I. Copper and copper alloys
Copper and its alloys are widely used due to their excellent electrical conductivity, thermal conductivity, corrosion resistance and formability. These alloys can generally be divided into four categories: red copper, brass, bronze, and white copper.
II. Material properties of copper and copper alloys
1. Red copper
Red copper is a pure form of copper with a copper content of at least 99.5%.
It can be divided into pure copper and oxygen-free copper based on its oxygen content.
Cu 2 Oxides of O and CuO can form on the surface of red copper.
At room temperature, the copper surface is covered with Cu 2 Ó.
Under high temperatures, the oxide scale is composed of two layers: the outer layer is CuO and the inner layer is Cu. 2 Oh.
It is important to note that pure copper cannot be brazed in a reducing atmosphere containing hydrogen.
2. Brass
Brass refers to a copper-zinc alloy that has greater strength, hardness and corrosion resistance compared to red copper, while maintaining toughness and high corrosion resistance.
Brass metallographic diagram
3. Special brass
(1) Tin Brass:
Tinned brass contains approximately 1% tin (Sn) and the presence of tin does not alter the composition of the surface oxides.
The solderability of tinned brass is comparable to that of brass, making soldering easier.
(2) Lead brass:
Lead brass contains lead, which when heated forms a sticky slag that impairs the wetting effect and fluidity of the solder.
It is important to select the proper flow to ensure proper fluidity.
(3) Manganese brass:
The surface of manganese brass is composed of zinc oxide and manganese oxide.
Manganese oxide is relatively stable and difficult to remove, so it is necessary to use active brazing flux to ensure the wettability of the brazing filler metal.
4. Bronze
There are several types of bronze, each with different alloying elements, which affect their brazing.
When the added alloying element is tin, or a small amount of chromium or cadmium, it has minimal impact on weldability and is generally easier to weld.
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 weldability.
In these cases, it is necessary to use a special brazing flux.
For example, 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 alloying element is beryllium.
Although a relatively stable BeO oxide is formed, conventional brazing flux is sufficient to remove the oxide film.
5. White copper
White copper is an alloy of copper and nickel that has excellent comprehensive mechanical properties.
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.
III. Typical copper and copper alloy compositions and heat treatment
| Name | Code | Primary Chemical Composition (Mass Percentage, %) | Melting temperature/℃ | Heat treatment | |||||||
| ω（Cu） | ω（Zn） | ω（Sn） | ω（Pb） | ω（Mn） | ω（Al） | ω（Ni） | Others | ||||
| Pure Copper | T1 | ≤99.95 | – | – | – | – | – | – | 20.02 | 1083 | Annealing: 450~520℃ |
| T2 | ≤99.90 | – | – | – | – | – | – | 20.06 | 1083 | Annealing: 500 ~ 630 ℃ | |
| Oxygen-free copper | ATU1 | ≤99.97 | – | – | – | – | – | – | 20,003 | 1083 | Vacuum Annealing: 500℃ |
| TU2 | ≤99.95 | – | – | – | – | – | – | 20,003 | 1083 | ||
| SHIFT | ≤99.60 | – | – | – | 0.1~0.3 | – | – | 20,003 | 1083 | ||
| Brass | H96 | 95~97 | Rem. | – | – | – | – | – | – | 1056~1071 | Annealing: 600℃ |
| H68 | 67~70 | Rem. | – | – | – | – | – | – | 910~939 | Annealing: 600℃ | |
| H62 | 60.5~63.5 | Rem. | – | – | – | – | – | – | 899~906 | Annealing: 600℃ | |
| Tin brass | HSn62-1 | 61~63 | Rem. | 0.7~1.1 | – | – | – | – | – | 886~907 | Annealing: 600℃ |
| Lead Brass | HPb59-1 | 57~60 | Rem. | – | 0.8~1.9 | – | – | – | – | 886~901 | Annealing: 600℃ |
| Manganese Brass | HMn58-2 | 57~60 | Rem. | – | – | 1~2 | – | – | – | 866~881 | Annealing: 600℃ |
| Tin bronze | QSn6.5-0.1 | Rem. | – | 6~7 | – | – | – | – | P: 0.1~0.25 | ~996 | Annealing: 500 ~ 620 ℃ |
| QSn4-3 | Rem. | 2.7~3.3 | 3.5~4.5 | – | – | – | – | – | ~1046 | ||
| Bronze Aluminum | QAl9-2 | Rem. | – | – | – | 1.5~2.5 | 8~10 | – | – | ~1061 | Annealing: 700~750℃;Quenching880℃,Tempering400℃ |
| QAl10-4-4 | Rem. | – | – | – | – | 9.5~11 | – | Fe: 3.5~4.5 | – | Annealing: 700~750℃;Quenching920℃,Tempering650℃ | |
| Beryllium Bronze | QBe2 | Rem. | – | – | – | – | – | 0.2~0.5 | Ser: 1.9 ~ 2.2 | 865~956 | Quenching: 800℃, Aging: 300℃ |
| QBe1.7 | Rem. | – | – | – | – | – | 0.2~0.4 | Ser: 1.6 ~ 1.8 | – | Quenching: 800°C, Aging: 300°C | |
| Silicon Bronze | QSi3-1 | Rem. | – | – | – | 1~1.5 | – | – | Si: 2.75~3.5 | 971~1026 | Annealing: 600 ~ 680 ℃ |
| Bronze Chrome | QCr0.5 | Rem. | – | – | – | – | – | – | Cr: 0.5~1.0 | 1073~1080 | Extinguishing: 950 ~ 1000 ℃ |
| Aging: 400 ~ 460 ℃ | |||||||||||
| Cadmium Bronze | QCd1 | Rem. | – | – | – | – | – | – | CD: 0.9~1.2 | 1040~1076 | Annealing: 650℃ |
| Zinc Nickel Silver | BZn15-20 | Rem. | 18~20 | – | – | – | – | 13.5~16.5 | – | ~1081 | Annealing: 700℃ |
| Manganese Nickel Silver | BMn40-1.5 | Rem. | – | – | – | 1~2 | – | 39~40 | – | 1261 | Annealing: 1050~1150℃ |
4. Brazing properties of copper and copper alloys
Brazing of copper and copper alloys mainly depends on the following factors:
- The stability of the oxides formed on the surface.
- The influence of the brazing heating process on material properties.
- The sensitivity of the material to stress cracking.
Pure copper surfaces can form two oxides, Cu2O and CuO. At room temperature, a copper surface is covered by Cu2O, while at high temperatures, the oxide film is divided into two layers, with CuO on the outside and Cu2O on the inside. Copper oxides are easy to remove, so pure copper welds well.
Oxygenated copper is copper refined using pyrometallurgy and electrolytically resistant copper. It contains 0.02% to 0.1% oxygen by mass, which exists as copper oxide, forming a eutectic organization with copper. This eutectic organization is distributed in the copper matrix in globular form.
If oxygenated copper is brazed in a reducing atmosphere containing hydrogen, the hydrogen quickly diffuses into the metal, reducing the oxide to produce steam. This vapor forms cavities within the copper crystals and expands rapidly, leading to hydrogen embrittlement. In severe cases, the copper material may fracture.
If the atmosphere contains carbon monoxide and moisture, the carbon monoxide can reduce the vapor to hydrogen, which then diffuses into the metal, resulting in hydrogen embrittlement. Therefore, oxygenated copper should not be brazed in ammonia-reducing, endothermic or exothermic decomposition atmospheres.
Prolonged heating of oxygenated copper above 920°C will cause copper oxide to accumulate at grain boundaries, decreasing the strength and ductility of the copper. Therefore, during brazing, the material must avoid prolonged exposure to temperatures above 920°C.
Copper cannot be heat treated to increase strength, therefore cold working methods are often used to increase its strength. Cold hardened copper will soften when heated between 230°C and 815°C. The degree of softening depends on the temperature and the duration at this temperature. The higher the brazing heating temperature, the softer the cold work hardened copper becomes.
Oxygen-free copper has low oxygen content, and there are no eutectic constituents of copper and copper oxide in copper. Its electrical conductivity and cold workability (such as deep drawing and spinning) are better than those of deoxidized copper.
Oxygen-free copper can be brazed in a protective atmosphere containing hydrogen without hydrogen embrittlement. Oxygen-free copper hardened by cold working also softens during heating.
Ordinary brass can be divided into three categories: low brass (mass fraction of zinc less than 20%), high brass (zinc fraction greater than 20%), and alloy brass. When the mass fraction of zinc in brass is less than 15%, the surface oxide mainly consists of Cu2O, which contains small ZnO particles.
When the mass fraction of zinc is greater than 20%, the oxide mainly comprises ZnO. Zinc oxide is also easy to remove, so brass brazing is very good. Brass is not suitable for brazing in a protective atmosphere, especially vacuum brazing. This is because zinc has a high vapor pressure (reaching 105Pa at 907°C).
During brazing in a protective atmosphere, especially vacuum brazing, the zinc in the brass volatilizes, the surface turns red and affects both the brazing and the inherent properties. If brazing must be done in a protective atmosphere or in a vacuum, a layer of copper or nickel must be previously galvanized on the surface of the brass parts to prevent zinc volatilization. However, the coating can affect the strength of the welded joint.
Brazing brass requires the use of a flux.
Tinned brass has approximately 1% ω (Sn). The presence of tin does not affect the surface oxide composition. The brazing of tinned brass is comparable to that of brass and is easy to braze.
Lead brass forms a sticky residue when heated, which impairs the wetting action and fluidity of the brazing material, therefore, an appropriate flux must be chosen to ensure the wetting action of the brazing material. When lead brass is heated, it tends to stress crack. Its sensitivity to hot cracking is directly proportional to the lead content.
Therefore, the internal stress of lead brass must be minimized during brazing, such as by annealing before soldering to remove the stress caused by component processing. The heating temperature should be as uniform as possible to reduce thermal stress. The brazing effect is weak when ω (Pb) > 3%. For lead brass with ω (Pb) > 5%, brazing is not recommended.
The surface of manganese brass is composed of zinc oxide and manganese oxide. Manganese oxide is relatively stable and difficult to remove, therefore a highly active flux must be used to ensure the wettability of the brazing material.
QSn6.5-0.1 tin bronze forms two oxides on its surface: an inner layer of SnO2 and an outer layer of copper oxide. These oxides are easy to remove and the alloy welds well, making it suitable for various brazing methods, including gas shielded brazing and vacuum brazing.
Conventional fluxes can be used for air brazing. To prevent cracking, phosphor-containing tin bronze parts must be stress relieved at approximately 290-340°C before brazing.
Aluminum bronze contains a significant amount of aluminum (up to 10% by weight), forming an oxide layer on the surface composed mainly of aluminum oxide, which is difficult to remove. Therefore, brazing aluminum bronze is quite challenging. Aluminum oxide cannot be reduced in a protective atmosphere and cannot be removed by vacuum heating, requiring a specialized flux.
If aluminum bronze parts are welded in a quenched and tempered state, the brazing temperature should not exceed the tempering temperature. For example, the tempering temperature of QAl9-2 is 400°C.
If the brazing temperature exceeds 400°C, the base material will soften. If brazing is done at high temperatures, the brazing temperature must correspond to the quenching temperature (880°C), followed by tempering, to achieve the desired mechanical properties of the base material. This must be considered when selecting a brazing material.
Although a relatively stable BeO oxide forms on the surface of beryllium bronze, conventional flux still satisfies the requirement of removing the oxide film. Beryllium bronze is often used in situations where parts require elasticity.
To avoid decreasing this property, the brazing temperature must be below the aging temperature (300°C) or the brazing temperature must correspond to the quenching temperature, followed by aging treatment after brazing.
Silicon bronze, mainly QSi3-1 alloy with about 3% ω (Si), forms an oxide composed mainly of silicon dioxide on its surface. The same flux used for brazing aluminum bronze should be used for brazing silicon bronze. Stressed silicon bronze is extremely sensitive to thermal cracking and stress cracking under the action of molten brazing material.
To avoid cracking, the alloy must be stress relieved at a temperature between 300-350°C before brazing. A brazing material with a lower melting point should be chosen and a brazing method that heats uniformly should be used during brazing.
Chrome bronze and cadmium bronze contain small amounts of chromium or cadmium, which do not significantly affect the brazing process. When brazing chrome bronze, the heat treatment regime of the base material must be considered.
Brazing must occur below the aging temperature (460°C) or the brazing temperature must correspond to the quenching temperature (950-1000°C).
Nickel silver and manganese silver. Nickel silver contains nickel and phosphorus-containing brazing materials such as copper-phosphorus brazing material and copper-phosphorus-silver brazing material should be avoided when choosing a brazing material because phosphorus-containing brazing materials can easily form phosphorus phosphide. brittle nickel at the interface after brazing, reducing joint strength and toughness.
Nickel silver is extremely sensitive to both hot cracking and stress cracking under the action of molten brazing material. Therefore, the internal stresses of the parts must be removed before brazing, and a brazing material with a lower melting point must be chosen.
Parts must be heated evenly and free expansion and contraction of parts during heating and cooling must be allowed to reduce thermal stress during brazing.
Brazing of common copper and copper alloys
| turns on | Brazability | |
| Copper T1 | Great | |
| Oxygen-free copper TU1 | Great | |
| Brass | H96 | Great |
| H68 | Great | |
| H62 | Great | |
| Tin-bronze | HSn62-1 | Great |
| Manganese brass | HMn58-2 | Good |
| Tin-bronze | QSn58-2 | Great |
| QSn4-3 | Great | |
| Lead Brass | HPb59-1 | Good |
| aluminum bronze | QAl9-2 | Bad |
| QAl10-4-4 | Bad | |
| beryllium bronze | QBe2 | Good |
| QBe1.7 | Good | |
| silicon bronze | QSi3-1 | Good |
| chrome bronze | QCr0.5 | Good |
| cadmium bronze | QCd11 | Great |
| Zinc-copper-nickel alloy | BZn15-20 | Good |
| Nickel Copper Manganese Alloy | BMn40-1.5 | Difficult |
V. Filler metal for brazing
1. Silver-based brazing filler metal
Silver-based solder is widely used due to its moderate melting point, good processability, strong and tough qualities, conductivity, thermal conductivity and corrosion resistance.
The main alloying elements in silver-based solders are copper, zinc, cadmium and tin. Copper is the most important alloying element, as it reduces the melting temperature of silver without forming a brittle phase.
The addition of zinc further reduces the melting temperature.
Although the addition of tin can significantly reduce 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 normally no more 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/℃ | |||||
|---|---|---|---|---|---|---|---|---|---|---|
| Ag | Ass | Zn | CD | Sn | other | |||||
| BAg72Cu. | 72±1 | Rem. | – | – | – | – | 779~779 | 375 | 0.022 | 780~900 |
| BAg50Cu. | 50±1.1 | Rem. | – | – | – | – | 779~850 | – | – | – |
| BAg70Cu. | 70±1 | 26±1 | Rem. | – | – | – | 730~755 | 353 | 0.042 | – |
| BAg65Cu. | 65±1 | 20±1.1 | Rem. | – | – | – | 685~720 | 384 | 0.086 | – |
| BAg60Cu | 60 ±1 | Rem. | – | 10±0.5 | – | – | 602~718 | – | 720~840 | |
| BAg50Cu | 50±1.1 | 34±1.1 | Rem. | – | 10±0.5 | – | 677~775 | 343 | 0.076 | 775~870 |
| BAg45Cu | 45±1 | 30+1 | Rem. | – | – | – | 677~743 | 386 | 0.097 | 745~845 |
| BAg25CuZn. | 25±1. | 40±1 | Rem. | – | – | – | 745~775 | 353 | 0.069 | 800~890 |
| BAg10CuZn | 10±1 | 53±1.1 | Rem. | – | – | – | 815~850 | 451 | 0.065 | 850~950 |
| BAg50CuZnCd | 50±1 | 15.5±1 | 16.5±2 | – | – | – | 627~635 | 419 | 0.072 | 635~760 |
| BAg45CuZnCd | 45±1. | 15±1 | 16±2. | – | – | – | 607~618 | – | – | 620~760 |
| BAg40CuZnCdNi | 40±1 | 16±0.5 | 17.8±0.5 | – | – | Ni0.2±0.1 | 595~605 | 392 | 0.069 | 605~705 |
| BAg34CuZnCd | 35±1 | 26±1 | 21±2 | – | – | 607~702 | 411 | 0.069 | 700~845 | |
| BAg50CuZnCdNi | 50±1.1 | 15.5±1 | 15.5±2 | – | – | Ni3±0.5 | 632~688 | 431 | 0.105 | 690~815 |
| BAg56CuZnSn | 56±1 | 22±1 | 17±2 | 50.5 | 50.5 | – | 618~652 | – | – | 650~760 |
| BAg34CuZnSn | 34±1 | 36±1.1 | 27+2 | 30.5 | 30.5 | – | 630~730 | – | – | 730~820 |
| BAg50CuZnSnNi | 50±1 | 21.5±1 | 27±1.1 | 10.3 | 10.3 | Ni0.30~0.65。 | 650~670 | – | – | 670~770 |
| BAg40CuZnSnNi | 40±1 | 25±1 | 30.5±1 | 30.3 | 30.3 | Ni1.30~1.65 | 630~640. | – | – | 640~740 |
2. Copper and phosphorus solder
Copper-phosphorus brazing filler metal is widely used in brazing copper and copper alloys due to its favorable technological performance and economy.
Phosphorus performs two functions in copper:
First, it significantly reduces the melting point of copper.
Secondly, it acts as a self-welding flux during air brazing.
To further reduce the melting temperature of 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 alloys -nickel with a nickel content greater than 10%.
This type of filler metal can result in segregation when heated slowly, so it is best to adopt a brazing method with rapid heating.
Chemical composition and properties of copper and phosphorus solder
| Filler metal | Chemical composition (mass fraction) (%) | Melting temperature | Tensile strength MPa | Resistivity/μΩ·m | ||||
|---|---|---|---|---|---|---|---|---|
| Ass | P | Ag | Sn | other | ||||
| Bcu95P. | Rem. | 5±0.3 | – | – | 710~924 | – | – | |
| Bcu93P | Rem. | 6.8~7.5 | – | – | 710~800 | 470.4 | 0.28 | |
| Bcu92PSb | Rem. | 6.3±0.4 | – | – | Sb1.5~2.0 | 690~800 | 303.8 | 0.47 |
| Bcu91Ag | Rem. | 7±0.2 | 2±0.2 | – | – | 645~810 | – | – |
| Bcu89Ag | Rem. | 5.8~6.7 | 5±0.2 | – | – | 650~800 | 519.4 | 0.23 |
| Bcu80Pag | Rem. | 4.8~5.3 | 15±0.5 | – | – | 640~815 | 499.8 | 0.12 |
| HLAgCu70-5 | Rem. | 5±0.5 | 25±0.5 | – | – | 650~710 | – | – |
| HLCuP6-3 | Rem. | 5.7±0.3 | – | 3.5±0.5 | – | 640~680 | – | 0.35 |
| Cu86SnP | Rem. | 5.3±0.5 | – | 7.5±0.5 | 0.8±0.4 | 620~660 | – | – |
| Bcu80PSnAg | Rem. | 5.3±0.5 | 5±0.5 | 10±0.5 | – | 560~650 | – | – |
| Cu77NiSnP. | 77.6 | 7.0 | 9.7 | – | Ni5.7 | 591~643 | – | – |
3. Tin-based soft solder
When brazing copper with Sn-based solder, it is common for the intermetallic compound Cu6Sn5 to form at the interface between the solder and the base metal. Therefore, it is important to carefully consider the brazing temperature and dwell time.
When using a soldering iron, the composite layer is typically thin and has minimal impact on joint performance.
Brass joints soldered with tin-lead filler metal are stronger than copper joints soldered with the same filler metal. This occurs 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 | Melting temperature | Tensile strength | Stretching | |||
|---|---|---|---|---|---|---|---|
| Sn | Ag | Sb | Ass | ||||
| HL606 | 96.0 | 4.0 | – | – | 221 | 53.0 | – |
| Sn95Sb | 95.0 | – | 5.0 | – | 233 | 39.2 | 43 |
| Sn92AgCuSb | 92.0 | 5.0 | 1.0 | 2.0 | 250 | 49.0 | 2.3 |
| Sn85AgSb | 84.5 | 8.0 | 7.5 | – | 270 | 80.4 | 8.8 |
| Brazing filler metal | Chemical composition | Melting temperature | ||
|---|---|---|---|---|
| 97.0 | 3.0 | Sn | ||
| HLAgPb97 | 97.5 | 1.5 | – | 304-305 |
| HLAgPb97.5-1.0 | 92 | 2.5 | 1.0 | 310-310 |
| HLAgPb92-5.5 | 83.5 | 1.5 | 5.5 | 287-296 |
| HLAgPb83.5-15-1.5 | 97.0 | 3.0 | 15.0 | 265-270 |
4. Soft solder – cadmium-based solder
Chemical composition and properties of cadmium-based solder
| Filler metal | Chemical composition (mass fraction) (%) | Melting temperature/ | Tensile strength/MPa | ||
|---|---|---|---|---|---|
| CD | Ag | Zn | |||
| HL503 | 95 | 5 | 338~393 | 112.8 | |
| HLAgCd96-1 | 96 | 3 | 1 | 300~325 | 110.8 |
| Cd79ZnAg | 79 | 5 | 16 | 270~285 | 200 |
| HL508 | 92 | 5 | 3 | 320~360 | – |
5. Soft soldering – lead-free soldering
Lead-free solder for brazing copper pipes
| Brand | Composition (mass fraction) | Solid phase line/℃ | Liquid/℃ |
| AND | 95Sn-4.5Cu-0.5Ag | 226 | 360 |
| THERE IS | 94.5Sn-3Sb-1.5Zn-0.5Ag-0.5Cu | 215 | 228 |
| HB | 91.225Sn-5Sb-3.5Cu-0.275Ag | 238 | 360 |
| B.C | 96.25n-3.25Bi-0.2Cu-0.35Ag | 206 | 234 |
| OA | 95.9Sn-3Cu-1Bi-0.1Ag | 215 | 238 |
| AM | 95.45n-3Cu-1Sb-0.6Ag | 221 | 231 |
Strength of copper and brass joints soldered with soft solder part
| Weld mark | Shear strength/MPa | Tensile strength/MPa | ||
|---|---|---|---|---|
| copper | brass | copper | brass | |
| S-Pb80Sn18Sb2 | 20.6 | 36.3 | 88.2 | 95.1 |
| S-Pb68Sn30Sb2 | 26.5 | 2740 | 89.2 | 86.2 |
| S-Pb58Sn40Sb2 | 36.3 | 45.1 | 76.4 | 78.40 |
| S-Sn90Pb10 | 45.1 | 44.1 | 63.7 | 68.6 |
| S-P697Ag3 | – | 29.4 | – | 49.0 |
| S-Cd96Ag3Zn1 | 73.5 | – | 57.8 | – |
| S-Sn95Sb5 | 37.2 | – | – | |
| S-sn85Ag8Sb7 | – | 82.3 | – | – |
| S-Sn92AgSCu2Sb1 | 35.3 | – | – | – |
| S-Sn96Ag4P | 35,339.2~49.0 | – | 5,339.2~49.0 | – |
SAW. Brazing Flux
Commonly used brazing fluxes consist of a matrix of borax, boric acid, or a mixture of both, and are supplemented with alkali or alkaline earth metal fluorides or fluoroborates to achieve an appropriate activation temperature and improve removal ability. of oxide.
When heated, boric acid (H3BO3) decomposes to form boric anhydride (B2O3).
The reaction formula is as follows:
2H 3 BO 3 →B 2 Ó 3 +3H 2 Ó
The melting point of boric anhydride is 580°C.
It can react with oxides of copper, zinc, nickel and iron to form a soluble borate, which floats in the weld joint as slag. This not only removes the oxide film but also provides mechanical protection.
MeO+B 2 Ó 3 →MeO-B 2 Ó 3
Borax Na 2 B 4 Ó 7 melts at 741 ℃:
N/A 2 B 4 Ó 7 →B 2 Ó 3 +2NaBO 2
Boric anhydride and metal oxides react to form soluble borates. Sodium metaborate and borates combine to form compounds with a lower melting temperature, facilitating their rise to the surface of solder joints.
MeO+2NaBO 2 +B 2 Ó 3 >（NaBO 2 ） 2 Eu （BO 2 ） 2
The combination of borax and boric acid is a commonly used flux. The addition of boric acid can decrease the surface tension of the borax flux and increase its spread. Boric acid also increases the ability of flux residue to separate cleanly from the surface. However, when using borax-boric acid flux with silver filler metal, its melting temperature remains very high and its viscosity is still very high.
To further lower the melting temperature, potassium fluoride can be added. The main role of potassium fluoride is to decrease the viscosity of the flux and increase its ability to remove oxides. To further reduce the melting temperature and increase its activity, KBF 4 can be added.
The melting point of KBF 4 is 540 ℃, and the melt decomposition is:
KBF 4 →KF+BF 3
| Brand | Composition (mass fraction) (%) | Action temperature ℃ | Purpose |
| FB101 | Boric acid 30, potassium fluoroborate 70 | 550~850℃ | silver soldering flux |
| FB102 | Anhydrous potassium fluoride 42, potassium fluoroborate 25, boric anhydride 35 | 600~850℃ | The most commonly used silver soldering flux |
| FB103 | Potassium fluoroborate>95, potassium carbonate<5 | 550~750℃ | For soldering silver copper zinc cadmium |
| FB104 | Borax 50, boric acid 35, potassium fluoride 15 | 650~850℃ | Brazing with silver-based filler metal in a furnace |
VII. Smooth Solder Flux
1. Corrosive flow
| Number | Component | Purpose |
| 1 | ZnCl 2 1130g,NH 4 Cl110g,H 2 O4L | Brazing of copper and copper alloys, steel |
| two | ZnCl 2 1020g,NaCI280g,NH 4 CI,HCI30g,H 2 O4L | Welding of copper and copper alloys, steel |
| 3 | ZnCl 2 600g,NaCl170g | Covering agent for dip brazing |
| 4 | ZnCl 2 710g, NH 4 Cl100g, Vaseline 1840g, H 2 Ó 180g | Brazing of copper and copper alloys, steel |
| 5 | ZnCl 2 1360g,NH 4 Cl140g,HC185g,H 2 O4L | Brazing of silicon bronze, aluminum bronze, stainless steel |
| 6 | H3P04960g,H 2 0455g | Welded manganese bronze, Stainless steel |
| QJ205 | ZnCl250g,NH4Cl15,CdCl230,NaF6 | Brazing of copper and copper alloys with cadmium-based filler metals |
2. Weakly corrosive flow
| Number | Component | Purpose |
| 1 | Glutamic acid hydrochloride 540g, urea 310g, water 4L | Copper, brass, bronze |
| two | 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 |
3. Non-corrosive flow
The main component of non-corrosive flux is rosin.
There are three commonly used rosin streams:
- Inactivated rosin;
- Weakly activated rosin;
- Active rosin.
VIII. surface preparation
- Solvent degreaser or alkaline solution is applicable to copper and copper alloys.
- Mechanical methods, wire brushes and sandblasting can be used to remove oxides.
- Silicon brass;
- Chrome bronze and copper-nickel alloy;
- Aluminum bronze and beryllium bronze;
- Copper, brass, tin bronze.
IX. Brazing Process
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 required due to the low resistance of copper.
X. Brazing technology for copper and copper alloys
1. Copper
In copper brazing, 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 resin, weak corrosive flux or corrosive flux can be used.
It is important to note that pure copper should not be brazed in a reducing atmosphere, except oxygen-free copper, to avoid hydrogen embrittlement.
2. Brass
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 the brass, it cannot be brazed with inactive rosin. Furthermore, when brazing with phosphor copper and silver, FB102 flux must be used.
3. Manganese brass
For tin-lead brazing, a stream of phosphoric acid solution should be used. 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 welds must be welded with FB102 or FB103 flux. Other silver-based solders, as well as copper-phosphorus and copper-phosphorus-silver solders, must be soldered with FB102 flux. Brazing with FB104 flux in a protective atmosphere inside a furnace is recommended.
4. Beryllium bronze
When brazing beryllium bronze in its soft solder quench aging state, it is important to select a brazing filler metal with a melting temperature below 300°C. The preferred combination for this application is 63Sn-37Pb in combination with a weak corrosive flux or a corrosive flux.
Furthermore, brazing and solution treatment must be carried out simultaneously during the brazing process.
5. Chrome bronze
Soft soldering has minimal impact on the performance index of beryllium bronze, therefore soft solders and fluxes similar to those used for beryllium bronze can be utilized for brazing.
It is important to note that chrome bronze should not be brazed in the 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 silver solder with a lower melting temperature, such as BAgA0 CuZnCdNi.
6. Cadmium bronze and tin bronze
Tin-bronze brazing is similar to copper and brass brazing, but with the added benefit of avoiding hydrogen embrittlement and zinc volatilization during brazing in a protective atmosphere.
However, it should be noted that phosphorus-containing tin bronze is prone to stress cracking.
7. Silicon bronze
For soft welding, it is recommended to use a strong corrosive flux containing hydrochloric acid.
During brazing, there is a tendency to stress cracking and intergranular penetration of the filler metal. The brazing temperature must 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 best results, FB102 and FB103 are the recommended fluxes to use.
8. Aluminum Bronze
When performing soft welding, it is important to use a strong corrosive flux containing hydrochloric acid to remove the oxide film from 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 as short as possible. Coating the surface of aluminum bronze with copper or nickel can also prevent aluminum from diffusing into the weld.
9. White zinc copper
The welding process for white copper zinc is similar to that for brass. The following silver solders are commonly used for brazing: BAg56CuZnSn, BAg50CuZnSnNi, BAg40CuZnNi and BAg56CuZnCd, among others. Recommended fluxes for use are FB102 and FB103.
10. Manganese White Copper
To weld white copper zinc, a phosphoric acid solution flux can be used or the surface can be pre-coated with copper.
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 the phosphorus and nickel will form a brittle composite phase.
Resistance of copper and brass joints soldered with silver solder
| Filler metal | Shear strength/MPa | Tensile strength/MPa | ||
|---|---|---|---|---|
| copper | brass | copper | brass | |
| BAg45CuZn | 177 | 215 | 181 | 325 |
| BAg50CuZn | 171 | 208 | 174. | 334 |
| BAg65CuZn | 171 | 208 | 177 | 334 |
| BAg70CuZn | 166 | 199 | 185 | 321 |
| BAg40CuZnCdNi | 167 | 194 | 179 | 339 |
| BAg50CuZnCd | 167 | 226 | 210 | 375 |
| BAg35CuZnCd | 164 | 190 | 167 | 328 |
| BAg40CuZnSnNi | 98 | 245 | 176 | 295 |
| BAg50CuZnSn | – | – | 220 | 240 |
Mechanical properties of copper joints welded with phosphor copper and phosphor silver copper solders
| Filler metal | Tensile strength /MPa |
Shear force /MPa |
Bending angle (°) |
Impact resistance /J · cm-2 |
| BCu93P | 186 | 132 | 25 | 6 |
| BCu92PSb | 233 | 138 | 90 | 7 |
| BCu80PAg | 255 | 154 | 120 | 23 |
| BCu89PAg | 242 | 140 | 120 | 21 |
XI. 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 residual flux and clean the surface of the part.
The main reason for removing residue is to prevent corrosion of the part and, in some cases, to obtain a good appearance or to prepare the part for further processing.
XII. Brazing Materials
The strength of copper and brass soft brazed joints using various commonly used soft brazing materials is shown in Table 10.
Table 10: Strength of copper and brass soft soldered joints
| Brazing Material Class | Shear force /MPa |
Tensile strength /MPa |
||
| Copper | Brass | Copper | Brass | |
| S-Pb80Sn18Sb2 | 20.6 | 36.3 | 88.2 | 95.1 |
| S-Pb68Sn30Sb2 | 26.5 | 27.4 | 89.2 | 86.2 |
| S-Pb58Sn405b2 | 36.3 | 45.1 | 76.4 | 78.4 |
| S-Pb97Ag3 | 33.3 | 34.3 | 50.0 | 58.8 |
| S-Sn90Pb10 | 45.1 | 44.1 | 63.7 | 68.6 |
| S-Sn95Sb5 | 37.2 | – | – | – |
| S-Sn92Ag5Cu2Sb1 | 35.3 | – | – | – |
| S-Sn85Ag85B7 | 一 | 42.3 | – | – |
| S-Cd96Ag3Znl | 57.8 | – | 73.8 | – |
| S-Cd95Ag5 | 44.1 | 46.0 | 87.2 | 88.2 |
| S-Cd92Ag5Zn3 | 48.0 | 54.9 | 90.1 | 96.0 |
When brazing copper with tin-lead solder, non-corrosive fluxes such as alcohol rosin solution or a mixture of activated rosin and ZnCl2 + NH4Cl aqueous solution can be used. The latter can also be used for brazing brass, bronze and beryllium bronze.
When brazing aluminum brass, aluminum bronze and silicon brass, a flux consisting of zinc chloride in hydrochloric acid solution may be used. For brazing manganese bronze, a phosphoric acid solution can be used as flux.
When using lead-based solder, an aqueous zinc chloride solution can be used as flux, and for cadmium-based solder, FS205 flux can be used.
Hard brazing materials and fluxes for hard brazing
When brazing copper, silver-based solder and copper-phosphorus solder can be used. Silver-based solder has a moderate melting point, good processability and excellent mechanical, electrical and thermal conductivity properties. It is the most commonly used hard brazing material.
For applications that require high electrical conductivity, a silver-containing solder such as B-Ag70CuZn should be chosen. For vacuum brazing or brazing in a furnace with a protective atmosphere, silver-based solders without volatile elements, such as B-Ag50Cu and B-Ag60CuSn, must be used.
Solders with lower silver content are cheaper but have higher brazing temperatures and lower joint toughness, making them suitable for brazing applications with lower copper and copper alloy requirements.
Copper-phosphorus and copper-phosphorus-silver solders can only be used for hard brazing of copper and its alloys. B-Cu93P solder has excellent fluidity and is suitable for brazing parts in the mechanical, electrical, instrumentation and manufacturing industries that are not subject to impact loads.
The optimal gap size is 0.003-0.005 mm. Copper-phosphorus-silver solders (such as B-Cu70Pag) have better toughness and electrical conductivity than copper-phosphorus solder and are mainly used for high-conductivity electrical joints. The performance of various hard brazing materials commonly used for hard brazing of copper and brass joints is shown in Table 11.
Table 11: Performance of copper and brass hard-brazed joints
| Brazing Material Class | Shear force /MPa |
Tensile strength /MPa |
Bending angle /(°) |
Impact Absorption Energy /J |
||
| Copper | Brass | Copper | Brass | Copper | Copper | |
| H62 | 165 | – | 176 | – | 120 | 353 |
| B-Cu60ZnSn-R | 167 | – | 181 | – | 120 | 360 |
| B-Cu54Zn | 162 | – | 172 | – | 90 | 240 |
| B-Zn52Cu | 154 | – | 167 | – | 60 | 211 |
| B-Zn64Cu | 132 | – | 147 | – | 30 | 172 |
| B-Cu93P | 132 | – | 162 | 176 | – | 58 |
| B-Cu92PSb | 138 | – | 160 | 196 | 25 | – |
| B-Cu93Pag | 159 | 219 | 225 | 292 | – | – |
| B-Cu80Pag | 162 | 220 | 225 | 343 | 120 | 205 |
| B-Cu90P6Sn4 | 152 | 205 | 202 | 255 | 120 | 182 |
| B-Ag70CuZn | 167 | 199 | 185 | 321 | 90 | – |
| B-Ag65CuZn | 172 | 211 | 177 | 334 | – | – |
| B-Ag55CuZn | 172 | 208 | 174 | 328 | – | – |
| B-Ag45CuZn | 177 | 216 | 181 | 325 | – | – |
| B-Ag25CuZn | 167 | 184 | 174 | 316 | – | – |
| B-Ag10CuZn | 158 | 161 | 167 | 314 | – | – |
| B-Ag72Cu | 165 | – | 177 | – | – | – |
| B-Ag50CuZnCd | 177 | 226 | 210 | 375 | – | – |
| B-Ag40CuZnCd | 168 | 194 | 179 | 339 | – | – |
