## 2 The application of Additive Manufacturing (AM) in industry 4.0
### Different Types of AM  
The AM processes, which are commonly utilized in manufacturing parts from engineering materials for different purposes, are extrusion, sheet lamination, vat photo-polymerization, binder/material jetting, powder bed fusion, and direct energy deposition [40; 41]. Each of these processes is used to deposit different types of materials based on the energy source. The materials can change from polymers to ceramics and metallic compounds [42]. The extrusion method is mostly used for thermoplastics and requires high operating temperatures. The final parts usually suffer from high porosity but the low processing cost and flexibility in geometry increase its applications in making different mechanical parts. In addition, some researchers have tried to make ceramic-reinforced polymers with this method [43]. Vat photopolymerization is another process employing UV light to cure polymers layer-by-layer and the processing speed is high while it keeps the process' simplicity. In addition to polymers, researchers have tried to mix the polymers with ceramic particles to produce stronger mechanical objects with bio-applications [44]. Sheet lamination is another AM process, which assembles sheets of metal on top of each other in order to form a 3D object. In this process, different glues, welding, and brazing can be used to hold the sheets of material in place for a longer time, but ultrasonic welding is the most efficient and the most common [45]. The sheets are fed into the building area in the needed geometry and an ultrasonic head punches them against the previous layer and lightly welds them together. This process is known to be a cheap and fast process while the second material removal is needed after the parts are done [46]. Material and binder jetting are two distinct processes, but they work on the same principles that are binding materials to the main body of a part. In material jetting, polymers are usually melted and deposited in the shape of droplets to form the needed geometry. The molten polymers then undergo a curing process by heat, light, or chemical reactions to increase the bonding strength [47]. In binder jetting, there is a prepared bed of metallic powder laying under a jetting nozzle that disperses bonding polymers selectively on the surface of the metallic powder. After applying the polymer glue on the surface, a new layer of metallic powder is  
Figure 2: A flowchart of cloud manufacturing and the involved sections [17]. deposited, and the glue dispersion takes place. This cycle continues until the final shape is achieved. After that, the parts are sintered in furnaces with controlled atmosphere and different temperatures based on the metallic powders and the glue utilized to bond them together [48]. Usually, these two processes are considered fast, but the final product has some porosities. The best application of these processes is making selectively porous mechanical objects. Powder bed fusion appears in different shapes and selective laser melting (SLM) is one of the most popular ones. In SLM, metallic particles are fed in different layer thicknesses and a laser beam melts the desired regions of the surface. In the next step, a new layer of powder is distributed on the build plate and the laser source melts the powder until the deposition finishes and the final shape is achieved. The sources of melting beams can vary based on conditions and price of the utilized machines [49]. The most common sources are the laser, electron, and ion beams. On the other hand, the atmosphere of the chamber is controlled by purging some inert gases to minimize the oxidation process during melting and solidification of the powder [50]. The most common gases used in SLM are nitrogen and argon. In some rare cases, it has been observed that the chamber is slightly vacuumed. In addition, the interesting feature of SLM is its capability in fabricating functionally graded materials from premixed or separate powders [51]. The other widely used AM process is known as direct energy deposition (DED).