US3139334A - Atomized lead-aluminum alloy powder article - Google Patents

Description

United t s. aw

. 3,139,334 ATOMIZED LEAD-ALUMINUM ALLOY POWDER ARTICLE p Fritz Victor Lenel, Rensselaer, N.Y., assignor to Lead Industries Association, Inc., New York, N.Y., a nonprofit corporation of New York No Drawing. Filed Sept. 29, 1961, Ser. No. 141,577

1 Claim. (Cl. 75.5)

This invention relates to lead-base alloys having high strength and resistance to creep, and adequately high ductility.

Pure lead is soft and weak; .its rate of creep at room temperature is so high that in many applications lead cannot be used as a structural material in itself. The strength and creep resistance of lead can be somewhat impoved by alloying, particularly ifthe alloying element causes precipitation hardening as in conventional leadantimony and lead-calcium alloys. t

In precipitation hardening, an alloying element is added to the lead, which has higher solid solubility at temperature near. its melting point than at room temperature. The alloy is rapidly cooled from the high temperature and then aged.

However, the extent to which lead can be strengthened and made more creep resistant by this type of alloying is limited. For instance, at 86 F. antimonial lead .containing 6% Sb, has a creep rate of 1% in one year at a stress of 400 p.s.i.; and of 100% in one year at a stress of 860 {mi An age hardened lead-calcium alloy with 0.03% Ca. has creep resistance similar to the antimonial lead alloys.

In contrast, one of the lead base alloys according to the present invention, containing 0.55% aluminum, has, at 77 F., a creep rate of less than 0.5% in one year at a stress of 2500 p.s.i.; such alloy also has adequate tensile properties and ductility. I

The preferred properties of prior art age hardened leadbase alloys are obtainedby choosing a suitable chemical composition for the alloy and then giving the alloy an appropriate mechanical and thermal treatment. For the alloys according to the invention it is also necessary to choose a suitable chemical composition and appropriate mechanical and thermal treatments.

Incontrast to most conventional alloys, the alloys according to the invention are fabricated by atomizing them into a fine powder, then compacting the powder, and then extruding the powder compact.

Ithas also been previously proposed to strengthen lead by atomizing it into a lead powder, then compacting it and working the compact. The dispersion of the lead oxide present in the powder strengthens the lead but such lead is so brittle as to be impractical for many uses.

It is an object of the invention to provide a lead base alloy article having greater resistance to creep and having also high strength and adequate ductility. Other objects of the invention will be more apparent from the following description and claim.

A preferred method of producing the alloy of the invention is described in the following, for two lead-aluminum alloys containing 0.55 and 1.7 weight percent of aluminum respectiv ly, the remainder all lead.

. 0.15 inch in diameter.

The alloy is melted in a suitable crucible and heated to a temperature high enough to dissolve all the aluminum in the liquid lead. This temperature is in the range of 1500to 1800 F. The melting point of lead is 621 F. and the melting point of aluminum is 1219.4 F.

The molten alloy is then atomized in the following manner:

A thin stream of the molten alloy is produced by pouring it through a suitable nozzle. This stream of metal isbrokenup into very fine droplets by impinging a stream of air of suitable velocity upon the stream of molten metal. The droplets of molten alloy freeze into powder particles.

The resultant powder is collected in a suitable container and sifted through a 325 mesh sieve. Only that portion of the powder, finer than 325 mesh (less than '44 microns) is used in the preparation of the alloy.

The powder is compacted in the atomized (as-received) condition. No compacting was done on reduced powder since it has been found that .by the hydrogen-loss test, no readily reducible oxides exist in these powders. This is discussed more at length below.

The as-received powder is cold compacted into a compact 1 inch in diameter in a steel die under a pressure of 35,000-40,000. p.s.i. The resultant compact has a density of 98-99% of the density calculated by the rule of mixtures for a completely dense 99.45% Pb-0.5S% Al alloy. The compact is extruded by indirect extrusion into a wire 0.156 inch in diameter at a temperature in the range 300 to 450 F.

It should be understood that the above method of consolidating the powder into a solid shape is only one of several methods which may be used to produce a solid shape. For instance, the lead powder may be fed into a rolling mill and rolled into a sheet. Cold rolling the powder into a sheet, and then hot re-rolling the sheet in the same temperature range as used for extrusion, results in rolled sheet 'having properties similar to those for the extrusions described below.

The preferred alloy composition is 99.45% lead-0.55% aluminum. This alloy composition and other lead-aluminum alloys have been tested in the form of extrusions, The tensile properties and the creep rate has been determined for extrusions made from as-received powder at extrusion temperatures as follows.

The data obtained are shown in Tables I to V below.

TABLE I Mechanical Various Extrusion Temperatures [Characterized by severe embrittlement] Extrusion Temperature, F

Tensile Percent Strength, Elongation p.s.i. in 1 inch Powder map-m 3 TABLE 11 Creep Stresses of Various Rolled Lead Alloys and of 95% Lead-5% Copper Powder Extrusions Compared to 99.45% Lead-0.55% Aluminum Powder Extrusion Test D in Table II refers to copper-lead powder alloys as disclosed in copending application Ser. No. 99,995, filed April 3, 1961, in the names ofSchrade F. Radtke and Fritz' V. Lenel. g

a I TABLE III Mechanical Properties of As-Received Lead-Aluminum Powder Extrusions With Varying Amounts of- Aluminum l Weight, Extrusion Tensile Percent Test Percent Tempera- Strength, Elongation Aluminum ture, F. p.s.i. in 1 inch A 0. 55 450 5, 420 20. o B 1.7 450 3,680 28.0

TABLE 1v 7 Steady-State Creep Rate of the Alloy of Lead With 0.55% Aluminum at 77 F.

[Extrusion temperature 450 F.]

Steady-state Test Stress, p.s.i. Creep rate, Percent/year A 2,000 11 s; than B 2, 500 Lei Ethan o 3,000 1.8 D- I 3,200 3.1

TABLE V Creep Rates of the Alloy of Lead With Steady-State .'1.7% Aluminum at 77 F.

A practical use of this invention is for lead products which require resistance to creep, either at room or at slightly elevated temperatures. For radiation protection, lead maybeextruded into bricks or into other products.

For example, the extruded products may be used in leadsteel. or lead-concrete sandwich structures, which must keep close dimensional tolerances. The design of these I tron radiation is involved, because aluminum has a very low capture cross-section for neutrons and is therefore not apt to produce secondary radiation whenirradiated with neutrons.

It is believed that the improved strength and creep The strength and creep resistance of the alloy will depend upon the degree of dispersion of the aluminum particles in the particles of powder; the finer the dispersion, the stronger and more creep resistant the alloy. The degree of aluminum dispersion in the particles of powder will in turn depend on the rate of cooling of the alloy from the temperature where the aluminum is in solution. The finer the particles of powder, the faster the rate of cooling. It is therefore desirable'to use quite n p er- Workhas also been done on powders of ditferent particle size. Even though best results have been obtained with powder, all of which is below 44 microns, good results have been obtained with powders of larger particle size.

In' contrast to the lead-copper alloy powders the leadaluminum alloy powders have a very low hydrogen loss. The hydrogen loss is the percent loss in weight of a sample when it is heated in hydrogen at a certain temperature for a certain time (1020' F. for 30 minutes for lead). It is therefore an indication of the amount of oxide which can be readily reduced by hydrogen under the given reducing conditions. In lead-copper powders the hydrogen loss is due to the presence of lead oxide, or possibly of copperoxide, which can be readily reduced by hydrogen. The

low hydrogen loss in lead-aluminum powders means that oxide. Since A1 0 cannot be reduced under the condi- [Extrusion temperature 450 F.]

' Steady-state Test Stress, p.s.i. Creep rate,

Percent/year tions of the hydrogen loss test, the hydrogen loss is low. It is possible that a considerable portion, or possibly even all,'of the aluminum' in the lead-aluminum powder is in the form of aluminum oxide.

It is not inconceivable that the higher creep-resistance of the lead-aluminum powder extrusions, compared with the lead-copper powder extrusions, is due to the fact that aluminum oxide is a more effective strengthener. than metallic copper or that the aluminum oxide is more finely divided in the lead than the metallic coppen. It is also possible that the higher strength and'creep resistance of the alloy with 0.55% Al compared with those of the 1.7% A1 'is due to the form in which-the aluminum is which the aluminum is present'tinetallic' aluminum vs;

aluminum oxide) or due to the distribution of thealuminum or'its oxide (coarse vs. fine).

Work has also been done to determine the range of composition of the Pb-Al alloys, which have these desirable properties. This Work indicates that. the aluminum content should-run from 0.1% to 2.0% aluminum by weight, remainder lead. I

What is claimed ist' 'A worked lead base powder article, the particles of said powder being obtained by atomizing ,a molten leadaluminum alloy in whichthe. aluminum is completely dis solved in the lead, said particles consisting of a fine dispersion in a lead matrix of a form of aluminum selected from the group consisting of aluminum and aluminum oxide, and said lead-aluminum alloy, before atomizing,

consisting essentially of from 0.1 (02.0% by weight of resulting compacted form, said article having a substantially lower creep rate than that of pure lead.

References Cited in the file of this patent UNITED STATES PATENTS Williams July 12, 1927 Williams June 3, 1930 Cooper May 2, 1939 McKim Sept. 11, 1962 Giraitis et al Apr. 16, 1963 6 FOREIGN PATENTS Great Britain Oct. 7, 1959 OTHER REFERENCES Campbell et al.: The Alloys of Aluminum and Lead, Canadian Journal of Research, vol. 18, sec. B. 1940, pages 281-287.

Claus et al.: Zur Kenntnis der Systeme AlPbCu und Al-Pb, Metallwirtschaft, 18, 1939,. pages 957-960.

Hansen: Constitution of Binary Alloys, 2nd ed., 1958, McGraw-Hill Book Company, Inc., N.Y., pages 122-124.