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Abstract

Additive Manufacturing (AM) in whole and especially Selective Laser Melting (SLM) are already making a revolution in the way parts are designed and manufactured with the ability to produce lightweight parts with unprecedented complex geometries which are otherwise impossible to manufacture. These advantages are becoming more apparent and acknowledged in the manufacturing world and are reflected in a constant and rapid yearly growth (e.g. an 80% increase in the number of sold metal AM machines in 2017 compared to the year before). This thesis represents an experimental study focused on two major limitations of the SLM process: i) accumulation of tensile residual stresses (TRS) and ii) non optimal microstructures in SLM parts. In order to address these limitations, a novel hybrid additive manufacturing process entitled 3D Laser Shock Peening (3D LSP) was developed and patented. 3D LSP repetitively introduces Laser Shock Peening (LSP), a well-known surface post treatment, during the building phase of SLM in order to introduce beneficial compressive residual stresses (CRS) and subsequently decrease part distortion, improve microstructure, increase fatigue life and reduce crack density. The thesis is composed of 10 chapters. The first three chapters give an introduction to SLM and LSP, the problem statement, and a thorough analysis of the state of the art. Results obtained during this PhD are described in six chapters associated to six papers. Among the 6 papers, some have been published (2), are about to be published (1), have been submitted (2), or are planned to be submitted (1) for publication in international peer-reviewed journals. In the fourth chapter (first paper), LSP treatments were applied on SLM samples made out of two different steel grades in their rough as – built (AB) surface state and led to a clear conversion of detrimental TRS into more beneficial CRS. In the next chapter (paper), the accumulation of CRS due to the repetitive nature of the 3D LSP process is quantified. The LSP effects on the microstructure and the ability to improve recrystallization kinetics of the treated material is then shown in Chapter 6. In Chapter 7, the 3D LSP method is applied on a titanium alloy (Ti6Al4V) with a demonstrated 75% reduction in geometrical distortion compared to the SLM AB part. The next (8th) chapter demonstrates that 3D LSP can produce 316L steel parts with unrivaled fatigue life, i.e. a 15 times increase compared to the SLM AB state (non-machined surface), and more than 57 times increase compared to conventionally made parts (machined surface). The 9th chapter addresses the ability to reduce crack density in SLM parts made of a nickel based super alloy (CM247LC). 3D LSP showed a reduction of up to 95% compared to the SLM AB state. The final chapter summarizes all investigated effects of 3D LSP and gives a vision on potential future research. The thesis stands as a series of initial, feasibility studies showing the potential of a novel hybrid additive manufacturing process. They represent stepping stones towards more detailed and focused future research in each of the investigated fields.

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