Internal stresses and shape distortions in generatively manufactured implants
The reliability of biomedical implants has to meet the highest standards. Shape distortions and internal stresses can limit the range of application of generative manufacturing techniques. These effects are caused by materials transformations during the production process which typically involves a layer-by-layer generation of complex components. The interaction between process parameters and the mechanical properties of the manufactured components have been studied in order to assess their reliability. Internal stresses were studied experimentally for the Selective Laser Melting (SLM) technique which can be employed to manufacture metallic implants, based e.g. on c.p. Ti or TiAl6Va4. The incremental center hole drilling technique is used in order to determine residual stresses which resul t from strong temperature gradients which occur during the manufacturing process. The results show that process parameters like the scan strategy and heat treatments can be used to control residual stresses and to improve reliability and form accuracy of SLM manufactured implants. In the case of stereolithography, shape distortions and residual stresses are caused by the polymerization shrinkage and the temporary local heating of the resin during processing. The complex interplay of increasing stiffness, decreasing flowability, and volumetric changes due to polymerization shrinkage and thermal expansion during the curing reaction was described with a four-parameter Burgers model. In order to obtain the required parameter set for the model, dedicated mechanical experiments as well as therma l and volumetric analyses of the resin were carried out. The Burgers model can be used in finite-element-simulations of the building process of a component. Experiments and simulations yielded qualitative similar results for a »diagnostic H« component.