GB2234986A - Grain refining of copper-based alloys - Google Patents

Abstract

The invention provides a method of grain refining a copper-based alloy, especialy one having a beta phase as the primary phase on solidification, by arranging that a melt of the metal to be grain refined contains boron, and casting the boron-containing melt to produce grain refinement of the copper-based alloy. The invention is characterised by arranging that the melt which is cast also contains a reactive material (preferably calcium) which is more reactive with boron than boron is with iron. It has been found this makes the grain refinement process more tolerant of the presence of iron impurities, and that it also reduces the occurrence of hard spot defects in the cast product; and it has further been discovered that the incidence of hard spots can be eliminated, by casting from a sufficiently high temperature, e.g. from above about 1020 degrees C in the case of an alpha-beta brass. Grain refiners for practising the method are also disclosed.

Description

Grain Refining of Copper-Based Alloys This invention relates to grain refining metals, and is more especially concerned with grain refining copper-based metals.

The most common technique used in industrial practice to produce grain refinement of a metal is to add, to a melt of the metal, a small quantity of a suitable material, known as a grain refiner, which has the effect that, on casting the metal, it has a grain size which is substantially smaller than in cast products produced by casting from melts which have not been treated in this way. The technique is widely practised in the aluminium industry.

In the copper industry, it is at present used much less, but in recent years there has been an increasing interest in applying the technique to copperbased metals; see, for example, International Patent Specifications nos. WO 87/01138 and WO 87/02070.

It is known that certain copper-based metals, for example alpha-beta brasses, can be grain refined by adding boron to a melt of the metal to be grain refined, and then casting the thus-treated melt. The boron may be added to the melt by means a suitable master alloy, for example a copperboron alloy; copper-2 mass % boron alloys are commercially available for this purpose.

Unfortunately, known methods of grain refinement involving the addition of boron can give rise to the formation, in the grain refined alloy, of defects which are known as "hard spots1,. These hard spots are inclusions of impurities of very high hardness, and their presence in such alloys is highly undesirable, notably because of their detrimental effect on subsequent polishing.

In addition, it has been found that, while the presence of small quantities of iron does not actually prevent grain refinement from taking place, it does impair it, and once the level of the iron impurity exceeds a certain level (for example about 0.3 mass % in the case of alpha-beta brasses), then known grain refiners cease to provide a worthwhile effect at the low addition levels necessary to prevent hard spot formation.

According to the present invention, there is provided a method of grain refining a copper-based metal, by arranging that a melt of the metal to be grain refined contains boron, and casting the boron-containing melt to produce grain refinement of the copper-based metal, characterised by arranging that the melt which is cast also contains a material (herein referred to as the reactive material) which is more reactive with boron than boron is with iron and which does not poison the grain refinement ability of the boron.

It has been discovered in accordance with the invention, surprisingly, that by the inclusion in the melt of the active material, it is possible to reduce or even entirely eliminate the incidence of hard spots in the grain refined product. We believe that hard spots formed in a copper-based metal as a result of grain refinement involving the addition of boron consist predominently of iron boride particles. Further, without wishing to be bound by this theory, we believe that the reduction in the incidence of hard spot formation achievable by practice of the present invention is due to preferential formation of boride(s) of reactive material, which are less deleterious than the iron boride particles which otherwise would have formed.

It will be appreciated that the invention is of most benefit when applied to a copper-based melt which contains iron, because of the benefit it can then provide in eliminating or reducing the incidence of hard spots in the grain refined product. However, it can also be applied more generally, because of the reassurance it provides that the product will contain relatively few, if any, hard spots, even where the iron content is not known.

The method of the invention may be practised on any type of copper-based metal which is capable of being grain refined by known procedures involving the addition of boron to the melt before casting. In particular, the method may be practised on copper-based metals which are alloys having a beta phase as the primary phase on solidification, especially dual-phase copper-based alloys. Examples of particular types of dual-phase copper-based alloys to which the method of the present invention may be applied are: (a) Alpha-beta brasses Brasses are copper-based alloys which contain zinc. Apart from the incidental impurities, they may also contain small proportions of one or more additional alloying elements. Alpha-beta brasses are brasses whose zinc content (between about 30 and 45 mass %) is such that both alpha and beta phases are present.

(b) Aluminium bronzes These are copper-based alloys which contain, as main alloying elements, up to about 11 mass % aluminium, optionally with additional alloying elements such as one or more of iron, manganese and nickel, along with the incidental impurities. Normally, both the alpha and beta phases would be present.

We have made the further surprising discovery in accordance with the present invention that the method of the invention can be effective in the presence of iron at levels at which boron alone would be ineffective. Thus, for example, we have found that with alpha-beta brasses the method of the invention can be successfully practised even when the iron content in the melt being treated is greater than 0.3 mass %, and generally with iron levels up to about 1 mass Z. This is of substantial economic significance, because it means that grain refined products can be produced from less pure, and thus cheaper, starting materials.

Examples of reactive materials which may conveniently be used in the method of the invention are magnesium and calcium, calcium being preferred.

The amount of boron in the melt should be at a level sufficient to produce grain refinement. At the same time, one should avoid allowing the boron content to rise substantially above the level at which further boron additions fail to produce an increase in the grain refinement effect. In view of these considerations, we prefer that the melt should contain, immediately prior to casting it, 0.0003 to 0.005 (preferably about 0.0007) mass % of boron.

As a result of numerous tests, we have found that, to minimise the degree of hard spot formation, the atomic ratio of the reactive material to boron in the melt, immediately prior to casting it, should preferably be at least 1:6; and that the ratio desirably should be not more than 10:1. Thus, when the reactive material is calcium, and the boron level is within the preferred range stated in the previous paragraph, we prefer that the calcium level in the melt, immediately prior to casting, should be within the range from 0.0002 to 0.2 mass %.

Preferably, the reactive material is introduced into the melt by means of a master alloy. The use of a master alloy enables the reactive material to be added with good recovery, giving both economy and predictability of results.

The master alloy preferably comprises copper; this enables the alloy to be grain refined without substantially affecting its composition. Such a master alloy preferably also contains aluminium: introduction of aluminium in the small amounts resulting from the use of such a master alloy is generally not deleterious to the alloys of the kind to which the invention is applicable, and we have found that the presence of aluminium in the master alloy can, to a surprising degree, enhance the ability of the master alloy to introduce the reactive material into the melt. We have found that best results are obtained when the mass ratio of copper to aluminium in the master alloy is in the range from 45:55 to 55:45, preferably about 50:50.

Preferably, the master alloy also contains boron; for example in an amount of from 0.2 to 3 mass %. We believe that, when boron is introduced in this way, the reactive element can then provide maximum protection to the boron from iron impurities in the melt. However, boron may instead be added by means of a separate master alloy (eg copper-boron, or copper-boronaluminium), or by any other suitable means. Where the master alloy does contain boron as well as the reactive material, the information given above concerning the presence of copper and aluminium as preferred constituents of the master alloy will be equally applicable.

If it is desired to achieve a relatively high level of reactive material in the melt being treated (for example, where there is a high iron impurity level in the melt), then it may be convenient to add a master alloy containing reactive material but no boron, as well as a master alloy containing both boron and reactive material.

Although we prefer the use of one or more master alloys for introducing boron and reactive material into the melt, it is possible to add them by other means. For example, boron may be introduced by adding potassium borofluoride (KBF ); calcium may be added as the metal itself. In 4 particular, both boron and calcium could be added to the melt by introducing a mixture of KBF and calcium in a container comprising copper foil; or, one 4 or more of the materials mentioned in the previous sentence could be added in briquetted form, e.g. a briquette pressed from calcium with KBF or 4 copper-boron; or the boron and reactive material could be introduced into the melt as a compound of the boron and the reactive material, for example as calcium boride (CaB ), rather than as boron and reactive material 6 uncombined with each other.However, in practice, we have found that none of these methods is capable of introducing the reactive material and boron into the melt with as good, predictable recoveries as can be achieved by the use of one or more master alloys as described above and later in the description.

While the inclusion of the reactive material in the melt in accordance with the method of the invention will at least reduce the extent to which hard spots occur in the cast product, we have found that it does not always eliminate them entirely, even when the atomic ratio of reactive material to boron is within the preferred range of from 1:6 to 10:1. However, we have additionally discovered that it is possible, when using boron as a grain refiner for a copper-based metal, to eliminate the occurence of hard spots in the product substantially entirely, by arranging that the temperature from which the melt is cast is sufficiently high. In the absence of a reactive material in accordance with the method of the invention, the necessary temperature is so high that its use would introduce further problems which would make the measure industrially and commercially unacceptable.In particular, the use of such high temperatures would result in excessive damage to the refractories used for containing such melts, and also the energy costs would rise excessively. Furthermore, compositional changes, for example loss of zinc from brasses, become increasingly troublesome at higher temperatures.

A surprising additional benefit of the use of a reactive material in accordance with the method of the invention is that the temperature from which it is necessary to cast the melt to ensure that there will be substantially no hard spots in the cast product is substantially lower than it would have been in the absence of the reactive material. Thus, for example, we have found that where the melt is an alpha-beta brass and the reactive material is calcium present in an atomic calcium to boron ratio within the preferred range of 1:6 to 10:1, then the occurrence of hard spots can be eliminated substantially entirely if the melt is cast from a temperature greater than about 1020 degrees C.

The method of the invention is especially useful when the melt is cast to die-casting ingot form. Such ingots (which would normally be alpha-beta brass) can then be remelted and used to make water fittings, for example.

In such an application, the fine-grained properties of the alloy, with relatively low or zero incidence of hard spots, will give the product good polishing and plating properties.

The present invention also comprehends a grain refiner for use in the method of the present invention, the grain refiner comprising, in master alloy form, both boron and a material (herein referred to as the reactive material) which is more reactive with boron than boron is with iron, but which does not poison the grain refining ability of the boron.

As is known in the art, certain materials have a poisoning effect on boron, notably the transition elements generally, and in particular chromium, manganese, vanadium and, especially, zirconium, and care should be taken to ensure that the levels of any such poisoning materials in the grain refiner are not at a sufficiently high level to cause an excessive reduction in the performance of the grain refiner. Of course, the same considerations should also be taken into account in forming the melt which is to be grain refined.

In view of this, we prefer, in particular, that the grain refiner should be zirconium-free.

For reasons which will be clear from the foregoing disclosure relating to the method of the invention, the following are preferred features relating to the master alloy according to the invention: 1. that the reactive material should comprise magnesium or calcium, most preferably calcium; and 2. that the atomic ratio of the reactive material to boron should be at least 1:6, the atomic ratio preferably also being not more than 10:1.

The boron and the reactive material may, if desired, be contained in different master alloys. Thus, for example, the boron could be contained in copper-boron, or copper-aluminium-boron; and the reactive material could be contained in an alloy with copper (e.g. calcium-copper), or with aluminium (e.g. calcium-aluminium), or with both copper and aluminium (e.g. calciumcopper-aluminium). However, we prefer that they should both be contained in a single master alloy. Preferably, the single master alloy comprises copper, and preferably also contains aluminium, preferably being based on copper-aluminium, in which case the preferred mass ratio of copper to aluminium is in the range of 45:55 to 55:45, preferably being about 50:50.

The preferred boron content for the single master alloy is 0.2 to 3 mass %, preferably about 1.5 mass %. The calcium content for the single master alloy can usefully be from 0.1 to 10 mass %; preferably it is about 5 mass A particularly preferred grain refiner according to the present invention comprises a master alloy substantially consisting of about 1.5 mass Z boron, 5 mass % calcium, balance (apart from incidental impurities) copperaluminium, the mass ratio of copper to aluminium being about 50:50.As a result of extensive tests, we have found that such a master alloy has the following surprising and desirable combination of properties: (a) low melting point, resulting in easy dissolution of the master alloy in the melt being treated; (b) a sufficiently high density to enable the master alloy to be easily submerged into copper-based melts; and (c) friability, enabling the master alloy to be comminuted after casting, so that, when added to a melt to be treated, its dissolution into the melt will be achieved more easily.

This combination of properties results in surprisingly high, and predictable, recoveries of the calcium and boron in the melt. However, we believe that these properties are due to the fact that the master alloy is based on copper-aluminium, with the mass ratio of copper to aluminium being in the preferred range of 45:55 to 55:45, rather than to the particular percentages of boron and reactive material.

Where it is desired to achieve a relatively high calcium level in the melt being treated (for example, where there is a high iron impurity level in the melt), then the grain refiner may advantageously include both a single master alloy containg both boron and the reactive material and a master alloy containing the reactive material but no boron. In such a case, the statements made above relating to the former master alloy apply also to the latter master alloy, except for the absence of boron. In particular, an especially preferred form of the latter master alloy substantially consists of about 5 mass Z calcium, balance (apart from incidental impurities) copper-aluminium, the mass ratio of copper to aluminium being about 50:50.

Comminuted pieces of grain refiner in accordance with the invention can conveniently be introduced into the melt to be grain refined loose or with the pieces contained in a container made of material(s) (copper foil, for example) which is/are not deleterious to the melt being treated.

In many circumstances, it will be convenient to add the grain refiner in the form of a rod or cored wire. If in the form of a rod, it may consist of a single master alloy as described above. If in the form of a cored wire, the core may consist of such a single master alloy, preferably in comminuted form, or of a combination of different master alloys. A less preferred form of such a grain refiner is in briquetted form.

In order that the invention may be more fully understood, some embodiments in accordance therewith will now be described, by way of example only, with reference to the accompanying drawings, all of which are optical micrographs at a magnification of 53:1, except for those of Figs. 5 and 6, which are at a magnification of 106:1.In the drawings: Figs. 1 and 2 show an alpha-beta brass die casting alloy, respectively un grain refined, and grain refined by a method in accordance with the invention; Figs. 3 and 4 show the same alpha-beta brass alloy, respectively grain refined by a method in accordance with the invention, at a high grain refiner addition rate; and by a method in accordance with the known procedure of the prior art using copper-boron, at a similar addition rate; Figs. 5 and 6 show a dual-phase aluminium bronze alloy, respectively un grain refined, and grain refined by a method in accordance with the invention; and Figs. 7 and 8 show an alpha-beta brass die casting alloy which has a high iron content, respectively un-grain refined, and grain refined by a method in accordance with the invention.

Example 1 A grain refiner in accordance with the invention having the following composition was produced by a conventional melt alloying technique, followed by casting: 1.5 mass % boron 5.0 mass Z calcium balance: copper-aluminium (mass ratio 50:50), plus incidental impurities.

The master alloy was cast in the form of waffle plate, and was comminuted so as to have a particle size of 25 mm down.

15 kg of an alpha-beta die casting alloy having the following composition was melted in a pop-up induction furnace and brought to a temperature of just above 1020 degrees C: 38 mass % zinc 1.5 mass % lead 0.5 mass % aluminium 0.2 mass % iron, as an impurity balance: copper and additional impurities.

The master alloy was then added to the melt at an addition rate of 0.03 mass %, to give a concentration of boron of 0.0004 mass X, and of calcium of 0.0015 mass %. The addition was accomplished by dropping the master alloy onto the melt surface and stirring it once it had melted. Dissolution of the master alloy occurred within a few seconds. A sample of the melt was taken 5 minutes later and poured into a cylindrical graphite mould 150 mm in diameter by 150 mm high and having a cylindrical mould cavity 38 mm in diameter by 70 mm high, the sample being allowed to cool down to ambient temperature.

Next, a blank test was run by repeating the above-described test but without making any grain refiner addition.

In each case, the solidified sample was sectioned, polished, etched and examined. Figs. 1 and 2 show respectively the blank and the sample grain refined in accordance with the invention. The blank sample clearly has a prior beta grain size that is much coarser than that of the treated sample.

To aid interpretation, line 1 has been drawn in on Fig. 1 to show part of the boundary of a typical grain boundary of the un-grain refined sample, and line 2 has been drawn in on Fig. 2 to show the boundary of two adjacent and typical complete grains in the grain refined sample.

Example 2 The procedure described in Example 1 in relation to Fig. 2 was repeated, except that the rate of addition of the grain refiner master alloy was 0.10 mass %; this resulted in a concentration of boron of 0.0015 mass %, and of calcium of 0.005 mass %.

The test was then repeated, with the exception that, in place of the master alloy according to the invention, an addition of a proprietory copper-2 mass % boron master alloy was made in an amount such as to provide the same boron level in the melt, viz. 0.0015 mass In each case, a sample of the melt was sectioned, etched, polished and examined as in the Example 1. The respective samples are shown in Figs. 3 and 4. As can be seen, both samples were grain refined; the boundaries of two adjacent and typical grains are shown at 3 in Fig.3 and at 4 in Fig. 4.

However, the sample grain refined by the copper-boron master alloy had hard spots, a typical one being visible at 4a, whereas the sample grain refined by the calcium-containing master alloy in accordance with the invention was substantially free of hard spot defects.

Example 3 The procedure described in Example 1 in relation to Fig. 2 was repeated, but this time the melt was an aluminium bronze having the following composition: 9.45 mass % aluminium 2.69 mass % iron balance: copper and incidental impurities.

After the addition of the grain refiner in accordance with the invention, the concentration of boron in the melt was 0.0025 mass %, and that of calcium was 0.018 mass %. The test was repeated without any grain refiner additions. Samples from each test melt were taken as in Example 1. In each case, the solidified sample was sectioned, polished, etched and examined as in Example 1. Figs. 5 and 6 show respectively the blank sample and the sample grain refined in accordance with the invention. The blank sample clearly had a prior beta grain size that was much coarser than the treated sample. To aid interpretation, line 5 has been drawn in on Fig. 5 to show part of the boundary of a typical grain of the un-grain refined alloy, and line 6 has been drawn in on Fig. 6 to show the boundaries of two adjacent and typical complete grains in the grain refined alloy.

Example 4 The procedures described in Example 1 were repeated with the following exceptions: 1. The treated melt was a typical alpha-beta brass die casting alloy, having the following composition: 38 mass % zinc 2.0 mass % lead 0.4 mass % aluminium 0.28 mass % tin 0.40 mass % iron, as an impurity balance: copper and additional incidental impurities.

The relatively high iron content was not especially unusual for a die casting alpha-beta brass.

2. In the case where a grain refiner addition was made (in accordance with the invention), the grain refiner consisted of: (i) an addition of the same copper-aluminium-calcium-boron master alloy as used in Example 1, at the same addition rate, 0.03 mass Z; plus (ii) an addition of a master alloy having the following composition: 5.0 mass % calcium balance: copper-aluminium (mass ratio 50:50), plus incidental impurities.

Master alloy (ii) had been produced by a procedure analogous to that described in Example 1, and was used to increase the calcium level, in view of the high iron level. The addition rate was 0.27 mass %, which brought the calcium level in the melt to 0.015 mass Z.

Figs. 7 and 8 show respectively the blank and the sample grain refined in accordance with the invention. The blank sample clearly has a prior beta grain size that is much coarser than that of the treated sample. To aid interpretation, line 7 has been drawn in on Fig. 7 to show the boundary of a typical grain of the un-grain refined alloy, and line 8 has been drawn in on Fig. 8 to show three adjacent and typical grains in the grain refined alloy.

Claims (44)

Claims

1. A method of grain refining a copper-based metal, by arranging that a melt of the metal to be grain refined contains boron, and casting the boroncontaining melt to produce grain refinement of the copper-based metal, characterised by arranging that the melt which is cast also contains a material (herein referred to as the reactive material) which is more reactive with boron than boron is with iron and which does not poison the grain refinement ability of the boron.

2. A method according to claim 1, wherein the copper-based metal is an alloy which has a beta phase as the primary phase on solidification.

3. A method according to claim 2, wherein the copper-based metal is a dualphase copper-based alloy.

4. A method according to claim 3, wherein the copper-based metal is a dualphase aluminium bronze.

5. A method according to claim 3, wherein the copper-based metal is an alpha-beta brass.

6. A method according to claim 5, wherein the copper-based melt contains more than about 0.3 mass % of iron.

7. A method according to any one of claims 1 to 6, wherein the reactive material comprises magnesium.

8. A method according to any one of claims 1 to 7, wherein the reactive material comprises calcium.

9. A method according to any one of claims 1 to 8, wherein the melt contains, immediately prior to casting it, 0.0003 to 0.005 (preferably about 0.0007) mass % of boron.

10. A method according to any one of claims 1 to 9, wherein the atomic ratio of the reactive material to boron in the melt, immediately prior to casting it, is at least 1:6.

11. A method according to claim 10, wherein the said atomic ratio is not more than 10:1.

12. A method according to any one of claims 1 to 11, wherein the reactive material is introduced into the melt by means of a master alloy.

13. A method according to claim 12, wherein the master alloy comprises copper.

14. A method according to claim 13, wherein the master alloy also contains aluminium.

15. A method according to claim 14, wherein the master alloy is based on copper-aluminium.

16. A method according to claim 15, wherein the mass ratio of copper to aluminium is in the range from 45:55 to 55:45.

17. A method according to claim 16, wherein the mass ratio of copper to aluminium is about 50:50.

18. A method according to any one of claims 12 to 17, wherein the master alloy also contains boron.

19. A method according to claim 18, wherein the master alloy contains from 0.2 to 3 mass % boron.

20. A method according to any one of claims 1 to 19, wherein the melt is cast from a temperature which is sufficiently high to result in there being substantially no boride hard spots in the cast product.

21. A method according to claim 20, wherein the reactive material is calcium, the melt is an alpha-beta brass, and it is cast from a temperature greater than about 1020 degrees C.

22. A method according to any one of claims 1 to 21, wherein the melt is cast to die casting ingot form.

23. A method according to claim 1, and substantially as described in any one of the foregoing Examples 1 to 4.

24. A copper-based metal, whenever grain refined by a method in accordance with any one of claims 1 to 23.

25. A grain refiner for grain refining a copper-based metal, the grain refiner comprising, in master alloy form, both boron and a material (herein referred to as the reactive material) which is more reactive with boron than boron is with iron, but which does not poison the grain refining ability of the boron.

26. A grain refiner according to claim 25 which is substantially zirconiumfree.

27. A grain refiner according to claim 25 or claim 26, wherein the reactive material comprises magnesium.

28. A grain refiner according to any one of claims 25 to 27, wherein the reactive material comprises calcium.

29. A grain refiner according to any one of claims 25 to 28, wherein the atomic ratio of the reactive material to boron is at least 1:6.

30. A grain refiner according to claim 29, wherein the said ratio is not more than 10:1.

31. A grain refiner according to any one of claims 25 to 30, wherein boron and reactive material are contained in different master alloys.

32. A grain refiner according to any one of claims 25 to 30, wherein boron and reactive material are contained in a single master.

33. A grain refiner according to claim 32, wherein the said single master alloy comprises copper.

34. A grain refiner according to claim 33, wherein the said single master alloy also contains aluminium.

35. A grain refiner according to claim 34, wherein the said single master alloy is based on copper-aluminium.

36. A grain refiner according to claim 35, wherein the mass ratio of copper to aluminium is in the range of 45:55 to 55:45.

37. A grain refiner according to claim 36, wherein the mass ratio of copper to aluminium is about 50:50.

38. A grain refiner according to any one of claims 35 to 37, wherein the said single master alloy contains from 0.2 to 3 mass % boron.

39. A grain refiner according to claim 38, wherein the said single master alloy contains about 1.5 mass % boron.

40. A grain refiner according to any one of claims 35 to 39, wherein the said single master alloy contains about 5 mass % of calcium.

41. A grain refiner according to claim 40, comprising a master alloy substantially consisting of about 1.5 mass % boron, about 5 mass % calcium, balance (apart from incidental impurities) copper-aluminium, the mass ratio of copper to aluminium being about 50:50.

42. A grain refiner according to any one of claims 25 to 41, when in the form of comminuted pieces of the alloy.

43. A grain refiner according to any one of claims 25 to 42, when in the form of a rod or cored wire.

44. A grain refiner according to claim 25, and substantially as described in any one of the foregoing Examples 1 to 4.