Additive manufacturing is a process in which metallic components of almost any geometry and complex structures can be produced directly from 3D CAD data with considerable potential for weight reduction can be achieved using bionic design, optimized lightweight construction methods in the aerospace, automobile and medical industries. 

There are various types of metal additive manufacturing. Additive manufacturing process are characterized by the layer-by-layer joining of solid elements [1]. The various additive manufacturing processes can be according to the initial state of the material used, example: liquid, solid, powder [2]. Additive manufacturing processes such as Laser Powder Bed Fusion (LPBF) are becoming increasingly important due to the high degree of design freedom and tool-free, near-net-shape production. Despite the potential, the LPBF process presents several challenges that must be addressed to ensure consistent specimen quality.

Powder Bed Fusion (PBF) uses laser or electron beam or other heat source to scan and melt the desired part cross section in the powder layer spread on the build platform. Due to the layer-by-layer melting of the powder material and the subsequent solidification, a characteristic, anisotropic microstructure is formed. As a result of anisotropy, the rapid heating and cooling rates and potentially existing defects (e.g. pores), the mechanical properties are direction-dependent and differ from those of conventional components. This research project, which is carried out together with the Institute for Manufacturing Technology and Production Systems in RPTU, demonstrates the ability to manufacture lightweight and complex structures of AlSi10Mg components using Laser Powder Bed Fusion (LPBF) and to validate the fatigue properties of additive manufactured AlSi10Mg components. 

Due to the nature of the process, the surface quality of components produced using LPBF is inadequate for many applications, so that an additional step, typically post-machining is required to improve surface quality. One of these steps is milling, that is employed to overcome the surface irregularities and poor tolerances produced as a result of additive manufacturing (AM). However, there are also other post-machining methods like turning, reaming or grinding. Here, milling improves the surface topography but represents a time and cost-intensive additional effort. In addition, the fatigue properties of components manufactured using LPBF are generally worse than those of conventionally manufactured components. The reason for this is adverse residual stress and local volume defects.

In this project, post-machining of the printed components by using cryogenic cooling is investigated. Since it offers the potential to modify the mechanical and metallurgical properties within the workpiece edge zone, e.g. by introducing work hardening and residual compressive stresses. This has a positive effect on the fatigue behaviour of the components. The use of cryogenic machining therefore offers the possibility of creating a technical surface in a single post-processing step and simultaneously improving the fatigue properties of the component. With cryogenic post-processing, added value can be realized by optimizing the surface morphology and counteracting the potential disadvantage of LPBF.

Bibliography:

[1]  A. Gebhardt, Generative Fertigungsverfahren: Additive Manufacturing und 3D Drucken für Prototyping - Tooling - Produktion, 4. Aufl. München: Carl Hanser Verlag GmbH & Co. KG, 2013. doi: 10.3139/9783446436527.

[2]  J.-P. Kruth, M. C. Leu, und T. Nakagawa, „Progress in Additive Manufacturing and Rapid Prototyping“, CIRP Ann., Bd. 47, Nr. 2, S. 525–540, 1998, doi: 10.1016/S0007-8506(07)63240-5.

[3]  V. Seyda, „Werkstoff- und Prozessverhalten von Metallpulvern in der laseradditiven Fertigung“, in Light Engineering für die Praxis. , Berlin, Heidelberg: Springer Berlin Heidelberg, 2018. doi: 10.1007/978-3-662-58233-6.