When casting heat metal on a metal weapon, will it deal an additional 2d8 damage on a hit? From the following sentence from the spell description, I assume this to be the case:
A creature in contact with a metal object you can see takes 2d8 fire damage.
(I'm assuming the wielder doesn't lose his grip).
When you cast Heat Metal on an object you get the following effect, emphasis mine:
Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.
Anyone touching the object only actually takes damage from it once immediately when you cast the spell and thereafter when you use a bonus action to cause the damage again. Otherwise, touching the object (despite the heat) does not cause damage, so simply hitting someone with such a metal object wouldn't do any more damage than normal - you don't have to worry about accidentally giving your foes a boost by using the spell against them.
The above said, there are a couple of ways I would allow you to get this kind of effect if you were deliberately trying to.
Firstly, if you readied the spell to be cast at the moment the target object was used to hit someone, I'd rule that both the wielder and their target were both in contact with the object at the time of casting, so both would take the damage.
Secondly, if you were to cast it on a weapon you were holding yourself, then on subsequent turns you could in principle use your bonus action to cause the extra damage exactly when you hit someone with it (since a bonus action without timing specified can be used whenever you like on your turn). That would get you one hit per round with some extra damage.
Obviously not a recommended strategy for use on yourself/friends unless you've got fire resistance or immunity.
The spell causes 2d8 damage to the wielder of the weapon. This damage would not have happened otherwise, so, yes, in that sense, it causes "additional" damage.
It does not increase the damage done by the weapon to creatures struck by it, because the damage is inflicted at the moment the spell is cast or (on subsequent rounds within the spell's duration) when the caster expends a bonus action. The damage is not inflicted continuously and the rules do not allow for two actions to occur simultaneously (such as the caster spending a bonus action to inflict spell damage at exactly the instant that a weapon contacts a foe).
Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again. (emphasis mine)
If the weapon were used to impale a foe and left embedded in their body, then it would presumably inflict the fire damage on that target, but such impalement would have to be at the DM's discretion, as I do not believe the D&D5 rules cover it.
I think with cooperation between the caster and the wielder then yes definitely. Using hold action in combination with bonus action you would be able to make it work as a pseudo buff spell.
That being said if you wish to use heat metal as a buff spell casting it on your ally's plate armor and having him grapple the opponent would be much more effective, since you aren't giving him disadvantage or forcing him to roll a con save. As long as the grapple is taking place, the armor must be assumed to be at least somewhat in contact with your opponent so you both would be dealt the heat metal damage.
The intended use of the spell, as described, is to be cast in the enemy's weapon, so he takes damage unless he drops it.
Now ask yourself: do you think the spell is inteded to work in a way that means you cast it on the enemy sword, and if he does not drop it, he ATTACKS YOU WITH +2d8 FIRE DAMAGE?
If the answer is no (which I think it's the only sane answer), then the next question is: if the enemy sword does not provide him with +2d8 damage vs you... why would your own weapon provide 2d8 damage vs him? Other than... "but I'm the PC, I should be allowed to squeeze the rules!"
