Additive manufacturing(AM) or 3D-printing should not be thought as single manufacturing method, but as a group of different manufacturing methods, where parts are manufactured eg. by growing-, adding-, or joining material selectively. There are multiple different AM methods. Some of them are only slightly different from eachother while others have nothing in common.
Additive manufacturing standards EN ISO/ASTM 52900:2017 ja SFS-EN ISO/ASTM 52900:2017 ”Additive manufacturing -- General principles – Terminology” categorize AM processes to seven categories:
- Material Extrusion
- Material Jetting
- Binder Jetting
- Sheet Lamination
- VAT Photopolymerization
- Powder Bed Fusion
- Directed Energy Deposition
All of the AM methods do not fit to the categories defined on the standards, or can fit to multiple categories at the same time. One of the reasons behind this is the rapid developing of the AM-processes. The rapid developing of the method is affected by the fact, that many traditional manufacturing methods can be used to manufacture parts additively, either straight up, or as slightly modified processes. Of course only after the idea of "AM" has started to spread people have started to think the prosesses as Additive rather than just plain ordinary manufacturing processes.
In addition to that, by combining different AM processes or material reducing manufacturing processes with each other many benefits that are not possible with any single process can be realized. These hybrid prosesses do not really fit into any ISO/ASTM standard categories.
One example of processes that do not fit to the standard categories is Cold Spray- process, where metal powders are selectively positioned to the part surface by spraying them with high speeds (Mach 4) at temperatures below the material melting point. This method is being developed among others by GE (http://www.geglobalresearch.com/blog/cold-spray-ge-technology).