Additive manufacturing technologies

In 1986, Charles W. Hull has patented the first method of additive manufacturing called stereolithography or SLA (Charles W. Hull, 1986). Since then, there has been a signifi-cant amount of research on the topic. Nowadays, there are several techniques in additive manufacturing namely material extrusion, powder fusion, material jetting, binder jetting, direct energy deposition and sheet lamination etc. In the rest of this subsections, a short description of four of these techniques is provided.

Stereo Lithography (SLA)

In Stereo Lithography (SLA) or Vat Photo-polymerization (VP) a photosensitive liquid monomer, polymer or resin is cured or solidified using a controlled source of ultraviolet light, electron beam or laser. The light applied with the shape of each slice to polymerize the liquid into a solid layer. Then the platform moves downwards to make space for a new layer of liquid of the solidified layer. The process continues until the whole object is shaped layer by layer as shown in Figure 1 (Wong & Hernandez, 2012).

Figure 1. SLM process (Proform, 2018)

Part manufactured by SLM can be post-processed with light curing, to reach to better mechanical properties, and surface enhancement. SLM can be used for manufacturing ceramic parts by adding ceramic particles or using polymer-driven ceramifiable mono-mers (Ngo et al., 2018).

Possible defects in SLA are shrinkage, curling defect and distortions due to removing the part from the platform. Shrinkage is the direct effect of forming polymers from mon-omers. The curling defect in SLA happens as a result of shrinkage between the layers.

And finally, the process of removing the manufactured part from the platform may cause further distortions in the part due to the liberation of internal forces cumulated between layers (Bugeda, Cervera, Lombera, & Onate, 1995).

Material Extrusion

Material Extrusion AM (MEAM) is the process of softening the material and passing it through a nuzzle and deposit is layer by layer in order to manufacture a 3D part. A MEAM machine usually consists of a two-axis (x and y) CNC manipulator which moves the ex-truder and a platform which moves in z-axis which moves the manufacturing part down-wards to be ready for printing the next layer. The material can be in the form of solid filaments, powders or powder plus bounder liquid and the softening process is normally done by heating. The extrusion process can be done by either plunges, screws or wheels as shown in Figure 2 (Gonzalez-Gutierrez et al., 2018).

Figure 2. Material Extrusion Additive Manufacturing (Gonzalez-Gutierrez et al., 2018)

This technique can be used for manufacturing with metals, polymers, ceramics and com-posites. In case the material is used as the form of filaments, the process is called Fused Filament Fabrication (FFF) or Fused Deposition Modelling (FDM). FDM is the most com-mon method of AM. FDM machines are available from around one hundred Euros up to several thousand Euros, from desktop home versions up to industrial production ver-sions. The other reason is that the process of manufacturing is safe and simple and the filaments have a good variety of materials (Gonzalez-Gutierrez et al., 2018).

Cooling profile of FDM manufactured parts have a direct relationship with distortions and porosity in them. The mechanical properties of the parts are affected by the bond be-tween the layers of the manufactured part, which is, in turn, is affected by the temperature of the extruder and the temperature of the last layer of the part (Stavropoulos &

Foteinopoulos, 2018). Poor surface finish and mechanical properties are the main flaws of this technology. Using fibre reinforced filaments can be a solution for the latter problem (Ngo et al., 2018).

Powder Fusion

In powder fusion AM, a thin layer of fine powder which is spread and packed on the top of a descendant platform is fused together using pressure, heat or a binder. The source of the heat can be a laser beam or an electron beam. The fusion process can take place at two levels. In Selective Laser Sintering (SLS) the powder particles are not getting fully melted but they fuse together in molecular level. In a Selective Laser Melting (SLM) or Electron Beam Melting (EBM), the powder particles are melt and the fusion happens in a liquid phase, shown in Figure 3 (Stavropoulos & Foteinopoulos, 2018).

Figure 3. Powder bed fusion (Frazier, 2014)

The quality of parts is highly dependent on the powder shape, size, material and distri-bution. The other effective parameter is the chemistry and rheology of the binder, in the binder based processes, and the amount and flow of heat energy input to the system in the heat based processes. The heat sintering and melting process cause high residual stress in the manufactured parts. These stresses are the source of several defects in the parts, such as deformations, curling defect, lack of thickness, etc. Therefore, thermal and thermo-mechanical modelling of the process is of utmost importance for optimizing the

manufacturing process for minimizing the defects (Ngo et al., 2018; Stavropoulos &

Foteinopoulos, 2018). In the next subsection, a detailed description of defects in the SLM process is provided.

Post-processing procedures that usually take place in powder bed techniques are coat-ing, sintering and infiltration. Superior resolution, good surface quality and good mechan-ical properties of the parts manufactured with powder bed techniques make them one of the most favourable techniques, especially in metal AM (Ngo et al., 2018).

Direct Energy Deposition (DED)

The reason for calling this technique direct energy depositions (DED) that here the en-ergy is guided and focused to a narrow region and the material is deposited and melted simultaneously in the same region. There are several variations for these methods and this technology is mainly used for metal AM. The form of the material feed can be powder or filament and the energy source can be laser, electron beam, or electric arc (Stavropoulos & Foteinopoulos, 2018). Figure 4 is showing a simplified schematic of an electron beam DED.

Figure 4. A generic powder and electron beam DED system (Frazier, 2014) If a DED process uses metal wire filaments and electric arcs, it is called Wire and Arc additive manufacturing (WAAM) (Figure 5). While the powder bed based AM techniques are focused on fine details of the parts, WAAM systems are able to build larger parts (in the scale of 5.8𝑚 × 1.2𝑚 × 1.2𝑚) with higher deposition rates (3 to 10.63 kilograms per hour) (Ding et al., 2016). DED manufacturing systems normally consist of a robotic arm

and a turning table, therefore they normally have a minimum of five degrees of freedom.

Therefore, it is possible to manufacture parts which are difficult to manufacture with the other technologies. DED is also used for modifying parts and repairing cracks in metal parts (Pinkerton, Wang, & Li, 2008).

Figure 5. Wire and Arc Additive manufacturing (McAndrew et al., 2018)