Lampitella, Valerio (2022) Numerical simulation and experimental study of the powder spreading process in additive manufacturing. [Tesi di dottorato]

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Abstract

The term Additive manufacturing refers to a wide family of production technologies that can be classified based on a great number of features such as the operating principle and the material employed however, this thesis will focus on the Laser Powder Bed Fusion (LPBF) technologies and more specifically on the powder bed deposition stage. LPBF technologies (AM) have become commonly used method for the production of parts to be employed in critical applications in an ever-expanding list of fields ranging from aerospace and automotive to medical devices. Indeed, the popularity of this method of production resides in the undeniable advantages it allows, i.e., material savings, design flexibility, customization etc. and in how such advantageous features perfectly apply to cases where a complex design and reliable mechanical properties are required for small batches productions. However, a further affirmation of LPBF technologies is hindered by the incomplete understanding of the complex multiphysics involved. Indeed, the selection of the operating process parameters is not trivial in case of novel materials and requires a laborious trial and error approach that can in turn lessen, if not completely even out the aforementioned advantages. Another weakness of this technologies is the high volatility of the finished parts characteristics if compared with more traditional and stable methods that leads to issues such as process repeatability, internal defects of the printed parts, and non-uniformity of the properties within the building chamber. In fact, a Laser Powder Bed Fusion process comprises multiple stages: first, the feedstock material, in the form of micrometric powders, is spread in layers ranging from few microns to several dozens, then selected areas of the deposited layer are melted by a focused laser beam. The steps are repeated until the final part is completed. As mentioned before the work presented in this thesis will focus on the first step: the powder bed deposition. This step can be regarded as a sub process with his own input parameters and outputs which in turn can influence the successive step and the printing process as a whole. Indeed, the spreading process is the only form of control over the state of the powder bed that will be processed by the laser beam and any defect or discontinuity will affect the layer and consequently the final part. Moreover, the laser’s parameters (i.e., power, scan speed etc.) are set not taking into consideration the local variations in the powder bed characteristics and are unlikely to be ideal for the whole layer. Therefore, clarifying the relationship that links inputs and outputs means being able to obtain the desired characteristics of the powder bed trough an appropriate selection of the process parameters and ultimately grants more control over the final result. Such a fine level of control on the process has significant implications on both industrial and research applications. From the industrial point of view, it means increasing the reliability of the existing process, eliminating probable sources of defects, obtaining more uniform mechanical properties throughout the printing chamber and between successive prints. Moreover, the characteristics of the powder bed could be chosen as to enhance the laser-matter interaction, increasing the energy efficiency of the process. As for the research on the topic, clarifying and controlling the spreading mechanisms is pivotal in cases where the feedstock material is made up of particles with significantly heterogeneous characteristics (e.g., size, material, shape etc.) in order to avoid segregation phenomena or suboptimal spreading that can cause defects in the final part. This scenario is relevant when dealing with the printing of tailored materials or new alloys. In this light, the extent of the impact of a complete comprehension of the link between the spreading process parameter and the resulting powder bed appears evident. Therefore, the aim of this thesis is to investigate the powder bed deposition stage during a LPBF in order to deepen the understanding the spreading mechanism. The investigations have been carried out by means of both a numerical and experimental approach. The outline is the following: • Section 1: State of the art • Section 2: Experimental analysis • Section 3: Numerical model • Section 4: Application • Section 5: Future developments • Section 6: Conclusions

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