A method is provided during which a thermoplastic rib is arranged with a thermoplastic spar. A first flange of the thermoplastic rib is abutted against the thermoplastic spar. The first flange of the thermoplastic rib is ultrasonic welded to the thermoplastic spar using a tilted ultrasonic horn. A centerline of the tilted ultrasonic horn is angularly offset from the first flange of the thermoplastic rib by an acute angle during the ultrasonic welding.

A method is provided during which a thermoplastic rib is arranged with a thermoplastic spar. A first flange of the thermoplastic rib is abutted against the thermoplastic spar. The first flange of the thermoplastic rib is ultrasonic welded to the thermoplastic spar using a tilted ultrasonic horn. A centerline of the tilted ultrasonic horn is angularly offset from the first flange of the thermoplastic rib by an acute angle during the ultrasonic welding.

A method of forming z-channels in a fibrous ceramic preform includes mounting the preform in a tooling assembly, the tooling assembly comprising a first fixture and a second fixture, heating the preform mounted in the tooling assembly to a temperature above a glass transition temperature of a polymer binder within the preform to induce a softened state of the polymer binder, inserting a plurality of needles sequentially through respective first holes in the first fixture, the preform, and respective corresponding second holes in the second fixture, and removing the plurality of needles leaving behind a corresponding plurality of z-channels in the preform.