Electronic devices and other apparatuses that include plastic parts ultrasonically bonded to metal parts are disclosed. A first component or part includes a metallic surface region having a surface roughness with one or more surface irregularities. A second component or part formed from a plastic material that can include an energy director is attached to the metallic surface region via an ultrasonic bond, such that a portion of the plastic material is melted or otherwise formed into the surface irregularities to attach the two components or parts together. The parts can be housings, internal components or other items. The surface irregularities can be formed as a result of a chemical etching process, a machining process, or a combination thereof, and can result in the creation of undercuts or cavities in the metallic surface region.