The anisotropy behavior of metallic foams under Charpy impact tests

Currently, the automotive industry is looking for their new products to have a density as low as possible so that CO2 emissions decrease. Metallic foams have attracted a great deal of interest in this industry because of their multiple advantages. They can be produced at a relatively low cost and have advantageous properties, especially due to their ability to absorb energy. In the framework of this study, 42 specimens were tested to determine the impact energy and Charpy impact strength according to the cutting orientation. Before being notched according to the ISO 148 standard, their specific mass was determined. For the tests, an Instron CEAST 9050 Charpy test machine was used. The impact strength was determined according to the cutting orientation, and the results obtained by the mass density groups were compared. It could be observed that the cutting orientation of the specimens does not have a clear influence on the impact strength; this is due to the irregular shape of the closed cell, however, the grouping of the specimens on specific density samples has influenced the impact strength.


Introduction
In the last two decades, cellular materials have experienced a continuous spread in all top industries.Whether it is materials with a metallic [1,2], polymeric [3,4], or ceramic [5] matrix, this use is due to the high characteristics they develop.In this sense, among the most significant properties of cellular materials are low weight, high strength, stiffness, high-energy absorption capacities, good thermal and sound insulation, as well as good vibration damping [6][7][8].However, among all cellular materials, metallic foams (MFs) present the best properties.Also, due to continuous development, MFs are divided into several subcategories, namely conventional, composite, syntactic, or hybrid MFs [9][10][11][12].
Due to the simplicity of the production process, the most widespread are conventional metallic foams especially those based on aluminum alloys [13][14][15].Over the years, various teams of researchers have studied the mechanical behavior of MFs.Thus, the static properties in compression, bending, and sometimes tensile and shear were investigated in detail [16][17][18].In addition, due to the difficulty of the experimental setup, fewer reports of the properties are found in the impact field (low and high-speed tests) [19][20][21].Although they are among the most studied MFs, aluminum foams still face uncertainties regarding impact behavior.
Metal foam is characterized structurally by its cell topology, relative density, cell size, cell shape and anisotropy [22].The production process of the foams can influence their mechanical properties because the cells are not perfectly spherical and uniformly distributed; they are elongated and can influence the anisotropy of the structure.Likewise, the metallic foams with a skin layer show anisotropy due its skin on the X, Y, and Z direction in the compression tests [23].
The aim of the work is to determine the impact of the anisotropy of metallic foams on the density, impact energy and Charpy impact strength.In this sense, specimens cut in-plane at 7 different angles (range 0-90, in steps of 15) were tested and analyzed.The results will give a broader picture of mechanical impact properties in close relation to the cut orientation, and density aspects.

Materials and methods
Powder metallurgically prepared foaming precursor with composition AlMg1Si0.5 + 0.4 wt.% Ti (porosity below 3%) was used for the preparation of aluminum foam plates of size 296 mm × 296 mm × 7 mm.Foams were prepared from pieces cut of precursor of size 20 mm × 20 mm × 5 mm placed inside of a mold in such a way that they almost completely fill the mold to avoid the anisotropy of the foam properties on the X-Y direction.The mold was placed in the electric furnace, heated to 680 ℃ and held at the temperature for 10 minutes to create the foam.Afterward, the foam in the mold was cooled down to room temperature to freeze the foam structure.
In order to obtain the specimens, the plate was cut using a waterjet cutting machine Streamline SLV OEM 30HP.To investigate the anisotropy of metallic foams on the Charpy impact test, the specimens were cut in various orientations: 0°, 15°, 30°, 45°, 60°, 75°, and 90°, Figure 1a.Following the water jet cutting, parallelepiped specimens resulted.
Afterwards, the specimens were rectified in order to obtain a width of 10 mm, and finally, they were notched with the help of a milling cutter.Each parallelepiped specimen was weighed using the KERN scale, and finally, the density of the specimens was determined by taking into account the orientation of cutting from the plate.
Figure 2 shows the resulting densities for each orientation.The highest densities were recorded for the specimens oriented at 30 degrees, but the dispersion of the results is also greater.The lowest densities were recorded in the case of specimens cut at 90 degrees.Recorded results for the set of specimens cut at 60 degrees showed the smallest deviations from the average in terms of density.During the foaming process, a non-uniform foam microstructure is obtained with cells elongated in a certain direction.Therefore, for example, at 30 the specimens are cut perpendicular to the elongated pores, while with the increase of the cutting angle, the specimens tend to be cut along the elongated pores.Of course, the presence of pores (gaps in the material), smaller (samples cut at 30) or larger (samples cut at 90), results in a change in the density of the specimens.This is the reason for obtaining maximum densities for specimens cut at 30, respectively minimum for those at 90.

Figure 2. Density of the specimens taking into account the orientation of cutting
For the experimental study, 42 metallic foam specimens were tested using the CEAST 9050 Pendulum Impact System.Charpy impact test specimens have dimensions of 55 mm × 10 mm × 7.3 mm and have a V-notch milled on one face according to the ISO 148 standard [24].The angle of the notch is 45 and the radius of curvature at the base notch is 0.25 mm.The distance between the top of the notch and the end of the specimen is 8 mm.
The equipment consists of a 229.7 mm long impact pendulum combined with a 1.186 kg hammer at the end of the striker, possessed of a potential energy of 5 J and an impact speed of 2.9 m/s.The force signal measurements were forwarded to the computer software equipment through a data acquisition system connected to the hammer.The pendulum is raised to a defined height and released to fall.The difference between the initial and final height of the pendulum is directly proportional to the amount of energy lost as a result of the fracturing of the specimen [25][26].
The equipment was automatically calibrated and the specimens were placed horizontally on the supports, which have a span of 40 mm [27].
The Charpy impact strength of notched test specimens acN, in kilojoules per square meter kJ/m 2 , was calculated using Eq.(1).
where: WB is the energy at the break in J, h is the thickness of the specimen in mm and bN is the width of the specimen in mm.

Results and Discussion
The experimental investigation was carried out to observe if an anisotropy behavior of the metallic foams on the Charpy impact test takes into account the specimens cutting orientations.Figure 3 shows the specimens corresponding to each orientation, following the Charpy impact test.Most of the specimens were broken after the tests, and for a small part the hinge effect was observed.Following the tests carried out, CEAST 9050 Pendulum Impact System allows the visualization of different graphs that increase the understanding of the behavior of the metallic foams.
The results recorded for each set of specimens are presented below.The main charts presented are the impact force vs. deflection, and the impact energy vs. deflection of all specimens group.Each chart contains six curves, for all individual specimens from one cutting orientation group.
The value of the maximum impact force reaches 125 N in the case of a specimen cut at 45 degrees, but at the same time, specimens with closed cells with a larger air volume were encountered, which did not exceed 60 N in the case of the peak impact force (for one each cut specimen at 30, 60 and 75 degrees).At the same time, the energy absorbed by the specimen during the tests varied a lot from 0.1 J to 0.53 J. Regardless of the cutting orientation of the specimens, the results in terms of impact energy and impact force are dispersed, this is due to the inhomogeneous structure of the metal foams.
Likewise, when we look at all the impact force-deflection curves, a quasi-brittle behavior for all the specimens can be observed.Due to the scattering of the results, the Charpy impact strength was determined taking into account the global cross-section of the specimens.The results calculated for each set of specimens are presented in Table 1.Considering that the results did not show a clear influence on the cutting orientation of the specimens, the results being very scattered, grouping the specimens into density classes was considered.This grouping was made arbitrary in order to observe the influence of density on the impact properties.Most of the metallic foam specimens exhibit a density between 0.7 and 0.799 g/cm 3 .However, some specimens showed larger cells and a lower density (0.7 g/cm 3 ), while others with smaller cells showed a higher density (0.8 g/cm 3 ).This aspect can be observed very easily in Figure 2, especially for the upper limit of the error bars presented for cut orientations at 15 and 30.
Figure 11 shows the tendency to increase the Charpy Impact Strength with increasing specimen density: from 2.94 kJ/m 2 (class I), to 3.5 kJ/m 2 (class II), respectively to 4.26 kJ/m 2 (class III).

Conclusions
In this paper, the Charpy impact behavior of metallic foams was investigated.The impact strength was determined according to the cutting orientation and was found to be in the range 1.935 to 4.067 kJ/m 2 , showing a high dispersion of the results, regardless of the cutting orientation.Likewise, the results obtained by the mass density groups were compared.All the specimens experience quasi-brittle behavior.
The cutting orientation of the specimens does not have a clear influence on the impact strength; this is due to the irregular shape of the closed cell, however, the grouping of the specimens on specific density samples has influenced the impact strength, showing that the increasing of the density leads to a uniform distribution of the cells with spherical shapes, so better impact properties; while the decreasing of the density in our case tends to elongate large cells with lower values of the impact properties.

Figure 1 .
Figure 1.a) Aluminum foam plate with chessboard structure, and b) a detailed view of the specimens

Figure 3 .
Figure 3.The specimens after the impact test

Figure 11 .
Figure 11.Charpy impact strength according to the class of density

Table 1 .
Charpy impact strength according to the cutting orientation of the specimens