Hydrogen solubility drop on solidification (aluminum)

Now, there are a number of types of defects that you can have in aluminum, but most of them are not terribly harmful. We do have a problem with porosity and hydrogen. Here's the solubility of hydrogen as a function of temperature. With steel, when it melts it has a very high solubility. What are the units? Cubic centimeters of hydrogen per 100 grams of aluminum — that's one of the things we use in steel as a measure of how many cubic centimeters are dissolved in 100 grams. So you go from 0.5 down to — semi-log plot — you go down by a factor of 10 or more, a factor 10, 12, or 15 drop in solubility of hydrogen. What that does is give you porosity. You will never have, in a little weld — there's always more, you decompose the moisture into hydrogen and oxygen, you're going to get a few tenths of a percent hydrogen in the weld metal. This one, they had argon gas with no hydrogen — there's still a little bit of something because there's adsorbed moisture on the surface of the metal before you weld. Here you've got a quarter percent hydrogen in the shielding gas, here you've got one percent hydrogen in the shielding gas, and you get all this porosity.