In many places, owning a replica sword is perfectly legal (especially if it is not a real weapon). However, it is best to consult local laws. Some places, like the UK, for example, may allow people to own metal replica swords but not allow them to be carried around in most public places. For most places where a convention is being held, and cosplay is expected, people will probably not prevent you from carrying a safe replica weapon as an accessory/part of your outfit. I have not run into any trouble carrying replica swords at comic conventions and Renaissance Festivals in the United States.  It is important to double-check the rules for the places you plan to have your replica weapon, but usually, a fake weapon replica is legal. The more realistic it looks in terms of materials and overall appearance, the more hoops you may have to jump through. Many events and locations will have you mark a real metal replica with a tag or something similar so that security personnel knows it is not dangerous.