Influence of post-processing on Ti6Al4V lattice structures produced by Selective Laser Melting

Abstract

Selective laser melting (SLM) is a novel technique being increasingly used for the production of porous structures with a high degree of precision and near net shape. These porous materials are finding their use in the biomedical industry for implants. In this thesis, the effect of post-processing using hot isostatic pressing (HIP) and surface modification techniques, such as chemical etching and sandblasting on the developed microstructure and mechanical properties is studied. Cylindrical porous samples with a diamond unit cell were produced by the vector based approach from Ti6Al4V ELI powder using the SLM process. The samples were tested in the as-processed and post-processed condition for compression and fatigue life. A comparison between the as-processed vector approach and Stereo Lithography (STL) approach was also studied and discussed in detail. The post-processing was found to have a positive influence on the microstructure and mechanical properties of the Ti6Al4V lattice structures. The HIP treatment showed the best post-processing step allowing to relieve residual stress, reduce the process induced porosity and obtain the optimal lamellar (α+β) microstructure. The HIP post-processing resulted in a significant improvement of the fatigue life of the studied porous structures. HIP followed by surface treatment using sandblasting technique was found to be promising for further fatigue life improvement without any reduction in strength as compared to HIP and as-processed conditions. Chemical etching showed improved fatigue life at lower stress levels but decreased strength due to a reduction in relative density.
Additionally, the comparison between the vector and STL approach confirmed the importance of process parameter optimization required to improve the as-processed properties. In this study a combination of HIP and sandblasting methods allowed to improve fatigue properties of vector based samples, making them superior to STL as-processed condition. Thus, the application of the studied post-processing techniques can be further applied to other porous metallic structures in order to improve their mechanical performance.