CA2046372C - Copper-based alloy - Google Patents

Abstract

A copper alloy consisting of between 0.01 and 0.05% by weight tellurium and 0.001 and 0.01% by weight phosphorus is made by forming a hot melt of the alloy, casting the melt so that the melt solidifies rapdily at a speed of greater than 1.5 mm per second, with a high cooling rate which is greater than 20°C per second through the whole thickness of the casting.

Description

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DESCRIPTION OF INVENTION
"Improvements in or relating to a copper-based alloy"
THE PRESENT INVENTION relates to a copper-based alloy and more particularly relates to a copper-based alloy suitable for use in producing a radiator strip for an internal combustion engine. A radiator strip for an in-ternal combustion engine comprises a thin strip or ribbon of copper or copper alloy which is folded back upon itself and inserted between parallel tubes extend-ing between a header tank and a base tank of a radiator, which is a simple heat exchanger. This design of radi-ator is well-known, especially for use on motor cars and other vehicles, and many attempts have been made to pro-vide a suitable material for the copper strip enabling the copper strip to be very thin, but providing the copper strip with corrosion-resistance properties, and good thermal conductivity properties. Also, it is pref-erable that the alloy should have a softening temper-ature Which is as high as possible, since the strip may be subjected to high temperatures, and should not soften at those temperatures.
It is a well known fact that small additions of tellurium in solid solution or as small precipitates can increase the softening or re-crystallisation temper-ature of copper, without significantly lowering the thermal conductivity. Initially, to obtain the high softening temperature with copper including tellurium, ;. ; - , , , c' !i' n ~~ "~ fl, ~~.~ ~~ ~ ,e it was necessary to conduct an annealing step at a high temperature for a period of one hour followed by a rapid quench and subsequent cold working. In recent years, two U.S. P,atents have been published relating to this art. U.S. Patent 4,650,650 uses a copper alloy with 25-225 ppm tin and 25-225 ppm selenium or tellurium, together with 10-50 ppm of phosphorus. A solution an-nealing is necessary according to the teaching of this Patent. U.S. Patent 4,704,253 discloses a copper alloy with 10-150 ppm tellurium and 20-110 ppm phosphorus. In this case the hot rolling of a small ingot is followed by a rapid cooling. These techniques give rise to a .
material that may be brittle and which does not have the .
desired properties.
According to this invention there is provided a method of producing a copper alloy consisting of copper, between 0.001 and 0.05 by weight of tellurium, and be-tween 0.001 and 0.01°b by weight phosphorus, together with the incidental impurities, the method comprising the step of forming a hot melt of the alloy composition, casting the melt so that the melt solidifies rapidly at a speed of greater than 1.5 mm per second, with a high cooling rate, greater than 20°C per second through the whole thickness of the casting.
Preferably subsequently the casting is sub-jected Lo a rapid annealing at a high temperature greater than 700°C.
Conveniently the annealing step is carried out at; a temperature between 700 and 900°C.
Advantageously the annealing is carried out for a period of one second.
Preferably a cold rolling step is carried out between the casting step and the annealing step.
Conveniently the cold rolling step reduces the thickness of the casting by between 70 and Preferably a temper cold rolling step is carried out after the annealing step.
Advantageously the temper cold rolling step reduces the thickness of the alloy by between 8 and ~15~.
Preferably the hot melt is established by melt-ing the appropriate raw material and adding the tellur-ium just before the casting step.
Conveniently the tellurium is encapsuled in copper, the capsules being immersed under the surface of the melt just before casting the melt.
Preferably the casting is in the form of a strip or slab having a thickness of 20 to 30 milli-metres.
Advantageously the tellurium content of the alloy is 0.01 to 0.03 percent by weight.
Preferably the phosphorus content of the alloy is 0.002 to 0.006 percent by weight.
This invention also relates to an alloy when-ever made by a method as described above, and also relates to a heat exchanger incorporating a strip-of such an alloy.
In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way 4~ ~'' '~'~ i ~.~s~~d _!E_ of example, with reference to the accompanying drawings in which FIGURE 1 is a graphical figure illustrating the conductivity change during processing of three alloys respectively comprising, in parts per million, teller-ium 230 and phosphorus 30, tellurium 200 and phos-phorus 50, and tellurium 300 and phosphorus 60, FIGURE 2 illustrates softening curves for three alloys comprising, in parts per million, teller-ium 310 and phosphorus 60, tellurium 190 and phos-phorus 70 and tellurium 330 and phosphorus 60, and FIGURE 3 illustrates the properties of alloys of this invention, when compared with the properties of alloys provided by the prior art.
The present invention seeks to provide an alloy of phosphorus de-oxidised copper with 0.001-0.05 weight percent tellurium, preferably 0.01-0.03 weight percent tellurium and with 0.001-0.01 weight percent phos-phorus, preferably 0.002-0.006 weight percent phos-phorus.
It is known that the boiling temperature of tellurium is lower than the melting temperature of copper. Thus, if tellurium is simply added to a melt of copper, the tellurium will boil and a significant portion of the tellurium will be lost due to evapor-ation, thus giving inaccuracy in the final composition.
In the present invention, the necessar y raw material For making the alloy is melted in a con-ventional induction Furnace, and is de-oxidised by phos-phorus protected by a charcoal layer. The tellurium is added to the melt 3ust before the melt is cast. The _5_ tellurium can be added in any appropriate form, either a pure metal or as an alloy in copper, but preferably the tellurium is in the form of pure tellurium encapsuled in a small diameter copper tube. One end of the tube is inserted under the surface of the melt in the launder, and the tube is driven into the melt at the same rate that the tube is melted within the melt. This is ac-complished in the launder just before the casting step to avoid evaporation of the tellurium. A stirring equipment provides a homogenous melt. The melt is at a conventional temperature approximately 1120°C.
The melt is cast in a continuous production strip casting process, with a water cooled mould des-igned to obtain a very rapid cooling rate. The strip is east to have a thickness of 20-30 millimetres, although the strip may have any appropriate width. Rapid solid-ification is thus achieved, at a solidification rate equal to or greater than 1.5 millimetres per second, with a high cooling rate, which is greater than equal to 20oC per second through the whole thickness of the slab or strip.
Because the tellurium is added to the melt at the last possible moment, and also because the melt is then cooled very rapidly and is also solidified very rapidly, only a minimum amount of tellurium vapourises, thus ensuring that the final cast material has precisely the desired quantity of tellurium. The tellurium is found to be well distributed in the east material, thus providing a homogenous alloy.
It is to be appreciated that tellurium has a very low solubility in copper. Thus the solubility at , 800oC is 0.0075 weight percent, at 700°C is 0.0015 weight percent and at 600°C is 0.0004 weight per-cent. Tellurium segregates strongly in copper as the ~,~ . ., i .r copper is cooled and tends to be precipitated at grain boundaries. This may cause brittleness in the material.
Consequently, in the present invention, the solidifi-cation and cooling rates are high in order to get as much tellurium in solid solution as possible, and to avoid harmful precipitation of tellurium at grain bound-aries and/or in segregation patterns. It has been found that with the solidification rate of greater than 1.5 millimetres per second and the described cooling rate in excess of 20°C per second (corresponding to seg-regation distances of less than 10 to 17 microns) a fine distribution of the alloying elements is obtained in solid solution, with a nucleation of a finely dispersed precipitation. Furthermore, the rapid cooling ob-structs the diffusion of tellurium and growth of pre-cipitates and the fine dispersion is frozen-in.
Subsequently, the strip is subjected to a cold rolling step to a near finishing dimension. This roll-ing is carried out at a low temperature to avoid the diffusion and coarsening of precipitates that may occur at a higher temperature. The rapidly solidified struc-ture is not found to be brittle after the cold rolling has been performed, because the grain boundary part of the precipitates is minimised in this way. A cold roll-ing with more than 90~ reduction of thickness is poss-ible.
To obtain the right delivery temper, the strips are annealed before a final finishing rolling. The strip annealed in a high-speed strand nealer, which raises the temperature of the strip to a very high temp-erature, which is greater than or equal to 700°C, for a very short period of time, Such as approximately one second. The preferred annealing temperature is in the range of 700°C to 900°C. By utilising this tech-nique, the diffusion of tellurium can be controlled so (.'. s _., ~,, -7- . ~,~ Cj ' d that only a minor part of the tellurium is precipitated from solid solution. Consequently, the thermal con-ductivity is found to be increased as compared with prior art proposals, but without any decrease in the softening temperature. The amount of tellurium that is precipitated can only diffuse a very short distanee~dur-ing this rapid annealing, and this actually contributes to a very finely dispersed precipitation, which can even cause an increase of the softening temperature after the final temper rolling.
The described annealing step takes place in an oxide preventing atmosphere and thus the strip does not need subsequent pickling and is not depleted of the alloying element tellurium. The grain size after an-nealing may be of the order of 10 microns.
Subsequently, the strip is given a final cold rolling to give the strip the right temper before the strip is utilised in the production of a radiator for a motor vehicle. A thickness reduction of between 8 and 45% may be effected at this stage.
Various samples of alloys in accordance faith the invention have been made, and properties of the alloys have been measured. Electrical conductivity has been measured in percent IACS (International Annealed Copper Standard) where 100 IACS equals the electrical conductivity 58 m/ohm.mm2 and corresponds to a resist-ivity of 0.01724 ohm/mm2 metre (equals 17.24 nano ohm metre). It is to be appreciated that "percent IACS" is a very well established unit in this art.
Referring to Figure 1 of the accompanying drawings, the electrical conductivity of three alloys in accordance with the invention is shown in a graphical form. It is found that the electrical conductivity in J ' 'J

the "as east" state is approximately 96~ IACS. The cold rolling decreases conductivity according to the degree of reduction. With a 99~ reduction conduetivities of the order of 92~ IACS are typical. During the rapid an-nealing step the conductivity increases, and indeed the level of conductivity rises to a level of approximately 98% IACS which is greater than the conductivity of the alloy as cast. The finishing cold rolling, which may give a reduction of typically 30%, decreases the con-ductivity to some extent, but the final value is found, in the examples given, to be greater than or equal to 94% IACS. This is a very good conductivity.
Figure 2 illustrates the softening curves of three alloys, showing the hardness measured in terms of "HV" plotted against temperature during two-minute an-nealing. In each case the initial hardness is 105 or 115, and the temperature for 50~ softening is, in each case, greater than 450°C. This is known as the half-softening temperature. The high softening temper-ature shows that the tellurium content has been kept in solid solution and small, finely distributed precipi-tants of tellurium are present, which effectively hinder re-crystallisation.
A further prior art document that is of rele-vance, in addition to the two U.S. Patents discussed above, is J.S. Smart and A.A. Smith, Effect of Certain Fifth-period Elements on some Properties of High-Purity Copper. AIME Trans. Inst. Metals 152 (1943) which pro-vides a teaching relating to a copper alloy which in-cludes tellurium. The alloys taught in this document have a maximum half-softening temperature of 430°C, whereas the alloy of U.S. Patent 4,650,650 -has a half-softening temperature of 415°C and the alloy of U.S. Patent 4,704,253 has a half-softening temperature of 400°C.

- ~ ~ '' '.'~ ~:i Figure 3 is a graphical figure plotting four areas. The graph plots half-softening temperature in degrees C against electrical conductivity in per-cent LACS for various alloys. The large area indicates the properties of alloys in accordance with the inven-tion, and the smaller numbered areas indicate the prop-erties of alloys as disclosed by Smart and Smith, and in U.S. Patent 4,650,650 and 4,704,253. It can be seen that the present invention provides an alloy which has an improved half-softening temperature without any sig-nificant reduction in electrical conductivity.

Claims (10)

1. A method of producing a copper alloy consisting of copper, between 0.001 and 0.05% by weight of tellurium, and between 0.001 and 0.01% by weight phosphorus, together with the incidental impurities, the method comprising the steps of forming a hot melt of the alloy composition, casting the melt so that the melt solidifies rapidly at a speed of greater than 1.5 mm per second, with a high cooling rate greater than 20°C per second, through the whole thickness of the casting, cold rolling the casting and subjecting the cold rolled casting to a rapid annealing at a high temperature greater than 700°C, for a period of one second.

2. A method according to claim 1, wherein the annealing step is carried out at a temperature between 700 and 900°C.

3. A method according to claim 1 or 2, wherein the cold rolling step reduces the thickness of the casting by between 70 and 99%.

4. A method according to claim 1, 2 or 3, wherein a temper cold rolling step is carried out after the annealing step.

5. A method according to claim 4, wherein the temper cold rolling step reduces the thickness of the alloy by between 8 and 45%.

6. A method according to any one of claims 1 to 5, wherein the hot melting is established by melting copper and adding the tellurium just before the casting step.

7. A method according to claim 6, wherein the tellurium is encapsulated in copper, the capsules being immersed under the surface of the melt just before casting the melt.

8. A method according to any one of claims 1 to 7, wherein the casting is in the form of a strip or slab having a thickness of 20 to 30 millimeters.

9. A method according to any one of claims 1 to 8, wherein the tellurium content of the alloy is 0.01 to 0.03 percent by weight.

10. A method according to any one of claims 1 to 9, wherein the phosphorus content of alloy is 0.002 to 0.006 percent by weight.