Selective Laser Sintering and Melting as Additive Manufacturing Methods to Produce Alumina Parts; '3D printing' of Ceramics

摘要

Additive manufacturing (AM; aka 3D printing) has the potential to rapidly shape parts, without compromising geometrical properties. Variants of the following powder metallurgy (PM) process, which includes additive manufacturing as a shaping step, were explored to produce ceramic parts: (i) powder synthesis, (ii) additive manufacturing, (iii) binder removal and (iv) furnace sintering. In this study alumina (Al2O3) parts were produced, since Al2O3 is currently the most commonly used ceramic material for technical applications. Further, two AM methods were investigated as possible shaping steps for the production of alumina parts: indirect Selective Laser Sintering (indirect SLS) and direct Selective Laser Sintering/Melting (direct SLS/SLM).To explore indirect SLS, different powders, with a particle size of about 10-100 µm and consisting of an alumina and a polymer binder phase (i.e. composite alumina-binder agglomerates), were irradiated by a laser beam. Five different alumina-binder agglomerates were investigated: alumina-polyamide produced through ball milling, alumina-polystyrene produced through dispersion polymerization and alumina-polyamide, alumina-polypropylene and alumina-carnauba_wax-low_density_polyethylene produced through temperature induced phase separation. The laser irradiation, which melted only the binder phase, selectively consolidated the powder agglomerates layer by layer. After subsequent binder removal and furnace sintering, alumina parts, containing inter-agglomerate pores, were obtained. In order to reduce the inter-agglomerate pores, the possibility to include the following steps into the PM process was explored: (i) irradiating the powder layers multiple times instead of only once (i.e. remelting), (ii) cold, quasi and warm isostatic pressing the SLSed parts and (iii) infiltrating the parts obtained at different stages of the PM process. As a result, freeform shaped alumina parts with densities up to approximately 90% could already be obtained. In order to produce higher quality ceramics through indirect SLS, the inter-agglomerate pores should be avoided or completely eliminated.To explore direct SLS/SLM of alumina parts, an experimental setup was developed. The experimental setup enabled to selectively irradiate densely packed layers of alumina powder, with a particle size of about 0.3 µm and without binder phase, at uniform temperatures up to 800°C. After a subsequent furnace sintering step, alumina samples with a grain size smaller than 5 µm could be obtained. In order to produce high quality ceramics through direct SLS/SLM, the reliability of the of the experimental setup should be improved by homogenizing: (i) the powder deposition process, (ii) the consolidation temperature during selective heating of the powder layers and (iii) the preheating temperature of the powder layers.