GB1605248A

GB1605248A - Production of articles from powdered metals

Info

Publication number: GB1605248A
Application number: GB20745/43A
Authority: GB
Inventors: Leonard B Pfeil
Original assignee: Mond Nickel Co Ltd
Current assignee: Mond Nickel Co Ltd
Priority date: 1943-12-10
Filing date: 1943-12-10
Publication date: 1986-03-05

Description

(54) IMPROVEMENTS IN THE PRODUCTION OF ARTICLES FROM POWDERED METALS (71) We, THE MOND NICKEL COMPANY LIMITED. a British Company, of Grosvenor House, Park Lane, London W.1, and LEONARD BESSEMER PFEIL. a British Subject, of 10 Carisbrooke Road, Edgbaston, Birmingham, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement: This invention relates to the production of articles by sintering metal powders.

Our primary object is to produce a metal article in strip or sheet form of very accurate thickness or gauge and to do this rapidly by a continuous process. We also aim at making a porous product the porosity of which can be exactly controlled.

In general the metal powders used in powder metallurgy are of a freely flowing nature, and are either charged into moulds, each successive charge in a given mould being of uniform weight, or are applied over a moving surface and spread themselves over it by their capacity for flowing freely. The powders used generally have to be of controlled particle size. There are, however, many powders which will not flow sufficiently freely in the dry state to form a truly uniform layer on a support and so they form products of irregular thickness. In particular it is practically impossible to make powders of very small particle size into uniform layers. Moreover, so far as we are aware, it is both troublesome and expensive by any of the methods now in use so to control the porosity of a sintered product as to produce very fine (microscopic) pores occupying a predetermined proportion of the product and rendering it suitable for use as a permeable membrane.

According to this invention we form a slurry by suspending the metal powder in a liquid, and then we spread this slurry uniformly over a support, which is preferably a moving surface.

Next, we remove the liquid by vaporization so as to leave a layer of dried slurry on the support and this dried slurry is heated to sintering temperature to cause the metal particles to adhere to one another.

The slurry is prepared by mixing metal powder with a liquid until the suspension has a creamy consistency such that it will adhere to a glass rod as a smooth layer. Mixtures of metal powders may be employed and the powder need not be of uniform size; indeed, it may sometimes be advantageous, especially when it is desired to produce porous material, to employ mixtures of different particle size so as to combine the convenient processing characteristics of coarse powder with the fine pore size that is characteristic of the use of fine powder. A particular advantage obtained by the use of a slurry is that the particle size of the powder ceases to be of primary importance and any powder can be distributed more evenly over the support than if it is used dry.

Moreover, substances may be added to the slurry to impart solidity to the dried slurry before sintering or to act as spacing materials which will decrease the packing density of the dried slurry and increase the porosity of the sintered products. Particularly suitable substances for this purpose are those which are volatile at the sintering temperature or which can be removed coveniently by means of a solvent. For example, to a slurry of nickel powder, ammonium carbonate may be added and will volatilise during the sintering step, or again aluminium powder may be added, the aluminium being removed from the lightly sintered material by treatment with caustic soda.

In making the slurry, the proportion of metal powder to liquid must necessarily vary with the size and shape of the metal particles, the packing density of the powder, the rate of travel of the surface on which the slurry is to be formed into a layer, the thickness of that layer and the nature of the liquid. Water may be employed as the liquid medium, but other liquids may be used, for example organic liquids such as alcohols and aromatic hydrocarbons, in particular ethyl alcohol or benzene. In choosing a suitable liquid, attention must be paid to the acidity, which, for example, in the case of water should be kept low, and to the desirability of avoiding coagulation and settling of the solid particles.

Wetting and dispersion agents may advantageously be incorporated to assist in maintaining the powder suspended in the liquid.

It is desirable that the dried slurry should be as coherent as possible, and to give it cohesion small amounts of a plastic material, say less than one tenth of the metal powder by weight, may be incorporated in the slurry. The preferred plastic materials are the so-called thermoplastic resins which leave no residue when volatilised in hydrogen, and they may be used in conjunction with plasticisers.

Preferably the support is a travelling belt, and this may carry the layer through drying and sintering zones. One apparatus embodying such a belt is illustrated diagrammatically in Figures 1, 2 and 3 of the accompanying drawings, in which: Figure 1 shows the lay-out of the whole apparatus; Figure 2 is an enlarged view showing the initial formation of the slurry layer; and Figure 3 is a section on the line III - III in Figure 2.

In this apparatus, the slurry is introduced into a hopper-like container 1 mounted on a bed plate 2 which acts as a jig. The bed plate is formed with a trough having a longitudinally curved bottom surface 32 and upstanding walls 33, and an endless belt 3 carried by rollers 4 and 34 runs through this trough in close contact with the bottom surface 32. This belt 3 may advantageously be made from a chromiumcontaining alloy such as a nickel-chromiumiron alloy, e.g. the alloy sold under the registered trade mark "Inconel", since an adherent film of oxide can be formed on the surface which prevents unduly firm adhesion of the sintered material to the belt and thus facilitates its removal at the end of the sintering stage. The belt may be very thin, say 0.005 inch thick, so that it will be flexible and will easily adapt itself to the curvature of the surface 32.

The rear wall 1A of the container 1 extends into the trough into contact with the belt so that the slurry cannot escape backwards, and to ensure close contact the bottom edge of the wall may be formed by a rubber pressure pad 35 urged downwards by springs 36. A plate 5 is adjustably fixed to the front wall IB by means of a screw 6 passing through a slot 7 in the wall and is formed at the bottom with a knife edge 8 which also lies inside the trough. This plate is spaced away from the upper surface of the belt so as to leave a narrow slit through which the slurry is carried away by a belt as a layer 9, and the plate lies above the rising part of the surface 32 in order to ensure that the slurry is spread on the belt 3 while that belt is still bedded down on the surface 32. The arrangement shown results in the thickness of the layer 9 being substantially uniform; this thickness can be varied by adjusting the plate 5 towards or away from the surface 32. It is found that slurry of creamy consistency does not tend to flow provided that the belt is maintained horizontal as it passes onwards, so there is no need to provide any means for preventing the slurry from flowing laterally off the bels after it leaves the bed plate 2. If there is any unevenness in the surface of the slurry layer it may be eliminated at this stage by passing the belt over a vibrator 10 which vibrates at a very high speed in contact with the belt. As a rule this vibration is not required.

The evaporation of the liquid from the slurry may be conveniently effected by passing the moving belt carrying the wet slurry layer through a drying chamber in which the slurry is exposed to radiant heat, or over a hot plate, or in any other suitable way, provided that the vaporisation takes place sufficiently slowly and uniformly to avoid disintegration of the spread layer. In the apparatus illustrated, the belt travels through a drier 11 which contains a row of electric heating elements 12 mounted beneath a parabolic reflector 13. At the far end of this drier there is a fan 14 by which the vapours driven out of the wet slurry are removed from the drier. The duration of the drying depends on the speed of the belt and the length of the drier and these must be coordinated with the temperature and the nature of the solvent. For example, the drier may be 30 feet long and the belt may travel at 15 feet a minute, thus taking two minutes to pass through the drier, the temperature being maintained at 5fry. With layers up to 20 mils thick the drying time is in general in the order of one half to five minutes. If desired, the drying may be done at room temperature in which case a period of ten minutes or more is required. The maximum temperature employed during the drying must be such that there is no disintegration of the slurry layer, and in particular the temperature must not reach the boiling point of the volatile constituents. Provided that disruption of the layer is avoided, it is advantageous to dry the layer as quickly as possible, since in this way a higher belt speed can be used or the length of the drier may be reduced.

The dried slurry layer shrinks during the sintering and control of the shrinkage is of great importance in the production of a product of precise predetermined thickness and porosity. When a thin layer of dried slurry is sintered there is always a much more pronounced tendency for shrinkage to occur through the thickness than longitudinally or transversely. It is quite unusual, for example, for a shrinkage of more than l/4 ) to occur in a width of 2X', while the thickness may decrease in a considerably greater ratio. If, however, the dry slurry is stuck to the belt, longitudinal and transverse shrinkage can be prevented and such shrinkage as does occur is only in the thickness. For this reason an oxide film on a belt made from a chromium-containing alloy may with advantage be rendered discontinuous, for example by rubbing with emery cloth, since this promotes slight adhesion between the dried powder and the belt and thus prevents irregular shrinkage and cracking during sintering.

By incorporating a small amount of a plastic material in the slurry sufficient cohesion is imparted to the dried slurry layer to give cohesion until sintering sets in or to allow the layer to be rolled before the sintering. In this rolling operation the thickness of the layer is decreased and the amount of further shrinkage on sintering necessary for the desired porosity to be acquired is correspondingly reduced.

Various other steps play a part in the control of shrinkage, e.g. mixing different grades of powder in the slurry, varying the viscosity of the slurry, causing some consolidation of the wet slurry by vibration and so on. None of these steps, however, has an importance comparable with adhesion to the belt and pre-sintering rolling in the control of shrinkage.

In the apparatus illustrated, provision is made for lightly rolling the dried slurry, the belt being passed between calendering rollers 15.

The belt next passes through a tube 16 contained in an electric furnace 17 and here the powder is sintered. The temperature of the sintering furnace is adjusted to suit the nature of the metallic powder under treatment and also the speed of travel of the belt or other moving surface. In general when nickel powder is a major constituent of the dried slurry, the temperature may vary from 650 to 1,1000C. It will be appreciated that the rate of heating, temperatures and soon will depend on the type of powder being treated and on the porosity and mechanical properties desired in the article. It is also important that the sintered material should be rendered strong enough to handle but should not adhere so firmly to the belt as to be difficult to remove from it. The atmosphere in the sintering furnace must be such that the oxygen partial pressure is below the dissociation pressure at the furnace temperature of the oxides of the constituents being sintered. Pure hydrogen and cracked ammonia are suitable for nickel. Any plastic materials or volatile spacing materials are removed in the furnace.

The belt may take two minutes to pass through the furnace, the temperature inside the furnace being maintained at 1,000"C.

Immediately the belt leaves the furnace it passes through a cooler 25 in which it is cooled substantially to room temperature, and it is then coiled on a holder 27.

It will be appreciated that in order to allow a uniform slurry layer to be formed, the support to which the slurry is applied must be accurately made and present a surface over which the slurry will spread easily. When a metal belt such as that illustrated in Figure 1 is subjected to high tempertures during the sintering it tends to become distorted, particularly if it is more than, say, 1 inch wide, and thus to become unsuitable for further use. When there is a risk of distortion, we prefer to use two belts, one on which the slurry is spread and dried and another on which the slurry is sintered, the dried slurry being rendered sufficiently coherent to be transferred from the first belt to the second. The first belt need not be made of metal, but may, for example, be made of paper or cloth, provided of course that the material used is one into which the metal powder and liquid do not penetrate unduly.

An apparatus embodying two belts is illustrated diagrammatically in Figure 4 of the accompanying drawings.

In this apparatus a paper belt 18 is uncoiled from a rotary holder 19 and passes over a bed plate 2 similar to that of Figure 1. Slurry is applied to the belt in the same way as in Figure 1 and the sluny layer is similarly dried. The rolling by the rollers 15 is particularly useful, as it facilitates the next step, in which the consolidated slurry layer is stripped from the paper belt, and transferred to the sintering belt. At this stage the dried layer should be sufficiently coherent to permit stripping handling or rolling without the layer breaking up. The inclusion of a plastic material in the slurry is therefore more important than when only one belt is used.

The gap between the two belts is preferably short, say six inches, and a fixed plate or series of rollers of small diameter may be provided over which the spread and dried layer stripped from the first belt may pass on to the second belt. In the apparatus illustrated the stripping is effected by means of a fixed plate 20, the dried slurry layer passing over the upper surface of the plate and the paper belt passing below the plate and being wound up on a holder 21. The next step is to pick the dried slurry layer up on a second belt 22. This has to pass through the sintering zone so it must be made of heatresistant material, but on the other hand it need not have the smoothness of surface required by a belt on which the wet slurry is spread. This belt 22 passes round rollers 23 and 24, the roller 23 lying close to the edge of the plate 20. The layer should adhere to the belt 22 if a pronounced tendency to shrink longitudinally is to be overcome during the sintering operation, and to press the layer firmly into contact with the belt a jet of compressed air, which may be heated, is delivered downwards through a fish-tail nozzle 28. Alternatively, a light roller may be employed at this point to press the layer into contact with the second belt. An adhesive consisting, for example, of a thin film of the slurry mixture, may be applied to the belt instead of or in addition to the application of mechanical pressure.

The belt 22 must, of course, run at the same speed as the belt 18, and it passes through a sintering furnace 17 in the same way as the belt 3 shown in Figure 1.

The further treatment (if any) of the sintered products depends upon the properties required and usually involves mechanical consolidation.

We have found, however, that porous articles, e.g. membranes, can be produced without mechanical consolidation after the sintering operation. Thus, even if the material is not subjected to mechanical consolidation either before or after the sintering, a coherent product with good mechanical properties can be made by the use of a high sintering temperature with a long period of sintering, e.g. by sintering nickel powder at 1,000 C for 30 minutes.

The mechanical consolidation treatment may consist of one or more passes through a rolling mill. In considering this part of the process it must be remembered that when a porous strip, whether sintered or unsintered, is rolled the strip does not become longer as would ordinary metallic strip, but rather it is a matter of squeezing the air out of the strip.

However, with increasing reductions by rolling a stage is reached when some appreciable elongation begins to develop, and this occurs before the last of the air is squeezed out of the strip. With a slurry containing a plastic and the use of two belts, considerable reduction in the thickness of the dried slurry should be effected before the sintering, say by reducing a dried layer 8 mils thick to 6, 5 or possibly even 4 mils.

It is not permissible, however, when rolling the slurry on a paper belt to cause appreciable elongation. The permissible reduction without elongation is a function of the packing density of the dry slurry. After being sintered such a strip has some residual porosity but is now so highly ductile that further heavy reductions causing elongation are permissible. This subsequent consolidation is preferably effected in a separate mill and it may be done in one stage. Alternatively, it is sometimes advantageous to consolidate the material in a series of stages with or without intermediate heat treatment. The process enables complete control over the porosity to be exercised, in the first place by varying the severity of the mechanical consolidation. The porosity can also be controlled by subjecting the material to one or more additional sintering treatments after consolidation. This, if low porosity is required, heavy reductions up to, for example, 80% may be employed or one or more sintering heat treatments may be applied after reduction.

Such heat treatments serve not only to reduce the porosity but also to improve the temperature employed and the longer the time of heat treatment, the lower is the porosity and the higher the ductility of the article; in general the higher the ductility of the treated article. If a highly porous material is desired on the other hand, the reduction, if any, and the subsequent heat-treatment are sufficient only to impart the necessary mechanical properties to the article.

As an example, in the production of a porous strip the dried slurry may be reduced in thickness from 8 mils to 7 mils or 6 mils after the drying or as it passes from one belt to another and there may be no mechanical consolidation after the sintering. It is found that a rolling operation prior to the sintering accelerates the sintering operation. Thus if a strip 8 mils thick when dried would normally sinter down to, say, 6 mils thickness, a pre-sintering reduction from 8 mils to 7 mils may make the thickness of the sintered strip more nearly 5 than 6 mils.

By means of the invention it is possible to produce articles having parts differing in their physical characteristics. This may be done by locally consolidating the sintered product at one or both surfaces or by building up the articles in two or more layers. For instance a membrane-like surface may be formed on a more porous backing by first effecting some consolidation in the sintered product by cold rolling and the surface layer may be subsequently further consolidated by localised deformation such as that caused by shotblasting or the like. A laminated product may be formed by applying the slurry to the surface of a sintered strip produced in a previous operation instead of directly to the surface of a belt. This pre-sintered strip may be rolled if necessary to make its surface smooth, and it may be used in substitution for the first belt in a two-belt process or may itself be supported by a belt. In either case it is uncoiled from a holder and travels through the apparatus with the slurry. On passing through the sintering furnace the dried slurry, in sintering, will become united to the strip beneath it.

Alternatively, in a two-belt process a presintered strip may be introduced beneath the dried slurry layer at the point of transfer from the one belt to the other.

Another method of forming a laminated product is to feed slurry onto the dried layer, pass the double layer through a second drier and then sinter the double layer. This method may be used with slurries of different compositions.

As one example of a slurry suitable for use in a two-belt process, a suspension medium may be made by mixing 60 grams of methyl methacrylate with 15 grams diethylphthalate and making this up to 1 litre with a solvent in the form of benzene. The nickel powder is then added, the amount varying with the particle size of the powder. When this is 4.5cm the ratio of nickel powder to liquid may be about 2.5 to 1, e.g. 2,400 grams of nickel are added to 1,000 cc. of suspension medium.

In making up such a slurry containing a plastic it is desirable to avoid concentration of the plastic at the surface from which the more volatile constituents evaporate. For this purpose we prefer to formulate the mixture so that the plastic is precipitated in a finely divided form or as films round the metal particles. This end may be achieved by mixing the metal powder with a plastic dissolved in a minimum quantity of benzene and then, just before the spreading operation, introducing alcohol with thorough stirring so that much of the plastic is precipitated in a finely divided form.

When a slurry of this kind is treated in an apparatus of the kind illustrated in Figure 1 or Figure 4, dense non-porous strip may be formed by reducing the thickness of the dried layer by from 60 to 70% at the rollers 15, sintering for two minutes at 1 ,0000C, rolling the sintered strip to reduce it in thickness by from 25 to 30% and again sintering it for two minutes at 1,000 C. To produce porous strip from the same slurry, the rolling after the drying may be omitted and after the sintering the porosity is from 40 to 60%. The strip is, however, fragile, and to make it capable of being handled and at the same time to reduce its porosity it is rolled to about 50% reduction in thickness, the resulting porosity being 10 to 20%.

If only a single belt is used, a smaller quantity of plastic may be incorporated in the slurry or the plastic omitted altogether. For example, the slurry may have the following composition: nickel powder 1,000 grams ethyl alcohol 300 ccs.

benzene 50ccs.

methyl methacrylate 5 grams di-ethyl-phthalate 3 ccs.

Again the slurry may be made by mixing 2,000 grams of nickel powder (4.5cm) with 1,000 cc. of commercial methylated spirits.

Although the invention is particularly applicable to nickel powder, which is generally employed in particle sizes of from 1 to 10 microns, many other metal powders may be used and their particle size may be from 1 to 50 microns. Examples of other metals are iron, copper and the precious metals (including gold and silver) and mixed powders, e.g. nickel-iron and nickel-copper. Metals which by themselves are not particularly suitable, either having too high a melting point (as in the case of molybdenum and tungsten) or being too easily oxidisable (as in the case of chromium, titanium and zirconium), may be mixed with other metals. Moreover, non-metallic elements, e.g.

graphite, may be included in the slurry.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that WHAT WE CLAIM IS: 1. A method of producing a sintered metal product in which metal powder is formed into a slurry with a liquid, the slurry is formed into a uniform layer on a support, the liquid is removed by vaporisation to leave a dried layer, and this dried layer is sintered.

2. A method according to Claim 1 in which the support is a travelling belt which carries the layer through drying and sintering zones.

3. A method according to Claim 1 in which the support is a travelling belt which carries the layer through a drying zone, and thereafter the dried layer is transferred to a second belt by which it is carried through a sintering zone.

4. A method according to Claim 3 in which the first belt consists of paper, cloth or the like.

5. A method of producing metal strip in which metal powder suspended in a slurry with a liquid is formed into a uniform layer on a travelling support, the support carries the layer through a drying zone in which the liquid is evaporated to leave a dried layer and this dried layer is sintered.

6. A method according to Claim 5 which the dried layer is transferred to a second travelling support by which it is carried through a sintering furnace.

7. A method according to Claim 3 or Claim 6 in which the dried layer is positively pressed into contact with the second belt.

8. A method according to Claim 2 or Claim 5 in which the support on which the sintering is carried out consists of chromium-containing alloy with an oxidised surface film renderred discontinuous to control the adhesion of the layer to the support.

9. A method according to any of the preceding claims in which the uniform layer is formed by causing the support carrying the slurry to travel under a knife edge or inverted weir while travelling upwards over a curved surface.

10. A method according to any of the preceding claims in which the liquid in which the metal is suspended is ethyl alcohol, benzene or a like volatile solvent and the slurry has a creamy-like consistency such that it will adhere to a glass rod as a smooth layer.

11. A method according to any of the preceding claims in which a plastic material is incorporated in the slurry for the purpose of increasing the cohesion of the dried slurry.

12. A method according to any of the preceding claims in which the dried slurry is rolled before the sintering.

13. A method according to any of the preceding claims in which the sintered product is mechanically consolidated.

14. A method according to Claim 13 in which an article having parts varying in physical characteristics is produced by local consolidation of the sintered product at one or both surfaces.

15. A method axcording to any of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. of nickel powder to liquid may be about 2.5 to 1, e.g. 2,400 grams of nickel are added to 1,000 cc. of suspension medium. In making up such a slurry containing a plastic it is desirable to avoid concentration of the plastic at the surface from which the more volatile constituents evaporate. For this purpose we prefer to formulate the mixture so that the plastic is precipitated in a finely divided form or as films round the metal particles. This end may be achieved by mixing the metal powder with a plastic dissolved in a minimum quantity of benzene and then, just before the spreading operation, introducing alcohol with thorough stirring so that much of the plastic is precipitated in a finely divided form. When a slurry of this kind is treated in an apparatus of the kind illustrated in Figure 1 or Figure 4, dense non-porous strip may be formed by reducing the thickness of the dried layer by from 60 to 70% at the rollers 15, sintering for two minutes at 1 ,0000C, rolling the sintered strip to reduce it in thickness by from 25 to 30% and again sintering it for two minutes at 1,000 C. To produce porous strip from the same slurry, the rolling after the drying may be omitted and after the sintering the porosity is from 40 to 60%. The strip is, however, fragile, and to make it capable of being handled and at the same time to reduce its porosity it is rolled to about 50% reduction in thickness, the resulting porosity being 10 to 20%. If only a single belt is used, a smaller quantity of plastic may be incorporated in the slurry or the plastic omitted altogether. For example, the slurry may have the following composition: nickel powder 1,000 grams ethyl alcohol 300 ccs. benzene 50ccs. methyl methacrylate 5 grams di-ethyl-phthalate 3 ccs. Again the slurry may be made by mixing 2,000 grams of nickel powder (4.5cm) with 1,000 cc. of commercial methylated spirits. Although the invention is particularly applicable to nickel powder, which is generally employed in particle sizes of from 1 to 10 microns, many other metal powders may be used and their particle size may be from 1 to 50 microns. Examples of other metals are iron, copper and the precious metals (including gold and silver) and mixed powders, e.g. nickel-iron and nickel-copper. Metals which by themselves are not particularly suitable, either having too high a melting point (as in the case of molybdenum and tungsten) or being too easily oxidisable (as in the case of chromium, titanium and zirconium), may be mixed with other metals. Moreover, non-metallic elements, e.g. graphite, may be included in the slurry. Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that WHAT WE CLAIM IS:

1. A method of producing a sintered metal product in which metal powder is formed into a slurry with a liquid, the slurry is formed into a uniform layer on a support, the liquid is removed by vaporisation to leave a dried layer, and this dried layer is sintered.

15. A method axcording to any of the

preceding claims in which the article is built up in two or more layers by applying slurry either to a pre-sintered strip or to a dried slurry layer.

16. A method according to any of the preceding claims in which nickel powder is the predominating constituent of the sluny.

17. A sintered metal article made by a method according to any of the preceding claims.

18. A membrane having very fine pores and made by a method according to any of claims 1 to 17.