USRE18600E - Zinc base die casting alloy - Google Patents

Description

Reissuecl Sept. 20, 1932 UNITED STATES PATENT OFFHQE.

EDMUND A. ANDERSON AND GEORGE L. WERLEY, OF ."E'ALMERTON, PENNSYLVANIA, ASSIGNORS TO THE NEW JERSEY ZINC COMPANY, OF NEW YORK, N. Y., A CORPORA:

TION OF NEW JERSEY ZINC BASE DIE CASTING ALLOY No Drawing. Original No. 1,852,442, dated April 5, 1932, Serial No. 476,946, file 1i auiifiaifieo.

Application for reissue filed August 4, 1932.

This invention relates to zinc base die-casting alloys and has for its object the provision of an improved alloy of this character.

The mechanical requirements of the diecasting operation necessitate the construction of the melting pot and die from iron and steel. These metals are subject to attack by molten zinc and it has been found from experience that at least 0.25% aluminum must be added, to the Zinc in order to minimize this attack sufficiently to secure reasonable life from the apparatus. Alloys for die-.casting must possess a sufiicient degree of fluidity to properly flow into and completely fill the dies used, and 2% or more aluminum appears to be necessary to secure an adequate degree of fluidity. It has also long been known that aluminum increases the tensile strength of zinc and this, in itself, makes its presence in zinc base die-casting alloys desirable in quantities of from 2% to as much as 10 to 15%.

Zinc-aluminum alloys containing less than about 80% of aluminum undergo a structural change subsequent to solidification which is commonly known as a phase change or more specifically in this case as a eutectoid reaction. This phase change consists in the formation of two crystalline forms or phases from one phase previously existing, and is accompanied by certain changes in physical properties of the alloy, such, for example, as increase in density, hardness and tensile strength and decrease in ductility and impact strength. This phase change may occur during the cooling of the alloy after casting or may through certain'infiuences be retarded or inhibited. In such cases it may take place gradually over a period of months at ordinary temperatures.

A secondary stage of phase change which sometimes occurs is the growth or coalescence of the extremely small particles of the new phases as first formed into larger particles.

.This stage may be accompanied by softening Serial N 0. 627,534.

and lowering of tensile strength and by an increase in ductility and impact strength.

Zinc-aluminum alloys in this range of composition are also subject to a type of disintegration commonly known as intercrystalline oxidation. In extreme cases,under the influence of warmth and moisture, intercrys talline oxidation may completely penetrate specimens of these alloys and cause swelling,

Warping and even complete disintegration. Intercrystalhne oxldatlon 1s 1n some way as or the completeness of the reaction or in some,

other respect not fully understood. The particular effect produced by copper and magnesium on the phase change reacts favorably in increasing the resistance of the alloys to intercrystalline oxidation. Lead, while not exerting any marked influence on the phase change, very seriously diminishes the resistance of thesezinc-aluminum alloys to inter crystalline oxidation. Cadmium, though having a certain effect on the phase change, in the presence of lead usually'diminishes the resistance of these alloys to intercrystalline oxidation.

In the course of an exhaustive investigation of zinc base die-casting alloys, we have found that the tendency of zinc-aluminum alloys to undergo intercrystalline oxidation is greatly diminished by the elimination of harmful impurities, notably lead and tin and under certain conditions cadmium. This was taken advantage of in the preferred composition of the alloy described in the United States Patent of Peirce and AndersonNo. 1,596,761, dated o crystalline oxidation and when present together have a more or less additive beneficial eflect. I

Continued experiments with zinc metal con taining materially less lead than present in' high grade zinc metal indicated definitely that there were important possibilities in this direction. The first important discovery was that if the percentage of lead and other inipurities in the zinc metal (used in making up the zinc base alloy) was less than 0.02%, with tin substantially absent, the presence of 0.1% magnesium in an alloy containing 4% aluminum (and without copper) was sufficient to prevent intercrystalline oxidation, and that such an alloy had other desirable physical properties. The advantage gained by the elimination of copper from this alloy was primarily an increased impact strength with better retention of this impact strength upon ageing either at room temperature or elevated temperatures. This discovery constitutes the basis of a copending patent application, Serial No. 368,730, filed June 5, 1929 which has since issued into United States atent No. 1,77 9,525, of October 28, 1930). However, the elimination of copper from the 4% aluminum, 3% copper, 0.1% magnesium alloy resulted in a lower tensile strength which under some circumstances, might make the alloy less valuable.

We next discovered that it was not necessary to entirely eliminate the copper in order to'obtain the advantageous characteristics of the last-mentioned alloy. On the contrary, we found that additions of copper up to 1% are accompanied by slightly improved tensile strength apparently without any corollary disadvantages. This discovery forms the basis of our copending patent application Serial No. 47 6,452; filed August 19, 1930.

Our present invention is based on the discovery that in an alloy made with zinc metal of high purity and containing aluminum, for example 4%, the presence of a small amount of copper (without magnesium), for example 1%, is suflicient to give substantially complete resistance to intercrystalline oxidation. As compared with the 4% aluminum, 0.1% magnesium zinc base alloy (without copper), made with the same grade of high purity zinc metal, the alloy containing 4% aluminum and 1% copper (without magnesium) .has the advantage of a higher initial impactstrength with a somewhat higher impact strength after ten days exposure to air saturated with moisture at 95 C. The new alloy also has an appreciably higher tensile strength after such exposure.

he improved zinc base die-casting alloy of the present invention is, accordingly, substantially free of magnesium, and contains from about 2 to 10% aluminum, from about 0.05 to 2% copper, and is made up of a zinc metal base of high purity containing at least 99.98% zinc and preferably containing 99.99+% zinc. Our present preferred composition of the alloy of the invention is about 4% aluminum, about 1% copper and the bal ance zinc metal of high purity containing 99.99+% zinc. The zinc metal of high purity should contain not more than about 0.01% lead plus cadmium. Very satisfactory results have been secured with zinc metal containing 99.99+% zinc, less than 0.003% lead, less than 0.003% cadmium and less than 0.001% tin. Tin should be substantially excluded from the alloy and in no case should the tin content exceed about 0.001%.

The following tables illustrate the important physicalproperties of die-castings made of an alloy of the invention. Alloy No. 1 is of the composition described in United States Patent No. 1,596,761; alloy No.2 is of the composition disclosed in my copending patent application Serial N 0. 368,730, alloy No. 3 is of the composition disclosed in ourcopending patent application Serial No. 476,452; filed August 19, 1930; and alloy No. 4 is of the composition embraced within the present invention.

Composition Alloy No.1 Alloy No.2 Alloy No.3 gfg ig; 4

Pat. s.N. 368,730 S.N.476,452 on 1,596,761

Aluminum 4. 0% 4. 0% 4. 0% 4. 0% Copper 3. 0% 1. 0% 1. 0% Magnesium 0. 1% 0. 1% 0. 1% Quality of zinc H i g h High puri- High puri- High puribase. grade ty 99.99 99.99% ty 99.99%

Zn. Zn. Zn.

Properties as wet Tensile strength flat 46,200 39,400 42, 700 40,800 Tensile strength roun 45,600 37,300 43,100 41,800 Impact strength-- 108 12.0 169 288 Properties after ten days in steam at 95 C.

Tensile strength flat 27, 800 31,600 33,400 34,500 Tensile strength round 32,800 31,900 34,400 36,000 Impact strength. 8 142 137 170 Expansion wide 0.0076 0.0012 0.0008 0.0006 Expansion 54" wide 0.0068 0. 0016 0.0006 0.0007

The tensile strength is indicated in pounds per square inch for fiat and round test specimens. The impact strength is indicated in foot pounds per square inch. Linear expansion is indicated in inches'for test sections in. and in. in width.

We claim:

1. A zinc base alloy consisting of about 4% aluminum,about 1% copper and the balance zinc metal of high purity with at least 99.98% mm.

2., A zinc base alloy consisting of 2 to 5% aluminum, 0.05 to 2% copper, and in which the zinc base is zinc metal of high purity containing at least 99.98% zinc.

In testimony whereof we aflix our signatures.

EDMUND A. ANDERSON. GEORGE L. WERLEY.