T HE E FFECT OF Al 2 O 3 AND S TIRRING T IME ON D ENSITY AND P OROSITY OF A LUMINUM ADC12 FOAM

The instability of the foam forming during metallic foam manufacture commonly occurs, which will cause undesirable pores. The stability of the foam structure is one of the important factors. A stabilizer can maintain the foam cell during the melting process. In this study, the metal used is ADC12 with a 12 wt.% of Si element content


INTRODUCTION
The popularity of aluminum foam nowadays has increased.It can be investigated easily by the number of scientific articles that discuss it [1].Metal foams are known for their unique combinations of physical and mechanical properties, such as lightweight, higher specific strength & stiffness, improved elevated temperature strength, and excellent energy absorption capacity at very low plateau stress.The implementation of aluminum foam in the automotive and aerospace fields is unquestionable.Aluminum foam can be used as crash energy or vibration-absorbing material [2].Because of its lightweight material, aluminum foam can reduce the weight of vehicles.Massreduction vehicles achieve significant weight loss, reduce greenhouse gas emissions, and save intake fuel [3].All of these are the advantages of using metal foam.
Different methods have been developed to produce foams, which can be divided into two categories: direct foaming by introducing gas bubbles into a conditioned melt [4] and foaming with the help of blowing agents [5].Metal foam making using the molten metal can be called the direct method because liquid metal can form foam by directly injecting gas or gas from blowing agents [6].Gas that appears in molten metal will form gas bubbles and be dispersed in the liquid.Generally, gas bubbles formed in molten metal tend to immediately rise to the surface due to the high buoyancy forces in the high-density melt.However, this upward movement will be hampered by increasing the viscosity of the liquid metal.This condition can be conducted by adding fine ceramic powders or alloying elements to form stable melt particles [7].
Using foaming agent CaCO 3 can produce foam under optimum conditions, resulting in a density of 0.9 g/cm 3 with a rounded pore shape.[8].Too high a stirring speed, up to 2000 rpm, causes the porosity to increase, but the pore size decreases.The use of CaCO3 is considered very efficient and effective and thus can replace the role of conventional TiH2 foaming agents [9].The effect of porosity percentage is very significant on the compressive strength of the aluminum foam.The value of the porosity is higher; the foam strength will be low [10].
In a previous study, aluminum in the liquid will react to form aluminum oxide (Al2O3) and then continue with calcium carbide to become calcium-alumina [11].It will increase the viscosity of the melt so that the bubbles that form do not quickly come out to the surface.In this study, the formed aluminum oxide slag was cleaned, so how effective the addition of Al2O3 in forming porosity in the foam metal.
So, this study aims to understand the effect of additional alumina and stirring time on the density and porosity of foam aluminum, pore shape and size, and the strength of the aluminum foam as a foaming agent using calcium carbonate (CaCO3).

MATERIALS AND METHODS
This experiment uses base metal aluminum ADC12 and foaming agent calcium carbonate, as much as 300-gram dan 2 wt.%.The liquid metal stabilizer compound uses alumina (Al2O3) with a composition variation of 1, 2 and 3 wt%.All materials melt in an electric furnace.Each specimen was coded based on the stirring time and stabilizer composition, as described in Table 1.S denotes the sample, the first two or three numbers indicate the stirring time, and the last digit indicates the alumina composition.For example, if the specification is S1203, the stirring is carried out for 120 minutes with a 3 wt.%alumina composition.
First, the aluminum ingot was cut into smaller sizes and then put into an electric furnace until around 800 °C.After the metal has melted, the slag on the surface of the liquid is cleaned and then mixed with alumina and stirred with stirring variations of 60, 120, and 180 seconds with a constant rotational speed of 1200 rpm.The stirring process at a set time aims to obtain an evenly distributed alumina in the melt, then the foaming agent CaCO3 is added, stirred for 60 seconds, and held for 3 minutes.
At a temperature of 800 °C, CaCO3 has decomposed into CaO and CO2.CO2 gas will be trapped in melted molten aluminum and form bubbles in large quantities and will eventually be referred to as foam.The alumina slag that comes from the reaction of melted aluminum with the surrounding air is removed from the melt surface.It aims to observe the effect of the added alumina more clearly.It is different from what was done by previous researchers who used alumina that came from the reaction between the melt and air [11].Then, the same process was carried out for other research variations of the alumina composition.
Density and porosity can be calculated using Archimedes' principle [12].Density (ρsample) and porosity tests were carried out to determine the percent pore level formed.In the density test, the sample is weighed to determine the dry sample weight (Mk).Then, the sample is immersed in a container filled with water for 1 hour and weighed to determine the wet sample weight (Mb).Then, do the calculations using equation ( 1  The next step is to observe the pore morphology of foam aluminum.This observation includes the pore size and pore area fraction.This step is supported by the ImageJ application to obtain the values.The pore size is determined by measuring the pores' length and width and then averaged.

RESULT AND DISCUSSION
After adding the foaming agent and stabilizer, the dimension of the casting is higher than before, which shows that the pore is formed in the melt during solidification.It is possible to see the differences between the sample before and after foaming after cutting it in the transversal direction.The sample has no apparent porous structure without adding the foaming agent and stabilizer (Fig. 1).
The effect of the foaming process is that the casting colour is darker than the casting without the process.After the tranverse cut, the surface have porous (Fig. 2).The composition of Al2O3 influences the morphology, density, porosity and, compressive strength of the aluminum foam.

Morphology
Figure 3 shows that all samples form pores.The change in the morphology of this precursor material into foam was due to the addition of partition materials that have their respective purposes, namely CaCO3 as a foaming agent and Al2O3 as a foam stabilizer.
From the data shown in Table 1, it can be observed that there are differences in the size and fraction of the pore area in the sample cut layers.The S1201 specimen has the highest average pore size and the lowest pore area fraction.The sample with a longer stirring time of 180 seconds will produce a smaller pore size than others, with a larger pore area fraction.These differences will affect the percentages of porosity, density values, compressive strength, and energy present in each sample.The pore shape can be seen in Fig. 4. The alumina composition affects the pore wall thickness.The specimens with the same stirring time, namely 120 seconds for 2 wt% by weight (S1202) alumina, had thinner walls and more irregular foam shapes than samples with 1 wt.% of alumina composition.

Density and Porosity
Figure 5 shows the effect of alumina content on the density and porosity of aluminum foam.The blue curve shows 60 seconds of stirring, red for 120 seconds, and green for 180 seconds of stirring.The highest porosity was obtained in 2 wt.% of the alumina composition with a stirring time 120 seconds while adding 3 wt.% of alumina at stirring time 120 seconds which results in lower porosity than the addition of 2 wt.% of alumina.This exhibits that the additives added to the aluminum melt have yet to shown that increasing the levels of additives that increase porosity and reduce density.Different things are shown in the effect of increasing the stirring time.In Figure 6, the blue, red, and green lines indicate the alumina composition of 1, 2, and 3 wt.%consecutively.The curves show that increasing the stirring time can increase the porosity and decrease the density.It is due to the effect of the stirring process that can help distribute the foaming agents and additives more evenly in the aluminum liquid.In the 3 wt.%alumina composition, from the results of successive stirring for 60, 120, and 180 seconds, the density values obtained were 1.039, 0.954, and 0.928 gr/cm 3 consecutively, while for porosity were 61.68%, 64.82%, and 66.15%.According to the graphic trend, the same direction can be seen in other alumina compositions.

Compresive Strength of Aluminum Foam
The effect of adding Al2O3 on the compressive strength produced from all aluminum foam specimens can be seen in the figure presented in Fig. 7.The change of the compressive strength values for each sample as the effect of the Al2O3 content increases.The compressive strength values obtained from all samples increased with each difference in stirring time.During the 60second mixing time marked with the orange line, there was an increase and decrease in the compressive strength from 1.17 MPa to 0.90 Mpa, and then it increased again to 1.07 MPa.A significant difference in compressive strength values was obtained for a specimen with a stirring time of 120 seconds marked with a red line.The difference in the compressive strength value in sample 4 was 7.82 MPa, and it decreased very significantly to 0.76 MPa then experienced a significant increase again to 7.58 Mpa.
Figures 8 and 9 show the difference in the pressure test curve lines.Those lines tend to be smooth and go up and down.This indicates that there are differences in the mechanical properties of the samples themselves and that they can be classified into ductile or brittle samples.In the pressure test results, the specimen with a stirring time of 120 seconds had the highest pressure test value compared to other conditions, around 70.73 joules.Meanwhile, the non-smooth curve, as shown in the S1202 sample, includes samples with brittle properties, and you can also see that the energy value is the lowest compared to the other samples, namely 9.58 joules.The shape of the curve that goes up and down is because when pressing is done, the pore walls are immediately destroyed, and consequently, the pressure value at a certain point on the curve decreases.This is also described in previous research [14].Representative stress-strain curve from sample S1202 during compression testing (Fig. 8).The first zone is the linear elastic at strain values up to 3% or 4%.In this first zone, the pore shape has not changed permanently.The second zone is the plateau zone, which continues with strain values up to around 6-70%.Plateau is a state of little or no change following a period of activity or progress, but in this case, the stress continues to move up even though it is slower than the first zone.As the pressure increases in this zone, the pores begin to collapse and continue to collapse until the pores are flatter and have the properties of solid materials.The third zone is the densification zone, with a strain value of around 70% and above.In this zone, the pore wall and the opposite wall close together until they touch each other, and the pore completely collapses, indicated by a significantly increased stress value.
) after knowing the value of the hanging mass (Wt), the sample mass in water (Wb), and the specific gravity of water (ρwater) : …………(1) Porosity percentage can calculate using this equation follow, ....(2)Compressive strength is observed to obtain the ability of metal foam to accept loads.Compressive testing of aluminum foam uses a universal testing machine Shimadzu AG-X Plus.The 10 KN of load and 2.5 mm/minute of loading speed.The dimension of the compressive test sample is 20x20x30 mm according to the standard.Mechanical properties of commercial aluminum metal foam according to reference in the 0.04-14 MPa[13].Compressive strength was measured by going through a compressive test with a test speed of 2.5 mm/min at a temperature of 21.2 °C with a humidity of 52.9% RH.

Figure 2 .
Figure 2. (a) Specimen as cast before cutting, (b) aluminum with adition foaming agent

Figure 4 .
Figure 4.The differences in the pore wall thickness at stirring time 120 seconds with different compositions of alumina (wt.%)(a) 1, and (b) 2

Figure 5 .
Figure 5.The effect of alumina composition on (a) the density, and (b) the porosity value

Figure 6 .
Figure 6.The effect of stirring time on (a) the density and (b) the porosity value

Figure 7 .
Figure 7.The effect of addition Al2O3 on compressive strength of aluminum foam

Table 1 . Pores size and fraction in every specimen No Specimens Pore Size average (mm)
Note : *S is specimen, 60 is stirring time, 1 is stabilizer content