Ultrasonic additive manufacturing makes parts no other technique can.
In 1999, Dawn White, a researcher at Ford, invented a process called “ultrasonic-additive manufacturing” (UAM), also sometimes called “ultrasonic consolidation.” She started a company called Solidica in Ann Arbor, Mich., to manufacture parts by welding foil layers together using sound. This solid-state operation joins together thin layers of similar or dissimilar metals, one layer at a time.
While the UAM machine is building a part, integrated CNC milling intermittently cuts portions out of the as-yet-to-be-completed form. These areas produce features including deep slots, latticed internal structures, and other complex geometries. Solidica also built and sold UAM machines.
“Industry found Solidica’s UAM process useful, but at about 1 kW, it had limited power,” says Mark Norfolk. He was project manager at EWI, a manufacturing research and engineering company, in Columbus, Ohio, but recently became president of a new joint venture between Solidica and EWI called Fabrisonic LLC, also in Columbus.
“The limited-power application prevented the technique from welding together anything much harder than copper or aluminum,” says Norfolk. “EWI was working on building what it called ‘very-high-power ultrasonic-additive manufacturing’ (VHP UAM) to handle advanced materials, faster speeds, and larger parts.” As a nonprofit organization, EWI gets roughly 50% of its funds from the Federal government; its other revenue comes from fees for contract engineering and membership dues. The organization’s mission is to bring innovation to industry through advances in materials joining and allied technologies and to make its manufacturing customers more competitive.
“When Solidica began working closely with EWI, it became clear that a combination of the two technologies was the key to drive UAM forward,” says President of Solidica Ken Johnson. Together, the companies built a 9-kW machine. Fabrisonic recently produced its first UAM machine and delivered it in October. The company already has an order for a second machine.
When atoms diffuse
The UAM process starts with the machine feeding onto a base plate a strip of 0.005 to 0.015-in.-thick foil that is 1-in. wide. The base plate, in turn, is held stationary by an “anvil.” A cylindrical tool called a “horn” or a “sonotrode,” which rotates along the X axis, travels directly over the foil. The sonotrode is excited by a piezoelectric transducer, causing it to oscillate 20 to 30 microns at a 20-kHz rate.
The resulting vibration causes friction at the interface between the two foil layers. The friction breaks up oxides to produce virgin-metal-on-virgin-metal contact. The clamping force of the sonotrode along with the ultrasonic vibration of the foil causes atoms to diffuse across the interface between the foil pieces. The atomic bonding and the plastic deformation that results from shear vibration causes dynamic recrystallization of the material between the layers for a true solid state bond. The largest part that can be made on current machines is 6 × 6 × 3 ft. Small parts can be machined with channels down to 0.001 in.
Because UAM is a solid-state process, there is no melting. So there are no worries about degradation of the material or the embedding of delicate electronics. Elements that would be impossible to incorporate in a dense metal object by other techniques, because of their sensitivity to temperature or pressure, can be embedded by metal that flows around them in a low-temperature plastic state.
“The temperature rise is usually only 100 or 200°F above ambient,” says Norfolk. “This is nowhere near a melting point or a temperature where the metal would undergo a phase transformation. We typically join copper, aluminum, steel, stainless, nickel, and titanium. There is nothing preventing us from working with other alloys, these are just the ones we happen to be working with.”
Current UAM machines can go up to 7,500‑lb pressure to press layers together, depending on the material, says Norfolk. “Aluminum alloys typically take a few hundred pounds,” he says. “The machine staggers layers while welding them on, resulting in a seamless part. We bent a sample part made of aluminum and titanium to show how strong the joint is. All layers held tightly. In this case, combining aluminum and titanium gave the customer specific, custom mechanical properties not possible with any other metal.”
The machining spindle is supplied by a standard CNC carousel with 20 different cutting tools. “We can machine a lot of features into parts — we have a 4-in.-diameter fly cutter, all the way down to a 0.010-in.-diameter end mill,” says Norfolk. “It’s possible to put a lot of different levels of features into a part. The machine doesn’t run on anything special — just standard G-code.”
“Smart” materials, bulkheads, and reaction vessels
In another job, UAM embedded Nitinol wires into a metal matrix. “Nitinol is a shape-memory alloy and when the part bends, the alloy’s electrical resistance changes, so it can be read as a sensor,” says Norfolk. “Or, when the Nitinol is heated, it changes the material’s stress state. This is a fictitious example, but we could make a part for a car suspension where the stiffness would change depending on how much current goes through the part. This would let the car’s computer “tune” the suspension by changing its stiffness on-the-fly.
Nitinol is also widely used in medical devices. “Nitinol is so expensive it doesn’t make sense to make a whole tool out of it,” says Norfolk. “So for a medical instrument, it would be possible to embed a Nitinol part in an aluminum handle, say. The aluminum would be inexpensive, but there would be a good solid connection to the Nitinol for going into a human body.”
Other embedded smart materials include a smart plastic, PVDF, which can detect a contact. A good application might be embedding it in the bumper of a car to trigger an air bag. Many air bags now work from accelerometers sitting a few feet back from the bumper and the distance brings about a hundred-microsecond delay. The embedded PVDF gives a signal in 2 msec. “We are talking microseconds, but that is important in such applications,” says Norfolk.
There is also Galfenol. When the material undergoes stress, it changes the magnetic field around itself. For example, it would be possible to embed a Galfenol part in a truck axle such that the part could “read” the axle’s loading state.
Current UAM machines also include a gimble that allows building parts 30° about the X and Y axes for 3D parts, says Norfolk. “It’s typically used for repair operations where there is an existing part that needs material built up over a worn spot.”
A good example of a UAM part for aerospace is the bulkhead in a plane, says Norfolk. “Typically it is a highly webbed design. Currently, these are machined from big slabs of material 6 × 6 ft that are 5-in. thick. This is a waste because about 95% of the material is machined away. UAM lets a manufacturer start with a thin plate and add the webs just where they are needed. Besides costs, the issue with using billets is that there are few mills that can make large billets. Lead times for some of those parts can be years. However, foil of high-strength aluminum, titanium, or nickel alloy is readily available.”
UAM also works well in the manufacture of biofuels, says Norfolk. So-called reaction vessels to make biofuels often include a lot of intricate, internal pathways. These are channels that cannot be conventionally machined in.
So far, both of the machines sold to universities. Depending on size and options, machines can run from several hundred thousand to several million dollars. Fabrisonic was formed to commercialize the highly-researched technology.