Direction and location dependency of selective laser melted AlSi10Mg specimens
Introduction
Selective laser melting (SLM) belongs to the layer-wise, powder-bed based additive manufacturing technologies and is similar to selective laser sintering (SLS) regarding machines and equipment (Gibson et al., 2010). The distinction between these technologies is given by the binding mechanism of the powder particles and has been reported by Kruth et al. (2007). Due to the complete melting of the powder in SLM, this process is capable of manufacturing almost full dense parts within one manufacturing step. Extensive adjustments have been carried out in the last decade to optimize the SLM process towards obtaining full dense parts by modifying process relevant parameters. Its suitability for the production of high quality components has been proven for various materials by Yadroitsev and Smurov (2010). Considering the raw material AlSi10Mg, the process control optimization for SLM was undertaken by Kempen et al. (2011) and Kempen et al. (2015) by adjusting the applied laser power, scan speed and layer thickness in combination with a suitable scan pattern. Kleszczynski et al. (2013) revealed the importance of these individual parameters in terms of the achievable mechanical strength. It has been shown that the energy density, calculated based on the irradiation parameters, is not sufficient for the characterisation of the irradiation process. Aboulkhair et al. (2014) reported that the remaining porosity in AlSi10Mg components was a superimposition of metallurgical pores and keyhole pores, which varied in their ratio based on the selected process parameters. In addition, it was found that one option for a further increase in density was to expose the layer a second time, often referred to as laser re-melting.
As the particular properties which determine the material’s suitability for the welding process differ between various raw materials, the optimization for the SLM production has to be material specific. In terms of aluminum based raw materials, Louvis et al. (2011) reported the peculiarities which need to be overcome: the rapid creation of a protective oxide layer combined with its high reflectivity and heat conductivity. Following these initial optimizations, it was additionally found by Thijs et al. (2013) that the occurring texture can be modified by the irradiation pattern applied, with minimal remaining anisotropy in the case of a perpendicular alternation of the scanning direction of subsequent layers.
Based on the layer-wise manufacturing and its numerous directional dependencies (Hitzler et al., 2016), the fabricated components, at this point in time, cannot be considered as an isotropic material in terms of mechanical properties. This raises the issue of how quality in the context of material strength is measured for SLM fabricated parts. Good mechanical properties, in general, go hand in hand with a “defect-free” microstructure as well as grain size and orientation (Heine, 2011). The applicability of this relationship in the case of laser beam melted material under tensile loading was proven by Kleszczynski et al. (2013). Thus, the obtained density is commonly used as the first indicator for quality. Another option, often employed for conventional bulk material, is the conclusion from the surface hardness to the mechanical properties of the material (e.g. German standard for conversion of hardness values to ultimate tensile strength for steel DIN 50150:1976-12). Hence, surface hardness appears also to be a promising characteristic to ensure quality. However, as the applied laser parameter sets and the thermal environment present in the SLM process differ between core and contour, the approach to directly relate the surface hardness results to the overall mechanical properties needs to be proven first.
Shifting the focus back to the inherent anisotropy of SLM fabricated components, one major influencing factor was found to be the build direction during fabrication. Kempen et al. (2012) and Buchbinder et al. (2015) investigated the tensile strength of AlSi10Mg on samples fabricated in-plane and in build direction. Both studies revealed an enormous variation of the breaking elongation, which was reduced by roughly 40% for the in build direction orientated samples, and just minor deviations in the ultimate tensile strength. In addition, Manfredi et al. (2013) showed that the alignment in plane is negligible in terms of tensile strength. Aside from the static properties, Brandl et al. (2012) revealed that the high cycle fatigue strength also differed based on the build direction and the studies from Dai et al., 2016a, Dai et al., 2016b, which were based on Ti6Al4V, documented that even the corrosion properties altered with the alignment.
The build direction is by far not the only parameter affecting the SLM process. The thermal environment has also been reported to have a significant impact on the obtained mechanical properties. The variation of mechanical properties of AlSi12 in terms of different build rates, preheating temperatures and post-heat-treatments was studied by Siddique et al. (2015). It was emphasized that the highest tensile strength was obtained without additional heat input, such as substrate plate heating or stress relieving. The effect of grain coarsening, followed by a decrease of hardness, caused by preheating of the substrate plate was reported by Buchbinder et al. (2015) for AlSi10Mg. Although the base plate heating favored a coarser microstructure, it was found to be a promising parameter to control the resulting microstructure and to eliminate process related defects. However, above a certain temperature the increasing solubility of gases in the melt was reported by Buchbinder et al. (2010) as a detrimental effect in regard to the achievable density. It was implied by Weingarten et al. (2015) that the remaining porosity was partially caused by entrapped gas, likely to be hydrogen, resulting from dissolved water in the raw aluminum powder. Thus, the influence of drying the powder before the consolidation process was studied and a positive effect on the resulting density was reported. A further factor influencing the SLM process is the type and purity of the inert gas atmosphere. Consequently, the deviations of the tensile properties and porosity of AlSi12 for nitrogen, argon and helium atmospheres were examined by Wang et al. (2014). It was found that nitrogen and argon were similar, whereas helium reduced the ductility accompanied by an increase in porosity. Another aspect is the feasibility of manufacturing inclined and downfacing areas without support structures, which has been investigated by Mertens et al. (2014).
In addition, selective laser melted components can be treated similarly to casted ones, thus, there is a variety of certified thermal procedures available. Given this, for Ti6Al4V it was reported by Cain et al. (2015) that the remaining anisotropy could be lowered by employing subsequent heat-treatments. In terms of aluminum based cast-alloys, when undergoing a post heat-treatment, a further increase in hardness was reported by Buchbinder et al. (2010) as well as Kempen et al. (2015). However, it was found by Buchbinder et al. (2009) that the impact on the hardness was dependent on the substrate plate heating during manufacturing. In the case of fabrication without preheating, the post heat-treatment resulted in a decrease in surface hardness. Li et al. (2015) reported that the major advantage of post-heat-treatments on a selective laser melted aluminum based raw material is the enormous gain in ductility, up to 25%, but to the costs in yield and ultimate strength. The solution annealing process was described leading to a homogeneous distribution of the Si particles in the microstructure, whereas Prashanth et al. (2014) examined an inhomogeneous microstructure evolution and clear tendencies between scan track cores and hatch overlaps.
To sum up, the review of previous studies reveals high fluctuations of mechanical material properties and various process induced causes for anisotropy and therefore, further investigations need to be undertaken.
This study extends previous research on directional dependencies of SLM fabricated components by taking an increased number of alignments into account. Deviations between orientations and inhomogeneities within individual samples were examined. Tensile testing was carried out with attached strain gauges to gain further insight into the material behavior regarding its deformation. Moreover, tendencies in the tensile strength were directly linked with the fluctuations present in the surface hardness.
Section snippets
Manufacturing conditions
Within this study, a SLM 280HL machine (SLM Solutions GmbH, Lübeck, Germany) equipped with a 400 W Yb-fiber-laser and an available build space of 280 × 280 × 320 mm3 was utilized. Nitrogen was employed as the inert gas and the temperature of the mounting plate was kept constant at 200 °C throughout the entire manufacturing process. It needs to be highlighted that the substrate plate temperature is controlled on the mounting plate underneath, on which the removable substrate plate is tightened down (
Tensile test
Without reviewing the recorded data, a conspicuous effect was revealed by the area of failure occurrence (Fig. 3). Configurations (d), to (f)3 showed a distinctive tendency towards failing on the top end of the specimen, which was fabricated last during production; hence, indicating a decrease in material strength with increasing build height (z-direction in Fig. 2). This trend was evident for 92% of the tested samples out of these three
Conclusion
In this research the mechanical properties of selective laser melted AlSi10Mg samples were studied. Clear tendencies were observed for the area of failure occurrence in the tensile testing of non-post-heat-treated samples. The height dependent configurations showed a dominant rupture towards the upper ends, which were fabricated last during the manufacturing process (true for 22 samples of 25). For the in plane orientated configurations a less emphasized tendency of rupture on the ends, which
Acknowledgements
Sincere appreciation to Markus Hubbel, Tim Schubert, Daniel Krahl, Johann Hirsch, Christian Schillinger and Wilfried Salzwedel for their helpful support throughout the implementation and evaluation of the experiments.
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