Composite metal foams (CMFs) are intriguing materials, offering several big advantages over traditional solid metal. And while the welding of CMFs usually poses some challenges, it has now been been discovered that the use of an alternative type of welding works like a charm.
CMFs typically consist of hollow spheres made of one type of metal, contained within a solid matrix composed of either the same or a different metal. They're sort of like an Aero chocolate bar, if it were made of metal instead of chocolate.
One of their biggest selling features is the fact that they tend to be lighter than conventional solid metals, while offering comparable strength. Over the past few years, they have additionally been found to stop bullets, block radiation, and insulate against high temperatures.
Unfortunately, though, when separate pieces of CMF are welded together via conventional methods, they loose those desirable properties at the weld point.
"Traditional fusion welding uses a filler to connect two pieces of metal," said North Carolina State University professor of mechanical and aerospace engineering, Afsaneh Rabiei. "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."
Seeking an alternative, Rabiei and colleagues looked to a lesser-used technique known as induction welding. In a nutshell, it utilizes a radio-frequency electric current to energize an induction coil, creating a high-frequency electromagnetic field that causes certain metals to heat up to the melting point.
Lab tests showed how CMF's porosity works in its favor with induction welding. First of all, the electromagnetic field was able to reach deep into the material, as there was less solid metal through which that field had to pass. Additionally, the voids within the CMF kept the heat from spreading out from the weld site, so the material only melted right at the join.
"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," said Rabiei.
A paper on the research was recently published in the journal Advanced Engineering Materials.
Source: North Carolina State University
