US3294530A - Flash sintering - Google Patents

Description

3,294,530 FLASH SINTERING Samuel Storchheim, ForestHills, N.Y assignor to Alloys Research & -Manufacturing Corporation, :Woo'd- .side,N.Y., a corporation of Delaware No Drawing. Filed Jan. 22, .1965, Ser. No. 427,507

8-Claims. (Cl. 75-214) This appliation is a continuation-in-part of application Serial No. 288,607, filed June 13,1963, and now abandoned.

This invention relates generally to powder metallurgy, and more particularly to a novel method of consolidating and bonding metal particles to form-coherent bodies.

Powder fabricated metal parts aregenerally prepared by cold-pressing loose powders to about 50 or more percent of theoretical density and then sintering the resultant green :compactinafurnace for periods of time varying fromilO .minutes to'several .hours.

Heat is transferred to the-compacts in such furnaces by conduction, convection or radiation. When furnace sintering-is used for oxidation-sensitive materials such as alurninum,.an extremely dry protective atmosphere is ordinarily used in order to prevent oxide contamination of the compacts.

The temperature employed in sintering is determined by the metals under treatment and the characteristics desired in the finished piece. In the case of elemental metals and in many mixtures of metal particles, no part of the material is melted during heat treatment. In some instances, however, where a small amount of a low meltingpoint metal is used in conjunction with a high melting-point metal, as with hard carbide compositions, the lower melting-point metal may be fused during sintering. While no fixed rule can be laid down, in general temperatures disclosed in the literature for sintering operations for prealloyed powders is about two-thirds of the melting point of.

the alloy.

The main purpose of the present invention is to provide a low-cost powder metallurgical flash heating process which acts to consolidate and bond metal particulates with little or no bonding pressure, and within a relatively brief period.

Briefly stated, this object is accomplished by momentarily heating the particulates at a temperature above their melting point, with or without pressure, the particulates having oxide coatings thereon which prevent flow thereof. The total time at which the particulates are maintained above the melting point, for flash heating, will not exceed about one minute and will generally be between 5 and 6-0 seconds. By particulates is meant metal particles above or below maximum powder size (1000 microns) and of any known form, such as shot, needles, etc.

The invention is limited to those metal particulates, such as aluminum, magnesium, titanium, beryllium, zinc, and lead, in which a continuous oxide film is readily formed to enclose completely the individual particles. The oxide layer has a higher melting point than the molten metal contained therein, and acts to prevent the molten metal from flowing and forming the typical cast structure. No segregation of alloying elements takes place, for they remain uniformly dispersed. Moreover, there is no formation of the dendritic characteristic of cast materials, and there is no need, therefore, to soak or heat the material for prolonged periods to break up the dendritic structure before subsequent hot or cold working. Additional metal particulates, in an amount up to about 10% by weight, can be added to the primary metals. Such additional metal particulates are those metals known to form alloys with the primary metals. For example, in the case of aluminum as the primary metals, additional metals such as copper, magnesium, and silicon could 'be employed. In the case of such alloy systems, only the United States Patent 0 3,294,530 Patented Dec. 27, 1966 temperature. of the primarymetal, such as aluminum, need be exceeded during flash heating.

There are three basic situations which lend themselves to flash heating. First, there is the formation of parts from particulates, in pure or unalloyed form, such as elemental aluminum. Second, there is the formation of parts from ,prealloyed particulates, such as an aluminumcopper alloy in powderform. Third, there is the fabrication ofparts from a blend of different particles, such as aluminumand coppfirpowders, the meltingpoint of pure aluminum being 660 C. andth at of copper, 1083 C. In all instances,.wh en subjecting particulates to flash melting, an expanded ,molten constituentis formed which acts ,to craze the less expanded oxide casing without, however, destroying its integrity, whereby diffusion of the molten metal takes place through the oxide boundary to elfect interbonding of the particles.

EXAMPLE :1

In flash-melting a blend of aluminum and copper powder, where the relative copper content by weight is about 6%, the mixture is heated above the liquidus temperature of the aluminum but under the solidus temperature of the copper, to effect bonding.

EXAMPLE III In flash-melting an aluminum-coper alloy in which the relative copper content by weight is about 6%, the powdered alloy is subjected to a temperature above its melting point to effect bonding in less than a minute, as against a sintering period of about two hours at a temperature between solidus and liquidus.

In flash sintering, it is vital that the powders, whether in elemental or alloy form, have a relatively strong oxide film on their surface, bonding being effected in the molten state by diffusion through this film, which has a much higher melting point, without any rupture thereof which would cause flowing. If the oxide film that normally forms is too thin to effectively encase the molten metal for purposes of flash-melting, the particulates can be deliberately oxidized, as by exposure to steam, or by chemical treatment. Such oxidation also advantageously changes certain physical properties of the resultant bonded body, such as corrosion resitance, or thermal and electrical conductivity. This occurs by reason of the rnulti-cellular oxide matrix formed within the finished body.

The flash-melting process is applicable to the making of finished shapes as well as to continuous shapes, such as bar and strip. Subsequent to flash-sintering the coherent body may be worked, as by forging or rolling, to improve or alter its properties.

Because of the reduced bonding time, the flash-melting process effects substantial production economies. Moreover, eliminating the need for cold-pressing loose particulates or other compacting steps, further simplifies the manufacturing process. In some instances, the rapidity of the flash-melting process does away with the need for protective or reducing atmospheres. With flash-melting, the final density of the part can be varied from the powder density of the starting material to the full density of cast wrought material.

While there has been described a preferred method of consolidating and bonding metal particulates in accordance with the invention, it Will be appreciated that many changes and modifications may be made therein without, however, departing from the essential spirit of the invention as defined in the annexed claims.

Having thus described the invention, that which is desired to be claimed is:

1. In a method of forming articles comprising forming a metallic powder mixture containing from 90% to 100% by weight of a primary metal selected from the group consisting of aluminum, magnesium, titanium, beryllium, zinc, and lead, shaping the mixture and sintering, the improvement which substantially reduces the pressure required for shaping, provides an oxide coating over at least a portion of the primary metal particles and reduces the time required for sintering comprising heating the product at a temperature above the melting point of the primary metal and less than the melting point of the primary metal oxide to efiect melting thereof and maintaining the temperature for a period Within the range from 5 to 60 seconds, sufiicient to complete sintering.

2. A method as in claim 1 wherein the primary metal is aluminum.

3. A method as in claim 1 wherein the primary metal is magnesium.

4. A method as in claim 1 wherein the primary metal is titanium.

5. A method as in claim 1 wherein the primary metal is beryllium.

6. A method of preparing aluminous products comprising shaping a powder mixture containing at least 90% by weight of particulate aluminum and sintering the shaped mixture by heating it at a temperature in excess of 660 C, and not in excess of the melting point of aluminum oxide for a period of from 5 to 60 seconds, whereby upon cooling, a sintered object is obtained.

7. A method as in claim 6 wherein the metallic components of the powder mixture contain from 1% to 8% by weight of copper and corresponding from 99% to 92% by weight of aluminum.

8. A method as in claim 6 wherein shaping is carried out at an applied pressure of less than 10 p.s.i.

References Cited by the Examiner UNITED STATES PATENTS 2,796,660 6/1957 Inmann 75-2l4 2,894,838 7/1959 Gregory 752l2 3,066,391 12/1962 Vordahl 752l2 L. DEWAYNE RUTLEDGE, Primary Examiner.

R. L. GRUDZIECKI, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3,294,530 December 27, 1966 Samuel Storchheim certified that error appears in the above numbered pat- It is hereby t the said Letters Patent should read as ent requiring correction and the corrected below.

Column 2, line 40, for "coper" read copper line 57, for "resitance" read resistance column 4, line 14, for "corresponding" read correspondingly Signed and sealed this 12th day of September 1967.

( L) Attest:

ERNEST W. SWIDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents

Claims (1)

1. IN A METHOD OF FORMING ARTICLES COMPRISING FORMING A METALLIC POWDER MIXTURE CONTAINING FROM 90% TO 100% BY WEIGHT OF A PRIMARY METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, MAGNESIUM, TITANIUM, BERYLLIUM, ZINC, AND LEAD, SHAPING THHE MIXTURE AND SINTERING, THE IMPROVEMENT WHICH SUBSTANTIALLY REDUCED THE PRESSURE REQUIRED FOR SHAPING, PROVIDES AND OXIDE COATING OVER AT LEAST A PORTION OF THE PAIMARY METAL PRITICLES AND REDUCING THE TIME REQUIRED FOR SINTERING COMPRISING HEATING THE PRODUCT AT A TEMPERATURE ABOVE THE MELTING POINT OF THE PRIMARY METAL AND LESS THAN THE MELTING POINT OF THE PRIMARY METAL OXIDE TO EFFECT MELTING THEREOF AND MAINTAINING THE TEMPERATURE FOR A PERIOD WITHIN THE RANGE FROM 5 TO 60 SECONDS, SUFFICIENT TO COMPLETE SINTERING.