13th International Conference on Fracture June 16–21, 2013, Beijing, China -10- the material will act as a less fine grained and creep properties are then likely to be improved due to a grain size effect. In fatigue the 90° direction is more ductile [13, 14]. This fact could be beneficial if, for instance the material contains a number of defects and individual defects play an important role in crack initiation. A more ductile material can possibly accommodate an increased stress field around an inclusion through a larger local plastic zone avoiding cracking. The microscopy evaluation shows that the material is isotropic from a microstructure point of view in a plane parallel to the build plane. In the 90° direction the microstructure is different, as shown in microstructure photos and EBSD. Taking both previous tensile, current fatigue and creep results into account, the conclusion is that the material is orthotropic. Fracture surfaces indicate that both creep and fatigue testing at elevated temperature are very similar. Hence it cannot be excluded that creep play a role already at 450°C for this material, especially if the material is fine-grained. 5. Conclusions A nickel-based superalloy manufactured by selective laser melting was investigated by mechanical testing and microstructural evaluations. The material is observed to have a layered weld-like structure. Due to the manufacturing process properties, a columnar grain structure is present. It is shown from mechanical testing that the material is orthotropic with respect to mechanical properties. Fatigue properties in the 90° direction are comparable to hot rolled material at temperatures below temperatures where creep could be expected. However, in other directions < 90° the material is less resistant to crack initiation. The biggest concern with selective laser melted materials appears to be creep related. It is shown here that the creep properties are inferior to hot rolled material in all directions 0° ≤ α ≤ 90° and that the material cannot compare to a standard hot-rolled material regarding creep. It is shown that the grain size is small, and it is well known that changes in grain size have a strong influence on both mechanical strength and creep properties. 6. References [1] Chu J., Engelbrecht S., Graf G., and Rosen D.W., 2010. “A comparison of synthesis methods for cellular structures with application to additive manufacturing”. Rapid Prototyping Journal, 16, pp. 275-283. [2] Rosen D.W., 2007. “Computer-aided design for additive manufacturing of cellular structures”. Computer-Aided Design and Applications, 4, pp. 585-594. [3] Marchelli G., Prabhakar R., Storti D., and Ganter M., 2011. “The guide to glass 3D printing: developments, methods, diagnostics and results”. Rapid Prototyping Journal, 17(3), pp. 187 – 194. [4] Das S., Beama J.J., Wohlert M., and Bourell D.L., 1998. "Direct laser freeform fabrication of high performance metal components". Rapid Prototyping Journal, 4(3), pp. 112 – 117. [5] Kruth J.P., Froyen L., Van Vaerenberg J., Mercelis P., Rombouts M., and Lauwers B., 2004. “Selective laser melting of iron-based powder”. Journal of Materials Processing Technology, 149, pp. 616-622.
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