METALLIC MATERIALS STRENGTHENING VIA SELECTIVE LASER MELTING BY PULSED LASERS
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METALLIC MATERIALS STRENGTHENING VIA SELECTIVE LASER MELTING EMPLOYING NANOSECOND PULSED LASERS
The Selective Laser Melting (SLM) process is a manufacturing technique that facilitates the production of metallic parts with complex geometries and reduces both materials waste and lead time. The high tunability of the process parameters in SLM allows the design of the as-built part’s characteristics, such as controlled microstructure formation, residual stresses, presence of pores, and lack of fusion. The main parameter in the SLM process that influences these parts’ characteristics is the transient temperature field resulting from the laser-matter interaction. Nanosecond pulsed lasers in SLM have the advantage of enabling rapid and localized heating and cooling that make the formation of ultrafine grains possible. This work shows how different pulse durations can change the near-surface microstructure and overall mechanical properties of metallic parts. The nanosecond pulses can melt and resolidify aluminum parts’ near-surface region to form nanograined gradient structures with yield strengths as high as 250.8 MPa and indentation strengths as high as 725 MPa, which are comparable to some steel's mechanical properties. Knowing that the nanosecond pulsed lasers cause microstructure refinement for high-purity metals, the microstructure variations effects were also investigated for the cast iron alloy. Cast iron was used alone and mixed with born or boron nitride powders to induce the precipitation of strengthening phases only enabled under high cooling rates. Although producing parts with superior mechanical properties and controlling the precipitation of strengthening phases, the SLM process with nanosecond pulsed lasers is still accompanied by defects formation, mainly explained by the large thermal gradients, keyhole effect, reduced melt pool depth, and rapid cooling rates. Ideally, a smooth heating rate able to sinter powder grains, facilitating the heat flow through the heat-affected zone, followed by a sharper heating rate that generates a fully molten region, but minimizes ablation at the same time are targeted to reduce the porosity and lack of fusion. Then, a sharp cooling rate that can increase the nucleation rate, consequently refining the final microstructure is targeted in the production of strong materials in SLM with pulsed lasers. This work is the pioneer in controlling the transient temperature field during the heating and cooling stages in pulsed laser processing. The temperature field control capability by shaping a nanosecond laser pulse in the time domain affecting defects formation, residual strains, and microstructure was achieved, opening a wide research niche in the additive manufacturing field.
- Doctor of Philosophy
- Aeronautics and Astronautics
- West Lafayette