Optimization and Characterization of Laser Powder Bed Fusion of AlSi10Mg for Complex Aerospace Structures

Abstract

Manufacturing for the aerospace industry is dominated by the production of components which operate in extreme environments under high loads and must be as light-weight as possible. Aerospace components are typically highly complex and produced in relatively small quantities, whereby manufacturing typically involves specialized tooling, with high costs due to complexity. Additive Manufacturing (AM) presents a potential solution to these challenges. AM presents unparalleled design freedom, since increasing the complexity of a part does not increase the cost. Not only can AM be used to aid in the manufacturing of complex aerospace components, but it can enable new designs that can significantly reduce weight and increase the performance of assemblies and systems. In particular, laser powder bed fusion (LPBF) is one of the most widely adopted metal AM technologies, as it provides high feature resolution, while achieving fully dense parts, with tailored microstructure. Although there are a number of use cases for LPBF in aerospace, there are still a number of manufacturing and design constraints that have limited its adoption. This work adds to the body of knowledge towards addressing these limitations by investigating several topics important to the practical use of LPBF for the manufacturing of aluminum aerospace components with complex design architectures, particularly in considering thin walled structures with complex surface inclinations: tailoring surface topography, studying mechanical properties, and customizing design architectures for structural and performance criteria. A case study is presented considering a key structural member for a high powered sounding rocket. The structural member is re-designed for AM to demonstrate a new methodology for the design of lightweight structures under compressive loading. Simulations were deployed which used the stress response of an initial solid beam design to create a functionally graded hollow shell that was then augmented with surface lattices to further prevent thin wall buckling. After the part was printed from AlSi10Mg using LPBF, it was subjected to compression testing. A novel test fixture was designed and validated to allow fixed-end free-end buckling testing. The design exhibited exceptionally even stress distribution and very high buckling resistance, demonstrating a 98% increase in strength compared to the conventional part design, despite the two designs having equal mass. One of the largest design constraints of LPBF is the inability to print structures with steep overhangs. When these overhangs can be printed, the downward facing surface, known as downskin, typically has a very high surface roughness. To enable the printing of structures with steeper overhangs and better surface roughness, a number of process parameter combinations are tested across downskin angles ranging from 75◦ to as low as 20◦. A modeling approach is proposed for predicting the effect of process parameters on downskin roughness and the predicted performance is found to align closely with the experimentally measured roughness. A set of process parameters are identified that enable the printing of parts at angles as low as 20◦ with good surface roughness. The tensile properties of thin-walled structures printed from AlSi10Mg are then investigated using a select range of process parameters. The effect of part thickness, part orientation, and part finishing (machining vs. as-built) on the measured tensile properties of the samples is discussed. The Young’s modulus, yield strength, ultimate tensile strength, and elongation at break are characterized for these process parameters. The effect of overhang angle on tensile properties is measured, with significant reductions in performance found at low downskin angles. The design of a number of complex aerospace components which rely on the results of this work are then discussed. This includes multiple parts slated to fly on the 2nd ever Canadian built liquid rocket.