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Investigating and Modeling the Microstructure and Mechanical Properties of Additively Manufactured High Strength Alloys

Creative Commons 'BY-NC-SA' version 4.0 license

Laser-based powder bed fusion (LPBF) is an additive manufacturing method that fabricates parts layer by layer by fusing the raw material powder using laser as the energy source. Originally, LPBF was used for rapid prototyping due to its capability to fabricate prototypes whenever changes are introduced to the concept CAD model without lead-time or the need to modify the fabrication line. Due to the inferior mechanical properties such as strength and stiffness of the fabricated parts compared to conventionally manufactured parts, and the limited options of materials, additive manufacturing was not used to fabricate end user functional parts. However, the recent developments in the last decade such as the introduction of high-power lasers with more compatible wavelengths, and higher quality raw materials, provide the opportunity to produce functional parts with high mechanical properties. Currently, LPBF is capable of producing parts with near full relative density. However, these parts still suffer from high anisotropy, defects, residual stresses, and inferior tensile strength, and fatigue life. This dissertation investigates different approaches to render the manufacturing of load-bearing metallic parts using LPBF more effective and reliable. The work presented focuses on two alloys, 15-5PH stainless steel and Inconel 718, due to their comparability with LPBF and their use in aerospace and automotive industries where additive manufacturing can be used cost-effectively.

The microstructure and mechanical properties of LPBF parts are highly sensitive to the processing parameters during fabrication and post-processing. Metallic parts are sensitive to the laser power, laser scanning speed, hatch spacing, powder layer thickness, and the orientation of the part during fabrication. Several studies investigated the effect of the processing parameters on the relative density and mechanical properties of LPBF parts. It was found that the set of processing parameters required to obtain high relative density is usually different than the set of processing parameters required to optimize the mechanical properties. To achieve the full potential of LPBF parts this work aims at optimizing the microstructure and mechanical properties of parts fabricated with the set of processing parameters that maximizes the relative density of the part. Therefore, the first objective of this dissertation is to characterize the microstructure and mechanical properties of as-fabricated LPBF parts at room and elevated temperatures.

The second objective is to develop and investigate different post-processing heat treatments that aim to homogenize the microstructure, improve the reliability of the parts, and identify the treatments that have the potential to improve fatigue strength and life.

Finally, the last objective of this dissertation is to develop predictive models that assist in estimating the effect of different heat treatments on the mechanical properties of LPBF produce parts. A hybrid approach of physical and data-based derived models was used to evaluate the influence of post-processing heat treatment on the tensile strength of additively manufactured Inconel 718.

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