Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/0408—Light metal alloys
  • C22C1/0416—Aluminium-based alloys

Definitions

• This invention relates to aluminum alloy powder products, and it is more particularly directed to hot worked dispersion hardened compositions adapted for service at elevated temperatures.
• Powdered metal products having a ferrous or cuprous base have achieved considerable commercial importance in recent years.
• the development of aluminum powder products has lagged behind that of other metals, partly because of the difliculty in making acceptable articles at a reasonable cost.
• the major emphasis was placed on mixtures of aluminum and other metals which could not be produced conveniently by the usual melting and casting practices.
• US. Patent 1,944,183 describes a compressed powder product cnsisting of to 80% aluminum and to 90% silicon.
• the large proportion of silicon imparts properties which are characteristic of that element, since the composition is essentially a physical mixture of the two components.
• Another type of aluminum powder product has been introduced in recent years which is made from oxidecoated aluminum flakes, an example of such a product being described in Swiss Patent 259,878. These products have a strength and hardness which exceeds that of commercially pure aluminum, especially at elevated temperatures. These properties appear to be developed by the uniform distribution throughout the metal matrix of finely divided oxide particles derived from the oxide coating on the flakes. Hardening of the aluminum matrix in this manner is referred to as dispersion hardening and is distinguished from the hardening produced by cold working or by precipitation of a constituent from a supersaturated solid solution.
• the manufacture of the oxide-coated flakes presents some difficulties, particularly those associated with milling of the flakes in a ball mill or similar device. It is necessary either to mill the flakes in a dry or nearly dry condition or to follow the conventional practice of employing mineral spirits with or without a lubricant, such as stearic acid, and subsequently drying the mass of flakes. In addition to such ditficulties, the handling of the dry powder and the charging of it to a container for compaction presents some dangers and inconvenience.
• An alternative and cheaper method of making comminuted aluminum is that known as atomization, wherein the aluminum is blown from a specially designed nozzle into a gas-filled chamber and the molten particles are frozen almost immediately.
• the solid particles thus produced tend to be spheroidal in shape, are free from any lubricant, and a given volume of the particles has a higher apparent density than the same volume of flake powders. It has been found, however, that such atomized aluminum powder yields compacted powder products which have inferior high temperature strength and hardness compared to those made from fiaketype of powder.
• One of the objects of our invention is to provide a but worked powder product made from atomized aluminum alloy powder, the product being characterized by a high strength at elevated temperatures. Another object is to provide a hot worked aluminum alloy atomized powder product which does not require a preliminary thermal treatment to develop a high strength at elevated temperatures. Still another object is to provide a but worked atomized aluminum alloy product which retains its strength at elevated temperatures even upon long exposure. A further object is to provide a hot worked atomized aluminum alloy powder product that retains to a substantial degree at elevated temperature any effect of cold working produced during fabrication of the product. Another object is to provide a hot worked atomized aluminum alloy powder product which is of the dispersion hardening type, but which does not depend upon the presence of oxide particles to impart strength and hardness at elevated temperatures. Another object is to provide a hot worked atomized aluminum alloy powder product which has a higher modulus of elasticity than pure aluminum.
• a compacted and hot worked aluminum alloy product which is free from aluminum oxide except as an incidental impurity, and made from atomized particles of an alloy consisting of at least aluminum and from 2.5 to 20% by weight of iron as the essential components, and preferably from 5 to 10% of iron.
• the iron is present in the form of a substantially insoluble constituent such as FeAl in the binary alloy. Under the microscope the constituent generally appears in the form of dendrites and/or as a component of a eutectic or peritectic product.
• the iron constituent has an extremely fine size, the dendrite stems and branches having a thickness of not over 1 micron, and usually less than 0.4 micron while in the eutectic or peritectic prodnet, the longest dimension generally does not exceed 1 micron and usually it is less than 0.4 micron. These dimensions are much smaller than those which characterize the constituent as it appears in conventional castings. Upon hot working the atomized powder compacts, the size of the iron constituent particles is reduced to less than 0.4 micron, generally between 0.03 and 0.3 micron. It has been found to be essential to observe these dimensions in order to secure a high strength and hardness at elevated temperatures.
• the composition is considered to be of the dispersion hardening type. Moreover, there is no intentional addition of oxide or oxide coated particles to increase the strength and hardness, the properties of our hot worked product being dependent upon the size and distribution of the iron constituent.
• the fine aluminum alloy particles are produced by the atomization process which yields very small substantially equi-axed bodies having a cast structure.
• the process must be controlled in such a manner as to produce particles of a size such that the majority of them will pass through a 200 mesh scheen (74 micron opening) and that none or only va small proportion are larger than 100 mesh (145 micron opening).
• the atomization process must be set up so that the molten metal spray is so drastically chilled that the desired size of iron-containing constituent is produced.
• the thickness of the constituent in the chilled particles must be under one micron and, preferably, under 0.4 micron. This size is much smaller than that of the particles of iron constituent found in cast alloys or even in sintered powder mixtures of aluminum and iron having the same chemical composition.
• the hot worked powder product may be made by first forming a compact and working it or in some cases, such as extrusion, the entire operation may be carried out in one press in a more or less continuous manner.
• the atomized alloy powder either in a cold or a preheated condition, is charged to a compression chamber, the powder heated to a hot working temperature preferably between 700 and 900 R, if the powder is not already at such a temperature, and compressed.
• the pressure exerted will depend upon several factors such as the composition of the alloy, the mass being compressed and the temperature of which compression is effected. Generally, a pressure of 30,000 to 120,000 p.s.i. is adequate to form a suitable compact.
• the compact may be immediately hot worked, as by extrusion, or it may be transferred to metal working apparatus, such as a forging press or a rolling mill. Also, if desired, the compact may be cooled to room temperature :and later reheated for hot working. When the compact is to be rolled or forged, it is preferred that the compact be first extruded in a suitable shape and then rolled or forged.
• the hot working of the compacted alloy powder is preferably done within the temperature range of 700 to 900 F.
• the hot worked product may be handled or finished according to conventional practices, including cold'working, if desired. To facilitate further oold working, it may be desirable to heat the product to a sufiiciently high temperature to produce recovery rather than recrystallization.
• the strength of the hot worked product :atelevated temperatures is affected by the-sizev of theiron constituent, as mentioned above.
• -the strength increases 4 as the size of the constituent decreases.
• a finer dispersion of the constituent usually exists in the smaller sized atomized particles than in the larger sized particles.
• particles as large as mesh microns can be used, we prefer to employ particles of 200 mesh (74 microns) and finer.
• particles passing through a 325 mesh screen (43 microns opening) yield products having the highest strength, usually not less than 18,000 p.s.i. in tensile and 14,000 p.s.i. in yield strength at 600 F. 1
• the total amount should not exceed 10%, :and preferably the iron content should exceed that of the other elements.
• the foregoing elements appear to interact with the iron in such a manner that their solubility is actually less than it is in the binary alloys of these elements with aluminum. Such a reduction in solubility is advantageous in improving the strength of the hot worked product.
• the alloys containing these elements are prepared by forming a melt of the alloy and atomizing it asdescribed above. By controlling the atomization of the molten alloy the desired size of insoluble iron-containing constituent is obtained. These constituents should not exceed one micron in size and, preferably, not over 0.4 micron in thickness in the atomized particles.
• Another desirable property of products fabricated from our atomized alloy powders is the high modulus of elasticity that is attainable through selection of the amount of iron and the supplemental element added to the aluminum-iron base.
• Wrought commercially pure aluminum has a modulus of 9,900,000 p.s.i., but a hot worked powder product consisting of aluminum and 7.6% iron was found to have a modulus of 11,400,000 p.s.i. while a similar product containing 12.8% iron had a modulus of 12,700,000 p.s.i.
• the addition of certain elements that apparently form intermetallic compounds is also beneficial in increasing the modulus.
• a hot worked powder product composed of aluminum, 5.5% iron and 5% chromium had a modulus of 12,500,000 p.s.i.
• a high modulus can be achieved along with a high strength.
• the tensile properties that can be developed in binary aluminum-iron powder compositions are illustrated in the following examples.
• the iron was added to molten aluminum and the resulting alloy atomized to produce finely divided cast particles, the most of which passed through a 200 mesh screen (74 micron opening).
• the atomized powder of each composition was placed in the cylinder of an extrusion press, heated to about 800 F. and compressed under a pressure of about 100,000 p.s.i. against a blind die to form a compact about 7 inches in length and 4% inches in diameter.
• the compact was either removed from the cylinder, heated to 800 to 850 IF., and extruded to a rod inch in diameter, or the blind die was replaced by an extrusion die rand the compact extruded immediately into inch diameter rod.
• Tensile test specimens were cut from the rod, one portion was heated to 600 F. for 100 hours and tested; a second portion was heated at the same temperature for 1,000 hours, and tested and a thirdportion was heatedto 800? F. for a 100-hour period and tested.
• the composition l r r 1 The beneficial effect of decreasing the particle size of the atomized alloy powder on the strength of the ex- TABLE I Tensile properties of extruded Al-Fe powder products 100 Hrs. at 600 F. 1,000 Hrs. at 600 F. 100 Hrs. at 800 F. Alloy Percent Fe Tensile Yield Percent Tensile Yield Percent Tensile Yield Percent Strength, Strength, Elong. Strength, Strength, Elong. Strength, Strength, Elong.
• the foregoing property values may be compared with those of a commercial aluminum forging alloy designed for service at elevated temperatures having a nominal composition of 4% copper, 0.6% magnesium, 2% nickel and the balance aluminum.
• forgings of this alloy have a typical tensile strength at 600 F. of 5,500 p.s.i., a yield strength of 3,000 p.s.i. and an elongation of 60%. It is also significant that even at 800 F. the tensile and yield strengths of the extruded aluminum-iron specimens. exceeded those of the forged alloy at 600 F. It is therefore evident that the aluminum-iron atomized powder products possess 3r 0 some exceptional properties at high temperatures.
• p.s.i. p.s.i. in 4D 0 least one hardening element selected from the group composed of 0.1 to manganese, 0.1 to 10% nickel, 0.1 to 10% cobalt, 0.1 to 10% chromium, 0.1 to 10% titanium, 0. to 10% zirconium and 0.1 to 10% vanadium, the total amount of said elements not exceeding 10%, all percentages being by weight.
• the iron-containing constituent also being finely divided in the hot worked-article and having a maximum constituent size of 0.4 micron, said alloy being substantially free from elements which form solid solutions with aluminum,

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Citations (2)

• 1957
  • 1957-05-08 US US657702A patent/US2963780A/en not_active Expired - Lifetime
• 1958
  • 1958-04-30 DE DE19581408463 patent/DE1408463A1/de active Pending
  • 1958-05-07 CH CH5922458A patent/CH393752A/fr unknown

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