Improvements on an extrusion process of zinc-based alloys

Abstract: This process consists of the following steps: 1)- specil Zn-Al-Cu alloys, which comprise in percent amounts between 69-79 Zn, 20-30 Al and 1.0-8 Cu, are pre-heated; 2)- then extruded within a temperature range between 176°C and 275°C, at a speed comprised between 1 and 20 meters per minute; 3)- straightening thereafter the extruded shape, and letting same cool at ambient temperature.

Description: The present invention relates to an improved process for extruding shapes from zinc-based alloys. Zinc and zinc-based alloys have traditionally been considered unsuitable for extrusion processes, due to their mechanical characteristics. Numerous attempts have heretofore been made to develop zinc alloys having improved properties, in order to render them appropriate for extrusion, rolling and other industrial processes, for the production of formed articles. The present invention relates to improvements on extrusion process of superplastic zinc-based alloys with enhanced mechanical properties, by which extrusion is accomplished by controlling the superplastic behavior throughout the extrusion process by preventing the material to exceed the eutectoid temperature, avoiding thereby the γ -phase transformation, and guaranteeing the dimensional stability of the final product. Various improvements have been made on zinc-based alloys, to render them suitable for the production of formed articles. Thus, Hare U.S. Patent No. 3,676,115 ; Chollet et al U.S. Patent No. 3,741,819 ; Winter et al U.S. patent No. 2,169,441 ; Gervais et al U.S. Patent 3,793,091; Wayne U.S. Patent No.3,850,622 ; and Dollar U.S. Patents Nos. 3,966,505 and 4,029,525, refer to zinc-based alloys having improved physical properties, as well as to the various methods of manufacturing them. These alloys have proved to be suitable for different industrial processes, such as die-casting, rolling and extrusion. In particular, Mexican Patent No. 161483, issued to Universidad Nacional Autonoma de México (National Autonomous University of Mexico) refers to a low cost extrusion process of zinc-aluminum-copper alloys, whereby structural and architectural shapes having enhanced mechanical properties are obtained. The extrusion process covered by Mexican Patent No. 161483 uses zinc-based alloys comprising 69 - 77 Zn, 20-30 Al, and 1-8 Cu percentage weight, with trace impurities. The process essentially consists of pre-heating the alloy ingots within a temperature range of 150-350°C, extruding at speed between 1 and 20 meters/minute, straightening the extruded shapes immediately thereafter, and cooling same at ambient temperatures. The European Patent Application 0 216 519 refers to a method of making a superplastic zinc/aluminium alloy which is highly plastic at temperature of 240°C or above and which can be extruded at 150°-225°C. The present invention relates to improvements on an extrusion process for zinc-based alloys. More specifically, the present invention refers to improvements on an extrusion process of zinc-based alloys, such improvements consisting, on one hand, of preventing the temperature of the material from exceeding the eutectoid temperature, both during the pre-heating of the alloy ingot and during the whole extrusion process, and thus preventing the material from undergoing the γ - phase transformation, and on the other, subjecting the extruded product to cooling by forced-air and/or water at the exit of the extrusion die, in order to ensure that the extruded product is maintained out of the superplastic γ -phase, that is, within the α + β -phase, and lastly, receiving the extruded product on a conveyor table, in order to ensure the dimensional stability of the extruded product, prior to its straightening. Through pilot tests of the extrusion of Zinc based alloys applying the process covered by Mexican Patent No. 161483, it has been found that the extruded products obtained thereby present dimensional stability problems, due to the superplastic behavior of the material. In such tests, it has been determined that some regions of the material present the γ -phase, with superplastic behavior, characteristic of the Zn-Al eutectoid alloys, at the exit of the extrusion die, which in combination with the friction forces exerted upon the material by the receiving table, deform the extruded products. To overcome the above mentioned deficiencies, the present inventors have developed various improvements on the extrusion process of Zinc-based alloys, whereby optimal industrial results have been achieved. Such optimal industrial results have been obtained by restricting the temperature range throughout the entire extrusion process, to less than the eutectoid temperature of the Zinc-Based alloys. Also, said improved industrial results have been attained by closing the extrusion temperature range to within 176°C and 273°C. Additionally, the extrusion process for these alloys has been improved by cooling the extruded products with forced air and/or water, as the products leave the extrusion die, in order to prevent the material from reaching the eutectoid temperature. Finally, in order to avoid friction forces on the extruded products at the receiving table, the present inventors have devised an improvement consisting of utilizing a conveyor to maintain the product configuration uniform, thus preventing any dimensional variations an the final product. The present invention provides a process for extruding shapes from zinc-based alloys, of the type comprising preheating the alloyed ingots as well as the container and the extrusion die; extruding such ingots at a speed comprised between 1 and 20 meters per minute and applying the corresponding pressure loads; straightening thereafter the extruded shape, and letting same cool, characterized by: (i) selecting the temperature range of the preheating step and the extrusion step between 176°C and 273°C, while restricting the temperature range during the extrusion process to below the eutectoid temperature, (ii) applying rapid cooling with forced air and/or water to the extruded products as they leave the extrusion die, and (iii) using a mechanical conveyor to receive the extruded products as they leave the extrusion die prior to the straightening step, for maintaining the product configuration uniform. Preferably, said alloys are Zn-Al-Cu alloys and have the following formula by weight: 69-79 Zn, 20-30 Al, and 1.0-8 Cu. The improvements convered by this application will be better understood, by referring to the Zn-Al phase diagram of Fig. 1 annexed hereto, wherein it is shown that, for the chemical composition range of the eutectoid alloy referred to in Mexican Patent No. 161483, the superplastic γ -phase occurs at temperatures above 273°C, and in order to ensure that the extrusion process takes place in the non-superplastic range which corresponds to the α + β -phase, the working temperatures must be maintained below such eutectoid temperature. It must be noted that although the binary phase diagram of Fig. 1 refers only to the Zn-Al alloy, this eutectoid alloy is the base for the Zn-Al-Cu alloy referred to herein. The addition of copper in small amounts does not significantly affect the phase transformation temperature. Example 1 A Zn-Al-Cu alloy with nominal composition of Al-21%, Cu-2%, Zn-balance was extruded in a 1,800 ton aluminum extrusion machine, applying the parameters referred to in Mexican Patent No. 161483. The products obtained at the exit of the extrusion die presented high ductility characteristics, which resulted in poor configuration control, the product presenting dimensional irregularities including undulations and bulging. Example 2 For comparison purposes, a similar test using the same Zinc-based alloy was performed in the same extrusion machine of Example 1, applying the improvements of this patent application, that is, maintaining the temperature below 273°C throughout the process, cooling the extruded product rapidly at the exit of the extrusion die with forced air, and using a mechanical conveyor to receive the product as it left the extrusion die, prior to straightening. As a result of the improvements incorporated to the extrusion process, extruded products having optimal mechanical characteristics, including shape and dimensions within tolerances, were obtained in this test. Through the above comparative industrial test of this Example, it becomes evident that, with the improvements covered by this application, an extrusion process of Zinc-based eutectoid alloys, examplified, but not limited to the Zn-Al-Cu alloy, is acccomplished with optimal industrial results.

Claims: Process for extruding shapes from zinc-based alloys, of the type comprising preheating the alloyed ingots as well as the container and the extrusion die; extruding such ingots at a speed comprised between 1 and 20 meters per minute and applying the corresponding pressure loads; straightening thereafter the extruded shape, and letting same cool, characterized by: (i) selecting the temperature range of the preheating step and the extrusion step between 176°C and 273°C, while restricting the temperature range during the extrusion process to below the eutectoid temperature, (ii) applying rapid cooling with forced air and/or water to the extruded products as they leave the extrusion die, and (iii) using a mechanical conveyor to receive the extruded products as they leave the extrusion die prior to the straightening step, for maintaining the product configuration uniform. Process according to claim 1, characterized in that the alloys are Zn-Al-Cu alloys and have the following formula by weight: 69-79 Zn, 20-30 Al, and 1.0-8 Cu.

Inventors: GABRIEL TORRES V, RAMON GALVAN C, GABRIEL, TORRES V., RAMON, GALVAN C.
Applicants: FALMEX S A DE C V, FALMEX S.A. DE C.V.
Application year: 1992
Main classification: C22F1
IPC codes: C22F1, C22C18, B21C23