New fabrication method and properties of porous metals produced by ultrasonic control of microbubble size

Porous metals with a cellular structure have attracted attention because of their excellent characteristics. These include a higher stiffness to weight ratio and improved impact energy absorption. Porous metals are formed in the vapor phase, liquid phase, or solid phase. However, conventional processing methods pose some difficulties, such as large pore growth and process complexity. A novel fabrication method for closed-cell porous metals is presented herein. In the new method, a bubble generation mechanism creates small bubbles in molten metal with a cylindrical hollow ultrasonic step horn. The fabrication system consists of a gas supply system, a temperature regulator, and a microbubble generator, which can break down supplied gas into microbubbles by using ultrasound waves. This fabrication method can be used to readily produce closed-cell porous metals and to create lightweight alternatives for many industrial materials. Microbubbles have low buoyancy and oxide films are formed in molten metal. With the new method, the generated microbubbles remain within the molten metal until it solidifies by cooling, resulting in a closed-cell porous metal. The porosity of the fabricated porous metal is greater than 70%.