WO2000050656A1

WO2000050656A1 - Extrudable and drawable, high corrosion resistant aluminium alloy

Info

Publication number: WO2000050656A1
Application number: PCT/EP2000/001518
Authority: WO
Inventors: Ole Daaland; Lars Auran; Trond Furu
Original assignee: Norsk Hydro Asa
Priority date: 1999-02-22
Filing date: 2000-02-21
Publication date: 2000-08-31
Also published as: EA200100904A1; ES2198289T3; CA2356486C; DE60002990D1; DE60002990T2; AU2914400A; EA003950B1; BR0008407B1; BR0008407A; ATE241709T1; KR20010089609A; CN1359427A; CN1159468C; CA2356486A1; EP1155157A1; KR100650004B1; US20020007881A1; JP2002538296A; EP1155157B1

Abstract

An aluminium based, corrosion resistant alloy consisting of: 0,05- 0,15 % by weight of silicon, 0,06-0,35 % by weight of iron, 0,01-1,00 % by weight of manganese, 0,02-0,60 % by weight of magnesium, 0,05-0,70 % by weight of zinc, 0-0,25 % by weight of chromium, 0-0,20 % by weight of zirconium, 0-0,25 % by weight of titanium, 0-0,10 % by weight of copper, up to 0,15 % by weight of other impurities, each not greater than 0,03 % by weight and the balance aluminium.

Description

Extrudable and drawable, high corrosion resistant aiuminium alloy.

The invention relates to a high corrosion resistant aluminium alloy, especially an alloy intended to be used for manufacture of automotive air conditioning tubes for applications as heat exchanger tubing or refrigerant carrying tube lines, or generally fluid carrying tube lines. The alloy has extensively improved resistance to pitting corrosion and enhanced mechanical properties especially in bending and endforming.

The introduction of aiuminium alloy materials for automotive heat exchange components is now widespread, applications including both engine cooling and air conditioning systems. In the air conditioning systems, the aluminium components include the condenser, the evapora- tor and the refrigerant routing lines or fluid carrying lines. In service these components may be subjected to conditions that include mechanical loading, vibration, stone impingement and road chemicals (e.g.. salt water environments during winter driving conditions). Aluminium alloys of the AA3000 series type have found extensive use for these applications due to their combination of relatively high strength, light weight, corrosion resistance and extrudability. To meet rising consumer expectations for durability, car producers have targeted a ten-year service life for engine coolant and air conditioning heat exchanger systems. The AA3000 series alloys (like AA3102, AA3003 and AA3103), however, suffers from extensive pitting corrosion when subjected to corrosive environments, leading to failure of the automotive component. To be able to meet the rising targets/requirements for longer life on the automo- tive systems new alloys have been developed with significantly better corrosion resistance. Especially for condenser tubing, 'long life' alloy alternatives have recently been developed, such as those disclosed in US-A-5,286,316 and WO-A-97/46726. The alloys disclosed in these publications are generally alternatives to the standard AA3102 or AA1100 alloys used in condenser tubes, i.e. extruded tube material of relatively low mechanical strength. Due to the improved corrosion performance of the condenser tubing the corrosion focus have shifted towards the next area to fail, the manifold and the refrigerant carrying tube lines. Additionally, the tendency towards using more under vehicle tube runs, e.g. rear climate control systems, requires improved alloys due to the more heavy exposure towards the road environment. The fluid carrying tube lines are usually fabricated by means of extrusion and final precision drawing in several steps to the final dimension, and the dominating alloys for this application are AA3003 and AA3103 with a higher strength and stiffness compared to the AA3102 alloy. The new requirements have therefore created a demand for an aluminium alloy with processing flexibility and mechanical strength similar or better than the AA3003/AA3103 alloys, but with improved corrosion resistance. The object of this invention is to provide an extrudable, drawable and brazeable aluminium alloy that has improved corrosion resistance and is suitable for use in thin wall, fluid carrying tube lines. It is a further object of the present invention to provide an aluminium alloy suitable for use in heat exchanger tubing or extrusions. It is another object of the present invention to provide an aluminium alloy suitable for use as finstock for heat exchangers or in foil packaging applications, subjected to corrosion, for instance salt water. A still further object of the present invention is to provide an aluminium alloy with improved formability during bending and end-forming operations.

The objects and advantages are obtained by an aluminium-based alloy, consisting of 0,05 - 0,15 % by weight of silicon, 0,06 - 0,35 % by weight of iron, 0,01 - 1 ,00 % by weight of manganese, 0,02 - 0,60 % by weight of magnesium, 0,05 - 0,70 % by weight of zinc, 0 - 0,25 % by weight of chromium, 0 - 0,20 % by weight of zirconium, 0 - 0,25 % by weight of titanium, 0 - 0,10 % by weight of copper, up to 0,15 % by weight of other impurities, each not greater then 0,Q3 % by weight and the balance aluminium.

Preferably the manganese content is between 0,50-0,70 % by weight, more preferably 0,62 - 0,70 % by weight. The addition of manganese contributes to the strength, however, it is a major point to reduce the negative effect manganese have with respect to precipitation of manganese bearing phases during final annealing, which contributes to a coarser final grain size.

Addition of magnesium, preferably 0,15-0,30 % by weight, and more preferably 0,25 - 0,30 % by weight, results in a refinement of the final grain size (due to storage of more energy for recrystallization during deformation) as well as improvements the strain hardening capacity of the material. In total this means improved formability during for instance bending and endforming of tubes. Magnesium also has a positive influence on the corrosion properties by altering the oxide layer. The content of magnesium is preferably below 0,3 % by weight due to its strong effect in increasing extrudability. Additions above 0,3 % by weight are generally incompatible with good brazeabiiity.

In view of the polluting effect of zinc (ex. even small zinc concentrations negatively affect the anodising properties of AA6000 series alloy), the level of this element should be kept low to make the alloy more recycleable and save cost in the cast house. Otherwise, zinc has a strong positive effect on the corrosion resistance up to at least 0,70 % by weight, but for the reasons given above the amount of zinc is preferably between 0,10 - 0,30 % by weight, more preferably 0,20 - 0,25 % by weight.

Preferably the iron content of the alloy according to the invention is between 0.06-0.22 % by weight. In general, a low iron content, preferably 0,06 - 0,18 % in weight, is desirable for improved corrosion resistance, as it reduces the amount of iron rich particles which generally creates sites for pitting corrosion attack. Going too iow in iron, however, could be difficult from a casthouse standpoint of view, and also, has a negative influence on the final grain size (due to less iron rich particles acting as nucleation sites for recrystallization). To counterbalance the negative effect of a relatively low iron content in the alloy other elements has to be added for grainstructure refinement. However, another preferred iron content for many practical applications is 0,18 - 0,22 % by weight, giving a combination of excellent corrosion properties, final grain size and casthouse capability.

The silicon content is between 0,05-0,12 % by weight, more preferably between 0,06 - 0,10 % by weight. It is important to keep the silicon content within these limits in order to control and optimise the size distribution of AIFeSi-type particles (both primary and secondary particles), and thereby controlling the grain size in the final product.

For recycleability some chromium in the alloy is desirable. Addition of chromium, however, increases the extrudability and influences negatively on the tube drawability and therefore the level is preferably 0,05-0,15 % in weight.

In order to optimise the resistance against corrosion, the zirconium content is preferably between 0,02-0,20 % in weight, more preferably between 0,10-0,18% in weight. In this range the extrudability of the alloy is practically not influenced by any change in the amount of zirconium

Further optimising of the corrosion resistance can be obtained by adding titanium, preferably between 0,10-0,25 % by weight. No significant influence on the extrudability is found for these titanium levels.

The copper content of the alloy should be kept as low as possible, preferably below 0.01 % by weight, due to the strong negative effect on corrosion resistance and also due to the negative effect on extrudability even for small additions. In an effort to demonstrate the improvements associated with the inventive aluminium-based alloy over known prior art alloys, the extrudability, drawability, mechanical properties, formability parameters and corrosion resistance were investigated for a series of alloy compositions, see Table 1. The alloys have been prepared in a traditional way by DC casting of extrusion ingots. Note that the composition of the alloys have been indicated in % by weight, taking into account that each of these alloys may contain up to 0.03 % by weight of incidental impurities. Compositions were selected with varying amounts of the different major elements. Note that alloy 1 in Table 1 is the composition of the standard AA3103 alloy, which is used as reference alloy in the investigation.

Table 1 : Chemical composition of alloys (% by weight).

Alloy Fe Si Mn Mg Cr Zn Cu Zr Ti

1 0,54 0,11 1 ,02 - - - 0,03 0,01

2 0,24 0,08 0,67 0,29 - -

3 0,23 0,09 0,70 0,29 0,10 -

4 0,24 0,08 0,70 0,27 0,22 -

5 0,21 0,08 0,68 0,28 - 0,25

6 0,20 0,08 0,67 0,27 0,07 0,24

7 0,25 0,13 0,67 0,05 0,04 0,16 0,17

8 0,22 0,10 0,74 0,29 - 0,13

9 0,21 0,10 0,72 0,25 0,10 0,12 0,19

10 0,22 0,10 0,71 0,27 0,12 0,22 0,20

11 0,23 0,09 0,70 0,26 0,01 0,11 0,08

12 0,22 0,10 0,50 0,26 - 0,22

13 0,55 0,10 0,69 0,27 - 0,21

14 0,21 0,05 0,68 0,27 0,06 0,25

The following description details the techniques used to investigate the properties, followed by a discussion of the obtained results.

The composition of the billets were determined by means of electron spectroscopy. For this analysis a Baird Vacuum Instrument was used, and the test standards as supplied by Pechi- ney, were used. Extrusion billets were homogenised according to standard routines, using a heating rate of 100°C/hr to a holding temperature of approximately 600°C, followed by air cooling to room temperature.

Extrusion of the homogenised billets were carried out on a full scale industrial extrusion press using the following conditions: billet temperature: 455 - 490°C extrusion ration: 63 : 1 ram speed: 16.5 mm/sec die: three hole extrudate: 28 mm OD tube (extrudate water cooled)

The extrudability is related to the die pressure and the maximum extrusion pressure (peak pressure). Those parameters are registered by pressure transducers mounted on the press, giving a direct read out of these values.

The extruded base tube were finally plug drawn in totally six draws to a final 9.5 mm OD tube with a 0.4 mm wall. The reduction in each draw was approximately 36 %. After the final draw the tubes were soft annealed in a batch furnace at temperature 420°C.

Testing of mechanical properties of annealed tubes were carried out on a Schenk Trebel universal tensile testing machine in accordance with the Euronorm standard. In the testing the E-module was fixed to 70000 N/mm2 during the entire testing. The speed of the test was constant at 10 N/mm2 per second until YS (yield strength) was reached, whilst the testing from YS until fracture appeared was 40 % Lo/min, Lo being the initial gauge length.

Corrosion potential measurements were performed according to a modified version of the ASTM G69 standard test, using a Gamry PC4/300 equipment with a saturated calomel electrode (SCE) as a reference. The tube specimens were degreased in acetone prior to measurements. No filing or abrasion of the tube specimen surface was performed, and the measurements were done without any form of agitation. Corrosion potentials were recorded continuously over a 60 minute period and the values presented represents the average of those recorded during the final 30 minutes of the test.

To demonstrate the improved corrosion resistance of the inventive aluminium alloy composi- tion over known prior art alloys, the corrosion resistance was tested using the so-called SWAAT test (Acidified Synthetic Sea Water Testing). The test was performed according to ASTM G85-85 Annex A3, with alternating 30 minutes spray periods and 90 minutes soak periods at 98 % humidity. The electrolyte used was artificial sea water acidified with acetic acid to a pH of 2.8 to 3.0 and a composition according to ASTM standard D1141. The temperature in the chamber was kept at 49°C. The test was run in a Erichsen Salt Spray Chamber (Model 606/1000).

In order to study the evolution of corrosion behaviour, samples from the different alloys were taken out of the chamber every third day. The materials were then rinsed in water and subsequently tested for leaks by immersing tube specimens in water and applying a pressure of 1 bars. The test as described is in general use within the automotive industry, where an acceptable performance for condenser tubing is qualified as being above 20 days exposure. Data presented from the SWAAT corrosion testing is the 'SWAAT life'; first tube sample out of totally 10 tube samples (each 0,5 m long) to perforate in the test.

It was found that during extrusion of the different alloys, the extrusion pressures obtained for the tested alloys were equal or maximum 5-6 % higher compared with the 3103 reference alloy (equals alloy 1 ). This is regarded as a small difference and it should be noted that all alloys were run at the same billet temperature and ram speed (no press-parameter optimisation done in this test).

Surface finish after extrusion, especially on the interior of the tube, is particularly important in this application because the tube is to be cold drawn to a smaller diameter and wall thickness. Surface defects may interfere with the drawing process and result in fracture of the tube during drawing. All the alloys investigated during the tests showed good internal surface appearance.

Concerning drawing, most of the alloys drew well, i.e. same speed and productivity as for standard alloy 1. Note that a number of other alloys than given in Table 1 also were tested but they were not able to withstand the required number of draws without serious fracturing, and they were therefore excluded from further consideration. Basically the reason for these alloys having difficulties in drawing was related to microstructural features being incompatible with heavy drawing reductions (i.e. large grains or particle phases). Alloys surviving more than five draws have been included in this consideration.

Table 2 summarises the results of the draw ability test. Table 2.

Alloy Intended No. of draws without Comment no. of serious fracturing of draws tube

1 6 6 OK

2 6 6 OK

3 6 6 OK

4 6 6 OK

5 6 6 OK

6 6 6 OK

7 6 6 OK

8 6 6 OK, periodically breaks during last draw

9 6 5 considerable effort to finish last draw

10 6 6 OK

11 6 5 considerable effort to finish last draw

12 6 6 OK, periodically breaks during last draw

13 6 5 breaks at last draw

14 6 5 considerable effort to finish last draw

The characteristics of the alloys after annealing is given in Table 3.

Table 3.

Alloy YS UTS Elong. n-value^* Grain-size^** SWAAT Corr. pot. life

MPa MPa A10 (%) urn 1st out mV SCE

1 48 108 41.2 0.23 141 3 -730

2 51 113 36.1 0.24 82 7 -769

3 52 115 36.1 0.24 56 15 -755

4 53 117 37J 0.23 66 15 -760

5 46 112 36.0 0.25 88 57 -769

6 51 113 36.6 0.24 79 41 -782

7 42 99 43.0 0.24 92 30 -830

8 49 112 37.8 0.24 83 32 -797

9 57 119 33.9 0.22 48 32 -814

10 51 121 36.9 0.23 59 49 -819

11 51 112 37.1 0.23 48 28 -812

12 63 106 37.2 0.22 59 25 -745

13*^** 156 169 2.0 - - 21 -770

14 49 116 34.6 0.24 46 50 -775

* n-value means strain hardening exponent, obtained by fitting a Ludwik law expression to the true stress-strain curve in the region between yield and uniform strain. ** grain size measured along the drawing direction on longitudinal tube cross sections. *** alloy is tested in H14 temper condition.

From the results in Table 3 it can be seen that the mechanical properties, grain size and corrosion resistance are strongly alloy dependent. First of all, concerning mechanical properties the test alloys in general shows slightly higher UTS and YS values compared with the reference alloy 1. The measured n-values also are slightly higher which indicates better formability due to improved strain distribution during forming. Note also the refinement in grain structure obtained for the Long Life test alloys which influences in a positive way on the formability with less risk Orange peel' effects after extensive forming.

In terms of corrosion resistance (i.e. SWAAT life) of all the test alloys are superior compared to the standard alloy 1. Tubes of alloy 1 are seen to fail after only 3 days, while significantly longer lifetimes are found for the test alloys. A major feature in obtaining increased corrosion y lifetime, is a low iron content in the alloy. Additional elements like zirconium, titanium and especially zinc introduces a second level of corrosion protection by altering the oxide layer and changing the corrosion attack morphology. For alloys 5, 6, 10 and 14 a more than 10 times improvement in corrosion resistance is obtained compared with reference alloy 1, which is really a significant improvement. The superior corrosion resistance obtained in case of the test alloys is attributable in art to the mode of corrosion attack being limited to generally a laminar type. This extends the time required for corrosion to penetrate through a given thickness and thereby providing a long life alloy.

Concerning electrochemical corrosion potentials it can be seen from Table 3 that the test alloys generally have a more negative potential (more anodic) as compared to the reference alloy 1. Adding zinc, zirconium and/or titanium strongly drags the potentials to more negative values. The fact that these Long Life alloys have a more negative potential is important information with respect to corrosion design criteria, i.e. the importance of selecting appropriate material combinations in application were the tube is connected to a fin/header material (for instance in a condenser), is emphasised. In order for the tube not to behave sacrificial towards the fin/header, materials being more anodic than the Long Life tube needs to be selected.

Claims

1. An aluminium based, corrosion resistant alloy consisting of

0,05 - 0,15 % by weight of silicon, 0,06 - 0,35 % by weight of iron, 0,01 - 1 ,00 % by weight of manganese,

0,02 - 0,60 % by weight of magnesium, 0,05 - 0,70 % by weight of zinc, 0 - 0,25 % by weight of chromium, 0 - 0,20 % by weight of zirconium, 0 - 0,25 % by weight of titanium,

0 - 0,10 % by weight of copper up to 0,15 % by weight of other impurities, each not greater than 0,03 % by weight and the balance aluminium.

2. An aluminium based alloy according to claim 1 , characterised in that it contains 0,50 - 0,70 % by weight of manganese.

3. An aluminium based alloy according to claim 2, characterised in that it contains 0,62 - 0,70 % by weight of manganese.

4. An aluminium based alloy according to anyone of the claims 1 - 3, characterised in that it contains 0,15 - 0,30 % by weight of magnesium.

5. An aluminium based alloy according to claim 1-3, characterised in that it contains 0,25 - 0,30 % by weight of magnesium.

6. An aluminium based alloy according to anyone of the claims

1-5, characterised in that it contains 0,10 - 0,30 % by weight of zinc.

7. An aluminium based alloy according to claim 1-5, characterised in that it contains 0,20 - 0,25 % by weight of zinc.

8. An aluminium based alloy according to anyone of the claims

1-7, characterised in that it contains 0,05 - 0,12 % by weight of silicon.

9. An aluminium based alloy according to claim 1-7, characterised in that it contains 0,06 - 0,10 % by weight of silicon.

10. An aluminium based alloy according to anyone of the claims 1-9, characterised in that it contains 0,06 - 0,22 % by weight or iron.

11. An aluminium based alloy according to anyone of the claims 1-9, characterised in that it contains 0,06 - 0,18 % by weight of iron.

12. An aluminium based alloy according to anyone of the claims 1-9, characterised in that it contains 0,18 - 0,22 % by weight of iron.

13. An aluminium based alloy according to any one of the claims 1-12, characterised in that it contains 0,05 - 0J5 % by weight of chromium.

14. An aluminium based alloy according to anyone of the claims 1-13, characterised in that it contains 0,02 - 0,20 % by weight of zirconium.

15. An aluminium based alloy according to claim 14, characterised in that it contains 0,10 - 0,18 % by weight of zirconium.

16. An aluminium based alloy according to anyone of the claims 1-15, characterised in that it contains 0,10 - 0,25 % by weight of titanium.

17. An aluminium based alloy according to anyone of the preceeding claims wherein said copper content ranges below about 0,01 % by weight.