Microstructural and mechanical characterizations of metallic parts made by powder-bed fusion additive manufacturing technologies

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University of Alabama Libraries

Two typical powder-bed additive manufacturing (AM) technologies are selective laser melting (SLM) and electron beam additive manufacturing (EBAM). Due to the complex thermal history and the interactions among the thermal, mechanical, and metallurgical phenomena during the manufacturing process, further study is needed to comprehensively understand the manufacturing process and achieve finishing parts with desired properties for application in the industries. The primary objectives of this research are: (1) investigate the effects of the beam scanning speed and support structure in the EBAM; (2) determine the influences of build height and thermal cycles in the SLM; (3) estimate the distribution of the induced residual stress; and (4) model the microstructural evolution of Inconel 718 in the SLM. To achieve the research objectives, microstructure characterization technologies including optical microscopy, scanning electron microscopy, Energy-dispersive X-ray spectroscopy, and electron backscattered diffraction have been utilized, and nanoindentation and Vickers indentations were used to evaluate their mechanical properties. In addition, Vickers indentation method was adopted to estimate the residual stress, and the phase field method was developed to model the microstructural evolution. Based on the results obtained, it is found that (a) a typical columnar and equiaxed microstructure were observed in the X-Plane (side surface) and Z- Plane (Scanning Surface) of the AM parts, respectively. The γ phase of Inconel 718 presented a distinct {0 0 1} texture in the Z-plane and a strong {1 0 1} texture in the Y-plane. The α phase in Ti-6Al-4V had relatively weak textures of 〈0 0 0 1〉 and 〈1 1 2 ̅ 0〉 parallel to the z-axis. (2) Fine colonies of cellular dendrites with a cell spacing of 0.511 ~ 0.845 μm were observed in the Inconel 718, which implied a cooling rate of 1.74 ~ 3.88×〖10〗^7 K∙s^(-1) (°C ∙s^(-1)). (3) With increase of the build height, the width of the columnar structure increased from 75 μm till a stable state around 150 μm, and then slightly decreased to 112 μm when closing to the ending process, which results from the variation of the thermal gradient along the build height . (4) Under continue effects of thermal cycles, the morphology of Laves phase changed from coarse and interconnected irregular particles to discrete particles, and the maximum intensity of the texture also increased. (5) Equiaxed grains were formed at the bottom of the overhang region and then translated into wider columnar structures. The solid-gas support structure acted as a heat sink to enhance heat transfer and provided support for the overhang to avoid the occurrence of sink phenomena. (6) The phase field method is a powerful tool for simulation of microstructure evolution in SLM process. The manufacturing parameters significantly affected the thermal gradient which plays an important role in the dendrite growth and a larger temperature gradient resulted in a higher growth speed. Most of the columnar cellular dendrites have a preferred growth direction of 45-72° to the scanning plane. (7) The residual stress is unevenly distributed in the parts with no notable difference in the X-plane and Y-plane. The beam scanning speed and the build height did not show significant effects on the residual stress, while the right angle interface of the geometry induced a stress concentration. (8) The mechanical properties of the parts are comparable with the count-parts made by traditional methods. (9) The volume fraction of the porosity is below 2%, and no remarkable effects were found from the thermal cycles and the build height.

Electronic Thesis or Dissertation
Mechanical engineering