3D Printing Metal: Where we’ve been and where we’re going

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While much has been written on new developments in metal 3D printing, there have been several types of metal 3D printing systems producing otherwise impossible parts for many years. In order to fully understand where the industry is going, it’s important to first understand where it’s been, and the problems with the status quo.
Below, I’ve provided a brief overview of established and developing printing techniques.  In a follow-up post, I’ll explain how developments in metal 3D printing can impact the medical industry.

Powder Bed Techniques

Selective Laser Sintering Process
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In general terms, powder bed techniques build a metal part by melting one thin layer of powder metal at a time using either high powered lasers or electron beams. Laser-based processes include Direct Metal Laser Sintering (DMLS), also known as Selective Laser Sintering (SLS), and Selective Laser Melting (SLM). Electron beam based processes are generally known as Electron Beam Additive Manufacturing (EBAM).
Powder bed printing is generally performed within a closed inert gas environment or a vacuum chamber. A roller or rake system spreads a thin layer (~50-100 um) of metal powder across a movable platform to form the first layer of the powder bed. A laser or electron beam then melts or sinters select portions of the powder layer into a solid piece according to the design of the part (unfused powder provides support during the printing process and is cleared away from the part once printing is complete). The platform is then lowered slightly for each additional layer; for each layer, a new layer of powder is spread over the fused layer and surrounding powder bed.  
Powder bed fusion uniquely allows for the creation of complex shapes, without the need for supporting structures. The unfused powder in the bed supports all portions of the print, including deep overhangs that can be problematic for other forms of printing.
At the same time, these systems also have several disadvantages. Each print requires a large amount of powder (in order to fill the print bed).  However, the unfused powder can be recycled for later prints. Additionally, the enclosure required to maintain the inert atmosphere around the printer typically occupies a large footprint in a manufacturing space. Lastly, this process is unable to print hollow cavities into parts, given that any cavities would be filled with powder during the print process with no way to empty once the print completes.

Wire melting techniques

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While powder bed fusion involves moving a laser or electron beam across portions of a powder layer to create a shape, wire melting processes move a platform under a stationary heat source (here, a laser, electron beam, or plasma arc). To build each layer, a wire is fed to the heat source and melted into a pool on the platform or previous layers.
This process does not have many of the same downsides that come with powder bed techniques.  Wire melting can create closed, hollow shapes or closed meshes.  Additionally, for laser systems, the size of the controlled environment is much smaller—often just a nozzle to direct the inert gas along the wire feedstock, rather than sealing off the entire print area in an inert gas chamber.
At the same time, however, these techniques come with their own set of problems. Products printed using wire melting techniques often have a lumpy surface finish that may require subsequent machining.  Additionally, given that the laser and wire are stationary, and that there are limited degrees of freedom when moving the part (which, unlike with powder bed techniques, require support structures), this process is more limited in the size and structure of parts that it can print.

Blown Powder Techniques

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Blown powder printers are commonly referred to as Laser Engineered Net Shaping (LENS) or Direct Metal Deposition (DMD).
Blown powder techniques are similar to wire melting techniques, but rather than feeding a wire through the system, the printer uses inert gas to blow a stream of powder into the focal point of the laser, which in turn melts the powder to create a new layer of the part.  The powder nozzle can be mounted directly to the laser housing, resulting in a more compact printhead.
Blown powder shares several advantages with wire melting techniques: the nozzle focuses a smaller volume of inert gas only where it is needed, and the inert gases used to carry the metal powder offer the inherent protection. Additionally, blown powder systems’ more mobile and maneuverable print heads make it possible for these systems to repair or refurbish parts by printing onto existing surfaces. Powder systems can also use multiple powder compositions in a single print and/or mixtures to create high entropy alloys or novel composite systems.

Print and Sinter Techniques

Though it is hardly an official category at this point, the latest addition to metal additive manufacturing is a set of processes that directly print a powder form without melting it into place—effectively making a green body for sintering a solid part.
In general terms, the system prints out an amalgam made of a metal powder held together with a plastic matrix or binding agent—this is similar to how a plastic 3D printer would build a shape. Once the shape is printed, it is moved off the print platform and into either a furnace or a chemical bath to remove the binder. If the binder is removed via chemical bath, the remaining powder part is placed into a furnace to be sintered–a process where the metal powder is heated to a large percentage of its melting temperature, causing it to coalesce into a solid form without melting. If the part is placed into the furnace directly, the binder is burned off and the remaining metal powder is sintered into a metal part in a single step.
Sintering the entire body at one time allows for a more isotropic set of material properties in the part. That is, when you print an object layer by layer, there will be some alignment of material properties to the print direction since the solid surface of previous layers bias the solidification of any subsequent layers. That means the properties might be different in one direction (the print direction) than in another. Because all the layers in a print-and-sinter part are solidified at one time, this directionality is largely erased with print and sinter techniques.
While the specifics of this last technique are still in their developmental stages, there is great promise for print-and-sinter to drive down costs associated with previous forms of metal 3D printing. By removing the costly lasers, electron beams, and environmental controls required for other forms of metal printing, the cost of printers and the cost per print is greatly reduced.
Similarly, breaking out the sintering process allows for the use of more conventional furnace designs, rather than enclosures integrated into the print system, to maintain the environment around printed parts. Breaking out the sintering step could also allow for more efficient parallel processing of printed parts, where several printers create green bodies simultaneously and feed them into a single furnace to be sintered at the same time.
Several startups and expansions of FDM (plastic 3D printing) companies are currently developing techniques to print freestanding green bodies.  However, nobody has made print-and-sinter systems available for large-scale commercial use. It remains to be seen how the quality of items printed through these processes will compare to those made with previously developed metal printing methods as there are lingering questions about their print quality and if the tight geometric tolerances of previous techniques can be maintained through the separate sintering process. However, if such challenges can be overcome, print-and-sinter systems could greatly expand the feasibility and affordability of custom metal parts throughout many industries.

About the Author: 

Dr. Kimiecik specializes in failure analysis, failure prevention, metallurgy, and materials science. He completed his Ph.D. from the University of Michigan in 2015, where he studied microscale deformation of shape-memory alloys. During the course of his research, he helped to perfect several microscale analysis techniques that combined electron microscopy, digital image correlation, and “big data” analysis. Dr. Kimiecik has a particular interest in shape-memory/ superelastic alloys, microstructural development, fractography, small-scale deformation and engineering mechanics issues pertaining to industries including medical devices, transportation, aerospace, and oil and gas, among others.