| Selective Laser Melting(SLM)belongs to a family of technologies known as Additive Manufacturing or 3D printing,through which it is possible to directly manufacture parts with complex geometries from powders in one single step.Nowadays SLM has been widely used for fabricating components with complex geometries,but its potential for forming the multi-scale microstructures has not been well explored.With the main goal of producing complex materials by additive manufacturing in high qualities that this thesis will focus two essential topics about microstructure control and defects eliminating through delving into the micro-scale melt pool characteristics.SLM of pure tungsten was carried out and the consolidation issues were investigated.It was observed that balling of melted droplets at the laser focal points and entrapped cavities hindered the preparation of fully dense parts.An analysis of the balling mechanism revealed that SLM was a process where melt spreading and solidification competed with each other.The material intrinsic properties,the processing parameters and the balling phenomena were considered synthetically,melted tungsten droplets wetted its own solid substrate at a low speed driven by capillary force,but solidified simultaneously at a high speed driven by the existing steep temperature gradients.Melted droplets solidified before spreading completely and kept their globular geometry instead of creating a flat layer.Adjusting laser exposure time and using laser re-melting can be considered as possible approaches to further eliminate balling defects and improve density.Another reason of SLM defects can be correlated to the melt pool dynamics,oscillations and Plateau–Rayleigh instabilities.Accurate 3D images of defects inside SLM fabricated Co–Cr–Mo samples were provided by synchrotron radiation micro-CT imaging in this thesis,and with 3D reconstructed images distinctive morphologies of SLM defects spanning across the consolidated powder layers were generated.The accidental single layer defects form as gaps between adjacent laser melt tracks or melt track discontinuousness caused by inherent fluid instability under various disturbances.By stabilizing the melt pool flow and by reducing the surface roughness through adjusting processing parameters it appears possible to reduce the defect concentrations.Unique crystal textures formed by SLM of Co–Cr–Mo alloy were studied,when zigzag laser scan strategy is applied,columnar grains parallel with each other grow epitaxial from grains in the previous layers and across many fusion boundaries along the building direction.Orientation of these columnar grains are not purely <001> or <111>as reported previously,but a grain growth reflecting the concurrence of <001> and<011>.By rotating the scan direction every layer,the columnar grain structure could be fractionated as a result of the more complex and varied melt pool situation.The unique sub-grain patterns have been found in some particular alloys(316L,Al-Si,Co-Cr-Mo)SLM,the submicron-scale cellular,elongated cellular or even band structures are always coexisting.Furthermore,the cellular structures are symmetrical with hexagonal,pentagonal and square cellular patterns where the cellular size is only around 1?m.Single-layer and bulk 316 L SLM experiments are presented that reveals the forming mechanism of these sub-grain cellular microstructures.Complex cellular sub-micron patterns were formed by the local convection and Bénard Instabilities in front of the solid/liquid(S/L)interface(so-called mushy zones)affected by intricate temperature and surface tension gradients.This nonlinear self-organization phenomenon(Bénard Instability)occurring at the S/L interface is superimposed on the macro-grain solidification process to form the sub-grain patterns/structures. |
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