both alloys as the pore size decreases. This phenomenon can be explained in terms of the relative density and cell wall thickness of the open-cell type aluminum foams. In Section 3.2, the average cell wall thickness of the foams was calculated based on their structural conditions. Under identical porosity and relative density, it was found that the average cell wall thickness decreases as the pore size decreases. When the relative density is the same, the volume of aluminum alloy per unit volume is also the same, meaning that the volume of aluminum alloy forming the cell walls is equivalent. However, even if the volume of the cell wall material is the same, a smaller pore size results in a significant increase in the number of cell walls, as confirmed in Figure 4a,b. Under the same relative density of foams and assuming identical pore shapes, the number of cell walls per unit volume in foams can be mathematically represented.

$$\begin{gathered} n={\displaystyle \frac{1}{V_{cell}}} \\ {V}_{cell}={\displaystyle \frac{\pi }{6}}{d}^3 \\ N=\gamma n={\displaystyle \frac{6\gamma }{\pi d^3}} \end{gathered}$$

(3)

The number of cells $n$ per unit volume can be expressed as the reciprocal of the volume of a single cell. Assuming the cells are spherical and the number of faces per cell $\gamma$ is the same, the number of cell walls $N$ can be related to the pore size $d$ as shown in Equation (3). The total number of cell walls can vary depending on how many cells share each cell wall. However, assuming that the shape of the pores is consistent, the relationship between the total number of cell walls $N$ and the pore size $d$ remains unchanged. Therefore, under the same porosity (same relative density) condition, when the volume of the cell walls is the same, the number of cell walls increases as the pore size decreases. Numerous cell walls play a role in dispersing energy along multiple pathways when external impact is applied. This prevents the load from being concentrated and transmitted through a single pathway, thereby enhancing the overall stability of the structure [23]. This leads to more energy dispersion and absorption during the load transmission from external forces, thereby increasing the compressive yield strength. Research on the effect of the number of cell walls and cell wall thickness on compressive yield strength shows similar trends to many other studies [11,12,13,24]. However, most research proposes a reinforcement mechanism through changes in the relative density of the foam as the pore size varies. This study is distinct in that it analyzes the mechanical properties of open-cell type aluminum foams with varying pore sizes under the same porosity conditions.

3.3.2. Effect of Porosity

As shown in Figure 9a, for A356 open-cell type aluminum foams, the compressive yield strength of specimens with 55% porosity is higher than that of specimens with 62% porosity under the same pore size conditions. When the pore size is 0.7~1.0 mm, the compressive yield strength increases by approximately 31%, from 0.96 MPa to 1.26 MPa, as the porosity decreases from 62% to 55%. For a pore size of 1.0~2.0 mm, the compressive yield strength increases by 30%, from 0.79 MPa to 1.03 MPa, as the porosity decreases from 62% to 55%. In the case of 2.8~3.4 mm pore size, the compressive yield strength increases by about 28%, from 0.61 MPa to 0.78 MPa. This trend is similarly observed in A383 foams, as shown in Figure 9b, where both aluminum alloys exhibit a compressive yield strength increase of approximately 30% as porosity decreases from 62% to 55% under the same pore size conditions. As identified in Section 3.1, the stacking density of the space holder per unit volume decreases, and the porosity decreases when the number of ramming counts is lower. Additionally, as measured in Section 3.2, the average cell wall thickness increases by approximately 15~20% as the porosity decreases from 62% to 55% under the same pore size. Therefore, as the porosity decreases, the relative density and cell wall thickness of the foams increase, enabling them to withstand greater loads from external forces, thereby increasing the compressive yield strength.

3.3.3. Aluminum Alloy Type

Section 3.3.1 and Section 3.3.2 confirmed that the pore size and porosity of open-cell type aluminum foams significantly affect their mechanical properties. However, even with identical pore size and porosity, the mechanical properties ought to vary depending on the material of the foam. This is because the mechanical properties of open-cell type aluminum foams are influenced by the internal cell structure, while the characteristics of each individual cell wall are determined by the intrinsic properties of the material. As represented in Figure 9a,b, when the pore size is 0.7~1.0 mm and the porosity is 55%, the compressive yield strength of A383 foam is 2.40 MPa, whereas the compressive yield strength of A356 foam is lower at 1.26 MPa. This is also observed for pore sizes of 1.0~2.0 mm and 2.8~3.4 mm, where the compressive yield strength of A383 foams is higher than that of A356 foams. This trend is consistent in open-cell type aluminum foams with a porosity of 62% under the same pore size conditions. These results are attributed to the differences in the mechanical properties arising from the compositional variations of the aluminum alloys. Many researchers have conducted tensile tests on A356 and A383 alloys, and the reported ultimate tensile strength and elongation are shown in Figure 10 [25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43]. For aluminum alloys composed of various additive elements, the mechanical strength can vary depending on the casting process, heat treatment conditions, the content of the alloying elements, etc. However, as can be seen in the graph, the ultimate tensile strength of the A383 alloy is generally higher than that of the A356 alloy, while the elongation of the A383 alloy is lower compared to the A356 alloy. This aspect can influence the properties of open-cell type aluminum foams. Even with identical structural conditions, A383 open-cell type aluminum foams, which have higher mechanical strength in the materials, can withstand greater stress in the elastic region (i) during compression tests, as the internal cells bear the load from external forces. Thus, the compressive yield strength of open-cell type aluminum foams is influenced by both the structural characteristics of the foams and the inherent mechanical properties of the material.

3.3.4. Energy Absorption

In the plateau region (ii), the stress increases slowly or oscillates as the internal pores undergo repeated deformation or collapse during the compression test. The deformation behavior in this region can also vary depending on the structural conditions of the open-cell type aluminum foams. The energy absorption of open-cell type aluminum foams can be represented by the energy absorbed by the material up to a logarithmic strain of 0.4 (engineering strain of approximately 0.5), as shown in Equation (4). After the point of 0.4 logarithmic strain, the densification region (iii) is characterized by a rapid increase in stress as most of the pores within the foams have collapsed.

$$W={\int }_0^{{ϵ}_{T,40}}\sigma d\epsilon$$

(4)

$$\begin{gathered} W=Energy absorption \\ {ϵ}_{T,40}=Compressive true strain 40\% \\ \sigma =Compressive true stress \end{gathered}$$

The energy absorption of A356 and A383 open-cell type aluminum foams, calculated using Equation (4), is presented in Figure 11. The energy absorption W was calculated based on the logarithmic strain. For A356 open-cell type aluminum foams with 55% porosity, the energy absorption increased as the pore size decreased from 2.8~3.4 mm, 1.0~2.0 mm, to 0.7~1.0 mm, resulting in values of 0.65 MJ/m³, 1.07 MJ/m³, and 1.31 MJ/m³, respectively. This trend was also observed for a porosity of 62%, where the energy absorption increased from 0.61 MJ/m³ to 1.02 MJ/m³ and 1.26 MJ/m³ as the pore size decreased. In Section 3.3.1, it was confirmed that the compressive yield strength increased with decreasing pore size under equivalent porosity. This was attributed to the increased number of cell walls within the foams, which enhances energy dissipation. Consequently, in the plateau region (ii), where the deformation and collapse of pores occur after the elastic region (i), the amount of energy absorbed from external forces also increases. Therefore, under the same porosity conditions, the energy absorption up to a logarithmic strain of 0.4 increases as the pore size within the foams decreases. This trend is similarly observed in A383 foams.

Under the same pore size conditions, the compressive yield strength of the foams increases as the porosity decreases. In terms of energy absorption, as illustrated in Figure 11, for a pore size of 2.8~3.4 mm, the energy absorption of A356 foam with a porosity of 62% is 0.61 MJ/m³, whereas, with a porosity of 55%, the energy absorption increases to 0.65 MJ/m³. For a pore size of 1.0~2.0 mm, the energy absorption of A356 foam increases from 1.02 MJ/m³ at 62% porosity to 1.07 MJ/m³ at 55% porosity. Similarly, for a pore size of 0.7~1.0 mm, the energy absorption increases from 1.26 MJ/m³ at 62% porosity to 1.31 MJ/m³ at 55% porosity. These results indicate that, similar to the mechanism by which the compressive yield strength of open-cell type aluminum foams is enhanced by porosity conditions, the energy absorbed during the compression test increases as porosity decreases in foams with the same pore size. This is due to the higher relative density and increased cell wall thickness. This is also observed in A383 foams, showing that when the aluminum alloy composition is the same, the energy absorption of open-cell type aluminum foams is influenced by structural conditions. In other words, for open-cell type aluminum foams made from the same aluminum alloy, both the compressive yield strength and energy absorption increase as porosity and pore size decrease.

In contrast, when the aluminum alloy composition changes, the compressive yield strength and energy absorption can exhibit opposite trends due to differences in the stress–strain behavior in the plateau region (ii) during compression tests. As shown in Figure 9, the compressive yield strength of A383 foams is higher than that of A356 foams under the same structural conditions. However, as indicated in Figure 11, the energy absorption up to a logarithmic strain of 0.4 is lower for A383 foams compared to A356 foams. Previously, it was noted that for the same alloy composition, both the compressive yield strength and energy absorption increase with decreasing pore size and porosity. Conversely, when the aluminum alloy composition changes, a higher compressive yield strength does not necessarily correspond to higher energy absorption during the application of compressive loads. This result can be understood through the stress–strain curves (deformation behavior) for A356 and A383 open-cell type aluminum foams with a porosity of 55% and pore size of 0.7~1.0 mm, as shown in Figure 12. To clearly distinguish between the five specimens tested for compression under the same conditions, different colors were used to mark each specimen.

Under these conditions, the compressive yield strength of A383 foam is 2.40 MPa, while that of A356 foam is 1.26 MPa. Although the compressive yield strength of A383 foams is higher, their energy absorption is 1.02 MJ/m³, which is lower than 1.31 MJ/m³ of A356 foams. The energy absorption of open-cell type aluminum foams is represented by the area under the stress–strain curve up to a logarithmic strain of 0.4. In the stress–strain graph of A356 foams shown in Figure 12a, the stress in the plateau region (ii) gradually increases with strain. In contrast, the stress–strain graph of A383 foams in Figure 12b shows serrated stress–strain curves with stress oscillations in the plateau region (ii) as strain increases. These differences in compressive deformation behavior can be clearly observed through the images of specimen deformation during the compression test, as illustrated in Figure 13. In the case of the A356 foam specimen shown in Figure 13, the internal pores elastically sustain the structure as the strain increases, resulting in a continuous increase in stress in the plateau region (ii) of the stress–strain graph. Conversely, for the A383 foam specimen in Figure 13b, brittle fracture occurs as the internal pores collapse and fragments break off, leading to the serrated stress–strain curve. This deformation behavior is attributed to the differences in mechanical properties due to the alloy composition of A356 and A383 aluminum alloys. The higher mechanical strength of A383 aluminum alloy compared to A356 aluminum alloy provides an advantage in the compressive yield strength of open-cell type aluminum foams. However, as shown in Figure 10, the elongation of A383 aluminum alloy is significantly lower, approximately in the range of 1~3%, compared to the 5~10% of A356 aluminum alloy. This low ductility of A383 aluminum alloy hinders the elastic deformation of the cell walls under compressive load in A383 foams, causing brittle fractures and collapse with increasing strain. In contrast, A356 foams, with higher elongation, allow the cell walls to maintain elasticity under compressive load, resulting in a continuous increase in stress in the plateau region (ii) as strain increases. Therefore, although A383 open-cell type aluminum foam exhibits higher compressive yield strength, its lower energy absorption in the plateau region (ii), indicated by the serrated stress–strain curve, results from its low ductility.

3.3.5. Al-6Mg-(0, 2, 4, 6)Si Alloy Foams

The mechanical properties of open-cell type aluminum foams are influenced not only by the structural conditions of the foams but also by the alloy composition. To verify this, the mechanical properties of Al-6Mg aluminum alloys with varying Si additions (0, 2, 4, 6 wt.%) were compared, along with the mechanical properties of Al-6Mg-(0, 2, 4, 6)Si open-cell type aluminum foams.

Figure 14 shows the tensile test results for non-porous Al-6Mg-(0, 2, 4, 6)Si alloy bulk materials. The specimens were processed into rod form according to ASTM B557 small-size specifications and tested at a speed of 1mm/min using a universal testing machine (DTU-900MHN, Daekyung Tech Co., Inchen, Republic of Korea). The results indicate that the tensile yield strength of Al-6Mg alloy without Si content is approximately 90 MPa, whereas that of Al-6Mg-Si alloys is somewhat higher, ranging from 93 MPa to 96 MPa, but the difference is not significant. Additionally, the ultimate tensile strength of the alloys is also at a similar value. However, in terms of elongation, Al-6Mg aluminum alloy without Si exhibits a very high elongation of 15.5%, while the elongation drastically decreases to about 2.27% to 2.98% with the addition of Si. These results can be confirmed through the microstructure of Al-6Mg-(0, 2, 4, 6)Si alloys shown in Figure 15. Even a small amount of Si addition to Al-6Mg alloy results in the formation of the Mg₂Si phase, and as the Si content increases, the fraction of this phase also increases, leading to a sharp decrease in elongation. Based on these results, the compression test results of Al-6Mg-(0, 2, 4, 6)Si open-cell type aluminum foams were compared to understand how compositional changes affect the mechanical properties of the foams. In this study, the casting conditions (such as pouring temperature) applied to produce both the bulk materials and the open-cell type aluminum foams of Al-6Mg-(0, 2, 4, 6)Si alloys were almost identical. Neither the bulk materials nor the foams underwent any subsequent heat treatment processes.

The open-cell type aluminum foams made from Al-6Mg-(0, 2, 4, 6)Si aluminum alloys were manufactured using 1.0~2.0 mm space holders with two rammings. As confirmed in Section 3.1, when the manufacturing conditions are the same, the structural conditions of the foams are consistent, resulting in a measured porosity of approximately 55% for the manufactured foams, regardless of the alloy type. The test results are shown in Figure 16 and

Table 3. Porosity and compression test results of Al-6Mg open-cell type aluminum foams.

Alloy	Relative Density $\mathit{\epsilon }$	$\mathbf{Porosity} \mathit{\theta }$ (%)	Compressive Yield Strength (MPa)	Energy Absorption at 0.4 Logarithmic Strain (MJ/m3)
Al-6Mg	0.449 ± 0.005	55.1 ± 0.5	1.72 ± 0.05	1.75 ± 0.08
Al-6Mg-2Si	0.452 ± 0.005	54.8 ± 0.5	1.68 ± 0.08	0.83 ± 0.04
Al-6Mg-4Si	0.447 ± 0.004	55.3 ± 0.4	1.70 ± 0.07	0.83 ± 0.06
Al-6Mg-6Si	0.451 ± 0.006	54.9 ± 0.6	1.66 ± 0.04	0.75 ± 0.05

.

The compression yield strength of each open-cell type aluminum foam, measured using the 0.2% offset method, was 1.66 MPa to 1.72 MPa. The tensile yield strength of the bulk materials without pores showed almost no difference, indicating that the compression yield strength of the foams also exhibited similar levels. However, significant differences in deformation behavior were observed in the plateau region (ii) of the compression test results of Al-6Mg-(0, 2, 4, 6)Si foams. The tensile test results of the bulk materials without pores showed that the Al-6Mg aluminum alloy without Si content had a high elongation of about 15.5%, resulting in a continuous increase in stress with increasing strain in the plateau region (ii) of the stress–strain curve during the compression test of the corresponding open-cell type aluminum foam. Conversely, the foams made from Al-6Mg-(2, 4, 6)Si aluminum alloys, which have a low elongation of 2.27% to 2.98%, exhibit serrated stress–strain curves in the plateau region (ii), with stress increasing and decreasing repeatedly as strain increases. This compressive deformation behavior can be visually distinguished through the specimen images corresponding to different strains during the compression test, as shown in Figure 17. Figure 17a shows images of Al-6Mg open-cell type aluminum foam, indicating that while some deformation of the internal cells occurs as the crosshead descends, the high elongation allows the structure to elastically maintain its shape. In contrast, Figure 17b represents that Al-6Mg-4Si open-cell type aluminum foam with a low elongation of 2.68% shows relatively easy brittle fracture of the internal cells. Due to this difference in fracture behavior, even with the same structural conditions, the trends in stress–strain curves vary according to the alloy composition, as shown in Figure 16a. This variation significantly affects the absorbed energy value, which is represented in the lower region of the stress–strain graph. Therefore, as shown in Figure 16b, as the strain increases, the absorbed energy of Al-6Mg open-cell type aluminum foam is higher than that of Al-6Mg-Si foams. The energy absorption up to 0.4 logarithmic strain for Al-6Mg open-cell type aluminum foam is 1.75 MJ/m³, which is significantly higher than 0.75~0.83 MJ/m³ for Al-6Mg-Si foams. It was confirmed that the deformation behavior of the internal cells in the open-cell type aluminum foams varies significantly depending on the elongation of the base aluminum alloy, with high elongation leading to a substantial increase in energy absorption.

In the case of the Al-6Mg-Si alloy used in this study, which is a ternary aluminum alloy, relatively straightforward comparisons are possible. However, when the variety of alloying elements in aluminum alloys increases, various strengthening mechanisms can introduce additional factors that influence the mechanical properties during the manufacturing process, such as pouring temperature, cooling rate, heat treatment, etc.

4. Conclusions

This study evaluated the mechanical properties of open-cell type aluminum foams by manufacturing foams with varying porosities, pore sizes, and alloy compositions through the replication casting process.

• Pore size

Under the same aluminum alloy conditions, when the porosity is identical, as the pore size decreases, the number of cell walls increases, resulting in enhanced energy dispersion and absorption when external forces are applied. This leads to an increase in compressive yield strength and energy absorption.

• Porosity

When the pore size is constant, a decrease in porosity leads to an increase in cell wall thickness, which also results in higher compressive yield strength and energy absorption.

• Alloy composition

As confirmed by the comparison of the mechanical properties of A356, A383, and Al-6Mg-(0, 2, 4, 6)Si open-cell type aluminum foams, the mechanical properties of the foams are influenced by the intrinsic properties of the materials, even with the structural conditions are identical. Specifically, the deformation behavior of pores greatly affects the energy absorption values. Alloys with low ductility exhibit lower energy absorption due to the serrated stress–strain curve in the plateau region (ii).

Both structural conditions and alloy composition significantly affect the mechanical properties of open-cell type aluminum foams. However, due to limitations in the pore sizes or porosities achievable through the replication casting process, it is challenging to determine the weighted influence of each factor. Therefore, we are considering improvements to the manufacturing process to produce foams with a broader range of structural conditions and to quantitatively analyze the effect of each factor on the mechanical properties.