Pi Box Weld Development

The 787 aircraft is the most fuel-efficient in the Boeing fleet. In addition to the well-known composite construction, emphasis was placed on design for weight reduction of metallic components to drive increased fuel efficiency. A critical weight reduction comes from the use of a novel welded titanium seat track design.

The “Pi-Box” seat track design consists of butt welded Ti-6Al-4V sheet joined with a continuous laser weld to a machined Pi-shaped Ti-6Al-4V extrusion. To accommodate established floor support structure without additional weight, the extrusion has variable height along the length of the track. This requires precise guidance of the weld head to follow the curves of the part. The machined extrusion is designed with a sacrificial weld nub, minimizing the use of filler material.  This design feature allows for a high rate of welding, with over one quarter mile of welds per airplane.

Allowables for the design were developed in partnership with leading industry organizations. The butt weld allowables manufacturing was performed at TWI (The Welding Institute). Laser weld allowables were developed at EWI (Edison Welding Institute). Due to the size and quantity of parts scale-up testing was performed at EWI prior to development at the supplier. These tests were performed at roughly half of maximum production length. Testing was performed to validate the design and related attachments. The seat tracks make up a major component of the floor grid and are key passenger safety. Testing included 16G dynamic-load sled testing to verify component strength during extreme conditions. Nondestructive Inspection (NDI) techniques were developed by to ensure reliability of the process. Inspection techniques include ultrasonic, X-ray, and visual inspections of the weld.

This presentation will highlight the major lessons learned in development of the design, testing, and inspection methods.