How powder bed fusion works
Powder bed fusion (PBF) is a popular additive manufacturing process already making significant contributions to the aerospace and healthcare industries.
Prior to building an object using powder bed fusion, it is necessary to convert CAD data into object cross-sections in an .STL file. To build each new layer, the build platform is lowered incrementally. Simultaneously, a hopper containing material powder is raised, and a roller or vibrating blade spreads the next layer across the build platform. A laser, electron beam or thermal printhead, guided by digital data from the .STL file, selectively melts the metal powder, which fuses to the preceding layer as it cools. The layering process continues until the object is fully printed.
At the conclusion of the process, the remaining loose powder is blasted away, fully exposing the desired three-dimensional object. During powder bed fusion processes, waste is minimized compared to traditional subtractive manufacturing methods. Unused powdered metal is simply gathered and re-used for the next project.
A variety of different 3D printing techniques use the PBF process, including electron beam melting (EBM), direct metal laser sintering (DMLS), selective laser sintering (SLS), selective heat sintering (SHS) and selective laser melting (SLM).
Direct Metal Laser Sintering (DMLS)
The powder bed fusion process known as direct metal laser sintering (DMLS) uses digitized information from computer-aided design (CAD) software to guide a Yb fiber-optic laser to sinter, or partially melt, the metal powder. CAD data is converted to an .STL file that “slices” the digital object into ultra-thin cross-sections. The laser selectively sinters layers of metal powder to faithfully reproduce the 3D CAD model.
The DMLS process is distinguished from selective laser sintering, another PBF process, in that it uses powdered metal rather than thermoplastic or ceramic powders. The process works with many metals, including various titanium, Inconel and aluminum alloys. DMLS machines are used to fabricate functional prototypes and end-use parts. The grainy surface of an object can be controlled by printing layers as thin as 20 microns, approximately one-fourth the width of a human hair.
Direct Metal Laser Melting (DMLM)
The direct metal laser melting (DMLM) powder bed fusion process involves the full melting of metal powder into liquid pools. As with other PBF processes, an .STL file is generated from computer-aided design data, which guides the “printing” of sequential, micro-thin layers of fully melted metal powders. Various metals can be used, including titanium, cobalt-chrome and aluminum alloys.
When printing is complete, excess powder is removed, leaving a high-resolution object with a smooth surface that usually requires little or no post-processing.
End-use parts and functional prototypes are both dense and homogeneous. In fact, full melting results in densities approaching 100 percent. DMLM machines produce functional prototypes in a fraction of the time required using traditional casting and machining. Fast design cycles meet deadlines in competitive environments. Designers, unencumbered by long-standing limitations, often come up with highly intricate concepts that dramatically reduce parts requirements.
The DMLM process yields components with mechanical properties equal to or greater than those produced using traditional means. For example, wing brackets for the Airbus A350 XWB have been printed using this powder bed fusion process.
In aerospace, DMLM has already been used to fabricate parts for jet engines and other airplane parts. For example, Fortune highlighted how GE is printing fuel nozzles for its LEAP engines using DMLM technology. Thanks to its AM’s ability to produce complex geometric structures, the design of the 3D-printed fuel nozzle reduces 20 traditionally fabricated parts down to just one. Newer multi-laser DMLM machines further enhance productivity.
At Frankfurt’s Formnext Show in November of 2017, GE Additive addressed size limitations with the BETA version of a new meter-class printer developed in its ATLAS program (Additive Technology Large Area System). The new machine combines enhanced build rates and fine resolution with a scalable architecture capable of increasing ‘Z’ axis dimensions to more than 1 meter.
Electron Beam Melting (EBM)
Electron beam melting (EBM) is a powder bed fusion technology that melts metal powder through the use of a beam of electrons gathered and focused by electromagnetic coils. Given the inherent nature of the 3000-watt electron gun, the process must be carried out in a vacuum chamber. EBM is used to print sophisticated parts and components of many kinds, including those that supply the aerospace and medical industries.
In the medical industry, EBM technology is used to fabricate one-of-a-kind orthopedic implants. The EBM process yields objects with slightly rough surfaces, so some machining is appropriate when smooth surfaces are stipulated. However, the rough surface is actually an advantage when fabricating implants because it promotes bone adhesion.
High-quality mechanical properties are achieved, in part due to temperature consistency that strengthens fused layers. EBM machines use titanium alloys to produce parts for high-temperature, high-stress aerospace applications. EBM parts are already being used in new jet engine designs and in rocket engine prototypes.
Selective Heat Sintering (SHS)
The selective heat sintering (SHS) process operates at temperatures sufficient to fuse thermoplastic powders. Rather than using a laser or a beam of electrons to sinter the material, SHS uses a heated thermal printhead to accomplish the task. SHS machines typically operate at lower temperatures and consume less energy than their laser-powered counterparts.
As with other powder bed fusion processes, layers of powdered material are sequentially added with the use of a roller or blade. Since the SHS process requires less heat, shrinking, warping and other dimensional distortions are minimized. However, this relatively low-temperature process often requires the use of support structures, which must be removed in post-processing.
The SHS process is commonly used to produce concept prototypes rather than fully functioning ones. Some SHS printers are compact enough to fit on a desktop.
Selective Laser Sintering (SLS)
The powder bed fusion process known as selective laser sintering (SLS) uses a laser as a heat source to achieve the partial melting of a wide variety of materials, including polymers, glass and ceramics. Although partial melting requires lower temperatures than full melting, the process still promotes fusion of both particles and material layers. This powder bed fusion is also facilitated through the use of a build platform and chamber heated to temperatures close to those achieved during the sintering process. Carefully controlled cool-down periods maximize fusion and dimensional accuracy. Nitrogen-filled chambers control oxidation to further enhance quality.
Unlike SHS, this laser-based process does not typically require model support, with the bulk powdered material supplying any needed support during printing. In some SLS machines, layer-by-layer temperature control further optimizes the quality and relative porosity of the 3D-printed object.
The SLS process can produce objects with complex geometries that are dimensionally accurate. In addition to common single-component powders, SLS can also use dual-component powders; a laser melts away the outer layer as particles in the inner layer fuse. Sealants are sometimes used in post-processing to address surface porosity.
Selective Laser Melting (SLM)
As with other powder bed fusion processes, selective laser melting (SLM) begins with CAD data converted into an .STL file. This digitized information is delivered to the SLM machine so cross-sections of the desired object are sequentially printed. The selected powdered material is delivered to the print bed by a roller or vibrating blade. A laser fully melts each ultra-thin layer of powdered build material, causing it to automatically fuse to the previous layer as it cools. Material that is not melted is gathered for reuse.
SLM’s high-temperature requirements add to energy costs, and the overall energy efficiency of the process is less than with some other PBF processes.
It is possible to use a variety of metal powders in the SLM powder bed fusion process, including titanium, aluminum, cobalt-chrome, maraging steel and stainless steel. The high temperatures required to fully melt certain superalloys introduce certain internal stresses and dimensional distortions which must be allowed for. The use of support structures and a heated build chamber filled with inert gas reduces distortions. It is also possible to introduce later heat treatments to minimize internal stresses.