Study On The Preparation Technology Of Al-Zr Master Alloy By Molten Salt Reaction Method

The method of molten salt reaction has become one of the most popular techniques to prepare for Al-Zr master alloy.This method has been widely researched and applied for its simplicity and low cost.However,the studies on the formation and transformation mechanisms of the Al3Zr phases during the different preparation process of molten salt reaction,melting and solidification are far less than systematic.The formation mechanisms of some particular morphologies of Al3Zr remain unclear.And the preparation technology,which can effectively control the morphologies,size and distribution of Al3Zr phase in the Al-Zr master alloy,is need to be developed.The processing windows for avoiding the defects in the master alloy,such as agglomeration and settlement are still undefined.So the preparation technology for the Al-Zr master alloy needs to be optimized.Based on the present situation,in this paper,the reaction mechanism at the Al-K2ZrF6 interface,the formation and transformation mechanisms of Al3Zr phases,and the behavior of agglomeration as well as the settlement of Al3Zr phase during the reaction,melting and solidification processes are investigated by controlling the conditions of different process.And the controllable preparation of Al-Zr master alloy was realized.The main results are as follows:(1)The temperature of Al-K2ZrF6 interface is raised about 10 ℃ when adding 125.1g K2ZrF6 powder into the melt(780.8g)at 800℃.A zirconium accumulation layer with the thickness of about 2-3 μm formed on the aluminum side of the interface during the reaction process between the K2ZrF6 and aluminum at the temperature of 800℃。Many initially formed Al3Zr phases(with the size of 0.1-12μm)distributed in this layer,most of which exposed in the interface.These Al3Zr phases show various kinds of morphologies such as spherical,cubic,flake,cuboid and severe agglomerated particles and so on.The effective yield of Zr element is more than 90%as the melt aluminum and K2ZrF6 react at different temperatures(800℃,950℃,1000℃ and 1200℃)for 25min in the medium frequency induction furnace.(2)The granular Al3Zr phases,which formed during the reaction process at low temperatures,continuously dissolve into the aluminum melt with the melt temperature increase from 800 ℃ to 1250℃.With the increment of temperature,the quantity of the Al3Zr granules in the molten melt decreased,the size and the length-width ratio increased.The Al3Zr granules disappear in the molten melt as the temperature is higher than the liquidus temperature of this alloy.On the other hand,during the process with temperature decreasing from 1250℃ to 800℃,the coarse strip-like Al3Zr phase begins to precipitate from the melt and grow slowly as the temperature lower than the liquidus temperature.The quantity and the size of those coarse strip-like Al3Zr phases increase with the decreasing of the temperature.(3)The quantity and morphology of the Al3Zr phase(with the fine acicular-flaky shape and the complex acicular-flaky shape)are related with the solubility of Zr element in the molten melt at the casting temperature as the Al-5Zr master alloy casting at different temperatures(800-1250℃)in the water-cooled copper mold.There is no acicular-flaky shape Al3Zr phase observed in the master alloy at the casting lower than 850℃.The fine acicular-flaky and complex acicular-flaky shape Al3Zr phase appear in the master alloy as the casting temperature in the range of 900 ℃ to 1000℃ and the quantity of that kind of Al3Zr phase increases with the increase of the casting temperature.A large number of fine acicular-flaky Al3Zr phases begin to form in the master alloy as the casting temperature higher than 1000℃.(4)When the casting temperature of the Al-5Zr master alloy is 1250℃,the morphology of primary Al3Zr evolves from coarse plate strip to fine acicular flake with the cooling rate increasing from 1.9℃/s of refractory mold to 50.0℃/s of copper mold.In the solidification process of Al-5Zr master alloy,as the cooling rate in the two-phase region rises from 1.9℃/s to 2.89℃/s and 50.0℃/s,the nucleation number per unit area of prymary Al3Zr increases from 23mm-2 to 54mm-2 and 309mm-2,the spread of size distribution is diminished from 70-1140μm to 30-750μm and 11-350μm,while the corresponding macroaxis growth rate are 5.4μm/s,53.2μm/s,and 38.4μm/s,respectively.(5)The settlement behaviour of Al3Zr during the solidification process is dependent upon the melt viscosity,particle shape and solidification time.The Al3Zr phases present less obvious settlement on all 3 kinds of mould with the casting temperature of 800 ℃.When the casting temperature changes in the range of 850℃ to 950℃,the settlement of Al3Zr phases in the ingot of refractory mold intensifies for the decrease of the melt viscosity.However,no obvious settlement behavior is found in the ingots of the copper and graphite mould for the limited solidification time.The settlement behavior of Al3Zr in the ingots of the 3 kinds of mould is not obvious with the temperatures increading from1000℃ to 1250℃ for change Al3Zr phase shape.