There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

Additive manufacturing of metallic components – Process, structure and properties

Unlike conventional materials removal methods, additive manufacturing (AM) is based on a novel materials incremental manufacturing philosophy. Additive manufacturing implies layer by layer shaping and consolidation of powder feedstock to arbitrary configurations, normally using a computer controlled laser. The current development focus of AM is to produce complex shaped functional metallic components, including metals, alloys and metal matrix composites (MMCs), to meet demanding requirements from aerospace, defence, automotive and biomedical industries. Laser sintering (LS), laser melting (LM) and laser metal deposition (LMD) are presently regarded as the three most versatile AM processes. Laser based AM processes generally have a complex non-equilibrium physical and chemical metallurgical nature, which is material and process dependent. The influence of material characteristics and processing conditions on metallurgical mechanisms and resultant microstructural and mechanical properties of AM proc...

Laser additive manufacturing of metallic components: materials, processes and mechanisms

Additive manufacturing (AM) of nickel superalloys typically leads to large columnar grains forming along the build direction, resulting in poor and anisotropic tensile performance. In this work, a novel AM process involving directed energy deposition (DED) combined with in-situ rolling was applied to fabricate a typical nickel superalloy – Inconel 718. The results revealed hierarchical layered refined grains formed in the microstructure, due to the unique dynamic and meta-dynamic recrystallization induced by the in-situ rolling during DED. Through this, an average grain size of ∼8 µm and much weaker texture was achieved for the in-situ rolled samples. Tensile test showed that a significant increase of over 50% in the yield strength was achieved for the in-situ rolled samples at both room temperature and 650°C, while maintaining a good uniform elongation above 10%.

Hierarchical layered and refined grain structure of Inconel 718 superalloy produced by rolling-assisted directed energy deposition

Limited research has attempted to reveal the different modes of the melt pool formation in additive manufacturing This paper aims to study the mechanisms of surface roughness formation, especially on the aspect of melt pool formation which determine the surface profile and consequently significantly influence the surface roughness,In this study, the conditions under which different modes of melt pool formation (conduction mode and keyhole mode) occur for the case of as-fabricated Hastelloy X using direct metal laser solidification (DMLS) are derived and validated experimentally Top surfaces of uni-directionally built samples under various processing conditions are cut, grinded, polished and etched to reveal their individual melt pool morphologies Similarly, up-skin (slope angle  90°) melt pool morphologies are also investigated to compare the differences Surface tension gradients and resultant Marangoni flow, which dominate the melt flow in the melt pool, is also calculated to help better evaluate the melt pool shape forming,Two types of melt pool formation modes are dominating in DMLS: conduction mode and keyhole mode Melt pool formed by conduction mode generally has an aspect ratio of 1:2 (depth vs width) and is in elliptical shape Appropriate selection of scanning laser power and speed are required to maintain a low characteristic length and width ratio to prevent ballings Melt pool formed by keyhole mode has an aspect ratio of 1:1 or less High-energy contour promotes formation of key-hole-shaped melt pool which fills the gaps between layers and smoothens the up-skin surface roughness Low-energy contour scan is necessary for down-skin surface to form small melt pool profiles and achieve low Ra,This paper provides valuable insight into the origins of surface quality problem of DMLS, which is a very critical issue for upgrading the process for manufacturing real components This paper helps promote the understanding of the attributes and capabilities of this rapidly evolving three-dimensional printing technology and allow appropriate control of processing parameters for successful fabrication of components with sound surface quality

Melt pool morphology and surface roughness relationship for direct metal laser solidification of Hastelloy X

Additive manufacturing processes, of which Selective Laser Melting (SLM) is one, provide an increased design freedom and the ability to build structures directly from CAD models There is a growing interest in using optimization methods to design structures in place of manual designs Three design optimization problems were addressed in this paper The first related to axisymmetric structures and the other two addressing important design constraints when manufacturing using SLM These solutions were developed and applied to a case study of a turbine containment ring Firstly, many structural components such as a turbine containment ring are axisymmetric while they are subjected to a non-axisymmetric load A solution was presented in this paper to generate optimized axisymmetric designs for a problem in which the mechanical model was not axisymmetric The solution also worked equally well for generating a prismatic geometry with a uniform cross section, requiring no change in the procedure from axisymmetric designs to achieve this Secondly, the SLM process experiences difficulties manufacturing structures with internal voids larger than a certain upper limit A method was developed that allowed the designer to provide a value for this upper limit to the optimization method which would prevent the generation of internal voids larger than this value in any optimized design The method calculated the sizes of all the voids and did not increase their size once they reached this limit It was also aware of voids near each other, providing a minimum distance between them Finally, in order to remove the metal powder, that fills the internal voids of structures built using SLM to reduce unnecessary weight, a method was developed to build paths to join the internal voids created during the optimization process It allowed the analyst to nominate suitable path entrance locations from which powder could be removed, then found the shortest path connecting all voids and these locations For axisymmetric structures it also distributed this path around the circumference to avoid generating weak points

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Axisymmetric structural optimization design and void control for selective laser melting

The stress-strain response of samples of Ti64 and Ti550 at strain rates from 10−1 s−1 to 103 s−1 and samples of Ti811 and Ti153 at a strain rate of 103 s−1 have been assessed. It has been found that the influence of the imposed strain rate on the stress-strain response of Ti64 and Ti550 alloys is very similar – in both alloys the yield stress increases with increase of strain rate and the energy absorbed to fracture increases. At high strain rates localised deformation occurs in the form of shear bands in Ti64 and Ti550 but no shear banding was seen in Ti811 and Ti153. The fracture surfaces of Ti64 and of Ti550 show an increased tendency to brittle failure and an increase in necking with increase of strain rate. The influence of alloy microstructure and composition on the response to changes in imposed strain rate are discussed in terms of adiabatic heating and the factors controlling the flow stress in these alloys.

Response of titanium alloys to high strain rate deformation

The influence of carbon addition on the aging response of quenched Ti–13Cr (wt-%) has been investigated using hardness tests, tensile tests, optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been found that carbon refines beta grain size, leads to fine and homogenous alpha precipitation and reduces grain boundary alpha. The carbon addition accelerates the rate at which hardening occurs during aging and increases the peak hardness of the aged specimens. A significant improvement in room temperature tensile strength and ductility has also been achieved in the carbon containing alloy after aging at 500°C. The effects of carbon on the aging response appear to be attributed to dislocations introduced by carbides during quenching, elastic strain created in the matrix by carbides and gettering effect of Ti2C carbides. The influence of each of those mechanisms has been demonstrated through experiments and the factors giving r...

Xinhua Wu

Papers

Hierarchical layered and refined grain structure of Inconel 718 superalloy produced by rolling-assisted directed energy deposition

Melt pool morphology and surface roughness relationship for direct metal laser solidification of Hastelloy X

Axisymmetric structural optimization design and void control for selective laser melting

Response of titanium alloys to high strain rate deformation

Influence of carbon on aging response and tensile properties of eutectoid beta titanium alloy Ti–13Cr