Types of 3D Printing in Metal
Metal powder is the backbone of metal 3D printing. Though it’s difficult and dangerous to handle in its raw state, its unique features make it the preferred metal stock type. The vast majority of metal 3D printing technologies utilize metal powder. As a result, the major differences between types of metal printers relate to how they fuse the powder into metal parts. These methods vary greatly, ranging from using high energy lasers to fuse loose powder to extruding bound metal powder filament. In this article, we’ll take a look at the most heavily used types of metal 3D printing, how they work, and why they’re beneficial.
Powder Bed Fusion
Known by many names, powder bed melting is currently the most common type of metal 3D printing. These machines distribute a fine layer of powder over a build plate and selectively melt a cross section of the part into the powder layer. There are two distinct types of powder bed melting techniques: Selective Laser Melting and Electron Beam Melting.
Selective Laser Melting (SLM)
Also known as: Direct Metal Laser Sintering (DMLS), Selective Laser Sintering (SLS), Direct Metal Printing (DMP), Laser Powder Bed Fusion (LPBF).
The majority of Powder Bed Fusion machines are Selective Laser Melting (SLM) machines. SLM machines use high powered lasers to fuse metal layers into parts. After a print, an operator removes the part (or parts) from the powder bed, cuts the part away from the build plate, and post processes the part. It’s the current standard for metal printing -- most companies in Metal AM today sell SLM machines.
As the most mature variety of metal 3D printing, SLM is often considered the standard that other technologies are evaluated against. SLM printed parts are great for precise, geometrically complex parts that would not be otherwise machinable. They fit into a wide variety of applications: from dental/healthcare to aerospace. Build volumes range from very small (100mm cube) to large (800mm x 500mm x 400mm) and print speed is moderate. Precision of these machines is determined by laser beam width and layer height. Most materials available to be 3D printed today can be used on an SLM machine.
While these machines are groundbreaking, a wide variety of facility and post processing requirements limit these machines to industrial users. SLM machines require trained professionals to operate them. Because of its intricate process, many parts need to be printed and tweaked a few times to yield results. After printing, most parts require significant post processing and heat treatment. In addition, the metal powder that these machines use is both extremely dangerous and expensive to handle: most fully baked SLM machines cost upwards of 1M dollars to implement and a dedicated technician to run.
Electron Beam Melting (EBM)
EBM machines use an electron beam instead of a laser to fabricate parts. GE Additive is the only company producing EBM machines. The electron beam yields a less precise part than SLM, but the process as a whole is faster for larger parts. These machines have almost all of the same constraints, costs, and issues as SLM machines, but are used more heavily in aerospace and medical applications than anywhere else. Similarly to SLM, EBM machines cost upwards of 1M to set up and require a dedicated technician to run.
Direct Energy Deposition
Direct energy deposition uses metal feedstock and a laser to fabricate parts. Unlike powder bed fusion, the stock (which can be powder or wire) and the laser both sit on a single print head that dispenses and fuses material simultaneously. The resultant parts are very similar to Powder Bed Fusion, with a few key differences and opportunities.
Also known as: Laser Material Deposition (LMD), Blown Powder
The sibling of Selective Laser Melting, Direct Energy deposition also uses a laser and metal powder to fabricate metal parts. Instead of spreading powder on a bed and melting it with a laser, DED machines precisely blow powder out of a print head onto a part, using an on-head laser to fuse it to the part in construction.
As both machines use metal powder and a laser, parts printed with DED are very similar to those printed by SLM with one key exception: DED machines can utilize their unique powder distribution system to “heal” non printed parts that have deficiencies. Their available materials, post processing and powder management requirements are analogous to SLM, and machines also cost in the 1M dollar range.
Also known as: Electron Beam Additive Manufacturing, or EBAM
Wire DED machines use a laser to melt feedstock in a very similar manner to their powder DED relatives -- however, their feedstock is metal wire instead of blown powder. It’s a niche technology used with larger build volumes (as large as 5m x 1m x 1m) and faster print times at the expense of precision and quality. As a result, Wire DED parts are designed to be significantly larger and less precise than powder bed machines. These machines cost several million dollars per unit and are extremely uncommon in the space.
Binder Jetting is a large scale, high fidelity method of metal 3D printing that may replace SLM as the premier loose powder based method of 3D printing. The field has exploded from a single manufacturer to a variety of companies (including AM industry leaders) in the last two years. Due to its speed and scalability, it may be the technology that propels metal additive manufacturing capabilities into production volumes.
The technology behind metal binder jetting reflects what a conventional (2D) printer uses to quickly jet ink onto paper. First, a binder jetting machine evenly distributes metal powder over its print bed, forming an unbound layer. Then, a jetting head much like one in a 2D printer distributes binding polymer in the shape of the part cross section, loosely adhering the powder. The process repeats until the machine yields a finished build of completed parts.
Parts printed on Binder Jetting machines require a post processing step called “sintering” to become fully metallic. In this process, the printed part is heated in an oven to just below its melting temperature. The binding material burns away and the metal powder unites into a full metal part. This process can be done in batches, meaning that it doesn’t significantly affect throughput.
Binder Jetting holds two main advantages over Selective Laser Melting. First, machines CAN print much faster by using multiple heads to jet in several places simultaneously. Second, the machine can make tens or even hundreds of the same part in one build. These parts can be sintered in a large furnace to achieve a manageable batch production infrastructure. As a result Binder Jetting is significantly faster on a per part basis than any other type of metal printing. With this speed (and powder management requirements) comes massive costs -- currently, the only machines in this space cost well over a million dollars.
Bound Powder Extrusion
Also known as: Atomic Diffusion Additive Manufacturing, Bound Powder Deposition
Bound Powder Extrusion (BPE) is an exciting newcomer to the metal additive manufacturing space. Unlike almost every other major 3D printing process, BPE machines do not use loose metal powder. Instead, the powder is bound together in waxy polymers in the same way that metal injection molding stock is created. The result is a material that’s much safer and easier to use than loose powder: bound powder extrusion material can be handled by hand and does not require the safety measures that loose powder machines do. BPE filament is extruded out of a nozzle in a manner very similar to standard FFF 3D printing, yielding a “green” part that contains metal powder evenly distributed in waxy polymer. After printing, BPE has two post processing steps: first, the polymer is mostly dissolved in a “wash” machine; second the washed part is sintered in an oven (similar to binder jetting). During the sintering process, the part shrinks to account for the space opened up by the dissolved binder, yielding a fully metallic part.
As a filament based printing process, the part constraints of BPE parts closely mirror those of conventional FFF plastic printing: it works well for almost all part geometries, and can print with open cell infill. Parts printed on BPE systems still often require post-processing -- heat treatment for parts that need advanced properties (though this is required for every metal), and post machining/polishing for enhanced surface finishes -- but there’s no powder management and reduced facility requirements. BPE machines leverage a simpler process to be much more affordable than all other major types of metal 3D printing, with machines costing between $120,000 and $200,000. The Markforged Metal X uses this process -- to learn more about this process, check out this article on the Metal X process.