A solid-state stir welding process was developed at NASA Marshall Space Flight Center to join large pieces of very high-strength metals such as titanium and Inconel.

Ultrasonic Stir Welding (USW) improves current thermal stir welding processes by adding high-power ultrasonic (HPU) energy at 20 kHz frequency.Ultrasonic Stir Welding equipment. Source: NASA

Axial, frictional, and shear forces are reduced while travel rates are increased with the addition of ultrasonic energy. Wear on the stir rod is also reduced, which translates into extended stir rod life.

The new process decouples the heating, stirring, and forging elements found in the friction stir welding process, allowing for independent control of each process element and, ultimately, greater process control and repeatability. Closed-loop temperature control can be integrated into the system so that a constant weld nugget temperature can be maintained during welding.

Heating is effected with a specially designed induction coil. The control system pulses the HPU energy of the stir rod on and off at different rates from 1-second pulses to 60-millisecond pulses. This pulsing capability allows the stir rod to act as a mechanical device (moving and stirring plasticized nugget material) when the HPU energy is off, and allowing the energized stir rod to transfer HPU energy into the weld nugget (to reduce forces, increase stir rod life, etc.) when the HPU energy is on.

Uses include hardware for severe environments, launch vehicles, and aircraft in aerospace applications; pistons, struts, and vehicle structure in automotive applications; shipbuilding and platforms in marine applications; and bridges, trains, and pressure vehicles in civil applications.