1 Introduction

Cellular metallic materials have attracted attention in the last decades with the availability of practical manufacturing technologies and improved understanding of their physical, chemical and mechanical properties. Aluminum is one of the very few metals with applications from utensils to aircraft parts. Although aluminum surface is protected by the oxide layer, this layer can still be damaged in aggressive environments leading to fast corrosion. Different additives can be introduced into the aluminum matrix to improve its microstructure and other properties [1, 2].

Porous aluminum alloy is a very popular advanced composite material because of its attractive physical and mechanical properties, such as light weight, high strength, good energy absorption, excellent shock resistance, damp shock absorption, etc. Based on the connectivity of cells, cellular metals can be categorized as either closed- or open-celled. Cellular metals are also dominantly used for construction, auto parts, electronic equipment, aerospace, and other broad variety of fields [3,4,5,6,7].

Rare earths (RE) elements incorporated into the aluminum structure inhibit its recrystallization, grain refinement and influence the precipitation process. RE elements additions also improve mechanical and physical properties of aluminum as a conductor [8,9,10,11,12]. Additions of such RE elements as Er, Sc, Ce and La were studied with regard of their beneficial effects on the mechanical properties of aluminum alloys. The most benefits were obtained at low RE elements concentrations [13,14,15,16,17,18,19,20].

However, the effects of CeO2 addition on the microstructure and mechanical properties of the porous aluminum alloys obtained by powder metallurgy have not been investigated systematically. In this study, we used SDP technique to prepare 2A12 porous aluminum alloys with different porosity. We studied the influence of CeO2 addition on phase evolution, microstructure and mechanical properties of porous Al alloys at room temperature.

2 Materials and methods

The initial materials were 2A12 aluminum alloy powder (see Table 1 for its chemical composition) with 99.9% purity and 8–15 μm particle sizes, hydrosoluble filler with 99.9% purity and 40–55 μm particle diameters and CeO2 powder with 99.99% purity and 10–15 nm diameters. The microstructure of CeO2 powder is shown in Fig. 1.

Table 1 Chemical composition of the 2A12 aluminum powder, wt%
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Fig. 1
figure 1

Microstructure of the CeO2 powder

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The porous aluminum alloys with 30 (group A) and 50% (group B) porosity were prepared by SDP. CeO2 content for different samples is shown in Table 2. The composite powders were prepared by V-blender mixer for 3 h at the milling speed of 30 r/min. The two action pressures were adopted to make green compacts at 200 MPa. The samples had the dimension of Φ40 × 5 mm. Wu et al. [21] measure the liquid volume fraction and his results indicated that the actual freezing range of 2A12 aluminum alloy powder is 509–648.3 °C. Thus, we sintered the green compacts in vacuum at 650 °C for 270 min. The sintering was performed using melted samples the liquid metal viscosity is sufficient penetrate between WSG particles. After sintering, the alloys were cooled to the room temperature at 10 °C/min. The pore former of the sintered samples was dissolved in water at a temperature of 60 °C.

Table 2 Nominal composition of specimen powders (mass fraction %)
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The X-ray diffraction (XRD) analysis was performed at room temperature using D/MAX-2550X diffractometer with Cu-Kα radiation. The surface and fracture microstructure of polished samples were studied using scanning electron microscopy (SEM, FEI Nano230) equipped with energy dispersive spectroscopy (EDS). The chemical analysis was performed by electron probe micro-analysis (EPMA) using a JXA-8530F. The tensile and compression strengths were tested with an INSTRON 3369 with the loading speed of 0.5 mm/min.

3 Results and discussion

3.1 XRD analysis

XRD patterns of all samples in the group A are shown in Fig. 2 in comparison with the CeO2-free alloy #1. Four phases were detected at low CeO2 content: metallic Al, Al2O3, AlCuMg and MgAl2O4. As the CeO2 content increases, new phase Al11Ce3 appears in the XRD patterns. Incorporation of the CeO2 in the aluminum matrix is not evident because of the very negligible XRD peak shifts.

Fig. 2
figure 2

The XRD patterns of the alloys from the A-group with different CeO2 content

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The following reactions may occur when the CeO2 was added to 2A12 aluminum [22]:

$$3/2{\text{CeO}}_{2} + 2{\text{Al}} \to {\text{Al}}_{2} {\text{O}}_{3} + 3/2{\text{Ce}} + \Delta {\text{H}}_{1} ,$$
(1)
$$11{\text{Al}} + 3{\text{Ce}} \to {\text{Al}}_{11} {\text{Ce}}_{3} + \Delta {\text{H}}_{2} .$$
(2)

Both reaction (1) and (2) are exothermic with ΔH1 = −44.1 kJ/mol and ΔH2 = −574 kJ/mol. The Al–Al11Ce3 eutectic is formed in the Al–rich side of the Al–Ce binary phase diagram at 913 K. The heat released from the Al–CeO2 reaction may cause the eutectic melting of Al and Al11Ce3. Because of that the heat is rapidly lost to the surrounding aluminum matrix, and the melting only occurs locally and resolidification follows. It is suggested that the microstructure consisting of round Al–rich phase enclosed by Al11Ce3 may result from the local melting and resolidification of material with a hypereutectic Al–Al11Ce3 composition. From the thermodynamic point of view, the addition of CeO2 may reduce the sintering temperature of 2A12 alloy and promote the sintering process.

3.2 Microstructure analysis

Figure 3 shows the SEM images of 2A12 aluminum with different porosity and CeO2 amounts. Less white phase is observed (see Fig. 3a) as the CeO2 amount changed, implying that the aluminum alloy achieved a good compactness. The microstructures of samples in the Group A can be described by two different colored areas. The content of white precipitated phase increased (see Fig. 3a–c): CeO2 addition reduced grain size of aluminum powder and increased the white precipitated phase content (see Fig. 3d–f). However, it is not obvious that the CeO2 has grain refining effect on the casting aluminum alloy. The pore distribution of porous aluminum is uniform, and it has open-pore structure, which can effectively dissolve the pore-forming agent. The samples showed no obvious collapse after sintering, and the pores were uniformly distributed and had diameters ~ 80 μm. The white precipitated phases have granular or rod-like shape and are mainly observed at the granule boundaries.

Fig. 3
figure 3

SEM micrographs of porous aluminum with different CeO2 content. a A + alloy 1, b A + alloy 3, c A + alloy 6, d B + alloy 1, e B + alloy 3, f B + alloy 6

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The results of EDS are show in Fig. 4, and the gray part can be described as Al–Cu–Mg intermediate phase while the white part can be a mixture of Al–Cu–Mg and Al11Ce3 phases. These results agree with the XRD results. Solubility of RE elements in aluminum matrix is very small [22,23,24] and typically RE elements cannot be detected by just EDS.

Fig. 4
figure 4

EDS analysis of selected areas of the alloy 6 from the group A

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Results of EPMA analysis of the alloys from group A are shown in Fig. 5. Distribution of Al, Cu, Mg, O and Ce is shown by the color ranging from blue to red. Aluminum (see Fig. 5b) is found in all parts of the sample with the exception of the pores. Aluminum content is small in the dark grey area (see Fig. 5a). Spectral analysis shown in Fig. 5c showed that the dark grey area scorrespond to the oxidized aluminum judging by the distribution of O shown in Fig. 5e. Distribution of Cu, Mg and Ce is almost identical to the position of white precipitated phase in Fig. 5a. These white phases are located between aluminum granules. According to the analysis described above, the white precipitated phase consists of Al–Cu–Mg intermediate phase and Al11Ce3.

Fig. 5
figure 5

The elements distribution of A + alloy 6. a selected area, b Al, c Cu, d Mg, e O, f Ce

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3.3 Mechanical properties analysis

The tensile and compression strengths of sintered samples with different porosity and CeO2 additions were measured at room temperature (see Fig. 6), and the strain rates are 0.001/s and 0.0015/s, respectively. Both tensile and compression strengths for both groups A and B first increased but then decreased with the increasing CeO2 content (see Fig. 6a). The tensile and compression strengths of aluminum with 30% porosity are 32.8 and 70.6 MPa, respectively. The maximum values of tensile and compression strengths were 48.1 and 77.8 MPa, respectively, at CeO2 content equal to 0.2 wt%. CeO2 content increase from 0 to 0.2 wt% resulted in the fastest tensile and compressive strengths increase: 48.2% and 11.2%, respectively. Group B samples with 50% porosity showed similar results (see Fig. 6b). The best tensile and compression strengths were obtained again at 0.2 wt% of CeO2 and are equal to 24.9 and 32.6 MPa, respectively.

Fig. 6
figure 6

The tensile and compression strengths of sintered samples with different porosity and CeO2 additions, a 30% porosity, b 50% porosity

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Under the same conditions, the compressive strength of porous aluminum is much higher than its tensile strength, which is related to the structural characteristics of porous aluminum. Because of the pores presence, the tensile strength of the porous aluminum is sensitive to the crack due to the stress concentration, while the compression strength is not affected by this. In addition, the content of CeO2 has no influence on the pore size and morphology of 2A12 alloys. Since certain conditions are required for particles to grow during the sintering process, we believe that alloys can achieve better properties with the CeO2 addition.

Ce can either form solid solution with aluminum and its alloys, or accumulate in the interface between grain and phase boundaries, or form an intermetallic compounds with aluminum. The metallic Ce and Al have h.c.p. and f.c.c. crystal structures, respectively. Addition of Ce into the Al structure can form limited solid solution [25]. However, Ce can accumulate at the interface with the aluminum alloy to form the intermetallic compound. This can affect the distribution of precipitated phases and the deformation resistance of aluminum alloys, both of which are beneficial to the mechanical properties enhancement.

The improvement of mechanical properties of sintered samples can be explained by the fracture mechanism. SEM images of fracture microstructure are shown in Fig. 7. However, the number and depth of toughness nest increased with increase the CeO2 content, and the intergranular and transgranular fractures coexisted in the alloy. The EDS analysis of toughness nest of the sintered alloys with CeO2 is shown in Fig. 7d. The fracture area of the sample corresponds to the area of the precipitated phase, thus, we believe that CeO2 addition can effectively promote Al–Cu–Mg phase separation from the Al11Ce3 phase. The Al11Ce3 phase causes the lattice distortion in the aluminum matrix with high distortion energy. This phase can strengthen both the grain and phase boundaries. However, the stress concentration appeared at very high Al11Ce3 content diminishing the performance of the sintered alloys. Thus, only the certain amount of CeO2 can improve the mechanical properties of 2A12 porous aluminum alloys [26,27,28].

Fig. 7
figure 7

The fracture micrographs of porous aluminum with different CeO2 contents. a A + alloy 1, b A + alloy 2, c A + alloy 6, d EDS of red area in b

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4 Conclusions

The micron porous aluminum alloys with different porosities were successfully prepared by the cold pressing-vacuum sintering-dissolution. The CeO2 changed the microstructure of porous aluminum alloys and resulted in the formation of Al11Ce3 phase, which enhanced the mechanical properties of the hosting matrix. When the CeO2 addition was 0.2 wt%, the tensile and compression strengths of porous aluminum achieved the maximum values.