ah is that your aluminum? soldering aluminum needs special solder and flux vs steel. throw the magnesium into the alloy and i'm not sure exactly- you could research it. there are special soldering rods for aluminum.

aluminum quickly forms a thin oxide layer that has to be removed for proper wetting.

flux helps but you may need to sand it a bit or hit it with a wire brush to remove oxides. akso be careful about flux fumes especially for aluminum

this is a steel ring. im doing it outdoors so fumes arent much of an issue i dont think, but its rly cold which maybe messes with it, idk.

but i just noticed the flux i have says it's for copper so maybe thats not good either

if it's not sticking (wetting) you need better pre-cleaning, more pre-heat before applying the solder, more flux, or different solder

i would say instead of trying to lay the solder on there, try applying flux, heating, then touch the solder onto the boiling flux

what kind of solder and flux are you using? and is the metal clean and shiny when you start?

it should not need to get red hot. that will oxidize the surface and boil off all the flux, then you'll have to clean the metal again

out of curiosity i just tried with one of my stainless steel rings i made and was having the same problem with rosin flux and both silver, and lead/tin solder

i know it's possible to solder steel because some electronic component leads are made of steel- it's just a matter of finding the right chemistry

generally those component leads are pre-coated with tin. i think you will probably have to research it