Open-cell steel foams are potential candidates for a wide range of applications due to their combination of mechanical and physical-chemical properties. The open-cell structure exposes an increased surface area for interaction with fluids flowing through the foam. Applications of interest range from lightweight construction material and crash absorbers in the transportation sector to heat conducting components such as heat sinks. Most current research concerns the manufacturing of open-cell aluminum foams because they pose an obvious choice for lightweight construction and require less effort in production. Stainless steel foams expand the list of possible applications due to their higher strength and volumetric heat capacity. In contrast, reproducible manufacturing of open-cell steel foams is challenging because of higher melting temperatures, high viscosity of the melt and chemical reaction with the mold material.
This study presents a novel manufacturing process for open-cell stainless steel foams with pore densities of 10, 13 and 15 ppi using investment casting. The influence of microstructure on the mechanical properties under quasi-static compression (plateau stress, energy absorption and strain hardening) of an austenitic (AISI 304), a super duplex (AISI F55) and a martensitic stainless steel with metastable austenite (AISI 420) were investigated. Microstructure was characterized prior and subsequent to mechanical testing using light microscopy and SEM.
The manufacturing process yields open-cell foams with relative densities in the range of 10 - 20 %, solid struts being circular in shape and defect-free surfaces. Electro-polishing is used to further minimize the influence of near-surface deformation on the resulting microstructure, especially in steel samples containing metastable austenite. During compression, martensitic transformation of metastable austenite affects the strain hardening and leads to brittle fracture of single struts.