Various People said:
A bar of metal is no more or less suitable as a weapon then a stick.
...
A sword is just a metal bar that is sharp.
...
if you advance a club's material from wood to metal you get... an metal baseball bat, instead of a wood one
IMO, a lot of people here are oversimplifying the change of material from wood to metal to the point of being ludicrous.
The difference in weight between a steel or iron bar and a wooden bar is huge. A 1" diameter, 4' length bar of steel will weigh over 40 pounds. Yes, the balance may be the same proportionally, but the forces required to move them around are very different. Also, a wooden quarterstaff will have a certain amount of give that the wielder will be used to working with. Changing the material will change both the amount of flex and response time of the weapon. If you don't believe me, go to a construction site and find a piece of rebar. It would certainy hurt someone, but it's nothing like a quarterstaff.
A metal club and wooden club are extremely different in terms of design, size, shape, and weight. The fact that a wooden baseball bat and a metal baseball bat feel the same to a hitter is only because a large amount of engineering (and a much different manufacturing process) has gone into the metal bat. A wood bat is a solid piece of wood. Weight and balance are determined by the overall size. A metal bat is hollow, and has been machined to have whatever weight and balance the manufacturer wants it. This is why metal bats can come in a much wider variety of widths without affecting the weight. Also, note that all metal bats are aluminum, not the steel or iron that you find in D+D. A baseball bat made out of steel would be either too heavy to be usable, of so thin that you would worry about denting it.
Even if you wanted to assume that someone in the D+D universe was able to design a hollowed quarterstaff with proper weight and obtain the correct material, it would be impossible for a regular weaponsmith to make. Hammering a metal end cap onto a wood rod and nailing it in place is relatively simple. Creating a long, hollow, straight piece of metal is not. Unless a wizard in your campaign has researched Elohna's Aluminum Extrusion Process or Mordenkainen's Magical Lathe, I simply don't see it as being feasible.
As to the comment that a sword is just a sharpened metal bar, I don't even know where to start.
Bottom line: a metal quarterstaff is not something that should exist in the D+D universe. If you wanted one, it would definetally be an exotic weapon.