# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 3 Results and Discussion
### Articles Analysis
#### 3.2.3 Challenges and Outlook  
Parts made by additive manufacturing generally require further processing to achieve the characteristics specified in the design plans. Hybrid manufacturing is very useful as it reduces production time and costs as, in many cases, it avoids the need for post-treatment. Case studies can be found in industries such as aerospace, automotive and medical. With these new hybrid additive-subtractive manufacturing techniques, there is a broad consensus that parts with complex geometries are first produced by additive manufacturing and then the surface finish is improved to meet the specifications of each sector. Moreover, the process can be carried out through several repeated additive-subtractive sequences, so that the improvement in surface quality can reach areas that would be inaccessible once the part is finished [1; 2; 5; 11; 21; 46; 50; 51; 52; 58]. [PERSON] et al. and [PERSON] et al. provide detailed evidence of how hybrid processes improve surface quality and precision in metallic and ceramic contexts [49; 59]; however, other studies support this advantage across different applications. Furthermore, [PERSON] et al. highlight the role of intermediate machining in reducing distortion and enhancing surface uniformity [57], while [PERSON] et al. show that machining WAAM repairs reduces stress concentrators by refining deposited profiles [56]. [PERSON] et al. emphasize the value of adaptive toolpaths in correcting geometric deviations in LMD processes [21], and [PERSON] et al. confirm that finishing improves surface integrity and removes thermal defects [54]. Moreover, [PERSON] et al. [1] point out as the main challenge the development of predictive models for the cutting forces in the milling of parts manufactured by LPBF, given the nonlinearity of this process. The complexity of the interactions between the additive and subtractive subprocesses must be addressed, focusing on grain characteristics and crystallographic structure. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Ref.** & **Q** & **Cit.** & **Year** & **Sector** & **Applications** \\ \hline
[1] & Q1 & 128 & 2021 & Aerospace industry & Aircraft engine parts \\  & & & & Power system components \\
[2] & Q1 & 103 & 2021 & The general manufacturing industry & Molds with CC channels \\  & & & & & Stamping dies with CC channels \\
[40] & Q1 & 57 & 2020 & Aerospace industry & Titanium alloy parts \\
[44] & Q1 & 38 & 2021 & Aerospace industry & Manufacturing of monolithic structures \\
[45] & Q1 & 38 & 2021 & Aerospace industry & Manufacturing and repair of high \\  & & & & & added-value parts \\ \hline \end{tabular}
\end{table} 
Table 8: Sectors and applications. Five most cited articles. Hybrid manufacturing systems entail substantial initial investment costs, primarily due to the need for specialized equipment and highly skilled operators [21; 47]. Nevertheless, opinions are divided regarding their overall economic viability. [PERSON] et al. posit that the utilization of hybrid additive and subtractive manufacturing methodologies can yield cost-effectiveness in high-value applications, such as aerospace Maintenance, Repair, and Overhaul (MRO), particularly when evaluated through activity-based costing models that account for lifecycle benefits [58]. Conversely, alternative studies posit that, in the absence of such niche contexts, the elevated per-unit cost and diminished scalability render hybrid manufacturing less viable for small to medium production volumes [21; 47]. Furthermore, [PERSON] et al. [2] outline a path of work and evolution by highlighting that one of the main current challenges is the complexity and high cost of hybrid manufacturing machines. The future of the mold industry with conformal cooling channels is promising, but it requires the development of more economically affordable solutions. They also anticipate that the manufacturing of CC channels by hybrid additive and subtractive manufacturing will expand to gas turbine engines, gears, and other systems and parts with special self-cooling requirements.