US2180292A - Zinc base alloy - Google Patents

Description

Patented Nov. 14, 1939 UNITED STATES ZING BASE ALLOY William H. Finkeldey, Hastings on Hudson, N. E, assignor to The Edes Manufacturing Company,

Plymouth, Mass, a corporation setts oi'. Massachu- No Drawing. Application July 26, 1938, Serial No. 221,346

'IClaims.

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 now produced from ordinary commercial zinc containing about .10% to .30% cadmium, about .015% iron and about 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 coarse 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 oi! 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 diflicult to work mechanically. Furthermore, these alloys have undesirable shrinkage properties on casting.

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 surface qualities and good etching properties, being much superior to known zinc compositions of comparable nature in these combined qualities.

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

A further object of the invention is to provide zinc base alloy compositions which are free from appreciable coarse grain growth or selective grain growth, 1. e., growth of some grains to a much larger size than their neighbors, when subjected to heating-or repeated heating at temlarge grain size and softness.

from, although at least .005%

peratures near or above their recrystallization temperatures.

According to the present invention, I have found that the grain structure, hardness and toughness characteristics of zinc base alloys may 5 be improved to an extraordinary extent and that their resistance to coarse grain growth and selective grain growth upon heating in the wrought state may be increased to a point substantially eliminating these'characteristics, by the provision of new compositions containing a base of very high purity zinc and certain small percentages of aluminum, magnesium and manganese. Zinc of very high purity when used alone is of little or no value for the production of cast or wrought products, because of its very I have found, however, that when zinc of very high purity is alloyed with from about .10 to 1.75% aluminum, from about .001 to .035% magnesium, and from 2 about .002 to .250% manganese, compositions are obtained which possess a very fine and uni-, form grain structure, malleability, high hardness and wearing qualities in the cast or wrought condition, and extraordinary resistance to coarse grain growth and selective grain growth upon heating. Additions of as low as 002% of manganese to the molten compositions have a marked effect in preventing the growth of large grains. on annealing products wrought thereof manganese should be included to obtain the best products for etching and like treatments.

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 101% of lead, iron and cadmium. A suitable zinc metal of this preferred type is the "Bunker Hill brand of zinc of which the fol- 40 lowing is a typical analysisz' I Per cent Cadmium .0002 Iron .0008 Lead if .0008 Copper 0 .0009 Magnesium .0015 Zinc Remainder tent when in a molten state, they separate out as thezinc 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 effect on the homogeneity of cast or wrought products made from the alloys. Furthermore, the very 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 inter-crystalline oxidation and un desirable 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 effect 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, magnesium and manganese within the ranges of proportions indicated above by any suitable 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 qualities 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 surface irregularities and shrinkage cavities. The cast metal can be rolled, or otherwise mechanically worked, satisfactorily, and the wrought material has high hardness qualities and very fine grain structure, even after repeated heating or annealing operations, due'to the effects of aluminum, magnesium and manganese, when used in the proper proportions, in

hardness before, and especially after, heating the wrought material. Similarly, raising the magnesium content to .003% and above increases the hardness and resistance to deformation and wear. The manganese content, in addition to its beneficial effect on homogeneity and grain structure, also improves the hardness of the alloys after heating in the wrought state, its hardening effect apparently reaching a maximum at about, .05%. In producing wrought zinc products from compositions lying near the higher limits of the alloying ingredients, I have found some tendency to encounter difllculties in polishing the products where such operations are required, possibly due to metallic inclusions in the metal which may be compounds of zinc and aluminum or zinc and magnesium or zinc and manganese.

The optimum alloys for fine and homogeneous grain structure, hardness, toughness, good grinding and polishing properties in the wrought state, and freedom from coarse grain growth upon heating are provided by preferred compositions comprising a base of high purity zinc alloyed with about .35 to .'75% aluminum, about .003 to .02% magnesium and about .005 to .15% manganese.

The following table gives compositions and comparable hardness data with respect to rolled plates of .065" thickness, made from (A) ordinary commercial zinc, (B) a special hard grade 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 A3" diameter ball, and maintaining the load for 10 seconds time.

Composition (percentages) v 353222 After Gd Fe Pb Mg Cu Balance aggg 22 600 F.

A Ordinary commercial zinc 18 015 30 0015 Zinc.. 74-75 55-56 B Hard grade commercial zinc 26 .015 .31 009 .d0.. 83 73 C High purity zinc (Bunker Hill) 0002 0008 0008 0015 0009 .do. 17-22 9-19 Rockwell hardness- 41 Mg Mn Balance Mm Before heating heating at D 1 Zinc-Al-Mg-Mn alloy 25 010 76-77 2 do 45 015 74-77 3 45 015 -81 4 45 015 84-85 5 45 015 6 45 015 82-83 7 50 003 74-76 8 50 010 80-81 9 50 020 83-84 10 55 010 82-84 11 55 010 81-84 12 75 010 83-85 13 '1. 50 010 86-87 lending hardness, toughness and small grain structure without causing brittleness and in preventing coarse grain growth and selective grain growth.

Maintaining the aluminum content of the alloys in excess of 25% results in an inc ease of The hardness and toughness of the wrought products produced from my alloys, before and after heating, are much superior to the same qualities of the other alloys. In addition to desirable hardness and toughness characteristics, my alloys possess a small and homogeneous grain structure which renders them much more valu- 2,1so,2e2

metallic aluminum in the form of rolled sheet.

able than 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, ground and polished plates made from my alloys flnd valuable fields of use as photo-engraving plates and as plates'to be used for lithography, due

to their 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, 1. e., any suitable apparatus or procedure may be used as long as the metal possesses the prescribed chemical composition. In a 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 aluminum, manganese, and magnesium all have deoxidizing properties, and sincethe amounts of manganese and magnesium added are small, I

prefer to alloy the zinc first with aluminum, to

take ofl any de-oxidation oi the zinc bath which may be necessary, and thus avoid any loss of alloying efiect oi the elements manganese and magnesium through their acting as de-oxidizers. 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 and manganese are so low that in many cases theywould probably be reported as "traces in analyses made by ordinary wet analytical chemical methods. It is, therefore, desirable to carry out such determinations spectrographically, or by other special methods capable of determining such small amounts.

The following example is illustrative of a preferred procedure for making the alloy of my invention, having the aproximate composition of: aluminum 0.45%, manganese 0.05%, 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 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, ten pounds, fourteen ounces of a zinc-manganese-master alloy containing 4.63% manganese is next dissolved and stirred into the bath. When this operation is complete, one pound, twelve and a 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, magnesium, and manganese, I find that very small amounts of copper, from .01 to less than may be included in my alloys without destroying their valuable properties. 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 .10% to 1.75% aluminum, from about .001 to 035% magnesium and from about .002 to .25% manganese, substantially all of the remainder being high purity zinc containing at least 99.99% zinc.

2. As a new composition of matter, a zinc alloy containing from .35 to .75% aluminum, from .003 to .02% magnesium, and from .005 to .15% manganese, substantially all of the remainder being high purity zinc containing at least 99.99% zinc.

3. A zinc base alloy consisting of from .35 to .75% aluminum, from .003 to .02% magnesium, from .005 to .15% manganese and zinc of very high purity containing less than .01% of lead, iron and cadmium.

4. A zinc base alloy consisting of from .10 to 1.75% aluminum, from .001 to .035%, magnesium, from .002 to .25% manganese and high purity zinc containing less than .01% of lead, cad.- mium and iron.

5. A zinc base alloy consisting of about .45 to .55% of aluminum, about .01 to .02% of magnesium, about .005 to .05% of manganese and high purity zinc containing less than .01% of lead, cadmium and iron.

6. A zinc'base alloy consisting 01.10 to 1.75% of aluminum, .001 to .035% of magnesium, .002 to .25% of manganese, and the balance high purity zinc. p

7. A zinc base alloy consisting of .35 to .75%

of aluminum, .003 to .02% of magnesium, .005;

to .15% of manganese and the balance high purity zinc.

WILLIAM H.