Lightweight metal matrix syntactic foam

Dr. Neil Canter, Contributing Editor | TLT Tech Beat August 2015

Researchers developed a metal matrix syntactic foam, prepared by a casting method, based on the magnesium alloy AZ91D and the filler silicon carbide.

KEY CONCEPTS
• Syntactic foams are composite materials containing hollow particles filled with a solid material.
• A metal matrix syntactic foam has been developed from a magnesium alloy and the filler silicon carbide that has a density less than water.
• The mechanical properties of the metal syntactic foam are comparable to several foams with higher densities and the dampening properties are superior to the base magnesium alloy.

ONE OF THE APPROACHES TO IMPROVING FUEL ECONOMY and efficiency is to reduce the weight of machinery. This trend is driving producers of metal substrates to develop lighter weight materials with superior strength.

In a previous TLT article, research describing the development of the world’s lightest metal was discussed (1). The ultralight density material is based on nickel and contains a unique microlattice cellular architecture that exhibits a density of 0.9 milligrams per cubic centimeter. The strength of this ultra-light metal is remarkably good, and those who developed it believe that it can be used in aerospace structural components.

One metal that is under evaluation due to its light weight is magnesium. Nikhil Gupta, associate professor of mechanical and aerospace engineering at New York University’s Polytechnic School of Engineering in New York City, says, “Magnesium alloys display densities in the range of 1.7 grams per cubic centimeter, which is approximately 37% lower than the density of aluminum.”

With aluminum finding large opportunities in automotive applications, a move to magnesium would seem to be logical but has been limited due to the difficulty processing the metal into specific parts.

A material that has become of interest due to its potentially useful properties is a metal matrix syntactic foam. Gupta says, “Syntactic foams are composite materials in which hollow particles are filled in a solid material. For example, ordinary foams such as kitchen sponges are made by filling gas porosity in a plastic. Use of hollow particles such as carbon can be used to maintain the porosity yet not sacrifice strength. In metal matrix syntactic foams, a metal or alloy is filled with ceramic hollow particles.”

A second material property that has become of interest along with light weight is high dampening. Gupta says, “Vibration damping is very important for automotive structural components, especially for military vehicles, which are designed for rough terrains and rugged operating conditions.”

A series of metal matrix syntactic foams have been studied in the literature, but all of the materials prepared have displayed inferior properties. For the first time, a magnesium-based metal matrix syntactic foam has been produced that displays good mechanical strength, high damping parameters and a low density.

FLOATS ON WATER
Gupta and his collaborators at Deep Spring Technology have developed a metal matrix syntactic foam based on the magnesium alloy AZ91D and the filler silicon carbide. The metal composite is prepared by a casting method. 

Gupta says, “The hollow silicon carbide particles are filled in a mold of desired size and shape and then the molten alloy is poured into the mold. Once the mold completely fills, the alloy is allowed to cool and solidifies producing the solid composite material.”

The metal matrix syntactic foam is described by Gupta as looking somewhat like Swiss cheese. He says, “On the cut surface, you can see the dimples due to the hollow particles.”

The density of the magnesium matrix syntactic foam is 0.92 grams per cubic centimeter, which is less than water. Figure 2 shows an image of the matrix syntactic foam floating in water.


Figure 2. A metal matrix syntactic foam shows promising mechanical properties that combined with a density less than water may lead to its use in automotive applications. (Figure courtesy of New York University.)

Mechanical property testing is conducted using quasi-static compression tests. Gupta says, “The strength of this syntactic foam is comparable to several existing varieties of aluminum and magnesium syntactic foams displaying higher densities in the 1.2-1.3 grams per cubic centimeter range.” The researchers found that a single sphere’s shell can withstand a pressure of over 172 megapascals before it ruptures, which is one hundred times the maximum pressure in a fire hose.

The dampening properties of the magnesium matrix syntactic foam are superior to the base magnesium alloy, AZ91D. Gupta says, “The magnesium matrix syntactic foam has a higher energy absorption capacity and a much-improved performance at higher temperatures over 430 C. Silicon carbide is a ceramic and helps to improve the stability of the magnesium matrix syntactic foam at high temperature.”

Gupta considers this material to be unique because this is the first time any metal composite has been fabricated with such low density without having gas porosity in the matrix. He says, “The low density of the magnesium matrix syntactic foam may enable this material to compete for applications with polymer matrix composites. One of the advantages of the magnesium matrix syntactic foam is that it can be used at temperatures in excess of 400 C while polymer matrix composites are not stable above 120 C.”

Potential applications for the metal matrix syntactic foam include in non-load bearing automotive components such as engine cover castings, alternator covers and air intake manifolds. Gupta cautions, “A lot of testing needs to be conducted before any new material can be used in these applications. Information on fatigue life, fatigue failure mode, corrosion and impact still need to be generated.”

Gupta indicates that this research effort has been focused on finding out how the density of a material could be reduced while still achieving useful physical properties. Future research now moves to determining how to further improve the properties of the metal matrix syntactic foam and developing some alternatives.

Gupta says, “We plan to heat treat the AZ91D matrix syntactic foam to determine how this process will improve the material’s physical properties. But use of metal syntactic matrix foams is not dependent upon achieving the highest possible strength or stiffness but whether they can be engineered to provide the desired level of properties for applications as diverse as structural components to crumple zones. Newer, non-flammable magnesium alloys also will be evaluated as replacements for AZ91D to better understand the properties of metal matrix syntactic foams prepared.”

Additional information on this research can be found in a recent article (2) or by contacting Gupta at ngupta@nyu.edu. 

REFERENCES
1. Canter, N. (2012), “Ultra-light metal,” TLT, 68 (3), pp. 10-11.
2. Anantharaman, H., Shunmugasamy, V., Strbik, O., Gupta, N. and Cho, K. (2015), “Dynamic properties of silicon carbide hollow particle filled magnesium alloy (AZ91D) matrix syntactic foams,” International Journal of Impact Engineering, 82, pp. 14-24.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat can be submitted to him at neilcanter@comcast.net.