Showing papers by "Hengzhi Fu published in 2005"

Unidirectional Solidification of a Nbss/Nb5Si3 In-Situ Composite

Abstract: The directionally solidified specimens of Nb-13.52 Si-22.60 Ti–6.88 Hf–2.54 Cr–2.24 Al alloy were prepared in an electron beam floating zone melting furnace at the withdrawing rate of 0.1, 0.3, 0.6, 1.0, 2.4 and 6.0 mm/min. All the primary Nb solid solution (Nbss) columns, Nbss + (Nb)3Si/(Nb)5Si3 eutectic colonies and divorced (Nb)3Si/(Nb)5Si3 plates or chains align well along the longitudinal axis of the specimens. With increasing of the withdrawing rate, the microstructure is gradually refined, and the amount of Nbss + (Nb)3Si/(Nb)5Si3 eutectic colonies increases. Both the room temperature ultimate tensile strength σb and fracture toughness KQ are improved for the directionally solidified specimens. The tensile fracture occurs in a cleavage way.

Progress of Directional Solidification in Processing of Advanced Materials

Abstract: Most of materials have long been considered to be mechanical and/or physical anisotropy. Permitting materials to grow along specific orientation by means of directional solidification technique can optimize their structural or functional properties. The present paper attempts to introduce the research work in the field of processing of some advanced materials by innovative directional solidification techniques performed at State Key Laboratory of Solidification Processing and with author’s intended research work. The paper deals with the specific topics on directional solidification of following advanced materials: column and single crystal superalloys under high thermal gradient, Ni-Cu alloys under deep supercooling of the melt, intermetallic compounds with selected preferential crystal orientation, superalloys with container less electromagnetic confinement, high Tc superconducting oxides, high temperature structural ceramics, continuous cast single crystal copper and copper-based composites. The relevant solidification phenomena, such as morphological evolution, phase selection, peritectic reaction and aligned orientation relationship of crystal growth for multi-phases in the processing of directional solidification, are discussed briefly. The trends of developments of directional solidification technique are also prospected.

Continuous Casting and Directional Solidification of Titanium Alloys with Cold Crucible

Abstract: Titanium alloy billets were continuously cast and directionally solidified under vacuum and compelling cooling conditions by using round or rectangular cross-section cold crucible. It is found that hot crack is caused by friction between the skull and the inner wall of crucible, a coating of CaO and CaF2 flux can eliminate the hot cracks. The experimental results show that the withdrawal velocity, where the changes was setting as 5, 3, 1, 0.5mm/min respectively, is the significant factor that affects the macrostructure and solidification front shape from concave to flat and then to convex. Directional columnar grains and a single crystal can be obtained when the velocity is controlled to either 1 or 0.5mm/min under a round cross-section crucible, and columnar grains can be also obtained at the speed of 2mm/min under a rectangular cross-section crucible. Finally, the temperature fields during the process are calculated and the trend of solidification front is in good agreement with the experiment. Specimens composed of columnar grains or single crystal exhibit excellent tensile properties. Introduction Titanium alloys are promising engineering materials, which are applied to aerospace and shipbuilding industries owning to their high specific strength, hot resistance and corrosion resistance. However, there are some difficulties in controlling the chemical elements and temperature in melting titanium alloys. Recent directional solidification of titanium alloys experiments were performed in optical floating furnace [1-3] or in resistance furnaces using Al2O3[5], Y2O3[4] or CaO [5] crucible. But titanium will react with all commercial ceramic mould materials, which in turn affect the microstructure and mechanical properties. Water-cooling cold crucible provides an effective method for melting titanium alloys because of its high speed in melting, non-contamination and continuous casting [6,7]. Cold crucible continuous casting and directional solidification includes induction melting, soft contact, continuous casting and directional solidification. Not only aluminum, steel and titanium alloys [8], but also photovoltaic multi-crystalline and single silicon are continuously cast by cold crucible [9]. But directional solidification of titanium alloys with cold crucible has not been reported elsewhere. This paper investigates the methodology of continuous casting and directional solidification with cold crucible when it was applied to Ti-6Al-4V alloy. Experimental method The experimental apparatus is schematically shown in Fig.1. This apparatus comprises: -a furnace chamber made of steel, in which a vacuum can be established by an exhaust pump -a water-cooling segment copper crucible with 8 pieces of vertical slits and 30mm in the internal diameter and 130mm in height. -a four-turn water cooling coil surrounding the crucible and supplying 50 kHz frequency alter current generated by a transistor generator, of which the maximum output power is 100kW -a withdrawal system and a feeder system, both of them can be controlled respectively. The procedure is as follows. A Ti-6Al-4V alloy billet is fixed on feeder and a primer is placed in the crucible. A vacuum of 1Pa is established, then Argon gas is introduced to 200 Pa. Power is gradually Materials Science Forum Online: 2005-01-15 ISSN: 1662-9752, Vols. 475-479, pp 2575-2578 doi:10.4028/www.scientific.net/MSF.475-479.2575 © 2005 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-11/03/20,13:00:55) transmitted to the charge until it reaches 50 to 60kW. The supply of raw materials and the withdrawal of the billet will be started at certain velocities simultaneously after 5 minutes later. The fresh billet is cooled by Ga-In liquid room-temperature alloy in the cooler. Fig.1 Scheme of the apparatus a) Process and equipment b) Round cross-section c) Rectangular cross-section Results and discussion The surface of the billets. Billets are cast under different conditions with round or rectangular cold crucible, and pictures of the surface are shown in Fig.2. Hot cracks appear on the surface of 1) and 2) in Fig.2, but cracks are free on the others. Fig.2 Surface of the billets 1)-4) are cast by round crucible, 5) is cast by rectangular crucible 1)v=5, 2)v=3, 3) v=1, 4) v=0.5, 5) v=2 (mm/min) Analyzing the first appearance position and the shape of those hot cracks, it is deduced that hot crack will appear when friction between the skull and the crucible inner wall is higher than the strength of the skull. In order to decrease the friction, the inner wall is coated by flux of CaO and CaF2 mixture, in which crack free billets are made by this method, as shown of 3), 4) and 5) in Fig.2. The macrostructure of the billets. These billets are sectioned longitudinally, polished, and etched in a solution of H2O +HF+HNO3. The macrostructure are shown in Fig.3. Withdrawal velocity is an important factor affecting the macrostructure. In the round cold crucible, the grains are small when the velocity is 5mm/min, and the grains become larger when the velocity decreases to 3mm/min. There are only three grains and the grain boundaries are parallel to the growth direction and the solidification front is flat when the velocity is 1mm/min. It is surprised that there are only one grain when the velocity is set to 0.5mm/min, and the solidification front is convex, this billet is a single crystal. The solidification front changes from concave to flat and then to convex with decreasing of velocity. In the rectangular cold crucible, columnar grains and flat solidification front are obtained at the velocity of 2mm/min, as shown in 5) of Fig.3. c) slit coil steel chamber Evac Ar melt pool water Ga-In alloy crucible

Interface Morphologies and Microstructure of a Single Crystal Superalloy under High Thermal Gradient Directional Solidification

Abstract: The interface morphologies single crystal superalloys CMSX-2 has been studied over a range of cooling rate with large variations in withdrawal speed in directional solidification. The superfine cellular structure was obtained under both high thermal gradient up to 1000K/cm and fast withdrawal rate up to 1mm/sec. The high rate directional solidification results in reduction in primary and secondary dendrite arm spacing, refinement of λ’ phase, reduced microsegregation of alloying elements and smaller size of γ-γ’ eutectics.

Investigation on a Fabrication Technique of TiAl Sheet

Abstract: A TiAl sheet fabrication technique from Ti/Al foils was studied experimentally. Firstly, the reaction sequence between Ti and Al foils were studied with DSC and then a three stage solid transformations processing was carried out. At the first stage, Al diffused into Ti foils and formed Al3Ti. The heating temperature was selected based on a ruler that the pure Al was consumed by solid diffusion with a short time and avoiding the melting of Al. The resulted microstructure consists of pure Ti and Al3Ti. At the second stage, part of Al in Al3Ti diffused into pure Ti and formed the high temperature phase. At the third stage, the sample was heated into the high temperature -phase zone and hold a given time and then cooling with the furnace. A full density γ-TiAl based alloy sheet with lamellar microstructure was successfully fabricated under above processing. The lamellar orientations are aligned around 0-45o compared to the longitudinal direction of the sheet.

Skull Variation during the Induction Skull Melting Processing of γ-TiAl Alloy

Abstract: The ratio of skull weight to charge weight (Rs) and the skull size during the induction skull melting (ISM) processing of TiAl alloy were investigated. The effects of inputting power, charge weight, and holding time on them were studied theoretically. An experiment was carried out. The theoretical and experimental results are in good agreement.

Investigation on the Electromagnetic Contactless Shaping and Directional Solidification of Plates of Stainless Steel and Supperalloy under Vacuum

Abstract: In this study, the distribution of electromagnetic pressure and the coupling between shaping force and temperature field are investigated in electromagnetic contactless shaping process for plates of stainless steel and superalloy. The distributions of electromagnetic pressure on outlines of melt cross sections are very different along sides than at corners. Under the vacuum, the cooling ability is not strong enough, and therefore the position of melt in inductor is too low, which causes an unbalance between electromagnetic pressure and static pressure. Experiments show the insert of a screen shell from the bottom of inductor can change the distribution of magnetic field very much and make a good and stable coupling between shaping force and temperature field.