when you hit copper why does it heat up?

A number of reasons.

Mostly you are trasferring energy and the lowest form of energy is heat.

Some of the energy will be used up in deforming the copper.

Some will be a shock wave travelling down the copper and back and making the hammer "recoil."

Heat is caused by friction and also a certain amount is caused by a sudden compression of the material. PV=NRT.

I would say the answer has to do with the pressure you are applying to the metal. I don't see how you have enough friction to heat something up when The motion is perpendicular to the object. However when you apply a pressure to an object with a fixed volume, the pressure will increase. So think of it backwards,if the metal would simply deform like Plato, no heat would be created. But because the metallic bonding is keeping the form, there is heat evolved.

Heat is a measure of how much the molecules in a substance are vibrating. The faster they're moving the hotter something is, so when you add more energy to a substance by, say, hitting it with a hammer, the energy will cause the molecules to move faster and the copper to get hotter.

Molecules within are frictionated, causing dispersion of bonds that radiate energy, thus releasing heat

Because of the friction.