US2180291A - Zinc base alloy - Google Patents

Description

Patented Nov. 14, 1939 UNITED STATES ZINC BASE ALLOY William H. Finkeldey, Hastings on Hudson, N. Y.,

assignor to The Edcs Manufacturing Company, Plymouth, Mass, a corporation of Massachusetts No Drawing. Application July 26, 1938, Serial No. 221,345

3 Claims.

This invention relates to zinc base alloys.

Prior to the present invention zinc alloys have been provided for die-casting which embody a base of very high purity zinc and fairly large proportions (from about 3 to 11%, usually more than 4% of aluminum and copper, manganese or magnesium. These alloys are unsuitable for the production of rolled or wrought zinc products, most of which are produced from ordinary commercial zinc containing about .18-to .30% cadmium, about .015% iron and about .30% lead, with or without about 009% magnesium. Important disadvantages of the ordinary zinc alloys, however, lie in their shrinkage when cast, their coarse grain structure, their lack of homogeneity in the cast and wrought condition, and their grain growth and drop in hardness when heated after being rolled or otherwise mechanically worked.

Other zinc alloys for the production of wrought products are known which consist of either ordinary or high grade zinc metal, substantial percentages of cadmium, and manganese or magnesium. In their lower ranges of alloying ingredients, these alloys are low in hardness, have coarse grain structure in the cast state, and are subject to structural changes and considerable softening on annealing in'the wrought state, so that their utility for many purposes is limited. In their higher ranges of alloying ingredients, they are difiicult to work mechanically. Furthermore,

these alloys have undesirable shrinkage properties on casting. 7

An object of my invention is to provide new zinc base compositions which may be cast and rolled or otherwise worked into improved wrought products characterized by small and uniform grain structure, high hardness and toughness, good etching properties, and greater freedom from coarse grain growth upon annealing than known zinc compositions of comparable nature.

Another object is to provide new zinc base compositions which may be used for producing valuable wrought products and also for producing zinc castings that are fine-grained, hard, tough and free from objectionable shrinkage cavities.-

I have discovered that valuable zinc base alloys for use in the production of cast and, particularly, wrought products, having certain properties superior to known zinc base alloys, may be obtained by compounding zinc of very high purity with certain percentages of. aluminum and magnesium. Zinc of very high purity, when used alone, is of little or no value for these purposes, because of its very large grain size and its softness. I have found, however, that when zinc of very high purity is alloyed with from about .10

to 1.75% aluminum and from about'.001 to .035%

magnesium, compositions are obtained which possess small and uniform grain structure, malle- It is important that the zinc metal used as the base for my alloys be of very high purity, preferably the highest grade of zinc commercially available containing at least 99.99% zinc and less than .01% of lead, iron and cadmium. A suitable zinc metal is the Bunker Hill brand of zinc, of which the following is a typical analysis:

. Per cent Cadmium .0002 Iron .0008 Lea .0008 Copper -I .0009 Magnesium .0015 Zinc I Remainder I have found that lead and iron in substantial proportions are harmful impurities in certain wrought zinc products due to their tendency to form lead-rich and iron-rich segregations which render the structure of the metal non-homogeneous. While lead and iron are soluble in zinc to a limited extent when in a molten state, they separate out as the zinc solidifies after being cast, in the form of globules or droplets of lead in the case of lead and in the form of fine particles of iron-zinc crystallites in the case of iron. The content of these impurities in my zinc base alloys is so low that they have no noticeable efiect on the homogeneity of the cast or wroughtproducts made from the alloys. low cadmium and lead content of my alloys is an important factor in ensuring the stability of products made from them. It is well known in the die-casting industry that intercrystalline oxidation and undesirable structural changes occur in zinc-aluminum alloys, particularly in the presence of lead, cadmium, tin or antimony. These metals are either absent from my alloys or are present in such small amounts that no adverse efiect has been noticeable within the lifetime of products made from the alloys,

Using very high purity zinc as a base, I alloy it with aluminum and magnesium within the ranges of proportions indicated above by any suit-.

able alloying method. The molten metal compositions are unusually free of oxide and dross, which, if carried into castings and products wrought therefrom, would adversely affect quali- Furthermore, the very.

ties of the products and cause trouble, for example, in polishing, etching or other surface treatments. Moreover, the molten metal flows freely into molds when cast and forms dense and hard cast products characterized by small and uniform grain structure and unusual freedom from shrinkage cavities. The cast metal can be rolled, or otherwise mechanically worked, satisfactorily, and the wrought products have high hardness qualities and good grain structure, even after annealing, which I attribute to the efiects of aluminum and magnesium, when used in the proper proportions, in lending hardness, toughness and small grain structure without causing brittleness and in preventing, to some extent, coarse grain growth.

Maintaining the aluminum content of the alloys in excess of .25% results in an increase of hardness before, and especially after, heat-v ing the material. Similarly raising the magnesium content to 003% and above increases the hardness and resistance to deformation and wear. In producing wrought zinc products from compositions lying near the higher limits of the alloy.- ing ingredients, I have found some tendency to encounter difilculties in polishing the products where such operations are required, possibly due I to metallic inclusions in the metal which may be compounds of zinc and aluminum or of zinc and magnesium.

The optimum alloys for good grain structure, hardness, toughness and good grinding and polishing properties in the wrought condition are provided by preferred compositions comprising a- .of commercial zinc, (C) very high purity zinc (Bunker Hill brand), and (D) certain illustrative examples of my improved alloys. Hardness figures were obtained before and after heating at 600 F. to indicate the reaction of the several plates to heating operations. These figures were obtained on a Rockwell hardness testing machine, reading the B scale on the dial, using a 100 kg. load, a V diameter ball, and

The hardness and toughness of the wrought productsproduced from my alloys, before heating in the preferred examples. In addition to deof any of the other alloys, and also after heating inthe preferred examples. In addition to desirable hardness characteristics, my alloys possess a grain structure and homogeneity which render them much superior to the other alloys, particularly for making wrought zinc products and for uses in which surfaces of wrought zinc products are ground, polished, etched or otherwise treated to produce desirable surface characteristics. Rolled plates-made from my alloys find valuable fiields of use as photo-engraving plates and as plates to be used in lithography, due to their improved structure and physical and chemical qualities.

Any of the usual methods of alloying may be used in preparing the zinc base compositions provided by my invention, i. e., any suitable apparatus or procedure may be used as long as the metal possesses the prescribed chemical composition.

Ina preferred procedure, high purity slab zinc is first melted in a crucible or other suitable apparatus and the molten metal raised to a temperature suflicient above its melting point (800- 900 F.) to permit easy incorporation of the alloying ingredients, which may be in the form of master" alloys of predetermined composition. Aluminum may also be added as pure aluminum sheet clippings, which dissolve in the molten zinc reasonably fast. Since both aluminum and magnesium have deoxidizing properties, and since the amounts of magnesium added are small, I prefer to alloy the zinc first'with aluminum,'to take oil any de-oxidation of the zinc bath which may be necessary, and thus avoid any loss of alloying effect of the element magnesium through its act as a de-oxidizer. The small amount of aluminum consumed in the de-oxidation reaction is not important in view of the relatively large amount of aluminum added.

The percentages of magnesium are so low that maintaining the load for 10 seconds time. ferred procedurev for making an alloy having the Composition (percentages) Rockwell hardness- Balance Mm Before heating Cd F6 Pb MK Cu heating at me A Ordinary commercial zinc .18 .015 30 .0015 Zinc 74-75 55-56 B Hard grade commercial zinc .26 .015 .31 .009 Z .d0 83 73 0 High purity zinc .0002 .0008 .0018 .6315 .0009 do... 17-22 9-19 Rockwell hardness- Al Mg Balance Before heating heating at 600 D 1 Zinc-Al-Mg alloy .25 .007 Hlghpurity zinc (as o) 90 68-69 2' --d0 .25 .01 re 90 69-70 .45 .015 c.0 -L 89 75 .45 (.0 88-89 76-78 .50 ,.003 ro 88 6869 .50 .01 n 89 70-77 It is therefore, de-

approximate composition of: aluminum 0.45%, magnesium 0.010% balance high-purity zinc containing not more'than 0.01% of lead plus iron plus cadmium as impurities.

One thousand pounds of high-purity zinc is first melted in a crucible, the metal raised to a temperature between 840 and 880 F. and the bath skimmed free of dross and oxides accumulated during the melting operation. While holding the metal within the foregoing temperature range, four pounds, eight ounces of pure metallic aluminum in the form of rolled sheet clippings is added to the molten zinc bath and dissolved in the same, solution being aided by stirring. After the aluminum is thoroughly dissolved and stirred into the zinc, one pound, twelve and one-half ounces of a zinc-magnesium master alloy containing 5.6% magnesium is added to the melt. After this has been thoroughly incorporated into the melt, the metal is skimmed and ready for casting.

In addition to high purity zinc, aluminum and magnesium, I have found that very small amounts r ga of copper, from .01 to less than .20%, may be included in my alloys without destroying their valuable properties and, indeed, with improvements in properties in some cases. I contemplate that copper or other metal which does not substantially change the characteristics of my alloys may be included in minor proportions without departing from the spirit of the invention.

I claim:

1. As a new composition of matter, a zinc alloy containing from about .25 to 1.75% of aluminum, and from about .003 to .035% of magnesium, the remainder being high purity zinc containing at least 99.99% of zinc.

2. A zinc base alloy composed of from .25 to 1.75% of aluminum, from .003 to .035% of magnesium and the balance high purity zinc.

3. A zinc base alloy composed of aluminum in an amount between .35 and 375%, magnesium in an amount between .003 and .02% and the balgiililce zinc metal containing at least 99.99% of 'WILLIAM H. FINKIEHLDEY.

CERTIFICATE OF CORRECTION. Patent No. 2,180,291.- v November 11;,19 9.

WILLIAM H. FINKEIDEY.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring Correction as follows: Page 2, sec- 0nd column, line 5, strike out the words and syllable in the preferred examples. In addition to de-" and insert instead the cor nma and words are considerably greater than in the case; and that the said Letters" Patent should be read with this correction therein that the. same may conform to the record of the case in the Patent Office.

Signed and sealed this 19th day of December, A. D. 1959.

Henry Van Arsdaie (Seal) Acting Commissioner of Patents.