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Title:
ALUMINUM ALLOY WITH IMPROVED EXTRUDABILITY AND CORROSION RESISTANCE
Document Type and Number:
WIPO Patent Application WO/2021/077209
Kind Code:
A1
Abstract:
There is provided an extruded and brazed product with improved corrosion resistance by having low coarse recrystallized grain formation as well as a method for making same. The extruded and brazed product comprises an aluminum alloy comprising in weight percent Mn 0.6 – 0.75; Fe 0.11 – 0.16; Si 0.10 – 0.19; Cu < 0.01; Zn < 0.05; Ti < 0.05; optionally a grain refiner; optionally Ni < 0.01; and the balance being aluminum and inevitable impurities.

Inventors:
PARSON, Nicholas Charles (Kingston, Ontario K7L 4V3, CA)
GUAY, Raynald (Jonquière, Québec G7S 2G6, CA)
Application Number:
CA2020/051370
Publication Date:
April 29, 2021
Filing Date:
October 14, 2020
Export Citation:
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Assignee:
RIO TINTO ALCAN INTERNATIONAL LIMITED (Montreal, Québec H3B 0E3, CA)
International Classes:
C22C21/00; B21C23/02; C21D1/34
Attorney, Agent or Firm:
NORTON ROSE FULBRIGHT CANADA LLP (Suite 2500Montreal, Québec H3B 1R1, CA)
Download PDF:
Claims:
WHAT IS CLAIMED IS:

1. An extruded and brazed product comprising: an aluminum alloy comprising in weight percent

Mn 0.6 - 0.75;

Fe 0.11 - 0.16;

Si 0.10 - 0.19;

Cu < 0.01 ;

Zn < 0.05;

Ti < 0.05; optionally a grain refiner; optionally Ni < 0.01 ; and the balance being aluminum and inevitable impurities; wherein less than 15% of the extruded and brazed tube width includes coarse recrystallized grains.

2. The extruded and brazed product of claim 1 , wherein each of the inevitable impurities is present at a maximum of 0.05 and the total inevitable impurities comprises less than 0.10.

3. The extruded and brazed product of claim 1 or 2, wherein the aluminum alloy comprises less than 0.01 Ni.

4. The extruded and brazed product of any one of claims 1 to 3, wherein the aluminum alloy comprises less than 0.05 Mg.

5. The extruded and brazed product of any one of claims 1 to 4, wherein the aluminum alloy comprises less than 0.05 Cr.

6. The extruded and brazed product of any one of claims 1 to 5, wherein the aluminum alloy comprises between 0.64 to 0.72 Mn.

7. The extruded and brazed product of any one of claims 1 to 6, wherein the aluminum alloy comprises between 0.10 to 0.14 Si.

8. The extruded and brazed product of any one of claims 1 to 7, wherein the aluminum alloy comprises between 0.12 to 0.16 Fe.

9. The extruded and brazed product of any one of claims 1 to 8, wherein the aluminum alloy comprises between 0.011 to 0.024 Ti.

10. The extruded and brazed product of any one of claims 1 to 9, wherein the extruded and brazed product is an extruded and brazed tubing.

11. The extruded and brazed product of claim 10, wherein the extruded and brazed tubing is or comprises micro-multiport tubing.

12. A method for producing an extruded and brazed product comprising: a) providing billets comprising an aluminum alloy comprising in weight percent

Mn 0.6 - 0.75;

Fe 0.11 - 0.16;

Si 0.10 - 0.19;

Cu <0.01 ;

Zn <0.05;

Ti <0.05; optionally a grain refiner; optionally Ni < 0.01 ; and the balance being aluminum and inevitable impurities; b) homogenizing the billets with at least one heat treatment, the heat treatment comprising a treatment temperature in the range of 540°C to 590°C for at least one soaking period from 1 to 8 hours to obtain an homogenized aluminum alloy; c) extruding the billets into the product to obtain an extruded product; and d) brazing the extruded product to obtain the extruded and brazed product.

13. The method of claim 12, further comprising, before providing the billets, casting the aluminum alloy into the billets.

14. The method of claim 12 or 13, further comprising, after homogenizing and before extruding, cooling the billets.

15. The method of any one of claims 12 to 14, wherein each of the inevitable impurities of the aluminum alloy is present at a maximum of 0.03 and the total inevitable impurities comprises less than 0.10.

16. The method of any one of claims 12 to 15, wherein the aluminum alloy comprises less than 0.01 Ni.

17. The method of any one of claims 12 to 16, wherein the aluminum alloy comprises less than 0.05 Mg. 18. The method of any one of claims 12 to 17, wherein the aluminum alloy comprises less than 0.05 Cr.

19. The method of any one of claims 12 to 18, wherein the aluminum alloy comprises between 0.64 to 0.72 Mn.

20. The method of any one of claims 12 to 19, wherein the aluminum alloy comprises between 0.10 to 0.14 Si.

21. The method of any one of claims 12 to 20, wherein the aluminum alloy comprises between 0.12 to 0.16 Fe.

22. The method of any one of claims 12 to 21 , wherein the aluminum alloy comprises between 0.011 to 0.024 Ti. 23. The method of any one of claims 12 to 22, wherein the extruded and brazed product is a tubing.

24. The method of claim 23, wherein the tubing is a micro-multiport tubing.

25. An extruded and brazed product obtainable or obtained by the method of any one of claims 12 to 24.

Description:
ALUMINUM ALLOY WITH IMPROVED EXTRUDABILITY AND CORROSION

RESISTANCE

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from US provisional patent application 62/925,314 filed on October 24, 2019 and herewith incorporated in its entirety.

TECHNICAL FIELD

The present disclosure relates to aluminum alloy-based extruded and brazed products and methods for producing same.

BACKGROUND OF THE ART

Aluminum alloys provide corrosion resistance to manufactured parts, and are used for example in the automotive industry as well as in heat exchangers and air conditioning applications. They are used in tubing because of their good extrudability while being light weight and offering moderate strength. Long-life corrosion resistant alloys have typically used high Mn contents or additions of Ti, which are detrimental to extrudability and can reduce extrusion speeds and die life. It is a challenge to improve extrudability without hindering the long-life corrosion performance of the alloys. Improvements are desired.

SUMMARY

The present disclosure concern aluminum alloy having increasing extrudability characteristics as well as aluminum products comprising same having increased corrosion resistance.

In a first aspect, the present disclosure provides an extruded and brazed product comprising an aluminum alloy comprising in weight percent Mn 0.6 - 0.75; Fe 0.11 - 0.16; Si 0.10 - 0.19; Cu < 0.01 ; Zn < 0.05; Ti < 0.05 with the balance being aluminum and inevitable impurities. In the extruded and brazed product, less than 15% of the product’s width includes coarse recrystallized grains. In an embodiment, each of the inevitable impurities is present at a maximum of 0.03 and the total inevitable impurities comprises less than 0.10. In another embodiment, the aluminum alloy comprises less than 0.01 Ni. In still another embodiment, the aluminum alloy comprises less than 0.05 Mg. In still a further embodiment, the aluminum alloy comprises less than 0.05 Cr. In yet another embodiment, the aluminum alloy comprises between 0.64 to 0.72 Mn. In still a further embodiment, the aluminum alloy comprises between 0.11 to 0.14 Si. In some embodiments, the aluminum alloy comprises between 0.12 to 0.16 Fe. In additional embodiments, the aluminum alloy comprises between 0.011 to 0.024 Ti. In some embodiments, the extruded and brazed product is an extruded and brazed tubing, such as, for example, a micro-multiport tubing.

In another aspect, the present disclosure provides a method for producing an extruded and brazed product. First, a billet is provided, the billet comprising an aluminum alloy comprising in weight percent Mn 0.6 - 0.75; Fe 0.11 - 0.16; Si 0.10 - 0.19; Cu < 0.01 ; Zn < 0.05; Ti < 0.05 with the balance being aluminum and inevitable impurities. Then, the billet is homogenized with at least one heat treatment. The billets are then extruded into the product and the product is brazed to obtain the extruded and brazed product. The method can further comprise, before providing the billets, casting the aluminum alloy into the billets. In an embodiment, the method further comprises, after homogenizing and before extruding, cooling the billets. In an embodiment, each of the inevitable impurities of the aluminum alloy is present at a maximum of 0.03 and the total inevitable impurities comprises less than 0.10. In an embodiment, the aluminum alloy comprises less than 0.01 Ni. In another embodiment, the aluminum alloy comprises less than 0.05 Mg. In a further embodiment, the aluminum alloy comprises less than 0.05 Cr. In still a further embodiment, the aluminum alloy comprises between 0.64 to 0.72 Mn. In still another embodiment, the aluminum alloy comprises between 0.10 to 0.14 Si. In yet another embodiment, the aluminum alloy comprises between 0.12 to 0.16 Fe. In still another embodiment, the aluminum alloy comprises between 0.011 to 0.024 Ti. In an embodiment, the extruded and brazed product is a tubing, such as, for example, a micro-multiport tubing.

In a third aspect, the present disclosure provides an extruded and brazed product obtainable or obtained by the method described herein.

Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of dispersoid volume fraction of three aluminum alloy compositions (content in Fe and Si is provided in the figure legend) as a function of soak time (hours).

FIG. 2 is an example of standard alloy AA3012A exhibiting coarse recrystallized grains through the outer wall thickness.

FIG. 3 shows the surface of sample in Figure 2 after macroetching showing coarse recrystallized grains on surface. FIG. 4A shows the surface grain structure revealed by Poulton’s macrotech of different tubes obtained from a commercial operation. The material on the left is the “as extruded” tube, the material in the middle has been submitted to a simulated braze thermal cycle of 2 min. at 605°C whereas the material on the right has been submitted to a simulated braze thermal cycle of 4 min. at 605°.

FIG. 4B shows the surface grain structure revealed by Poulton’s macrotech of brazed tubes obtained from a commercial operation. The material has been submitted to a simulated braze thermal cycle of 4 min. at 625°.

DETAILED DESCRIPTION

The present disclosure concerns Al-Mn-Si-Fe extrusion alloys having improved extrudability as well as products comprising same exhibiting long-life corrosion resistance. The aluminum alloys of the present disclosure exhibit improved extrudability. The extruded and brazed products made from the alloys of the present disclosure exhibit a fine post-brazed grain structure and/or tolerance to extended homogenization and brazing cycles. As used in the context of the present disclosure, a “fine post-brazed grain structure” refers to a structure consisting mainly of residual fine grains produced during the extrusion process and the corresponding absence of coarse recrystallized grains formed during the braze cycle. The expression “fine as-extruded grain structure” refers to the a structure consisting mainly of residual fine grains produced during the extrusion process and before any brazing cycle. Still according to the present disclosure, the term “coarse recrystallized grains” refers to grains with a width across the extruded surface (i.e. perpendicular to the extrusion direction) higher than 200 microns or grains with a thickness extending through the entire outer wall thickness of the tube. Figure 2 shows an example of the grain structure of alloy AA3012A after sizing and brazing where coarse recrystallized grains are present extending through the wall thickness. Figure 3 shows the appearance of the tube surface of the same sample as in Figure 2 after macroetching, revealing coarse recrystallized grains on the tube surface with a width higher than 200 microns.

The alloys of the present disclosure are especially useful in making extruded (e.g., aluminum) products. “Extruded aluminum products” refers to products made from the aluminum alloy of the present disclosure which have been pushed through a die at elevated temperature to obtain a desired cross section.

The extruded aluminum products of the present disclosure are brazed to other components, for example to create a heat exchanger. “Brazing” as defined herein is the process of metal- joining two or more items by melting and flowing a filler metal into at least one joint. A “brazed product” is defined as having been subjected to brazing.

As indicated herein, the chemistry of the aluminum alloys of the present disclosure favors retention of a fine post-brazed grain structure in the outer wall of the product (e.g., tube) and thus prevents or limits recrystallization or “coarse grain formation” during high temperature brazing. Recrystallization at this stage replaces the desirable fine grain structures resulting from extrusion and replaces it with a coarse grain structure where one coarse grain can occupy the entire tube wall thickness. This condition offers a direct corrosion path through the material and is detrimental to the corrosion resistance of the tubing. Thus, recrystallization into coarser grains has to be avoided, prevented or limited.

In a first aspect, there is provided an aluminum alloy comprising in weight percent Mn about 0.6 to about 0.75; Fe about 0.11 to about 0.16; Si about 0.10 to about 0.19; Cu less than about 0.01 ; Zn less than about 0.05; Ti less than about 0.05; optionally a grain refiner; optionally Ni less than about 0.01 ; and the balance being aluminum and inevitable impurities.

The aluminum alloy of the present disclosure is an Al-Mn-Si-Fe alloy and thus includes Mn. However, the Mn content of the aluminum alloy of the present disclosure is lower than standard corresponding “long-life” Al-Mn-Si-Fe alloys. This reduction in Mn content provides reduced flow stress and improved extrudability. Mn is also important for the formation of Al- Mn-Fe-Si dispersoids and for providing increased self-corrosion protection along with adequate mechanical strength. Mn can be present in the aluminum alloy of the present disclosure in weight percent from about 0.6 to about 0.75, from about 0.61 to about 0.74, from about 0.62 to about 0.73, from about 0.63 to about 0.72, from about 0.64 to about 0.71 , from about 0.65 to about 0.70, from about 0.66 to about 0.69, from about 0.67 to about 0.68, from about 0.6 to about 0.74, from about 0.6 to about 0.73, from about 0.6 to about 0.72, from about 0.6 to about 0.71 , from about 0.6 to about 0.70, from about 0.6 to about 0.69, from about 0.6 to about 0.68, from about 0.6 to about 0.67, from about 0.6 to about 0.66, from about 0.6 to about 0.65, from about 0.6 to about 0.64, from about 0.6 to about 0.63, from about 0.6 to about 0.62, from about 0.6 to about 0.61 , from about 0.61 to about 0.75, from about 0.62 to about 0.75, from about 0.63 to about 0.75, from about 0.64 to about 0.75, from about 0.65 to about 0.75, from about 0.66 to about 0.75, from about 0.67 to about 0.75, from about 0.68 to about 0.75, from about 0.69 to about 0.75, from about 0.70 to about 0.75, from about 0.71 to about 0.75, from about 0.72 to about 0.75, from about 0.73 to about 0.75, from about 0.74 to about 0.75 or from about 0.64 to 0.72. The aluminum alloys of the present disclosure also include Fe which is beneficial for increasing the resistance to coarse recrystallized grain formation after homogenization. Fe also plays a role in controlling the distribution of Al-Mn-Fe-Si dispersoids. Furthermore, Fe reduces the solubility of Mn and facilitates the formation of Al-Mn-Fe-Si dispersoids. However, excessive levels of Fe can be detrimental to pitting corrosion resistance by providing active cathode sites. Fe can be present in the aluminum alloy of the present disclosure in weight percent from about 0.11 to about 0.16, from about 0.12 to about 0.15, from about 0.13 to about 0.14, from about 0.12 to about 0.16, from about 0.13 to about 0.16, from about 0.14 to about 0.16, from about 0.15 to about 0.16, from about 0.11 to about 0.15, from about 0.11 to about 0.14, from about 0.11 to about 0.13 or from about 0.11 to about 0.12.

The Si present in the aluminum alloys of the present disclosure promotes Al-Mn-Fe-Si dispersoid formation and contributes to the distribution of the Al-Mn-Fe-Si dispersoids. In addition, Si reduces the tendency for reduction in the volume fraction of dispersoids with extended homogenization times. As shown in the Examples, it was surprisingly found that Si provided remarkable control of the post-brazed grain size structure control under severe processing conditions to obtain desirable low recrystallization. However, excessive Si levels can lower the bulk melting point of the alloy and reduce extrudability. Si can be present in the aluminum alloys of the present disclosure in weight percent from about 0.10 to about 0.19, from about 0.11 to about 0.19, from about 0.12 to about 0.19, from about 0.13 to about 0.19, from about 0.14 to about 0.19, from about 0.15 to about 0.19, from about 0.16 to about 0.19, from about 0.17 to about 0.19, from about 0.18 to about 0.19, from about 0.10 to about 0.18, from about 0.11 to about 0.18, from about 0.12 to about 0.18, from about 0.13 to about 0.18, from about 0.14 to about 0.18, from about 0.15 to about 0.18, from about 0.16 to about 0.18, from about 0.17 to about 0.18, from about 0.10 to about 0.17, from about 0.11 to about 0.17, from about 0.12 to about 0.17, from about 0.13 to about 0.17, from about 0.14 to about 0.17, from about 0.15 to about 0.17, from about 0.16 to about 0.17, from about 0.10 to about 0.16, from about 0.11 to about 0.16, from about 0.12 to about 0.16, from about 0.13 to about 0.16, from about 0.14 to about 0.16, from about 0.15 to about 0.16, from about 0.10 to about 0.15, from about 0.11 to about 0.15, from about 0.12 to about 0.15, from about 0.13 to about 0.15, from about 0.14 to about 0.15, from about 0.10 to about 0.14, from about 0.11 to about 0.14, from about 0.12 to about 0.14, from about 0.13 to about 0.14, from about 0.10 to about 0.13, from about 0.11 to about 0.13, from about 0.12 to about 0.13, from about 0.10 to about 0.12, from about 0.11 to about 0.12, from about 0.10 to about 0.11. The aluminum alloys of the present disclosure can include, in some embodiment, Cu. However, if present, the Cu content is limited to less than 0.01 wt. % as it can reduceself- corrosion resistance.

The aluminum alloys of the present disclosure can include, in some embodiments, Zn. Extruded tubes for heat transfer applications are frequently coated with a galvanically sacrificial layer of Zn. The Zn may be deposited by arc spray, use of a Zn containing flux or by plasma spray and the Zn diffuses into the tube surface during heating to the braze temperature. The Zn concentration in the base alloy is limited to less than 0.05 wt. % as it can interfere with the behaviour of the sacrificial coating if present in a higher concentration.

A grain refiner may be optionally included in the aluminum alloys of the present disclosure to solidify aluminum alloys with a fully equiaxed, fine grain structure, in the form of Ti, TiB or TiC. When TiB is used as a grain refiner, this may result in a B content of up to 0.01 wt. % in the alloy.

The aluminum alloys of the present disclosure can include, in some embodiments, Ti. However, a high content of Ti can be detrimental to extrudability and can reduce extrusion speeds and die life, therefore the concentration of Ti, if present, is limited to less than 0.05 wt. %. For example in weight percent less than about 0.030, less than about 0.027 or less than about 0.024. As indicated above, it may be desirable to add low levels of Ti to extrusion alloys as a grain refiner during casting either as Ti or combined with B as a TiB grain refiner or with C as a TiC grain refiner.

The aluminum alloys of the present disclosure can include, in some embodiments, Ni. However, since Ni can reduceself-corrosion resistance, the content of Ni is less than 0.01.

In the aluminum alloys of the present disclosure, Mg is optionally present but is kept relatively low for extrudability and brazeability of the alloy, less than 0.05 wt%.

In some embodiments, the balance of the alloy includes aluminum and inevitable impurities. In some embodiments, each of the inevitable impurity is present at a maximum of 0.05 (and in some embodiments 0.03) and the total inevitable impurities comprises less than 0.10.

The extruded and brazed product of the present disclosure include Al-Mn-Fe-Si dispersoids. The Al-Mn-Fe-Si dispersoids are submicron particles that play a role in the deformation behaviour, recrystallization behaviour and resulting mechanical properties of products comprising the aluminum alloys of the present disclosure. In some embodiments, the dispersoids allow the fine as-extruded grain structure to be retained in the outer wall of a tube, after typical cold sizing and brazing treatments, for example combining the tubing with fins and header tubes to make a brazed heat exchanger. Without wishing to be bound to theory, the retention of the fine as-extruded grain structure in the outer wall of the shape, after brazing, contributes to the corrosion resistance by presenting a more tortuous corrosion path through walls of the shape.

In an embodiment, the extruded and brazed products include less than 15% coarse recrystallized grains across the tube width, preferably less than 12%, most preferably less than 10% when subjected to severe brazing and/or less than 5% recrystallization, preferably less than 3%, most preferably less than 1% when subjected to standard brazing (such as, for example, standard controlled atmosphere (CAB) brazing). The percentage referring to the percentage of the outer tube wall consisting of coarse recrystallized grains. In an embodiment, less than 15%, 14%, 13%, 12%, 11% or 10% of the extruded and brazed product width is occupied by coarse recrystallized grains when subjected to severe brazing and/or less than 5%, 4%, 3%, 2% or 1% recrystallization when subjected to standard controlled atmosphere (CAB) brazing which is widely used for the production of aluminum heat exchangers. The percentage referring to the percentage of the outer tube wall width consisting of coarse recrystallized grains.

The extruded and brazed products can be provided in any shape or form. In some embodiments, the extruded and brazed products can be in the form of a tube or a plurality of tubes. In some specific embodiments, the extruded and brazed products can be or comprise micro-multiport tubing (MMP). When the extruded and brazed products are tubing or tubes (such as MMP), they can have a wall thickness of equal to or less than about 0.4 mm, 0.3 mm or 0.2 mm.

The present disclosure also provides a method for producing extruded and brazed products. The method comprises working the aluminum alloy of the present disclosure into the aluminum product. The working step can include casting the aluminum alloy directly into an intermediary billet intended for extrusion.

In some embodiments, methods of the present disclosure first provides billets comprising an aluminum alloy as described herein. Then, the billets are homogenized with at least one heat treatment, the heat treatment comprising a treatment temperature in the range of 540°C to 590°C and for at least one soak time ranging from 1 to 8 hours to obtain an homogenized aluminum alloy billet. Next, the billets are extruded into products such as tubing. The product (tubing) is then optionally coiled, uncoiled, cold sized, assembled and then brazed to obtain the brazed product (tubes forming part of a heat exchanger). The brazing step can comprise at least one brazing cycle.

In one embodiment of the method, before providing the billets, the aluminum alloy of the present disclosure is cast into the billets. In one embodiment of the method, after homogenizing and before brazing, the homogenized aluminum products are cooled down, preferably at a cooling rate of 300°C/h or less.

EXAMPLE I EFFECT OF MN AND FE ON RECRYSTALLIZATION OF BRAZED TUBES

The alloys A to E (chemistry detailed in Table 1) were direct chill (DC) cast as 101 mm billets. Alloy A represents the existing state of the art, and is the benchmark of comparison. The concentration of Mn in the experimental alloys was increased compared to alloy A, alloys B and C had 0.64 wt. % Mn, and alloys D and E had 0.70 wt. % Mn. The concentration of Fe was increased compared to alloy A only in alloys C and E, to 0.14 and 0.15 wt. % respectively.

Table 1 : The compositions of alloys A to E in weight percent, the balance being Al and inevitable impurities

The billets B to E were homogenized using four treatments, the first treatment (TR1) was 2 h at 550°C, the second treatment (TR2) was 6 h at 550°C, the third treatment (TR3) was 2 h at 560°C, and the fourth treatment (TR4) was 6 h at 560°C. Billet A was only homogenized with TR1 and TR2. The billets were then cooled at 300°C/h. The cooled material was then extruded into mini microport (MMP) tubing with an outer wall thickness of 0.35 mm using a billet temperature of 480°C and an exit speed of 77 m/min. Lengths of the tubing were cold sized by rolling to give a thickness reduction of 4 % to replicate commercial tube sizing. Simulated brazing cycles of 2.5 min at 605°C (cycle 1) and 625°C (cycle 2) were then applied, and the grain structures were assessed by macro-etching the external flat surface of the tube and measuring the proportion of the tube width occupied by coarse recrystallized grains, where the term “coarse grains” refers to grains with a width on the extruded surface > 200 microns or grains with a thickness extending through the entire wall thickness The results are shown in Table 2. Table 2: Results (in percentage) of tube width occupied by coarse recrystallized grains for alloys A to E

The extent of undesirable coarse recrystallized grains increased with increasing homogenization soak time/temperature, and increasing braze temperature. Alloy A retained a fine grain structure when homogenised for 2hrs/550°C and brazed at 605°C. However, it gave significant recrystallization when the soak time was increased to 6 hours of soak at 550°C, and 605°C braze. Increasing the braze temperature to 625°C gave excessive recrystallization for both soak times. Therefore variations in braze temperature and homogenization soak time, which are possible in commercial operations, could result in excessive coarse recrystallized grain when using alloy A.

Under the experimental conditions tested, acceptable targets for coarse recrystallized grain formation are zero coarse recrystallized grain formation with the standard brazing treatment at 605°C and <15 % after the more severe treatment at 625°C. The latter represents formation of single coarse recrystallized grains at the tube nose (ends) where the strain is more concentrated during sizing. In this example, Alloy B performed slightly better than alloy A, in terms of coarse recrystallized grain formation. However, the performance, when brazed at 625°C, was unacceptable for homogenization temperatures in the range of 550 to 560°C. Alloy C gave significantly better resistance to coarse recrystallized grain formation along with Alloy E, suggesting that increasing the Fe content is beneficial. Alloy D, with an increased Mn content compared to alloy B but the same Fe content, gave unacceptable behaviour at the higher braze temperature, suggesting that increasing the Mn content alone is not sufficient to prevent coarse recrystallized grain formation.

EXAMPLE II EFFECT OF SI ON RECRYSTALLIZATION OF BRAZED TUBES

The alloys A, F, G and H (chemistry detailed in Table 3) were DC cast as 101 mm diameter billets. Alloy A represents the existing state of the art, and is the benchmark of comparison. Alloys F, G, H had increasing concentrations of Si 0.08, 0.14, and 0.19 wt. % respectively. Table 3: The compositions of alloys A, F, G and H in weight percent the balance being Al and inevitable impurities

The alloys were homogenized for 6 h at 580°C to represent a high temperature long-soak cycle. The billets were then cooled at 300°C/h. The cooled material was then extruded into mini microport (MMP) tubing with an outer wall thickness of 0.35 mm using a billet temperature of 480°C and an exit speed of 77 m/min. Lengths of the tubing were cold rolled to give thickness reductions of 4% to replicate commercial tube sizing, and 10% to investigate excessive sizing. Then an extreme braze cycle of 2.5 min at 625°C was applied. The grain structures were assessed by macro-etching the flat surface of the tube and measuring the proportion of the tube width occupied by coarse recrystallized grains. The results are shown in Table 4.

Table 4: Results (in percentage) of proportion of tube width occupied by coarse recrystallized for alloys A, F, G and H.

As expected, alloy F, which has a similar composition to alloy A but with an increased Fe content, fully recrystallized to a coarse grain structure. However, increasing the Si from 0.08 to 0.14 wt. %, in alloy G, provided remarkable control of the post-brazed grain size, and the trend continued with alloy H with 0.19 wt. % Si. Therefore, slight increases in Si content can provide post-brazed grain structure control under severe processing conditions. Increasing the Si content from 0.08 to 0.19 reduces the melting point by 4°C, which could have some impact on extrudability. Therefore, further increases in Si beyond 0.19 wt. % are undesirable EXAMPLE III AL-MN-FE-SI DISPERSOID MODELING

Without wishing to be bound to theory, the mechanism of controlling the post-brazed structure and preventing coarse recrystallized grain recrystallization seems to be due, at least in part, to grain boundary pinning by submicron alpha - Al-Mn-Fe-Si dispersoid particles, which are presumed to form during homogenization. The pinning effect is proportional to the volume fraction/particle radius. The effects of composition and homogenization cycle observed in these experiments were probably due to changes in these two parameters. Using a proprietary homogenization model developed to predict dispersoid growth and solute diffusion across a dendrite arm, it is possible to predict the effects of composition on the dispersoid distribution. Figure 1 shows how the volume fraction of dispersoids varies with Fe and Si contents for a 0.70 wt.% Mn base alloy during homogenization at 550°C. With a base level of 0.08 wt. % Si, increasing Fe from 0.10 to 0.15 wt. % increased the volume fraction, but this starts to reduce after 2-3 hours soak, meaning that extended homogenization times can reduce the ability to prevent coarse recrystallized grain formation. When the Si content is increased from 0.08 to 0.13 wt. %, the initial dispersoid volume fraction is lower but continues to increase with longer homogenization times. This can offset the effect of extended soaking, which can occur under production conditions.

EXAMPLE IV CORROSION RESISTANCE TEST

Alloys A, B, C, D, E, F, G and H were homogenised as described above and extruded into a 30 x 1.4 mm strip using a billet temperature of 480°C and an exit speed of 75 m/min. Commercial alloy variants corresponding to AA3102 and an established commercial long life alloy were also processed for comparison. The material was water quenched at the die exit. A simulated braze cycle of 5 mins at 605°C was applied to 100 mm coupons. These were degreased in alcohol and then 4 coupons per alloy exposed in the SWAAT corrosion test (ASTM G85) for 20 days. The mean pit depth was measured for each sample based on the 6 deepest pits per coupon selected by eye. The results after 20 days exposure in the accelerated corrosion test are shown in Table 5. A low pit depth is desirable and is an indicator of superior resistance to pitting corrosion in service. The established commercial long life alloy, based on AA3012A, performed the best in SWAAT but the experimental alloys B-E, including the inventive alloys C, E, G and H all performed better than the state of the art alloys A and F and the standard commercial alloy AA3102 Table 5. SWAAT Test Results

EXAMPLE V FLOW STRESS TEST

The extrudability, or potential extrusion speed of Al-Mn type alloys is controlled by the alloy flow stress at elevated temperature. A lower flow stress is an indicator of potentially higher extrusion speed and reduced die wear. Billets of alloys C and E were homogenized to a cycle of 2 hrs/550°C followed by cooling at 250°C/hr and alloys F, G and H were homogenised to a cycle of 2 hrs/580°C followed by cooling at 250°C/hr. A sample of the established commercial long life alloy was also processed to a standard commercial practice. Cylindrical samples 10 mm dia. x 10 mm in length were machined. Triplicate samples were tested in hot compression using a Gleeble 3800 machine. The samples were heated at 100°C/min to 450°C and held for 5 mins before deforming in compression at a strain rate of 1/sec to a strain of 0.8. The recorded load was converted to true stress and the value at a strain of 0.7 was extracted as a measure of the flow stress. The mean flow stress of alloys C, E, G and H was 7-10% lower than the existing established commercial long life alloy, corresponding to a significant improvement in extrusion performance in all cases.

Table 6. Flow stress measured at 450°C at a strain rate of 1/s EXAMPLE VI GRAIN STRUCTURE OF BRAZED TUBES AT A COMMERCIAL SCALE

The alloy composition whose chemistry is detailed Table 7 was direct chill (DC) cast as 203 mm diameter billets. The billets were then homogenized (4hrs/550°C) and cooled (215°C/hr).

Table 7. The composition of the alloy used in weight percent, the balance being Al and inevitable impurities.

The material was extruded into a microchannel tube with a 0.3 mm wall on a commercial extrusion press. The microchannel tube surface was zinc arc sprayed at the press exit prior to passing through a water quench. The tubing was coiled at the press and then processed through an offline cut-to-length and sizing operation where a reduction was applied to the tube thickness.

Simulated braze thermal cycles of 2 min. at 605°C, 4 min. 605°C and an extreme cycle of 4min. 625°C were applied using a laboratory furnace. Figures 4A and 4B show the corresponding surface grain structures as revealed by Poultons macroetch. The “as extruded tube” exhibited only fine grain. The post brazed grain structure after all three treatments was fine except for a narrow band of coarse grain along one size of the tube. The band width corresponded to 6% of the tube width in all three cases.

As can be seen therefore, the examples described above and illustrated are intended to be exemplary only. The scope is indicated by the appended claims.