Aluminum Alloy For Electric Conductor

Abstract

This aluminum alloy for electric conductor consists of a ternary Al-Mg-RE alloy containing less then 0.6 weight percent of one or more of rare earth metals and has improved castability, weldability, fatigue strength and antisoftening characteristics on heating at a high temperature and is applicable to the continuous casting and rolling method and the rolling method using large ingots for the purpose of easily manufacturing aluminum alloy conductor having excellent characteristics.

Description

This invention relates to improvements in aluminum alloys for electric conductors, more particularly to electric conductive aluminum alloys having electric conductivity equivalent to that of the conventional electric conductive aluminum (hereafter abridged as ECAl) and more excellent mechanical strength, fatigue strength and antisoftening characteristic at a high temperature than ECAl, and electric conductivity and mechanical strength equivalent to those of the conventional electric conductive binary Al-Mg alloy or ternary Al-Mg-Si alloy and higher castability, weldability, fatigue strength and antisoftening characteristic at a high temperature than these alloys and applicable to the continuous casting and rolling method or the rolling method using large ingots for the purpose of manufacturing electric conductive wires having improved characteristics.

Aluminum and aluminum alloys have recently been used not only for wires of transmission lines and distribution lines, but also for electric alloy conductive materials in various fields. Various characteristics which have not heretofore been considered are required for the aluminum and alloys thereof, in addition to such basic characteristics as electric conductivity, tensile strength, elongation and bendability, etc. For example, the conventional electric conductive aluminum (ECAl) used for transmission line and distribution line is deficient in fatigue strength and antisoftening characteristic at a high temperature so that improved electric conductive materials are desired. The electric conductive binary Al-Mg alloy and ternary Al-Mg-Si alloy have been used for special transmission line and distribution line, rotor bars for generator, windings for transformer, bus bars for electric resistance heating furnace, etc. These alloys, however, are deficient in weldability, fatigue strength and antisoftening characteristic at a high temperature and thus could not be used for a long duration and applied to welding operation for obtaining highly reliable welds. Thus, excellent weldability, fatigue strength and antisoftening characteristic at a high temperature have recently been required for the aluminum alloys.

Binary Al-Mg alloy and ternary Al-Mg-Si alloy used as the conventional electric conductive material contain a comparatively large amount of Mg, which fact not only makes degassing in molten state considerably difficult but also enlarges the solidification temperature range with the result that excessive unstable gas remains in their ingots and causes fine defects even when a high level of technique and skill is used.

Ingots having no defects could not be obtained by such method as the continuous casting and rolling method and the rolling method using large ingots wherein degassing of occluded gas becomes ineffective, which fact will decrease the fatigue strength of the final product. Thus, it is extremely difficult to apply the electric conductive binary Al-Mg alloys and ternary Al-Mg-Si alloys to the continuous casting and rolling method and the rolling method using large ingots. Moreover, the electric conductive binary Al-Mg alloy and ternary Al-Mg-Si alloy have disadvantages that they contain magnesium which is unstable and liable to be oxidized at high temperature and thus are inferior in weldability and could not be used for the conventional resistance welding in air for the purpose of obtaining highly reliable welds, and that they are difficult to use at a high temperature since their tensile strength remarkably decreases when they are heated at a high temperature of about 500.degree. C. As the result of our studies of aluminum alloys for electric conductors to obviate the above disadvantages we found that a ternary aluminum alloy consisting of 0.02-0.8 weight percent of magnesium, 0.6-3.0 weight percent of one or more rare earth metals such as cerium, lanthanum, neodymium, praseodymium, samarium, etc., or of any composition thereof such as Misch metal (all these are hereafter abridged as RE), and the remainder, aluminum, had excellent electric conductivity and tensile strength and was superior in heat resistance at a comparatively low temperature of the order of 150.degree.-200.degree. C. We have this Al-Mg-RE alloy registered as U.S. Pat. No. 3,278,300. This ternary alloy has an improved work hardening effect owing to minute dispersion of the intermetallic compound of Al-Mg-RE into the matrix of the aluminum solid solution, but has a disadvantage that it contains a comparatively large amount of expensive RE and makes the final product expensive. Moreover, this ternary alloy is comparatively difficult of degassing in casting and of application to the continuous casting and rolling method or the rolling method using large ingots for the purpose of obtaining ingots having no defects. This ternary alloy has good heat resistance at a comparatively low temperature of the order of 150.degree.-200.degree. C., but is not quite satisfactory in antisoftening characteristic at a higher temperature in the neighborhood of 500.degree. C., and moreover requires a fairly high level of technique to make good weld on it. Moreover, this ternary alloy has a disadvantage that its final product becomes hard and brittle when processed by a method in which its component inclusive of impurities, remain in the forced state of solid solution in the final product, such as continuous casting and rolling method, or by a method in which it is given a very high degree of cold working, such as rolling method using large ingots, thus making the manufacture of the electric conductive material by these two methods extremely difficult and increasing the working costs.

In order to obviate the above disadvantages, further researches were made on many aluminum alloys for electric conductors and the inventors have succeeded in obtaining excellent aluminum alloys for electric conductors, free from the above-mentioned disadvantages.

The principal object of the invention is to provide aluminum alloys for electric conductors which are equal in electric conductivity and superior in mechanical strength, fatigue strength, and antisoftening characteristic at a high temperature to any conventional conductive aluminum.

Another object of the invention is to provide aluminum alloys for electric conductors which are equal in electric conductivity and mechanical strength and superior in castability, weldability, fatigue strength, and antisoftening characteristic at a high temperature to the binary Al-Mg alloy or the ternary Al-Mg-Si alloy for electric conductors.

A further object of the invention is to provide aluminum alloys for electric conductors having improved characteristics and applicable to the continuous casting and rolling method or the rolling method using large ingots.

In other words, the inventors have further investigated the ternary Al-Mg-RE alloy disclosed in the U.S. Pat. No. 3,278,300 and found out that reduction in quantity of RE added to this alloy improves its castability, weldability, fatigue strength, and antisoftening characteristic at a high temperature and that said alloy can easily be made into electric conductive materials having improved characteristics by the continuous casting and rolling method or the rolling method using large ingots.

The inventors have succeeded in obtaining aluminum alloys which are equal in electric conductivity and mechanical strength and superior to castability, weldability, fatigue strength and antisoftening characteristic at a high temperature to the conventional electric conductive aluminum or the binary Al-Mg alloy or the ternary Al-Mg-Si alloy, by adding to aluminum 0.02-2.0 weight percent, preferably 0.05-1.5 weight percent, of magnesium and less than 0.6 weight percent, preferably 0.1-0.5 weight percent, of RE.

Addition of magnesium to the alloys of the invention increases mechanical strength which is suitable for the electric conductive conductive material, while addition of RE serves to stabilize the gas occluded in the molten alloy to improve the castability, and make the alloy fit for continuous dynamic casting and casting into large ingots. Moreover, the alloys of the invention have an advantage that their fatigue strength becomes improved owing to improved soundness of ingots, that magnesium occluded therein in the solid solution becomes thermally stabilized to remarkably improve the weldability and antisoftening characteristic at a high temperature, that the state of compositions in forced solid solution or the severe working does not increase brittleness of the alloy after the cold working, and that the electric conductive material can be manufactured from the alloys of the invention in any easy manner with the aid of the continuous casting and rolling method or the rolling method of using large ingots.

The reason for limiting the amount of magnesium to 0.02-2.0 weight percent is that in the amount of less than 0.02 weight percent, magnesium becomes stable in aluminum at a high temperature irrespective of addition of RE and thus makes it possible to effect casting and welding substantially without any difficulty but the mechanical strength becomes insufficient for the electric conductive material and that in case magnesium content is more than 2.0 weight percent, the alloys become unsuitable as the electric conductive material because of remarkable decrease in electric conductivity.

The reason for limiting the amount of RE to less than 0.6 weight percent is that although even such small amount of RE as recognizable by the analysis can remarkably improve castability, weldability, fatigue strength and antisoftening characteristic at a high temperature, addition of 0.6 weight percent or more of RE does not present any improvements in proportion to its high cost, instead, it tends to deteriorate castability, weldability, fatigue strength and antisoftening characteristic at a high temperature and the work hardening effect of the alloys is increased to an unnecessary extent, thus making it very difficult to manufacture the electric conductive material by the continuous casting and rolling method or the rolling method using large ingots and requiring a higher degree of cold working.

In the alloys of the invention, any one or more of rare earth metals such as Ce, La, Nd, Pr, Sm, etc., or any composition thereof, such as Misch metal (herein called "RE") can produce the same effect. The alloys of the invention may contain impurities not more than 0.5 weight percent of copper, not more than 0.6 weight percent of silicon, not more than 0.6 weight percent of iron, not more than 0.2 weight percent of zinc and not more than 0.1 weight percent of manganese without substantially impairing the characteristics thereof.

The preferable composition of the alloys of the invention, which are equal in electric conductivity and superior in mechanical strength, fatigue strength and antisoftening characteristic at a high temperature to the conventional electric conductive aluminum (ECAl), is 0.05-0.1 weight percent of magnesium, 0.1-0.3 weight percent of RE and the remainder, aluminum. The preferable composition of the alloys of the invention which are equal in electric conductivity and mechanical strength and superior in castability, weldability, fatigue strength and antisoftening characteristic at a high temperature to binary Al-Mg alloy for electric conductors, is 0.15-0.5 weight percent of magnesium, 0.1-0.5 weight percent of RE and the remainder, aluminum. The preferable composition of the alloys of the invention which are equal in electric conductivity and superior in castability, weldability, fatigue strength and antisoftening characteristic at a high temperature to the ternary Al-Mg-Si alloy for electric conductors, is 1.0-1.5 weight percent of magnesium, 0.1-0.4 weight percent of RE and the remainder, aluminum.

The improved castability of the alloys of the invention renders it possible to manufacture from these alloys electric conductive materials having excellent characteristics in an easy manner with the aid of the continuous casting and rolling method. That is, the electric conductive material having more excellent characteristics may easily be manufactured by a continuous casting and rolling method comprising continuous casting the alloys of the invention in molten state into a mould, solidifying by quenching the molten metal until its temperature becomes 300.degree.-580.degree. C., subjecting to the solidified casting at least 70 percent of plastic deformation while cooling at a cooling speed of at least 50.degree. C./min. to a temperature of at most 250.degree. C., and effecting subsequent cold working.

In the above-mentioned continuous casting and rolling method the reason for limiting the quenching temperature in case of solidifying the molten cast to 300.degree.-580.degree. C. is that in case of the temperature of more than 580.degree. C., the forced state of solid solution is not sufficient to improve the mechanical strength of the final product and that the temperature less than 300.degree. C. is too low to effect the subsequent rolling step. The reason for giving the solidified casting at least 70 percent of plastic deformation while cooling at a cooling speed of at least 50.degree. C./min. to a temperature of at most 250.degree. C. is that in case of effecting the plastic deformation it is necessary to effect rolling at a high temperature at the beginning of the rolling step and effect rolling at a low temperature at the end of the rolling step in order to prevent the defects in the material and thus improve the mechanical strength thereof, that at a cooling speed of less than 50.degree. C./min. the recovery of plastic strain takes place, thus decreasing the mechanical strength of wire rods, that less than 70 percent of plastic deformation makes it impossible to obtain mechanical strength necessary for the final product, and that at a temperature higher than 250.degree. C. at a time immediately after the continuous rolling step it is impossible to obtain not only sufficient mechanical strength of the wire rod but also uniform characteristics of the wire owing to difference in cooling speeds at several locations of coiled wire rod.

The most preferable conditions for the continuous casting and rolling method are a quenching temperature range of 400.degree.-500.degree. C., a cooling speed of 200.degree.-600.degree. C./min. during plastic deformation, a plastic deformation of at least 90 percent and a temperature immediately after the working effect of 180.degree. C. or under.

The alloys of the invention make also it possible to easily manufacture aluminum alloy conductors with the aid of the rolling method using large ingots, thereby improving the characteristics of the electric conductive material. That is, the aluminum conductor having improved characteristics such as tensile strength, fatigue strength, etc., may easily be manufactured by a rolling method using large ingots comprising casting the alloys of the invention in molten state into a large mold having a short side of at least 12.7 cm., reheating the casting obtained and subjecting hot rolling and subsequent cold rolling with flat rolls to sad casting to obtain a strip, and slitting said strip along the rolled direction thereof into bars or subsequently shaping said bars into wires of circular in section.

In the above-mentioned rolling method using large ingots the reason for limiting the dimension of the short side of the ingot to at least 12.7 cm. is that for the ingot having the short side of at least 12.7 cm. the characteristics of the electric conductive material are improved and the manufacturing cost becomes less expensive and that for the ingot having the short side of smaller than 12.7 cm. the characteristics of the electric conductive material are not improved and such small ingot can effectively be cooled by means of the mould to effect a proper degassing and thus obtain an ingot having substantially no defects, with the result that necessity of applying the alloys of the invention to such small ingot becomes meaningless.

The most preferable conditions for the above-mentioned rolling method using large ingots for the purpose of obtaining electric conductive material having improved characteristics in less expensive manner are a length of the short side of the ingot of 20.3-50.8 cm., a temperature for starting the hot rolling step of 400.degree.-500.degree. C., a reduction of hot-rolling step of at least 90 percent, a temperature at the end of the hot-rolling step of at most 350.degree. C., a reduction of cold rolling step of at least 30 percent, a size of the product at the end of the cold rolling step of 0.5-1.3 cm. and a reduction of shaping and drawing into wire is at least 50 percent.

It will be seen that the alloys of the invention have excellent castability so that they can be applied to the above-mentioned continuous casting and rolling method and rolling method using large ingots for the purpose of improving characteristics of the electric conductive materials manufactured. It is to be understood that the alloys of the invention may also be applied to the conventional hot working methods such as methods of extruding the alloys from billets rolling with grooved rolls, from square bars, etc. for the purpose of obtaining wires or bars having any desired dimensions. However, the above-mentioned continuous casting and rolling methods and rolling method using large ingots are most preferably applied to the alloys of the invention in order to further improve the characteristics of the electric conductive material manufactured.

The invention will be further explained in detail with examples.

EXAMPLE 1

By the use of alloys of the invention, conventional binary Al-Mg alloys for electric conductors (5005 alloy), conventional ternary Al-Mg-Si alloys for electric conductor (6063 alloy), and ternary Al-Mg-RE alloy containing excessive RE each having a composition shown in table 1, billets were produced and extruded into 5 mm..times.10 mm. bars. The electric conductivity and tensile strength of these bars were measured and the tension test at room temperature was carried out after heating the bars at 500.degree. C. for 50 hours. The results are shown in table 1. The RE was Misch metal available in market and was directly added to the aluminum base metal added with magnesium. ##SPC1##

As seen from the table 1, the alloys of the invention consisting of the binary Al-Mg alloy and less than 0.6 weight percent of RE have excellent tensile strength in case the electric conductivity is equivalent to that of the conventional electric conductive aluminum (ECAl) and extremely small rate of decrease in tensile strength after heating at a high temperature for many hours.

It is also apparent from the table 1 that the alloys of the invention have excellent tensile strength in case the electric conductivity is equivalent to that of the binary Al-Mg alloy for electric conductors such as 5005 alloy and that of the ternary Al-Mg-Si alloy for electric conductors such as 6063 alloy and extremely small rate of decrease in tensile strength after heating at a high temperature for many hours. On the contrary the conventional ternary Al-Mg-RE alloy for electric conductors, added with at least 0.6 weight percent of RE, begins to increase its rate of decrease of tensile strength after heating at a high temperature for many hours in case the addition of RE reaches at about 1.0 weight percent and the advantageous effect caused by the addition of RE could not be appreciated.

EXAMPLE 2

The alloys of the invention, conventional binary Al-Mg alloys for electric conductors and ternary Al-Mg-RE alloys for electric conductors containing excessive RE were made into billets which were extruded into 8-mm. diameter wires to measure the tensile strength at welds of each wire when the wire has been subjected to butt weldings for 100 times. The results are shown in table 2. ##SPC2##

It is clearly shown in the above table 2 that the weldability of the alloys of the invention is far better than that of the conventional binary Al-Mg alloy for electric conductors, that the mechanical strength at the welds is remarkably improved, and that the range of values of the mechanical strength is also decreased.

The weldability of the conventional ternary Al-Mg-RE alloy for electric conductors containing excessive RE is better than that of the conventional binary Al-Mg alloy for electric conductors, but is worse than that of the alloys of the invention. It seems that the addition of excessive RE causes bad influence upon welds owing to the presence of magnesium, RE and intermetallic compound thereof in aluminum.

EXAMPLE 3

Alloys of the invention, conventional binary Al-Mg alloys for electric conductors and ternary Al-Mg-RE alloys containing excessive RE each having a composition shown in table 3 and in molten state were subjected to degassing with the aid of chlorine gas and to holding and then cast into a metallic mold to obtain ingots. Comparison was made of weldability of these ingots by investigating the state of defects occurred in the ingots and by measuring specific gravity at the optimum portion in the ingots. The results are shown in table 3 in which A represents the sound state, B, the presence of small blowholes, and C, the presence of large blowholes. ##SPC3##

As seen from the table 3, the conventional occurrence Al-Mg alloys produce ingots with defects while the alloys of the invention containing small quantity of RE is capable of properly fixing gas remained in the metals to decrease occurrence of defects during solidification thus obtaining sound ingots without any defects. The conventional ternary Al-Mg-RE alloys contain a large amount of gas when they are in the molten state after the degassing treatment has been completed so that it becomes difficult to fix all of the gas by means of RE, thereby occurring blowholes.

EXAMPLE 4

The alloys of the invention, conventional binary Al-Mg alloys for electric conductors, and ternary Al-Mg-RE alloys containing comparatively large quantity of RE, each having a composition shown in table 4 were by the following methods.

1. Method in which the alloys were cast while cooling by water into 10 cm..times.10 cm. bars which were hot-rolled with the aid of grooved rolls into wire rods (hereinafter called "WRR method").

2. Method in which the alloys were continuously cast into 22 cm..sup.2 bars which successively were hot rolled with the aid of grooved rolls into wire rods (hereinafter called "CCR method").

3. Method in which the alloys were cast while cooling by water into 25.5 cm..times.76 cm. large ingots which were hot-rolled with the aid of flat rolls and then slit along the rolled direction into square bars which were subsequently shaped into wires of circular in section (hereinafter called "SRS method").

The wire rods manufactured by the above-mentioned three methods were made into wires of 3.5 mm. diameter by means of cold wire-drawing. The specific gravity of each ingot and the tensile strength, bending limit and fatigue limit strength of each wire were measured. The results are shown in table 4. ##SPC4##

It is apparent from the table 4 that for the conventional binary Al-Mg alloy for electric conductors the CCR and SRS methods make it more difficult to obtain the sound ingot than the WRR method while the bending characteristic of the worked wire becomes remarkably bad. On the contrary the alloys of the invention ensure stabilizing effect of the proper content of RE which permits of obtaining the sound ingots without any defects by means not only of WRR method but also of CCR and SRS methods. Moreover, the alloy wire of the invention manufactured by the CCR and SRS methods has more excellent fatigue limit strength than that of the alloy wire of the invention manufactured by the WRR method.

The alloy wire of the invention manufactured by the WRR, CCR and SRS methods has more excellent bending limit and fatigue limit strength than the conventional binary Al-Mg alloy for electric conductors. The conventional ternary Al-Mg-RE alloy containing comparatively large quantity of RE and manufactured by the WRR, CCR and SRS methods are capable of obtaining better ingots than the conventional binary Al-Mg alloy and has more improved tensile strength, bending limit and fatigue limit strength than the latter alloy. Those characteristics of the former alloy are worse than those of the alloy of the invention. It seems that the addition of excessive RE causes deterioration of the above-mentioned characteristics.

EXAMPLE 5

The alloys of the invention consisting of 0.1 weight percent of Mg, 0.5 weight percent of RE and the remainder, aluminum in the molten state were subjected to the continuous casting and rolling method based on the manufacturing conditions of the CCR method shown in table 5 and then cold-drawn into 4.0 mm. diameter wires to measure numbers of detecting defects during the wire drawing step and characteristics of the wire manufactured. The results are shown in table 5. ##SPC5##

As seen from the table 5, the wire manufactured from the alloys of the invention in accordance with the CCR method which lies within the scope of the invention is capable of preventing defects from occurring during the wire-drawing step and has excellent tensile strength and bending limit. On the contrary the wire manufactured from the alloys of the invention in accordance with the CCR method which lies out of the scope of the invention, that is, in case the casting temperature is too high or too low or the cooling speed during the rolling is too low or the temperature of the material immediately after the working is too high, is not capable of preventing defects from occurring during the wire drawing and deteriorates the tensile strength and bending limit thereof.

EXAMPLE 6

The alloys of the invention consisting of 0.4 weight percent of Mg, 0.1 weight percent of RE, and the remainder, aluminum in the molten state were subjected to water-cooled casting step to obtain ingots having dimensions shown in table 6. The ingots obtained were heated again and made into strips by hot-rolling and cold rolling steps. The strips were slit along the rolled direction into bars which were shaped by grooved rolls into wires of circular in section to measure the tensile strength and fatigue limit strength. The results are shown in table 6. ##SPC6##

It is apparent from the table 6 that the wire manufactured from the alloys of the invention in accordance with the SRS method which lies within the scope of the invention has excellent tensile strength and fatigue limit strength, and that the wire manufactured from the alloys of the invention in accordance with the SRS which lies out of the scope of the invention deteriorates the tensile strength and fatigue limit strength owing to insufficient rate of cold working.

As explained hereinbefore the alloys of the invention consisting of the conventional binary Al-Mg alloy containing small quantity of RE ensures remarkable improvement as aluminum alloy conductors in castability, weldability, fatigue strength and antisoftening characteristic at a high temperature without impairing their basic characteristics such as electric conductivity, tensile strength, bendability, etc. and are applicable to the continuous casting and rolling method and rolling method using large ingots for the purpose of easily manufacturing aluminum alloy conductor having improved mechanical strength and antisoftening characteristic at a high temperature and further provide the important advantage that the alloy of the invention and the alloy conductor manufactured from the alloy by means of the continuous casting and rolling method and rolling method using large ingots are suitable not only for transmission line and distribution line but also for windings of dynamoelectric machine and transformer and for bus bars for high-temperature application, that said alloy and alloy conductor can effectively be manufactured without using a large amount of expensive RE, and that said alloy and alloy conductor are so excellent in weldability that reliable welds can be obtained.

Claims

What we claim is:

1. Aluminum alloy for electric conductor consisting of from 0.02 to 2.0 weight percent of magnesium, from 0.04 to 0.57 weight percent of one or more of rare earth metals, and remainder of aluminum.