Cell size controlling of closed-cell aluminum foams

Closed-cell Al foams have been widely applied in fields like transportation, aerospace, and construction due to its lightweight and energy-absorbing characteristics. However, the cell size of closed-cell Al foams produced by various methods is typically larger than 4 mm, which significantly limits their mechanical properties and shaping capabilities. This review highlights the latest advancements in reducing cell size and the corresponding improvements in performance. For melt foaming method, the cell size was reduced to approximately 0.6 mm by increasing the viscosity of the molten metal, controlling the outgassing of the foaming agent, and adjusting insulation parameters. For static gas injection method, the cell size was reduced from 15 to 5 mm by optimizing factors such as gas flow rate and nozzle diameter. Al foams with cell sizes smaller than 1 mm were achieved through dynamic gas injection methods by optimizing the motion pattern of the gas injection nozzle. Additionally, methods such as powder metallurgy, friction stir welding and emerging additive manufacturing techniques were also explored for controlling the cell size. The findings reveal that as cell size decreases, the mechanical properties of Al foams improve significantly, with plateau stress reaching 30 MPa for foams with 1 mm cells and exhibiting short-range ordered stacking structures. It was also demonstrated that small cell sized Al foams can be formed through thermal processing techniques such as casting, welding and hot-pressing. Future directions for further reducing the cell size of Al foams and enhancing their mechanical properties were also discussed.