# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 3 Results and Discussion
### Articles Analysis
#### 3.2.1 Processes and Materials
The study led to the conclusion that hybrid manufacturing, in which the machine process is not interrupted, reduces total cycle time by 68%. It also reduced the average relative porosity by 83% and the relative elongation at failure by 71%, compared to the additively manufactured part. Hybridization does not affect the crystalline structure. With these results, the future of hybrid additive and subtractive manufacturing, although it must still mature, is promising. The study by [PERSON] et al. [43] proposes a topology optimization (TO) framework with the objective of automating the design of complex parts manufacturable on five-axis machine tools. For this purpose, multi-axis machining constraints are established, with the consequent reduction of time and material in trial-and-error processes, which are inevitable due to the mismatch between design and manufacturing models. Although the study focuses on the machining process, the author directs it toward hybrid manufacturing, first by including the concept \"hybrid manufacturing\" among its keywords, and then, in the conclusions, by pointing out as future work the extension of the proposed method to hybrid additive and subtractive manufacturing. Although briefly, it clearly defines what the hybrid process should be, alternating additive and subtractive subprocesses. To extend the method, the defined concept of inaccessibility measure field (IMF) must be applied to the additive subprocess, both in terms of accessibility and in cases of over-deposition or under-cut. Clearly, the study is focused on a context of five-axis CNC manufacturing, in which hybrid flows are included. [PERSON] et al. [44] address in an experimental study the influence of the residual stress (RS) produced by additive manufacturing on the stresses and distortion induced by the subsequent machining of these parts. To carry out the study, the DED manufacturing of thin walls is simulated by finite element software, followed by a high-speed milling phase. The material considered in the simulations is Inconel 625. In the simulations, one hypothesis is used in which the residual stresses generated by additive manufacturing are disregarded, and another hypothesis in which the stresses produced by the DED process are incorporated. The work confirms the importance of the residual stresses generated by the rapid heating and cooling cycles of DED on the stresses and distortion produced by subsequent milling. It also concludes that the design of the machining strategy is crucial in the effect caused by high-speed machining of DED parts. [PERSON] et al. [45] carry out an investigation on the wear processes of turning tools for parts made of Ni alloys, specifically IN718, manufactured by DED. In the experiments, additive and subtractive subprocesses are also combined with heat treatments to evaluate their effect. Adhesive and abrasive wear are identified as the main types of tool wear. In the case of the as-deposited material, the wear is due to the low surface quality, although no tool failures occur. When heat treatment is applied to the DED part, although the machinability is similar to that of the forged material, high-speed turning causes rapid tool failure, and surface quality depends to a greater extent on the cutting parameters. Working with a hybrid machine (Matsuura LUMEX Avance-25, Fukui, Japan), [PERSON] et al. [4] manufacture managing steel 1.2709 parts in a hybrid way, using SLM and five-axis milling. Subsequently, they carry out a statistical optimization study of surface roughness. For this purpose, they rely on the Taguchi method. The study was carried out distinguishing between contour surfaces and hatch, or filling surfaces, and varying the laser power, the scanning speed, and hatch distance. Energy input and energy density were also considered. In milling, variations of feed per tooth, cutting speed, and radial depth were considered.