Abstract:
A phosphorus-copper based brazing alloy comprising by weight: 4.0% to 8.0% phosphorus; about 0.1% to about 8% tin; 0% to about 2% antimony; and the balance copper.  
     A process of using this alloy to produce a brazed joint with a raised shoulder and with little black oxide.

Description:
[0001]    This is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/027,090, filed Dec. 20, 2001, which is a continuation-in-part of U.S. patent application Ser. No. 09/913,000, filed Jul. 25, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to novel brazing filler metal compositions. Two families of alloys (phos-copper and phos-copper-silvers) are used to braze copper and its alloys. Silver brazing alloys (composed primarily of silver, copper, zinc, tin, nickel, manganese, and cadmium) are used to braze ferrous and non-ferrous metals and alloys. These brazing alloys are designed to work at low temperatures and to provide strong, ductile joints.  
           [0003]    Researchers in the past have discovered that additions of other elements, such as tin and antimony, to phos-copper and phos-copper-silver, and nickel, manganese and lithium to silver brazing alloys, have increased the properties of these two families in important ways, adding strength to some and changing melting temperatures in others. For example, U.S. Pat. No. 5,066,456 discloses that the addition of tin and antimony up to six percent each to a phosphorus copper based alloy lowers brazing temperatures.  
           [0004]    Applicant has discovered that an alloy containing by weight: 6.0% to 7.0% phosphorus, about 2% to about 8% tin, about 0% to about 2% antimony; and the balance copper, produces a brazed joint with greatly improved and previously unexpected properties.  
           [0005]    The first of such new and unexpected properties is that the alloy forms a large cap, or shoulder, during the brazing process, and does so at a temperature that has not been possible with phosphorus/copper brazing alloys in the past without the addition of silver to the composition of the alloys. In fact, these new alloys will form a cap or shoulder similar to or superior to a silver-containing phos/copper alloy and will do so at slightly lower liquidus temperatures than the silver-containing alloys. The liquidus temperatures of brazing alloys more closely represent the most important characteristic of where the alloy flows (the working temperature).  
           [0006]    Air conditioning coils, heat exchangers, water coolers and other copper coils are manufactured by connecting copper tubing and fittings by brazing with copper-phosphorus or copper-phosphorus-silver brazing alloys. These alloys produce strong, ductile brazes, but the industry has long experienced a relatively high percentage of leaks after brazing. Most leaks are caught on the production floor during testing and are repaired. This double work of brazing and testing is very costly. More damaging, very tiny leaks can evade factory testing and end up as warranty work in the field that is both expensive and damaging to the company&#39;s brand image.  
           [0007]    The phos/copper alloys now on the market all range within a solidus temperature of 1310° F. to a liquidus temperature of 1500° F. Alloys of even higher phosphorus content, up to 8%, are now in used to enhance productivity because of their lower operating temperature cost considerations. The non-silver alloys in this group are the most commonly used in industry and contain 7.1% to 7.4% phosphorus, the balance being copper. The fact that these alloys flow and join very well is problematic in that they also flow very thinly. Torch and furnace brazing is performed as rapidly as possible to achieve good productivity. While these alloys are quick to braze, they are difficult to observe for soundness. The entire 360° of the brazed joint must be carefully viewed by the operator, for it is here that a correction, if needed, should be made. These thin-flowing alloys produce only a very small cap, or shoulder, around the pipe at the fitting junction. The alloys are thin-flowing in that they flow like a heavy coating of paint, instead of more thickly as in a putty used to seal a ⅛ crack).  
           [0008]    Even a skilled brazer cannot tell 100% of the time that he has a totally leak-free connection by visually looking at his completed braze. In some places on a given braze connection, the brazing alloy can be seen to be in places as a shoulder between the two parts, while in other places the alloy drops in the adjoining area (the capillary) without forming any noticeable shoulder. When viewing this closely, the operator can often see that the joint appears to be 100% sound, but he can&#39;t be certain of it.  
           [0009]    Most air conditioning companies submerge the copper coil, which comprises perhaps 100 brazes, into a water tank, and air pressure is added to this coil to determine if there are any leaks. The now-in-use phos/copper alloys, as described above, could be modified to form an advantageous cap by lowering the phosphorus content significantly. However, doing so is not feasible as the liquidus temperatures rise to a point of endangering the copper being brazed.  
           [0010]    It is noteworthy that silver in the range of 6-15%, when added to the phosphorus/copper alloys described above, lowers the solidus temperature to 1190° F., allows the phosphorus contents to be reduced as much as 1.5%, allows the alloy to flow in a much thicker manner, and effects a noticeable cap or shoulder to the brazed area. The popular 15% silver-phos-copper alloy has the consistency of hot taffy when hot enough to braze, and easily forms a large cap or shoulder at the joint area. This visible fillet is quickly seen by the operator and any omission can be remedied. However, the addition of silver is quite expensive.  
           [0011]    Another serious deterrent to being able to observe the quality of copper tubing brazed with phos-copper or phos-copper-silver brazing alloys is the formation of a black oxide that is formed on the actual braze surface and on the adjacent copper pipe. Because the braze and the copper pipe all turn black, it is difficult to closely inspect the actual braze.  
         SUMMARY OF THE INVENTION  
         [0012]    A process of using a phosphorus/copper/antimony/tin brazing alloy to produce a brazed joint with a raised shoulder and with little black oxide, comprising the steps of:  
           [0013]    a) melting an alloy comprising by weight:  
           [0014]    1. about 4.0% to about 8.0% phosphorus;  
           [0015]    2. about 0.1% to about 8% tin;  
           [0016]    3. 0% to about 2% antimony; and  
           [0017]    4. the balance copper;  
           [0018]    b) applying the melted alloy to a joint to be brazed;  
           [0019]    c) allowing the melted alloy to cool; and  
           [0020]    d) forming a raised shoulder of solidified alloy about the joint without the substantial production of black oxide.  
           [0021]    A principal object and advantage of the present invention is that it produces a substantial raised shoulder or cap about the brazed joint, which is a visible sign to the operator that the joint is sound.  
           [0022]    Another principal object and advantage of the present invention is that it produces a brazed joint without significant black oxide, which can obscure the operator&#39;s view of the soundness of the joint.  
           [0023]    Another principal object and advantage of the present invention is that the addition of silicon adds important benefits to phos-copper, phos-copper-silver, and silver brazing alloys. The melting range of these brazing filler metals can be reduced significantly, saving cost by reducing the use of expensive fuel gases and the time required of the brazing process.  
           [0024]    Silicon addition to these alloys also creates the advantage of lowering the surface tension when they are in the molten state. This allows the brazing alloys to better penetrate tightly fitting parts and to achieve fuller coverage of the surfaces to be brazed. For example, leaks in copper coils used in air conditioning systems are often caused by small voids within a braze, connecting with one another, to form a path wherein refrigerant gases escape.  
           [0025]    Silicon additions also offer the advantage of changing the color and texture of phos-copper and phos-copper-silver brazes from a dull, grainy, brown finish to a very smooth finish of bright silver color. To achieve this color and texture change, these alloys require a tin or antimony content of not less than 0.1 % or greater than 10% individually or in combination.  
           [0026]    Silicon addition also increases the average tensile strength of phos-copper alloys. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a cooling curve plotting liquidus temperatures of a 40%-silver brazing alloy against the weight percent of added silicon.  
         [0028]    [0028]FIG. 2 is an elevational view of a brazed copper joint that has been brazed with a phos-copper alloy of the prior art.  
         [0029]    [0029]FIG. 3 is an elevational view of a brazed copper joint that has been brazed with a phos-copper-15% silver alloy of the prior art.  
         [0030]    [0030]FIG. 4 is an elevational view of a brazed copper joint that has been brazed with an alloy of the present invention.  
         [0031]    [0031]FIG. 5 is a chart showing the tensile strength of various phos-copper alloys in the presence and absence of silicon. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0032]    [0032]FIG. 2 shows a copper joint J that has been brazed with a phos-copper alloy of the prior art. Note the absence of any substantial shoulder about the brazed joint.  
         [0033]    [0033]FIG. 3 shows a copper joint J that has been brazed with a phos-copper-15% silver alloy of the prior art. Although the joint has a substantial shoulder or cap S, there is also substantial black oxide O about the joint which obscures the operator&#39;s view of the soundness of the joint.  
         [0034]    [0034]FIG. 4 shows a copper joint that has been brazed with an alloy of the present invention. A substantial cap or shoulder S is present, and, in addition, there is little or no black oxide present.  
         [0035]    The alloy of the present invention preferably comprises: about 4.0% to about 8.0% phosphorus; about 0.1% to about 8% tin; 0% to about 2% antimony; and the balance copper.  
         [0036]    Most preferably, the alloy comprises either 6.75% phosphorus and about 2% tin; or 6.7% phosphorus and about 6% tin.  
         [0037]    The solidus temperature of the alloy is about 1178° F. and the liquidus temperature is about 1247° F.  
         [0038]    The alloy may also contain about 0.01% to about 3% by weight silicon, the benefits of which are disclosed in U.S. patent application Ser. No. 09/913,000, which is a parent to the present application.  
         [0039]    Table I shows working examples of the change in liquidus temperature of various phosphorus-copper brazing alloys with the addition of various amounts of silicon. All values are percents by weight.  
                                                     TABLE I                       Alloy   % Cu   % P   % Si   Liquidus ° F.                                1   92.9   7.1   0.0   1477       2   92.8   7.1   0.1   1465       3   92.6   7.1   0.3   1448       4   92.4   7.1   0.5   1430       5   91.9   7.1   1.0   1395       6   91.2   7.1   1.7   1343                  
 
         [0040]    Applicant expects that the benefits of the addition of silicon will also extend to other alloys having a composition of from 89.0% to 94.0% copper, 5.0% to 10.0% phosphorus, and 0.01% to 3.0 % silicon.  
         [0041]    The substantial reduction of required brazing temperatures effects considerable savings of both fuel gases and cycle time of brazing operations. The lower brazing temperatures also lessen the degree of annealing caused to parent copper and brass metals with the phos-copper alloys. Such annealing causes the metal to soften and to become very weak. Also, in some very hot brazing furnaces, slight melting of the copper or brass base parts can occur.  
         [0042]    Addition of silicon as in Table I also results in a brazing alloy with lowered surface tension when melted.  
         [0043]    Addition of silicon as in Table I also results in smoother surfaces after brazing.  
         [0044]    As silicon is added, in the presence of tin (0.1% to 6.0%) and/or antimony (0.1% to 6.0%) a color change is also effected to a bright silver color finish after brazing.  
         [0045]    [0045]FIG. 5 shows that the addition of a small amount of silicon also significantly increases the tensile strength of phos-copper alloys.  
         [0046]    Table II illustrates working examples of liquidus temperatures of two phos-copper alloys containing silver, with and without silicon. All values are percents by weight.  
                                                                         TABLE II                                   Alloy   % Cu   % P   % Ag   % Si   Liquidus ° F.                                        1   87.9   6.1   6.0   0.0   1485           2   87.4   6.1   6.0   0.5   1438           3   80.0   5.0   15.0   0.0   1462           4   79.5   5.0   15.0   0.5   1416                      
 
         [0047]    Applicant expects that the benefits of the addition of silicon will also extend to other alloys having a composition of 59% to 92% copper, 5.0% to 10.0% phosphorus, 0.1% to 18.0% silver, and 0.01% to 3.0% silicon.  
         [0048]    A significant reduction in brazing temperature is apparent in both of the silver alloys containing silicon.  
         [0049]    Addition of silicon as in Table II also results in a brazing alloy with lowered surface tension when melted.  
         [0050]    Addition of silicon as in Table II also results in smoother surfaces after brazing.  
         [0051]    As silicon is added, in the presence of tin (0.1% to 6.0%) and/or antimony (0.1% to 6.0%), a color change is also effected to a bright silver color finish after brazing.  
         [0052]    [0052]FIG. 1 is a graph of liquidus temperatures of an alloy composed of 2% tin, 30% copper, 28% zinc, and 40% silver with the addition of various amounts of silicon.  
         [0053]    Applicant expects that the benefits of the addition of silicon will also extend to other alloys with compositions of 20.0% to 40.0% copper, 5.0% to 80.0% silver, 10.0%% to 30.0% cadmium, 0.10% to 4.0% manganese, 0.50% to 4.0% nickel, 1.0% to 30.0% zinc, 0.50% to 10.0% tin, and 0.05% to 3.0% silicon.  
         [0054]    The plotted curve shows meaningful reductions in liquidus temperatures throughout the range of silicon. As silicon is added to this alloy, a color change is also effected. Without silicon, the alloy has a color of bright silvery yellow. At 0.5% silicon, the color is a lighter yellow, becoming a very light bronze at 1.5% silicon. When the alloy is composed of 2.8% silicon, the color is very close to the color of sterling silver. With variations in tin and silver content, various changes in color are effected with the addition of silicon to the silver brazing alloy family. This has definite advantages in decorative applications such as jewelry and other arts.  
         [0055]    Experiments have also shown that the addition of 0.01% to 3.0% silicon to soft solders lowers the surface tension of the solders, allowing better flow into tight connections and hardening the solders, adding strength and wear resistance. Silicon addition may also affect electrical conductivity of the solder. Soft solders come in many different alloys: lead-tin, tin-antimony, tin with copper or silver. Other metals used are zinc, aluminum, bismuth, nickel, cadmium, and indium.  
         [0056]    An example of an alloy in common usage is composed of 6% Ag, 6. 1% P, balance Cu. It has a solidus temperature of 1190° F., a liquidus of 1465° F., and is ductile. One embodiment of the present invention is of an alloy of 3% Ag, 2% Sn, 6.1% P, 0.1% Si, and the balance Cu. It has a solidus temperature of 1109° F. and a liquidus temperature of 1477° F. This alloy is quite ductile, forms a large shoulder or cap, and has a bright color when brazing is being performed. Brazing is accomplished at a lesser temperature, the braze is more visible to the operator to enable corrections, and the alloy is ductile enough to braze brass alloys that heretofore were brazed only with silver/phos/copper alloys containing from 6 to 15% silver.  
         [0057]    The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it is therefore desired that the present embodiment be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than to the foregoing description to indicate the scope of the invention.