# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 3 Results and Discussion
### Articles Analysis
#### 3.2.1 Processes and Materials
[51] recognize the great potential of metal additive manufacturing, in this case PBF-LB/M (Laser-Based Powder Bed Fusion of Metals), they investigated its combination with subtractive manufacturing, integrated in a hybrid manufacturing flow, to overcome the porosity of the additively produced material and improve its surface roughness, factors that reduce its corrosion and fatigue resistance. The study is specifically applied to the effect on the fatigue resistance of AISI 316L produced through hybrid additive and subtractive manufacturing. It also compares it with the same parameters of wrought 316L. The study confirms that the as-built material has low fatigue strength, while the machining of the additively manufactured part significantly increases its fatigue strength (up to 87%). Since most of the major defects are found in the 250 \(\upmu\)m closest to the surface, this allows for improvement in surface quality and integrity. Among the routes analyzed by [PERSON] et al. [51] are the following hybrid routes: PBF-LB/M + CNC Milling; PBF-LB/M + HIP (Hot Isostatic Pressing) + CNC Milling; PBF-LB/M + Annealing + CNC Milling. These are compared with conventional routes such as: Wrought (316L) + Heat Treatment + CNC Milling; PBF-LB/M (as-built); PBF-LB/M + Annealing; PBF-LB/M + HIP. [PERSON] et al. [52] conducted an experimental study in which they analyze various manufacturing sequences, combining and alternating casting, milling, and additive manufacturing DMD, to evaluate the interface strength and the resulting mechanical properties. Among the studied flows, the following fit hybrid additive and subtractive manufacturing: Casting + CNC milling + DMD; Casting + heat treatment + CNC milling + DMD. The authors conclude that the hybrid sequences are suitable for the manufacturing and repair of Inconel 718 parts. When comparing them, it is observed that the first, without heat treatment, presents a smoother transition, a better interface strength, and less thermal affectation. In contrast, the route with heat treatment before machining presents worse mechanical and interface behavior due to the precipitation of undesired phases (such as 8) and heterogeneous microstructures. The work of [PERSON] et al. [53] is oriented toward hybrid additive and subtractive manufacturing with materials that require the use of sacrificial support structures. For this reason, they propose a systematic methodology of topology optimization (TO) that allows removal while overcoming accessibility problems. Their approach is validated through the simulation of the manufacturing of several parts, in Ti-6A1-4V, from the aerospace, robotics, and automotive sectors: GE bracket, +Z (Powder Bed Fusion + 3-axis CNC machining); quad-copter (Powder Bed Fusion + 14-orientation CNC milling); upright example (Powder Bed Fusion + Multi-tool CNC milling with vise fixture). The authors conclude that the approach ensures the successful removal of the support structures from the design phase. To achieve this, they apply the idea of the 'inaccessibility measure field' (IMF) to the near net shape parts produced in the additive phase of hybrid additive and subtractive manufacturing. [PERSON] et al. [21] developed a hybrid additive and subtractive manufacturing methodology for high added-value parts, within a single CAM environment. In the work, an API (Application Programming Interface) is developed for the additive manufacturing module, with options for Planar LMD and three-axis and five-axis LMD. A methodology for predicting the shape manufactured by LMD is also developed, for the subsequent milling process. To validate the methodology, a blisk made of Hastelloy(r)X over Inconel(r)18 is selected for manufacturing. Work is carried out in the NX(tm) environment from Siemens. Milling is then applied to provide the geometric, dimensional, and surface quality required in the aerospace industry context. Finally, deviations from the designed part are studied.