# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 3 Results and Discussion
### Articles Analysis
#### 3.2.1 Processes and Materials
The milling force changes tensile stress into compressive stress, and its extended gradient distribution transforms into regional distribution. Finally, the result of the experimental work confirms the conclusions of the simulation. [PERSON] et al. [58] developed a theoretical cost model of hybrid additive and subtractive manufacturing, as a tool for the MRO (Maintenance, Repair, and Overhaul) sector,especially focused on the aerospace sector. The study is centered on the manufacturing of parts. Their intention is to integrate in the model the benefit of cost reduction. In the work, analytical techniques are applied to help the aerospace sector increase its competitiveness. Although the process is not addressed from a technical point of view, it is described clearly and precisely: additive and subtractive subprocesses are alternated in the same machine, where the part can remain fixed, to achieve free and complex geometries with the high quality required by the sector. The difficulty lies in identifying the costs of the different phases of the hybrid process, understanding its complexity. This will allow the cost-benefit balance to be adjusted, optimizing the different variables. The authors apply their model to the case of an aerospace bracket manufactured in Titanium 6 Al-4V. The work of [PERSON] et al. [59] is novel, as it designs a series of rules for a hybrid additive and subtractive manufacturing process, SLM and high-speed micromilling. The intention of the set of rules developed is to serve as guidance during the design phase. To define them, different elements recognizable in any manufacturable geometry are analyzed: cylinders, overhangs, chamfers, radii, holes, etc., both from the additive point of view and from the hybrid additive and subtractive perspective. The designed rules are applied in the manufacturing of different parts in Managing steel 1.2709 on the hybrid machine Lumex Avance-25 (Matsuura Machinery GmbH, Wiesbaden, Germany), using SLM and high-speed micromilling. The injection mold with conformal cooling stands out, as it perfectly synthesizes the concept of hybrid additive and subtractive manufacturing. The authors consider the advantages of applying the established rules in hybrid manufacturing to be demonstrated. Figure 9 shows an example of the application of hybrid additive and subtractive manufacturing in the production of molds with conformal cooling (CC) channels. Table 7 provides a summary of the five most cited articles that address a specific hybrid manufacturing flow, breaking down the main additive and subtractive subprocesses and indicating the work material. As it can be seen, although the additive subprocesses are varied, it is clear that the subtractive subprocess generally integrated into the different hybrid flows is milling. In turn, the materials column clearly synthesizes the materials on which hybrid additive and subtractive manufacturing is focused. Figure 9: Injection mold with conformal cooling channels [59]. A synthesis of the reviewed studies allows us to conclude that, in order to achieve parts manufactured by hybrid additive and subtractive manufacturing with adequate physical and mechanical properties, it is essential to overcome some challenges, among which the following are worth highlighting: [1; 5; 54; 55; 56; 11; 57; 5; 14; 40; 42; 11; 5; 5; 15]  
Controlling grain orientation and phase formation so as to improve the microstructure of the material, as this directly influences the mechanical properties. Optimizing the process parameters to minimize porosity, as this can reduce the mechanical strength and durability of the parts. Controlling process conditions such as cooling rate and material composition to ensure adequate mechanical properties such as tensile strength and hardness. Addressing these challenges comprehensively is key to achieving high quality parts.