Abstract:
Disclosed is an aluminium alloy brazing material which is substantially lithium-free and calcium-free, and has the composition, in weight percent: Si 5.0 to 14.0; Fe 0.1 to 0.7; Mn 0.2 to 1.5; Mg max. 2.0; Zn max 1.0; optionally a wetting agent as alloying element up to 1%, and balance Al and inevitable impurities.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This claims the benefit of U.S. provisional patent application No. 60/649,582 filed Feb. 4, 2005, European patent application number 05075292.2 filed Feb. 4, 2005 and European patent application number 05076694.8 filed Jul. 22, 2005, all of which are incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to aluminium alloy brazing material which is substantially lithium-free and calcium-free, and to a brazed assembly including, as a joining material, a resolidified aluminium alloy brazing material.  
       BACKGROUND OF THE INVENTION  
       [0003]     Brazing is commonly used to assemble a complex structure, such as a heat exchanger, made of aluminium or aluminium alloy components. Generally, a clad layer of brazing material is provided on at least one component, and forms a fillet of resolidified material joining two components, following brazing. Alternatively, a fillet of aluminium alloy brazing material is added to the structure before brazing, to form a joining fillet of resolidified material in the brazed product. In recent years, there has been much improvement in the corrosion resistance of aluminium alloy sheets and tubes used in such structures. As a result, the present inventors have perceived that the corrosion resistance performance of the structure may become determined by the corrosion resistance of the fillet, rather than by the corrosion resistance of the base materials.  
         [0004]     There is little in the literature describing study of corrosion of the resolidified clad or fillet. In unpublished work, we have found that (i) the addition of Zn to the clad material is ineffective to improve corrosion resistance of the brazed product (somewhat contradicting some published work mentioned below) and (ii) adding Cu to the clad decreased the corrosion resistance of the tube to which the clad was applied (the reason for this is believed to be that the beneficial potential difference between diffusion zone and clad disappears, which results in poor core corrosion performance).  
         [0005]     Clad materials typically have a high Si content, for example 10 wt %. In one published study, Chen, Wu &amp; Li, “The effects of trace elements on Al—Si alloys brazing filler metal”, Hanjie Xuebao (1985) vol. 6, no. 2, pp. 55 (in Chinese), small additions of Na, Sr, La or Ce improved the corrosion behaviour, while Bi made it worse. This was measured by comparing the strength of the joint before and after corrosion testing. Kuroda &amp; Tohma, “Electrochemical properties of Al—Si brazing filler, Aluminium Alloys—Their physical and mechanical properties”, Proceedings of the 6 th  International Conference on Aluminium Alloys, ICAA-6, Japan, 1998, pp. 1543, report the effects of alloy elements in the clad (filler) alloy on the corrosion behaviour of the filler material. The effect of Si, Zn and Cu was studied. Corrosion was found preferentially in the eutectic phase. The conclusion reached was that a clad (filler) alloy containing a large amount of Zn may give better performance against pitting of the core. Similarly Takemoto, Okamoto &amp; Kurishima, “Sacrificial anode type Al-10Si-1Mg brazing filler metals for suppression of corrosion of brazed 3003 aluminium alloy”, Transactions of JWRI (1986), vol. 15, no. 2, pp. 111, added 0.5% Fe to a filler containing 10% Si, 1% Mg and different levels of Zn or Sn, to study the corrosion behaviour of the AA3003 core alloy. No effect of Fe was found, while Zn and Sn improved the corrosion performance of the core alloy. The same authors Takemoto &amp; Okamoto, “Effect of iron content in brazing filler metals on corrosion of brazed aluminium”, Transactions of JWRI (1986), vol. 15, no. 2, pp. 101, have reported no effect of the iron content, up to 1.4 wt %, in the 10 wt % Si filler alloy on the corrosion behaviour of AA3003 or pure aluminium. A slight effect was found on AA1100.  
         [0006]     Outside the field of brazing materials, there have been studies on the corrosion behaviour of precipitates or intermetallic compounds, which are often found in AA3xxx type alloys. These precipitates are typically Al (Mn, Fe) Si and Al (Mn, Fe), the iron being usually present as an impurity in commercial alloys. Nisancioglu &amp; Lunder, “Significance of the electrochemistry of Al-base intermetallics in determining the corrosion behaviour of aluminium alloys”, Aluminium Alloys—Physical and mechanical properties, Charlottesville USA (1986), pp. 1125, discuss these questions, and show that Mn has an effect to reduce the corrosion potential of the AlMnFe phase. It was also shown that, in alloys with 0.7 wt % Fe, Si levels up to 0.3% proved to be beneficial.  
         [0007]     Zamin, “The role of Mn in the corrosion behaviour of Al—Mn alloys”, Corrosion (1981), vol. 37, no. 11, pp. 627, studied the corrosion behaviour of laboratory-cast Al—Mn alloys, and found that the corrosion behaviour improves with the increasing Mn/Fe ratio. A similar effect was shown by Fukuzuka, Shimogori &amp; Fujiwara, “Relationship between the initiation of microscopic pitting corrosion and the composition of the Al6Mn x Fe 1-x  intermetallic compounds in aluminium-manganese alloys”, Boshoku Gijutsu (1979) vol. 28, pp. 323 (in Japanese).  
         [0008]     In the light of the disclosure of the invention below, attention is also drawn to U.S. Pat. No. 4,648,918, which describes an abrasion resistant aluminium alloy which is used for the production of mechanical parts by extrusion and has a content of 7.5 to 15 wt % of Si, 3.0 to 6.0 wt % of Cu, 0.3 to 1.0 wt % of Mg, 0.25 to 1.0 wt % of Fe and 0.25 to 1.0 wt % of Mn, balance Al and impurities. Secondly, mention is made of U.S. Pat. No. 4,854,495, which discloses, in the production of electronic components and semiconductor components where soldering of ceramic and metal is desired, the use of a high-melting jointing material comprising a core material comprised of aluminium-1.3% manganese alloy with a skin layer comprised of aluminium-10% silicon-2% manganese alloy.  
       SUMMARY OF THE INVENTION  
       [0009]     The object of the present invention is to provide aluminium alloy brazing material having improved corrosion resistance of the resolidified clad or fillet in the brazed structure, bearing in mind that in a commercially produced alloy, iron is inevitably to be found.  
         [0010]     According to the present invention there is provided an aluminium alloy brazing material which is substantially lithium-free and calcium-free, and comprising the composition, in weight percent:  
                                                       Si   5.0 to 14.0, preferably 7.0-12.0           Fe   0.1-0.7, preferably 0.2-0.6           Mn   0.2-1.5, preferably 0.3-1.0, more preferably 0.4-0.8           Mg   max. 2.0           Zn   max. 1.0, and                      
 
         [0011]     optionally a wetting agent as alloying element up to 1 wt. %, and  
         [0012]     balance Al and inevitable impurities. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0013]      FIG. 1  is the sole figure and shows a brazed structure. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     The invention is based on the finding that, in a brazing material, for example a clad or filler layer, containing a high Si content, Mn can play a significant role in improving the corrosion performance of the resolidified brazing material in a brazed product.  
         [0015]     The term “substantially free” means having no significant amount of that component purposely added to the alloy composition, e.g. at a level of &lt;0.005% and more preferably absent, it being understood that trace amounts of incidental elements and/or impurities may find their way into a desired end product.  
         [0016]     Particularly it is preferred that the Mn/Fe ratio in weight percent is at least 1, preferably at least 2 weight percent. It is also believed to be desirable to select the amount of Mn such that, in an intermetallic phase in the resolidified brazing material containing Mn and Fe, the ratio of Mn/Fe in weight percent is at least 0.4, preferably at least 0.5, more preferably at least 0.7 weight percent. All percent compositions given in this specification are weight percents unless otherwise indicated.  
         [0017]     The invention further provides a product suitable for brazing, having a base (core) of aluminium or aluminium alloy and a clad layer on said base having a lower melting point than said base, wherein the clad layer is aluminium alloy brazing material of the invention as described above.  
         [0018]     The invention further provides a brazed assembly comprising at least two members joined by an aluminium alloy brazing material of the invention as set out above. Typically this brazing material is in the form of a clad layer on one of the members or a fillet joining the two members.  
         [0019]     In the aluminium alloy brazing material of the invention, the amount of Si is selected in a conventional manner, to provide the desired brazing properties of the alloy.  
         [0020]     The amount of Fe depends primarily on the origin of the alloy material.  
         [0021]     The amount of Mn is in the range 0.2 to 1.5%, because below 0.2% the effect of improved corrosion resistance is not found. Preferably the amount of Mn is at least 0.3%, and more preferably at least 0.4%, to provide improved corrosion resistance. With a view to the properties of the alloy, the amount of Mn should be not more than 1.5%, preferably not more than 1.0%. The preferred maximum is 0.8%, since above this level the improved corrosion resistance may be less.  
         [0022]     The amount of Mg is chosen in accordance with the intended type of brazing of the particular product. For CAB brazing, a relatively low level of Mg may be present, e.g. up to 0.4%. For vacuum brazing, a higher level, up to 2.0% preferably not more than 1%, is suitable.  
         [0023]     Zn is an impurity element which can be tolerated to a level of up to 1%, and is preferably not more than 0.5%.  
         [0024]     The brazing material comprises optionally a wetting agent as alloying element in a range of up to 1 wt. %, and preferably up to 0.8 wt. % in order to improve the wettability of the brazing material during the brazing process, in particular during vacuum brazing or controlled atmosphere brazing (CAB) in the absence of a brazing flux material. Preferably the wetting agent is selected from the group consisting of lead, bismuth, antimony, tin, silver, thallium, indium, and any mixture thereof.  
         [0025]     The balance of the composition is aluminium and inevitable impurities, and preferably maximum impurities in total preferably 0.20 wt %, with no element more than 0.05 wt %.  
         [0026]     Preferred embodiments of this aluminium alloy are also substantially sodium-free and beryllium-free.  
       EXAMPLES  
       [0027]     The invention will now be illustrated by non-limitative examples.  
         [0028]     For the purposes of study, seven alloys suitable for the use as brazing filler materials (clad or fillet) were cast, having the compositions given in Table 1. Alloys 1, 2 and 3 are comparative, and have an increasing amount of Fe without Mn. Alloys 4, 5, 6 and 7 have an increasing amount of Mn, with constant Fe.  
                                                                           TABLE 1                           Amounts in wt. %, balance is aluminium       plus inevitable impurities.                                    Melting           Alloy                   Temperature           No.   Mn   Fe   Si   Mg   (° C.)                            1   &lt;0.01   0.06   10.2   0.03   578.2           2   &lt;0.01   0.26   10.2   0.03   577.6           3   &lt;0.01   0.52   10.1   0.03   576.8           4   0.21   0.28   10.1   0.03   579.0           5   0.42   0.28   10.2   0.03   578.6           6   0.49   0.27   10.0   0.03   579.1           7   0.81   0.28   10.1   0.03   579.5                      
 
         [0029]     Just before casting, 1 kg/ton AlTiB5/1 was added as a grain refiner.  
         [0030]     DSC (Differential Scanning Calometry, also known as Differential Scanning Calorimetry) was used to determine the melting temperature (onset of melting) of the alloys, since the effect of Mn on the melting point was not known. The melting points show that the melting temperature will not affect the brazing properties of these alloys.  
         [0031]     These filler alloys were used to form fillets in a brazed structure, shown in  FIG. 1 , in which the sheet  1  and the angular coupon  2  are AA3003 aluminium alloy. The brazing alloy forms the fillet  3 . In each sample about 85 mg filler alloy was used, and the samples were brazed using 2 g/m 2  flux.  
         [0032]     Corrosion tests were performed on both the filler alloys as cast, and on the brazed fillets. Polished samples were exposed to a NaCl solution (3%, twice for 90 minutes) light microscopy pictures were taken before and after corrision the same location. Corrosion was qualitatively established from the visible local attack around second phase particles. For the alloys as cast, it was found that the corrosion performance deteriorates with increasing Fe content, for alloys 1, 2 and 3. On the other hand, the corrosion attack clearly decreased with increasing Mn content in the alloys 4, 5 and 7. The corrosion performance of alloy 6 was similar to that of alloy 5.  
         [0033]     Similarly, using light microscopy images, corrosion was inspected for the fillets of the brazed product shown in  FIG. 1 . Corrosion after 90 and 180 minutes exposure in 3% NaCl was studied. While there was not much difference between the corrosion attack on alloys 1 and 2, there was much more corrosion for alloy 3. All four of alloys 4, 5, 6 and 7 showed improved corrosion resistance, compared with the fillet of alloy 2. The optimum effect was achieved for alloys 5 and 6, with alloy 7 showing a less good performance than alloys 5 and 6.  
         [0034]     From these results, improved corrosion resistance is expected for the brazing material of the invention, when used as a clad layer as a member on an aluminium or aluminium alloy base (core) in a construction subjected to brazing, with resolidification of the material of the clad layer. The brazing material of the invention can be applied for example to constructions such as heat exchangers.  
         [0035]     Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made without departing from the spirit or scope of the invention as herein described.