There are others more knowledgeable than me, but my understanding is that blobs of weld are laid into the die and then ground down but this does leave a couple of problems in my mind; 1) When patching the bodywork of a car, a skilled automotive welder can lay blobs of weld into the interface between old and new metal in such a way that you have no idea there is a patch there. That being the case, why couldn't the mint make a better job of it? 2) When did they invent welding anyway?* * Just searched the internet. In the Bronze Age apparently...  Welding may have been around since the bronze age but it was carried out by a blacksmith who would heat the two component parts to white heat and after laying them together  bash brutaly with a big hammer, not the sort of treatment conducive to repairing coin dies.