What is metal 3D printing?
Metal 3D Printing is a manufacturing technique that allows fabrication of parts by adding material layer-by-layer. For this reason, the term Additive Manufacturing (AM) is commonly used in the industry in contrast to conventional subtractive manufacturing methods.
The feedstock material used in AM is either in powder or in wire form and the general principle is that the material is melted by a focused heat source and solidified into the desired shape.
AM techniques were conceived more than 20 years ago; but, at first, their use was limited to a few applications, like rapid prototyping. With the advancement of the technology, it has been possible to create parts with improved mechanical properties and geometries that are not possible with traditional manufacturing techniques.
The energy and aerospace sectors are already taking advantage of this technology, and applications in the medical, electronics and food industries are also growing every year.
For example, GE Aviation opened a whole division dedicated to AM in 2016, and parts of its turbine engines are already being manufactured with AM . Siemens is also a big player in this field and is already using AM to build gas and steam turbines . Check out the video by GE Aviation:
How does metal 3D printing work?
Despite the different metal 3D printing techniques, they all use the same general approach. The process starts by generating a CAD model of the desired object and using topology optimisation methods and 3D scanning.
This model is virtually sliced to translate its information into coordinates that the focused heat source of the 3D printing machine will follow repeatedly, depositing layers of material.
After the printing process is completed, a post-processing step must be done in order to obtain the finished part. The post-processing can include heat-treatments, machining, surface treatments and the elimination of support structures.
Finally, quality assurance measurements are made including dimension analyses and surface analyses.
Metal 3D printing technologies
Most metal 3D Printing technologies can be classified between powder bed and powder/wire fed systems. In the powder bed systems, the feedstock material used is in powder form; a thin layer of powder is spread in a flat surface and subsequently, certain parts of the surface are melted or joined with the shape of the vertical cross-section of the final part (Fig. 2). This process is repeated as many times as required and in every iteration a layer is created. The most popular processes in this category are Selective Laser Melting (SLM) and Electron Beam Melting (EBM).
It is worth mentioning that there exist some methods where direct melting of the feedstock material is not required, this is the case for the binder jetting metal 3D printing technology. Here, the printing head sprays a binder adhesive on top of the powder where required, the process is repeated in the same way as in the SLM process.
A curing step for solidification is usually performed by introducing the part to an oven, where the binder is decomposed and the metal powder grains are sintered (partially melted together), resulting in a low porosity structure.
The powder/wire fed systems can use either metallic powder or wire as the feedstock material. The difference compared to the powder bed methods is that the material is continuously delivered by the printing head and melted to form the adequate shape. The most popular process in this category is the Direct Metal Deposition (DMD) method.
There exist multiple DMD methods, some of them use a laser as the heat source for melting the material, for instance, the Laser Metal Deposition process. Other methods use an electric arc as the heat source, like for example the Wire Arc AM process.
This technique brings some advantages, mainly because the deposition speed can be higher, but also because of its wide range of uses: it can be used for creating new objects, for repairing or to add a new element on the surface of another object.
However, this technique is vastly more complicated than powder bed systems and nowadays it is mostly under development, with only a few products actually being commercialised.
Main process problems
Regardless of the great potential of the technology, metal 3D printing also has some limitations. Currently, driving down the cost is the main challenge. In this sense, there are several open fronts.
On one hand, increasing 3D printing speed will be important. Nowadays, building relatively small parts (around 1kg) can take a whole day, and bigger parts can take weeks.
Automatisation of the process will also be necessary to make it competitive. Although the AM process was designed from the beginning with a high level of autonomy, there is still a long way to go in software development and integration of the different process steps (CAD model generation, 3D printing and post-processing).
Here, tools like machine learning and digital twins are also expected to play a big role in the future.
Another limitation is to build large parts. The SLM and EBM methods, which are some of the most reliable, are carried out inside a protected chamber, which limits the size of the printed objects. However, the DMD technology is showing great progress in this matter.
Recently, the start-up company MX3D showed the potential of DMD for manufacturing large metal parts by printing an entire bridge:
Nowadays, metal 3D printing is only possible with a selected range of metals and this significantly restricts the range of applications. In general, developing the technology for processing new materials is expensive and takes a lot of time, but it also has huge potential benefits.
For example, the technology for 3D printing of copper is still under development, but when this technology will be ready it could revolutionise the electronics industry.
 “Additive Manufacturing Machines & Materials | GE Additive.” [Online]. [Accessed: 19-Sep-2018].
 “Additive Manufacturing (3D-printing) in the energy sector – Spare parts for gas and steam turbines and other rotating equipment – Siemens Global Website.” [Online]. [Accessed: 19-Sep-2018].
 B. Mueller et al., Added Value in Tooling for Sheet Metal Forming through Additive Manufacturing. 2013.
 Tomas Kellner, “Big Data Meets 3-D Printing: Big Data to Monitor Laser-Printed Jet Engine Parts – GE Reports,” 2013. [Online]. [Accessed: 18-Sep-2018].
 B. Graf, S. Ammer, A. Gumenyuk, and M. Rethmeier, Design of Experiments for Laser Metal Deposition in Maintenance, Repair and Overhaul Applications, vol. 11. 2013.
 “Laser metal deposition (LMD) | TRUMPF.” [Online]. [Accessed: 19-Sep-2018].
 “Home – MX3D.” [Online]. [Accessed: 19-Sep-2018].
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