## 2 The application of Additive Manufacturing (AM) in industry 4.0
### Applications and Industries  
The three key features of AM processes discussed in Section 2.4, i.e., time, cost, and flexibility, result in benefits which interest many industries in using AM, especially industries which are in need of rapid prototyping and/or component manufacturing, which require low quantities of parts to be produced with certain specifications because industries dealing with rapid prototyping and component manufacturing are challenged by complex and customized parts production and on-demand manufacturing of components and prototypes. According to [PERSON]'s report [106], AM parts manufactured in automotive and aerospace industries have taken over 20% of the whole AM market. Following is a summary of the industries, which are interested the most in AM processes:  
* Aerospace: AM techniques are ideal for producing aerospace components as they need small batches of components, which have complex geometries, which is necessary for airflow and heat dissipation functions [107]. Furthermore, on-demand and on-site manufacturing are needed to be established for astronauts to produce parts for repair or maintenance of space stations. Moreover, AM is capable of producing parts with a low weight-to-strength ratio, which is necessary for airplanes and space shuttles. Since the materials used in the aerospace industry are expensive and AM processes are known for having less waste material, AM has become popular among manufacturers in the aerospace industry.
* Medical: One of the first signs of AM appearing in the medical industry was producing medical implants [9]. In addition to high complexity in design, medical implants have the patient-specific necessity. As it is mentioned in Section 2.3, AM, compared to traditional techniques, is more cost-effective for manufacturing small batches of parts, which is typical in the medical industry. Manufacturing patient-specific implants reduce the cost and time of surgeries as well [108]. Hip stems with functional gradation in porosity characteristics have been made from Ti6 Al4V by laser engineered net shaping (LENS) [109].
* Automotive: Complexity and low weight-to-strength ratio is a necessity for a part in the automotive industry as well. AM is not only used for prototyping for automobile parts, but its advantages have also made it able to be used for AM of actual components and vehicles [110]. For Example, Optomec used LENS to reduce the material, time, and cost of manufacturing of Red Bull Racing car components including drive shaft spiders and suspension mounting brackets [111].
* Architectural: From AM of historical buildings [112] to the construction of a village on the moon [113] the architectural industry has benefited from AM in two ways: models and construction. AM of models is an ideal tool for architects as it allows them to improve their designs on a smaller scale and refine their architecture plans. AM also benefits the construction industry by altering the three key features, that is decreasing production time and cost, and increasing flexibility.  
There are other applications of AM which do not fall into the above-mentioned categories. Due to the flexibility and multifunctionality, e.g., load-bearing, while being lightweight, of AM lattice structures [113], they have been thoroughly analyzed for energy absorption applications [86]. AM has also been introduced to other industries such as food [114] and clothing [18], because of its flexibility and capability of manufacturing custom products on demand. Figure 7 illustrates how AM advantages are employed for different industries.