Effect of Calcium Addition on the Aluminum Alloy A201 Foam

To study the effect of calcium on the porosity, sphericity, and microstructure of Al–Cu foams, a gradient calcium content (0.5, 1.0, 1.5, and 2.0 wt%) is added into aluminum A201 alloy (Al–Cu alloy) for the cellular aluminum foam preparation via the melt foaming method (TiH2 is used as a bubbling agent). Thereafter, the porosity of the prepared Al foams and their pore size are found to increase, and the sphericity of pores tends to approach to unit with calcium addition. This phenomenon is then explained through the modified thermophysical properties (i.e., surface tension and viscosity) of A201 alloy melts caused by the calcium addition. After the microstructure analysis, a bunch of secondary phases in the A201 + Ca alloy foams, e.g., CalAl2Si2, Al2Cu, and Al20CaTi2 as well as an eutectic phase with a great copper content Al8CaCu4 are detected by means of scanning electron microscopy/energy‐dispersive X‐Ray spectroscopy and X‐Ray diffraction analysis.


Experimental Section 2.1. Aluminum Foam Preparation
The commercial aluminum A201 alloy (the chemical composition is shown in Table 1) was used for the Al foam preparation by the melt foaming method.The aluminum ingot (850 g) was put into a steel crucible and melted by heating it to 800 °C in a resistance furnace.Then calcium pellets (4.3, 8.5, 13, and 17 g; purity of calcium is 99.9%) were added into the melt and stirred at 300 rpm for 10 min to ensure its full mixture.Further, the molten sample was cooled down to 675 °C, and bubbling agent TiH 2 in the form of a pellet (constantly 13 g, preheated at 400 °C for 1 h) was added into the prefabricated melt followed by an immediate stop of stirring after 90 s.To ensure a good decomposition of the bubbling agent TiH 2 , the rapid stirring with a rotation speed of 1000 rpm was settled.Then, the steel crucible was withdrawn from the furnace for water cooling.The in-detail description of the foam preparation setup was reported elsewhere. [15,16]o analyze the prepared sample, samples with Φ15 mm Â 15mm were taken off through the wire cutting and cleaned with kerosene, and dried.2D CT scan (60 kV, 134 μA, and 50 μm, Hiscan XM Micro X-ray computed tomography) was used for the characterization of the pore structure.The commercial software Avizo as well as ImageJ were used to analyze the structural parameters of the pores.Avizo was used to obtain the pore structure parameters, where the mean values of porosity, relative pore size, and solid material cell thickness were determined.The CT images were randomly cut and reconstructed into 3D structures.The calculation of the sphericity is defined based on the results of the CT.For a pore of volume V and surface area S, the shape factor [17] is defined as Equation (1).The distribution of the variation of the sphericity of the sample with the relative diameter was counted in a randomly cut small piece in the sample (200 pixels Â 200 pixels Â 200 pixels) (70 μm per pixel).

Shape factor
The phases in the obtained samples were analyzed via X-Ray diffraction analysis (XRD) (D8-Discover).The parameters were standard CuKα radiation (λ = 1.54 Å), a 2θ-range from 7.5°to 90°with a scan step of 0.013°and a holding time of 30 s per step.To study the microstructure of Al foam, the Al foam samples were embedded into epoxy and perpendicularly cut and polished via noncrystallizing colloidal silica polishing suspension.Scanning electron microscopy in combination with energydispersive X-Ray spectroscopy (EDX) was used for morphology and chemical composition analyses (Ultra55, Zeiss NTS GmbH).

Pore Structure of A201 Foam
The various sizes and shapes of pores in the prepared A201 þ Ca foams are observed (see Figure 1).The size and shape of the pore are mostly related to the Al foam preparation (molten state).During the molten state, foam is considered to consist of a dispersion of gas bubbles, therefore, the basic parameters (size and shape) of the bubbles are correlated with the thermophysical properties of the liquid.For instance, the thermophysical property of surface tension directly determines the diameter of the bubble. [3,18]In the current investigation, the calcium addition is one of the main factors that changes the thermophysical properties of the molten Al alloy.
The porosity of the investigated aluminum A201 þ Ca foams gradually increased from 70.1 % with the addition of calcium in an amount of 0.5 wt% to 87.7% at 2.0 wt% Ca, wherein a more pronounced increase in porosity (11.1%) occurred with 0.5-1.0wt% Ca (see Table 2).These results contradict those obtained by Song et al. [13,19] for pure Al+Ca foams.They [13,19] reported a change in the porosity trend with a maximum of 75% with 0.75 wt% Ca.Song et al. [13] explained that the increase in viscosity due to the addition of calcium, on the one hand, strengthens the liquid film of bubbles, and reduces the rate of its thinning and rupture.On the other hand, when the viscosity is too high, the bubbles would undergo difficulty releasing from the bubbling agent.Hence, the increased viscosity stabilizes the bubbles.But in fact, the foam structure could be influenced by many factors, such as bubbling agent, temperature, cooling velocity, foaming time, and the composition of the alloy.Furthermore, the average pore size also shows an increasing trend with calcium addition, as shown in Table 2.
The most influential factor in porosity is bubble diameter.According to Equation (2), the bubble diameter is related to   the thermophysical properties (surface tension and density). [20]o, the addition of calcium could directly change the morphology (including size and sphericity) of bubbles by changing the surface tension and density of the molten Al alloys, i.e., in the solid state, porosity, size, and sphericity.Hur et al. [21] stated that the addition of calcium reduces the surface tension of liquid aluminum, while it increases the viscosity of liquid aluminum.It is worth to mention that, according to the study of Hur et al. the surface tension of molten Al is strongly decreased with 0.5 wt% Ca addition.While the further Ca addition from 0.5 to 2 wt% only slightly decreased the surface tension.It may indicate that the bubble size variation was less affected by surface tension in the Ca addition range of 0.5-2 wt%.
where d B is bubble diameter (cm), σ is surface tension of liquid (mN m À1 ), d n is nozzle diameter (cm), g is the acceleration of gravity (m s À2 ), and ρ L is the density of liquid (g m À3 ).Except for the surface tension and density, in the present Al foam preparation process, the melt foaming method with the addition of TiH 2 as a bubbling agent was applied.Since the density of TiH 2 (3.75 g cm À3 ) is greater than molten aluminum, so the bubble-releasing process would conduct at the bottom of the molten alloy.Consequently, when bubbles are released from the TiH 2 agent, they will immediately flow up.This floating process or its velocity is strongly related to the dynamic viscosity of the molten Al alloys, as depicted in Equation ( 3).And the velocity also plays a crucial role in the coalescence of bubbles.Hence, the size of the bubbles or porosity depends on the surface tension, density, as well as viscosity.As already mentioned above, the surface tension was not much changed in the Ca addition range of 0.5-2 wt% and the small addition of calcium has not significantly changed the density of A201 alloy.Hence, the viscosity can be the most essential factor for the size of the bubble where V is the velocity of bubble in liquid (m s À1 ) and μ L is dynamic viscosity of liquid (mPa s).The porosity is found to increase by the addition of calcium from 0.5 to 2.0 wt%, this result is slightly different from the study of Song et al. [13,19] As shown in Equation ( 2), when the surface tension is decreased with calcium addition, the diameter of the bubbles should also consequently decrease.While based on the study of refs.[22-25], the diameter of the bubble can be increased with the higher viscosity.Yuan et al. [25] also reported a slight bubble size rising with the increasing viscosity of the liquid in a small viscosity range of 20-50 mPa s.Kovtas et al. [27] investigated the influence of the viscosity and surface tension on the bubble's dynamics.They found that the bubble size distribution became bigger and narrower with the increasing viscosity.Based on the literature survey, we can conclude that the viscosity has a positive effect on the bubble size, and the present experimental obtained results showed a good consistency with the previously reported results.The increased porosity and pore size with the addition of calcium is considered as caused by the effect of the increased viscosity over-compensated the effect of the decreased surface tension on the bubble size.
Except for the abovementioned parameters, the distribution of sphericity was also found to correlate with Ca addition.Figure 2 presents the sphericity of bubble as a function of equivalent diameter.The sphericity of Al foam after 0.5 and 2.0 wt% Ca addition is in the range of 0.5-1.0(1.0 indicates the perfect sphere).The value of sphericity is improved through the Ca addition.
The deformation of bubbles in the molten liquid can be ascribed to the surface pressure distribution and surface tension.E. Loth [28] summarized the relationship between the bubble shape and Weber number, as presented in Equation ( 4).
Weber number is a number that could be affected by the surface tension, density, and bubble velocity.And the velocity of the bubble is correlated to the viscosity.Therefore, the shape of the bubble depends on the thermophysical properties of the liquid where ρ f continuous-phase density (g cm À3 ); w is bubble velocity (m s À1 ); and d is diameter of bubble (m).
The spherical bubbles could exist only when the Weber number is much less than unity.On the other hand, when the Weber number is close, equal, or much greater than unity, the bubble shape can be unsymmetrical including a spherical cap with a steady wake, a spherical cap with an unsteady wake, and an oblate ellipsoidal cap (aft end maybe dimpled). [28]The decreased velocity of the bubble led to a smaller Weber number, consequently, it resulted in a higher sphericity.Kovtas et al. [27] also observed that with increasing viscosity and decreasing surface tension, bubbles became rounder.

Microstructure of Al201 Foams with Various Ca Content
The microstructures showing secondary phases formed by various contents of Ca in aluminum foams are presented in Figure 3. Regardless of porosity, the microstructures are coarse-grained and rather homogeneous.They consist of α-Al solid solution dendrites and precipitates of intermetallic phases of different morphology ("Chinese script" eutectics, spheroids, plates, etc.) distributed in interdendritic regions or dispersed in the α-Al matrix.Spot EDX measurements of the different precipitates were performed at different locations on the cross-sections.Even though the precipitates are rather small, and their surroundings can influence the chemical composition, the EDX results for the corresponding precipitates are rather equable.Thus, for simplicity, the revealed phase compositions are assigned to the phases considered below.For example, the bright-contrast plate-like precipitates (in Figure 3) contain 71.34 at% Al and 28.66 at% Cu, which is likely to be Al 2 Cu, or the bright-contrast eutectic-like precipitates (in Figure 3d,f,h) contain 2.5 at% Ca and 8.0 at% Cu, which is likely to be Al 8 CaCu 4 .
The main alloying element of A201 aluminum alloy-Cupartially dissolved in the α-Al solid solution forms an intermetallic phase with aluminum, θ-Al 2 Cu, which, at a low Ca content, precipitates in the form of thin plates (see Figure 3b) and tends to enlarge at a higher Ca content (see Figure 3f ).Considering the Ca addition, other phases to form are Al 4 Ca, Al 2 CaSi 2 , and Al 8 CaCu 4 .It is worth noting that Al 4 Ca was not detected by the microstructure investigation, since most of either Ca or Al 4 Ca could be spent on the formation of ternary phases, e.g., Al 2 CaSi 2 and Al 20 CaTi 2 as discussed below.Due to the presence of Si in A201 aluminum alloy, the Ca addition leads to the formation of either Al 4 Ca or Al 2 CaSi 2 or both, depending on the Ca content (as shown in Figure 3b,d,f,h).That can also be justified by the phase diagram of the Al-Ca-Si system. [29]At a given Si content and about 0.5 wt% Ca, the two phases in equilibrium are α-Al and Al 2 CaSi 2 (see Figure 3b), while at a higher Ca content, the alloy compositions move into the three-phase area of α-Al, Al 4 Ca, and Al 2 CaSi 2 (see Figure 3d,f,h).The amount of Al 2 CaSi 2 consequentially increases with increasing Ca content.
To discuss the Al 8 CaCu 4 phase formation one can refer to the phase diagram of the Al-Ca-Cu system. [30]In the range of 0.5-2.0wt% Ca, the three phases in equilibrium are α-Al, θ-Al 2 Cu, and eutectic Al 8 CaCu 4 .Being in line with the phase diagram, the micrographs in Figure 8 clearly show that the amount of the eutectic Al 8 CaCu 4 phase gradually increases with the Ca addition.These results are consistent with those of Huang et al. [10] who indicated that the eutectic Al 8 CaCu 4 phase in the Al-4.84Cu-1.26Caalloy (wt%) forms a network with a volume fraction of 21.8%.Also, a small amount of the α-AlFeMnSi phase [31,32] was found to be unevenly distributed within the α-Al matrix.Severe cracking and local porosity are observed at the locations close to the α-AlFeMnSi precipitates (as shown in Figure 4-7).Khalid et al. [33] also pointed out that the interface between pure Al and α-AlFeSi shows a significant intrinsic brittle nature.
Moreover, the globular areas of the bubbling agent Ti surrounded by the Al 20 CaTi 2 phase are also found in all cases (well-seen in Figure 3d,h), the same phenomenon was also reported in the study of He et al. [15] This phenomenon may be related to various surface tension and viscosity that prevents bubble formation.Hence, the bubbling agent TiH 2 could contribute to the formation of the secondary Al 20 CaTi 2 phase by the interaction of remaining Ti from the decomposed TiH 2 with stabilized aluminum melt or with Al 4 Ca intermetallic.Similar behavior was observed by Amsterdam et al. [34] who also pointed out that the Al 22 CaTi 2 (The stoichiometry was determined by scanning and transmission electron microscopy, and the structure is assumed to be body-centered tetragonal. [34]However, a more detailed experimental description is missing.Therefore, the Al 20 CaTi 2 phase with a face-centered cubic structure of Al 20 CeCr 2 [35] is considered in this work.)precipitates are usually located close to the Al 4 Ca precipitates, but the former is more dispersed than the latter.Since the absence of Al 4 Ca was observed by the microstructure investigation, it can be assumed that Ca or most of the Al 4 Ca phase (if formed) is spent on the formation of Al 20 CaTi 2 .Nevertheless, by the microstructure observation, it can be assumed that the amount of Al 20 CaTi 2 is rather constant regardless of the Ca content in the scale of 0.5-2 wt%.The XRD studies of the aluminum foams, as shown in Figure 8, confirm the microscopic observation.In the Ca-free matrix alloy, apart from the α-Al solid solution, the higher relative volume is only occupied by the precipitates of the θ-Al 2 Cu phase.In A201 þ Ca alloys, the volume of θ-Al 2 Cu drastically decreases.The XRD spectrums of the examined A201 þ Ca alloys also revealed the reflections from the phases Al 8 CaCu 4 , Al 2 CaSi 2 , and Al 20 CaTi 2 , however, the quantitative analysis is limited due to the low intensity and severe overlap of the corresponding reflections.Nevertheless, considering also the obtained microstructures, it can be assumed that with an increase in the Ca content, the increase in the volume of the Al 8 CaCu 4 precipitates is more pronounced compared to other phases.

Conclusion
In the present study, a gradient calcium content (0.5, 1.0, 1.5, and 2.0 wt%) was added into the A201 alloy for cellular foam preparation by using the melt foaming method with TiH 2 as the bubbling agent.Subsequently, its effects on the foam including the bubble's porosity, size, structure, and microstructure of Al foams were studied and discussed in detail.
With the addition of calcium from 0.5 to 2.0 wt% into the A201 alloy, the porosity of the Al foams was increased.And the pore size was consistent with porosity changing tendency.This phenomenon was explained by considering the influence of the thermophysical properties (surface tension and viscosity) on the bubble's formation.These two thermophysical properties, i.e., surface tension and viscosity increase the bubble size.The addition of calcium into molten A201 alloy has a negative effect on the surface tension.The addition of calcium increases the viscosity of the molten A201 alloy.Although the bubble size is related to surface tension and viscosity in tandem, calcium has a slightly decreased surface tension in the calcium addition range of 0.5-2.0wt%.Therefore, the size of the bubble was mainly related to the viscosity of molten alloy and showed an increased tendency with calcium addition.
Furthermore, a bunch of various second phases were detected in the Al-Cu-Ca foams, e.g., CalAl 2 Si 2 , Al 2 Cu, and Al 20 CaTi 2 .Except for these second phases, a eutectic phase Al 8 CaCu 4 has also been detected, with the calcium addition, its quantity increased and the size became finer.

Figure 1 .
Figure 1.The photographs of the prepared aluminum A201 + Ca foams with various calcium content in the scale of 0-2 wt%.

Figure 2 .
Figure 2. Distributions of equivalent diameter and sphericity of microspores in different aluminum A201 + Ca foams with the calcium addition of a) 0.5 wt%, b) 1.0 wt%, c) 1.5 wt%, and d) 2.0 wt%.Color column on the right side of graph corresponds to the number of the micropores.

Figure 4 .
Figure 4.The elemental mapping on cell wall with 0.5 wt% Ca addition.

Figure 5 .
Figure 5.The elemental mapping on cell wall with 1.0 wt% Ca addition.

Figure 6 .
Figure 6.The elemental mapping on cell wall with 1.5 wt% Ca addition.

Figure 7 .Figure 8 .
Figure 7.The elemental mapping on cell wall with 2.0 wt% Ca addition.

Table 1 .
Chemical compositions of primary commercial A201 alloy and Fe crucible, wt%.

Table 2 .
The characteristics of aluminum A201 + Ca foam porous structures.