The fastest growing segment of the 3D printing/additive manufacturing is metal 3D printing. In just the last decade—and in particular the last few years—the technology has matured, and applications have evolved from prototyping to production for a number of high-profile applications. The aerospace and medical industries have been on the vanguard of this evolution, but their success, further advancements from market incumbents, and new technologies suggest opportunities for metal 3D printing will expand rapidly. Metal 3D printing for electronics, although still a relatively small piece of the overall market, may soon be a growing part of this wave.
While most of the headlines have been grabbed by “Powder Bed Fusion” technologies, there are in fact a wide range of ways to achieve metal 3D-printed parts, with more on the way. For example, printed circuitry is in the early stages of commercialization. So is metal extrusion printing, which bears similarities to the desktop plastic machines (think “Makerbot”) that have become most popular in the consumer market. For the purposes of this article, we’ll focus on those technologies that are more fully commercialized.
Powder Bed Fusion—Laser Melting
In powder bed fusion via laser melting, layers of powdered metal are selectively melted by a laser or electron beam. This process offers significant design freedom and has a global accuracy of .005 inches on the first inch and .002 inches for additional inches. Powder bed fusion is available for aluminum, stainless steel, Inconel, titanium, cobalt chrome, and tool steel. Tighter tolerances can be achieved through build orientation and post-process finishing. A few limitations to powder bed fusion via laser melting are that it requires supports due to residual stress and has a roughness average (Ra) of 250-400. It is also relatively expensive. Powder bed fusion via laser melting is best for applications involving functional prototyping and end use parts that are highly complex and have small features (Figure 1).
Figure 1: This heat exchanger printed with a laser powder bed process shows the sorts of complex, organic, yet highly accurate shapes achievable through a laser powder bed process.
Powder Bed Fusion—Electron Beam Melting
This is another powder bed fusion process, however it utilizes an electron beam rather than a laser, and is done in a vacuum environment at higher temperatures than laser powder bed. This process has less residual stress and fewer support requirements compared to the laser powder bed fusion. Like laser powder bed, it allows for significant design freedom and strength approaching forged parts. EBM parts have a surface finish that is a good deal rougher (Ra 600-800) and is also a bit less accurate (typical). It is fairly expensive, but less expensive than laser melting for bulkier parts in most applications. EBM also offers a more limited set of materials, with titanium, cobalt chrome, and Inconel as commercially available options. This approach is best for complex parts that don’t require extremely tight tolerances.
Figure 2: This sample part printed by an EBM process shows both the complexity achievable, but also the rough surface finish.
Infiltrated Binder Jetting
Infiltrated binder jetting is a process in which a binding agent holds a scaffold matrix of metal powder together. The parts are then placed into an oven where the metal is sintered and molten metal infiltrates the glued-together scaffold matrix, much like how a sponge sucks up water. This process features some of the design freedoms offered by powder bed fusion technologies at a significantly lower price point. This is because much of the metal melting is done using conventional metal stock (not powder) and a furnace, two input costs that are less expensive than powdered metal, and a high-powered laser or electron beam.
The process cannot deliver the same feature accuracy as direct metal printing. Also, the mixed material approach isn’t ideal for certain applications (e.g., aerospace or medical). It also offers an as-printed surface finish that is fairly rough (Ra 600), requiring post-processing in many circumstances. Ideal applications include non-precious jewelry and next-generation castings that are currently made of iron, bronze, or steel that could benefit from greater complexity.
Directed Energy Deposition
In directed energy deposition (DED), powdered metal is jetted into a melt pool created by a focused laser. The melting takes place outside the print head, with a focused laser melting powdered metal material jetted in front of it. A key strength of this process is its rapid powder deposition rate and accuracy, which makes DED an option for larger parts. Additional strengths include the ability to build on existing structures and a relatively wide range of materials. This process is not known to deliver tolerances on par with powder bed or binder jetting and its surface finish is the roughest of all the processes mentioned. As a result, its primary applications are large, complex parts that might otherwise be cast.
Figure 3: The teeth of this rack and pinion were printed directly onto a stock ring and plate, a unique capability relative to other metal printing technologies.
Wax Print-to-Cast
With wax print-to-cast, a wax or high-temperature resin is jetted to create a “pattern” that is used to create a silicone mold for casting in a variety of metals, especially precious metals like silver, gold, bronze, brass, and copper. In this way, wax print-to-cast is oftentimes very cost-effective for jewelry production. It also provides significant material breadth, as most any metal can be melted and cast. However, the multi-step nature of wax to cast printing makes it more ideal for multi-unit printing rather than single-unit production. Additionally, maximum print size is fairly small relative to other processes—typically under 6” cubed.
Sand Print-to-Cast
Sharing a number of similarities to wax print-to-cast, sand print-to-cast involves printing a pattern, but in this case the pattern is created when adhesive is sprayed onto sand particles to create a shape. In this way, the printer can either create the “cores” that will go into a sand mold, or print the mold itself. Either way, once the sand mold (sometimes referred to as a sand casting) is built, molten metal can be poured into it and cooled to create a new part in the mold’s void. From there, the sand mold can be broken with a hammer, leaving the finished part inside. This process offers a faster timeline to casting fairly large, complex parts. Ideal applications include complex production castings (complex cores, engine blocks).
Print-to-Plating
With print-to-plating, parts are made by plating printed resin or plastic parts with metal, using an electrolytic or electroless nickel-copper plating of plastic or resin parts. This process lends a nice metal finish to parts, which can provide benefits for strength, aesthetics, or electromagnetic interference shielding. Depending on the part geometry, this approach also can provide cost advantages relative to other metal printing technologies. With that said, this process is time consuming and does not provide the incremental strength to be considered a true substitute for a metal part.
Conclusion
Metal 3D printing is emerging as a viable solution for an expanding list of applications. Between continued advancement of the well-commercialized technologies presented here and the growing list of emerging technologies, you can expect more metal 3D printing headlines in the near future.
Filed Under: 3D printing • additive manufacturing • stereolithography, Industrial automation
