Laser-Powder Bed Fusion (L-PBF) is an additive manufacturing technique used to melt metal material into solid three-dimensional parts. While offering a high degree of design freedom, L-PBF still has technical restrictions, like the achievable surface roughness, resolution and the need for support structures in overhanging areas. [1] Currently, L-PBF is used mainly to produce small batches of parts and prototypes. [2] In order to fully industrialize the technology, the research campus in Aachen is investigating possible future applications in turbomachinery while developing the corresponding processes with industry partners. Sealing systems, like honeycomb seal strips in gas turbines often require time-consuming joining and assembly operations that can be avoided by building up the structure monolithically using L-PBF. The following process development study proves the feasibility of manufacturing honeycombs with L-PBF using the Nickel-based super-alloy Inconel 718 (IN718) on an EOS M290 machine. Here, we have evaluated the economic aspects of different build orientations of the seal strips. Afterwards, we conducted a systematic parameter study with continuous and pulsed wave laser emission and investigated the resulting wall thicknesses. A reduction in wall thickness of about 30% can be observed when a modulated laser is used.
Laser powder bed fusion (L-PBF) of entire assemblies is not typically practical for technical and economic reasons. The build size limitations and high production costs of L-PBF make it competitive for smaller, highly complex components, while the less complex elements of an assembly are manufactured conventionally. This leads to scenarios that use L-PBF only where it’s beneficial, and it require an integration and joining to form the final product. For example, L-PBF combustion swirlers are welded onto cast parts to produce combustion systems for stationary gas turbines. Today, the welding process requires complex welding fixtures and tack welds to ensure the correct alignment and positioning of the parts for repeatable weld results. In this paper, L-PBF and milled weld preparations are presented as a way to simplify the Tungsten inert gas (TIG) welding of rotationally symmetrical geometries using integrated features for alignment and fixation. Pipe specimens with the proposed designs are manufactured in Inconel 625 using L-PBF and milling. The pipe assembly is tested and TIG welding is performed for validation. 3D scans of the pipes before and after welding are evaluated, and the weld quality is examined via metallography and computed tomography (CT) scans. All welds produced in this study passed the highest evaluation group B according to DIN 5817. Thanks to good component alignment, safe handling, and a stable welding process, the developed designs eliminate the need for part-specific fixtures, simplify the process chain, and increase the process reliability. The results are applicable to a wide range of components with similar requirements.
The industrial use of laser powder-bed fusion (L-PBF) in turbomachinery is gaining momentum renderingthe inspection and quali?cation of certain post-processing steps necessary. This includes fusiontechniques that allow to print multiple parts separately to take advantage of e.g. various print orientationsand join them subsequently. The main motivation of this study is to validate the tungsten inertgas (TIG) welding process of L-PBF manufactured parts using industrial speci?cations relevant for gasturbines to pave the way for the industrial production of modular build setups. For this, two commonlyused nickel-based super alloys for high-temperature applications, Inconel 718 and Inconel 625 are chosen.Since their defect-free printability has been established widely, we focus on the suitability to be joined usingTIG welding. The process is evaluated performing microstructural examination and mechanical testsin as-built as well as heat-treated samples. The welds are assessed by applying a general weld quali?cationapproach used at Siemens Gas and Power. It was found that both materials can be joined via TIGwelding using standard weld parameters causing minimal defects. A solution annealing heat treatmentbefore welding is not necessary for a positive outcome, but still recommended for Inconel 718.
The production of high-temperature components is of great importance for the transport and energy sector. Forging of high-temperature alloys often requires expensive dies, multiple forming steps and leads to forged parts with large tolerances that require machining to create the final shape. Additive manufacturing (AM) offers the possibility to print the desired shapes directly as net-shape components. AM could provide the advantage of being more energy-efficient compared to forging if the energy contained in the machining scrap exceeds the energy needed for powder production and laser processing. However, the microstructure and performance of 3D-printed parts will not reach the level of forged material unless further processes such as hot-isostatic pressing are applied. Combining AM and metal forming could pave the way for new process chains with little material waste, reduced tooling costs and increased part performance. This study investigates the hot working properties and microstructure evolution of Ti–6Al–4V pre-forms made by selective laser melting and electron beam melting. The results show that both materials are hot workable in the as-built state. Due to its martensitic microstructure, the SLM material shows a lower activation energy for hot working than EBM and wrought material and a faster globularization during forming, which is beneficial for hot forming since it reduces the forming forces and tool loads.
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