I work in a factory and my employer has a tendency of playing mind games or mind manipulation. He has threatened to decrease my wages although I still weld. It is not by choice that he has changed my jobs but I still tig weld is this legal? Iowa is the state I am from

Yes, your employer may reduce your wages. The only legal requirement is that you be paid at least minimum wage. I couldn't find anything at Iowa's DOL website that even requires advance notice of a wage reduction in your State.