Abstract :
[en] This joint PhD thesis between MSM (Mechanics of Solids and Materials) and MMS (Metallic Materials Science) teams, presents an in-depth investigation into the Directed Energy Deposition process of High-Speed Steel M4. Through a blend of computational modeling (MSM) and experimental work (MMS), this research explores the complex interrelations between process parameters, thermal history, and the resultant microstructural and mechanical properties of DED parts fabricated in High Speed Steel HSS M4. This study was supported by a FNRS PDR Project “Lasercladding” and additional funding from an ULiege Faculty grant, alongside collaborative efforts with SIRRIS.
The thesis begins by establishing a foundational understanding of the DED process, exploring its potential defects, and underscoring the necessity of a modeling tool capable of optimizing parameters to mitigate these defects. Chapter 2 introduces a 2D thermal model, developed to comprehend the thermal dynamics within a 36-layer bulk deposit of HSS M4. This model, validated against measured thermal histories and melt pool sizes, presents out the basis for a study into thermal effects on microstructure.
Progressing to Chapter 3, the research transitions to a 3D thermomechanical model, confronting the challenges posed by deviations in measured thermophysical properties from those traditionally reported for cast or forged samples. A series of compression tests across various temperatures and strain rates further enriches the understanding of the stress-strain behavior of M4 HSS, laying the groundwork for subsequent analyses.
In Chapter 4, the research focuses on the optimization of the DED process, specifically through the implementation of a variable laser power strategy aimed at achieving uniform microstructures at different deposit depths. The relationship between melt pool size, remelting events, and microstructure is examined, employing the Newton-Raphson iterative method to fine-tune the laser power function for optimal melt pool dimensions. This approach is validated by real experiments, which demonstrate the possibility of obtaining homogeneity of the microstructure in the properties of materials throughout the depth of the sample.
The thesis concludes with an evaluation of the mechanical properties of the optimized samples, utilizing microhardness and nanoindentation tests. These assessments reveal significant improvements in homogeneity, attributable to the refined thermal management and microstructure optimization strategies developed through this research.
This work focuses on the DED process and its impact on HSS M4. It highlights the critical role of accurate thermal and mechanical modeling in advancing AM technologies. The insights and methodologies developed here offer support for ongoing work in both research teams, providing a foundation for further exploration and innovation in metal additive manufacturing.
