Workings of SLS Software

Similar to other AM processes, information about the designed component is collected from a computer-aided design (CAD) file and converted into standard triangle language (STL) file format for the SLS machine to read. There are other types of files, but the STL file is the standard for every additive manufacturing process. The STL file creation process mainly converts the continuous geometry in the CAD file into a header, small triangles, or coordinates triplet list of x, y, and z coordinates and the normal vector to the triangles (Wong & Hernandez, 2012). This process can be inaccurate therefore the smaller the triangles, the closer the component will be to the original design. By using software to generate a component, allows designers to transfer the processing information to any place in the world, perhaps even outer space. Therefore, AM is also called digital manufacturing, solid free-form fabrication (SFF), or e-manufacturing (Gu et al., 2012) Parameter selection

The user selects the process parameters according to the specific powdered material intended for processing. Adjustments to the process parameters lead to variations in the resulting build quality.

Process parameters affect quality aspects such as dimensional accuracy, mechanical strength, porosity, surface roughness, processing time etc. (Mierzejewska, 2015). Each material is unique

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(material properties, particle size, shape and distribution), hence the material specific parameters need to be fully understood and carefully selected in order to build parts of acceptable quality (Van Staden et al., 2016). Thus far, the establishment of process and material specific parameters has been difficult. Researchers have made attempts to determine optimal configuration parameters for specific AM process-material combinations, however there has been a lack of general conclusions of wider applicability (Townsend et al., 2016). Table 3.4 lists the range of parameters that the user can adjust according to the material.

Table 3.4: Standard selection of SLS process parameters, adapted from Kruth et al., (2005).

Material Laser Scan Environment

Composition Mode Scan speed Preheating

Powder density Wave length Hatch spacing Pressure

Morphology Power Layer thickness Gas type

Particle size distribution Frequency Scan strategy O2 level

Thermal properties Pulse width Scan sectors Humidity

Flow properties Offset Pulse distance

Spot size Scaling factors Powder, environment and layering

The environment in the build chamber needs to be controlled perfectly in order to limit the formation of the thermal stresses and surface oxidation of components. The melting occurs within an inert environment of either argon, nitrogen or in special cases helium (Loeber et al., 2011). The powder is placed in the build chamber, which is first pumped down to a vacuum pressure of about 100 Pa and then filled with an inert gas to a pressure of 0.5 bar, which reduces the initial oxygen amount to around 0.02 % (Wang et al., 2002b). Over-pressurising the build chamber prevents atmospheric oxygen to leak back into the chamber. Oxygen will react with the melting powder, resulting in oxidation and even a fire (Hagedorn-Hansen, 2017).

Before starting a new build, the user fills the powder feed piston with the adequate amount of new or recycled powder, which was recovered from previous build. The user is also required to install a component known as the build plate or base substrate. The plate is needed; as SLS builds require a solid, clean surface as a build foundation. The plate may be dulled through a sandblasting process in order to limit initial layer laser reflection (Hagedorn-Hansen, 2017). Certain points of reference will be created by the SLS machine on the build plate, therefore the plate must be perfectly flat and firmly secured to the build platform (Van Staden et al., 2016). The build plate may be pre-heated to temperatures ranging between 100-300 in order to lower the thermal gradient during processing (Murr et al., 2012). A higher preheating temperature results in less crack formation (Kempen et al., 2013)

The laser selectively melts the powder according to the geometric information stored in the STL file about a specific powder layer. The unmelted powder is left untouched as it provides support for subsequent layers. After a layer scan is completed, the build platform is lowered down in the z-direction by a distance equal to one single powder layer. This distance is called the layer thickness and is generally between 20-100 µm thick (Ruan et al., 2006). The powder feed piston is raised up in the z-direction, pushing fresh powder to the build surface. A powder roller or scraper mechanism transfers the powder from the feed piston's surface to the build platform. The roller distributes the powder equally on the lowered build platform, creating a new, levelled powder layer. The next layer scan is initiated and the newly deposited powder layer is fused to underlying layers by careful controlling processing parameters such as laser power, scanning speed and scan spacing (Loh et al., 2015). A schematic of the workings of a SLM/SLS system is shown in Figure 3.7. The thickness of

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the powder layer is critically important. A too thin layer will cause the binder in the powder to overheat and evaporate, whereas a too thick layer will increase the risk of delamination. The ideal powder layer thickness needs to be determined experimentally (Kempen et al, 2014).

Figure 3.7: Schematic of the basic workings of a SLS machine.

Laser and mirrors

In the past, laser powder bed fusion systems used either a single CO2 or Nd:YAG laser. These are standard Gaussian lasers, which are also implemented in other laser machining operations such as laser welding and laser cutting. However, in recent years, improvements in positioning systems, materials and other process parameters, meant the laser technology became a limiting factor. Recent developments have led to an improvement in laser technology, which notably improves the laser beam’s quality and available resolution. Ytterbium fibre and disc lasers systems are regarded as the future standard laser technologies. SLS systems have developed to incorporate more powerful and multiple sets of lasers (Shellabear & Nyrhilä, 2004). The laser positioning system is shown in a schematic in Figure 3.8.

Figure 3.8: Mirror and lens laser positioning system used for laser fusion technologies

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The SLS laser positioning system consists of a series of lenses and reflecting mirrors (Sames et al., 2016). Galvanometers move laser-grade mirrors with low mass and inertia in order to deflect the laser beam according to the specific geometry received from the STL file. In a two-mirror setup, the focused laser beam is projected onto the first mirror which then reflects the beam to the second beam where it is positioned and reflected to the powder bed’s surface (Deckard, 1990). A series of lenses control the focus spot of the laser beam on the surface of the powder bed. A smaller focus spot will generally yield better geometric accuracy of the laser-fused components. The laser spot size will greatly affect the dimensions of the melt pool that will form on the powder surface (Thomas, 2009).