Degree

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Document Type

Dissertation

Abstract

Material processing by laser is increasingly applied in industrial applications for its outstanding characteristics, such as localized heating, high efficiency, and high manufacturing precision. In this study, two kinds of laser material processing strategies were investigated, namely laser surface engineering and laser-powder-bed fusion additive manufacturing, with pure titanium and copper alloys as target materials, respectively.

For laser surface engineering related studies, the work includes the investigation of the dynamic interactions between titanium and pure nitrogen or ambient air under transient laser processing conditions. Thanks to the in-situ synchrotron X-ray diffraction tests, the high-temperature reaction steps between titanium and pure nitrogen/ambient air were successfully revealed. For the titanium-nitrogen interaction case, under a set of gas environments containing different concentrations of nitrogen, the titanium-nitrogen reaction products were examined for compositions, phases, and microstructures. Besides, considering the biomedical application of titanium, the above laser processed surfaces (titanium-nitrogen cases) were also studied for its biocompatibility and corrosion property. For the improved properties, a dense and thick titanium-nitride dendritic layer was found to be desirable. For the titanium-air interaction situation, the microstructures and composition distributions of the reaction products (under transient laser processing conditions) on and within the titanium samples were also reported, which led to the estimation of high-temperature diffusion coefficients for nitrogen and oxygen into the titanium substrate. Moreover, the formation mechanisms of both titanium nitrides and titanium oxides were also discussed.

For laser-powder-bed fusion additive manufacturing research, Cu-10Sn alloy and Cu-1.5Cr-0.5Zr alloy (wt.%, C-18150) parts were examined. After additive manufacturing preparations, the compositions, microstructures, together with mechanical, thermal, and corrosion properties were investigated under both as fabricated and heat-treated conditions. For the Cu-10Sn alloy, the as-fabricated samples exhibited smaller grain sizes and higher compression strength than those of vacuum annealed samples, which is consistent with the Hall-Petch equation. Thermal conductivity of the as fabricated samples was higher than that of the vacuum annealed samples ascribed to the two-phase constituents of the former. The corrosion rate of the as fabricated samples was almost two times higher than that of the vacuum annealed samples due to the differences in passive layers, intergranular corrosion, and internal galvanic corrosion. For the Cu-1.5Cr-0.5Zr alloy, aging treatments were performed on the samples with three differing fabrication orientations (vertical, angled (45˚), and horizontal to the building direction). Thermal conductivity of the as fabricated samples was significantly reduced due to the extremely fast cooling rate of the laser powder bed fusion process, which made the samples remain as supersaturated state. However, aging above 500 ℃ for 2 hours could dramatically recover the thermal property of the as fabricated samples. Building direction exhibited little influence on the tensile strengths but had obvious effect on ductility of the as fabricated samples. After aging treatments, tensile strengths were improved while ductility of the samples were declined. The highest tensile strength was obtained after aging treatment at 500 ℃ for 2 hours.

Date

3-3-2021

Committee Chair

Guo, Shengmin

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