Patent Publication Number: US-2002007881-A1

Title: High corrosion resistant aluminium alloy

Description:
[0001] The invention relates to a high corrosion resistant aluminum 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 tines. The alloy has extensively improved resistance to pitting corrosion and enhanced properties in bending and endforming.  
       [0002] The introduction of aluminium 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 evaporator 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 automotive 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 U.S. Pat. Nos. 5,286,316 and WO97/46726. The alloys disclosed in these patents are generally alternatives to the standard AA3102 or AA1100 alloys for condenser tube uses, 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 body/vehicle tube runs, e.g. rear climate control systems, requires improved alloys due to the more heavy exposure towards the mad 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 or AA3103 with a higher strength and stiffness compared with 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 significantly improved corrosion resistance.  
       [0003] 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 (including grain size) during bending and end-forming operations.  
       [0004] The objects and advantages are obtained by an aluminium-based alloy, comprising 0.06-0.35% by weight of iron, 0.05-0.15% by weight of silicon, 0.01-1.0% by weight of manganese, 0.02-0.60% by weight of magnesium, 0.05-0.70% by weight of Zn, one or more of the elements zirconium, titanium, chromium or copper, up to a maximum of 1.30% by weight, up to 0.15% by weight of other impurities, each no greater than 0.03% by weight and the balance aluminium.  
       [0005] Preferably the iron content of the alloy according to the invention is between 0.10-0.20% by weight. In this way the corrosion resistance is increased due to smaller amounts of iron rich particles which generally creates sites for pitting corrosion attack. The relatively low iron content, however, 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 grain-structure refinement.  
       [0006] Addition of magnesium (preferably 0.15-0.30% 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 content is preferably below 0.3% by weight due to its strong effect in increasing extrudability. Additions above 0.3% by weight are also incompatible with good brazeability.  
       [0007] Preferably the manganese content is between 0.50-70% by weight, in order to counterbalance the increase in extrusion pressure obtained when adding magnesium, and reducing the negative effect of manganese with respect to precipitation of Mn bearing phases during final annealing.  
       [0008] 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 recyclable 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.  
       [0009] For recycleability acceptance of 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.  
       [0010] In order to optimise the resistance against corrosion, the zirconium content is 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.  
       [0011] Further optimising of the corrosion resistance can be obtained by adding titanium, preferably between 0.10-0.16% by weight. No significant influence on the extrudability is found for these titanium levels.  
       [0012] 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 substantial influence on extrudability even for small additions.  
       [0013] In an effort to demonstrate the improvements associated with the inventive aluminium-based alloy over known prior art alloys, the extrudability, drawability, mechanical properties (including 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 0.02% by weight of incidental impurities. Compositions were selected with varying amounts of magnesium, chromium, zinc, zirconium and titanium. In Table 1 is also shown the compositions of the standard alloys AA3003 and AA3103, which were used as reference alloys in the investigation.  
               TABLE 1                          Chemical composition of alloys (weight %)                                                     Alloy                                           desig-                                           nation   Fe   Si   Mn   Mg   Cr   Zn   Cu   Zr   Ti               AC1   0.24   0.08   0.67   0.29   —   —   —   —   —       AC2   0.23   0.09   0.70   0.29   0.10   —   —   —   —       AC3   0.24   0.08   0.70   0.27   0.22   —   —   —   —       AC4   0.21   0.08   0.68   0.28   —   0.25   —   —   —       AC5   0.20   0.08   0.67   0.27   0.07   0.24   —   —   —       AC6   0.20   0.08   0.69   0.28   0.21   0.25   —   —   —       AC7   0.20   0.09   0.68   0.29   0.22   0.11   —   —   0.05       AC8   0.21   0.10   0.69   0.27   0.18   0.23   —   —   0.16       AC9   0.25   0.13   0.67   0.05   0.04   0.16   —   —   0.16       AA3103   0.54   0.11   1.02   —   —   —   0.03   —   0.01       AA3003   0.59   0.27   1.05   0.01   —   —   0.08   —   0.01                  
 
     
    
    
     [0014] The following description details the techniques used to investigate the properties, followed by a discussion of the obtained results.  
     [0015] 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 Pechiney, were used.  
     [0016] 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.  
     [0017] 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)                      
 
     [0018] 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.  
     [0019] The extruded base tube were finally plug drawn in totally 6, 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.  
     [0020] 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.  
     [0021] 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.  
     [0022] To demonstrate the improved corrosion resistance of the inventive aluminium alloy composition over known prior art alloys, the corrosion resistance was tested using the so-called SWAAT test (Acidified Syntetic 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 D 1141. The temperature in the chamber was kept at 49° C. The test was run in a Erichsen Salt Spray Chamber (Model 606/1000).  
     [0023] In order to study the evoluton 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 is qualified as being above 20 days exposure.  
     [0024] Extrusion data for the alloys are given in Table 2 below.  
               TABLE 2                          Extrusion data for long life alloy matrix (3 hole die)                             Peak   Die                             Alloy   Chemical composition (wt %)   pressure   pressure                                                         designation   Fe   Si   Mn   Mg   Cr   Zn   Cu   Ti   (kN)   (kN)               AC1   0.24   0.08   0.67   0.29   —   —   —   —   2573   1395       AC2   0.23   0.09   0.70   0.29   0.10   —   —   —   2584   1424       AC3   0.24   0.08   0.70   0.27   0.22   —   —   —   2597   1464       AC4   0.21   0.08   0.68   0.28   —   0.25   —   —   2536   1373       AC5   0.20   0.08   0.67   0.27   0.07   0.24   —   —   2559   1415       AC6   0.20   0.08   0.69   0.28   0.21   0.25   —   —   2599   1470       AC7   0.20   0.09   0.68   0.29   0.22   0.11   —   0.05   2594   1495       AC8   0.21   0.10   0.69   0.27   0.18   0.23   —   0.16   2599   1508       AC9   0.25   0.13   0.67   0.05   0.04   0.24   —   0.16   2552   1385       3103   0.54   0.11   1.02   —   —   —   0.03   0.01   2399   1281       3003   0.59   0.27   1.05   0.01   —   —   0.08   0.01   2481   1288                  
 
     [0025] As seen from Table 2 the extrusion pressures obtained for alloys AC1 to AC9 are approximately 5-6% higher than for alloys AA3103 and AA3003. 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).  
     [0026] Surface finish after extrusion, especially on the interior of the tube, is particularly important in this application because the tube is to be cold drawl 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 investigated alloys in the test matrix showed good internal surface appearance.  
     [0027] The alloys drew well (same speed and productivity as for standard 3003/3103 alloys), except for AC6, AC7 and AC8 for which it was not possible to do make more than two draws. For these alloys the tube fractured on the third draw. It is emphasised that the fractures were microstructure related (too high Cr content) and not due to bad internal surface appearance of the tube.  
     [0028] The characteristics of the alloys after annealing is given in Table 3 (test results for alloys AC6, AC7 and AC8 based on tubes annealed after the second draw).  
               TABLE 3                          Characteristics of the alloys after drawing and soft annealing.                                             Alloy   YS   UTS   Elong. (A10)       Grain-size**   SWAAT life   Corrosion pot.       designation   (MPa)   (MPa)   (%)   n-value*   (um)   (days)   (mV SCE)                                                     AC1   51   113   36.1   0.244   82    7   −769       AC2   52   115   36.1   0.236   56   15   −755       AC3   53   117   37.1   0.232   66   15   −760       AC4   46   112   36.0   0.250   88   57   −769       AC5   51   113   36.6   0.237   79   41   −782       AC6   41   112   32.1   0.238   (71)   (82)   −810       AC7   39   112   30.0   0.234   (67)   (82)   −765       AC8   —   —   —   —   (102)    (no perfor.)   —       AC9   42   99   43.0   0.238   92   30   −830       AA3103   48   108   41.2   0.232   141     3   −730       AA3003   48   108   39.8   0.241   70    3   −754                                  
 
     [0029] From the results in Table 3 it can be seen that the mechanical properties, grain size and corrosion resistance are alloy dependent. First of all, the corrosion resistance (in terms of SWAAT life) of all the test alloys AC1 to AC9 are superior compared to the standard alloys AA3003 and AA3103. Note that the SWAAT life, given for each alloy in Table 3, represents the first tube parallel to fail out of totally 10 parallels mounted in the SWAAT chamber. The mechanical properties of the test alloys are slightly higher compared with the standard alloys, note also the refinement in grain structure obtained for the test alloys compared with AA3103.  
     [0030] Analysis of the SWAAT data shows that tubes of alloys AA3003 and AA3103 failed after only 3 days. Adding Mg (and at the same time reducing Mn and Fe) is seen to double the SWAAT life (AC1). Cr addition also improves the corrosion resistance (AC2 and AC3). Addition of Zn furthermore improves the corrosion resistance significantly (AC4, AC5 and AC9).  
     [0031] The best alloy combination with respect to corrosion are found for a relatively high Zn content (˜0.25%), and no Cr or Ti added. Acceptance of Cr in the alloy may be desirable related to recycleability, and although a reduction in SWAAT life is found when adding Cr the alloy still maintains good performance (AC5). Alloys AC4 and AC5 tubes completed 57 and 41 days of exposure in SWAAT, respectively, before failure. This is really a significant improvement.  
     [0032] 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 lamellar type. This extends the time required for corrosion to penetrate through a given thickness and thereby providing a long life alloy.  
     [0033] As can be seen from Table 3 the electrochemical corrosion potentials of the test alloys AC1 to AC9 are generally decreased (more negative) as compared to the standard alloys AA3103/AA3003. In order for the tube material not to behave sacrificial towards the filler metal (for instance when connected to cladded header in a condenser) it is recommended to select clad materials that matches the electrochemical potential. This is the usual methodology applied when designing components/systems against corrosion, and this will curb any attack of the tube due to galvanic corrosion.