Issue34

A. Riemer et alii, Frattura ed Integrità Strutturale, 34 (2015) 437-446; DOI: 10.3221/IGF-ESIS.34.49 445 Parts consisting of stainless steel show the largest values for weight. Consequently, they shouldn’t be manufactured by Selective Laser Melting. The aluminium alloy represent excellent lightweight properties and due to the lack of processing parameters, efforts have to be made in order to realise the part production by the use of EN AW-7075-T651. The parts consisting of titanium alloy deliver small masses. Consequently, the laser melted Ti-6-4 should be employed in lightweight applications. In this study, the design with the thickness value of 0.8 mm for outer walls was selected for fabrication by SLM. This variant has well-balanced values regarding low displacement and weight. Part manufacturing and non-destructive testing Figure 10a show the bicycle cranks in their final design positioned on the substrate platform. These parts are connected to the platform by support structures. Supports are used to keep the SLM parts and the overhanging surfaces in position, to transfer the heat from the melt pool and to counteract the distortion that occurs due to the residual stresses. Figure 10 : Final product positioned on the substrate platform a) and the nondestructive test results from the computer tomography b) , c) and d) . In order to examine the bicycle crank in its interior area Computer Tomography (CT) scans were performed. The maximum resolution in this analysis was limited to 200 µm. The aim was to exclude imperfections like macro pores, cracks and distortions of internal structures. The interior chambers as well as holes that are required in order to remove the powder material out of the chambers are presented in Fig. 10b. Fig. 10c shows that no collapse of the overhanging areas occurs. The bicycle crank following fabrication corresponds to the CAD model. Fig. 10d illustrates the final design in half- section where the thin walls exhibit constant thickness. C ONCLUSIONS he present work proves that the Selective Laser Melting technique has promising opportunities for the future. The fracture mechanical properties may be optimised by the application of subsequent treatments. The residual stresses which occur in the titanium alloy have a very high influence on the crack growth. Parts consisting of Ti-6-4 should be heat treated (for example at 800°C) in order to achieve a significant improvement in crack growth performance and to exceed the crack growth data that was found for conventionally processed material. By means of the crack growth simulation was found that the remaining lifetime may be increased by a factor of 25. Stainless steel shows a low level of dependency upon heat treatment that aims at stress relieving. The crack growth performance is influenced to a large extent by the microstructure. This can be deduced from the different threshold values from the both orientations used in this study and from the increased threshold values following HIP process that results in a modified microstructure. The studies focused on the optimisation of the bicycle crank show the large lightweight potential of the SLM technique. Especially, personalised, delicate and geometrically complex parts are obvious candidates for fabrication by SLM. T