**2. Materials and Methods**

Materials: Zr powder (China, MACKLIN, purity > 99%, d50: 48 μm), 2024Al alloy powder (China, Sinopharm Group Chemical Reagent Co., average particle size: 48 μm), the alloy composition is shown in Table 1 and NaCl space-holder powders (China, Shanghai Wokai Biotechnology Co., purity: A.R., average size: 100~500 μm) were used, the morphology of powders is shown in Figure 1a–c.

**Table 1.** Chemical composition of the 2024Al alloy (mass fraction, %).


**Figure 1.** (**a**–**c**) SEM images of the raw powders: (**a**) 2024Al, (**b**) Zr, (**c**) NaCl. (**d**,**e**) the sample surface morphology of the sample.

Experimental procedures: First, 2024Al-Zr metal powder with Zr content of 5–30 wt.% was put into a planetary ball mill to mix the powder uniformly with a ball-to-ball ratio of 4:1 and a speed of 100 r/min for 3 h. In the second step, a 50–70 vol.% NaCl space holder was mixed uniformly with the above powder mixture with the same ball milling parameters as in the first step. The powder particle size was not changed before and after mixing. The final mixed powder was placed in the stainless-steel mould (inner diameter 40 mm, outer diameter 100 mm, height 100 mm), and the cold-pressed block with a height of 10–12 mm was obtained by applying constant pressure (400 MPa). The block was heated to 650 ◦C at a heating rate of 5 ◦C/min in an argon atmosphere for 1 h [21]. The obtained sample was placed in water at 50 ◦C to remove NaCl. The sample surface morphology of the sample is shown in Figure 1d,e. The preparation and sintering process curves of the sample are shown in Figure 2.

**Figure 2.** Schematic illustration showing the fabrication techniques for porous materials.

Characterisation of the porous composite: A rectangular compressed sample with a size of 10 × 10 × (10–12) mm was obtained through the wire electrical discharge machining. The porosity of the sample can be calculated by the following equation [22]:

$$P = 1 - \frac{m}{v\rho\_s} \tag{1}$$

where *m* is the mass of the sample, *v* is the volume of the sample, and *ρ<sup>s</sup>* is the theoretical density of the composite material, which can be calculated by the following equation [23]:

$$\rho\_s = \frac{1}{\frac{m\_{Al}}{\rho\_{Al}} + \frac{m\_{Zr}}{\rho\_{Zr}}} \tag{2}$$

where *mAl*, *mZr* are respectively mass fractions of 2024Al and Zr, the 2024Al is 2.78 g/cm<sup>3</sup> and Zr is 6.49 g/cm3. In this study, to facilitate the description of the two types of pores studied and consequently the two sizes, we will use "intergranular pore" which corresponds to the pores between the particles (2024Al and Zr) in the matrix material and range from 1 to 10 μm. "Pores" correspond to the leached NaCl space holder which ranges from 100 to 500 μm. The intergranular porosity can be calculated in the following equation [24]:

$$P\_m = P - P\_n \tag{3}$$

where *Pm* is the intergranular porosity, *P* is the total porosity, *Pn* is the volume fraction of NaCl.

The compression test was carried out by the CMT-4204 material creep testing machine at room temperature. The previously mentioned rectangular compressed specimen of dimensions 10 × 10 × (10–12) mm was placed in the middle of the loading table at the strain rate of approximately 3.3 × <sup>10</sup>−<sup>3</sup> <sup>s</sup>−1. When the material was compressed to the identified region, the compression stress–strain curve of the sample was obtained. The energy absorption capacity of the material is calculated by the following equation [25]:

$$
\omega = \int\_0^{c\_0} \sigma \, de \tag{4}
$$

where *ω* is the energy absorption capacity, *e*<sup>0</sup> is the densification strain and *σ* is the compressive stress. The compressive yield stress is defined as the first peak of the compressive stress–strain curve, and the plateau stress is set as the average stress in the range from 20% to 30% strain.

The micro-hardness of the pore wall of the sample was measured by a Vickers hardness tester with a load of 0.98 N (0.1 Kgf) along the vertical line. The hardness test was conducted at least ten test times, and the results were averaged. The phase composition of the composite was analysed through an Empyrean sharp shadow X-ray diffractometer at the scanning rate of 10◦/min with Cu-Kα radiation; the obtained porous material was processed by wire cutting, mounting, polishing and cleaning, and the tissue was observed under an optical microscope. After it was determined that the surface of the material was free of scratches, the microstructure of the sample was observed by Zeiss Gemini 300 scanning electron microscope (SEM), and the phase composition was analysed by EDS.

## **3. Results and Discussion**

#### *3.1. Effect of Zr Content on Phase and Structure Morphology of Composites*

Figure 3 presents the XRD pattern of the composites as an increase in the Zr content. As Zr content increases, the relative intensity of the Al peak decreases and the relative intensity of the Al3Zr peak increases. The specimens were confirmed to be composed of α-Al(fcc), Al3Zr(D023) and Al2Cu(C16) phases, indicating that Zr reacted with Al to form Al3Zr under the designed sintering process. The result was consistent with the Al-Zr binary phase diagram [26]. In the case where the mass fraction of Zr is less than 53%, Al and Zr will form a single-phase Al3Zr intermetallic compound under equilibrium conditions. Moreover, there is no new phase formed involving the elements Na and Cl, reflecting that NaCl has no effect on the composition of the composite after sintering. It can be employed as an excellent space holder.

**Figure 3.** XRD patterns of the composites with different Zr contents.

Figure 4 shows the SEM image and results of the EDS quantitative analysis of composites with different Zr content. It can be concluded that the grey part is Al3Zr and the morphology is mainly a fine block. In the case of Zr content 30 wt.%, the point scan result is Al3Zr, which is consistent with the result of XRD, indicating that Zr and Al in this content will only form Al3Zr and there is no other substance. An increase in Zr content also causes the appearance of Al3Zr agglomerates with short rod morphology.

**Figure 4.** The points show the SEM images and results of EDS quantitative analysis of the Al3Zr/2024Al composites prepared with different Zr contents. (**a**,**b**) 10Zr; (**c**) 20Zr; (**d**) 30Zr.

Figure 5 is the SEM image of the pore wall of Al3Zr/2024Al porous composites with different Zr contents (space holder content is 60 vol.%). With the increase of Zr content, the number of Al3Zr in the pore wall gradually increases, and the main components in the pore wall gradually change from 2024Al to Al3Zr. As shown in Figure 5a,b, the Al3Zr are mainly formed at the contact interface between Zr and 2024Al. In the region of the Al3Zr phase, the microstructure was more uniform and regular compared with other regions, and there are fewer defects in the reaction area. However, obvious powder gaps and defects can be found between the mechanically bonded 2024Al powder particles, there is an obvious gap between the 2024Al powders, and there is much unevenness in the pore wall. When the Zr content continues to increase, the defect and gap in the material are declined and the distribution of Al3Zr in the pore wall is more dispersed. In the case of Zr content 20 wt.%, the gap between 2024Al particles caused by mechanical bonding cannot be observed. It was supposed that the bonding mode of the material was metallurgical bonding. As the content of Zr further increases, a large number of 2024Al react with Zr. Almost the whole pore wall was occupied by Al3Zr particles. It demonstrates that the material is mainly Al3Zr. However, the amount of Al3Zr possibly causes serious brittleness in the samples. The qualitative relationship between the content of Al3Zr and the mechanical properties of the material will be investigated in the future.

**Figure 5.** The SEM images of the composite pore wall were prepared with different Zr contents. (**a**) 5Zr; (**b**) 10Zr; (**c**) 15Zr; (**d**) 20Zr; (**e**) 25Zr; (**f**) 30Zr.

Figure 6 shows the SEM images of the pore walls of the porous composites. In the case of composition Zr5, there are obvious gaps (Intergranular pores) between the crystals in the pore wall. This phenomenon occurred since the 2024Al in the sample melted at the sintering temperature of 650 ◦C and flowed out due to the low interfacial tension, which resulted in the precipitation of some aluminium alloy on the outside of the sintered sample. In contrast, the powder is tightly connected, and the gap is small, and the surrounding defects are greatly reduced at the composition Zr20. It illustrates that the Al3Zr formation under the action of surface tension will prevent the outflow of molten 2024Al, even liquid phase sintering also can keep the shape of cold pressing, and the reaction heat between particles will accelerate the bonding between particles.

**Figure 6.** The SEM images of the pore walls of the porous composites with Zr content of (**a**,**b**) 5Zr and (**c**,**d**) 20Zr.
