**1. Introduction**

Porous aluminium, as a widely used structural and functional material [1], has become a highly practical topic due to its low density, high specific surface area, high specific strength and light weight [2–5]. Its properties such as vibration damping, energy absorption, flame retardancy, sound absorption and heat dissipation have been widely studied [6–8]. The preparation methods of porous aluminium generally include the gas injection method [9], foaming method [10], mould casting method [11], deposition method [12] and space holder method [13]. However, the mechanical strength of pure aluminium and aluminium alloys prepared by the above methods is poor, which limits the application of porous materials. Therefore, to enhance the mechanical strength of the material, some researchers added the second phase to the porous aluminium to prepare composites. The in situ synthesis method is widely used because of its uniform distribution and tight bonding. Inoguchi [14] prepared in situ Al3Ti/Al porous composites and the compression properties of porous materials improved by the combination of Al phase and Al3Ti. Atturan [15] prepared an A357-TiB2 porous composite and the mechanical strength is much higher than Al porous materials. In situ synthesis of reinforced particles in porous aluminium enhances the mechanical properties of the composites.

In recent years, in situ generation of Al3X, such as Al3Ni, Al3Fe, Al3Ti, and Al3Zr, has been used in different composites due to their attractive properties. But among these, Al3Zr has attracted much attention due to its low density (4.11 g/cm3), high melting point (1580 ◦C), high specific strength, high specific stiffness, excellent corrosion and wear resistance [16,17]. The lattice matching values of Al3Zr (tetragonal structure) and α-Al (FCC structure) in the a and c/2 directions are 93% and 99.2%. The relatively high wettability contributes to a better dispersion of particles in the matrix, better grain refinement and better properties of composite [18]. Gupta and Danie [19] added K2ZrF6 salt into the aluminium melt to produce Al3Zr particles, the Al3Zr particles dispersed uniformly and refined

microstructure during the melting process, which significantly improved the strength of the material. Pourkhorshid [20] used mechanical alloying (MA) and a hot extrusion process to prepare Al/Al3Zr composites, the Al3Zr began with the nucleation of the metastable phase and then transformed into a stable tetragonal Al3Zr structure. The tensile yield stress of obtained Al-10 wt.%Al3Zr composites is 103 MPa, which is about twice that of pure aluminium (53 MPa). It can be seen that the in situ Al3Zr has a good effect on the mechanical properties of composites. Nevertheless, the studies on the application of Al3Zr to improve the mechanical properties of porous aluminium have not been fully investigated. In the present study, the effects of Zr and NaCl space holder content on the microstructure of porous Al3Zr/2024Al composites with space holder and in situ synthesis method were investigated. Changes in the compressive and energy absorption characteristics were also studied.
