scholarly journals Simulation of the Columnar-to-Equiaxed Transition in Alloy Solidification - The Effect of Nucleation Undercooling, Density of Nuclei in Bulk Liquid and Alloy Solidification Range on the Transition

2008 ◽  
Vol 139 ◽  
pp. 129-134 ◽  
Author(s):  
H.J. Dai ◽  
H.B. Dong ◽  
H.V. Atkinson ◽  
Peter D. Lee
Keyword(s):  
Dendritic Structure ◽  
Bulk Liquid ◽  
Structure Evolution ◽  
Alloy Solidification ◽  
Solidification Range ◽  
Nucleation Undercooling ◽  
Grain Structures ◽  
Columnar To Equiaxed Transition ◽  
Equiaxed Grains ◽  
The Relationship

A coupled cellular automaton-finite difference (CA-FD) model is used to simulate the detailed dendritic structure evolution of the columnar-to-equiaxed transition (CET) for Al-Cu alloys during solidification. The effects of material properties (nucleation undercooling, density of nuclei in bulk liquid and alloy solidification range) on the CET are investigated. Simulated results reveal that: (1) equiaxed grains form at an earlier stage with a smaller critical nucleation undercooling; (2) CET is promoted if the density of nuclei in bulk liquid is increased; (3) extending the alloy solidification range promotes the CET. Finally, CET maps corresponding to different alloy concentrations are constructed, illustrating the relationship between processing conditions and the resulting grain structures for alloys with different solidification ranges.

scholarly journals The Interaction between Grains during Columnar-to-Equiaxed Transition in Laser Welding: A Phase-Field Study

Metals ◽  
2020 ◽  
Vol 10 (12) ◽  
pp. 1647
Author(s):  
Lingda Xiong ◽  
Chunming Wang ◽  
Zhimin Wang ◽  
Ping Jiang
Keyword(s):  
Laser Welding ◽  
Phase Field ◽  
Numerical Results ◽  
Model Alloy ◽  
Columnar Dendrite ◽  
Nucleation Density ◽  
Al Alloy ◽  
Nucleation Undercooling ◽  
Columnar To Equiaxed Transition ◽  
Equiaxed Grains

A phase-field model was applied to study CET (columnar-to-equiaxed transition) during laser welding of an Al-Cu model alloy. A parametric study was performed to investigate the effects of nucleation undercooling for the equiaxed grains, nucleation density and location of the first nucleation seed ahead of the columnar front on the microstructure of the fusion zone. The numerical results indicated that nucleation undercooling significantly influenced the occurrence and the time of CET. Nucleation density affected the occurrence of CET and the size of equiaxed grains. The dendrite growth behavior was analyzed to reveal the mechanism of the CET. The interactions between different grains were studied. Once the seeds ahead of the columnar dendrites nucleated and grew, the columnar dendrite tip velocity began to fluctuate around a value. It did not decrease until the columnar dendrite got rather close to the equiaxed grains. The undercooling and solute segregation profile evolutions of the columnar dendrite tip with the CET and without the CET had no significant difference before the CET occurred. Mechanical blocking was the major blocking mechanism for the CET. The equiaxed grains formed first were larger than the equiaxed grains formed later due to the decreasing of undercooling. The size of equiaxed grain decreased from fusion line to center line. The numerical results were basically consistent with the experimental results obtained by laser welding of a 2A12 Al-alloy.

Formation of Grain Structures of Thermal Sprayed Nickel Coatings and the Coating Properties

2005 ◽  
Vol 502 ◽  
pp. 517-0
Author(s):  
Kenji Murakami
Keyword(s):  
Steel Substrate ◽  
Nickel Coatings ◽  
Grain Structures ◽  
Heat Treated ◽  
Columnar Grains ◽  
Different Temperatures ◽  
Plasma Sprayed ◽  
Equiaxed Grains ◽  
Sprayed Coatings ◽  
Lamellar Interfaces

Pure nickel powder was low pressure plasma sprayed onto a steel substrate held at different temperatures during spraying. The as-sprayed coatings consist of columnar grains whose axes are nearly perpendicular to the lamellae composing the coatings. As the coating temperature becomes higher, the length of the columnar grains increases and is longer than the thickness of the lamellae, indicating the growth of the grains across the lamellar interfaces during spraying. On the other hand, the coatings that were heat treated after spraying consist of coarse equiaxed grains. The coatings that experienced high temperatures during spraying or the heat treated coatings have large porosity and contain large globular pores. The hardness, apparent density and the tensile strength of the coating itself were the highest for the coating prepared at a low temperature and became low on heat treatment. The thermal conductivity in the direction perpendicular to the coating was the largest for the coating that consisted of long columnar grains.

scholarly journals Crystalline modification of a rare earth nucleating agent for isotactic polypropylene based on its self-assembly

2018 ◽  
Vol 5 (5) ◽  
pp. 180247 ◽  
Author(s):  
Yuanming Zhang ◽  
Tingting Sun ◽  
Wei Jiang ◽  
Guangting Han
Keyword(s):  
Hydrogen Bond ◽  
Rare Earth ◽  
Isotactic Polypropylene ◽  
Self Assembly ◽  
Dendritic Structure ◽  
Nucleating Agent ◽  
The Self ◽  
Structure Evolution ◽  
Crystalline Modification ◽  
Assembly Structure

In this paper, the crystalline modification of a rare earth nucleating agent (WBG) for isotactic polypropylene (PP) based on its supramolecular self-assembly was investigated by differential scanning calorimetry, wide-angle X-ray diffraction and polarized optical microscopy. In addition, the relationship between the self-assembly structure of the nucleating agent and the crystalline structure, as well as the possible reason for the self-assembly behaviour, was further studied. The structure evolution of WBG showed that the self-assembly structure changed from a needle-like structure to a dendritic structure with increase in the content of WBG. When the content of WBG exceeded a critical value (0.4 wt%), it self-assembled into a strip structure. This revealed that the structure evolution of WBG contributed to the K β and the crystallization morphology of PP with different content of WBG. In addition, further studies implied that the behaviour of self-assembly was a liquid–solid transformation of WBG, followed by a liquid–liquid phase separation of molten isotactic PP and WBG. The formation of the self-assembly structure was based on the free molecules by hydrogen bond dissociation while being heated, followed by aggregation into another structure by hydrogen bond association while being cooled. Furthermore, self-assembly behaviour depends largely on the interaction between WBG themselves.

Study on Production and Environmental Factors Based on GMM of Dynamic Panel System

2016 ◽  
Vol 693 ◽  
pp. 1922-1934
Author(s):  
B. Li ◽  
R. Yi ◽  
T. Li
Keyword(s):  
Land Use ◽  
Industrial Structure ◽  
Structure Evolution ◽  
Dynamic Panel ◽  
Urbanization Level ◽  
Industrial Structures ◽  
Intensive Land Use ◽  
The Relationship ◽  
Stimulation Of ◽  
New Urbanization

This paper measured the new urbanization level and the degree of intensive land use in 28 provinces of China from 2006 to 2012 through building index system, and built the economic model by utilizing GMM system to demonstrate the relationship between new urbanization, industrial structure evolution and intensive land use based on panel data. The result indicates that the industrial structure evolution is conducive to intensive land use. The development of new urbanization and the intensive land use generate structural contradictions, but it will promote intensive land use indirectly through stimulation of the industrial structure evolution. At the regional level, new urbanization inhibit the intensive land use in the eastern regions and promote it in the western regions, while the effect in the central region is not significant. Industrial structures in some regions stimulate the intensive land use directly or indirectly.

3D Characterization of Continuously Cast Slabs

2007 ◽  
Vol 537-538 ◽  
pp. 555-562
Author(s):  
Mihály Réger ◽  
Balázs Verő ◽  
Árpád Szélig
Keyword(s):  
Dendritic Structure ◽  
Transfer Model ◽  
Chemical Compositions ◽  
Research Activity ◽  
Casting Technology ◽  
Transition Position ◽  
Continuously Cast ◽  
Columnar To Equiaxed Transition ◽  
Semi Empirical

This paper deals with the characterization of solidification, cooling and expected properties of continuously cast slabs. Semi-empirical models based on theoretical consideration and on the results of heat transfer model are used for characterization of the following: surface and inner temperature distribution of the cast semis, liquid sump depth and shape, liquid motion intensity resulting in centerline segregation, parameters of the primary dendritic structure (primary and secondary dendritic spacings, columnar to equiaxed transition position, CET). This method provides an opportunity to make a realistic comparison between the solidification and final properties of semis with different chemical compositions as a function of applied casting technologies. The final goal of this research activity in the future is to define a comprehensive quality function for optimizing continuous casting technology.

Comparison of Microstructure and Mechanical Properties of Semi-Solid Castings Produced Using Billets Made by EMS and SEED

2014 ◽  
Vol 217-218 ◽  
pp. 332-339 ◽  
Author(s):  
Xiao Kang Liang ◽  
Da Quan Li ◽  
Pascal Côté ◽  
Stephen P. Midson ◽  
Qiang Zhu
Keyword(s):  
Mechanical Properties ◽  
Cross Section ◽  
Dendritic Structure ◽  
Casting Process ◽  
Grain Structure ◽  
High Quality ◽  
Microstructure And Mechanical Properties ◽  
Grain Structures ◽  
Heat Treated ◽  
Semi Solid

The spheroidal grains in billets used for semi-solid casting are generally manufactured by electromagnetic stirring (EMS) during the casting process. This method however, is not economically applicable for small quantities of the thixo billets. Swirled Enthalpy Equilibration Device (SEED) has been developed as a rheocasting process, and the SEED process is of interest for developing new thixo alloys, as well as for optimizing the thixocasting processes for high quality components. The objective of this paper is to compare the microstructure and mechanical properties of aluminum alloy 319s billets and castings produced using EMS and SEED feed materials. The experimental results show that for as-cast billets made from SEED process, a well-developed spheroidal grain structure is distributed throughout the cross-section of the billet, while for as-cast EMS billets, the grain structure is inhomogeneous, i.e., a dendritic structure was present adjacent to the surface of the billet, while a uniform, spheroidal structure was present at the centre. After the thixocasting process, however, the both SEED and EMS billets have well-developed, spheroidal grain structures. Mechanical properties of thixocast and T61 heat treated components are comparable for the both SEED and EMS billets.

On the Relationship between Inclusions and Pores, Part II: Dendritic Structure, Pressure Drop in the Liquid and Pore Precipitation

2014 ◽  
Vol 790-791 ◽  
pp. 302-307 ◽  
Author(s):  
Arash Safavi Nick ◽  
Hasse Fredriksson
Keyword(s):  
Electron Microscopy ◽  
Thermal Analysis ◽  
Pore Formation ◽  
Pressure Field ◽  
Primary Dendrite ◽  
Dendritic Structure ◽  
Steel Sample ◽  
Continuously Cast ◽  
The Relationship ◽  
Scanning Electron

There is a relationship between pores and inclusions. As hypothesis goes, inclusions have an affinity to gather inside the pores and form clusters. Focus of this paper is how solidified dendritic structure affects the pressure field in the liquid and pore precipitation in austenitic stainless steel. Steel sample is a continuously cast bloom. Temperature profile and width of the mushy zone of the strand is modeled according to a constant temperature at the strands surface. Thermal analysis has been performed with differential thermal analysis (DTA) and differential scanning calorimeter (DSC). Dendrite arm spacing (DAS) is measured with light optical microscopy (LOM) and scanning electron microscopy (SEM). DAS is represented as the weight average of the distance between parallel sets of primary dendrite stems. Pressure field is calculated based on Darcys law. Pore formation is described through segregation of the gas components and pressure field in the liquid.

An Experimental Investigation Into the Effects of Grain Transport on Columnar to Equiaxed Transition During Dendritic Alloy Solidification

1999 ◽  
Vol 121 (2) ◽  
pp. 430-437 ◽  
Author(s):  
J. W. Gao ◽  
C. Y. Wang
Keyword(s):  
Mushy Zone ◽  
Chloride Solution ◽  
Packed Bed ◽  
Model Alloy ◽  
Alloy Solidification ◽  
Pile Up ◽  
Thermally Driven ◽  
Columnar To Equiaxed Transition ◽  
Vertical Test ◽  
Dendritic Alloy

An experimental study has been conducted to investigate the effects of grain transport on the columnar to equiaxed transition (CET) in dendritic alloy solidification. Using the aqueous ammonium chloride solution as a transparent model alloy, experiments were performed in a vertical test cell with cooling from the top, resulting in unidirectional columnar crystals growing downwards. Ahead of the columnar front, equiaxed nuclei were observed to originate mostly by fragmentation of the columnar dendrites in the presence of a thermally driven flow in the melt beneath the columnar mushy zone. Being heavier than the liquid, these fragments fall into the bulk melt where they may grow or remelt. The survived equiaxed crystals finally settle towards the floor and pile up to form an equiaxed bed. The CET occurs when the bottom equiaxed packed bed rises and eventually obstructs the columnar mushy zone growing from the upper surface. Therefore, the CET in the present configuration was predominantly controlled by the sedimentation of equiaxed crystals. A parametric study by varying initial concentration, cooling rate, and superheat was performed.

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