Innovation in Metalworking: Welding Metal Foam Without Melting Its Bubbles

Composite metal foams (CMFs) are foams that consist of hollow, metallic spheres – made of materials such as stainless steel or titanium – embedded in a metallic matrix made of steel, titanium, aluminum or other metallic alloys. The resulting material is both lightweight and remarkably strong, with potential applications ranging from aircraft wings to vehicle armor and body armor. In addition, CMF is better at insulating against high heat than conventional metals and alloys, such as steel. The combination of weight, strength and thermal insulation means that CMF also holds promise for use in storing and transporting nuclear material, hazardous materials, explosives and other heat-sensitive materials. But to realize many of these applications, manufacturers would need to weld multiple CMF components together. And traditional welding techniques melt the bubbles that make CMF desirable. Researchers have now shown that a technique called induction welding, that relies on electromagnetic fields, can weld CMFs without impairing the bubbles in the foam. This illustration shows how it works. Note that the CMF is cool enough to touch with your bare hand, only inches away from the site of the weld. Credit: Afsaneh Rabiei, NC State University

North Carolina State University researchers have discovered a new welding method for composite metal foam (CMF), preserving its light, strong, and thermally insulating properties, vital for numerous applications.

Researchers at North Carolina State University have now identified a welding technique that can be used to join composite metal foam (CMF) components together without impairing the properties that make CMF desirable. CMFs hold promise for a wide array of applications because the pockets of air they contain make them light, strong and effective at insulating against high temperatures.

Characteristics and Challenges of CMF

CMFs are foams that consist of hollow, metallic spheres – made of materials such as stainless steel or titanium – embedded in a metallic matrix made of steel, titanium, aluminum or other metallic alloys. The resulting material is both lightweight and remarkably strong, with potential applications ranging from aircraft wings to vehicle armor and body armor.

In addition, CMF is better at insulating against high heat than conventional metals and alloys, such as steel. The combination of weight, strength and thermal insulation means that CMF also holds promise for use in storing and transporting nuclear material, hazardous materials, explosives and other heat-sensitive materials.

However, in order to realize many of these applications, manufacturers would need to weld multiple CMF components together. And that has posed a problem.

“Traditional fusion welding uses a filler to connect two pieces of metal,” says Afsaneh Rabiei, corresponding author of a paper on the new research and professor of mechanical and aerospace engineering at NC State. “This is problematic, because the metal being melted to fuse two pieces of CMF is solid, so it lacks the desirable properties of the CMF on either side of it. In addition, any type of welding that uses direct heat to melt the metal results in some of the porosity in the CMF being filled in, which impairs its properties. In short, that means that most forms of traditional welding don’t work well with metal foams.”

Breakthrough in CMF Welding Technique

However, the researchers have now identified a form of welding that works very well. It’s called induction welding, and it uses an induction coil to create an electromagnetic field that heats the metal for welding.

“Because CMF is only 30-35% metal, the electromagnetic field is able to penetrate deeply into the material – allowing for a good weld,” Rabiei says. “The air pockets that make up the remaining 65-70% of the CMF serve to insulate the material against the heat. This allows induction welding to heat up the targeted area for joining two pieces of CMF, but prevents the heat from spreading out from the site of the join. That helps to preserve the CMF’s properties.

“This is an important step forward because CMF’s properties make it attractive for a wide range of applications, but it’s essential to have a means of welding the CMF components without impairing the properties that make it attractive in the first place.”

The paper, “A Study on Welding of Porous Metals and Metallic Foams,” was recently published in the journal Advanced Engineering Materials.

Reference: “A Study on Welding of Porous Metals and Metallic Foams” by Afsaneh Rabiei, John Cance and Zubin Chacko, 16 November 2023, Advanced Engineering Materials.
DOI: 10.1002/adem.202301430

The paper was co-authored by John Cance and Zubin Chacko, Ph.D. students at NC State. The work was done with support from the U.S. Department of Transportation’s Pipeline and Hazardous Materials Safety Administration, under grant number PH957-20-0075.