**1. Introduction**

Nowadays, the Selective Laser Melting (SLM) technology is widely used in different do‐ mains of the industry, such as aerospace, automotive, consumer goods and medical field. This additive manufacturing technology method offers an important series of advantages as compared to the conventional manufacturing methods that consists in the capability of realizing different parts with high complexity of the shape, the reduced manufacturing time,

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etc. By using the SLM method, it is possible to manufacture finite parts without necessarily using other supplementary manufacturing processes. The mechanical properties of the realized parts are acceptable, being dependent on the composition and the size of the metallic powder grains, and also on the internal structure of the parts, process parameters, and the manufacturing strategy used. These factors also have an important influence on the surface roughness and the accuracy of the manufactured parts.

The working principle of the SLM process is easy to understood, as could be observed in **Figure 1**. There are several steps that have to be fulfilled. Before starting the process, an inert gas (nitrogen or argon) will be introduced in the SLM machine chamber through a circulating system as presented in **Figure 1**, until an inert atmosphere will be obtained inside the working chamber (the maximum level of oxygen admitted inside is 0.1%). Furthermore, the inert gas will be circulated through the system during the entire process until the end with a level of 0.1% of oxygen maintained constant.

 **Figure 1.** Working principle of the SLM process [1].

The process continues with the deposition of the raw material from the powder container over the building platform by using a wiper that moves along the Y-axis direction. The thickness of the deposited material is around 20–100 μm for the first few layers. The process is repeated until the powder covers the entire building platform uniformly. Then the layer thickness will be set to a constant value of 30–50 μm and the process continues with the scanning of the first layer according to the first slice of the model, and then this will continue with the scanning of the next slice and so on until the part will be finished on the machine.

The building platform is moved along the Z-axis after the scanning process of every layer. Finally, when the maximum height of the building packet is reached, the building platform is moved to the start position and the manufactured packet is removed from the platform after sucking the non-melted metallic powder and after eliminating the metallic supports that were generated within the control software of the SLM system. The metallic supports have a wired structure and are needed in the manufacturing process in order to maintain the built part onto the manufacturing platform.

etc. By using the SLM method, it is possible to manufacture finite parts without necessarily using other supplementary manufacturing processes. The mechanical properties of the realized parts are acceptable, being dependent on the composition and the size of the metallic powder grains, and also on the internal structure of the parts, process parameters, and the manufacturing strategy used. These factors also have an important influence on the surface

The working principle of the SLM process is easy to understood, as could be observed in **Figure 1**. There are several steps that have to be fulfilled. Before starting the process, an inert gas (nitrogen or argon) will be introduced in the SLM machine chamber through a circulating system as presented in **Figure 1**, until an inert atmosphere will be obtained inside the working chamber (the maximum level of oxygen admitted inside is 0.1%). Furthermore, the inert gas will be circulated through the system during the entire process until the end with a level of

roughness and the accuracy of the manufactured parts.

 **Figure 1.** Working principle of the SLM process [1].

the next slice and so on until the part will be finished on the machine.

The process continues with the deposition of the raw material from the powder container over the building platform by using a wiper that moves along the Y-axis direction. The thickness of the deposited material is around 20–100 μm for the first few layers. The process is repeated until the powder covers the entire building platform uniformly. Then the layer thickness will be set to a constant value of 30–50 μm and the process continues with the scanning of the first layer according to the first slice of the model, and then this will continue with the scanning of

0.1% of oxygen maintained constant.

162 New Trends in 3D Printing

The SLM technology looks quite simple in theory, but the process control is not so simple from the experimental point of view. Experts' opinions regarding the technological parameters that have a significant influence onto the accuracy and the mechanical properties of a manufactured part made by SLM are quite different [1]. There are researchers who state that these charac‐ teristics are directly influenced by the laser system. So, the laser power or the laser beam diameter is important when speaking about the accuracy or porosity of a manufactured part [2, 3]. There are other researchers that state that the optical system in close connection to the scanning strategy is the most important when speaking about the accuracy or porosity of the manufactured part using the SLM equipment. The possibility of cooling-down the optics or optic's design is one of the most important issues in this case [4, 5]. Other researchers state that the scanning speed, the layer thickness or the building temperature have a direct influence over the SLM process and the accuracy or porosity of the manufactured parts as well [6–9]. There are also a series of researchers which consider that the properties of the raw material (particle size, particle distribution, etc.) are important within the SLM process, especially in the case when the resulted porosity of the material has to be precisely controlled [10–11]. Other researchers state that innovative technologies should be used in combination with the classical manufacturing technologies in the so called "hybrid technologies" [12–15]. This means, for example, that the parts have to be manufactured by using the SLM technology and finishing of the part can be done on the same equipment by using conventional manufacturing methods, such as milling. In this way, the accuracy of the manufactured part and the surface roughness will be improved significantly. Looking on all these various researches reported by the authors, it is very difficult to state who is right and who is not right regarding the best control of the SLM process.

The existence of an MCP Realizer II SLM 250 equipment (see **Figure 2**) at the National Centre of Innovative Manufacturing from the Technical University of Cluj-Napoca (TUC-N) facili‐ tated the start of different research activities in this field at TUC-N with the aim of improving the Selective Laser Melting (SLM) process capability for a better transfer of the technological gained knowledge to different partners from the industrial and medical fields. Reaching this goal has been also facilitated by the fact that in the period 2010–2013 at the Technical University of Cluj-Napoca, within a postdoctoral project financed by European Commission (E.U.), it was possible to activate with the program entitled "Research regarding the manufacturing of metallic parts by Selective Laser Melting (SLM) technology."

 **Figure 2.** MCP Realizer II SLM 250 system from the Technical University of Cluj-Napoca.

Part of the results obtained in this postdoctoral program, in cooperation with the SLM Solutions GmbH Company from Luebeck, Germany (best practice examples) are presented in this chapter of the book.
