Zinc-based alloys and their application to industrial processes

Abstract

 The present invention relates to zinc-based alloys having remarkably improved physical properties, characterized in that their composition is in a percentual ratio by weight of Zn:69-79, Al:20-30 and Cu:1-8. The present invention also refers to zinc-based alloys which, besides having the above mentioned composition, have incorporated therein between .001 and 1 per cent by weight of Mg. Due to their characteristics, these alloys are very appropriate for various transformation processes, among which are included smelting, pressure injection, direct or reverse extrusion, sheet rolling, open and closed die forging, precision forging, stamping, stretching, wire drawing, machining, and even the novel process of liquid metal forging.

Description

[0001] For more than thirty years, it has been known that alloys which are close to the zinc-aluminum eutectoid show a high level of plasticity which has been denominated super-plasticity.

[0002] Various patents have been granted in the United States and in other countries to cover Zn-Al eutectoid alloys to which other metals are added in different proportions, and which show the aforementioned superplastic characteristics, cited, among other authors, by W.A. Backofen, pioneer in this field.

[0003] The purpose in adding other metals to base alloys of the Zn-Al eutectoid, is to improve their physical properties, including their mechanical strength. Also, various thermal-mechanical processes have been applied to improve the superplastic conformation characteristics of the above mentioned alloys.

[0004] In addition to having better physical and mechanical properties that those of the Zn alloys presently used in industry, the new alloys covered by this invention are particularly suited for use in countries having large zinc deposits and where bauxite and other constituents of aluminum are scarce or non-existent. This is for the obvious reason that the alloys covered herein use smaller percentages of aluminum.

[0005] The National Autonomous University of Mexico, jointly with the assignee of this invention, acting in the framework of the program "Alternatives for Aluminum" sponsored by the Organization of American States, have worked for several years in the development of zinc-based alloys.

[0006] As a result of said work, it has been found that by the addition of amounts between 1 and 8 per cent by weight of copper, an improvement in the physical properties of the alloys are improved, particularly with respect to their mechanical strength. It has also been found that by adding amounts between .001 and 1 per cent by weight of Mg to the aforementioned alloy, its mechanical strength is increased even more, even though its superplastic properties may diminish.

DETAILED DESCRIPTION OF THE INVENTION

[0007] The present invention is realized by preparation of Zn-Al-Cu alloys in ratios by weight of Zn:69-79%, Al:20-30% and Cu:1-8%, or alternatively Zn:69-79, Al:20-30%, Cu:20-30% and Mg:.001%, utilizing 99.99% purity Zn, Al with a minimum purity of 99.5%, Cu with a purity of 99.7%, and Mg with a minimum purity of 99.9%, in order to guarantee the final characteristics of the alloys.

[0008] The use of raw materials with minimal impurities is recommended for smelting, the most suitable being: first melt Al, special high-grade Zn, Cu as cable or cathod scrap, and high-grade Mg. Also, pollution of the Zn-Al-Cu alloy with iron, cadmium and magnesium should be avoided as much as possible, as well as contamination with iron and calcium of the Zn-Al-Cu-Mg alloy.

[0009] A comparative study of the alloys covered by this invention, applied in the case of the Zn-Al-Cu alloy to sand-cast, and pressure-injected pieces, with respect to smelting pieces obtained form zamak, aluminum, brass, bronze, grey iron and steel, shows the advantages of the alloys of this invention, as it appears on the following Table:

[0010] From the preceding Table, it appears that the Zn-Al-Cu alloys of the invention have an intermediate density between Al and Cu, and a tensile strength above that of alternative materials. Additionally, it has been demonstrated, through tests performed in accordance with ASTM-B117 standard for saline chamber, that the alloys of this invention have a corrosion resistance above that of all other materials included in the Table, said resistance being thrice that of aluminum and twice that of copper. For these reasons, the alloys of this invention are particularly suitable for applications on which high mechanical strength is required, and for uses in highly agressive environments, such as coastlines and maritime areas.

[0011] The alloys of this invention are especially attractive to substitute bronze and brass pieces, due to the lower cost and the lower specific weight of the former.

[0012] Additionally, the surface-finishing tests performed on the alloys of this invention, showed an excellent adherence to paint, including electrostatic paint, as well as a good acceptance of the copper base to obtain chrome and nickel finishings.

[0013] The above described characteristics and advantages of the alloys covered by this invention, will be better understood by the examples appearing below, all of which refer to pilot tests conducted at an industrial level, and which show the high industrial potentials of said alloys.

EXAMPLE 1

EXTRUSION OF Zn-Al-Cu ALLOY

[0014] One inch diameter round bars were extruded in a 1,800 Ton pressure extruder, preheating the Zn-Al-Cu alloy ingots which were composed of Zn: 7, Al:21 and Cu:2 per cent by weight, as well as the equipment used, to a maximum temperature of 273°C, maintaining the temperature throughout the extruding process below 273° C, performing the process at an extrusion speed between 1 and 20 meters per minute, cooling rapidly the extruded section with forced air at the exit of the extrusion die, and using a mechanical conveyor prior to drawing or stretching.Extruded sections having optimal characteristics were obtained, compared with Series 6030 aluminum extruded sections, for although density of the alloy of the invention was almost twice that of the aluminum section, its comparative mechanical strength was twice that of the aluminum, the cost of the alloy of the invention and its operating costs being much lower than those of aluminum.

EXAMPLE 2.- PRESSURE INJECTION OF Zn-Al-Cu-Mg ALLOY

[0015] Octagonal thin-walled electrical connection boxes made of the alloys of the invention composed of Zn:76, Al:21, Cu:2 and Mg:1 per cent by weight, were injected in a 150 Ton sealing power "ITAL-PRESSE" machine, in H-13 steel one cavity molds.The injected pieces obtained showed better mechanical strength and corrosion characteristics when compared with the connection boxes which are now in the market.

[0016] It should be noted that the electrical connection boxes which normally could be manufactured by pressure-injection of aluminum, silicium or zamak, present various disadvantages in comparison with those made with the invented alloys, since the latter exhibit twice the mechanical strength of aluminum and thrice its corrosion resistance. With regard to zamak, the alloys of this invention have higher corrosion resistance and higher mechanical strength. Finally, in comparison with silicium and aluminum alloys, the alloys of this invention, in addition to the aforementioned advantages, have much better electrical conductivity.

[0017] On the other hand, the small grain-size of the pressure injected pieces made with the alloys of this invention, allow for a much higher detail resolution than those injected with conventional alloys, a better surface quality being obtained in the product, besides having much lower operating costs due to energy savings and other factors.

EXAMPLE 3.- SHEET ROLLING OF Zn-Al-Cu ALLOY

[0018] Various sheet rolling tests were conducted with the alloys of the invention,having a composition of Zn:76%, Al:22% and Cu:2%, the initial material being in the form of a plate, which was run for the purpose through a rolling mill.

[0019] Both the plate and the milling rolls were preheated to a temperature of appoximately 260° C. The dimensions of the plate were 17 x 60 x 90 cms. The plate was subjected to several sheet rolling operations, varying the number of rolling steps between 28 and 42, including water for cooling purposes in one or more steps.

[0020] Between the various sheet rolling steps, process temperatures were changed between 180° C and 295° C, in order to determine the optimal quantity of the product according to the number of rolling steps or series of steps, and according to the temperature levels.

[0021] Sheets having different thicknesses were obtained through these tests, said thicknesses varying between 3.5 and 8.55 mm., depending on the number of rolling steps. Said sheets were rolled-up and left to cool at ambient temperature.

[0022] The sheets that were obtained showed good cohesion, rigidity and uniformity characteristics, and when subjected to tests with 100 kg loads for 30 to 40 seconds, the results were: from 35 to 55 RB hardness, and between 150 and 350 MPa in tensile mechanical strength. It was also evidenced, through laboratory tests that the sheet obtained has superplastic characteristics, with the corresponding technological advantages in comparison with conventional copper and aluminum sheets.

[0023] In addition, through the above mentioned tests it was possible to determine that the material covered by this invention can be sheet-rolled as efficiently using the sheet-rolling lubricants traditionally employed and without using said lubricants, which constitutes an additional advantage of the alloys of this invention.

[0024] Although this invention has been described in reference to preferred embodiments, it may be put into practice by those who are skilled in the art in other ways, on the understanding that all such variations should be considered comprised within the spirit and scope of the above specification and examples, as well as within the scope of the appended claims.

Claims

1.- Zinc-based alloys, characterized in that the ratio of their composition is between 69 and 79 per cent by weight of Zinc, between 20 and 30 per cent by weight of Aluminum, and between 1 and 8 per cent by weight of Copper.

2.- The alloys referred to in Claim 1 above, characterized in that they additionally have incorporated therein between .001 and 1 per cent by weight of Magnesium.

3.- The application of the alloys referred to in Claims 1 or 2 to mold injection processes.

4.- The application of the alloys referred to in Claims 1 or 2 to extrusion processes.

5.- The application of the alloys referred to in Claims 1 or 2 to sheet-rolling processes.

6.- The application of the alloys referred to in Claims 1 or 2 to forging processes.

7.- The application of the alloys referred to in Claims 1 or 2 to wire drawing processes.

8.- The application of the alloys referred to in Claims 1 or 2 to metal stretching processes.