US2948609A

US2948609A - Manufacture of metal articles from

Info

Publication number: US2948609A
Authority: US
Publication date: 1960-08-09
Anticipated expiration: 1977-08-09

Description

MANUFACTURE OF METAL ARTICLES FROM TUNGSTEN METAL POWDER Tivadar Millner and Jenii Neugebauer, Budapest, Hungary, assiguors to Egyesult Izzolampa es Villamossagi Reszvenytarsasag, Budapest, Hungary, a Hungarian enterprise No Drawing. Filed Apr. 21, 1955, Ser. No. 503,021

Claims priority, application Hungary May 6, 1954 3 Claims. (Cl. 7'5214) This invention relates to the manufacture of metal articles such as tungsten bars, wires or filaments, and

' purities remaining in the rods after their major part has been volatilised during sintering. It is known, for example, that, besides such traces of alkali and silicon, traces of aluminium remaining in tungsten rods produce a large-crystal structure, and advantageous properties are obtained.

The above phenomenon is nowadays greatly exploited in the manufacture of tungsten wires. However, owing to conditions during manufacture and working of tungsten rods, it often happens that the extremely small quantity of the order of 0.001% or even less of useful impurities together with the excess additives and impurities is not evenly distributed in the separate tungsten particles or through the full cross-section of the sintered tungsten rod or, further, in the body of the tungsten wire along its length. The result is that, although the structure of such tungsten rods is large-crystalline near its surface, the interior is small-crystalline, thus making the rod non-homogeneous. The drawn tungsten wire is thus more brittle at certain points than elsewhere. This lack of homogeneity is due to the fact that the required small traces of useful additives and impurities are not always distributed with the desired uniformity in the tungsten, but accumulate at certain recognised points for example, at the boundaries of crystallites or fibres.

According to the standard method of manufacturing tungsten metal powder, the hydrogen reduction of W powder provided with useful additives to W O then to W0 and finally to metal powder is carried out in such a manner that the particle size of the W0 powder is generally one hundredth that of the W0 grains. As a result of a further reduction, metallic grains are obtained, the particle size of which is much greater than that of the initial W0 grains. If, however, the process of reducing W0 powder is not disturbed, the additives initially present on the surface of the W0 grains keep their relative position thereon and also their local concentration in the fine W0 grains, and become unevenly distributed in the nascent and growing metallic grains, i.e., they retain their high local concentration.

If, following the usual course of tungsten manufacture, such powder is compressed into porous bars (e.g. of an approximate porosity of 55%) and sintering effected by passing a slowly increasing current through them, the major proportion of the additives and impurities are gradually volatilised before the pores of the barsclose materially, ie before porosity decreases by about 30%. If the current, and hence the temperature, rises, the pores of the bars close rapidly, whereby traces :Patented' Aug. 9, 1960 of additives become trapped in the consolidating bar. If a considerable quantity of additives and impurities become trapped in the bar, the latter swells up due to vapour pressure in the bar when the latter reaches a temperature near the melting-point of tungsten. Consequently, the bar is unsuitable for working. If a smaller quantity of such additives is retained, the vapour pressure does not swell up the.bar but breaks down the cohesion of its crystallites, thereby making working uneven and producing non-homogeneous wire.

In order to secure better mechanical working characteristics and wire uniformity, it is necessary to rid the bar of additives to the greatest possible extent before the pores close. However, these additives should not be entirely volatilised.

An extremely small quantity, of the order of 0.001% or even less, of these useful substances is sufficient to obtain the required properties. To obtain a highly uniform wire, it is important that even these small traces are present in the tungsten in a most uniform distribution.

The present invention aims at minimising some at least of the undesirable characteristics of sintered tungsten articles, as hitherto produced, by introducing into the sintering process a delay stage during which removal of the excess additives or impurities can take place, whereafter sintering is taken to completion in the normal manner.

The said delay stage may consist of prolonged heating at moderate temperature, during which excess additives (i.e. those not used up in, say, crystal grain structure improvement) are volatilised through the pores of the partly sintered article. Alternatively, the delay stage may comprise immersion in a bath of dissolving agent for the additives following on an initial or pre-sintering heating stage.

In this specification, the term moderate range of temperature is to be understood as 'a range of temperature corresponding to the passage of current through the bar of between 30 and 60% of the current required to fuse the bar.

In the normal process of sintering tungsten bars, an excess of additives-sometimes exceeding 0.01%are trapped in the bar within the temperature range for rapid shrinkage, and their distribution is usually rather uneven. It has been found that this excess of additives over the specific requirements is not only unevenly distributed but actually disadvantageous in that, even if it does not always cause swelling of the bar, mechanical working is made more diificult. These additives would in time have escaped by volatilisation from the bar through the pores, and are normally concentrated at the grain boundaries. Although such bars usually consolidate during further sintering and appear to be satisfactory, their mechanical working is diflicult and their mechanical properties are uneven.

Comparative tests between the conventional process and that according to the present invention were carried out as follows:

Tungsten powder was prepared according to standard practice, using additives for obtaining a large crystal structure in the subsequent sintered bars. Bars were pressed from the powder and heated gradually in accordance with normal practice by passing current therethrough for a period not longer than 15-20 minutes and at amperages up to 92% of the current required to fuse them. The bars were held at the temperature corresponding to maximum current for 5 minutes. The sintering process was then completed, and the resultant bars exhibited the following features:

(a) A large crystal structure generally on the surface only, the interior remaining small-crystalline;

(b) Could not be easily and properly worked whilst maintaining the uniformity requirements of mass production;

On re-crystallisation, suitable large-crystalline structure and strength for drawing to thick wires,

(:1) When drawn to thin wires, low tensile strength and uneven metallurgical properties.

Tungsten bars were also made from the aforesaid metal powder by the process according to the present invention. This differed from the standard process in that the bars were heated by passing current therethrough in he usual way to raise the temperature gradually in minutes up to the moderate range-corresponding to about 4050% of the fusion current; then held at this current within the moderate range of temperature for a delay period of from /4 to 1 hour; and finally sintering in the usual manner. The results were bars which exhibited the following features:

(0) Their entire length and full cross-section showed a large-crystal structure;

(b) Ease of mechanical working;

(c) On recrystallisation, large crystalline structure and suficiently uniform strength for drawing to thick wires;

(d) Capable of being drawn to thin wires (e.g. having a diameter of below (e) Much improved tensile strength, elasticity and evenness in thin wires compared with those drawn from bars sinteredby the standard process.

One specific example of the process according to the present invention is particularly described below by way of illustration only:

Tungstic acid or W0 prepared with 0.2% NaCl, 0.3% KCl, 0.4% SiO 0.05% A1 0 and 0.03% Fe o and having a particle size of between /2-1 /2,u, was reduced in a current of hydrogen at a temperature rising to 820 C. to metal powder having a particle size of /22,u. From this powder bars were pressed measuring 10 x 9 x 280 mm. and weighing 200 grams, i.e. having a porosity of about 55%. These bars were heated for 56 minutes by passing a current through them up to 45% of the melting current. The observed shrinkage was 6%. Heating continued for one hour at a current of 46% of the melting current, this constituting the delay stage and giving a shrinkage of 12%. The bars were then heated in the usual manner for about 5 minutes at currents up to 92% of the melting current, and sintering continued at this current for a further 5 minutes. The finished bars showed a shrinkage of 18-19%, and were then swaged and drawn into wires in the usual way.

The following characteristics were observed:

(a) Large-crystal structure for substantially the entire surface and full cross-section of the finished bars;

(b) Marked economy of material in swaging and draw- (c) High degree of uniformity after drawing;

(d) Wires of 0.6 mm. diameter were composed of large crystals on recrystallisation;

(2) Thin wires of about 12 diameter had a tensile strength exceeding by 10% that of similar wires drawn according to the usual method. Their elongation was about 2%, and was entirely consistent and even. Their specific resistance was more uniform than that of wires drawn according to the usual method. While only about 60% of the thin filaments made in the usual way met the requirements of mass production on uniformity, 95% of the wires made according to the present invention proved to be satisfactory.

The foregoing results indicate that, by subjecting the pressed bar to prolonged heating within the moderate range of temperature, the pores of the bars close sufliciently slowly to allow excess additives to volatilise.

It was further found that bars made in the usual way showed a linear decrease in dimension (shrinkage) of 5-7% within about 10 minutes until the moderate range of temperature was reached, Whereafter the shrinkage increased to 18-40% within a further 10 minutes until sintering was complete. On the other hand, bars made from the same powder, but according to the present invention, showed also a shrinkage of 57% for about 10 inutes until the moderate range of temperature was reached, but during a 30 minute delay period of heating within the moderate range of temperature, shrinkage was only about 12% during sintering at a high temperature for a further 10 minutes or so it was about l820%. Furthermore, the impurity content of bars made in the usual way was of the order of 0.030.0l%. On the other hand, in bars made according to the present invention, this content was much smaller, and usually impossible to determine, i.e. less than 0.001%.

The higher the moderate range of temperature, the faster the excess additives are volatilised from the bar. Therefore, it seems advisable in practice to choose the highest possible moderate range of temperature. This, however, is accompanied by more rapid closure of the pores. Experiments showed that the degree of volatilisation of the undesirable excess additives does not mainly depend on whether the prolonged heating within the moderate range of temperature takes place at a somewhat lower temperature for a longer period or at a somewhat higher temperature for a shorter period.

In the process according to the present invention various combinations of time and temperature limits for the prolonged heating delay stage are possible. The main factors determining the selection of the duration and temperature, for the normal additives used nowadays, are the sufficiently rapid volatilisation of the excess additivessay, volatilisation sufficiently complete within a few hoursand the temperature at which the closing of the pores is not too rapid-say, closing not effectively completed within 510 minutes. The most practical range of heating current is between 40% and 50% of the melting current. These values correspond with heating for a delay period of about 10 hours on the one hand and about 10 minutes on the other hand.

In order to secure uniformity and good physical and mechanical properties in very thin wires, it is more advantageous to carry out heating at a lower temperature for a longer time than at a higher temperature for a shorter time. The value of the moderate temperature, at which the best relationship between the rates of volatilisation and of closing of the pores is obtained, is influenced to a certain extent by the quality and quantity of the additives, as well as by the grain size of the tungsten powder. It is advisable, therefore, to determine separately the moderate range of temperature for each tungsten powder of different character. This may in practice be achieved by determining, on the one hand, the maximum value of shrinkage of the bars during sintering under standard conditions and, on the other hand, the current required to fuse the bar; then determining the current at which shrinkage increases, within an hour, from /3 to /2 or of the maximum value of shrinkage normally 18%). If the current is 11% of the melting current, the useful current range for the delay stage in a process according to the present invention is from (n5)% to (n+5)%.

The process according to the present invention may also successfully be performed when the delay stage of heating within the moderate range of temperature is split into two or more periods at different temperatures, e;g. the pressed bar is heated within the useful current range first at a lower current value and then at a higher value for a considerable time. Alternatively, good results can be obtained by continuously raising the current within the useful range for the delay stage.

In a modified procedure according to the invention, compressed powder tungsten bars are heated to about the lower limit of the moderate range of temperature and immersed for a delay stage in a dissolving agent to dissolve out the excess quantity of additives and impurities.

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This method has an advantage over the vola'tilising process in cases where the pores of the bars close rapidly even at low temperatures. In such cases it is more advantageous to adopt the said washing process in place of the volatilising procedure. But the washing process may also be advantageously adopted in many other cases.

In the practical operation of the modified procedure according to the present invention, the usual heating of a porous tungsten bar is interrupted when the current passing through the bar reaches a value of about 35 40% of the fusion current, and the delay stage in the process starts with immersion of the bar in a dissolving agent chosen in dependence on the particular additives and impurities presentfor example, hydrofluoric acid solution of moderate concentration-and kept there for, say, 24 hours. The bar is rinsed in a slightly acid solution or distilled water for a longer period, e.g. three days, the rinsing fluid being changed regularly. After rinsing, the bar is dried thoroughly and heated in a reducing atmosphere at a temperature of about 1000 C., thereafter completing the sintering in the usual manner. By this modified procedure the excess additives are brought to the surface of the bar by the initial heating (during which the temperature reaches about 2000 C.) and are re. moved during a delay period by the dissolving agent from the surface pores.

In a second specific example of the process according to the present invention, which will now be particularly described by way of illustration only of the said modified procedure, tungstic acid or W prepared as particularly exemplified above, was pressed into porous bars measuring 15 x 15 x 450 mm. and weighing 800 grams. The porosity of such bars is approximately 55%. The bars are heated by passing through them a current which is increased slowly up to 35% of the melting current in about 20 minutes. The heating process is then interrupted for the delay stage, and the bars, having shrunk by 7%, are immersed in a 40% hydrofluoric acid solution and kept there for 24 hours. Thereafter the bars are kept in water containing hydrochloric acid for a day and in distilled water for two days, the bath being frequently changed. The washed bars are dried at a temperature of 100 C. and heated in a current of hydrogen at a temperature of about 1000" C. Thereafter sintering is continued by passing current through the bars, the current increasing slowly in 10 minutes from 35% to 92% of the melting current, sintering being completed by heating the bars for minutes with a current of 92% of the melting current. The finished bars having shrunk 18-19% are then swagedand drawn into wire in the usual manner. The resultant wire exhibited the same improved characteristics as that produced by the prolonged heating process described earlier in this specification.

This latter method is to be distinguished from the known washing procedure according to which tungsten powder containing additives is washed in a hydrofluoric acid solution and distilled water, and bars and wires are then made from this washed metal powder. By washing the metal powder, only such foreign matter as is found on the surface of the individual grains is removed, the traces of additives and impurities which become absorbed or occluded in the growing metal grains during the reduction process are not touched by the dissolving agent. That is why bars made of washed metal powder usually include similar excess quantities of additives, as bars made of an unwashed powder. By pre-sintering the tungsten bar itself before washingas envisaged in the present invention-the major part of the additives and impurities embedded between the grains escapes to the surface pores and can be dissolved therefrom.

The dissolving process just described requires in case of a single bar a considerably longer treatment period 6 for each bar than volatilising by prolonged heating at moderate temperature. As, however, in a mass production plant a large number of bars are continuously being produced, the time required for the dissolving process can be spread over the successive batches.

We claim:

l. A method of manufacturing tungsten articles from tungsten metal powder containing non-tungsten ingredientsin the form of additives and impurities, which comprises pressing porous bars from said powder, sintering said bars by heating them by means of an electric current of gradually increasing strength up to a temperature below fusion temperature of the bars, but higher than a moderate range of temperature which corresponds to the passage of current through said bars of between a lower limit of 30% and an upper limit of 60% of the current required to fuse said bars; removing excess quantities of non-tungsten ingredients from the still porous bars by heating said bars at temperatures in the range between said lower limit and said upper limit, effecting said removal of said excess quantities of non-tungsten ingredients up to an extent ensuring that the finished bars c0ntain non-tungsten ingredients in a useful quantity not exceeding the order of magnitude of 0.001%.

2. A method for manufacturing tungsten articles from tungsten metal powder containing non-tungsten ingredients in the form of additives and impurities, which comprises pressing porous bars from said powder, sintering said bars by heating them by means of an electric current of gradually increasing strength up to a temperature below fusion temperature of the bars, but higher than a moderate range of temperature which corresponds to the passage of current through said bars of between a lower limit of 30% and an upper limit of 60% of the current required to fuse said bars; working the resulting sintered bars by mechanical means; removing excess quantities of non-tungsten ingredients from the still porous bars by heating said bars at temperatures in the range betweensaid lower limit and said upper limit, eifecting said removal of said excess quantities of non-tungsten ingredients up to an extent ensuring that the finished bars contain non-tungsten ingredients in a useful quantity not exceeding the order of magnitude of 0.001%.

3. A method of manufacturing tungsten articles from tungsten metal powder containing non-tungsten ingredients in the form of additives and impurities, which comprises pressing porous bars from said powder, sintering said bars by heating them by means of an electric current of gradually increasing strength up to a temperature below fusion temperature of the bars, but higher than a moderate range of temperature which corresponds to the passage of current through said bars of between a lower limit of 30% and an upper limit of 60% of the current required to fuse said bars; working the resulting sintered bars by mechanical means; removing excess quantities of non-tungsten ingredients from the still porous bars by heating said bars at temperatures in the range between said lower limit and said upper limit, eifecting said removal of said excess quantities of non-tungsten ingredients up to an extent ensuring that the finished bars contain non-tungsten ingredients in a useful quantity not exceeding the order of magnitude of 0.001%; and finally. sintering the bars by heating, by an electric current not exceeding 92% of the melting current.

Tungsten, by Li and Wang, 2nd edition, page 206, publjshed by Reinhold Publishing Corp., New York, N. Y.

Claims

1. A METHOD OF MANUFACTURING TUNGSTEN ARTICLES FROM TUNGSTEN METAL POWDER CONTAINING NON-TUNGSTEN INGREDIENTS IN THE FORM OF ADDITIVES AND IMPURITIES, WHICH COMPRISES PRESSING POROUS BARS FROM SAID POWDER, SINTERING SAID BARS BY HEATING THEM BY MEANS OF AN ELECTRIC CURRENT OF GRADUALLY INCREASING STRENGTH UP TO A TEMPERATURE BELOW FUSION TEMPERATURE OF THE BARS, BUT HIGHER THAN A MODERATE RANGE OF TEMPERATURE WHICH CORRESPONDS TO THE PASSAGE OF CURRENT THROUGH SAID BARS OF BETWEEN A LOWER LIMIT OF 30% AND AN UPPER LIMIT OF 60% OF THE CURRENT REQUIRED TO FUSE SAID BARS, REMOVING EXCESS QUANTITIES OF NON-TUNGSTEN INGREDIENTS FROM THE STILL POROUS BARS BY HEATING SAID BARS AT TEMPERATURES IN THE RANGE BETWEEN SAID LOWER LIMIT AND SAID UPPER LIMIT, EFFECTING SAID REMOVAL OF SAID EXCESS QUANTITIES OF NON-TUNGSTEN INGREDIENTS UP TO AN EXTENT ENSURING THAT THE FINISHED BARS CONTAIN NON-TUNGSTEN INGREDIENTS IN A USEFUL QUANTITY NOT EXCEEDING THE ORDER OF MAGNITUDE OF 0.001%.