Papers by Keyword: Directional Solidification

Abstract: In the present work, solidification of hypoeutectic, eutectic and hypereutectic Al-Cu alloys was illustrated by synchrotron X ray imaging, and the CET of hypoeutectic alloy was picked out to thorough investigated. The mechanism of hypoeutectic dendrites fragment behaviors among the nucleation area was studied by in-situ imaging and first-principles computation. The results show that the density difference between the fragments and the enriched melt leads to the movement of the fragments. The ejected fragments contributed to the columnar-eutectic transition and expanded the breadth of mush zone in front of the solid/liquid interface.

Abstract: In this paper, an experiment model for the directional solidification of Lead/Tin alloy is built and the effects of different-shape seeds on the microstructures on the solidification microstructure are investigated. In a casting process, the temperature and concentration fields will affect the microstructures of materials and this influence is the key point of improving their mechanical and physical properties. It is not easy to control the morphology of solidifying microstructures. The scheme of directional solidification can make the microstructures grow along a fixed direction and it is also the base of single-crystal growth. In the experiment, a poly-grain seed with the same initial concentration of the solidifying casting is used to induce the columnar growth at the bottom portion of the casting, which could avoid the equiaxed growth due to the high undercooling or cooling rate there. In the experimental analysis, we studied the influences of different geometry seeds on the constrained growth, the preferential growth direction of dendrite, the grain size, the temperature gradient, the growth rate, the primary arm spacing and the secondary arm spacing. From the microstructure observation, the adding seed casting reduced the chill-affected and extended the directional solidification zone. This is expected to have the better or more complete structure of directional solidification. Keywords: Directional Solidification, Seed, Heat Transfer and Microstructure

Abstract: Hydrogen is a harmful element in aluminum. How to accurately measure hydrogen content in molten aluminum alloy has been significant for improving the metallurgical qualities of aluminum products. Currently, Reduced Pressure Test (RPT) based on the famous Sievert’ law is the most commonly used method to measure the hydrogen in aluminum. However, due to the changes in solidification morphology of alloys under different solidification conditions, hydrogen will be closed more or less in the sample by the developed dendrites, which results in the dispersion of test results. According to the principle of solute redistribution, an improved reduced pressure technology under directional solidification was proposed in this paper. The solidification interface was pushed on along the single direction by controlling the temperature gradient of solid-liquid interface. Hydrogen is released in front of the solid-liquid interface through the solute redistribution. The result shows that the hydrogen content increases and the mean variance decreases with the increasing of the temperature gradients. The method was proved to be of higher accuracy.

Abstract: The development of preferred grain orientation has been investigated in the directionally solidified samples of Tb_0.3Dy_0.7Fe_1.95 as a function of pulling speed viz. 10, 40, 70 and 100 cm/h. The study indicates that at lower solidification rate (10 cm/h) growth of and texture components are preferred, whereas, texture component becomes dominant at higher pulling rate (100 cm/h). However, as the solidification progresses, growth of texture component is observed subduing the other components. Consequently, the magnetostriction improves from 1100 to 1350 micro-strains with higher pulling speed.

Abstract: This paper reports the effect of Ho on the microstructure, texture and magnetostrictive properties of Tb_0.3Dy_0.7-xHo_xFe_1.95 (with x = 0, 0.05, 0.1, 0.15 and 0.2) alloys. The alloys were vacuum induction melted and directionally solidified by modified Bridgman technique. Ho addition has been found to improve the magnetostriction at lower concentration due to absence of deleterious pro-peritectic (Tb,Dy)Fe3 phase. The static strain co-efficient is found to be better in Ho added alloys and the prevalence of strong and texture is attributed to be the reason.

Abstract: In order to obtain the directional microstructure of different supersaturation and growing velocity, three simulations is calculated with different initial temperature. When the initial temperature is 1576K, and the supersaturation and growing velocity are smaller. The average space length of columnar crystals is bigger, and microsegregation is smaller. when the initial temperature is 1574K, the supersaturation and growing velocity increase. The average space length of columnar crystals decreases, and microsegregation increase; when the initial temperature falls to 1566K, the supersaturation and growing velocity reach 1.176 and 3.238 cm per second. The microsegregation decreases rapidly. The average space length of columnar crystals is the smallest. The solute curve of columnar crystal is like a “U” figure. When the growing velocity is bigger than V_A, the phenomenon of solute trapping takes place.

Abstract: In order to obtain the directional microstructure of different supersaturation and growing velocity, three simulations is calculated with different initial temperature. When the initial temperature is 1576K, and the supersaturation and growing velocity are smaller. The average space length of columnar crystals is bigger, and the directional microstructure grows by the wide columnar crystals. Microsegregation is smaller; when the initial temperature is 1574K, the supersaturation and growing velocity increase. when the initial temperature falls to 1566K, the planar interface comes back, and microsegregation decreases rapidly. The directional microstructure grows by the thinnest columnar crystals. At the same time, the transverse solute profiles and solute boundary layer are also talked in this paper.

Abstract: This paper gives a detailed study of temperature distribution on Ti46Al0.5W0.5Si alloy directionally solidified by cold crucible. Based on experiment and numerical calculation, the heating process under different power was investigated, and the operating efficiency of cold crucible was roughly calculated. Further, the temperature field of Ti46Al0.5W0.5Si alloy under high power was numerically calculated, the finite element model predicted the temperature distribution at different moment. The results give a significance guide on cold crucible directional solidification.

Abstract: The ingot of Ti47Al alloy was prepared by a newly developed rapid directional solidification, the microstructure and compressive properties of the ingot was observed and tested. The results show that the macrostructure consisted mainly of coarse columnar grains parallel to the axial direction, with the size of 0.5mm wide and 10mm in length. The direction of lamellar is almost perpendicular to the growth direction in the longitudinal section and no dendritic core is found. The average ultimate compressive strength of the specimens with grain growth parallel/perpendicular to the compressive direction is 1233.3 and 861.7 MPa respectively. The fracture mode for specimens with grain growth parallel to the compressive direction exhibits predominantly translamellar fracture, however, that for specimens with grain growth perpendicular to the compressive direction exhibits predominantly delamenation or interlamellar fracture.

Abstract: The morphologicalstability of a planar interface with the inclusion of the anisotropic surface tension is studied. The dispersion relation is obtained by means of the multi-variable expansion method. The result shows that the morphological stability of the solidification system in the extremely short wave region depends on the undercooling. As the anisotropic surface tension parameter increases the perturbation growth rate of the interface increases and the solidification system in the extremely short-wave region tends to destabilize.