# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 3 Results and Discussion
### Articles Analysis
#### 3.2.1 Processes and Materials
It is concluded that the application of the analysis method and the selection of the most favorable combination of parameters allow for the reduction of surface roughness to 9.0 \(\upmu\)m (40% compared to the values obtained with the manufacturer's recommendations) and the roughness of contour surfaces to 7.5 \(\upmu\)m (37.5%), which confirms the usefulness of the Taguchi method [4]. In the study by [PERSON] et al. [47], hybrid additive and subtractive manufacturing is addressed from the perspective of a hybrid process chain. This is done by considering the repair of a gas turbine blade, manufactured in 316L stainless steel. The research develops a hybrid process planning methodology aimed at improving the life-to-value ratio. The method is based on simulation and experimentation. Although it is concluded that the results in terms of surface roughness are similar, it is emphasized that the selected process chain does affect different KPIs (Key Performance Indicators), such as tool wear or total manufacturing time. [PERSON] et al. [48] address in their study hybrid additive and subtractive manufacturing, LMD and cryogenic milling. They justify it on the basis that the latter leaves surfaces clean of coolants that could affect the subsequent additive process within the hybrid process. The experimental part was carried out with cryogenic CO\({}_{2}\) milling of Ti-6 Al-4V parts. The influence on surface integrity and tool wear was evaluated. The study alternated hybrid and subtractive sequences, AM-SM-AM (additive manufacturing-subtractive manufacturing-additive manufacturing). The additive subprocess was carried out with an ABB IRB 4400 multi-axis robot and the subtractive one with an ABB IRB 6660 robot, integrated into the same station. [PERSON] et al. [49] seek with their study the reduction of the staircase effect and the improvement of surface quality in parts manufactured through hybrid additive and subtractive manufacturing, FDM (Fused Deposition Modeling) and dry milling. They use polymeric materials, PEEK (Polyetheretherketone) and CF/PEEK (Carbon Fiber Reinforced Polyetheretherketone), for their work, in which they evaluate the relationship between raster angle and layer thickness and spindle speed, depth of cut, and feed rate per tooth. Dry milling is used, considering the surface requirements of molds and medical implants. The study demonstrates that the dry milling subprocess can substantially improve the surface quality of the parts. [PERSON] et al. [50] configured a robotic WAAM system, with the possibility of adaptation to hybrid additive and subtractive manufacturing of large-sized parts in AlSi12. They emphasize the monitoring system to ensure compliance with quality requirements and aim for the industrialization of the system. The research covers the entire process, from design and process planning to WAAM deposition and precision milling. Everything is configured and modeled beforehand with Siemens NX. It is confirmed that the best results are obtained with a tilt angle of 0\({}^{\circ}\) and a lead angle of 0\({}^{\circ}\), \(-5^{\circ}\), and \(-15^{\circ}\). A digital twin of the manufactured part will be the basis for future adaptations of the process. The review by [PERSON] et al. [5] on hybrid additive and subtractive manufacturing concludes that the complexity of the parts, as well as of the manufacturing strategies and toolpaths, is an inherent part of the process. For this reason, the crucial importance of CAM software and the CAM strategies developed to date by different industries is analyzed and highlighted for a successful application. However, the quality of the result is linked not only to CAM but also to the associated processes, mentioning DED and milling, the materials, applied heat treatments, intricate geometries, or the level of the machine tools used. As [PERSON] et al.