# Emerging Trends in Hybrid Additive and Subtractive Manufacturing
## 1 Introduction
These include the development of digital twins, the study of hybrid trajectories with five or more axes, and the development of software tools that truly integrate all stages of the process [6]. Regarding the evolution from a historical perspective, polymeric parts, although not structural, were already being additively manufactured in real-life applications by Bell Helicopter and Boeing starting in 1990. Since then, other major companies in the sector, such as Airbus, Lockheed Martin, GE Aviation, Northrop Grumman, and Honeywell Aerospace, have incorporated parts produced using additive manufacturing. Currently, even the ESA (European Space Agency) and NASA (National Aeronautics and Space Administration), and newer companies, including Space X and Relativity Space, use additive manufacturing. Combustion chambers, fuel tanks, injectors, air ducts, brackets, and clips are some of the parts and components manufactured. [PERSON], vice president of additive manufacturing at Airbus, identified 70,000 pieces that would be flying in aircraft and satellites by 2022 [13]. In 2015, Boeing filed a patent application for 300 parts produced using additive manufacturing [14]. The first titanium part was additively manufactured by Airbus in 2017, authorized by the EASA (European Union Aviation Safety Agency), and integrated into the A350 XWB. However, the need for high precision and high geometric, dimensional, and surface quality of functional parts, enabling, for example, assembly operations, requires the use of machining subprocesses [15]. Multitasking machines, which initially integrated different subtractive processes and over time have come to integrate additive and subtractive processes, have a great impact on the reduction of labor costs, as these are reduced, as well as on machinery costs. The time reduction involved in performing all the subprocesses in the same machine without having to remove the part, along with the reduction in required software tools, also represents a significant cost reduction [16]. Since the 1980s, machine tool manufacturers have been presenting their new proposals, as shown in Figure 1[16]. In recent years, powerful companies such as Mazak and DMG Mori, and other growing companies such as Ibarnia, have developed hybrid additive and subtractive manufacturing machines that alternate between DMD (direct metal deposition) or LMD (laser metal deposition) and machining, especially milling [17]. Indeed, as shown in Figure 2, it was especially from 2014 onwards that a race began among the main machine tool manufacturers to launch their additive and subtractive hybrid manufacturing proposals [6]:  
Hybrid additive and subtractive manufacturing, while offering great potential, must address challenges to continue advancing, especially in the interaction phases of the additive and subtractive subprocesses. For this reason, studies are emerging on the effect of machining with lubricants on the subsequent addition of material using LMD. Some studies point to dry machining, while others propose solutions such as the introduction of cleaning phases [17]. One of the most promising fields of application for hybrid additive and subtractive manufacturing is the manufacture of stamping dies [18] and injection molds [2]. The geometric freedom provided by LMD manufacturing, combined with the dimensional, geometric, and surface quality enabled by milling, allows for the manufacture of conformal cooling channels, which allow for faster, more homogeneous, and more efficient cooling. Studies show that this hybrid process also allows achieving the machining characteristics of these parts [18]. Moreover, additive and subtractive hybrid manufacturing machines are beginning to be configured for a wider variety of hybrid processes. Those that integrate  
Figure 1: Timeline illustrating key developments in multitasking machine tools. PBF (Powder Bed Fusion), which is more oriented toward the production of complete parts, often combine it with milling.