Yan Qing Su

Investigation on a Fabrication Technique of TiAl Sheet

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.

Effects of Technical Parameters on Initial Silicon Melting in Round Cold Crucible Continuous Casting

The effects of technical parameters on initial silicon melting in cold crucible continuous casting were studied. These parameters include the materials, the shape and the position of the base, the mass of the silicon that set on the top of the base. Through experimental and theoretical analysis, the optimized parameters were finally given: the base graphite with obconical shaped should be put at the level of the second turn of the coil, and the initial silicon with 10g should be put on the base. The mechanism of these parameters affecting on the initial melting are discussed and revealed.

Study of Solid/Liquid Interface of Ti46Al0.5W0.5Si Ingot Directionally Solidified by a Near-Rectangular Cold Crucible

In this study, Ti46Al0.5W0.5Si ingots were directionally solidified by a near-rectangular cold crucible under different process parameters. These process parameters include the electromagnetic stirring, the crucible configuration and the molten drop, all of them have important effect on the S/L interface. The effects of the parameters on the solid/liquid (S/L) interface morphology were investigated, and the mechanisms of the parameters influencing the S/L interface were discussed and revealed. Results showed that the typical S/L interface of the ingots was presented as a curved ‘W-type’ shape. The uneven temperature distribution in the front of the solidified interface is the main reason for a curved S/L interface. Further, the requirements for obtaining a planar S/L interface in the process of cold crucible directional solidification were given, which provided a guide for the future work.

Effects of Parameters on the Stability of Silicon Melt Pool in Cold Crucible Continuous Casting

Effects of parameters on the stability of silicon melt pool in cold crucible continuous casting were discussed. The results indicate that the extinguishing of induction heating is mainly caused by the side wall feeding, low superheat degree and volume of the pool. The hot melt splashes are caused by the higher input power and uncompletely melted silicon of the dome top surface. The pool was shown to be stabilized as the raw materials were fed in the center area of the dome surface, the superheat degree and volume of the pool were properly increased, the pool was controlled in the effective heating range of the coil and the input power was set properly

Temperature Distribution and Operating Efficiency in Rectangular Cold Crucible Directional Solidifying Ti46Al0.5W0.5Si Alloys

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.

Microstructure and Mechanical Properties of a Rapid Directionally Solidified Ti47Al Alloy

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.

Grain Growth Behaviors of Multicrystalline Silicon during Cold Crucible Continuous Casting

A square shaped silicon ingot was cast by cold crucible continuous casting. The grain growth behaviors during different growth stages were investigated. In the first, grains nucleate at the base surface in the centre and in the three phase junction at periphery, and then grow upwards and inclined upwards separately. In the second, grain grow competitively, two distinguished zones are formed. In the final, a cap is formed on the liquid free surface, and precipitates are present.

Microstructure Evolution of Directionally Solidified Al-25at.%Ni Peritectic Alloy

Microstructure evolution of peritectic Al-25at.%Ni alloy during directional solidification with pulling velocity ranging from 2 to 500m/s is investigated. The directional solidified alloy is composed of Al3Ni2, Al3Ni phase and eutectic (Al3Ni+Al) phase. When pulling velocity ranges from 2 to 5m/s, Al3Ni phase grows into an integral matrix. Majority of primary Al3Ni2 is consumed by peritecti reaction and transformation behind the peritectic interface with pulling velocity ranging from 2 to 20 m/s. While pulling rate increases, major Al3Ni phase direct solidifies from liquid. With cooling rate increasing, Al3Ni2 phase content firstly decreases and then increases, while the Al3Ni phase content decreases throughout.

Permanent Mold Suction Casting for TiAl Based Alloys

A novel precision casting technique for TiAl based alloy was described in this work, called the permanent mold suction casting technology，and the blades of Ti-47Al-2Si、Ti-47Al-2W-0.5Si、Ti-47Al-2W-0.5Si -0.5B were get by this technology. The structures of these blades were studied. The results of this study indicate that microstructure is sensitive to heat treatment and composition. The average grain size is 30 um before heat treatment. The grain size of TiAl based alloy is grossed by heat treatment at 1573 for 5h and furnace cooling, especially Ti-47Al-2Si. The addition of boron, the grain size of Ti-47Al-2W-0.5Si-0.5B is smaller than Ti-47Al-2W-0.5Si. The γ-TiAl based alloys have equiaxed grain in cross section, but the surface microstructure of γ-TiAl based alloys have more fine equiaxed grain than inside due to high cooling rate of mold.

Microstructure Evolution of Ti-Al Peritectic System during the Initiated Stage of Directional Solidification

The microstructure evolution of Ti-Al peretectic system in transient stage and steady state in directional solidification was predicted via theoretical analysis. The solute distribution controlled by diffusion at and ahead the solid-liquid interface will determine whether the properitectic and peritectic phases can nucleate and grow ahead of the opposing solid phase. The formation of banding structure is possible in a certain composition range. At the steady state, a microstructure selection map was set up based on interface response function model. The microstructure of TiAl alloys with different aluminum content was studied with Bridgman directional solidification method. Some evidence in the experiment has been found to support the theoretical prediction.