EP1287175A1

EP1287175A1 - Corrosion resistant aluminium alloy

Info

Publication number: EP1287175A1
Application number: EP01940521A
Authority: EP
Inventors: Lars Auran; Trond Furu; Ole Daaland
Original assignee: Norsk Hydro Technology BV; Norsk Hydro Elektrisk Kvaelstof AS
Current assignee: Yara Technology BV; Norsk Hydro Elektrisk Kvaelstof AS
Priority date: 2000-05-22
Filing date: 2001-05-21
Publication date: 2003-03-05
Also published as: IS6629A; AU2001274064A1; EP1158063A1; KR20030013427A; CN1443249A; CA2409870A1; JP2003534455A; WO2001090430A1; RU2002134484A; BR0111053A; NO20025562D0; NO20025562L; US20030165397A1

Abstract

An aluminium-based alloy consisting of 0,05-1,00 % by weight of iron, 0,05-0,60 % by weight of silicon, less than 0,50 % by weight of copper, up to 1,20 % by weight of manganese, 0,02 to 0,20 % by weight of zirconium, up to 0,50 % by weight of chromium, 0,02 to 1,00 % by weight of zinc, 0,02 to 0,20 % by weight of titanium, 0,02 to 0,20 % by weight of vanadium, up to 2,00 % by weight of magnesium, up to 0,10 % by weight of antimony, up to 0,02 % by weight of incidental impurities and the balance aluminium, the total amount of Ti plus Cr plus V being less than 0.3 % by weight and the amount of V being lower than the amount of Cr, said aluminium-based alloy exhibiting high corrosion resistance and high extrudability.

Description

Corrosion resistant aluminium alloy

The present invention is directed to a group of corrosion resistant and extrudable aluminium alloys with improved elevated temperature strenght, especially to a AA3000 series type aluminium alloy including controlled amounts of titanium, vanadium and zirconium for improved extrudability and/or drawability.

In the prior art, aluminium is well recognized for its corrosion resistance. AA1000 series aluminium alloys are often selected where corrosion resistance is needed.

in applications where higher strengths may be needed, AA1000 series alloys have been replaced with more highly alloyed materials such as the AA3000 series types aluminium alloys. AA3102 and AA3003 are examples of higher strength aluminium alloys having good corrosion resistance.

Aluminium alloys of the AA3000 series type have found extensive use in the automotive industry due to their good combination of strength, light weight, corrosion resistance and extrudability. These alloys are often made into tubing for use in heat exchanger or air conditioning condenser applications.

One of the problems that AA3000 series alloys have when subjected to some corrosive environments is pitting corrosion. This type of corrosion often occurs in the types of environments found in heat exchanger or air conditioning condenser applications and can result in failure of an automotive component where the corrosion compromises the integrity of the aluminium alloy tubing.

In a search for aluminium alloys having improved corrosion resistance, more highly alloyed materials have been developed such as those disclosed in U.S. Patent nos. 4,649,087 and 4,828,794. These more highly alloyed materials while providing improved corrosion performance are not conducive to extrusion due to the need for extremely high extrusion forces.

U.S. Patent no. 5,286,316 discloses an aluminium alloy with both high extrudability and high corrosion resistance. This alloy consists essentially of about 0.1 - 0.5 % by weight of manganese, about 0.05 - 0.12 % by weight of silicon, about 0.10 - 0.20 % by weight of titanium, about 0.15 - 0.25 % by weight of iron, with the balance aluminium and incidental impurities. The alloy preferably is essentially copper free, with copper being limited to not more than 0.01 %. Although the alloy disclosed in U.S. Patent no. 5,286,316 offers improved corrosion resistance over AA3102, even more corrosion resistance is desirable. In corrosion testing using salt water - acetic acid sprays as set forth in ASTM Standard G85 (hereinafter SWAAT testing), condenser tubes made of AA3102 material lasted only eight days in a SWAAT test environment before failing. In similar experiments using the alloy taught in U.S. Patent no. 5,286,316, longer duration than AA3102 were achieved. However, the improved alloy of U.S. Patent no. 5,286,316 still failed in SWAAT testing in less than 20 days.

Accordingly, it is a first object of the present invention to provide an aluminium alloy having improved combinations of corrosion resistance and hot formability.

A still further object of the present invention is to provide an aluminium alloy which has good both hot- and cold- formability and corrosion resistance.

Other objects and advantages of the present invention will become apparent as a description thereof proceeds.

In satisfaction of the foregoing objects and advantages, the present invention provides a corrosion resistant aluminium alloy consisting essentially of, in weight percent, 0,05 - 1 ,00 % of iron, 0,05 - 0,60 % of silicon, less than 0,50 % of copper, up to 1 ,20 % of manganese, 0,02 - 0,20 % of zirconium, up to 0,50 % of chromium, 0,02 to 1 ,00 % of zinc, 0,02 - 0,20 % of titanium, 0,02 - 0,20 % of vanadium, up to 2,00 % of magnesium, up to 0,10 % of antimony, up to 0,02 % of incidental impurities and the balance aluminium.

Considering in more detail the amounts of the individual components, iron preferably is between 0,05 - 0,55 %, more preferably, between 0,05 - 0,25 %. Reducing the Fe content improves the corrosion resistance. Silicon is preferably between 0,05 and 0,20 %, more preferably, not more than 0,15 %. Copper is below 0,50 %, as this elements normally negatively influences the extrusion speed and the corrosion resistance. But in some circumstances some copper might be needed to adjust the electro-potential of the allay. Preferablly the Cu-content is below 0,05 % by weight. Zirconium is preferably between 0,02 and 0,18 %. Zn should always be present in at least 0,02 % by weight in order to improve the general level of corrosion resistance and preferably zinc content is between 0,10 and 0,50 %, more preferably between 0,10 and 0,25 %. Ttitanium is preferably between 0,02 and 0,15 %, and vanadium is preferably between 0,02 and 0,12 %. The preferred amount of manganese is highly dependent on the intended use of the article because manganese impacts extrudability, especially with thin sections. For applications with these type of alloys in which the corrosion resistance and excellent extrudability is the primary concern, manganese is preferably present in amounts between 0,05 - 0,30 % by weight. Fe is preferably present in amounts between 0,05 - 0,25 % by weight. For these applications the preferred amount of chromium is between 0,02 and 0,25%. The magnesium amount is preferably below 0,03 %. Zn is preferably present in amounts between 0,10 - 0,5 % by weight. By making an appropriate selection of the amount of these elements it is possible to have an alloy with good extrusion characteristics, with good mechanical properties and superior corrosion resistance. When the alloy is intended to be used in applications, in which after extrusion further deformation processes will be used in order to obtain a final product, such as cold deforming as e.g. drawing and/or bending, and where higher strength is required, it is preferred to have the amount of manganese between 0,50 and 0,80 % by weight. In this application chromium is preferably between 0,02 and 0,18 % by weight and magnesium below 0,30 % by weight, for brazeability reasons. The Fe content should be kept low for improved corrosion resistance. To further improve corrosion resistance 0,10 - 0,5 % Zn is added. Likewise, controlled additions of V, Zr and Ti each not more than 0,2 % by weight are made to further improve, corrosion resistance.

If the alloy is to be used in high temperature applications the role of V, Ti and especially Zr becomes important. The amounts added of each of these elements will depend on the functional requirements, however, the amount of zirconium is preferably between 0,10 and 0,18 % by weight. Further it is preferred in these applications to use post heat treatment of the cast alloy in that it is heated to a temperature of between 450 and 550°C with a heating rate of less than 150 °C/hour, and maintain the alloy at that temperature for between 2 and 10 hours. The final product may also for certain applications and especially after cold working, require a "back annealing" treatment consisting of heating the work piece to temperatures between 150 and 350 degrees Centigrade and keep at temperature for between 10 and 10000 min.

Improved corrosion resistance.

Zr and Ti in solid solution, are used separately to improve corrosion resistance in low alloy highly extrudable alloys e.g. for use in extruded tubes for automotive AC systems. The useful maximum additions of Zr and Ti when added separately is less than 0,2% by weight. Above this level primary compounds are formed that reduces the level of these elements in solid solution. In addition, the primary compounds from Zr and Ti (AI3Zr, AI3Ti) may initiate pitting corrosion as they are more noble than the Al matrix. Both Zr and Ti will upon solidification go through a peritectic reaction. The product of this reaction is revealed as a highly concentrated region of the elements in the centre of the grain (large positive partition ratio). These regions or zones will upon rolling or extrusion form a lamellae structure parallel to the surface of the work piece and slow down the corrosion in the through thickness direction.

Additions of both Zr and Ti in combination, will give larger and more concentrated zones and hence improve corrosion resistance.

V is an element with much the same behaviour and effect as Zr and Ti, but has up to now not been used much in these type of alloys. V will improve the mechanical properties in the same way as Zr and Ti, but do not have the same effect on corrosion unless the Zr-content is higher than the V-content.

Combination of all three elements will give the most optimal balance of the corrosion, strength and workability properties, if the total content of Zr, Ti and V is kept below 0,3 % by weight. .

Improved elevated temperature mechanical properties and formability.

The transition elements such as Zr, Ti, and V are known to improve formability by increasing the work hardening coefficient ("n"). The "n" increases with increased amount of the transition elements almost linearly up to some 0,5%. By combining Zr, Ti and V up to 0,45% of the transition elements may be added without the formation of deleterious primary particles of the type AI3Zr, as opposed to below 0,2% if only one of the elements is added. But it has found otherwise that above a total level of 0,3 % by weight some characteristics are negatively influenced.

Zr, Ti and V, and in particular Zr are known to impede the tendency of recrystalization, provided optimum heat treatment before high temperature processing. The ability to retard recrystalization is related to the number and size of small coherent/semi-coherent precipitates that are stable at temperature up to 300- 400 degrees Centigrade for prolonged times. The fine polygenized structure that will result from back annealing at temperatures in the 150 to 350 degrees Centigrade range will have higher mechanical strength than the corresponding recrystalized structure resulting in the absence of such transition elements. The density of these precipitates increases with increased amount of the transition elements, therefore combining the three elements would improve the mechanical property in the temperature range from ambient temperature to approx. 400 degrees Centigrade. Experimental results.

In order to prove the above mentioned statements about improvement of a number of characteristics of the alloys according to the invention, a number of experiments has been carried out, which are described below. From these results it will become clear that there is an additive effect of the simultaneous use of the three elements Zr, Ti and V up to a content of maximum 0,3 % by weight. It is supposed that this is due to the comparable behavior of the different elements Zr, Ti and V in an aluminium alloy, such as solvability, crystal structure, etc., which at the same time allowing higher effective amounts to be used than in case of only one or two of these elements.

Billets with different content of Zr, V and Ti were cast using the laboratory casting equipment at Sunndalsøra. For each alloy, four billets with a diameter of 95 mm and a length of 1.1 m were produced. At the beginning of the casting the casting speed was 115 mm/min, increasing to 240 mm/min after 15 cm cast billet. The temperature in the launder was set to be 705°C and the temperature was recorded during casting. Grain refiner (Ti₅B-wire) were added in the furnace before the casting.

After casting each billet were cut, producing three samples for extrusion and two samples for spectrographic analysis (first one sample for spectrographic analysis, then two samples for extrusion, then the second sample for spectrographic analysis (i.e. -in the middle of the billet) and finally the third sample for extrusion). Samples from the as-cast material (-middle of the billet) was etched to reveal feathery crystals, in addition samples were prepared to show grain structure and particle structure. Hardness and conductivity measurements were carried out for each alloy on specimens (2 cm x 2 cm x 1 cm) that were grinded to a grit size of 2000.

Extrusion experiments were carried out with a 8 MN vertical extrusion press, producing a tube with outer diameter 6 mm. Four extrusion trials were carried out for each alloy variant and the first three were cooled in air while the fourth were cooled in water. Samples for further investigations were taken from the first, the third and the fourth extrusion trial. The samples were taken from close to the end of the extruded profile, avoiding the very end (~2 m). In the following a presentation of the results from the investigations of the as-cast material and the extruded material will be given. The extruded material was tested in a SWAAT-test and in addition mechanical testing was carried out. A presentation of the results from these tests will also be given.

The results of the experiments are partly given as diagrams in the annexed drawings, partly as tables. In the figures there is:

Fig. 1 a diagram showing for the alloys 1-11 in the Y-axis the electrical conductivity (in MS/m) in function of the total amount of Ti, V and Zr (wt % in X-axis),

Fig. 2 a diagram showing for the alloys 1-11 in the Y-axis the main extrusion force (in kN) in function of the total amount of Ti, V and Zr (wt% in X-axis), Fig. 3 a diagram showing for the alloys 1-11 in the Y-axis the yield strength (round dots) and the ultimate tensile strength (square dots) in function of the total amount of Ti, V and Zr (wt% in X-axis).

Fig. 4 a diagram showing for the alloys 41-56 in the Y-axis the electrical conductivity (in

MS/m) in function of the total amount of Ti, V and Zr (wt % in X-axis), Fig. 5 a diagram showing for the alloys 41-56 in the Y-axis the break through pressure

(in kN) of the alloy as cast in function of the total amount of Ti, V and ZR (wt % in X-axis),

Fig. 6 a diagram showing for the alloys 41-56 in the Y-axis the breakthrough pressure (in kN) of the alloy after homogenizing at 470°C for 1 hour in function of the total amount of Ti, V and Zr (wt % in X-axis), Fig. 7 a diagram showing for the alloys 41-56 in the Y-axis the yield strenght (in MPa) of the alloy after extrusion in function of the total amount of Ti, V and ZR (wt % in

X-axis), Fig. 8 a diagram showing for the alloys 41-56 in the Y-axis the ultimate tensile strenght

(in MPa) of the alloy after extrusion in function of the total amount of Ti, V and ZR (wt % in X-axis), Fig. 9 a diagram showing for the alloys 41 -56 in the Y-axis yield strenght (in MPa) of the alloy after extrusion and subsequently homogenizing at 470°C for 1 hour in function of the total amount of Ti, V and ZR (wt % in X-axis), Fig. 10 a diagram showing for the alloys 41-56 in the Y-axis the ultimate tensile strenght

(in MPa) of the alloy after extrusion and subsequently homogenizing at 470°C for 1 hour in function of the total amount of Ti, V and ZR (wt % in X-axis), Fig. 11 a diagram showing for the alloys 41-56 in the Y-axis the ultimate tensile strenght

« (in MPa) of the alloy after homogenizing at 470°C for 1 hour and subsequently extrusion in function of the total amount of Ti, V and ZR (wt % in X-axis),

. The as-cast material

The as-cast material represents the starting point for the extrusion process and the following mechanical and corrosion testing. An investigation of the starting material has been carried out, and the results are shown in the following. Samples from the as-cast material were investigated to find the actual chemical composition and to reveal the microstructure (grain structure and particle structure) in the various alloys. The chemical composition of the material was obtained by spectrographic analysis, and the results are listed in Table 1 (alloys 1-11), Table 2 (alloys 20-35) and Table 3 (alloys 41-56).

10

15

20

25

30

35 Table 3

Conductivity measurements were also carried out, and the results are shown in Figure 1 and 4. The electrical conductivity is seen to decrease approximately linearly with increasing content of the alloying elements Zr, Ti and V. And, as seen from the figures, the effect of the various alloying elements is additive for this property.

2. The extrusion testing

To investigate the effect of the addition of Ti, V and Zr on the force during extrusion, all the alloy variants were extruded with the same extrusion conditions and the max force on the ram were measured. The temperature in the container and the ram speed was recorded during the trials and were found to be ~430°C and 1.8-1.9 mm/s respectively. The temperature in the container and the ram speed were not seen to be stable from one experiment to the next. The values of the maximum force as found from the^' experiments are shown in Figures 2, 5 and 6. The values shown in the figure are averages of four extrusion trials.

3. Mechanical testing of the extruded tubes

The results from the tension testing of the extruded tubes are shown in Figures 3,.4. As can be seen from the table and the figure, the variations in stress with changing alloy are small. The stress at max load is seen to increase slightly with increasing content of alloying elements, while the effect on the yield stress is not clear. This qualitative evalutation of the results were confirmed by a statistical analysis. 4. SWAAT-Testing

The specimens for the SWAAT testing were taken from the first of the four extrusion trials for each alloy. Specimens of 30 cm length were cut from the extruded tubes and then placed in a SWAAT chamber. The results from the SWAAT test are shown Tables 4 and 5.

Table 4

Table 5

O ω n o r Λ o n n

0 σ) Φ

Claims

1. An aluminium-based alloy consisting of 0,05-1 ,00 % by weight of iron, 0,05-0,60 % by weight of silicon, less than 0,50 % by weight of copper, up to 1 ,20 % by weight of manganese, 0,02 to 0,20 % by weight of zirconium, up to 0,50% by weight of chromium, 0,02 to 1 ,00 % by weight of zinc,

0,02 to 0,20% by weight of titanium, 0,02 to 0,20 % by weight of vanadium, up to 2,00 % by weight of magnesium, up to 0,10 % by weight of antimony, up to 0,02 % by weight of incidental impurities and the balance aluminium, the total amount of Ti plus Cr plus V being less than 0.3 % by weight and the amount of V being lower than the amount of Cr, said aluminium-based alloy exhibiting high corrosion resistance and high extrudability.

2. The alloy of claim 1 , wherein said iron content ranges between 0,05 - 0,55 % by weight.

3. The alloy of claim 2, wherein said iron content ranges between 0,05 - 0,25 % by weight

4. The alloy of any one of the preceding claims, wherein said silicon content ranges between 0,05 - 0,20 % by weight.

5. The alloy of any one of the preceding claims, wherein said silicon content ranges between 0,05 - 0,15 % by weight.

6. The alloy of any one of the preceding claims wherein said copper content ranges below 0,05 % by weight.

7. The alloy of any one of the preceding claims wherein said zirconium content ranges between 0,02 and 0,18 % by weight.

8. The alloy of any one of the preceding claims wherein said zinc content ranges between 0,02 and 0,50 % by weight.

9. The alloy of claim 8 wherein said zinc content ranges between 0,10 and 0,50% by weight.

10. The alloy of claim 9 wherein said zinc content ranges between 0,10 and 0,25 % by weight.

11. The alloy of any one of the preceding claims wherein said titanium content ranges between 0,02 and 0,15 % by weight.

12. The alloy of any one of the preceding claims wherein said vanadium content ranges between 0,02 and 0,12 % by weight.

13. The alloy of any one of the preceding claims wherein said manganese content ranges between 0,05 and 0,30 % by weight.

14. The alloy of claim 13 wherein said chromium content ranges between 0,02 and 0,25 % by weight.

15. The alloy of any one of the claims 13 or 14 wherein said magnesium content ranges between 0,00 and 0,03 % by weight.

16. The alloy of any one of the claims 1 to 12 wherein said manganese content ranges between 0,50 and 0,80 % by weight.

17. The alloy of claim 16 wherein said chromium content ranges between 0,02 and 0,18 % by weight.

18. The alloy of any one of the claims 16 or 17 wherein said magnesium content ranges between 0,00 and 0,30 % by weight.

19. The alloy of any one of the claims 1 to 12 wherein said zirconium content ranges from

0,10 to 0,18 % by weight.

20. The alloy of claim 18 which after casting has been heated with a rate of less than 150 °C/hour heating rate to a temperature of between 450 and 550°C, and kept at said temperature from 2 to 10 hours.

1. The alloy of claim 19 that after cold forming has been annealed with controlled and slow heating rate to temperatures of between 150 and 350 degrees C and kept at temperature for between 10 and 10000 min.