US2918368A

US2918368A - Production of powder-metallurgy articles

Info

Publication number: US2918368A
Application number: US585060A
Authority: US
Inventors: Bishop Edward
Original assignee: Birmingham Small Arms Co Ltd
Current assignee: Birmingham Small Arms Co Ltd
Priority date: 1955-05-31
Filing date: 1956-05-15
Publication date: 1959-12-22
Anticipated expiration: 1976-12-22

Description

Dec. 22, 1959 s 0 2,918,368

PRODUCTION OF POWDER-METALLURGY ARTICLES Filed May 15, 1956 A TTORNEY;

7 PRODUCTION OF POWDER-METALLURGY ARTICLES.

Edward Bishop, Sheffield, England, assignor to The Bin mingham Small Arms Company Limited, Birmingham, England, a British company Application May 15, 1956, Serial No. 585,050

Claims priority, application Great Britain May 31, 1955 16 Claims. (Cl. 75-222) This invention relates to-the production of powder metallurgy articles with ducts, for example as used for the supply of liquid or gaseous coolant to articles, such for instance, as gas-turbine blades, that are subjected to very high temperatures.

The blades of gas turbines must operate at temperatures that tend to early destruction of the blades, even when the blades are made of stainless steel or other heatresistant alloys. The formation of the blades formed by powder metallurgy methods enables duct positions to be established as the powder is assembled for compacting and permits ducts to be formed that could not be made in the hard blade material by other methods. Such ducts may be simply to provide a circulation of coolant, but advantage may also be taken of the ability to control the porosity of powder metallurgy articles to allow coolant supplied internally to exude from-the pores and effect'what is known as sweat cooling. The cooling may be directed to any part of the'surface that is par ticularly vulnerable to the action of the hot gases by appropriately locating an internal duct in the neighbourhood of such part.

However, whether used for mere circulation or for sweat cooling, any duct is generally of very fine bore, both 7 the assembly of the powder to be'compacted an element of the required section and shape, of a material that disperses or disappears on subsequent heat treatment, e.'g. on sintering the compact. wires'or other elements vaporise,-and leave voids to form the desired ducts. Again polystyrene is converted to the gaseousmonomer, and waxes volatilise. 'But the fine section of elements of such materials militates against theirready' and accurate location during the assembly of the powder and also against maintenance of their location during the heavy reduction in bulk during compacting of the powder.

The object of the invention is to provide for the assembly of the powder together with its duct-forming elements in a manner less consuming of time and labour than the existing method of inserting the elements instages of the assembly. 7

According to the present invention, duct-forming elements for a ducted powder metallurgy article are supported by the walls of a box so as to extend in predetermined positions in the space enclosed by the box, powder is packed in the box to cause surrounding of the elements by thepowder, a freezable liquid is introduced.

to occupy the voids between the powder grains, the whole is subjected to freezing to solidify the assembly of pow-' der, elements, and liquid, the solid assembly is separated from the box and confined in. a die, the solidified For instance, cadmium F 2,918,368 Patented Dec. 22, 1959 liquid is thawed and removed and the assembly of powder and elements is compacted by pressure applied transversely to the length direction of the elements, and subsequently treated to remove or disperse the duct forming elements.

In general, the elements will be supported by and extended between opposite walls of the box, with locating' and supporting holes in these walls to determine the location of the elements across the box space. Threading of the elementsinto the holes is a simple operation. If

the elements are straight, they may be readily tautened,

and in any event secured (as by soldering or gluing) after threading through the holes; alternatively they may be secured by wedges, or by clamp plates and the like.

The box may be of such shape as to approximate to] the finished shape of the article, to minimise or avoid machining.

Preferably the box is made with removable side-walls to facilitate separation of the solidified contents of the box after freezing. The element supporting walls may likewise be removable from the side walls. The portions of the elements through the holes in the opposite walls must be detached to enable those walls to be separated from the block, and projecting portions of the elements can be trimmed close to the surfaces of the block then exposed.

The'supply of powder to the box is performed gently to avoid disturbance of the duct forming elements, and

the box may be vibrated to assist in packing which avoids. any mechanical operation within the box likely to displace the elements.

The addition of liquid to the block does not disturb space in the die cavity so created, may be packed with metal powder to prevent the block from collapsing when the frozen liquid is removed prior to compacting and/or sintering. Where a block is made of such size that pow-- der must be added at the ends of the block where the 'duct forming elements protrude (as may frequently be the case) the block should be of such dimensions that the added end portions may be removed after theblock is sintered to allow entryto the ducts. I

Alternatively where it is desired to have ducts over part of the length of the finished article only, the block may be arranged so to require machining at one end only,

so as to expose'one end only of the ducts.

The freezable liquid should be one that will not corv rode or react with the powder or the material of the duct forming elements, and the box and die material should be similarly free from attack by the liquid.

Liquids with freezing points in the range 20 C. to

+ C. and corresponding boiling points in the range +20 C. to 300 C. are generally suitable, but liquids with high vapor pressures are preferred. Benzene has been found to be suitable, and also water, naphthalene and paradichlorobenzene.

Theliquid used may be removed from the block when the block is in the die by the application of heat and/or a reduction of pressure so as to vaporise the liquid,

Conveniently a space above the die cavity may be evacuated and the whole block allowed to heat to room temperature, no other heat being applied, if benzene is used.. I 7

The duct forming elements may be copper wires, or may be formed of cadmium, polystyrene or other suitable material.

A method for producingducted powder metallurgy articles in accordance with this invention,,ancl apparatus for performing the said method will now be particularly de-. scribed with reference to the accompanying drawing, wherein the sole figure is a diagrammatic perspective view of a preforming box employed in the performance of the said method. t

I r r to p e a d cted po der. m ta lurgy article of rectangular shape and having longitudinal ducts, a preforming box such as is shownby way of example in the drawing is employed. The preforming box illustrated consists of a base plate 1, together with two side plates 3, the assembly being conveniently held together in known manner by countersunk screws. The box is provided with two end plates 5 each provided with a plurality ofholes 8.

Two end plates 11 forming the ends of. the powder block are provided, being a sliding fit between the side plates 3 of the box. a plurality of holes 9.

The

holes

8 and 9 in the

plates

5 and 11, are arranged to correspond so that a duct-forming element such as a wire may be threaded through all four holes (.one in each plate 5, 11) to lie in a straight line along the length of the box.

Duct forming elements, which conveniently are copper Wires of suitable diameter, e.g. .048 inch, are threaded through the four plates, so that each hole is occupied by a wire.

The ends of the wires on the outer faces .of the. plates 5. are then bent over, and clamp plates 15are secured over the bent ends by means of screws 17 acting'in threaded holes 13. The positions which some of i the Bach end plate 11 is provided with wires occupyare indicated in chain lines in the drawings as at 21.

The plates 11 are then positioned sufficiently. far apart as is required by the block to be formed.

The duct forming elements are then tightened by use of tensioning screws 19 acting through tapped holes 7 against the ends of the box members 1, 3. i

The joints of the cavity formed between the box members 1, 3 and the end plates 11 are then sealed, by painting the joints with paraffin wax, which also serves to retain the end plates in position.

The cavity is then charged with metal powder, e.g. stainless steel powder, andthe box vibrated: topromote even packing. i

placing in dry ice. 5.4 C. but the block may be allowed to cool to a much lower temperature such as .40 C. to permit thc necessary operations to be performed before the benzene melts.

Screws 19 are then removed followed by the-box sides 3. A block of solid benzene and metal powder together with duct forming elements, plates 11, end plates 5, plates 15 and screws 17 remain. The solid block is then sawn through just inside end plates 11, thus removing plates "11, 5 and 15 and screws 17, as two separate assemblies. These can then be dismantled separately. j

Alternatively after the removal of box sides, 3,, the duct forming elements may be sawn through or cut outside the end plates 11, leaving any desired length of wire exposed, and the end plates 11 then pulled off.

The block is then placed in the die cavity, any spaces being filled with metal powder tamped into place, and the benzene is fimoved.

Benzene boils at about 80.4" C. at atmospheric pres sure, but by evacuating the die cavity and thus lowering the pressure, the liquid may melt and then boil at much lower temperatures. Conveniently the cavi ty is evacuated and the block allowed to warm to room tempera? 4 u the evacuation be c nued unt a l the benzene has evaporated otf.

When the benzene is all evaporated, as may be gauged from the fact that the evacuation pump ceases to draw off benzene, the block is ready for compacting.

Compacting is carried out by compressing the block transversely to the duct elements, so that the elements do not become displaced relatively in the block. With the assembly of powder and elements closely confined in its dimension in the direction of the applied pressure, but

the degree is calculable and therefore may be allowed for in the design of the end plates of the preforming box.

The elements themselves may become distorted in cross-section by the pressure applied, but this factor is considered to be discountable. For example it has been found that 0.048 dia. copper wires in stainless steel powder compacted at 30 tons per square inch were ovalised to approximately .040 by .050, so that the crossesectional area was not materially diminished.

t The block after compacting is ready for sintering, the sintering temperature being such that the duct forming elements are melted if not vaporised and diffused into the block, leaving fine bores to mark their places.

After sintering the blocl; is ready for machining or in the case of articles made to finished size, for coining.

Itwill readily be seen that the present invention pro-. vides a method of providinga finished article with a pinralit-y of holes throughout its length such as, would be extremely expensive by conventional means.

I claim:

1. The method of producing ducted powder-metallurgy articles which comprises charging metal powder into a prcforming box having wire duet-forming elements of fusible material supported by the walls of the box so i as to cause surrounding of the wire elements by the metal powder, introducing a freezable liquid to occupy voids between the metal powder grains, subjecting the whole to freezing, separating the solid frozen assembly from the box and confining it in a die, thawing and removing the solidified liquid, compacting the assembly of metal powder and wire elements and subsequently subjecting the assembly to a sintering process at temperatures such as to disperse the wire duct-forming elements.

2. The method of producing ducted powder-metallu'gry articles according to claim 1 wherein the wire duct-forming elements are secured tothe walls of the box by A freezable liquid, e.g. benzene is then poured into the powder in the cavity, and the whole box cooled by.

The benzene solidifies at about glueing.

3. The method of producing ducted powdeometallurgy articles according to claim l wherein the wire ductforming elements are secured to the .Walls of thejjbox by ing elements are secured to the walls of the box by clamping. 1

6. The method of .producing ducted powder-metallurgy articles according to claim 1, wherein the packing of the preforming box with metal is assisted :by vibrating the preforming box.

t 7. The method of producingducted .powdenmetallurgy articles according to claim l wherein metal powder is added to the die cavity to, occupy any space not taken up by the solid thawed.

8. The method of producing ductedpowder-metallurgy articles according to claim 1, wherein the freezable liquid is benzene.

9. In a method of producing a ducted powdered-metalasscmbly; before the frozen liquid is lurgy article, the steps of surrounding a series of small diameter duet-forming elements with metal powder to be sintered, introducing liquid into said powder and freezing the resultant mass, thawing out the mass within a confining die to remove the liquid, compacting the powder, and subsequently sintering the powder at a temperature that disperses said elements to leave fine ducts in their places in the sintered article.

10. In a method of producing a ducted powderedmetallurgy article, the steps of providing a receptacle having a plurality of spaced fusible small diameter elements sized to correspond to the desired ducts and extending in a selected direction through the space within said receptacle, charging sinterable metal powder into said receptacle to surround each of said elements, introducing a freezable liquid that is substantially inert with respect to said powder and said elements into said receptacle to occupy the voids between the metal powder grains, subjecting the mass within the receptacle to reduced temperature until it is all solid, separating the frozen mass from the receptacle and introducing it into a confining die, removing the solidified liquid by thawing, compacting the remaining mass by compression thereof transversely of said direction of the elements, and sintering the compacted mass at sufficiently, high temperature to melt and disperse said elements to leave the desired ducting in the sintered article.

11. In the method defined in claim 10, said metal powder being stainless steel.

12. In the method defined in claim 10, said liquid being benzene.

13. In the method defined in claim 10, the step of vibrating the receptacle during introduction of said metal powder.

14. In the method defined in claim 10, said liquid being vaporized during removal from said mass.

15. In the method defined in claim '14, the step of evacuating the die cavity to remove the evaporated liquid.

16. In the method defined in claim 10, said elements being vaporized during sintering of the compacted mass.

References Cited in the file of this patent UNITED STATES PATENTS 1,708,421 Henry Apr. 9, 1929 1,715,920 Henry June 4, 1929 2,685,507 Blood Aug. 3, 1954 2,751,293 Haller June 19, 1956