A ultrasonic welding method of joining dissimilar-material workpieces, such as sheet materials, and the joined components formed thereby. The method includes applying ultrasonic energy to a thermoplastic piece to fill a hole of a dissimilar piece to form a weld point that is made up with polymer from the thermoplastic piece. In general, the geometry of the thermoplastic piece is not altered during the process. The dissimilar piece generally has a higher melting temperate and can be metal, thermoset polymers, or other thermoplastic material. The welded pieces can be arranged in a lap, laminate, or double lap configuration. In some embodiments, the hole of the dissimilar sheet material includes undercut features that improve the mechanical interlock between the dissimilar pieces. In some embodiments, the weld point has a mushroom cap to improve mechanical interlock.