Development of Closed Cell Metallic Foam Using Casting Techniques

Abstract

The research sited in this paper involves the development of a new metal foam composite material using casting techniques. This work included the design of the material and the development of a process to produce the metal foam. The materials used to produce the foam consisted of low carbon steel hollow spheres and an aluminum alloy. The foam is comprised of steel hollow spheres packed into a random dense arrangement, with the interstitial space between spheres infiltrated with a casting aluminum alloy. Using prefabricated hollow spheres assures a uniform pore size and cell wall thickness. Casting a metal into the interstitial space provides a solid media to add structural support to the foam.

The goal of this research has been to develop metal foam that demonstrates improvements in product uniformity and mechanical properties over the currently available foams. To accomplish this goal, the study included the identification of the various technologies used to manufacture metal foams, the assessment of the improvements needed to augment the quality of foamed metals, and the design of a new product and processing technique that substantiates these goals. The experimental equipment was designed and procured, while the raw materials were obtained. Then the hollow sphere foam samples were successfully produced. Using these samples a series of characterization studies was done to qualify and quantify the results. These findings were then compared to presently published data to gauge the relative success of the work.

The hollow sphere metal foam developed in this study displayed significant improvements in the measures of compressive strength and energy absorption capacity, all the while maintaining the characteristic properties of cellular metals. The improvements were measured against the next best existing technology. The newly developed foam averaged 67 MPa over a region of 10 – 50% strain, with densification beginning at approximately 50% strain. The value for energy absorption is 30 MJ/m3 at 50% strain. This foam also has a strength to density ratio on level with the best reported results to date. The combination of these properties gives opportunity for use in previously unidentified applications, such as an energy absorption media for buildings subject to seismic motion. This foam can also be designed in such applications as automobile crumple zones, as structural members in air and space craft, and in biomedical prosthesis.

Several areas for improvement have been identified for this technology. The bonding strength between sphere and matrix needs improvement, and different material choices and processing changes have been identified in this research to achieve these improvements. The packing density of the spheres can be improved, and a new method of vibrating the sphere arrangement prior to molding may increase the packing density. The porosity of the aluminum matrix can be reduced, and the design of the casting mold and processing conditions can be modified to reduce undesirable porosity. Additional testing methods have been identified to further characterize the foam and reveal insights for further improvement. The iterative process of sampling, characterization, and analysis will continue to improve this product to satisfy the objectives of this research program.