Laser Melting
Selective Laser Melting (SLM) is a process that creates three dimensional parts by fusing fine metallic powders together. The process uses a high powered Ytterbium fiber laser to fuse the powder; the metal is fully melted into a solid homogeneous mass. The technology was invented by F&S – Stereolithographietechnik GmbH and the Fraunhofer ILT. The popularity of metal-based processes is growing.
SLM technology is used for functional testing of production quality prototypes as well as for manufacturing of complex end-use parts in low-volume, and for building highly complex organic structures. In addition, SLM is broadly applied to produce light-weight and lattice structures. Due to the technology’s inherent geometric complexity benefits and excellent material properties, many industries have been benefiting from this technology, e.g. the automotive, aerospace, tooling, jewelry and the medical industry.
Principle of Layer Generation
SLM uses fine metal powders to build a part layer by layer. The process is executed under a closed atmosphere of shielding gas in order to achieve maximum part quality. First, a layer of powder is placed on the building platform. The ytterbium laser traces the cross section of the part, thereby fluidizing and melting the respective particles (> melting temperature) together. The remaining powder serves as additional support structures. Upon completion of one cross-section, the platform is lowered by the height of one layer and another load of raw material is placed upon the existing model. Each time the laser liquefies a new layer of powder, both, the existing material and the raw material melt and a new shape is generated. This procedure is repeated until the part is finished.
Build Chamber Volume
The SLM 500HL system by SLM Solutions GmbH offers a build chamber sized 500x280x325 (x/y/z in mm), followed by SLM 250HL with a volume of 250x250x300. The processing area can be exploited optimally by stacking pieces on top or next to each other.
Build Time and Build-up Rate
The build time depends on the layer thickness and the processed material. With finer layers, more accurate parts can be produced; higher layers however require stronger laser modules. On average, build times are 7-30 cm³/h for SLM. In general powder bed fusion processes take longer for part construction than other AM processes due to the pre-heat and cool-down cycles involved.
Surface Quality and Accuracy
The surface quality is a function of the layer thickness, powder shape and size as well. Finer particles form a larger surface area and absorb laser energy more efficient than coarse particles. To achieve the best trade-off between dimensional accuracy, surface quality, build-up rate and mechanical properties, temperature of the powder bed, laser power, scan speed and scan spacing must be optimally aligned. Thanks to the focused high power beam, parts with a density of over 95% can be built. The minimal layer thickness is about 20 µm. With SLM machines layer thicknesses of 25 µm and surface finishes about Ra = 5 µm can be realized.
Material
Processing Material: A huge variety of metal powders are already available for both processes. In order to be processable, the grains should be round and their diameter should be between 10 and 45 µm. The used alloys are free of binders or other additives. In general, almost all metals can be processed with SLM. Tool steel H13, stainless steel 316 L, Titan TiV4, Titan, Cobalt Chrome, Inconel, Aluminum and Gold are materials which are approved for SLM processes. If the process parameters for a special material have not been standardized yet, manufacturers offer to develop the corresponding work specifications in cooperation with the customer.
Support Material: The support structure is needed to absorb the occurring stresses during the building process. The support structure is built from the same material as the part, and is removed after production.
Post-Processing
Ordinary post processing steps are support material removal, shot peening and polishing. As parts built by SLM are comparable to conventionally built parts, they can be reworked the same way, including machining, welding, eroding, etc.
Research initiatives
Current research initiatives aim at the development of new materials as well as understanding and advancement of existing materials. This also comprises the analysis of material properties and material processing in order to realize required property profiles, facilitating the production and processing of industrial materials.
