US2342037A - Powder metallurgy - Google Patents

Description

aseaoar y rownaa rea pear George D Cremer, P na. Manor, N. Y., assignor to Hardy Metallurgical Company, New. N. Y., a. corporation of Delaware No Drawing. Application December 10, k Serial N0. $22,426

, Claims.

. This invention is concerned with powder met- .allurgy and particularly with the powder metallurgy of aluminum although it may be employed advantageously in the formation of compacts from a great variety of metal powders.

In powder metallurgy, a loose mass of metal powders is compressed in a die or mold.' Thereafter the resulting compact is ejected, usually in a green or unsintered condition, although sintering of the compact may take place during compression or after compression while the compact is still in the die. The pressure employed to form the compact is high, 1. e. 5 to 75 tons per square inch, and this forces the metal powder against the die wall to suchan extentrthat the compact sticks in the die and requires considerable'pressure to eject it. In some instances the compact sticks in the die so tenaciously that the force required for ejection breaks the compact. Thus a compact may be released suddenly from a die with such force that it is shattered into pieces. In less aggravated instances a considerable portion of metal is left as a skin on the die I wall. This destroys the accuracy of configuration of the compact and also requires frequent die cleaning.

iYork,

ed. In other: wordsflthe coating of overlapping -metal flaiges isstrongfin a direction transverse to the major surfaces of the flakes but relatively v weakin-the 'plane of those surfaces.

Flakes of non-metallic substances are, in eral,,insumci ently strong and lack the required ductility, malleability and resiliency. on the other hand, almost any metal that is sufllciently malleable to v be hammered into flakes is admirably suited for the practice of the invention.

In some instances it is desirable to employ metal flakes per se without addition agents. However, especially in the case of aluminum it is desirable to employ metal flakes that have been treated with a suitable organic lubricant or leafing agent, say, stearic acid or aluminum stearate. These agents tend to'hold the flakes in proper orientation withthe die wall in an overlapping condition and alsofaid as lubricants. 20.

" The-metal flakes, with or without the leafing agent may be employedasa dry powder. How- It has been proposed heretofore to lubricate the walls of the die with oil and the like, but this involves contamination of the compact, and may have a deleterious efiect during sinterlng. Moreover, in many instances such lubricants do not prevent the compact from sticking.

As the result of my investigations, I have discoveredthat the above-described sticking difli-' culties (which; are particularly serious with aluminum powders and aluminum alloy powders but which may also occur with other metal powders) may be eliminated by lining the mold or die, preparatory to introductionof the metal powder for the compact, with fine metal flakes which lie flat against the mold surfaces and are at least in part over-lapping. The entire interior of the mold or die may be so lined if desired but, in any case where sticking is likely to occur or ejection pressure may be high, the walls of the mold that extend in the direction in which the compact is pressed (and elected) should be so lined, in which case the majoraxes of the metal flakes extend in the direction of election.

' My invention is based upon the fact that a thin coating of overlapping flakes of metal of reasonably high malleabllity is not broken when the metal powder is compressed in the die, so

,ev'eninimost instancea itis desirable to paint the flakes-on 'thcinsideofj the die. The vehicle employed wittftheeflakes A h H td rorm'me paint preferabl'yxis; suchythat itgdoles notudissolve the leaflng agent thjerefrom. gjlfhus-l aluminum flakes coated with' stefaric acidare conveniently suspended in a volatile vehicle, for-example benzene, which does not dissolvethe 'le'afing agent -in this instance the-stearic acidi However, other vehicles (say, acetone) in which the leaflng agent (say,

stearic acid) is soluble, are also satisfactory. These and other features of my invention will be more thoroughly understoodin the ligh of the following detailed description of my pref ntpact which is sinter'ed after it ejected/Ta suitable lubricant comprises one volume of al flake (such as is used in paint), one yo ume ofinum stearic acid powder and two volumes of benzene or other volatile vehicle in which the s aric acid is substantially insoluble. The stearic acid powder and aluminum flake is thoroughly suspended in the benzene and applied to the die wall by means of an automatic. wiper or spray or, in the case of a laboratory operation, with a brush.

The benzene evaporates almost immediately leaving the wall thoroughly covered with overlapping flakes of aluminum powder together with some stearlc acid. Thereafter the aluminum powder which is to form the compact is introduced into the die and compressed therein under a relatively high pressure, say, 40 tons persquare inch.

The compact is then ejected from the press in the usual fashion, say by moving the lower plunger of the press upwardly with respect to the die.

The presence of the oriented overlapping metal flakes on the die wall'reduces the pressure required for ejection to but a small fraction (say, of that required when customary lubricants such as oil or graphite are employed and completely eliminates the jamming" and "cold sintering which usually characterizes the compression of aluminum or aluminum alloy powders in a mold. Moreover, the compacts do not tend to rupture or develop flaws during ejection and the die walls are maintained in a substantially clean condition over long operating periods.

Numerous aluminum alloys, particularly age hardening aluminum alloys of the 17S and 24-S variety, are not strongly resistant to corrosion. In forming compacts of such alloys by powder metallurgical methods, it is desirable to line the mold with pure aluminum flakes. These flakes form a thin film on the outside of the com pact which is considerably more resistant to corrosion than the compact itself, thereby increasing the corrosion resistance of the resulting finished piece.

Parts compressed in accordance with my invention may be sintered in accordance with the usual practice. Thus, the invention is adapted to hot compression as well as to the more customary ins; compression of metal powders in a die and the subsequent ejection of the resulting compact from the die, the improvement which comprises lining the-die with a thin fllm of metal flakes lying flat against the die wall preparatory to compressing the powders therein, said flakes being of a metal of the group consisting of aluminum, copper and brms.

2. In a powder metallurgical process involving compression of metal powders in a die and the subsequent ejection of the resulting compact from the die, the improvement which comprises lining the die with a thin film of overlapping malleable metal flakes filmed with oxide and lying flat against the die wall preparatory to compressing the powders therein, said flakes being of a metal operation in which sintering takes place ur'ter the compact has been ejected from the die.

In the formation of compacts other than those of aluminum, the metal flake die lining isalso highly desirable. Thus, in one instance a 70-30 atomized brass powder was compressed at tons per square inch in a die lined with oriented aluminum flakes. Thereafter the resulting compact was elected and subjected to the usual sintering procedure, during which the minute film of flake aluminum on the outside of the compact scaled off to leave a smooth clean brass surface.

It will be understood that th application of the invention is not limited to the use of aluminum flake and that flakes of a variety of other metals may be employed. Thus, fine brass and copper flakes may be used either as a suspension in a vehicle or in a dry condition and with or without a leafing agent.

Metal flakes of the size (say minus 300 mesh) customarily employed in paints, lacquers and the like are well adapted to the practice of the invention. These flakes customarily are produced in i a stamping operation and the surfaces thereof usually are coated with a thin film of oxide which in turn is coated with a small proportion, say, 1%% on the weight of the metal, of a leafing agent that tends to cause the flakes to orient themselves with respect to the surface of a liquid fllm in which they are suspended. it is believed of the group consisting of aluminum, copp r and brass.

3. In a powder metallurgical process involving compression of metal powders in a die and the subsequent ejection of the resulting compact from the die, the improvement which comprises lining the walls of the die which extend in the direction in which the compact is pressed and ejected with a thin film of overlapping metal flakes prior to introducing the bulk of the metal powder into the die, said flakes being of a metal oi the group consisting of aluminum, copper and brass.

e. In a powder metallurgical process involving compression. of metal powders in a die and the subsequent ejection of the resulting compact from the die, the improvement which comprises lining the die with a thin film oi overlapping metal flakes coated. with a leafing agent and laid flat against the die wall preparatory to compressin the powders in the die, said flakes being of a metal or the group consisting of aluminum, copper and brass.

5. In a powder metallurgical process involving compression of metal powders in a die and the subsequent election of the resulting compact from the die, the improvement which comprises lining the walls of the die which extend in the direction in which the compact is elected with a thin. film of overlapping metal flakes laid 'fiat against the die wall, prior to introducing the bulk oi. the metal powder into the die, the flakes being coated with a leaflng agent and being introduced into the die as a suspension in a volatile vehicle in which the leaflng agent is substantially insoluble, said flakes being of a metalof the group consisting of aluminum, copper and brass.

GEGRGE l). CREMTER.