Hybridizing AM with Machining

Hybrid manufacturing

Growing demands from consumers has generated increasingly stringent requirements on manufacturing process. Reducing energy consumption and the environmental impact while improving productivity, is becoming more and more important. Existing manufacturing processes (additive and subtractive ones) suffer from drawbacks that cannot be completely eliminated due to technological constraints. Machining is restricted by the tool size which limits the shape and features sizes that can be machined. Additive manufacturing on the other hand leads to poor surface quality due to the “stair-case” effect while the mechanical properties of parts usually do not compete with those obtained using conventional means.

Combining two or more manufacturing processes to form a hybrid manufacturing process is a clever way to overcome these limitations.  The idea is to exploit synergistically the advantages of each process separately to create parts which cannot be obtained using the individual processes.

The International Academy for Production Engineering (CIRP) has suggested an open and a narrow definition of Hybrid Manufacturing [1]:

Open definition: a hybrid manufacturing process combines two or more manufacturing processes into a new combined set-up whereby the advantages of each discrete process can be exploited synergistically

Narrow definition:  a hybrid manufacturing process comprises a simultaneous acting of different processing principles on the same zone.

A classification of the various hybrid manufacturing processes is depicted in fig. 1.

Figure 1. Hybrid Manufacturing Classification

Hybridization is still a recent research topic which has been mostly investigated for polymers and metals but is still scarce for ceramics. Below some examples of promising hybrid processes are reviewed and their applicability to ceramics discussed.

Additive / subtractive hybridization

Fused deposition modeling of ceramics or robocasting are cheap and versatile technologies to print 3D parts in ceramics. Unfortunately the finish surface remains poor which strongly hinders the industrial use of these parts. To alleviate this drawback, Stanford University and Carnegie Mellon University reported a hybrid system called shape deposition manufacturing (SDM) as shown fig 2a, which combines additive /subtractive process [2]. Seoul National University also developed a similar approach to SDM where deposition and micro-machining are carried out in a single machine to solve the problems of referencing (figure 2b). The principle was demonstrated with polymers, ceramics like hydroxyapatite as well as nanocomposite polymer-based materials. The error in the fabricated parts using hybridization was 0.17% which is incredibly better than the 23.32% for parts obtained using deposition only.

     
Figure 2. (a) Process cycle of SDM combining additive and subtractive process exemple of a printed part smoothed through post-machining (from [2])

Another example of additive/subtractive technology is the combination of conventional powder bed approaches with machining. Parts obtained using powder bed additive manufacturing (Electron Beam Melting – EBM, Laser Beam Melting – LBM, binder jetting) suffer from a poor finish surface. Finishing the part after building completion is a highly challenging task especially for complex designs involving internal structures. Hybridization of powder bed systems with milling aims at circumventing this limitation. The idea is to mill the outer surface of the part when not after all layers have been completed but during build after each preset number of layers.  Using this approach, internal structures can be smoothed out and the dimensional accuracy of parts become comparable to machining centers.

This approach has been successfully applied to metals and some hybrid systems combining LBM and spindle milling are already available on the market [3]. The principle is shown in figure 3.

Figure 3. Principle of LBM – Milling hybridization (from [3]) 

Instead of using a spindle to refine the outer of each printed layer, a laser is also a highly suitable choice. Because of the fineness of the laser spot (typically less than 50 µm) and the fact that laser milling is a non-contact operation, the powder bed is only marginally affected during the machining operation (on the contrary to mechanical milling). In this case, the laser operates in a pulsed mode and material removal is obtained through ablation. Since binder jetting is capable of printing a variety of materials, this technology is also applicable to ceramics. Some machines are already available on the market (figure 4) and their relevance to ceramics is currently under investigation [4].

References

[1] Z. Zhu, V. Dhokia, A. Nassehi, and S. Newman, “A review of hybrid manufacturing processes – state of the art and future perspectives,” International Journal of Computer Integrated Manufacturing, vol. 26, no. 7, pp. 596–615, 2013

[2] W.-s. Chu, C.-s. Kim, H.-t. Lee, J.-o. Choi, J.-i. Park, J.-H. Song, K.-H. Jang, and S.-H. Ahn, “Hybrid manufacturing in micro/nano scale: A Review,” International Journal of Precision Engineering and Manufacturing-Green Technology, vol. 1, no. 1, pp. 75–92, 2014

[3] http://www.lumex-matsuura.com/english/contents/lumex03.html

[4] https://optec-laser-systems.com/