**Preface**

Although research efforts around manufacturing processes are increasingly concentrated on additive manufacturing, conventional processes still have a very high margin of progression, which deserves to be investigated. Furthermore, machining presents levels of surface quality not yet attainable by most additive manufacturing processes, which is why some more advanced equipment even incorporates both technologies. Given that machining continues to be a topic around which there is substantial research, it was considered appropriate to develop this Special Issue that brings together a batch of twelve excellent works, some of which are reviews. Thus, this Special Issue is intended to be extremely useful to researchers starting their work on these themes. The other articles also provide new data in terms of research, which may be useful to anyone who researches or teaches in this field of knowledge.

After a brief contextualization at the beginning of this Special Issue, some broad-spectrum review articles are presented. The first addresses hybrid manufacturing, which, as previously mentioned, is beginning to assume increasing importance in the metalworking industry and is also a hot topic of research. A retrospective of the most recent developments is carried out, allowing those new in the topic to evolve quickly by reading this review. Next, another review is presented, focusing on the difficulties presented in the machining of fiber metal laminates, a multimaterial that is beginning to gain considerable importance, mainly in the aeronautical and aerospace industries. Another review then follows, this one now focused on the conventional and unconventional machining of Inconel alloys, a nickel alloy used more and more frequently in very demanding applications and one that is traditionally demanding in terms of tools in machining operations. In fact, this alloy is also the main focus of the following two articles, the first studying the wear behavior of milling tools coated with TiN/TiAlN through PVD, using HiPIMS technology. Concerning the machining of Inconel alloys, the behavior of turning tools is also studied in the next paper, using PCBN tools. The Special Issue continues with a paper studying the wear behavior of coated tools in UNS S32101 Duplex Stainless Steel milling operations. This duplex stainless steel is traditionally a material that is difficult to cut due to its mechanical strength and elevated ductility, with the chip tending to adhere to the tools. Drilling, and the respective consequences of this operation when applied to fiber metal laminates, is also covered in this Special Issue, through a study focused on the phenomena of delamination between the different layers of these laminates when they are exposed to the characteristic forces of the drilling process.

Environmental sustainability in machining processes is a constant concern, mainly due to the lubricants and coolants used. This Special Issue presents two studies, one focused on the use of CO2 as a coolant in machining operations, and another review focused on the environment and on how to make machining more environmentally friendly using specific techniques and products.

Finally, the economic aspect of machining is addressed, through two different approaches to estimating the time and cost of machining operations. One of the approaches is more conventional, while the other uses artificial intelligence algorithms to calculate the estimated machining time in certain operations. The results obtained allow the prediction that the models will converge to increasingly reliable results that are closer to reality.

With a wide range of subjects, all of them sharply focused on current machining concerns, this Special Issue is expected to be of great usefulness to all those looking to deepen their knowledge in this important area of metalworking production.

> **Francisco J. G. Silva, Filipe Daniel Fernandes, and Vitor Fernando Crespim Sousa** *Editors*
