PRODUCTION OF FLAT PRODUCTS FROM PARTICULATE MATERIAL

Abstract: A process for producing flat products from gas atomised particulate material comprises the steps of forming a relatively smooth castable slurry comprising a suspension of such particulate material in a solution of a film-forming binder material, depositing a coating of the slurry onto a substrate or support surface and drying the coating to form a flexible flat product. The dried flat product may be bonded onto the substrate or support surface by the drying process or, alternatively, may be removed therefrom and roll-­ bonded to a suitable substrate for subsequent compaction and sintering.

Description: This invention relates to a process for producing flat products from particulate material. By the term "flat products" as used herein is meant products in strip, sheet or like form or products produced therefrom which have retained a generally flat appearance. A process for the production of strip from metal powder is known from EP-A-0162555 and EP-A-0176200 in which a suspension of powdered metal in a solution of a film-forming binder material in water is coated in the form of a slurry onto a support surface, dried and removed from the support surface as a thin, flexible strip. This strip is subsequently compacted within a rolling mill and sintered to produce the final strip product. It is also known from AU-B-409355 to apply a coating of slurry of metal powder produced in any suitable manner as by electrolytic precipitation, by air atomization, by grinding or by purchasing it on the market and a binder onto the surface of a steel sheet to clad the same. The slurry is dried and the sheet rolled and sintered to produce a steel sheet clad with the metal, e.g. copper. However, no particular process is disclosed which employs as a starting material spherical gas atomized powders. Hitherto, process operators have favoured the use of powder consisting, essentially, of irregular shaped particles as are produced, for example, by water atomisation techniques. It has been established that irregular shaped particles bind together more effectively than do spherical particles thereby producing relatively higher green strengths in the compacted strip. In addition the increased surface area of irregular particles provides greater particle contact area after compaction thereby increasing the surface area over which diffusion processes can occur during subsequent sintering resulting in greater strength for the sintered strip. In the alternative gas atomisation process, the cooling rate of the molten droplets produced during atomisation is sufficiently slow for the surface tension forces to spheroidise the particles before solidification. Where materials having relatively low freezing points are required, e.g. braze materials, this effect is exaggerated. Gas-atomised powders are generally more widely available than water atomised powders and also tend to contain less impurity since they are conventionally atomised using pure inert gases such as argon. Water atomised powders are more likely to be oxidised or otherwise contaminated by dissociation products of water, or any dissolved impurities the water may contain. There are, therefore, advantages which would accrue from the use of gas-atomised powders for the production of certain strip products where the absence of impurities is important, e.g. strips for use in brazing applications if problems associated with compaction and sintering of strip produced from gas-atomised powders can be overcome. One particular problem which does occur during the roll compaction process arises as a consequence of the fact that spherical powder particles produce a strip in which the particle content tends to "flow" producing large extensions with relatively little particle interaction. Hence the green strength of the compacted strip and surface area contact of the particulate content of the strip are both low resulting in a strip having inadequate physical properties following first compaction and first sintering. The present invention sets out to provide a process in which flat products can be produced from a slurry containing spherical gas atomised powders. According to the present invention, there is provided a process for producing flat products from a start material comprising particulate material in which a relatively smooth slurry comprising a suspension of particulate material in a solution of a film-forming binder material in water is cast onto a substrate and the cast slurry coating is dried; the slurry used in the process comprising a suspension of gas atomised particulate material in a solution of a film-forming binder material in water; and the process including the steps of roll binding the dried coating to the substrate; sintering the roll bonded product; and subsequently removing the substrate from the roll-bonded sintered product. The substrate may be removed by, for example, a chemical pickling or electro-chemical process. The flat product produced by the process may comprise braze material. Examples of substrate material include pure iron strip, nickel and nickel alloy strip. The invention will now be described by way of example only with reference to production of a bimetal, i.e. flat product without removal of the substrate. EXAMPLE 1 A pre-alloyed gas-atomised nickel-based powder of composition by weight 22.5% manganese, 7% silicon, 5% copper, balance nickel and particle size within the range 110-45 microns (140 to 325 mesh (BS 410)) was made into a smooth, castable slurry using a 0.215% solution of high molecular weight cellulose, to achieve the required viscosity and denseness to prevent the powder particles settling out. The slurry was cast as a layer of approximately 0.4mm thickness on a nickel strip substrate, and dried. After drying, a satisfactory bond was present between the cast slurry layer and the nickel substrate. The coated substrate was then subjected to compaction in a rolling mill to cause the powder content of the dried slurry layer to become at least partially embedded into the surface of the substrate. The roll-compacted product was subsequently sintered at temperatures of between 900°C and 1000°C. If required, the resulting flat product could readily have been subjected to further cold rolling and heat treatments. EXAMPLE 2 A pre-alloyed gas-atomised nickel alloy powder containing by weight 2% boron and 3.5% silicon, balance nickel, of particle size 140 mesh (110 microns), containing 14.5% of 325 mesh (45 microns) was made into a slurry identified in Example 1 above, and cast onto a nickel substrate. Mesh sizes referred to herein are British Mesh Standard BS 410. It will be noted that the powder used in this Example contained a higher proportion of fines than did the powder used in Example 1. The substrate coated with the cast slurry layer was compacted and a reasonable physical bond achieved. Sintering of the compacted material at a temperature of 1040°C produced a strip in which the bond between the substrate and cast strip was satisfactory. A further compaction produced no evidence of cracking, and the integrity of the material appeared reasonable after a subsequent sinter at 1050°C. EXAMPLE 3 A different substrate was then tried, namely 0.003" (0.08mm) finished iron strip. A pre-alloyed gas-atomised nickel powder containing by weight 13% Cr, 2.8% B, 4% Si, 4% Fe balance nickel of particle size less than 45 microns was made into a slurry using regular cellulose binder at a concentration of 0.7%. A separate slurry of pure iron was produced using a cellulose binder previously found to produce a rough surface finish after sintering. One example of such cellulose binder is methyl hydroxyethyl cellulose. Samples were cast to an optimum gauge of 0.35mm, followed by rolling and sintering. The flexible strip was then satisfactorily rollbonded to the sintered iron substrate and subsequent sintering at various temperatures yielded an optimum temperature of 1000°C. Two further compaction and sintering stages were carried out, producing a good quality bimetal, with no signs of delamination or surface cracking. From the foregoing Examples, it is apparent that by careful selection of the particle size of the powder and, the physical properties of the substrate (e.g. relative softness, denseness etc), compaction pressures and sintering temperatures, flat products can successfully be produced from gas atomised particulate material.

Claims: A process for producing flat products from a start material comprising particulate material in which a relatively smooth slurry comprising a suspension of particulate material in a solution of a film-forming binder material in water is cast onto a substrate and the cast slurry coating is dried; the slurry used in the process comprising a suspension of gas atomised particulate material in a solution of a film-forming binder material in water; and the process including the steps of roll binding the dried coating to the substrate; sintering the roll bonded product; and subsequently removing the substrate from the roll-bonded sintered product. A process as claimed in Claim 1 characterised in that the substrate is subsequently removed by a chemical pickling or electro-chemical process. A process as claimed in Claim 1 or Claim 2 characterised in that the flat product produced by the process comprises braze material. A process as claimed in any one of the preceding claims characterised in that the substrate material comprises pure iron strip, nickel or nickel alloy strip.

Inventors: BELLIS, JOHN, BROOKS, NIGEL JOHN
Applicants: MIXALLOY LIMITED
Application year: 1987
Main classification: B22F3
IPC codes: B22F3, B22F7, B22F5