WO2008002111A1

WO2008002111A1 - Molten salts for the refining of mg alloys

Info

Publication number: WO2008002111A1
Application number: PCT/KR2007/003201
Authority: WO
Inventors: Sang Ho Ahn; Woo Jin Park; In Ho Jung; Dong Kyun Choo; Jin Won Kim
Original assignee: Posco; Research Institute Of Industrial Science & Technology
Priority date: 2006-06-30
Filing date: 2007-07-02
Publication date: 2008-01-03
Also published as: RU2009100952A; CN101501177A; KR100760581B1; RU2417266C2; CN101501177B

Abstract

The present invention relates to a molten salt for refining magnesium alloys, especially Sr- containing magnesium alloys, and more specifically, relates to a molten salt for refining magnesium alloys to effectively remove impurities and to minimize the loss of Sr from Sr- containing magnesium molten alloy melt. This molten salt is a LiCl-KCl binary molten salt, a LiCl-KCl-NaCl ternary molten salt, or a binary or ternary molten salt containing 20% or less of MgCl2 or CaCl2 based on the total weight thereof. The molten salt may include 10 wt% or less of impurities in the working procedure.

Description

MOLTEN SALTS FOR THE REFINING OF MG ALLOYS

Technical Field

[1] The present invention relates to a molten salt for refining magnesium alloys, especially Sr-containing magnesium alloys, and more specifically, relates to a molten salt for refining magnesium alloys to effectively remove impurities and to minimize the loss of Sr from Sr-containing magnesium molten alloy melt. Background Art

[2] Sr-containing magnesium alloys are receiving attention as next-generation material due to its lightweight, high strength, and high creep properties even at high temperatures on the formation of Sr-containing intermetallic compounds upon the casting thereof.

[3] Based on these properties, the application of Mg-Al-Sr alloys to parts for automobiles or airplanes, in particular, high-temperature magnesium alloy products, is drastically increasing these days. Such parts are manufactured by melting an Mg-Al-Sr magnesium alloy ingot in a furnace and then mainly conducting die casting. Thus, in order to produce final products having high quality, the preparation of an Mg-Al-Sr magnesium alloy ingot having high quality is essential.

[4] The magnesium alloy ingot is prepared in a manner such that at least 99% pure Mg ingot is melted at about 700⁰C in a furnace, added with a highly pure alloying metal, including Al or Zn, to thus adjust the final alloy composition thereof, and then solidified in the form of a magnesium alloy ingot. However, because magnesium is highly affinitive for oxygen, various impurities and inclusions, including MgO, Mg N , MgAl 2 O 4 , intermetallic compounds, etc., are inevitably generated upon the melting of magnesium in the furnace and the addition of the alloying metal. Further, because such inclusions have specific gravity similar to molten magnesium, separation thereof through floating or precipitation requires a long period of time. Hence, with the goal of increasing the productivity of the magnesium alloy, there is a need to decrease the period of time required to remove the inclusions. To this end, the inclusions are floated and separated through gas bubbling using Ar, and a chloride-based molten salt is added to the surface of molten magnesium to thus adsorb and remove the inclusions. Furthermore, some molten salts, having higher specific gravity than magnesium, are responsible for adsorbing and removing various impurities while being precipitated in the molten magnesium.

[5] An example of the molten salt being used for the preperation of the magnesium alloy is CaCl -MgCl . Although the composition of the molten salt varies slightly depending on the manufacturer, CaCl and MgCl are used in large amounts.

[6] To prepare the Mg-Al-Sr magnesium alloy ingot, Sr metal, having high quality, is added to molten magnesium to thus uniformly melt it. Sr, having a melting point of 777⁰C, takes a long time to melt in the molten magnesium near 700⁰C. Therefore, after Sr is added, a propeller or screw is provided in the furnace to thus apply rotary power to the molten magnesium, thereby decreasing the Sr melting time period. Disclosure of Invention Technical Problem

[7] However, during the working procedure, various impurities, which precipitate in or float to the surface of the molten magnesium, intrude in the magnesium molten alloy melt, and undesirably take a long period of time to remove after the Sr is melted. In the case where a molten salt presently commonly available for removing such impurities is used, Sr is melted and combined with the molten salt, and thus the concentration of Sr in magnesium is remarkably decreased.

[8] Thus, to increase the productivity of the Sr-containing magnesium alloy ingot, a refining molten salt having a novel composition should be developed.

[9] The present invention has been made keeping in mind the above problems occurring in the related art, and provides a molten salt for refining magnesium alloys, which can effectively remove impurities while minimizing the loss of Sr upon the preparation of Sr-containing magnesium alloys. Technical Solution

[10] According to the present invention, a molten salt for refining magnesium alloys may be a LiCl-KCl binary molten salt, a LiCl-KCl-NaCl ternary molten salt, or the binary or ternary molten salt containing 20% or less of MgCl or CaCl based on the total weight thereof.

[11] The molten salt may include 10 wt% or less of impurities, such as chlorides, fluorides, nitrides, and oxides of Fe, Al, Rb, La, Ce, Nd, Ba, Br, Sr, Cs, Mn, Co, Ni, Zn, and Cr, or intermetallic compounds, in the working procedure.

[12] The composition of the molten salt for refining magnesium alloys according to the present invention is obtained on the basis of the following principle.

[13] The melting of Sr in the magnesium molten alloy melt and combination therewith is considered to be due to the substitution with MgCl as a molten salt so that it is thus melted in the form of SrCl in the molten salt, as represented by Reaction 1 below.

[14] Reaction 1

[15] MgCl (molten salt) + Sr (Mg alloy) → Mg (Mg alloy) + SrCl (molten salt)

[16] The reason why the above reaction takes place is that SrCl is more thermody- namically stable than MgCl . Comparing the thermodynamic stabilities thereof, molten salts may be more stable toward LiCl in the sequence of MgCl < CaCl < NaCl < SrCl < KCl < LiCl. Thus, when, as the molten salt for refining magnesium alloys, a molten salt having a minimum of MgCl or CaCl is formulated, the loss of Sr is expected to be minimized. [17] Further, to effectively eliminate various impurities from the magnesium molten alloy melt using the molten salt, the molten salt should be present in a solution state at temperatures for refining the melt. As such, the molten salt should have low viscosity (i.e., high fluidity) such that the reaction rate of the molten salt is increased in the magnesium molten alloy melt and the impurities are removed within a short time. The molten salt having low viscosity typically has a low melting point.

Advantageous Effects

[18] According to the molten salt for refining magnesium alloys constructed as above, impurities may be effectively removed, and as well, the loss of Sr may be minimized, upon the preparation of the Sr-containing magnesium alloy. Mode for the Invention

[19] The salt compositions able to effectively remove impurities while minimizing the loss of Sr in the molten magnesium on the basis of the facts described in the Technical Solution were estimated and subjected to experiments. The results of some compositions, coinciding with the purpose of the present invention, among various experimental results, are shown in Table 1 below. Unless otherwise specifically defined below, % indicates wt%.

[20] As experimental conditions, the magnesium alloy was composed of

96%Mg-3%Al-l%Sr, the weight ratio of magnesium alloy to molten salt was set to 10:1, and the working temperature was set to 700⁰C. Further, after the reaction with the molten salt, the components of the magnesium alloy and the components of the molten salt were investigated, and thus the degree of loss of Sr was determined.

[21] Table 1

[22] [23] As is apparent from Table 1, in Comparative Examples 1 and 2, including MgCl and CaCl molten salts as main components, most of the Sr, of the initially used 96%Mg-3%Al-l%Sr magnesium alloy, was observed to have been lost, and thus remained in an amount of 1 ppm or less.

[24] However, in Example 1, using a 45%LiCl-55%KCl binary molten salt, the magnesium alloy after the reaction was determined to be

96%Mg-3%Al-0.99%Sr-0.01K, from which Sr was seen to be lost only in an amount of about 0.01%. As such, the appropriate composition of the molten salt was 35~55%LiCl-45~65%KCl.

[25] In Example 2, using a 41%LiCl-50%KCl-9%NaCl ternary molten salt, the magnesium alloy after the reaction was determined to be Mg-3Al-0.79Sr-0.1 INa, from which Sr was seen to be lost in an amount of about 0.21%. As such, the composition of the molten salt was shown to be appropriate when LiCl and KCl were included at a weight-part ratio of 0.6-1.5: 1 and 15% or less of NaCl was included based on the total weight of the molten salt.

[26] In Example 3 using 42.5%LiCl-52.5%KCl containing 5% of CaCl , the magnesium alloy after the reaction was determined to be Mg-3Al-0.63Sr-0.17Ca, from which Sr was seen to be lost in an amount of about 0.37%. Further, in Example 4 using a 42.5%LiCl-52.5%KCl molten salt containing 5% of MgCl , the magnesium alloy after the reaction was determined to be Mg-3Al-0.54Sr-0.17Ca, from which Sr was seen to be lost in an amount of about 0.46%. As such, it could be seen that the loss of Sr in the magnesium molten alloy melt was effectively maintained at a level of not more than 50% when the molten salt was composed such that LiCl and KCl were included at a weight-part ratio of 0.4-1.7: 1, and 20% or less of CaCl or MgCl and 15% or less of NaCl were included based on the total weight of the molten salt.

Claims

[1] A molten salt for refining magnesium alloys to remove impurities from magnesium molten alloy melt, comprising:

35-55 wt% of LiCl and 45-65 wt% of KCl [2] A molten salt for refining magnesium alloys to remove impurities from magnesium molten alloy melt, comprising:

LiCl and KCl at a weight-part ratio of 0.6-1.5:1, and 15% or less of NaCl based on total weight of the molten salt. [3] A molten salt for refining magnesium alloys to remove impurities from magnesium molten alloy melt, comprising:

LiCl and KCl at a weight-part ratio of 0.4-1.7:1, and 20% or less of CaCl or

MgCl and 15% or less of NaCl, based on total weight of the molten salt.