Because I ran out of shielding gas I wanted to figure out how the atmospheric pressure affects oxidation and so I performed two tests one at normal atmospheric pressure and one at -20inHg, both without shielding gas.
Test at normal pressure:
Test at -20inHg:
You can see the difference between both videos while melting and on the melted spot.
At normal pressure the powder is sparking and smoking while being heated and after melting you can see the brown oxidation.
At -20inHg the heating reaction is less intensive and after melting there is no such sign of oxidation.
Both tests were performed without shielding gas. Because of the good result with the vacuum it could be possible to do it completely without shielding gas. I would really appreciate that because purging an 80L chamber needs a lot of shielding gas and shielding gas is not cheap, so the entire process would be a lot less expensive without it.