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Introduction
Selective laser melting (SLM) is a freeform fabrication process that allows the layer-wise production of parts in metallic materials ([5] Badrosamay et al. , 2009; [12] Levy et al. , 2003). SLM parts usually offer mechanical properties comparable to the properties of conventionally produced parts from bulk materials ([1] Abe et al. , 2001; [11] Kruth et al. , 2005; [13] Rehme, 2006; [14] Rehme and Emmelmann, 2007).
Furthermore, the layer-wise process allows the production of structures with a degree of complexity well above the possibilities of conventional processes such as drilling, casting, etc. ([9] Hollander et al. , 2003; [10] Kerckhofs et al. , 2008; [13] Rehme, 2006). Depending on the process under consideration (selective laser sintering (SLS), SLM, 3D printing and other additive processes), undercuts provide no or at least less problems for production.
During the last few years, the industry has recognised these advantages particularly for the production of small- and medium-sized lots and for customised applications. That is why additive processes are more than ever used for a so-called rapid manufacturing ([17] Spierings and Bachofen, 2008; [19] Spierings and Levy, 2009b), where the parts are directly produced for the end-application and not only as a prototype.
As the interests in several industries like medical and aerospace branches are rising, it is essential to be able to assess the quality of a layer-wise produced part in order to compare the different machines, materials and processes available. The quality of a part contains parameters out of at least three main topics, which are to some degree interdependent: mechanical parameters (density, mechanical strength, elongation to rupture, fatigue strength), surface quality and dimensional accuracy. The dimensional accuracy is typically adjustable by the machine software and the surface quality is also dependent of the machine technique (scan-strategy, process atmosphere, etc.). Furthermore, the grain size distribution of the powder used affects all topics. Inappropriate powders lead to worse densities ([5] Badrosamay et al. , 2009; [18] Spierings and Levy, 2009a) and the particles welded to the surface of a part affect the dimensional accuracy and the surface quality. The latter two are easily measurable, but the derivation of the mechanical parameters is time consuming and expensive.
It is commonly known that all powder-based processes (SLS, SLM),...