Fabrication of Porous Aluminum with Directional Pores through Continuous Casting Technique
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Lotus-type porous aluminum with slender directional pores is fabricated via a continuous casting technique in pressurized hydrogen or a mixed gas containing hydrogen and argon. The influence of solidification conditions such as hydrogen partial pressure, solidification velocity, temperature gradient, and melt temperature on the porosity and pore size is investigated. The porosity and pore size increase upon increasing the hydrogen partial pressure or the melt temperature, whereas the porosity and pore size decrease upon increasing the solidification velocity or the temperature gradient. Furthermore, the mechanism of pore formation in lotus aluminum is examined based on the results of an improved model of hydrogen mass balance in the solidification front, which was originally proposed by Yamamura et al. The results from the present model agree with the experimental results. We conclude that the diffusion of hydrogen rejected in the solidified aluminum near the solid/liquid interface is the most important factor for pore formation because the difference in hydrogen solubility between solid and liquid aluminum is very small.
Previously Reported Porosity and Pore Size Results in Porous Aluminum and Its Alloys with Directional Pores Fabricated in a Hydrogen Atmosphere
Materials (mass pct)
Pore Diameter (μm)
First Author (Year) [Refs.]
Al (99.8 pct pure)
difficult to produce regular highly porous Al
Al-(2−8) pct Fe
Al3Fe precipitates promote pore formation
0.18H2 + 0.22Ar
Al-2 pct Mg
Al-4 pct Mg
Al-4 pct Cu
Al-16 pct Si
Al-(10−20) pct Cu
H2 + Ar mixture
Al-(1,8) pct Fe
Al-7 pct Si
Ar + 6.25 pct H2
Al-(4−18) pct Si
The hydrogen concentration dissolved in the molten metal should exceed several at. pct.
The metal should hold a smaller solid solubility of hydrogen so that the solubility gap between liquid and solid metals becomes larger.
The system of hydrogen with copper, magnesium, and several transition metals such as iron, nickel, cobalt, and chromium can satisfy these conditions, and produce highly porous lotus metals. However, the hydrogen solubility in liquid and solid aluminum is only about 2 pct of those for magnesium, copper, and transition metals.[17,18] Consequently, previous researchers considered the fabrication of highly porous lotus aluminum difficult; none of the previous reports successfully fabricated lotus aluminum with a porosity above 5 pct.
On the other hand, extensive research on porous aluminum with directional pores has been conducted considering pores to be defects.[5,6,8, 9, 10, 11, 12,14] The formation of pores during aluminum solidification is a major drawback because porousness is detrimental to the mechanical properties, and reduces both fatigue performance and total elongation. Experimental studies on the formation of pores during solidification of aluminum alloys have classically correlated the final pore structure with the solidification processing parameters such as the initial hydrogen solubility, solidification velocity, etc. The two main causes of porosity are hydrogen and inadequate feeding to compensate for the volumetric shrinkage upon solidification. When a process is kinetically complicated, distinguishing between the relative importance of these two factors is difficult. However, a few in situ observations of pore formation during aluminum solidification have clarified that pore growth during the initial stage is primarily controlled by a hydrogen diffusion mechanism, while growth in the second stage, is controlled by volume shrinkage.
The morphology of the growth of lamellar and rod eutectics in duel phased alloys is affected by the solidification conditions such as solute concentration, solidification velocity, temperature gradient in the solidification front (hereafter referred to as the temperature gradient), molten temperature, etc.[19,20] Consequently, solute diffusion controls the morphology of both the microstructure and the porous structure. Thus, to increase porosity and to clarify the pore formation mechanism in lotus aluminum, it is reasonable to consider the competitive growth between directional pores and solid metal because the hydrogen flux is related to the solidification conditions.
Several experimental investigations have examined the effects of partial pressure and solidification velocity on the porosity and pore size in metals (copper,[15,21] stainless steel, and magnesium) to evaluate the porosity and pore size from the solidification processing parameters. On the other hand, pore growth in lotus metals has been theoretically evaluated from the viewpoint of competitive growth between directional pores and solid metal. Based on the mass balance between insoluble hydrogen due to the solubility gap and formed pores corresponding hydrogen, Yamamura et al. and Liu et al. have theoretically investigated the porosity of lotus metals as a function of hydrogen solubility, which depends on the partial gas pressure and molten temperature. The calculated porosities with the appropriate fitting parameters agree well with the experimental results. However, the conclusive formula to calculate porosity depends on only the initial hydrogen concentration between the liquid and solid phases, and is independent of the solidification conditions. On the other hand, Drenchev et al. have proposed a comprehensive model for pore formation in lotus copper using classical nucleation theory; they analyzed the diffusion boundary layer ahead of the solid/liquid interface with respect to the diffusion process and structure formation. The calculated porosities and pore diameters did not agree with the experimental results because heterogeneous nucleation modeling of the pores was insufficient.
In the present work, the influence of solidification conditions (solidification velocity, hydrogen partial pressure, temperature gradient, and melt temperature) on pore formation of lotus aluminum is experimentally and theoretically investigated to elucidate the pore formation mechanism as well as to control porosity. We fabricated lotus aluminum with slender unidirectional pores using a continuous casting technique under controlled solidification conditions. In particular, the present work clarifies that the porosity and the pore size are affected significantly by the solidification velocity, which is decreased by one to two orders of magnitude compared to those for copper, magnesium, and transition metals. Based on this knowledge, lotus aluminum with a porosity as high as 40 pct is obtained for the first time.
2 Experimental Procedure
The effects of different characteristics on pore morphology were examined. The influence of the solidification rate was probed by melting pure aluminum and unidirectionally solidifying in a mixture of hydrogen 0.25 MPa and argon 0.25 MPa with a constant temperature gradient and melt temperature of 9.7 K/mm and 1223 K (950 °C), respectively, while changing the solidification velocity from 0.5 to 0.9 mm/minute. The impact of hydrogen partial pressure was investigated by melting aluminum and unidirectionally solidifying in three different atmospheres: hydrogen 0.5 MPa, argon 0.5 MPa, or mixed gas (0.5 MPa) consisting of hydrogen (0.25 MPa) and argon (0.25 MPa) when the solidification velocity, temperature gradient, and melt temperature were set to 0.9 mm/minute, 9.5 K/mm and 1223 K (950 °C), respectively.
Additionally, the impact of the temperature gradient on pore morphology was probed by changing the temperature gradient while melting pure aluminum and unidirectionally solidifying in a mixed atmosphere of hydrogen (0.5 MPa) and argon (0.5 MPa). The temperature gradient was controlled by adjusting the contact area between the mold and cooling block; the contact area with the cooling block was decreased by introducing a groove on the contact surface of the mold, suppressing the temperature gradient. The temperature gradients were set to 7.2, 9.1, 10.1, or 14.5 K/mm. The solidification velocity and molten temperature were maintained at 0.5 mm/minute and 1373 K (1100 °C), respectively.
The influence of melt temperature on pore morphology was examined by melting pure aluminum in a 0.5 MPa hydrogen atmosphere. The melt temperatures were changed in the range from 1173 K to 1273 K (900 °C to 1000 °C). Because the melt temperature significantly influenced the temperature gradient, a constant temperature gradient of 9.5 ± 0.5 K/mm was achieved by changing the contact area between the mold face and the cooling block face. On the other hand, the solidification velocity was kept constant at 0.9 mm/minute.
The solidified aluminum ingot slabs were cut with a spark-erosion wire cutting machine (AQ325L, Sodik Corp., Yokohama, Japan) either parallel or perpendicular to the solidification direction. An optical microscope (Digital HD Microscope, VHX-200, Keyence Corp.) and a scanning electron microscope (SEM; JSM-6360T, JEOL Corp., Tokyo, Japan) were used for cross-sectional observations, while an image analyzer (WinRoof, Mitani Corp., Fukui, Japan) was used to obtain the porosity and pore diameter.
3 Experimental Results
Lotus aluminum can be fabricated via unidirectional solidification in a hydrogen atmosphere. Although hydrogen has a low solubility in aluminum, this work demonstrates that a porosity as high as 30 pct can be realized via a unidirectional solidification with a slow solidification velocity. Typically, the porosity and pore size are the parameters used to characterize pore morphology of lotus metals. On the other hand, solidification conditions are characterized by processing parameters, including solidification velocity, atmospheric hydrogen pressure, temperature gradient, melt temperature, etc. The porosity and pore diameter of lotus aluminum depend on the solidification velocity, hydrogen partial pressure, temperature gradient, and melt temperature. Below we discuss the influence of each processing parameter on the morphology parameters (the porosity and pore size) in lotus aluminum.
4.1 Effect of Hydrogen Partial Pressure on Porosity and Pore Size
The porosity and pore size increase as the hydrogen partial pressure increases (Figure 5). The dependence of porosity on hydrogen partial pressure has the same tendencies as those reported for lotus copper and lotus stainless steel. The effect of hydrogen pressure on the porosity can be explained in terms of Sieverts’ law because the experimental results show that the porosity is approximately proportional to the square root of the hydrogen partial pressure.
On the other hand, considering the increase in pore size due to the increase in hydrogen partial pressure, it is surmised that insoluble hydrogen is rejected at the solid/liquid interface when the aluminum melt solidifies, and this rejected hydrogen accumulates in the liquid. Hydrogen then inspissates, enhancing pore growth near the solid/liquid interface.
4.2 Effect of Solidification Velocity on Porosity and Pore Size
The present work demonstrates that both porosity and pore size in lotus aluminum decrease as the solidification velocity increases (Figure 3). Park et al. have investigated the effect of the solidification velocity on the porosity and pore size in lotus copper fabricated by a continuous casting technique. They found that the pore size decreases as the solidification velocity increases, but the porosity is nearly independent of the solidification velocity. Park et al. have suggested that when the solidification velocity increases, the number of pore nucleation sites increases due to an increase in the degree of supercooling, and consequently, the average pore diameter decreases. Similarly, the decrease in pore size in lotus aluminum can be also explained by supercooling.
However, supercooling cannot explain the decrease in porosity. The pores, which are considered to nucleate heterogeneously, are grown by hydrogen diffusion of the insoluble hydrogen rejected at the solid/liquid interface. The amount of hydrogen diffusing from the liquid and solid into the pores increases as the solidification velocity decreases because long distance diffusion occurs more significantly. This is the reason why the porosity increases with decreasing solidification velocity.
4.3 Effect of Temperature Gradient on Porosity and Pore Size
To date, the lotus metal fabrication while controlling the temperature gradient near the solid/liquid interface has not been systematically investigated during directional solidification. However, one study has observed the difference in the pore morphology with and without a blower (existent or nonexistent cooling) during directional solidification using continuous zone melting. Ikeda et al. fabricated lotus stainless steel, and found that the porosity and pore size increase as the temperature gradient decreases without a blower. Their finding is qualitatively consistent with the present result described in Figure 7.
According to Yamamura et al.’s model (hereafter Yamamura’s model), the porosity was evaluated by considering the influence of the temperature gradient on hydrogen solubility and the resultant change on the pore volume in lotus copper. Because the soluble hydrogen content in solidified copper decreases with decreasing temperature, more hydrogen is evolved, increasing the porosity at lower temperatures. It is considered that the steeper the temperature gradient, the larger the porosity is. On the other hand, Boyle-Charles law predicts that the pore volume decreases as the temperature decreases. Thus, as the temperature gradient increases the volume of pores decreases, thereby decreasing the porosity.
Property Parameters of Aluminum Used in the Calculation with Respect to the Present Model
density of liquid phase
density of solid phase
hydrogen solubility in the solid phase
1.05 × 10−5exp(−2188/T)
4.4 Effect of Melt Temperature on Porosity and Pore Size
As the melt temperature increases, the concentration of hydrogen dissolved in the liquid phase increases along the liquidus. When the temperature decreases during directional solidification, the concentration of hydrogen decreases along the liquidus up to the equilibrium solubility of hydrogen. Therefore, the porosity of lotus metals should be independent of the melt temperature. Consequently, the effect of melt temperature on pore morphology has not been investigated. On the other hand, in Yamamura’s model the porosity is evaluated by considering the hydrogen concentration dissolved in the liquid phase due to the change in the melt temperature. Considering that the initial concentration of hydrogen dissolved in the liquid phase changes in accordance with the melt temperature, the porosity in lotus aluminum is evaluated with change of the melt temperature of aluminum based on Yamamura’s model. However, Yamamura’s model is not suited to lotus aluminum because the model was originally designed for lotus copper, which has a sufficient hydrogen solubility, and lotus aluminum has a low hydrogen solubility.
To fit the calculated value to the experimental value at the melt temperature 1173 K (900 °C), a was set to 1.0 using the parameters in Table II. Figure 10(b) shows the dependences of the measured and evaluated porosities on the melt temperature. Although both porosities exhibit positive temperature dependences, the calculated porosity is much smaller than the measured one, suggesting more hydrogen than the increment of hydrogen concentration due to the increase in the melt temperature flows into pores to increase the porosity. Consequently, other factors should also be considered.
4.5 Mechanism of Pore Formation in Lotus Aluminum
As mentioned above, the porosity of lotus aluminum in the present work depends on not only hydrogen partial pressure but also the solidification velocity, temperature gradient, and melt temperature. These dependencies are similar to those of dual-phase formation in eutectic alloys; the flux of solute atoms due to solidification conditions affects the morphology of the phases. This suggests that a change in the hydrogen flux by different solidification conditions significantly affects pore formation in lotus aluminum.
Hydrogen Solubility Difference in Liquid and Solid Metals (Aluminum and Copper), and Diffusion Coefficients in Liquid
The porosity and pore diameter decrease as the solidification velocity and temperature gradient increase, but increase as the hydrogen partial pressure and melt temperature increase. Previous works have reported the effects of solidification velocity and hydrogen partial pressure on pore formation of lotus metals. However, the influence of solidification velocity on pore formation in lotus aluminum differs from previous reports using other lotus metals such as copper and stainless steel. Similar to the fabrication of lotus copper or stainless steel, the pore diameter decreases as the solidification velocity increases. However, unlike for lotus aluminum, the porosity for lotus copper or stainless steel is independent of solidification velocity. Such a difference is attributed to the hydrogen solubility between the liquid and solid phases. For example, the hydrogen solubility gap between the liquid and solid phases in aluminum (4.93 × 10−4 mol pct under H2 0.1 MPa) is about 40 times smaller than that of copper (2.09 × 10−2 mol pct at H2 0.1 MPa) (Table III), but there is not a significant difference in the diffusion coefficients.[27,28] Therefore, the supersaturated hydrogen atoms in aluminum have to migrate a relatively long distance toward the pores to contribute to pore formation and growth. For a fast solidification velocity, the hydrogen atoms in aluminum cannot migrate a sufficient distance to grow large pores. On the other hand, for a slow solidification velocity, the hydrogen atoms in aluminum can migrate a longer distance, allowing more supersaturated hydrogen atoms to contribute to pore formation and growth, producing a larger porosity.
Here the volume fraction in the equilibrium phase diagram is used as the volume fraction f. However, because the porosity of lotus aluminum depends on the solidification conditions, the volume fraction of aluminum phase f is obtained from the experimental results. In the present work, Yamamura’s model for mass balance of lotus copper is modified by considering hydrogen diffusion, i.e., the terms indicating the mass of hydrogen are substituted using Eqs.  through .
[Initial amount of hydrogen contained in the liquid in the volume element]
+ [Amount of hydrogen contained in the liquid flowing into the volume element to compensate for solidification shrinkage]
− [Amount of hydrogen contained in the liquid flowing out of the volume element due to pore formation]
+ [Amount of hydrogen dissolved in solid copper around the pore with a length of l + dl].
Moreover, even if all of hydrogen atoms rejected in the solidified metal evolves into pores, the maximum porosity calculated by Yamamura’s model is less than 15 pct, which is far below the maximum experimental porosity of 40 pct. This discrepancy suggests that the contribution of hydrogen diffusion in the melt rejected in the solid phase near the solid/liquid interface is critical. Thus, control of solidification conditions such as the solidification velocity, hydrogen partial pressure, temperature gradient, and melt temperature is crucial to increase the porosity of lotus metal with a low hydrogen solubility.
Lotus-type porous aluminum is fabricated by the continuous casting technique in pressurized hydrogen or a mixed gas of hydrogen and argon. The porosity and pore size increase with increasing hydrogen partial pressure or melt temperature, while those decrease with increasing solidification velocity or temperature gradient. Since the hydrogen solubility is low in aluminum, low solidification velocity is crucial to obtain high porosity of lotus aluminum. It is concluded that the diffusion of hydrogen rejected in the solidified aluminum near the solid/liquid interface is the most important for pore formation.
The present work was supported by the Global-COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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