You really have to give more detail about the specific model. Some can be welded back together and the fire control group made to only function in semiauto and everything is groovy. Some can't be welded back together at all (cost effectively vs. remanufacturing a semi-auto only receiver). So kick us some models and we can be of more help.
You also have to consider, a "de-milled" receiver is not a firearm. Welding one back together is creating of a firearm. Perfectly legal to do so for yourself as a SEMI-AUTO, but it gets a little hairy if you're paying someone to do so for you, as legally they'll be the manufacturer, not for personal use.