Characterization of porosity in sintered metals with 3D X-ray computed tomography to determine optimal production parameters for selective laser melting
B. Plank - Characterization of porosity in sintered metals with 3D X-ray computed tomography to determine optimal production parameters for selective laser melting - Master/Diploma Thesis, FH OÖ Fakultät Wels, Austria, 2008, pp. 161
This thesis investigates the characterization of porosities in sintered metals with 3D X-ray computed tomography (CT) and the improvement of the parameters of a plant in the process of the selective laser melting (SLM), which is used for Rapid Tooling.
The analyzed samples were produced with SLM layer by layer out of steel powder with the material number 1.4404 (X2CrNiMo17-12-2). The variation of the parameters in fabrication has a significant influence of the porosity of the components. These parameters include the laser speed, the focus diameter, the overlap of laser-lines, the pattern of laser lines and the layer thickness. Furthermore, the samples were generated with two different types of laser, a Nd:YAG and a fiber laser.
The investigation of the porosity was indirectly performed through the gravimetric density determination accordance the Archimedes' principle and directly using CT. CT is a non-destructive method using X-rays and a reconstruction algorithm to determine internal inhomogeneities of a component in a relatively short time. Such inhomogeneities are for example cracks, pores, shrinkage holes, density gradients, which can be visualized in 3D.
As a result of this work, diagrams could be drawn, which show the change of the density against the fabrication parameters based on the CT data and the analysis of the gravimetric density determination. Furthermore, 3D-visualizations of the inner pore structures, which were verified with micrographs by target metallography, are shown.
The accuracy of CT-evaluation is in absolute terms ±0.5 % porosity, which yields a component density in the used material of ±0.04 g/cm³. When comparing the density using CT with the determined gravimetric density, a standard deviation of ±0.027 g/cm³ was observed. With optimized parameters, the Nd:YAG and fiber lasers reached densities of 7.903 g/cm³ and 7.864 g/cm³ respectively.
In order to further improve the SLM process, it must first be determined to what extent the findings of this work can be applied to other materials. Moreover, it would be interesting to investigate how higher material densities might affect mechanical properties such as elongation and toughness.