I'm sure this was explained in some comics through the years, but If adamantium and to a lesser degree vibranium are nearly indestructible, how do you mine and melt down the metal do that you can forge objects with it? If it can be melted down, then any kind of weapon that can generate enough heat directed at the shield should be able to damage it, deform it, or destroy it. The Human Torch should theoretically be able to melt it down if he wanted to. If the metal can't be meleted down by heat, then you really cant make it maleable enough for it to be fashioned into the objects you want to make out of it. If it can be melted down then it really isn't that indestructible after all.

https://www.quora.com/How-was-Captai...ible-to-deform

Cap's shield (in the comics) is made of Vibranium and some other mystery alloy which combined to make it that hard, and vibranium can be melted but by unconventional means. But usually normal vibranium is not as hardy as Cap's shield is.

Adamantium is similar, an amalgamation of certain chemicals that is moldable in liquid form only initially, and once it is shaped is nigh indestructible.