Vortrag (20 Min., 5 Min. Diskussion, 5 Min. Raumwechsel)
Laser Powder Bed Fusion (LPBF) is an Additive Manufacturing (AM) technique for the layer wise build-up of complex three dimensional structures. Geometrical restrictions, compared with traditional design, are removed and, thanks to the high solidification rate encountered in this process, LPBF opens new possibilities for processing advanced materials and creating lightweight structures. These features are key enablers for the development of high performance optical components for space applications.
The hypereutectic aluminum-silicon alloy AlSi40 is a highly interesting material for the manufacturing of optical mirrors for space applications. Its high specific stiffness significantly reduces the wave front error. Moreover, its thermal expansion coefficient very close of that of the Nickel-phosphorus used as optical coating lowers the stresses between substrate and coating during temperature incursions. However, due to limited ductility and low fracture toughness processing AlSi40 by LPBF raises several issues. Therefore, parameter development, part and support structure design as well as post processing need to be investigated to achieve a stable, reproducible and scalable manufacturing process. In prior investigations Mueller et al. as well as Hilpert et al. presented LPBF process development and basic material characterization for AlSi40. Hence, the feasibility of processing AlSi40 by LPBF has already been shown.
Within this contribution process optimization for manufacturing large optical components and resulting effects on microstructure and crack formation are presented. Firstly, an overview of material properties of LPBF AlSi40 is given. Mechanical, thermo-physical as well as microstructural properties were acquired with respect to the developed process chain consisting of LPBF, heat treatment and hot isostatic pressing. Secondly, the influence of the parameters preheating temperature and volumetric energy density on crack initiation, crack propagation and microstructure is analyzed. For material and part characterization nondestructive (CT) and destructive testing (OM, SEM, EDX) methods are applied. Finally, crosslinks between process conditions and part design were drawn in order to create design and manufacturing guidelines for fabricating optical components made of AlSi40 by LPBF.