A reader writes: Dear Dr. Ron, I need to measure the void content of an alloy. Is there an easy way to do it?
After a little thought, it occurred to me that the densities of the voided and unvoided material will likely hold the answer. I derived the result below. Assuming we know the density of the unvoided material, we can measure the density of the voided material with the Wet Gold Technique, discussed in recent posts, if the voids are not connected (closed cell.) If the voids are connected (open cell), you could machine the foam to the shape of a rectangular parallelepiped and determine the density of the foam as the mass divided by the volume.
As an example, let’s say you have a closed cell aluminum foam. We use the wet gold technique to measure its density at 1.5g/cc. The density of solid Al is 2.7g/cc.
So the volume fraction of voids is:
Sadly, this technique could not be used to find void content in solder joints, or in BTC (e.g., QFN) thermal pad connections (which are so handily mitigated by using solder preforms.) The derivation for the above equation follows: