# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 3 Results and Discussion
### Articles Analysis
#### 3.2.1 Processes and Materials
After a hybrid additive and subtractive manufacturing process, which requires continuous five-axis work due to the complexity of the selected geometry, it is concluded that the deviation does not exceed 10% compared to the predicted geometry. [PERSON] et al. [54] developed a state of the art on the machining of Ti-6 Al-4V parts manufactured additively, which led them to directly address various hybrid additive and subtractive manufacturing processes (DED/PBF-LB / EBM + Turning/Milling). Porosity, residual stresses, surface defects, or anisotropy resulting from the additive subprocesses, and cutting forces, heating during the process, and hardness, resulting from the machining process, are some of the aspects studied. [PERSON] et al. [11] conducted an extensive review of the scientific literature on hybrid additive and subtractive manufacturing oriented toward the production of complex parts. They present a comprehensive summary of the different hybrid flows addressed in the reviewed articles. This allows the authors to approach the process solidly from a technological point of view, considering challenges and opportunities. Beyond overcoming the weaknesses of the independent additive and subtractive subprocesses through their combination, they highlight the versatility of hybrid manufacturing, its ability to reduce the waste of high added-value materials, such as aluminum and titanium alloys used in the aerospace industry, and the possibility of performing the entire process in a single machine. In their work, they identify the issue addressed by many authors concerning the residual stresses caused by the additive subprocess, with the resulting problems of distortion or requirements for heat treatments. The work of [PERSON] et al. [55] develops a novel approach compared to most studies on hybrid additive and subtractive manufacturing. In the hybrid process, the additive manufacturing of ceramic precursors is combined--that is, polymeric materials loaded with ceramic materials--by Blade Coating (2D printing) or DIW (Direct Ink Writing) (3D additive manufacturing), followed by the application of the subtractive subprocess, laser cutting, and ending with the application of the thermal process, pyrolysis, which converts it into ceramic material. This last step is what leads the authors to call it 4D-additive-subtractive manufacturing. In their experimental development, they work with PDMS (Polydimethylsiloxane) materials with ceramic particles of ZrO\({}_{2}\), AlON, or Al\({}_{2}\)O\({}_{3}\). In the study by [PERSON] et al. [56], ER705-6 wire is used to repair S355J2+N steel parts by additive manufacturing WAAM, given the metallurgical compatibility of both materials. The repaired specimens present central cracks caused for the experimental approach. After the material addition process, a subtractive subprocess, CNC milling (pyramid shaping), is applied. Finally, they were subjected to HCF (High-Cycle Fatigue) loading to evaluate the results and draw conclusions. It is demonstrated that the hybrid process reduces the stresses in the crack area and reduces premature detachment. Pyramid shaping milling helps to reduce stresses. The process as a whole limits the tendency of the crack to propagate. [PERSON] et al. [57] analyzed the effect of the machining subprocess, within hybrid additive and subtractive manufacturing, on the stresses generated by the additive manufacturing of 316L stainless steel and, therefore, on the resulting deformation of the part. The experimental approach was carried out using a three-axis LMD machine (Beijing Beiyi Machine Tool Co., Ltd., Beijing, China). The hybrid process was also simulated, considering temperature and stresses, using Finite Element Method through ANSYS Mechanical APDL19.2. Thanks to the simulation, it is confirmed that the direction has a significant influence on the stress distribution in the additively manufactured thin walls. Their milling significantly alters both the values and the distribution of the residual stresses.