Brazing of beryllium copper

When the required joint strength refractory metals but metals usually take soldered connection, the beryllium copper brazing is relatively simple, inexpensive and more importantly, the reinforcing mechanism inherent in the brazing is completed so as beryllium copper, is not There will be permanent weakening of the alloy, such as tin phosphor bronze or zinc white copper, which is strengthened by cold work hardening and produces permanent softening after brazing.
Brazing is defined by the process of joining. During the joining process, the filler or solder metal melts above 425 degrees and is filled into the gap by capillary action. The soldering temperature is between soldering and fusion welding. For beryllium copper alloys, the typical brazing temperature is above the age hardening temperature, which is approximately equal to the solution annealing temperature.
Beryllium Copper Metallurgy According to the requirements of use, beryllium copper is supplied in two alloy grades. Both high strength (C17000, C17200C17300) and high conductivity (17410, C17500, C17510) are strengthened by heat treatment.
First, the alloy must be dissolved and annealed to dissolve the bismuth into the solid solution, and then participate in the age hardening stage. The solution annealing is always cooled to room temperature in a fast cooling manner. The most common is water quenching, but the thin section of the workpiece can be used. Forced air cooling, after quenching, the workpiece is in annealed or A state. Under these conditions, the workpiece is soft, easy to mold, and has a low electrical conductivity. If the alloy is cold worked, it is called hard or H state, and the solution annealing and quenching steps of the alloy are usually completed by the alloy manufacturer.
The second step is age hardening (often referred to as heat treatment or aging). At this point, a hard, microscopic, ruthenium-rich site is formed in the matrix of the alloy below the solid solution temperature. The distribution of the number of hard spots depends on the temperature and time of aging and thus the high strength of the alloy. After annealing and hardening, the strength and electrical conductivity of the annealed alloy increase, while the ductility and formability decrease. The state of the alloy is represented by AT or HT.
During the brazing process, if the temperature at which the workpiece is heated is above the age hardening temperature, but below the annealing temperature, the workpiece will just be overaged, and the strength will begin to decrease. According to the temperature and time of the workpiece heating, due to overaging The loss of strength can be extremely significant - up to 50%.
Increasing the temperature of brazing, entering the temperature range of solution annealing, annealing or softening the workpiece, if the workpiece is rapidly cooled from this temperature, the workpiece can still be aged. If rapid cooling is not possible, the workpiece is reannealed and aged after the soldering is completed. In this case, you must be very careful. The temperature of solution annealing shall not exceed the melting point of the solder metal.
Cleaning of the cleaning surface before and after brazing is critical to obtaining a dense brazed joint. The polished surface should be free of dirt, oxides of grease stains. The use of flux in the brazing process provides protection, but it does not replace the proper surface preparation. Solvent or evaporation degreasing is effective for solving organic contamination, but it is also necessary to remove the oxide scale of the workpiece by brushing or pickling. [next]
The component shall be brazed immediately after cleaning, otherwise it will be stored under protective conditions. When it is inevitable to extend the shelf life, it must be plated with gold, silver or nickel at a thickness of at least 0.013 mm. It is impossible to provide sufficient protection time by pre-coating tin, solder or zinc because they melt at a temperature lower than the brazing temperature.
In order to prevent corrosion after soldering, the residue must be removed with hot water and thoroughly cleaned. Some residues need to be cleaned with a warm and diluted combo acid solution. Care should be taken to avoid excessive corrosion of the base metal.
The choice of the filler metal of the solder alloy depends on factors such as corrosion resistance, strength, appearance, electrical conductivity, fluidity and price, in addition to the shape of the beryllium copper alloy and its parts. Gold and copper-based solder alloys are generally not suitable due to their high melting point. Table 2 lists the solder alloys recommended for beryllium copper. Some alloys in the surface melt at a certain temperature range. These tight tolerance brazing, alloys containing volatiles such as zinc or cadmium , are not suitable for vacuum brazing.
The price of the solder alloy is very high depending on its content. Copper-phosphorus brazing alloy, although its strength is inferior to the commonly used high-silver alloy, it occupies the advantage of low cost. When brazing copper and steel are brazed, the niobium will diffuse into the brazing alloy, resulting in loss of wettability with the steel interface. . To avoid this problem, nickel-containing fillers such as Bag-3 should be used.
High strength beryllium copper solder brazing itself, beryllium copper, or other copper alloy into contact with the solder, it is recommended to use a basic, fluoroboric acid flux, named AWS3A, it begins to melt at 315 degrees, and the oxide is dissolved, Fully melted at 590 degrees, used at 620-870 degrees in the temperature range. When bismuth copper is brazed to aluminum bronze, it is recommended to use the chloride-containing flux AWS4A, a high-temperature brazing process, and when nickel-silver is used, a boron-modified AWS5A flux is required, which is effective at 870 degrees or more.
The brazing step is divided into two common methods for brazing beryllium copper alloys depending on the temperature at which they are operated. Low temperature operation is recommended for joining small parts of approximately equal size, rapid heating and brazing temperatures of less than one minute. Cooling down inside is the key to good integration. Heat only on the connected area, followed by forced air or water cooling. If handled with care, overheating is avoided and the hardness reading is at a low RB overshoot when not re-aged. High-temperature brazing is used for large workpieces or workpieces with similar dimensions. Conventional heating methods can be used. The choice of heat source depends on the required heating rate, the shape, throughput, and cost of the workpiece. Flame heating and induction heating. Of course, metals with different thermal conductivity must be carefully operated to ensure that all joint surfaces are uniformly heated, and the workpiece is rapidly cooled from the brazing temperature to age harden the alloy to achieve the highest mechanical properties for high-temperature brazing of high-conducting beryllium copper alloys. Welding requires the filler metal to flow in the 900-950 degree temperature range. For AT or HT high-conducting copper, when the filler is 620 degrees, the hardness RB decreases by 10-15 points. If the brazing time is extended, it will drop more. [next]
For high-temperature brazing of high-strength beryllium copper, the brazing material should be melted above 760 degrees. AWS Bag-8 meets this requirement, and the tissue assembly should be heated to 790 degrees to ensure that the filler can flow. On the eve of water quenching, the component assembly to be welded should be cooled to 730-745 degrees and then hardened at 315.
For high-temperature brazing of high-conducting beryllium copper alloys, filler metal is required to flow in the temperature range of 900-950 degrees. For AT or HT high-conducting copper, when the filler is 620 degrees, the hardness RB decreases by 10-15 points. If the brazing time is extended, it will drop more.
The brazing effect in the furnace is better. Before the workpiece is installed, the furnace first goes to the temperature and uses a reducing atmosphere, such as decomposing ammonia, to reduce oxidation, but flux is still needed to ensure the filler metal is fully wetted.
Resistance brazing utilizes the advantages of beryllium copper high-conductivity Lhasa, a low-conductivity filler metal that concentrates heat to the joint. This technique is widely used for brazing of precious metal contacts and beryllium copper reeds, and the simple shape produces uniformity. Current density, while uniform temperature is necessary for good bonding.
In induction heating brazing, the coil must be carefully designed to ensure that all bonded surfaces sense a uniform temperature. Note that the high conductivity alloy is heated at a lower speed.
Brazing cleaning should be carried out in a well-ventilated environment, preventing workers from being exposed to dusty particles. The design of the beryllium brazing joint is no different from other copper alloys. The general concept is dominant and is included in a few routines. Proper joint clearance always compromises between the two, one is to allow the flux to flow away, the other is to ensure that the capillary of the filler metal is sucked, the gap must be uniform, preferably between 0.04-0.08mm, due to the solder metal Filling, if possible, helps the flow of the filler by slight sliding or vibration of the workpiece. To ensure proper clearance at the temperature of the brazing, the change in the clearance dimension of the gap with temperature should be calculated.
When connecting metals with different coefficients of thermal expansion, the thermal strain should be considered. In the brazing area, the coefficient of expansion of beryllium copper is 18.0×10 ^-6 /degree Celsius, which is about the same as other copper alloys and filler metals. When connecting beryllium copper to steel plates A ductile filler is required to adjust the thermal strain.