Although additive and subtractive manufacturing processes seem to be on quite the opposite ground, in fact, as these technologies develop, the benefits of hybrid manufacturing are becoming increasingly evident. Hybrid manufacturing systems, equipped with additive and subtractive technology simultaneously, could represent a great technological advance for the industry. These two technologies can be seen as complementary and able to take advantage of the advantages of each.
The joint use of additive and subtractive technologies is not a new concept. For example, post-processing of metal parts made by 3D printing often involves CNC machining to give them mechanical precision and a better surface finish. However, there is another way to combine these two processes, resulting in what we call hybrid manufacturing.
Hybrid manufacturing consists of joining an additive and a subtractive technology in the same machine with the aim of taking advantage of the advantages of each of these technologies: the geometric complexity of the additive processes with the high precision of the subtractive processes. In this way, it is possible to obtain an additive and machined part in one step, speeding up the entire production process. Naturally, the design of a hybrid part must take into account the requirements of each of these technologies.
Applications of hybrid systems can be varied as manufacturing small series of metal parts or repairing damaged or worn parts.
Directed energy deposition
Directed energy deposition (DED) is one of the additive processes that can be used in hybrid technologies. This process consists of fusing the material with a laser or electron beam as it is deposited through a nozzle on the construction platform. The deposited material can then be machined to obtain a better surface finish and greater mechanical precision. Alternatively, it is also possible to add material over a machined part to increase the geometric complexity of the part.
Another of the advantages of the DED additive process has to be the fact that it is possible to use a 5-axis system in the print head, enabling the printing of geometrically complex areas without the need to place support structures.
DED is suitable for the production of large parts and repair of mechanical components, and these two characteristics are quite advantageous when we compare this process with laser metal powder melting. On the other hand, the fusion of metallic powder allows obtaining parts with better finishing and dimensional accuracy.
It was with the objective of making a hybrid part that we started this project, using a CNC machined part to which we added a castle by the laser metal powder fusion process. We can’t call it hybrid manufacturing, but hybrid printing, since we used two different equipment, a CNC milling machine and the Eplus3D EP-M150 3D printer, to make this part.
Another aspect that we wanted to study with this experiment was the bonding capacity between two steels with different chemical compositions. We used a 1.1730 steel construction for the CNC machined part and the hybrid castle was printed with metallic stainless steel 1.4404 (316L) powder.
The project started with the 3D modeling of the part and the study of its attachment to the construction wafer of the 3D printer. We opted for a simple geometry of a parallelepiped with an inner box, to which an additive castle was added, that is, a part that would have to be made using 3D printing. Inside, this castle is made up of a series of thin 1 mm-thick blades which, despite being a simple geometry, would be very difficult to be executed in any other way.
Another important aspect of this study was how to tighten the hybrid parts to the construction wafer. On the one hand, the printer does not have a parts straightening system, as happens in numerical control machines and, on the other hand, it would be difficult to add a tightening system that would not interfere with the printer’s operating dynamics. Furthermore, the construction wafer is fixed to the printer by four screws, which does not guarantee its performance.
It was decided to make several holes in the wafer thinking about the various possibilities of fixation in future hybrid prints.
The construction wafer (and all of its holes) is symmetrical about its central axis. To avoid positioning errors during its tightening on the printer, an arrow indicating its position in relation to the recoater was added, that it, in relation to the mobile mechanism that transports the metallic powder from the feed cylinder onto the construction wafer.
Once the printing is finished, it is necessary to remove the metallic powder that was not cast. Only after removing this powder can we remove the construction wafer from the printer. The unfused powder is sieved and reused for future printing. After removing all the metallic powder, the construction wafer is removed from the machine.
The first hybrid printing project was successful. We can say that the project had about 70% of preparation and 30% of execution, being the study phase of fixing the pieces to the printing wafer that gave us the most work to define.
Although the laser metal powder melting process is not as versatile as the DED process, we can prove that it is possible to combine CNC milling and laser metal powder melting successfully.
However, it is important to note that we have some limitations here to be taken into account. Hybrid bonnets must grow from a perfectly horizontal and rectified plane to ensure that the bonnets are welded to their base. It would not be possible to carry out this experiment if the printing plane were a surface with a certain three-dimensional profile.
It is also critical to ensure that there are no parts that are above the print plane so that they do not interfere with recoater movement. In other words, if we want to hybrid print a part with more complex geometry, we have to think about how to ensure the connection between parts without interfering with the printer’s dynamics.
It was also possible to demonstrate that it is possible to make a hybrid impression using similar metals, but with different chemical compositions.