Solidification progress and heat transfer analysis of gas-atomized alloy droplets during spray forming
TL;DR: In this article, the authors used the Newtonian heat transfer formulation coupled with the classical heterogeneous nucleation and specific solidification process to predict gas and droplet velocities, droplet temperature, and fractional solidification with flight distance during spray forming, and found that the corresponding solid fractions formed during recalesced, segregated, and eutectic solidifications are linearly related to the degree of undercooling.
Abstract: In order to predict gas and droplet velocities, droplet temperature, and fractional solidification with flight distance during spray forming, the Newtonian heat transfer formulation has been coupled with the classical heterogeneous nucleation and the specific solidification process. It has been demonstrated that the thermal profile of the droplet in flight is significantly affected by process parameters such as droplet size, initial gas velocity, undercooling, and superheat. With increasing droplet size or initial gas velocity, the onset and completion of solidification are shifted to greater flight distances and the solidification process also extends over a wider range of flight distances. It has been found that the corresponding solid fractions formed during recalesced, segregated, and eutectic solidifications are linearly related to the degree of undercooling and that those solid fractions are insensitive to droplet size, initial gas velocity and superheat.
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TL;DR: In this article, the thermal behavior during laser deposition with LENS is simulated numerically by using the alternate-direction explicit (ADE) finite difference method (FDM), and the simulation results showed that deposited material experiences a significant rapid quenching effect during the initial stages of deposition and can attain a very high cooling rate.
Abstract: Laser-engineered net shaping (LENS) is a rapid direct manufacturing process. The LENS process can be analyzed as a sequence of discrete events, given that it is a layer-by-layer process. The thermal history associated with the LENS process involves numerous reheating cycles. In this article, the thermal behavior during laser deposition with LENS is simulated numerically by using the alternate-direction explicit (ADE) finite difference method (FDM). The simulation results showed that deposited material experiences a significant rapid quenching effect during the initial stages of deposition and can attain a very high cooling rate. With an increase in deposit thickness, the rapid quenching effect decreases and eventually disappears. The effects of the processing parameters on the thermal behavior of deposited materials were also simulated and analyzed. The objective of this study is to provide insight into the thermal history during the LENS process, where the ability to correlate process parameters to microstructural evolution is a motivating force.
196 citations
TL;DR: In this article, the microstructures of the as-received powder and cold spray processed (CSP) ultrafine-grained (UFG) 6061 depositions were characterized by different electron microscopy techniques.
Abstract: Gas-atomized 6061 aluminum powder was used as feedstock for deposition using a high pressure cold-spraying process. The microstructures of the as-received powder and cold spray processed (CSP) ultrafine-grained (UFG) 6061 depositions were characterized by different electron microscopy techniques. It was found that there is segregation of solute elements at the particle grain boundaries, which is increased after cold spraying (CS). Various microstructural features were observed in both directions (parallel and perpendicular) of the CSP layer, including low-angle grain boundaries, clustered-small-cell walls, and dislocation tangle zones. The results also indicated that a combination of different recrystallization mechanisms (i.e., continuous and geometrical) may contribute to the formation of nano and UFG structures during CS.
73 citations
TL;DR: In this paper, the authors developed a reliable and controlled experimental technique by which the transfer of heat from a high temperature droplet to a significantly cooler gas can be assessed and used to understand and improve gas atomization systems.
71 citations
TL;DR: In this article, the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al-20Si-5Fe-2X alloys was studied.
58 citations
15 Jan 2019
TL;DR: In this article, phase components of four widely used steels (316L, H13, P20 and 18Ni300) were characterized by X-ray diffraction (XRD), synchrotron radiation (SR-XRD) and scanning electron microscopy (SEM).
Abstract: The determination of microstructural details for powder materials is vital for facilitating their selective laser melting (SLM) process. Four widely used steels (316L, H13, P20 and 18Ni300) have been investigated to detail their powders’ microstructures as well as laser absorptivity to understand their SLM processing from raw material perspective. Phase components of these four steel powders were characterized by X-ray diffraction (XRD), synchrotron radiation X-ray diffraction (SR-XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were utilized to reveal the surface structure of these four steel powders. It is found that phase components of H13, P20 and 18Ni300 are mainly composed of martensite and a small amount of austenite due to the high cooling rate during gas atomization processing, while 316L is characterized by austenite. XPS results show that the four steel powders all possess a layered surface structure, consisting of a thin iron oxide layer at the outmost surface and metal matrix at the inner surface. It is found that the presence of such oxide layer can improve the absorptivity of steel powders and is beneficial for their SLM process.
50 citations
References
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Book•
01 Jan 1955
TL;DR: The flow laws of the actual flows at high Reynolds numbers differ considerably from those of the laminar flows treated in the preceding part, denoted as turbulence as discussed by the authors, and the actual flow is very different from that of the Poiseuille flow.
Abstract: The flow laws of the actual flows at high Reynolds numbers differ considerably from those of the laminar flows treated in the preceding part. These actual flows show a special characteristic, denoted as turbulence. The character of a turbulent flow is most easily understood the case of the pipe flow. Consider the flow through a straight pipe of circular cross section and with a smooth wall. For laminar flow each fluid particle moves with uniform velocity along a rectilinear path. Because of viscosity, the velocity of the particles near the wall is smaller than that of the particles at the center. i% order to maintain the motion, a pressure decrease is required which, for laminar flow, is proportional to the first power of the mean flow velocity. Actually, however, one ob~erves that, for larger Reynolds numbers, the pressure drop increases almost with the square of the velocity and is very much larger then that given by the Hagen Poiseuille law. One may conclude that the actual flow is very different from that of the Poiseuille flow.
17,321 citations
Journal Article•
01 Jun 1978TL;DR: In this paper, the authors evaluated the applicability of the standard κ-ϵ equations and other turbulence models with respect to their applicability in swirling, recirculating flows.
Abstract: The standard κ-ϵ equations and other turbulence models are evaluated with respect to their applicability in swirling, recirculating flows. The turbulence models are formulated on the basis of two separate viewpoints. The first perspective assumes that an isotropic eddy viscosity and the modified Boussinesq hypothesis adequately describe the stress distributions, and that the source of predictive error is a consequence of the modeled terms in the κ-ϵ equations. Both stabilizing and destabilizing Richardson number corrections are incorporated to investigate this line of reasoning. A second viewpoint proposes that the eddy viscosity approach is inherently inadequate and that a redistribution of the stress magnitudes is necessary. Investigation of higher-order closure is pursued on the level of an algebraic stress closure. Various turbulence model predictions are compared with experimental data from a variety of isothermal, confined studies. Supportive swirl comparisons are also performed for a laminar flow case, as well as reacting flow cases. Parallel predictions or contributions from other sources are also consulted where appropriate. Predictive accuracy was found to be a partial function of inlet boundary conditions and numerical diffusion. Despite prediction sensitivity to inlet conditions and numerics, the data comparisons delineate the relative advantages and disadvantages of the various modifications. Possible research avenues in the area of computational modeling of strongly swirling, recirculating flows are reviewed and discussed.
5,396 citations
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01 Jun 1990
TL;DR: In this article, Atom Transfer at the Solid/Liquid Interface Morphological Instability of a Solid/ Liquid Interface Solidification Microstructure: Cells and Dendrites SolidificationMicrostructure, Eutectic and Peritectic Solute Redistribution Rapid Solidization Microstructures Summary Appendices Symbols Index
Abstract: Introduction Atom Transfer at the Solid/Liquid Interface Morphological Instability of a Solid/Liquid Interface Solidification Microstructure: Cells and Dendrites Solidification Microstructure: Eutectic and Peritectic Solute Redistribution Rapid Solidification Microstructures Summary Appendices Symbols Index
3,439 citations
Journal Article•
2,679 citations
Book•
01 Jan 1969
TL;DR: In this article, a unified treatment of momentum transfer (fluid mechanics), heat transfer and mass transfer is presented, with a focus on modern applications of the basic material, and many new homework exercises at the end of each chapter.
Abstract: Providing a unified treatment of momentum transfer (fluid mechanics), heat transfer and mass transfer. This new edition includes more modern applications of the basic material, and to provide many new homework exercises at the end of each chapter.
1,973 citations