This article describes briefly the technology involved with forming ultra-light aluminum foam panels.

Fig. 1 Foamed aluminum sandwich structures offer energy absorption, lightweight construction, and thermal barrier insulation for many applications.
Metal foams (Fig. 1) are produced by compacting a powder mixture of metal and foaming agent to high density and then heating the compacted foam percursor material to a temperature near the metal melting point. Gas bubbles create voids within the expanding body of semisolid metal and are retained during solidification, resulting in a lightweight structure with a high degree of porosity (a range of 50 to 90 vol %).

Originally developed by the Fraunhofer Institute for Applied Materials Research (IFAM), the metal foam products (U.S. Patent 5,151,246; 1992) can be produced in several different configurations:

  • Complex 3-D foam parts can be molded to net-shape. The preform is placed inside the cavity of a forming tool. The mold is then heated to near the melting point and the part expands to fill the entire mold cavity resulting in a 3-D shape. A skin of dense aluminum forms on the outer surface of the part and aids in joining the foams to other materials.

  • Sandwich panels of aluminum foam cores inside metal face sheets can also be produced. Panels up to about 3 feet x 5 feet x 0.5 inch thick have been produced for prototype vehicles. They have shown the high specific-stiffness and weight reduction advantages of aluminum foam sandwich panels in convertible body structures. These panels are formed by roll cladding aluminum face sheets onto an extruded billet of foam precursor. The resulting sheet is heated to activate the foaming agent. This results in expansion of the foam core thickness by about 400%, yielding 80 vol % porosity. Sandwich panels having complex, contoured surfaces are produced by press-forming the semi-finished panel into the desired shape before the foaming step.

  • Metal tubes filled with aluminum foam have also been produced. These tubes were fabricated by inserting powder preforms of foam precursors into the tubes and heating then to initiate the foaming reaction, thus filling the inside of the tubes with foam. The aluminum foams can be metallurgically bonded to the inner surface of the tubes, providing an excellent interfacial bond.



Fig. 2 The processing route for metal foaming.
The Process
The metal foaming process is a powder metallurgical process in which commercial powders are mixed with small quantities of a powdered foaming agent via conventional techniques (Fig. 2). The mixture is compacted to a semi-finished product of low-porosity (90 - 95% dense) by applying compaction techniques such as extrusion. Provided the process parameters have been chosen appropriately, the result of the compaction process is a foamable, semi-finished product that can be worked into sheets, profiles, etc., by applying conventional deformation techniques. During the final heat treatment at temperatures near the melting point of the corresponding alloy, the materials expands and develops its highly porous, closed-cell structure. In addition, to aluminum and its alloys, other metals such as steel, copper, zinc and lead can also be formed.

Properties
The density of aluminum foams typically ranges from 0.5 to 1 gram/cc, but lower densities can also be achieved. Due to its closed porosity, aluminum foam floats in water. The strength and other properties of foamed materials can be tailored by adjusting the following parameters:

  • specific weight
  • alloy composition
  • heat treatment
  • morphology of pores

Due to their porous structures, foams have high specific stiffness. Electrical and thermal conductivities of metal foams are considerably reduced, but still in the typical range of metallic materials. Aluminum foams have good mechanical damping and sound insulation properties. Metal foams provide excellent energy absorption features at a higher strength level as compared to foamed polymers and the wide range of service temperature and the non-flammability of the materials are important advantages. Finally, the recycling ability of the foamed metals is an important factor. Metal foams are easily processed by sawing, drilling, milling, etc., and can be joined by adhesive bonding, brazing, and TIG and laser welding techniques.

Applications
Aluminum metal foams are being applied in various automotive applications including firewalls and kick-up panels, floor panels, energy absorbing bumpers, door side-impact bars, front crash rails, space frame components and roll bars. While production costs are about 20% higher than conventional aluminum or steel auto panels, the powder product is more than ten times stiffer at half the weight. Railroad applications include locomotive collision posts and crash cages, crush buffer zones and side impact barriers on rail passenger cars.

The foam panels are also applicable in military systems such as lightweight armor for army trucks and personnel carriers, mine blast containment and water-tight doors on ships. IH

For further information on aluminum foam materials, contact Dr. Dennis Claar, Technical Director, Fraunhofer USA, Center for Manufacturing and Advanced Materials Delaware, 501 Wyoming Rd., Newark, DE 19716. Phone: 302. 369.6721; fax: 302.369.6763; or email: dclaar@fraunhofer.org.