Waterford Institute of Technology: Office of Research and Innovation

Metallic Foams

The group is conducting research on the mechanical properties of novel metallic foams and is currently collaborating with Northwestern University , USA, as part of this initiative

Novelty

Unlike conventional metallic foams these materials have been produced by infiltrating ceramic (mullite) shell performs via squeeze casting. This work has been undertaken in collaboration with Prof. David C. Dunand and Dr. Dorian K. Balch, Department of Materials Science and Engineering, Northwestern University.

Characteristics of Foam

Diameters range from 12 to 75mm, with a 45mm mean size. The wall thickness of shells was approximately 10% of the sphere diameter. These mullite shells had the following nominal composition (wt. %): 59% silica, 39% alumina, 1.5% titanium oxide, and 0.5% iron oxide.

Density of Material

Material	Mullite shells	Al alloy 7075	Al 7075 foam
Density (g.cm^-3)	0.6 - 0.8	2.80	1.66

Microstructure of Al Alloy 7075 based Foam

Optical micrographs indicate excellent filling of space by the aluminium alloy, with very little porosity between mullite shells. Approximately 10% of shells have been infiltrated. A significant amount of dark grey inclusions containing silicon that has been removed from the shell walls by the molten alloy and precipitated during cooling are evident. These inclusions are both blocky and script-like, and located both inside and outside filled shells together with small amounts of a light grey precipitate containing Al, Cr, and Fe.

Optical Micrograph Al 7075 Foam
Optical Micrograph Al 7075 Foam

Optical Micrograph Al 7075 Foam
Optical Micrograph Al 7075 Foam larger

Optical Micrograph Al 7075 Foam
Optical Micrograph Al 7075 Foam largest

Mechanical Characterisation

The mechanical response of these foams under compressive loading has been characterised at room temperature using both conventional and interrupted test studies. Small testpieces (2x2x4mm) were manufactured using the machining stage of a microtomography facility that enabled surface quality, similar to that achieved by standard materialographic techniques, to be obtained. This high surface quality permitted the use of optical microscopy to track the evolution of surface deformation and damage as a function of engineering strain as indicated below.

Stress strain curve for interrupted compression test and corresponding micrographs

stress strain curve - stress strain curve

Click on picture for enlargement

1	2	3	4

3-D Visualisation

Optical micrographs obtained from indexed surfaces have enabled the deformed testpieces to be viewed in 3-D, as presented below.

file: TP4, Strain = 3%, Image, 1,522KB TP4, Strain = 3%
(Image, 1,522KB)

file: TP7, Strain = 11%, Image, 1,522KB TP7, Strain = 11%
(Image, 1,522KB)

Microtomography of Metallic Foams

This work has been undertaken in collaboration with Prof. Peter W. Voorhees and Dr. Jens Alkemper, Department of Materials Science and Engineering, Northwestern University (NU). A semi-automatic microtomography facility that was developed and patented by NU was used for these studies. This facility incorporated a machining and an optical microscope stage, with synchronised positioning of sample relative to cutter and microscope. The intention was to achieve 3-D reconstruction of the foam testpieces that had been subjected to different levels of strain, these being selected on the basis of key 'events' that were evident from the stress-strain curve for the Al 7075 based foam. However, full testpiece reconstruction was rendered impossible as a result of excessive wear on the diamond tooling due to the presence of the hard mullite shells. This wear caused undesirable abrasion and damage of the foam testpiece and resulted in a very significant deterioration in the quality of the machined surface. Notwithstanding this impediment to full testpiece reconstruction, it was possible to obtain serial sections covering a depth of approximately 0.2mm, each section representing a 10µm decrease in the thickness of the testpiece. Serial sections of a deformed Al 7075 based foam testpiece may be viewed below.

file: Serial Sections of Deformed Testpiece, PDF, 660KB Serial Sections of Deformed Testpiece
(PDF, 660KB)

These collaborations with Northwestern University have been facilitated by financial support that was secured through the Enterprise Ireland, International Collaboration Programme.