Method for purification of metal based alloy and intermetallic powders

Abstract

This invention relates to a method for purifying metal alloy and intermetallic powders. Particularly, the present invention relates to a method for the reduction or elimination of the content of the dissolved oxygen and to remove the metal oxide inclusions from metal alloy and intermetallic powders including the steps of: a) placing the metal in powder form into a reaction apparatus; b) introducing a suitable carrier substance to the metal powder; and c) introducing calcium vapour into the reaction apparatus to create a reaction between the metallic powder and calcium vapour thereby removing inclusions in the metal as shown in FIG. 11.

Description

TECHNICAL FIELD[0001]This invention relates to a method for purifying metal alloy and intermetallic powders. Particularly, although not exclusively the present invention relates to a method for the purifying titanium based alloy and intermetallic powders.BACKGROUND ART[0002]Metal, alloy and intermetallic powders produced by different processes such as liquid atomisation, mechanical milling, separation processes or a process that combines the above processes often contain a high content of dissolved oxygen (up to 10 wt % and metal oxide inclusions such as Al2O3. Such high content of dissolved oxygen and metal oxide inclusions can severely limit the range of practical applications of the metal, alloy and intermetallic powders because the high oxygen content and oxide inclusions can deteriorate the mechanical and other properties of the articles to an unacceptable level.[0003]Therefore in many situations, it is essential to further purify the metal, alloy and intermetallic powders to s...

AI Technical Summary

Benefits of technology

[0054]The advantage of generating Ca vapour for this reaction process is that low grade calcium can be purchased from suppliers such as Aldrich and therefore at low cost. However, when this low grade solid is converted into a vapour, it is of very high purity. Also, the vapour can be added continuously to the titanium rich powder to provide a more efficient reaction and eliminates the steps of having to extract out any impurities in the solid.

[0055]The advantage of the use of a liquid carrier is that the titanium rich metallic or intermetallic powder particles are suspended in a liquid which can serve as a conductor for the deoxidant atoms to interact with the whole surface of each of the powder particles to be deoxidised, and thus allowing maximum effectiveness of the process. Specifically, the extraction of the dissolved oxygen and the oxygen associated with the metal oxide inclusions in the powder particles, and largely maintaining the original particle size and morphology. During holding the powder / carrier mixture at some temperatures, the powder particles may become more rounded, which is practically favourable for increased reaction surface.

[0056]This allows further reduction of the total oxygen content in a titanium rich powder to be below 0.4 weight %.

[0057]An advantage of this process is that it makes leaching of the reaction by-product, CaO and CaO.Al2O3 particles much easier.

[0058]Also, a slight extension of the application of the process allows the use of a fluidised bed to replace the liquid carrier to achieve similar enhanced reaction kinetics between Ca atoms and titanium rich powder particles.

[0059]A further advantage of this process is that it can be used to purify simple binary Ti—Al intermetallic or alloy powders, but also can be used to purify more complex titanium based intermetallic or alloy powders as well as other metal based metal, alloy and intermetallic powders derived from a number of methods of manufacture.

Problems solved by technology

Such high content of dissolved oxygen and metal oxide inclusions can severely limit the range of practical applications of the metal, alloy and intermetallic powders because the high oxygen content and oxide inclusions can deteriorate the mechanical and other properties of the articles to an unacceptable level.

However, the disadvantage of these methods is that they can not be applied to an active metal (such as titanium) based metal, alloy and intermetallic powders.

This is due to thermodynamics where hydrogen and carbon are not sufficiently capable of extracting dissolved oxygen or reducing some of the metal oxide inclusions such as Al2O3.

However, the above mentioned processes or a similar type have limitations when they are used for deoxdising metal, alloy and intermetallic powders, especially for those powders with fine particles of 100 micrometres or less in diameter.

This limitation prevents the use of low grade and low cost deoxidants, and thus can make the cost of the deoxidation process too high for many applications.

However, the disadvantage of this method is that the particles of the powder to be reduced or deoxided are packed in a bed.

Therefore, the effectiveness for the contact between the gaseous reductant and the powder particles to be reduced is limited because of the exposed surface area of each of the particles is reduced by the physical contacts with those particles surrounding it.

With high powder packing density which is often associated with fine particle sizes, the exposed surface area can be significantly limited and thus reduce the reduction effectiveness to a level that lead to impractical reaction times in order to achieve complete reduction or deoxidation.

Although these links can be broken by subsequent milling, it increases the costs of the process.

For some metal and alloy powders such as titanium alloys, the milling is often a difficult process because of their high ductility.

In addition, the milling process changes the size and morphology of the original powder particles, and this is not considered a desirable property.

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