GB2138447A

GB2138447A - Method for removing residual elements from metal powders

Info

Publication number: GB2138447A
Application number: GB08306903A
Authority: GB
Inventors: Griffith E Williams
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-03-14
Filing date: 1983-03-14
Publication date: 1984-10-24
Also published as: GB8306903D0

Abstract

A process for removing residual elements including dissolved, absorbed, adsorbed or otherwise occluded gases from metal powders comprises sequentially subjecting the metal powder to holding at the melting and boiling points of the residual elements to be removed while in a vacuum atmosphere. In the case of some gases, the process includes cryogenic cooling near absolute zero temperatures.

Description

SPECIFICATION Method for removing residual elements from metal powders This invention relates to a method for removing residual elements from metal powders. More particularly the invention is concerned with processes for producing metal powders having an extremely low gas and residual element concentration by means of a time-temperature vacuum method based on the melting and boiling temperatures of the residual elements to be removed.

For years, metallurgists have recognized the need for high purity metals in particular in the field of powder metallurgy as applied to the aircraft and aerospace industries. Although various methods and techniques are presently available for making relatively pure metals, the processes for converting these metals to powder form introduce gaseous inclusions and other contaminants, called residual or tramp elements which are detrimental to subsequent processes. Typical methods for producing high quality metal powders are disclosed in U.S. Patent No. 3,975,184, in U.S. Patent No. 3,963,812, in U.S. Patent No. 3,887,402 and in U.S. Patent No. 4,018, 633. However, in all of these processes, the amount of trapped gases and residual or tramp elements are either increased or unaffected.

The prior art teaches several methods for the removal of trapped gases. The most common process used in industry today is to heat the metal in a vacuum such as taught in U.S. Patent No. 3,954,458. This process is extensively used in the vacuum tube industry where residual gases entrapped in the giass and metal are released inside the vacuum tube over a period of time. The released gases raise the pressure inside the vacuum tube and adversely affect its operation. The removal of residual oxygen by heating in a reducing atmosphere such as hydrogen is taught in U.S. Patent No. 3,744,993 as well as in U.S. Patent No. 3,887,402 and in U.S. Patent No. 3,945,863. Another method for removing dissolved, absorbed or otherwise occluded gases from platinum is taught in U.S. Patent No. 3,511,640.In the latter there is described mixing the metal powder to be degassed with an inert metal oxide powder and heating to about 1000"C to 1800"C for a period of time to dissipate substantially all the gases.

The platinum powder is subsequently recovered by dissolving the metal oxide in an acid solution.

The present invention is concerned with a different method for the removal of gases and residual elements from metallic powders by a time-temperature vacuum process based on the melting and boiling temperatures of the gas or residual element to be removed.

The present invention provides a method for removing residial elements including dissolved, absorbed, adsorbed or otherwise occluded gases from metal powders, comprising the steps of: placing the metal powder in a vacuum chamber; evacuating the vacuum chamber to reduce the atmosphere surrounding the metal powder to a vacuum; subjecting the metal powder to the melting temperature of the residual element to be removed for a first predetermined period of time T1; elevating the temperature of the metal powder to the boiling temperature of the residual element to vaporize the residual element; holding the metal powder at the said boiling temperature for a second period of time T2; and returning the metal powder from the said boiling temperature to room temperature prior to removing the metal powder from the vacuum chamber.

In its preferred embodiment the invention comprises a method for removing gases and residual elements from a metal powder in which the metal powder is either heated or cooled in a vacuum atmosphere to the melting temperature of the residual gas or element to be removed.

The metal powder is held at the melting temperature of the residual gas or element for a period from 5 to 30 minutes then elevated to the boiling temperature of the residual for a period of about 1 to 10 hours. the metal powder is then returned to room temperature in a vacuum or inert atmosphere. When two or more residuals are to be removed the metal powder is subjected to the melting temperature and boiling temperature of each residual element to be removed in turn.

One advantage of this method is that it is capable of removing residual gases or elements from metal powders more efficiently than any other known process. Another advantage of the method is that most gases can be removed cryogenically thus not subjecting the metal powder to high temperature which could result in sintering or otherwise destroy the physical properties of the powder.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a flow diagram of the steps of a preferred method of the invention; Figure 2 is a graph showing the processing temperatures as a function of time for removing a residual element having melting and boiling points above room temperature but below the melting point of the powdered metal and other desirable elements; Figure 3 is a graph showing the processing temperature as a function of time for removing a residual gas having melting and boiling points below room temperature; Figure 4 is a graph showing the processing temperature as a function of time for removing two or more different residual elements; and Figures 5, 6 and 7 contain three tables listing the melting and boiling points of gases and residual elements.

The invention is a method for removing gases and residual elements from metal powders. The term "residual" as used herein means residual or "tramp" elements including dissolved, absorbed, adsorbed or otherwise occluded gases or any other unwanted elements. The basic method is shown in the flow diagram of Fig. 1. Metal powder made from a high purity metal is placed in a rotatable porous drum which is then inserted into a vacuum chamber as described in block 10. The porous drum may be made from a wire mesh or screen having apertures sufficiently small to retain the metal powder yet permit the gases or vapors evolved from the powders during the subsequent processing steps to escape and be removed by the vacuum pump. The drum may be mechanically or magnetically rotated by a motor external to the vacuum chamber using any of the various techniques known in the art.Magnet rotation of the porous drum is preferred over mechanical means since it does not require a rotating shaft or movable member passing through the walls of the vacuum chamber which could be a source of air leaks. The metal powder may be produced using any conventional method including those previously discussed. The vacuum chamber may include either cryogenic cooling capabilities or heating capabilities or both depending upon the residual to be removed. The vacuum chamber is then evacuated to a predetermined low pressure as described in block 12. Since the efficiency of the process for the removal of residuals is an inverse function of pressure within the vacuum chamber, the predetermined pressure is normally the minimum pressure obtainable within a reasonable period of time and depends upon the pumping capacity and minimum pressure capabilities of the vacuum pump..Preferably the predetermined low pressure is less than 50 micron of mercury but the method has been found to be effective in the removal of residuals at vacuum pressures in excess of 100 microns of mercury.

After the vacuum in the chamber has stabilized at the predetermined low pressure the rotation of the porous drum is initiated as described in block 14. Rotation of the porous drum tumbles the metal powder causing the particles to be momentarily levitated, thereby repetatively exposing all of the surfaces of the particles to the vacuum atmosphere. This enhances the release of entrapped gases and evolved residuals as well as preventing caking or sintering of the metal powder during the subsequent steps of the process.

With the pressure inside of the vacuum chamber stabilized at the predetermined pressure and the metal being tumbled in the rotating porous drum, the metal powder is heated or cryogenically cooled to the melting point (melting temperature) of the residual to be removed as described in block 1 6. After the metal powder reaches the melting temperature of the residual, it is momentarily held at that temperature for a period of time T1 usually ranging from 5 to 30 minutes as described in block 18.

The temperature of the metal powder is then elevated to the boiling point (boiling temperature) of the residual as described in block 20. After the metal powder reaches the boiling point (B.P.) of the residual, it is held at that temperature for a period of time T2 usually for 1 or more hours, suitably from 1 to 10 hours, as described in block 22. Although the residual will be removed by omitting the step of temporarily holding the metal powder at the melting temperature of the residual to be removed, tests have repeatedly shown that the removal of the residuals is significantly enhanced when this step is included. It is further understood that this method is limited to the removal of residual having melting and boiling points below the melting temperature of the metal powder being purified.Residuals having boiling points above the metal powder or any of its desired elements cannot be removed by this process since the metal powder would be melted or the desired element removed. For particular gaseous residuals such as oxygen (02), hydrogen (H2) and nitrogen (N2) gases, their removal from inside the vacuum chamber is accomplished by the boiling action of the gases as they are vaporized and are separated from the metal powders; the vaporized residuals are then exhausted through the vacuum pump port. The removal of these vapors may be enhanced during this period through the use of chips of a gettering material mixed in with the metal powder.

Gettering materials, such as barium, strontium, calcium magnesium, titanium and tantulum are known to selectively assist in the removal of these gases or vapors by absorption, adsorption, chemeosorption or any combination of these processes and prevent their re-entrance into the metal powders. These chips of gettering material are significantly larger than the particles of the metal powder so that they may be selectively removed at the end of the process.

If the process is for the removal of only one residual, the metal powder is returned to room temperature in the vacuum or inert atmosphere prior to being removed from the vacuum chamber as described in block 24. When more then one residual is to be removed from the metal powder, the steps given in blocks 1 6 through 22 are repeated for each residual. In the process of cryogenically removing more then one residual it is preferred to first process the metal powder at the melting and boiling temperatures of the residual having the lowest melting and boiling points. The remaining residuals are then removed in an ascending temperature order such that the last residual to be removed has the highest melting and boiling points.

A typical temperature-time schedule for the removal of a residual from the metal powder having a melting point (M.P.) and boiling point (B.P.) which is greater than room temperature is shown in Fig. 2. For this type of residual, the metal powder is heated in a vacuum atmosphere at the melting point of the residual to be removed for a period of time T1 ranging from 5 to 30 minutes The temperature of the metal powder is then elevated to the boiling point (B.P.) of the residual where it is held at that temperature for a period of time T2, usually from 1 to 10 hours.

It was found that by bringing the metal powder to the melting point of the residual to be removed for a short period of time, then bringing the metal powder to the boiling point of the residual, the residual is removed in a much shorter time when compared to going directly to boiling point. This has a distinct advantage since it reduces the processing time required to remove the residual. The metal powder is then allowed to return to room temperature in the vacuum atmosphere or in an inert atmosphere as previously described.

Fig. 3 is a time-temperature diagram for a residual having a melting and boiling point below room temperature. In this case the metal powder is cryogenically cooled in a vacuum atmosphere to the melting point of the residual desired to be removed. The metal powder is held at this temperature.for a period of time T, usually ranging from 5 to 30 minutes. The metal powder is then elevated to the boiling temperature (B.P.) of the residual where it is held for a period of time T2 usually ranging from 1 to 10 hours. The time T2 may be shorter than 1 hour or longer than 10 hours depending upon the degree of purification desired and the residual to be removed. At the end of time T2 the metal powder is allowed to warm up in the vacuum or in inert atmosphere to room temperature to prevent oxidation or other contamination of the powder.

Fig. 4 shows the time-temperature diagram for the removal of more than one residual from the metal powder, such as the removal of sodium chloride (Nac,) from a metal powder made from titanium sponge. In this case the metal powder is first heated in a vacuum atmosphere to the melting point of the sodium (M.P.Na) where it is held for a time T1 then elevated to the boiling point of sodium (B.P.N,,) for a time T2. The metal powder is then cryogenically cooled to the melting temperature of chlorine (M.P.CI) and held there for a second period of time T1. The temperature of the metal powder is then allowed to rise to the boiling point of chlorine (B.P.C,) where it is held for a second period of time T2 before allowing it to warm up to room temperature in the vacuum or inert atmosphere.The periods of time T1 and T2 have durations corresponding to those discussed with reference to Figs. 2 and 3.

The melting points (M.P.) and boiling points (B.P.) of some of the more common residual elements are shown on tables presented in Figs. 5, 6 and 7. Table 1 given in Fig. 5 lists the residuals having both melting and boiling points at temperature above 0 C. Table II given in Fig. 6 lists the residuals having melting and boiling points at temperatures below 0 C and Table Ill of Fig. 7 lists a couple of elements having a melting point below 0 C and a boiling point above O,C. The melting and boiling points (temperatures) listed on Tables I, II and Ill are those measured at atmospheric pressure (760 Torr) and are the temperatures used in the disclosed method.As will be noted, the melting temperatures of some of the gases, such as oxygen, hydrogen, and nitrogen approach absolute zero (- 273"C). Therefore in order to reduce the metal powder to these temperatures in accordance with the disclosed method, they must be cryogenically cooled using liquid helium.

Cryogenic cooling of metals have been previously used for purposes other than the removal of residuals. U.S. Patent No. 3,891,477 and U.S. Patent No. 3,185,600 disclose the use of cryogenic cooling to alter the microstructure of materials to improve resistance to wear and corrosion. Alternatively U.S. Patent No. 4,018,633 discloses cooling metal chips below the ductile-brittle transition point to increase impact fragmentation in the production of metal powders. None of the known prior art, however, teaches the use of cryogenic cooling for the removal of residuals.

The mechanism which produces the significant reduction in the residuals achieved by the disclosed method is bringing the residual to its boiling point and extracting the evolved vapors suitably with a vacuum pump. The following test results are provided to show the effectiveness of the process.

TEST NUMBER 1 This test was designed to reduce the chlorine (Cl) residual in a 50 gram sample of titanium powder. The sample was cryogenically cooled in a vacuum to the melting temperature of chlorine (about - 101 'C) and held at that temperature for approximately 5 minutes. The sample was then allowed to warm up to the boiling point of chlorine (about - 35"C). After 3 hours at - 35"C the sample was allowed to return to room temperature before removal from the vacuum chamber. The chlorine content of the sample prior to processing was 2,200 parts per million (ppm) and after processing the chlorine content was 50 parts per million.

TEST NUMBER 2 This test was designed to remove the oxygen residual from another 50 gram sample of titanium powder. The sample was cryogenically cooled at the melting point of oxygen (about - 218"C) for 5 minutes then at the boiling point of oxygen (- 183"C) for 3 hours. The oxygen content of the sample was 1 ,200 parts per million (ppm) prior to the test, and 47 parts per million (ppm) after being processed.

TEST NUMBER 3 This test was designed to remove hydrogen, nitrogen and oxygen residuals from another 50 gram sample of titanium powder. The sample was cryogenically cooled to the melting point of hydrogen (about -- 250'C) for about 5 minutes. The temperature was then raised to the boiling point of hydrogen (about - 253'C), the boiling point of nitrogen (about - 196"C) and the boiling point of oxygen (about - 183"C) in sequential order. The sample was held at each of these boiling points for approximately one hour.The before and after concentrations of the three residuals were as follows: Residual Before After Hydrogen 70 1 3 ppm Nitrogen 142 ppm 26 ppm Oxygen 1,200 ppm 63 ppm TEST NUMBER 4 This test was designed to remove the sodium (Na) residual from a 50 gram sample of titanium powder taken from the same batch as test 3. The powder was heated in a vacuum atmosphere to the melting point of sodium for 5 minutes then to the boiling point of sodium (about 882"C) for 2 hours. The concentration of sodium prior to the test was 1,570 parts per million and after the test the concentration was reduced to 800 parts per million.

In all of these tests, the concentration of the residuals was measured by an independent laboratory, Herron Testin Laboratories of Cleveland, Ohio.

The invention is applicable to removing any residual which has a melting and boiling temperature below the metal powder from which it is to be removed. Further the method is not limited to removing residuals from metal powders but also may be used to remove these same residuals from bulk materials, and in particular bulk materials in the form of thin metal sheets.

Claims

1. A method for removing residual elements including dissolved, absorbed, adsorbed, or otherwise occluded gases from metal powders, comprising the steps of: placing the metal powder in a vacuum chamber; evacuating the vacuum chamber to reduce the atmosphere surrounding the metal powder to a vacuum; subjecting the metal powder to the melting temperature of the residual element to be removed for a first predetermined period of time Tt; elevating the temperature of the metal powder to the boiling temperature of the residual element to vaporize the residual element; holding the metal powder at the said boiling temperature for a second period of time T2; and returning the metal powder from the said boiling temperature to room temperature prior to removing the metal powder from the vacuum chamber.

2. A method as claimed in Claim 1, wherein the step of subjecting the metal powder to the melting temperature of the residual element includes the step of heating the metal powder to the melting temperature of the residual element.

3. A method as claimed in Claim 1, wherein the step of subjecting the metal powder to the melting temperature of the residual element includes the step of cooling the metal powder to the melting temperature of the residual element.

4. A method as claimed in Claim 3, wherein the said step of cooling includes the step of cryogenically cooling the metal powder to the melting temperature of the residual element.

5. A method as claimed in Claim 4, wherein the said step of cryogenically cooling is performed with liquid helium.

6. A method as claimed in any of Claims 1 to 5, further including the step of tumbling the metal powder to enhance the release of entrapped gases and evolved vapors and to prevent sintering or caking when the metal powder is at the said melting and boiling temperatures.

7. A method as claimed in any of Claims 1 to 6, wherein the said first predetermined period of time T, is from 5 to 30 minutes.

8. A method as claimed in any of Claims 1 to 7, wherein the said second predetermined period of time T2 is from 1 to 10 hours.

9. A method as claimed in any of Claims 1 to 8, wherein the said step of placing the metal powder in a vacuum chamber further includes the step of mixing chips of a gettering material with the metal powder to assist in the removal of the vaporized residual element.

10. A method as claimed in any of Claims 1 to 9, wherein the metal powder has two or more residual elements to be removed, and wherein the method further includes repeating the said steps of subjecting, elevating and holding at the melting and boiling temperatures of each residual element.

11. A method for removing residual elements including dissolved, absorbed, adsorbed or otherwise occluded gases from metal powders, comprising the steps of: placing the metal powder in a vacuum chamber; evacuating the vacuum chamber to produce a vacuum atmosphere about the metal powder; heating the metal powder to the melting temperature of the residual element to be removed for a first predetermined period of time T,; elevating the temperature of the metal powder to the boiling temperature of the residual element to be removed, holding the metal powder at the said boiling temperature for a second predetermined period of time T2; and cooling the metal powder to room temperature in a non-contaminating atmosphere prior to removal from the vacuum chamber.

1 2. A method as claimed in Claim 11, further including the step of tumbling the metal powder to release entrapped gases and evolved vapors and to prevent sintering when the metal powder is at the said melting and boiling temperatures.

1 3. A method as claimed in Claim 11 or 12, wherein the said step of placing the metal powder in a vacuum chamber further includes the step of mixing into the metal powder chips of a gettering material to assist in the removal of the residual element.

14. A method as claimed in any of Claims 11 to 13, wherein the said first predetermined period of time T, is from 5 to 30 minutes.

1 5. A method as claimed in any of Claims 11 to 14, wherein the said second predetermined period of time T2 is greater than 1 hour.

1 6. A method for removing residual elements including dissolved, absorbed, adsorbed or otherwise occluded gases from metal powders, comprising the steps of: placing the metal powder in a vacuum chamber; evacuating the vacuum chamber to produce a vacuum atmosphere about the metal powders; cooling the metal powder to the melting temperature of the residual element to be removed for a first predetermined time period T,; increasing the temperature of the metal powder to the boiling temperature of the residual element to be removed; holding the metal powder at the said boiling temperature for a second period of time T2; and returning the metal powder to a temperature which will prevent moisture condensation on the powder prior to removal from the vacuum chamber.

1 7. A method as claimed in Claim 16, wherein the said step of placing the metal powder in a vacuum chamber further includes the step of tumbling the metal powder to enhance the release of entrapped and evolved gases and to prevent camking of the metal powder.

1 8. A method as claimed in Claim 1 6 or 17, wherein the said step of placing the metal powder in a vacuum chamber includes the step of mixing into the metal powder chips of a gettering material to assist in the removal of the residual element.

19. A method as claimed in any of Claims 16 to 18, wherein the said first predetermined period of time T1 is from 10 to 30 minutes.

20. A method as claimed in any of Claims 1 6 to 19, wherein the said second predetermined period of time T2 is greater than 1 hour.

21. A method according to Claim 1 for removing residual elements from metal powders, substantially as herein described with reference to Fig. 1 of the accompanying drawings.