# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 1 Introduction  
It is now generally accepted by the research community that the technical origin of additive and subtractive manufacturing lies in the search for synergy generated by the development of a hybrid flow that alternates several additive and subtractive manufacturing subprocesses, in order to reduce the limitations and take advantage of the strengths of both types of manufacturing [1, 2, 3, 4, 5], but also to generate capabilities arising from the new paradigm [6]. Although the maturity of additive manufacturing is much lower, given its shorter evolution time, since its first applications it has demonstrated its capacity to produce parts of complex geometry with different types of materials, including metallic ones [7]. Today there are many research groups and works presented in publications and conferences of the highest level on additive manufacturing, and even applied to sectors as cutting-edge and demanding as aerospace or on materials such as superalloys used in these sectors. For its part, research on the various subtractive manufacturing processes is much more abundant. Its considerable maturity, with a much longer evolutionary process, means thatits capabilities and characteristics are much better understood: geometric and dimensional precision, surface quality, and integrity are some of its strengths. However, in recent decades, research has proliferated on its application to new materials and alloys, process planning, and measurement and control systems. As for the hybrid concept, [PERSON] et al. [8] already established an important conceptual requirement, such that the subprocesses must interact with each other and affect the workpiece directly to produce effects different from those that would occur with the technologies involved applied independently. [PERSON] et al. [9] start from the premise that hybrid manufacturing is produced from the same machine and is also moving toward reaching a single CAD/CAE/CAM (computer-aided design/computer-aided engineering/computer-aided manufacturing) environment at the software tools level. In turn, while in the early years of the concept the debate on this point was greater [10] and some authors contextualized it in the field of production systems, as a mere combination of different manufacturing technologies along a manufacturing or supply chain, currently researchers focus their work almost exclusively on hybrid flows carried out on the same machine [11]. It is worth noting that at practical levels, given the complexity of the process and the high cost that hybrid machines still have, research work is often forced to propose a theoretical process on a single machine and, due to these limitations, to deconstruct it and execute it in parts on different machines. In this evolution of the concept and research work, we finally reached the point where, with few exceptions in recent scientific literature, it was established that the hybrid flow must be produced on a single machine, and that the result must be not only different but superior to that obtained using, for example, a hybrid manufacturing chain developed on different machines or even facilities [6]. Thus, with these foundations in place, lines of work are beginning to emerge, both in industry and in university and research institutions, focusing on different aspects of hybrid additive and subtractive manufacturing. The need, for example, to reduce material waste is particularly relevant in industries such as aerospace, where complex, high-value parts manufactured using subtractive manufacturing with high-cost superalloys are abundant [12]. The concept of hybrid additive and subtractive manufacturing is also generally related to the additive production of near net shape parts [3], whereby the subtractive subprocesses work in a finishing machining context. In addition, other concepts frequently appear linked to hybrid additive and subtractive manufacturing.