Weldment refers to a collection of metal pieces that have been welded together. Obviously, a part that is a good candidate for being part of a weldment must be metal. Weldment work must be done to metal pieces due to the fact that it relies on melting the metal in order to bond with another piece. This means that many metal parts are a great candidate for being in a weldment. Both large and small parts are great candidates. Essentially, the only limit is on how much the weldment quotes happen to be and the budget that is provided to pay for sourcing the work out.
As stated earlier, it is of utmost importance that the product that is to be welded must be made of metal. So be it aluminum, iron, or steel, the part must be made of metal. On top of that, it is important to make sure that welding the product is the best course of action for fabricating the product. If it is cheaper to use a different method of fabrication, it is a great idea to use that instead due to how time consuming welding a large number of parts can be. It is also important to consider how much stress the part will be under in normal operation.