Copper and copper alloys
Copper and its alloys are widely used due to their outstanding electrical conductivity, thermal conductivity, corrosion resistance, and formability. These alloys can generally be divided into four categories: red copper, brass, bronze, and white copper.
Material properties of copper and copper alloys
Red copper is a pure form of copper with a copper content of at least 99.5%.
It can be further divided into pure copper and oxygen-free copper based on its oxygen content.
Cu2O and CuO oxides can form on the surface of red copper.
At room temperature, the copper surface is covered with Cu2O.
Under high temperatures, the oxide scale is composed of two layers: the outer layer is CuO and the inner layer is Cu2O.
It is important to note that pure copper cannot be brazed in a hydrogen-containing reducing atmosphere.
Brass refers to a copper-zinc alloy that has greater strength, hardness, and corrosion resistance compared to red copper while still retaining toughness and high corrosion resistance.
Metallographic diagram of brass
Tin brass contains approximately 1% tin (Sn) and the presence of tin does not alter the composition of surface oxides.
The solderability of tin brass is comparable to that of brass, making it easy to solder.
Lead brass contains lead, which when heated forms a sticky slag that impairs the wetting effect and fluidity of solder.
It is important to select the proper flux to ensure proper fluidity.
The surface of manganese brass is comprised of zinc oxide and manganese oxide.
Manganese oxide is relatively stable and difficult to remove, so it is necessary to use an active brazing flux to ensure the wettability of the brazing filler metal.
There are various types of bronze, each with different alloy elements, which affects their brazability.
When the alloy element added is tin, or a small amount of chromium or cadmium, it has minimal impact on solderability and is generally easier to braze.
However, if the added element is aluminum, particularly when the aluminum content is high (up to 10%), the aluminum oxide on the surface is difficult to remove, causing a deterioration in solderability.
In such cases, it is necessary to use a special flux for brazing.
For instance, when silicon is added to form silicon bronze, it becomes highly sensitive to hot brittleness and stress cracking when exposed to molten solder.
Another example is when the added alloy element is beryllium.
Although a relatively stable BeO oxide forms, conventional brazing flux is sufficient for removing the oxide film.
White copper is an alloy of copper and nickel that boasts excellent comprehensive mechanical properties.
It contains nickel.
When selecting filler metal, it is important to avoid those containing phosphorus, such as copper-phosphorus filler metal and copper-phosphorus-silver filler metal.
White copper is highly sensitive to hot cracking and stress cracking when subjected to molten solder.
Brazability of common copper and copper alloys
|alloy||Copper T1||Oxygen free copper TU1||Brass||Tin-bronze
|Mn copper nickel alloy
Brazing filler metal – silver based brazing filler metal
Silver-based solder is widely utilized due to its moderate melting point, good processability, strong and tough qualities, conductivity, thermal conductivity, and corrosion resistance.
The main alloy elements of silver-based solders are copper, zinc, cadmium, and tin. Copper is the most important alloy element, as it reduces the melting temperature of silver without forming a brittle phase.
The addition of zinc further lowers the melting temperature.
While the addition of tin can significantly lower the melting temperature of silver-copper-tin alloys, this low melting temperature results in extreme brittleness and lack of practical use.
To avoid brittleness, the tin content in silver-copper-tin solder is typically not higher than 10%.
To further reduce the melting temperature of silver-based solder, cadmium can be added to the silver-copper-zinc alloy.
Chemical composition and main properties of silver based brazing filler metal
|Brazing filler metal||Chemical composition(weight %)||Melting temperature/℃||Tensile strength/MPa||Electrical resistivity/μΩ·m||Brazing temperature/℃
Brazing filler metal -Copper phosphorus solder
Copper-phosphorus brazing filler metal is widely utilized in brazing copper and copper alloys due to its favorable technological performance and cost-effectiveness.
Phosphorus serves two functions in copper:
First, it significantly lowers the melting point of copper.
Second, it acts as a self-soldering flux during brazing in air.
To further reduce the melting temperature of the Cu-P alloy and improve its toughness, silver can also be added.
It is important to note that copper-phosphorus and copper-rattan-silver filler metals can only be used for brazing copper and copper alloys and cannot be used for brazing steel, nickel alloys, or copper-nickel alloys with a nickel content greater than 10%.
This type of filler metal may result in segregation when heated slowly, so it is best to adopt a fast heating brazing method.
Chemical composition and properties of copper phosphorus solder
|Filler metal||Chemical composition (mass fraction) (%)||Melting temperature||Tensile strength MPa||Resistivity/μΩ·m|
Soft solder-Tin based solder
When brazing copper with Sn-based solder, the formation of the intermetallic compound Cu6Sn5 at the interface between the solder and base metal is common. Therefore, it is important to carefully consider the brazing temperature and holding time.
When using a soldering iron, the compound layer is typically thin and has minimal impact on the joint’s performance.
Brass joints brazed with tin-lead filler metal are stronger than copper joints brazed with the same filler metal. This is because the dissolution of brass in the liquid filler metal is slower, resulting in the formation of fewer brittle intermetallic compounds.
|Brazing filler metal||Chemical composition||Fusion temperature||Tensile strength||Elongation|
|Brazing filler metal||Chemical composition||Fusion temperature|
Soft solder – cadmium based solder
Chemical composition and properties of cadmium based solder
|Filler metal||Chemical composition (mass fraction) (%)||Melting temperature/||Tensile strength/MPa|
Soft solder – lead-free solder
Lead free solder for brazing copper tubes
|Brand||Composition (mass fraction)||Solid phase line/℃||Liquidus/℃|
Strength of Copper and Brass Joints Brazed with Part of Soft Solder
|Solder brand||Shear strength/MPa||Tensile strength/MPa|
The commonly used brazing fluxes consist of a matrix of borax, boric acid, or a mixture of both, and are supplemented with fluorides or fluoroborates of alkali or alkaline earth metals to achieve an appropriate activation temperature and improve oxide removal capabilities.
When heated, boric acid (H3BO3) breaks down to form boric anhydride (B2O3).
The reaction formula is as follows:
The melting point of boric anhydride is 580°C.
It can react with copper, zinc, nickel, and iron oxides to form a soluble borate, which floats on the brazed joint as a slag. This not only removes the oxide film but also provides mechanical protection.
Borax Na2B4O7 melts at 741 ℃:
Boric anhydride and metal oxides react to form soluble borates. Sodium metaborate and borates combine to form compounds with a lower melting temperature, making them easy to rise to the surface of solder joints.
The combination of borax and boric acid is a commonly utilized flux. The addition of boric acid can lower the surface tension of the borax flux and enhance its spread. Boric acid also enhances the ability of the flux residue to cleanly detach from the surface. However, when using borax-boric acid flux with silver filler metal, its melting temperature remains too high and its viscosity is still too high.
To further decrease the melting temperature, potassium fluoride can be added. The primary role of potassium fluoride is to lower the viscosity of the flux and enhance its ability to remove oxides. To further reduce the melting temperature and increase its activity, KBF4 can be added.
The melting point of KBF4 is 540 ℃, and the melting decomposition is:
|Brand||Composition (mass fraction) (%)||Action temperature ℃||Purpose|
|FB101||Boric acid 30, potassium fluoroborate 70||550~850℃||Silver solder flux|
|FB102||Anhydrous potassium fluoride 42, potassium fluoroborate 25, boric anhydride 35||600~850℃||The most widely used silver solder flux|
|FB103||Potassium fluoborate>95, potassium carbonate<5||550~750℃||For silver copper zinc cadmium solder|
|FB104||Borax 50, boric acid 35, potassium fluoride 15||650~850℃||Brazing with silver based filler metal in furnace|
Soft soldering flux
|1||ZnCl21130g,NH4Cl110g,H2O4L||Brazing copper and copper alloys, steel|
|2||ZnCl21020g,NaCI280g,NH4CI,HCI30g,H2O4L||Welding copper and copper alloys, steel|
|3||ZnCl2600g,NaCl170g||Dipped brazing covering agent|
|4||ZnCl2710g, NH4Cl100g, Vaseline 1840g, H2O 180g||Brazing copper and copper alloys, steel|
|5||ZnCl21360g,NH4Cl140g,HC185g,H2O4L||Brazing silicon bronze, aluminum bronze, stainless steel|
|6||H3P04960g,H20455g||Brazed manganese bronze, Stainless steel|
|QJ205||ZnCl250g,NH4Cl15，CdCl230，NaF6||Brazing of copper and copper alloys with cadmium based filler metals|
Weakly corrosive flux
|1||Glutamic acid hydrochloride 540g, urea 310g, water 4L||Copper, brass, bronze|
|2||Hydrazine monobromide 280g, water 2550g, non-ionic wetting agent 1.5g||Copper, brass, bronze|
|3||Lactic acid (85%) 260g, water 1190g, wetting agent 3g||Wrinkled bronze|
Non corrosive flux
The main component of non corrosive flux is rosin.
There are three commonly used rosin fluxes:
- Inactivated rosin;
- Weak activated rosin;
- Active rosin.
- Solvent degreasing or alkaline solution is applicable to copper and copper alloys.
- Mechanical methods, wire brushes and sandblasting can be used to remove oxides.
- Silicon brass;
- Chromium bronze and copper nickel alloy;
- Aluminum bronze beryllium bronze;
- Copper, brass, tin bronze.
Copper and its alloys can be brazed using various methods such as iron brazing, immersion brazing, flame brazing, induction brazing, resistance brazing, furnace brazing, contact reaction brazing, and others. However, during high-frequency brazing, a high heating current is necessary due to the low resistance of copper.
Copper and copper alloy brazing technology
When brazing copper, the coordination of filler metal and flux is as follows:
When soldering clean surfaces, especially with tin lead and tin silver solder, rosin flux can be used. For other surfaces, active rosin, weak corrosive flux, or corrosive flux can be utilized.
It is important to note that pure copper should not be brazed in a reducing atmosphere, except for oxygen-free copper, in order to avoid hydrogen embrittlement.
The filler metal and flux used for brazing brass are generally similar to those used for brazing copper. However, it should be noted that due to the presence of zinc oxide on the surface of brass, it cannot be brazed with inactive rosin. Additionally, when brazing with copper phosphorus and silver solder, FB102 flux must be used.
For tin-lead brazing, a phosphoric acid solution flux should be utilized. Lead-based brazing requires the use of a zinc oxide solution brazing flux. Q205 brazing flux is used for cadmium-based brazing. BAg45CuCdNi and BAg45CuCd solders should be brazed with FB102 or FB103 flux. Other silver-based solders, as well as copper phosphorus and copper phosphorus silver solders, should be brazed with FB102 flux. It is recommended to braze using FB104 flux in a protective atmosphere within a furnace.
When brazing beryllium bronze in its soft soldering quenching aging state, it is important to select a brazing filler metal with a melting temperature lower than 300°C. The preferred combination for this application is 63Sn-37Pb in combination with a weak corrosive flux or a corrosive flux. Additionally, brazing and solution treatment should be carried out simultaneously during the brazing process.
Soft soldering has minimal impact on the performance index of beryllium bronze, thus soft solders and fluxes similar to those used for beryllium bronze can be utilized for brazing.
It is important to note that chromium bronze should not be brazed in its solution aging state, but rather in the solution treatment state followed by aging.
When using a rapid heating method for brazing, it is recommended to use the silver solder with the lowest melting temperature, such as BAgA0 CuZnCdNi.
Cadmium bronze and tin bronze
Brazing tin bronze is similar to brazing copper and brass, but with the added benefit of avoiding hydrogen embrittlement and zinc volatilization when brazing in a protective atmosphere.
However, it should be noted that tin bronze containing phosphorus has a tendency towards stress cracking.
For soft soldering, it is recommended to use a strong corrosive flux containing hydrochloric acid.
During brazing, there is a tendency towards stress cracking and intergranular penetration of the filler metal. The brazing temperature should be below 760°C.
Silver solders with lower melting temperatures, such as BAg65CuZn, BAg50 CuZnCd, BAg40 CuZnCdNi, and BAg56 CuZnSn, can be used. The lower the melting temperature, the better.
For optimal results, FB102 and FB103 are the recommended fluxes to use.
When performing soft soldering, it is important to use a strong corrosive flux containing hydrochloric acid to remove the oxide film on the surface. The commonly used solder for this process is tin-lead solder.
For brazing, silver filler metal is typically used. To prevent aluminum from diffusing into the silver solder, the brazing heating time should be kept as short as possible. Plating the surface of aluminum bronze with copper or nickel can also prevent the diffusion of aluminum into the solder.
Zinc white copper
The soldering process for zinc white copper is similar to that of brass. The following silver solders are commonly used for brazing: BAg56CuZnSn, BAg50CuZnSnNi, BAg40CuZnNi, and BAg56CuZnCd, among others. The recommended fluxes for use are FB102 and FB103.
Manganese white copper
For soldering zinc white copper, a phosphoric acid solution flux can be used or the surface can be pre-plated with copper.
The brazing filler metals that can be used include BAg60CuZn, BAg45CuZn, BAg40CuZnCdNi, and BAg50 CuZnCd, among others.
It is not recommended to use copper-phosphorus silver solder, as phosphorus and nickel will form a brittle compound phase.
Joint Strength of Copper and Brass Brazed with Silver Solder
|Filler metal||Shear strength/MPa||Tensile strength/MPa|
Mechanical properties of copper joints brazed with copper phosphorus and copper phosphorus silver solders
|Filler metal||Tensile strength|
/J · cm-2
Post weld heat treatment
For age-hardenable copper alloys, such as beryllium bronze, that have undergone heat treatment, the only step after brazing is to remove the residual flux and clean the workpiece surface.
The primary reason for removing the residue is to prevent corrosion on the workpiece and, in some cases, to achieve a good appearance or prepare the workpiece for further processing.