Quality Management in Additive Manufacturing

One essential aspect in establishing additive manufacturing in industrial production is ensuring a high and reproducible quality of the printed components. The quality of the components is determined by the homogeneity and defect free microstructure as well as by the adherence to a narrow band of dimensional tolerances and a low surface roughness. The same material properties must be achieved as using conventional production processes.

In comparison with conventional forming processes, 3D printed components show more fluctuations in green density, which has a negative effect on the mechanical properties of the finished components. The microstructure of additive-manufactured components is examined at the Fraunhofer-Center HTL on a submicrometers to millimeters scale. The homogeneity of the microstructure is quantitatively recorded and evaluated by variance analyses (see Raether, F.; Schulze Horn, P.: Investigation of sintering mechanism of alumina using kinetic field and master sintering diagrams). Special electron microscopy and tomographic methods are used, which were developed for the characterization of conventionally produced green bodies. Based on the evaluation, measures for the optimization of the additive manufacturing processes are derived. This includes, improving the feedstocks, adjusting the printing parameters, optimizing the heat treatment or modifying the production equipment.

Even during the heat treatment of the printed components, various quality problems can occur, e.g. the introduction of defects during debinding (see Klimera, A.; Raether, F.; Schulze Horn, P.: In Situ Investigation of debinding of non-oxide ceramic) or distortion of shape during sintering (see Raether, F.; Seifert, G.; Ziebold, H.: Simulation of Sintering across Scales). In the latter case, the geometry and dimensions of components diverge from the specifications. The distortion can be caused by various factors such as gravity, frictional forces or density and temperature gradients. For optimization of heat treatment processes, systematic tools are used at Fraunhofer-Center HTL (see Raether, F. (Hrsg.): Sustainable heating processes (Publication available on request)). This ensures a reproducible and high quality of the components with low rates of waste. At the same time, the heat treatment processes are optimized concerning duration and energy efficiency (see Ziebold, H.; Raether, F.; Seifert, G.: Radical Time Reduction of Debinding Processes by Combined in-situ Measurements and Simulation) to save costs and achieve an improvement of the environmental balance.

The structure and quality of surfaces of additively manufactured components can be tested using various methods. An evaluation of their influence on the component properties is possible with FE simulations. Accordingly, damage-inducing stress concentrations at the component surface are identified and measures developed for their minimization. If necessary, free-form surfaces of green bodies, intermediate products or sintered components can be reworked using a 5-axis machining center.

The dimensional accuracy of the printed components represents another important quality aspect. For comparison of the effective component dimensions with the customer’s specifications, modern 3D scanners are employed. Interior structures are measured by computer tomography. Wherever necessary, printed components are reworked to achieve the desired dimensions and surface qualities. Finite element models were developed to precisely predict the sinter shrinkage of the additively manufactured components in all three dimensions and thus reduce finishing to a minimum (see Raether, F.; Seifert, G.; Ziebold, H.: Simulation of Sintering across Scales). Using the specifications from the simulation models, even complex green body shapes can be printed in a way, so that the finished components comply with the specified dimensions after sinter shrinkage.  

In addition to the quality management measures described, process-integrated methods for monitoring component and process quality during the printing process will play an important role in the future. Fraunhofer-Center HTL develops corresponding methods to monitor the quality of the print process in-situ and to adapt the process parameters directly in case of deviations.