This application relates to a multi-piece enclosure for a portable electronic device. The enclosure includes a metal part including a metal substrate and a metal oxide layer overlaying the metal substrate, the metal oxide layer having an external surface that includes openings that lead into undercut regions. The openings are characterized as having a first width, and the undercut regions are characterized as having a second width that is greater than the first width. The enclosure further includes a non-metallic bulk layer including protruding portions that extend into the undercut regions such that the non-metallic bulk layer is interlocked with the metal part.