Metal Additive Manufacturing without Melting
3D printing has already had a disruptive impact on the world of manufacturing by enabling parts with never-before-seen complexity to be made faster and brought to market in less time.
Ultrasonic additive manufacturing (UAM), developed by Fabrisonic, combines a unique room-temperature metal-deposition process with the ease of traditional CNC milling. The patented ultrasonic “print head” is integrated into 3-axis mills to create a hybrid additive-subtractive process where swapping from additive to subtractive is as easy as doing a tool change. So how does it work?
Welding Metal with Ultrasound?
Ultrasonic metal welding has been around since the 1950s with modern applications in everyday welding of battery tabs, thin-foil packaging and even electronic wires. An ultrasonic weld operation begins by pressing a thin-metal foil onto another metal component. While under a constant force, ultrasonic vibrations are applied to the thin-metal foil to generate scrubbing between the thin-metal foil and the mating surface of the metal component. This shearing motion cleans off surface oxides through friction to then allow direct contact of clean metal on clean metal (Fig. 1).
The result is a solid-state metallic bond with minimal heating of either component. The high shear rates and resultant plastic deformation promote diffusion and recrystallization at the interface, resulting in a strong metallurgical bond. Ultrasonic welding can be accomplished at very low temperature and without any special environments. For all metals, the bonding temperature is significantly below their respective melting temperature. In aluminums, for example, this peak temperature is always below 250°F. The solid-state nature is a key advantage of ultrasonic welds because it:
- Protects material properties of the incoming feedstock – Since the materials are only slightly heated, they do not experience changes in grain size, precipitation reactions or phase changes. The properties of the incoming feedstock are the same as the properties of the final part.
- Creates bonds between dissimilar metals without creating an undesirable brittle metallurgy – This capability differentiates UAM from fusion-based processes (Fig. 2) and enables Fabrisonic to print engineered materials with custom material properties or design properties to match an existing component. For instance, layers of molybdenum and Invar can be printed into an aluminum heat exchanger to match the CTE of a mounted electronic circuit.
- Embeds temperature-sensitive components in solid metal parts – Many electronic components (including microprocessors, sensors and telemetry) have been successfully embedded in solid metal parts using UAM. The low-temperature bond allows delicate components to be embedded in solid metal without the damage incurred in comparable fusion-based additive processes.
Ultrasonic Additive Manufacturing
UAM is ultrasonic welding on a semi-continuous basis where solid metal objects are built up to a net three-dimensional shape through a succession of welded-metal tapes stacked next to each other and then on top of one another much like layering of bricks. Through periodic machining operations, detailed features are milled into the object until a final geometry is created by removing excess material.
Figure 3 shows a rolling ultrasonic welding system. It consists of two 20,000-hertz ultrasonic transducers and the welding sonotrode where high-frequency ultrasonic vibrations are locally applied to metal foils, which are held together under pressure, to bond the foil feedstock onto the substrate. The vibrations of the transducer, generated via a pulsed electrical signal, are transmitted to the disk-shaped welding sonotrode, which in turn creates an ultrasonic solid-state weld between the thin metal tape and the substrate or previously welded layer. The continuous rolling of the sonotrode over the plate welds the entire layer width. This process is repeated layer-after-layer until a net-shape solid component has been created. CNC contour milling is then used to achieve required tolerances and surface finish.
What is hybrid?
UAM combines the advantages of additive and subtractive fabrication approaches to allow complex 3D parts to be formed with high dimensional accuracy and smooth surfaces, including objects with complex internal passageways (Fig. 4). ASTM International (American Society for Testing and Materials) defines additive manufacturing as the “process of joining materials to make objects from 3D-model data usually layer upon layer, as opposed to subtractive manufacturing methodologies” much like CNC milling.
While not formally defined under ASTM terminology, hybrid additive is generally considered to be a combination of additive (3D printing) and subtractive (CNC milling) technologies in a single machine. In practice, a Fabrisonic SonicLayer 4000 starts off life as a large 3-axis mill to which a patented ultrasonic welding head is seamlessly integrated. The system is then capable of both building material up to near-net shape via ultrasonic welding and completing fine details via onboard CNC milling.
How can UAM solve thermal problems?
Most of UAM’s history has revolved around the ability to 3D print heat exchangers utilizing dissimilar metals such as copper and aluminum (Fig. 5). In aerospace aluminums, Fabrisonic prints thermal-management devices with burst pressures in excess of 6,000 psi with hermetic seals tested to helium leak rates lower than 8.0-10 std cc/s. The channel diameters in these thermal-management devices can range anywhere from the micro scale (~0.010 inch) to the macro scale (>0.500 inch).
3D-printed metal heat exchangers allow:
- Complex flow-path designs optimized for higher efficiencies
- Integration of multiple components into a single part
- Designs with material gradients optimized for both strength and performance
Printing copper in high-heat flux regions leverages its ability to quickly conduct heat away. Unfortunately, copper is both expensive and heavy compared to other metals like aluminum. Aluminum alloys are lighter and less expensive, but they cannot match the thermal performance of copper. Fabrisonic has been bridging the gap between these two materials and their properties by printing both metals in the same part.
What about high-temperature use?
UAM has not been widely used for printing high-temperature parts. However, the solid-state nature of the ultrasonic bond is used frequently for cladding expensive metals for high-temperature equipment (Fig. 6). For instance, tantalum and nickel alloys have been 3D printed onto the surface of lower-grade steels. These alloys protect the surface of the steel from corrosive environments and allow higher operating temperatures.
Conventional cladding operations use arc welding to clad expensive metal A onto cheaper metal B. The two materials mix during the arc weld, however, diluting the properties of expensive metal A. This leads to many layers of arc welding being required to ensure the top surface meets property goals. These extra layers increase costs and delay schedules. With UAM, the weld is made in the solid-state, and there is no dilution. Thus, UAM can clad the same pipe with less than half the required volume of high-performance clad metal.
Embedding for High-Temperature Applications
Another application of UAM at higher temperatures was found in the nuclear industry. The combination of dissimilar-metal joining and subtractive capabilities of UAM enable fabrication of unique metal parts not possible with other processes. A high-flux isotope reactor, for instance, consists of a hot core filled with nuclear fuel rods surrounded by adjustable round-metal control panels that contain embedded neutron-absorbing materials.
The composite nature and selective spatial distribution of the neutron absorbers of the curved control panels make them costly to fabricate with traditional forming techniques via a combination of powder processing, welding, rolling, machining and explosive forming. UAM is able to “print” these composite control panels with greater accuracy, greater efficiency and reduced turnaround time compared to traditional methods (Fig. 7).
Embedding for Health Monitoring
Along the same lines, UAM has been used for embedding sensors into stainless steel piping for both oil-and-gas applications and rocket components. To embed small fiber sensors into a metal part, a channel path is cut using the CNC stage of the UAM machine. Next, the fiber is placed into the channel and material is consolidated over top using the additive stage. Metal flow in the UAM process (which is similar to metal flow in friction stir) creates a strong mechanical joint between the matrix and sensor material (Fig. 8), which in turn enables excellent strain coupling from the metal matrix for both stress and temperature measurements.
Optical strain gauges and traditional thermocouples have been embedded in aluminum, stainless steel and nickel alloys for monitoring strain in particular regions of parts as well as collecting distributed temperature measurements. Because UAM does not heat the part, all manner of sensors can be embedded in a 3D-printed metal component (Fig. 9). Sensors can then be used for health monitoring of the part during its entire service life because the sensors are soundly protected from the environment.
For more information: Contact Maureen Coffey, inside sales or Mark Norfolk, president, Fabrisonic LLC, 1250 Arthur E Adams Dr., Columbus OH 43221; tel: 614-688-5197; e-mail: email@example.com; web: firstname.lastname@example.org.
Want to learn more about a unique application of UAM used to build heat exchangers for NASA’s Jet Propulsion Lab? Check out the white paper here: http://www.industrialheating.com/uam.