High-Performance Porous Aluminum Alloys from Recycled A356 Scrap: Synergistic Foaming Approach Using TiH2 and Na2B4O7·10H2O

Porous aluminum alloys are widely used for lightweight structural materials such as marine structures, energy absorbers, and buoyant components. However, the conventional foaming agent TiH2 presents limitations such as high cost and elevated decomposition temperatures, which increase manufacturing costs and restrict industrial applicability. In addition, the utilization of recycled raw materials such as aluminum machining chips has emerged as an important challenge in material development for resource efficiency and sustainability. To address these issues, porous aluminum alloys were fabricated in this study using recycled A356 aluminum chips by incorporating TiH2 and a low-cost alternative foaming agent, Na2B4O7·10H2O (borax), either individually or in combination. The effects of foaming agent content (1, 1.5, and 3 wt.%) on pore characteristics, microstructure, hardness, and corrosion resistance were systematically investigated. TiH2 induced an increase in porosity due to hydrogen generation and also promoted grain refinement, which contributed to the improvement of hardness and corrosion resistance, while Na2B4O7·10H2O exhibited effective pore formation and hardness improvement at 1–1.5 wt.% but tended to deteriorate corrosion resistance as its content increased. In particular, combined addition of both agents at 1.5 wt.% showed excellent pore formation and corrosion resistance properties, with a relatively high pore area fraction (2.38%), porosity (27.0%), SDAS (48.1 ± 4.8 µm), hardness (59.35 ± 6.4 HV), corrosion potential (−1.039 V), pitting potential (−0.709 V), and corrosion current density (4.956 μA/cm2). This study demonstrated that Na2B4O7·10H2O (borax) foaming agent can be an economic alternative to TiH2, and shows that the performance of porous aluminum alloys can be effectively improved by optimizing the combination of recycled raw materials and foaming agents.