A framework to develop processing-microstructure-mechanical behavior relationships for metal additive manufacturing

Emine Tekerek

doi:10.17918/00001645

Laser powder bed fusion (L-PBF) is a promising Metal Additive Manufacturing (MAM) method which offers design flexibility in producing geometrically complex metallic parts in a short production time. During the LPBF process, the input material undergoes multi-physics nature and thermal cycles related to the laser-induced, layer-by-layer solidification and melting of the powder feed-stock material. This process leads to different microstructural evolutions and results to both material and geometrical defects in the as-manufactured state, as well as to uncertainty in the overall mechanical behavior of L-PBF produced as-build parts compared to the traditionally manufactured ones. Therefore, this dissertation focuses on damage mechanisms of L-PBF produced as-build AlSi10Mg alloys under monotonic tension and cyclic loading. Most emphasis in the literature has been given either the effects of defects on the mechanical properties of this material or fracture for performance considerations. In this dissertation, a technical workflow which leverages a critical combination of tools and methods, is presented which together investigate ways to both define and also reduce the parametric space which affects the quality of as-built AlSi10Mg alloys by implementing on axis, melt-pool scale, in situ monitoring data collected during the AM process were examined in conjunction with the formation of flaws. The research plan prioritizes on developing a design-of-experiments (DOE) approach capable of selecting the most important geometry, manufacturing, and material factors which could be varied parametrically to investigate the physical mechanisms involved in the solidification process in terms of crystallographic texture, grain size and sub-grain dislocation structures, phase transformations, secondary phase and precipitation formations and their role in fatigue of AM metals. Furthermore, the emphasis is placed on using nondestructive evaluation (NDE) combined with mechanical testing and characterization methods applied at a scale where damage incubation and initiation is occurring for this newly discovered microstructure. Specifically, a setup built inside a Scanning Electron Microscope (SEM) and retrofitted to be combined with characterization and NDE capabilities was developed with the goal to track the early stages of the damage incubation and microstructure-scale early crack initiation in this type of material. The characterization capabilities include Electron Backscatter Diffraction (EBSD) and Energy Dispersive Spectroscopy (EDS) in addition to X-ray micro-computed tomography ([mu]-CT) and nanoindentation, in addition to microscopy achieved by the Secondary Electron (SE) and Back Scatter Electron (BSE) detectors. Moreover, on axis, melt-pool scale, in situ monitoring data collected during the AM process were examined in conjunction with the formation of flaws. Extensions of the presented approach to include information from computational methods as well as their applicability to other material systems are discussed.

A framework to develop processing-microstructure-mechanical behavior relationships for metal additive manufacturing

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A framework to develop processing-microstructure-mechanical behavior relationships for metal additive manufacturing

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