**2. Selective laser melting**

SLM is an AM process that allows the manufacturing of all metal parts. This technology can easily realize complex geometries with interior features and channels. Regarding **Figure 1**, the manufacturing process starts with a thin layer of metal powder spread by a recoater along with the building platform. Then, a high-energy laser beam selectively fuses the deposited powder layer. The laser follows the contour defined in the STL file. Once a layer is completed, the building platform is lowered, the new powder is spread, and the laser melts this new layer. The process is repeated until the parts are completely manufactured. Due to the high temperatures necessary for melting, the process takes place in a protected atmosphere, normally argon, to prevent oxidation of the parts [1]. At the end of the process, the excess powder is removed; the parts still attached to the building platform undergo a stress-relieving job in an oven. This thermal treatment is necessary to reduce deformations of the parts caused by the high thermal stresses arising during the manufacturing process. Finally, the components are detached from the building platform and eventually subjected to surface finish treatment as polishing and shot peening [3].

Metal powder properties are important in the final quality and cost of the part built via SLM. The main properties influencing the process can be subdivided into three categories, which are as follows: [4]:


**Figure 1.** *Selective laser melting process.*

As far as the first point is concerned, fine particles enable high-density parts with good surface quality, while the spherical shape improves flowability and, hence, mechanical properties [5]. Irregular powder particles can lead to poor surface finish, low density, and increased defects [6].

Extremely important steps in the SLM process are the orientation of the part in the building platform and the design of the supporting structures. Supports have mainly three purposes, which are as follows:


Typical drawbacks are as follows:


A possible solution to reduce the number of supports consists of choosing an optimal building orientation. It is worth noticing that the staircase effect has to be also well-considered for specific applications. The generation of the staircase effect is described in **Figure 2**. The STL file format is a triangular approximation of the nominal CAD. If the layer thickness is too high or the inclination angle is too small, the staircase effect becomes more remarkable. On the other hand, overhanging surfaces is another important aspect to consider. These surfaces are areas not supported by solidified material during the building process. The heat-conduction rate of powder-supported zones is lower than the solid-supported zones, while the absorbed energy is higher. The melt pool created by the laser becomes too large and sinks into the powder. Therefore,

**Figure 2.** *Generation of the staircase effect.*

**Figure 3.** *Self-supporting angles.*

*Additive Manufacturing of RF Waveguide Components DOI: http://dx.doi.org/10.5772/intechopen.104106*

deformation occurs if these surfaces are not supported. Supporting structures are usually built with a low density during the manufacturing of the part, and they must be manually removed at the end of the manufacturing process. A clever solution is represented by self-supporting angles (**Figure 3**). Based on experimental results, downward sloping faces with angles α > 45° are self-supporting. At the same time, staircase effects can be reduced by increasing sloping angles. Moreover, in this way, the value of surface roughness decreases. On the contrary, angles lower than 30° should quickly be avoided since the staircase effect increases [7].

Materials commonly used in the SLM process are aluminum alloys, titanium alloys, stainless steel, Ni-based alloys, and cobalt-chromium alloys [1]. From an RF point of view, the most interesting ones are the aluminum ones as the AlSi10Mg alloy. This material exhibits high electrical conductivity, low-specific weight, high corrosion resistance, and good mechanical properties. The typical achievable accuracy guaranteed for this aluminum alloy is in the order of 0.1 mm [3]. However, better manufacturing accuracy has been observed in literature for components designed with an AM-oriented approach.
