|
What is an alloy?
|
|
A compound of two or more metals which usually has different
properties compared to the component metals.
|
|
|
What is an assembly gap?
|
|
A joint gap between the base metals to be brazed at room temperature.
|
|
|
What is a brazing gap?
|
|
A gap between the base metals to be brazed at the brazing temperature.
In cases of different base metals with a different coefficient of thermal expansion, the brazing gap may be different from the assembly gap.
|
|
|
What is a gas burner?
|
|
A gas burner is an instrument generating a flame to heat up metal parts for applications such as soldering, brazing and welding. For melting of metals and alloys with melting points up to 1,100°C fuel gases such as acetylene and propane are commonly used in combination with oxygen or compressed air.
|
|
|
What is a brazing atmosphere?
|
|
Brazing atmosphere is the atmosphere around the base metals during the brazing process. There are a few types of brazing atmospheres: open atmosphere, protective inert gas (i.e Argon, Nitrogen and Helium), reducing gas (i.e Hydrogen, Carbon Monoxide, dissociated Amonia) and vacuum brazing.
|
|
|
What is a Cd bearing alloy?
|
|
Cadmium (Cd) bearing alloys are free-flowing filler metals that have been the standard in the industry for many years. They have the lowest melting point of all silver brazing alloys and are suitable for Nickel, Steel and Copper based alloys. Besides Silver, the alloy contains Copper, Zinc and Cadmium. Since directly inhaling fumes containing cadmium can result in severe health problems, these alloys are restricted for use in many industries worldwide.
|
|
|
What is a Cd free alloy?
|
|
Due to health problems associated with Cadmium bearing alloys, the use of Cadmium-free (Cd free) alloys has drastically increased in recent years. Compared to Cadmium bearing alloys with the same silver content, the Cd free alloys have higher working temperatures but can operate at higher temperatures (200°C vs. 150°C).
|
|
|
What is capillary pressure?
|
|
The capillary pressure is the pressure inside the molten alloy which is caused by intermolecular forces in the molten brazing alloy. When the joint gap is tighter, the capillary pressure increases and causes the molten alloy to more effectively penetrate the joint gap.
|
|
|
What is the proper clearance for brazing?
|
|
When using the appropriate flux, a joint clearance between 0.05mm to 0.20mm will result in a good capillary action and high strength joint.
For a protected atmosphere and vacuum brazing, the joint clearance should be below 0.05mm.
When brazing different base metals with two different coefficients of thermal expansion, a precise calculation must be done in order to determine a close fit up clearance.
|
|
|
What is the flux active range?
|
|
In order to be effective, a flux must be molten and active before the brazing alloy melts. The flux needs to remain active until the alloy flows through the joint, cools down and then solidifies. Generally, the selected flux has to be active approximately 30°C below the solidus temperature and 90°C above the liquidus temperature of the alloy.
|
|
|
What is furnace brazing?
|
|
Furnace brazing is carried out at about 50°C -100°C higher than the working temperature of the brazing alloy and under a protective atmosphere of inert gas (i.e Nitrogen) or vacuum brazing without the need of a flux.
Furnace brazing is practical when the base metals can stay under a uniform heating temperature for a longer period of time and when the brazing material can be inserted prior to heating.
|
|
|
What is induction brazing?
|
|
In induction brazing, the base metals are placed in the center of induction coil and heated from the electromagnetic field that is created by the induced current.
Generally, induction brazing is applied in the open air and with a flux. The advantages of the induction brazing are: heating can be targeted to provide heat to very small areas; it creates clean and defined joints and can be used for high-precision applications; it reduces both oxidation and post-brazing cleaning; heating cycle is very short; and crucial variables such as time and temperatures are consistent and controllable.
|
|
|
What is vacuum brazing?
|
|
Vacuum brazing is often conducted in a furnace utilizing heat radiation. Several joints can be brazed simultaneously. The advantages of vacuum brazing are: a flux-free procedure, an excellent degree of flowing, extremely clean and uniform joints, a very high strength of the joint, and significant reduction in the residual metallurgical stresses of the base metals due to the slow heating and cooling cycles. Alloys containing high vapor pressure compounds, such as Cadmium and Zinc, can not be used in vacuum brazing.
|
|
|
What is solidus temperature?
|
|
The solidus temperature is a temperature at which the melting of the brazing alloy begins but the alloy is not necessarily completely melted. It is the lower temperature of the melting range. The alloy is completely solid at temperatures below this range.
|
|
|
What is liquidus temperature?
|
|
The liquidus temperature is the upper temperature at which the brazing alloy is still in the melting range and is not yet completely melted. Above this temperature, the brazing alloy is completely liquid.
|
|
|
What is melting range?
|
|
The melting range is the temperature range at which the alloy changes its state from solid to liquid. Eutectic alloy is exceptional since it has a particular melting point and not a melting range.
|
|
|
What is wetting?
|
|
Wetting is the spreading of a molten brazing alloy over the surface of the base metals being brazed. Appropriate wetting will occur if both base metals and the brazing alloy reach the working temperature of the alloy.
|
|
|
What is working temperature?
|
|
The working temperature is the lowest surface temperature of the base at which the brazing alloy wets the surfaces to be joined. It is a higher temperature than the solidus temperature of the brazing alloy and can be higher, lower or the same as the liquidus temperature. The actual working temperature is derived from the rate of heating, the type of the applied brazing alloy and its melting range.
|
|
|
How flux can be removed?
|
|
The best way to clean flux residue from the brazed joint is to soak the assembly in hot water or to apply a wet wire brush to crack and remove all residues. In case of severe residues, up to 25% of Hydrochloric acid [HCl] can be added to the water.
|
|
|
What is the strength of the brazed joint?
|
|
If correctly carried out, a brazed joint can be as strong as the base metals being brazed and sometimes even stronger. The joint strength varies with the base metals being brazed and the joint clearance and the applied brazing alloy. In general, a narrower gap between the base metals results in a stronger capillary action and joint.
|
|