# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 4 Conclusions  
This paper presents a systematic review of the emerging trends in hybrid additive-subtractive manufacturing that have emerged in the last 5 years. After an introductory review of the origin and evolution of hybrid additive and subtractive manufacturing, and establishing the concept of this as a new manufacturing paradigm, in which additive and subtractive manufacturing subprocesses are integrated to overcome limitations, leverage strengths, and generate new synergies, a systematic review was carried out, based on PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses), that allowed the selection of some experimental and review articles collected in specialized journals (classified as Q1 and Q2 within the ranking of their respective categories according to the Journal Citation Report), and peer-reviewed conference proceedings of recognized prestige, published in English, and open access. The selection was made from the SCI (Science Citation Index Expanded) and the CPCI-S (Conference Proceedings Citation Index-Science) databases of the Web of Science (WoS) Core Collection. After selecting the works, classification of the studies was carried out and, through detailed reading, the corresponding analysis and discussion of the findings was presented. The main conclusions are presented below. * The concept is generally established, having moved beyond the initial debates, as the lack of integration of CAM/CAx tools for several additive and subtractive manufacturing subprocesses, alternated with each other, interacting among them and within the same machine, to produce a different and superior result compared to what could be obtained through the mere combination of processes along a supply manufacturing chain. * The most studied manufacturing processes, both from an experimental and theoretical point of view and through simulations, are DED (direct energy deposition) in the case \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Q** & **Cit.** & **Year** & **Future Works** \\ \hline
[1] & Q1 & 128 & 2021 & Development of predictive models for the cutting forces \\
[2] & Q1 & 103 & 2021 & Development of affordable hybrid machines \\
[40] & Q1 & 57 & 2020 & Improvement of milling strategies \\
[41] & Q1 & 54 & 2020 & Development of process decision-making tools. \\
[3] & Q1 & 46 & 2022 & Development of simulation software \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Future works. Five most cited articles. of additive subprocesses and milling for the machining subprocesses integrated into hybrid additive and subtractive manufacturing flows. * The leading sector for the application and development of hybrid additive and subtractive manufacturing throughout the majority of the reviewed articles is the aerospace sector, followed by the sector of stamping dies and injection molds with conformal cooling channels. The biomedical and automotive sectors are mentioned much less frequently. * The most commonly used materials in both theoretical and practical or experimental developments are titanium and nickel alloys. To a lesser extent, some studies address stainless steels. * A lack of studies on standardization and certification processes is confirmed, as well as on approval by various national and international public agencies--for example, in the aerospace sector--of parts for real application. * From the analysis of the selected works, it is deduced that hybrid additive and subtractive manufacturing must mature, and for this, it is essential to significantly increase knowledge of the additive and subtractive subprocesses involved, both individually and in terms of their interaction and their application to materials such as the superalloys used in the aerospace field. * Researchers must identify or define, in the near future, hybrid parameters that make sense within the new concept of hybrid additive and subtractive manufacturing, rather than merely combining and studying interactions between traditional parameters of the integrated additive and subtractive processes.