Powder Bed Fusion (PBF) is the ASTM standard name for 3D printing techniques that build through a bed of powder (Direct Metal Laser Sintering, Electron Beam Melting, Direct Laser Melting and Selective Laser Melting). While PBF is a popular metal 3D printing process, there are commercially available 3D processes that do not use powders. One is Ultrasonic Additive Manufacturing (UAM). UAM uses commercially available metal foils. ASTM identifies this style of 3D printing as ‘sheet lamination.’ There are fundamental differences between PBF and UAM that make each suited to different manufacturing needs.
PBF prints by selective melting specific areas on individual layers of metal powder. A thin layer of powder is spread over a build plate. Then a laser beam melts specific areas of the powder where solid metal is required for a particular layer. The process is then repeated over and over, building up a solid 3D part. Once the build is complete, the loose unwelded powder is removed leaving the desired geometry.
PBF works well for complex organic structures that have intricate interwoven structure. But, be sure to consider the following points:
–Powder material. PBF works well with fusion weldable alloys. Since every layer is melted and resolidified, alloys known to be weldable are preferred. Be cautious with alloys susceptible to solidification cracking. Precipitation hardened alloys can be printed in some cases, however, they must be heat treated after the build to attain desired material properties. Typically, a single PBF system is restricted to one specific alloy as dissimilar metals create metallurgical problems and the systems are hard to ever truly get completely clean.
–Powder material properties. Vendors usually recommend choosing powders that have a specific particle size, shape distribution and metallurgical purity. A supply chain for these materials is developing, but understanding the complex interactions between all of the powder parameters will take years of research.
–Build size. PBF is suited for complex organic shapes. The final build will have a rough surface finish. As the part shape is governed by where the laser travels, each layer can be a mix of small islands, that when stacked, create hollow lattices and thin walls. In most cases, a build size is limited to about a cubic foot maximum.
–Residual stress. The method of build, being fusion, can result in parts with significant residual stresses. Depending on the application, this may not be an issue, or may require additional finishing operations to correct.
How UAM works
UAM is ultrasonic welding on a semi-continuous basis where solid metal objects are built up to a three-dimensional shape through a succession of welded metal tapes. Periodic machining operations deliver detailed features and form into the object until a final geometry is created.
The technology behind ultrasonic metal welding has been around since the 1950’s. 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 cause scrubbing of the mating faces. This shearing motion cleans off surface oxides through friction, then allows direct contact of pure metal on pure metal. This process results in a solid-state atomic bond with minimal heating. Ultrasonic welding can be accomplished at very low temperature and without special environments. In aluminums for example, this peak temperature is always below 250 °F. The solid-state nature is a key advantage of UAM as it:
–Protects material properties of the incoming feedstock. Since the materials are only slightly heated, the materials 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. Metal foils are widely available on the open market for prices approaching that of billet.
–Creates bonds between dissimilar metals without creating an undesirable brittle metallurgy. This capability differentiates UAM from fusion based processes and enables the machine to print engineered materials with custom material properties or 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.
–Embed 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 embed into solid metal without the damage incurred incomparable fusion based additive processes.