Ultrasonic Additive Manufacturing (UAM) works by welding thin strips of metal together with temperature, pressure and ultrasonic sound waves.

A huge thing that seperates UAM from the rest of the 3D printing processes is that UAM is a hybrid process of additive layer building and CNC machine material removal. The accuracy of some small details on the part depends mostly on the CNC machine used.

The Process

A rotating wheel, called the sonotrode, applies pressure and a ultrasonic oscillation to thin strips (not sheets) of metal foil that are laid down on previous layers. (The temperature is usually elevated, around 200 C, so that the metal bonds easily.) The pressure keeps the foil in place and presses down on the part, but the sound waves are caused by the wheel vibrating left-right at the foil. When enough of these foil strips are laid down, a "level" composed of a few layers of foil is completed. At each level, the CNC is used to remove material. The CNC completes fine detail work and then the process begins again with the sonotrode and foil.

Part Properties and Evaluation

Due to the many subprocesses, there are many things that need to be controlled in order have a good bond between metal foil layers and between the metal foil strips. The oscillation, pressure, sonotrode wheel speed, and temperature are the main factors.

Although oscillation is listed as a consideration, it is typically not adjustable. The sonotrode wheel dimensions and some other things affect oscillation amplitude of the ultrasonic waves. However, if the sonotrode wheel wears out, is damaged, or some other issue occurs, the sonotrode oscillation can change. To deal with the change, other controllable factors need to be altered.

The pressure, or similarly, the normal force, pushes the foil down onto previous layers. There is an optimizable amount of force to use. Too much force and you risk damages bonds on previous layers and strips below. Too little force and the foil strips don't really bond to the previous strips.

Temperature is also another optimizable parameter. Too much heat and you risk partially melting or heat affecting the metal strips, while too little heat will increase the flow stress of the metal, leading to problems with residual stresses and incomplete bonding on some layers or the entire finished part.

Speed of Sonotrode Wheel: controls the metal strip bonding. The wheel is not only applying a downward pressure on the strips, but also a ultrasonic wave to help the metals bond. Therefore, the wheel needs to move slowly enough to ensure the the strips can bond.

The strips need to bond with each other on the sides, and the strips need to bond to the strips on the previous layer.

Continuing the theme of optimization, too much time on one part of the foil (in other words, too slow of a speed), risks applying too much ultrasonic vibration and actually damaging the bonds that were formed previously.

One last consideration, missing from the aforementioned effects due to material properties, is the material type and thickness of the thin metal foil strips. Thinner metal foils form bonds easily, but they are also more fragile. Controlling the process parameters is even more important with thinner metal foils.

Why Use UAM?

The hybrid system of additive foil bonding and CNC refinement is an interesting idea, and offers the ability to create complex structures. Additionally useful is the ability to use multiple material types.

However, the main impact of UAM stems from the ability to lay down fiber and wiring between layers. Therefore, UAM parts could be made similar to composites like carbon fiber, but with more metal options. Most current composites needs to be wound within a matrix. This is difficult and tends toward nonmetallic materials (e.g. carbon fiber).

Additionally, having wiring in layers enables the rise of smart structures. Sensors can be implanted within the part itself due to the nature of having metal wiring that flows through the part. Clearly, there is more research that could be done on this, but the opportunity to make structures smart with ease would improve quality of life.