2 ADDITIVE MANUFACTURING APPLICATION

2.1 Additive manufacturing processes

The AM process involves a number of steps to progress from a virtual model to a physical part. Although, dependant upon the product requirements, different efforts have to be taken in the single steps, eight generic stages can be identified within each AM process [3] as is shown in Figure 1.

Figure 1: Eight stages of AM processes, according to [3].

AM process performance is always influenced by parameters in each one of these stages, starting from the virtual model over the build process itself to the final application.

While the stages of the AM process are more or less the same for all applications, the fundamental principles of the individual AM technologies differ greatly. Due to this, a classification of the different technologies is required, which most often follows the physical state of raw material used for the building process [2], which can be liquid, solid or gaseous. The solid

materials based technologies can be subdivided into powder-based fusing, cutting and adding of laminate material, fusing of solid material or binding of powder. In technical application, the systems using gaseous raw material are currently more often used for coating than for Additive Manufacturing of entire three-dimensional parts. Table 1 provides a summary of the different principles and common examples of technologies.

Table 1: Classification of AM technologies, according to [2].

Classification Description Example of AM

technology Liquid based Curing of liquid polymers by

use of light or laser Gaseous Chemical or physical

deposition from aerosol or these, particularly on those capable of processing metal powder.

2.2 Selective laser melting

The first commercialized powder bed fusion process was Selective Laser Sintering, which is today suitable for processing thermoplastic polymers and composites of these together with metals or ceramics [3]. Based on this process, various technologies were developed that are able to also process metal powders. One of these technologies is Selective Laser Melting. The basic principle of this method is to spread a layer of powder on a build platform; this powder is selectively fused in the area where the part is to be generated. After that the platform is lowered, a new layer is spread and fused again. This procedure is repeated until the final height of the product is reached and thus the product is generated layer by layer, surrounded by the residual powder. The general construction of a Selective Laser Melting machine is shown in Figure 2.

Figure 2: Principle of SLM process, according to [4].

An Yb-fiber laser, with a wavelength of 1030 nm, is usually used to fuse the metal powder. The optical system for positioning the laser beam on the powder bed contains, in addition to the laser itself, x- and y- scanning mirrors for positioning and a so called f-theta lens for correction of the focus length variation in different areas of the build envelope. With this laser beam the powder is completely melted, so that the fusion is based on a liquid phase sintering. In this way, almost completely dense products can be generated.

Contrary to the powder bed fusion of plastics, in metal processing support structures are required, which must be removed after the build process. One reason they are used is that they reduce distortion due to thermal gradients within the part, but the main function is to conduct heat from the part to the build platform, allowing the melt to cool and solidify. The build chamber in Selective Laser Melting is held at room temperature or higher (for further reduction of distortion), but far below the material melting point. For this reason a large amount of energy, up to 1 kW, has to be supplied by the laser source. In order to avoid oxidation of the melt, the process takes place under inert gas.

As “Selective Laser Melting” is a protected term, slight variations of metal processing systems are available under the names of “LaserCusing” or

“Direct Metal Laser Sintering”. Another system, called “Electron Beam Melting” works with an electron beam instead of a laser source.

The metal parts produced by these technologies show good mechanical properties and are used in various industries, for example the automotive and aerospace industries and for medical applications. One example for medical application is the production of dental structures for which the suitability of Selective Laser Melting as a production technique is well proven [5].

2.3 State of AM technologies

Compared to traditional technologies such as milling, forging or casting, AM technologies provide a number of advantages [6]: as no molds or tools are needed, AM processes are suitable for the production of small lots even down to lot size one. This enables the use of Additive Manufacturing for the production of customized products in consumer-oriented industries as well as in medical applications. Furthermore, AM technologies allow structures to be produced that are impossible or very difficult to realize with traditional technologies. Products with internal cavities, strictly defined porosity or surface structures are opening up a wide area of application, like production of molds with cooling close to the surface, lightweight products with high mechanical properties or biocompatible structures for implants.

However, AM processes do not only provide advantages. There are also a number of shortcomings to be mentioned when compared to traditional technologies. One of the most commonly stated is a so-called staircase effect on the part surface, Figure 3.

Figure 3: Staircase effect in AM parts.

The staircase effect results from the part being built from layers and depends mainly on the layer thickness. It leads to a deviation of the real surface from the target contour and can also cause insufficient surface quality [7]. Another shortcoming can be identified in the surface roughness,

which is quite high with most AM technologies and particularly derogates the usability of functional surfaces [8].

A look at the AM technology research landscape shows that a huge effort is currently being made to research the topics of material: new materials as well as material quality or material regeneration and recycling, mechanical properties and microstructure manipulation. Process tolerances and process automation are considered far less, although they have also been identified as important research fields [9].