According to the Merriam-Webster online dictionary, fusion occurs when two or more different elements merge into a new, unified entity or whole. This can occur when heat is used to liquefy or render the elements into a different form. Unification is the product of the atoms of each object or element fusing together to form a new elemental form.
The Merriam-Webster online dictionary cites penetration as an act that occurs when an object or form passes through or into another object. When this occurs, the two objects are not unified in the same way that fused objects are. Rather, when one object penetrates another, it goes inside or passes through the other object without ever forming a bond with the atoms in the object that it penetrates.
Some examples of fusion include bicycle frames and recycled plastics. When a bicycle frame is built, different parts of the frame are welded together using high amounts of heat. When this happens, the metals fuse together to form a single piece of metal. Recycled thermoplastic polymers are types of plastic that can be heated and reformed multiple times. When this happens, the different plastics are heated and fused together, and then formed to create a new plastic object.
An example of penetration is a bullet fired from a gun. When a bullet is fired from a gun, it has enough speed that it can penetrate flesh, wood, soil and some metals. Although the bullet is going into the object it penetrates, it does not become unified with the object and can be removed.