GB2169616A

GB2169616A - Grain refining metals

Info

Publication number: GB2169616A
Application number: GB08521133A
Authority: GB
Inventors: Winifried Reif; Magdy Abdel-Reihim; Mutiul Abidi; Gerhard Weber
Original assignee: London and Scandinavian Metallurgical Co Ltd
Current assignee: London and Scandinavian Metallurgical Co Ltd
Priority date: 1985-01-11
Filing date: 1985-08-23
Publication date: 1986-07-16
Also published as: GB8500768D0; GB8521133D0

Abstract

A tin-based or lead-based alloy (e.g. a bearing alloy) which is of a composition such that, on solidifying from a melt, it becomes dispersion strengthened by particles comprising SbSn cuboids and/or Cu6Sn5 needles is grain refined by arranging that the alloy, when in a molten condition prior to casting, contains nucleant particles of PbSe and/or PbTe, and that the melt is solidified so that the nucleant particles act as nucleant sites for the SbSn cuboids and/or Cu6Sn5 needles, to produce refinement of the SbSn cuboids and/or Cu6Sn5 needles. The Se and/or Te can be added as such or as a suitable master alloy, e.g. an antimony-based master alloy.

Description

SPECIFICATION Grain refining metals This invention relates to grain refining metals, and in particular relates to grain refining dispersion strengthened tin-based or lead-based alloys, and especially to grain refining alloys of those kinds which are bearing alloys.

Microstructural refinement of bearing alloys can give rise to considerable improvements in several properties of the alloys, for example casting properties as well as processing properties such as fatigue strength, hardness, wear resistance and ductility: (1) Abdel-Reihim, M., Abidi, M., Reif, W. and Weber, G., Metall 38 (1984) p.p. 729 - 734.

The mechanical properties are a function of the size of the particles and their distribution in the microstructure. In the case of a hard phase embedded in a softer matrix, i.e. dispersion hardening, the resulting properties are derived according to the rule of mixture: (2) Abdel-Reihim, M. Taha, M. and Farag, M., Aluminium 52 (1976) p.p. 259 - 264.

In reference (1), the inventors of the present invention have shown that good grain refinement of various lead-based and tin-based bearing alloys can be achieved by the addition of Ag2Se, electron probe microanalysis (EPMA) revealing that the crystallisation centres contained Ag and Se. They also showed in that work that the same bearing alloys and also Cu-Pb-Sn bearing alloys can be grain refined by means of ultrasonic treatment.

Ag2Se is rather expensive, and the use of ultrasonics requires the provision of extra equipment, involving extra expense. For these reasons, it would be desirable to be able to achieve grain refinement of the above-described kinds of tin-based and lead-based alloys at reduced cost.

The present inventors have now discovered, in accordance with the present invention, that tin-based and lead-based alloys which are of a composition such that, on solidifying on cooling from a melt they become dispersion strengthened by dispersed particles comprising SbSn cuboids and/or Cu6Sn5 needles, can be grain refined, by arranging that the alloy when in a molten condition prior to casting, contains nucleant particles of PbSe and/or PbTe, and that when the melt solidifies, the nucleant particles act as nucleant sites for the SbSn cuboids and/or CusSn5 needles, to produce refinement of the SbSn cuboids and/or Cu6Sn5 needles. The refinement of the latter grains, in turn, results in reduction of the grain size of the matrix.Thus, the present invention makes possible grain refinement of an important class of leadbased and tin-based alloys at relatively low cost, and without the need for substantial investment in extra equipment.

At present we envisage that the main application of the invention will be in grain refining tin-based or lead-based alloys which are of the kind described in the previous paragraph and which are bearing alloys, and the following description is given with reference to the grain refining of such bearing alloys.

However, it will be appreciated that the alloys which are to be grain refined by the method of the present invention will not necessarily be bearing alloys.

Examples of bearing alloys which are suitable for being grain refined by the method of the invention are bearing alloys in accordance with German Industrial Standard DIN 1703.

A preferred kind of lead-based alloy which may be grain refined by the method of the invention is a lead-based alloy which includes tin, antimony and copper, and on solidifying on cooling from a melt becomes dispersion strengthened by dispersed particles comprising SbSn cuboids and Cu6Sn5 needles.

An example of a type of such a lead-based alloy is a bearing alloy in accordance with the German Industrial Standard din 1703 - LgPbSn1O, e.g. one of commercial purity having a composition lead, 10 mass % tin, 15 mass % antimony, 1 mass % copper.

A first preferred kind of tin-based alloy which may be grain refined by the method of the invention is a tin-based alloy which includes antimony and copper (and normally also lead), and on solidifying on cooling from a melt, like the lead-based alloys mentioned in the previous paragraph, also becomes dispersion strengthened by dispersed particles comprising SbSn cuboids on Cu6Sn5 needles. An example of a type of such a tin-based alloy is a bearing alloy in accordance with the German Industrial Standard DIN 1703 -LgSn80, e.g. one of commercial purity having a composition tin, 12 mass % antimony, 6 mass % copper, 2 mass % lead.

Another preferred kind of tin-based alloy which may be grain refined by the method of the invention is a tin-based alloy which includes antimony and copper, and on solidifying on cooling from a melt becomes dispersion strengthened by dispersed particles comprising Cu6Sn5 needles, but substantially no SbSn cuboids. An example of a type of such a tin-based alloy is a bearing alloy in accordance with the German Industrial Standard DIN 1703 - LgSn89, e.g. one of commercial purity having a composition tin, 7 mass % antimony, 3 mass % copper.

The nucleant particles may be provided by inoculating a melt of the tin-based or lead-based alloy with a source of selenium and/or tellurium, and permitting or causing the selenium and/or tellurium so introduced to react with lead in the alloy to produce the nucleant particles.

In commercial practice this source will conveniently comprise at least one master alloy. Where selenium is to be added, an antimony-selenium master alloy may be used, preferably one comprising from 2 to 50 mass % selenium (about 5 mass % selenium being most preferred), balance (apart from impurities) antimony.

Similarly, where tellerium is to be added, an antimony-tellurium master alloy may be used, preferably one comprising from 40 to 70 mass % tellurium (about 50 mass % tellurium being most preferred), balance (apart from impurities) antimony.

As an alternative to inoculating a melt of the tin-based or lead-based alloy with a source of selenium and/or tellurium, the nucleant particles may be provided by inoculating a melt of the alloy with a material which comprises PbSe andlor PbTe. This method is of particular use where the alloy itself does not contain lead. However, even tin-based alloys suitable for grain refinement by the method of the invention often contain sufficient lead to give rise to sufficient PbSe and/or PbTe for nucleation. Thus, for example, all the alloys in DIN 1703 contain intentional amounts of lead, with the exception of one of the tin-based alloys, viz. LgSn 89. However, DIN 1703 permits the presence of up to 0.5 mass% lead as an impurity, and this amount of lead is sufficient to provide the required PbSe and/or PbTe nucleants.

We have found that the tin-based or lead-based alloy containing the nucleant particles can be held for up to 20 hours without any substantial reduction in the grain refining effect obtained on casting. We have also found that, when an alloy which has already been grain refined in accordance with the invention is re-melted and then re-solidified, grain refinement is again obtained, to a substantially undiminished degree, and indeed the degree of grain refinement can be substantially maintained even after ten such melting - solidifying cycles.

In order that the invention may be more fully understood, some embodiments in accordance therewith will now be described, by way of example only, in the following Examples, and with reference to the accompanying drawings, wherein: Figure 1 is a typical microstructure of a commercially pure Pb, 10 mass % tin, 15 mass % antimony, 1 mass % copper bearing alloy, which has not been grain refined, at a magnification of 200:1.

Figure 2 is a typical microstructure of the same commercially pure bearing alloy as in Figure 1, but grain refined in accordance with the invention by a 0.1 mass % selenium addition, also at a magnification of 200:1.

Figure 3 is a typical microstructure of a commercially pure tin, 12 mass % antimony, 6 mass % copper, 2 mass % lead bearing alloy, which has not been grain refined, at a magnification of 200:1.

Figure 4 is a typical microstructure of the same commercially pure bearing alloy as in Figure 3, but grain refined in accordance with the invention by a 0.1 mass % tellurium addition, also at a magnification of 200:1.

Figure 5 is a typical microstructure of a commercially pure tin, 7 mass % antimony, 3 mass % copper bearing alloy, which has not been grain refined, at a magnification of 2:1.

Figure 6 is a typical microstructure of the same commercially pure bearing alloy as in Figure 6, but containing 0.2 mass % lead as an added impurity, and grain refined in accordance with the invention by a 0.05 mass % selenium addition, also at a magnification of 2:1.

Example 1 About 50g of commercially pure PbSn10Sb15Cu1 bearing alloy in conformity with German Industrial Standard DIN 1703 -LgPbSn10, and having the following composition, was melted in a steel crucible and superheated to 500 degrees C.

Element Mass % Element Mass % Pb 73.80 As 0.26 Sb 14.94 Bi < 0.02 Sn 10.03 Zn < 0.02 Cu 1.02 Ni 0.04 Ag < 0.01 Fe < 0.01 Cd < 0.01 Al < 0.01 Commercially pure (99.6 mass %) selenium enclosed within a lead foil sealing cone was added, and the melt stirred by means of a ceramic rod. After a holding time of 20 minutes, the melt was poured into an unheated steel mould having 20 mm internal diameter and 30 mm height.

It was already known that the microstructure of this type of alloy when not grain refined comprises the hard phases, SbSn cuboids and Cu6SnS needles, which are embedded in a softer SbSn/Pb pseudobinary eutectic matrix: see ref. (1) above, and: (3) Jackson, J., Metallography, 14(1981) p.p. 107 - 118.

(4) Thwait, C., Metals Handbook, vol. 7, 8th edn., American Society for Metals, Metals Park, Ohio, (1972) p.p. 297 - 304.

The copper occasionally combines with antimony to form rods of Cu2Sb.

By reducing the particle size of the SbSn cuboids as well as of the Cu6Sn5 needles, by a grain refining method according to the present invention, the processing properties of the alloy can be considerably improved.

Figure 1 shows the microstructure of the alloy without addition. Here the cuboids, with rough surfaces, and the needles are embedded in the pseudobinary eutectic matrix with one of the phases having rib bon-like structure. The needles crystallise the cuboids and the cuboids in turn the matrix.

Electron Probe Micro Analysis (EPMA) of the above-mentioned alloy proved the existance of copper and tin in the needles, and antimony and tin in the cuboids as well as in the ribbons of the matrix. It was also detected that the other phase in the pseudobinary matrix consisted mainly of lead.

The above analysis and the earlier works (refs. 1, 3 and 4) have proved that the cuboids as well as the ribbons in the matrix are composed of SbSn compound. The ribbons in the pseudobinary matrix are embedded in a lead-rich phase. The EPMA also supported the suggestion that the needles are composed of Cu6Sn6 compound.

Figure 2 shows the effect of adding 0.1 mass % selenium to the alloy, in accordance with this Example: much finer cuboids with smooth surfaces and Cu6SnS needles were observed in the matrix. Scanning Electron Microscope (SEM) observation showed the cuboids having been nucleated on crystallisation centres. It was proved by the EPMA that the crystallisation centres consisted mainly of selenium and lead. From this, it was deduced that the crystallisation centre is made up of the compound PbSe.

It was found that repeating this Example, but prolonging the holding time up to 20 hours, and also remelting the case pieces up to ten times had no substantial effect on the degree of grain refinement obtained.

It was also found that repeating this Example, but replacing the selenium addition by an addition of 0.1 mass % tellurium (again enclosed within a lead foil sealing cone) resulted in a grain refined microstruc ture similar to that shown in Figure 2.

Further, when this Example was again repeated, but this time replacing the metallic selenium addition by corresponding amounts of the following alloys, a grain refined microstructure similar to that shown in Figure 2 also resulted: Sb, Se 5 mass % Sb, Te 50 mass % In each case, the amount of master alloy added was such as to introduce 0.1 mass % of Se or Te into the bearing alloy.

Example 2 About 10 kg of commercially pure SnSb12Cu6Pb2 bearing alloy in conformity with German Industrial Standard DIN 1703 -LgSn80 (which specifies the following composition, in mass %: Sn = 79 to 81, Sb = 11 to 13, Cu = 5 to 7 and Pb = 1 to 3) was melted in a steel crucible and heated to 500 degrees C.

Commercially pure (99.9 mass %) tellurium enclosed within a lead foil sealing cone was added to the melt. The alloy was then kept for 20 minutes at 500 degrees C, stirred with a graphite rod, and then a sample was poured into an unheated steel mould having 20 mm internal diameter and 30 mm height.

Figure 3 shows the microstructure of the alloy without addition. The SbSn cuboids and Cu6Sn5 needles as well as the Sn-rich matrix can be clearly seen.

Figure 4 shows the effect of adding 0.1 mass % tellurium to the alloy, in accordance with this Example: the SbSn cuboids and Cu6Sn needles are clearly of much reduced size. It was found that in this case the crystallisation centres consisted mainly of tellurium and lead. From this, it was deduced that the crystal lisation centre is made up of the compound PbTe.

It was found that repeating Example 2, but replacing the tellurium addition by an addition of 0.1 mass % selenium (again enclosed within a lead foil sealing cone) resulted in a grain refined microstructure similar to that shown in Figure 4.

We have also found that repeating Example 2, but this time replacing the metallic tellurium by corre sponding amounts of the following master alloys, also resulted in a grain refined microstructure similar to that shown in Figure 4: Sb, Se 5 mass% Sb, Te 50 mass % In each case, the amount of master alloy added was such as to introduce 0.1 mass % of Se or Te into the bearing alloy.

Example 3 About 50 g of commercially pure SnSb7Cu3 bearing alloy in conformity with German Industrial Stand ard DIN 1703 - LgSn89 (which specifies the following composition, in mass %: Sn = 88 to 90, Sb = 7 to 8, Cu = 3 to 4, and up to 0.5 impurities) was prepared from tin, antimony and copper, the latter being introduced as a SbCu50 master alloy. The bearing alloy was then melted in a steel crucible, and heated to 500 degrees C.

As a control, the alloy was poured into an unheated steel mould having 20 mm internal diameter and 30 mm height. Figure 5 shows the microstructure of the alloy without addition. The major phase, Cu6SnS needles, appears in a star-like configuration. The surrounding coured matrix consists of an Sn - Sb solid i solution having a spongy coral-like morphology. There are no SbSn cuboids.

The test was then repeated on a further 50 g sample of the alloy, but adding 0.05 mass % elemental selenium in the manner described in Example 1, and holding for 20 minutes before pouring into the mould. This test produced a cast sample having a microstructure very little different from that shown in Figure 5, and no grain refinement was achieved, owing to the absence of lead, and the consequent impossibility of forming PbSe.

The latter test was again repeated, the bearing alloy this time containing 0.2 mass % lead as an added impurity (still within the requirements of DIN 1703 - LgSn89). Figure 6 shows the microstructure of the sample produced. It will be seen that grain refinement had been successfully achieved. The Cu6SnS needles had become much finer, with a more or less H-shaped structure.

Successful grain refinement of LgSn89 containing the 0.2 mass% lead impurity was also achieved in further tests, using, in place of the elemental selenium: (a) elemental tellurium; (b) a Sb, Se 5 mass % master alloy; and (c) a Sb, Te 50 mass % master alloy.

It should be noted that, in practising the grain refining method of the present invention, care should be taken not to allow the nucleant particles to be poisoned. As will be seen from the Examples given above, the impurities present in the commercial bearing alloys used did not give rise to any such problems at the level at which they were present. However, tests have shown that increasing the amounts of impurities present in the bearing alloy used in Example 1 did result in coarsening of the microstructure, to a degree which varied from impurity to impurity. The metals silver, arsenic, bismuth and aluminium brought about only a slight coarsening of the cuboids and of the needles; cadmium and nickel resulted in a higher degree of coarsening; and zinc turned out to be the most destructive element to the refining efficiency of selenium.

It should also be noted that varying the degree of superheating of the nucleated alloy before casting, and also varying the temperature of the casting mould, can affect the microstructure of the solidified alloy. However, tests have shown that these factors can be varied by a substantial margin from those employed in commercial practice without giving rise to severe loss of grain refinement.

For example, when Example 1 was repeated but modified in that the temperature of the bearing alloy was varied over the range from 400 degrees C to 700 degrees C, no change in the microstructure of the cast alloy was observed. However, at 350 degrees C, which is close to the melting temperature of the alloy, much coarser cuboids as well as needles were observed. To put this in perspective, it should be noted that, in industrial production, the alloy is cast at between 480 degrees C and 520 degrees C.

Similarly, when the procedure of Example 1 was modified by increasing the temperature of the casting mould over the range from room temperature to 200 degrees C, the microstructure remained unchanged.

At a higher temperature, 300 degrees C, which is close to the melting temperature of the bearing alloy, the microstructure of the casting became slightly coarser.

From the foregoing, it will be seen that the invention provides an efficient method of grain refining important classes of tin-based and lead-based alloys, which method can be carried out economically under normal casting conditions.

As a result of grain refinement, the casting and processing properties can be improved. For example, the compressive strength can increase by up to 70% and the ductility by up to 50%. Other casting and processing parameters can also be improved by the refinement of microstructure.

Claims

1. A method of grain refining a tin-based or lead-based alloy which is of a composition such that, on solidifying on cooling from a melt, it becomes dispersion strengthened by dispersed particles comprising SbSn cuboids andlor Cu6SnS needles, the method comprising arranging that the alloy, when in a molten condition prior to casting, contains nucleant particles of PbSe and/or PbTe, and that the melt is solidified so that the nucleant particles act as nucleant sites for the SbSn cuboids and/or CuSSnS needles, to produce refinement of the SbSn cuboids andlor CuSSnS needles.

2. A method according to claim 1, wherein the tin-based or lead-based alloy is a bearing alloy.

3. A method according to claim 2, wherein the bearing alloy is in accordance with German Industrial Standard DIN 1703.

4. A method according to claim 2 or claim 3, wherein the bearing alloy is a lead-based bearing alloy.

5. A method according to any one of claims 1 to 4, wherein the alloy which is grain refined is a leadbased alloy which includes tin, antimony and copper, and on solidifying on cooling from a melt becomes dispersion strengthened by dispersed particles comprising SbSn cuboids and Cu6SnS needles.

6. A method according to claim 4 or claim 5, wherein the lead-based alloy is a bearing alloy in acaccordance with the German Industrial Standard DIN 1703 - LgPb10.

7. A method according to any one of claims 4 to 6, wherein the lead-based alloy is a lead, 10 mass % thin, 15 mass % antimony, 1 mass % copper bearing alloy of commercial purity.

8. A method according to claim 2 or claim 3, wherein the bearing alloy is a tin-based bearing alloy.

9. A method according to any one of claims 1 to 3 and 8, wherein the alloy which is grain refined is a tin-based alloy which includes antimony and copper, and on solidifying on cooling from a melt becomes dispersion strengthened by dispersed particles comprising SbSn cuboids and CuSSnS needles.

10. A method according to claim 8 or claim 9, wherein the tin-based alloy is a bearing alloy in accordance with the German Industrial Standard DIN 1703 - LgSn80.

11. A method according to any one of claims 8 to 10, wherein the tin-based alloy is a tin, 12 mass % antimony, 6 mass % copper, 2 mass % lead bearing alloy of commercial purity.

12. A method according to any one of claims 1 to 3 and 8, wherein the alloy which is grain refined is a tin-based alloy which includes antimony and copper, and on solidifying on cooling from a melt becomes dispersion strengthened by dispersed particles comprising Cu6Sn5 needles, but substantially no SbSn cuboids.

13. A method according to claim 8 or claim 12, wherein the tin-based alloy is a bearing alloy in accordance with the German Industrial Standard DIN 1703-Lg Sn89.

14. A method according to any one of claims 8, 12 and 13, wherein the tin-based alloy is a tin, 7 mass % antimony, 3 mass % copper bearing alloy of commercial purity.

15. A method according to any one of claims 1 to 14, wherein the nucleant particles have been provided by inoculating a melt of the alloy with a source of selenium and/or tellerium, and permitting or causing the selenium and/or tellerium so introduced to react with lead in the alloy to produce the nucleant particles.

16. A method according to claim 15, wherein the said source comprises at least one master alloy.

17. A method according to claim 15 or claim 16, wherein the said source comprises an antimonyselenium master alloy.

18. A method according to claim 17, wherein the antimony-selenium master alloy comprises from 2 to 50 mass % selenium, balance (apart from impurities) antimony.

19. A method according to claim 18, wherein the antimony-selenium master alloy comprises about 5 mass % selenium.

20. A method according to claim 15 or claim 16, wherein the said source comprises an antimonytellerium master alloy.

21. A method according to claim 20, wherein the antimony-tellurium master alloy comprises from 40 to 70 mass % tellerium, balance (apart from impurities) antimony.

22. A method according to claim 21, wherein the antimony-tellurium master alloy comprises about 50 mass % tellurium.

23. A method according to any one of claims 1 to 14, wherein the nucleant particles have been provided by inoculating a melt of the alloy with a material which comprises PbSe and/or PbTe.

24. A method according to any one of claims 1 to 23, wherein the total of the amount of any nucleant PbSe (measured as Se) plus the amount of any nucleant PbTe (measured as Te) is from 0.005 to 0.5 mass %.

25. A method according to claim 24, wherein the said total amount is about 0.1 mass %.

26. A method according to any one of claims 1 to 25, wherein the alloy containing the said nucleant particles is held in a molten condition for up to 20 hours prior to casting.

27. A method according to any one of claims 1 to 26, wherein the alloy containing the said nucleant particles has at least once previously been solidified from a melt containing the said nucleant particles.

28. A method according to claim 27, wherein the alloy containing the said nucleant particles has at least 10 times previously been solidified from a melt containing said nucleant particles.

29. A method of grain refining a tin-based or lead-based alloy, substantially as hereinbefore described in any one of the foregoing Examples 1 to 3.

30. A tin-based or lead-based alloy, whenever grain refined by a method in accordance with any one of claims 1 to 29.