## 2 The application of Additive Manufacturing (AM) in industry 4.0
### The Advantages of Additive Manufacturing over Traditional Manufacturing Approaches  
Different AM techniques have been developed not to replace all the traditional manufacturing methods, but to widen the selection range of processes for manufacturers and customers. Each process has its own advantages/disadvantages and the choice of which to employ is application-dependent. To review the advantages of AM processes, characterizing their key features is required. There are three important key features, i.e., time, cost, and flexibility, based on which the advantages of AM can be evaluated. One of the main purposes of using AM is to save manufacturing time and increase production speed. This will accelerate prototyping and reduces the time of production of spare parts and replacement parts [54]. Spare parts' supply chain performance can be improved by altering the location of manufacturing facilities and decentralizing manufacturing in different regional sites close to principle markets [55]. The benefits of distributed production related to spare parts include lower downtime, lower overall costs, lower capacity utilization, a reduced need for inventory management, higher robustness, and higher flexibility to supply chain variations [56]. Having manufacturing systems on-site enables rapid production of customized parts by eliminating the transportation time and cost of the parts. Unlike traditional manufacturing, where huge amounts of materials should be removed, AM applies materials proficiently by reusing the leftover materials for building the next part. Case studies have shown that the material waste in AM is reduced by 40% compared to traditional methods and 95% to 98% of the leftover materials can be recycled [57]. The cost-effectiveness of AM products could be related to the reduction of labor cost and avoiding costly warehousing as well. Moreover, AM does not require additional resources such as fixtures, cutting tools, jigs, and coolants. Plus, manufacturing to order reduces inventory risk, with no unsold finished goods.  
Flexibility can be referred to as part of the process. Meaning that designers are more flexible to design complex parts and have the freedom to easily alter the process parameters based on their needs. Since there is no-to-little tooling constraints in AM, parts with complex geometries can be manufactured and part functionality would not be restrained by manufacturing constraints. Moreover, it is possible to build a single part with varying properties, having more strength in one part and more ductility in another part [58; 59]. Furthermore, since the part quality is dependent on the process rather than operator skills, production can be exactly in line with customer demand. Figure 4 illustrates the interrelation of the three above-mentioned key features with the advantages of AM processes. All the benefits are related to one or two of the key features, such that any of the advantages are either to reduce the time and/or cost of the process or are a result of the flexibility of the AM processes. Despite all these advantages related to the AM technology, the process-properties-geometry correlation in AM components is very complicated and requires more investigations [60].  
Figure 4: The interrelation between the three key features and additive manufacturing (AM) advantages.