Choice of the right form of brazing alloy is one of the most important issues which affects the integrity of any joint assembly- other factors being joint design and proper fit; cleaning procedure to prepare surface; fluxing practice; proper cooling and flux removal; and method of heating. So, how to choose just the right form of brazing alloy? Let's know about it.
Some Criteria to Choose Brazing Alloy!
A brazing alloy is the metal which is added during the brazing process in order to complete a joint. Melting range for a brazing alloy has been defined by the minimum temperature at which the alloy starts melting (“solidus”) and the temperature at which the alloy becomes liquid (“liquidus”). For most purposes, the actual brazing temperature lies within 50°F to 200°F (30°C to 110°C) above the liquidus. While the melting range depends upon the chemical composition of alloy, it is also important to note that individual batch characteristics may differ slightly. Some alloys have a very narrow melting range and others have a relatively wider range. Those having narrow melting range are used for filling very narrow gaps while others with wider range are better used for filling larger gaps.
There are many forms of brazing alloys. Different alloy combinations change the melting temperatures as well as other properties of brazing alloy. However, a brazing alloy should fulfill the following basic criteria.
- It should flow evenly across the surfaces of the base metal, and through capillaries even if they are as small as 0.001-inch;
- It should resist alloy separation into solid and liquid phase during brazing.
- It should provide a strong bond by partially alloying with the base metals.
- It should meet conditions of corrosion resistance, ductility, thermal and electrical conductivity, or other desired properties.
High temperature brazing alloys like gold, nickel and copper can be used for brazing multiple joints at a time but care must be taken with the joint design and joint clearance. In such cases, heating time should be minimized to the time needed to bring all components to the heating temperature and for the molten alloy to flow quickly. high temperature brazing should be used for joining cobalt or nickel-based superalloys.
Comparison of Popular Brazing Alloys
Although copper, nickel and silver are the most frequently-used base metals for brazing alloys; aluminum and gold are also used for some brazing applications.
| Brazing Alloy | Brazing Temperature | Joint Clearance | Benefit |
| Copper | 2000° F | interference to 0.002” | Joint Strength, Low Cost |
| Silver | 1300° F | 0.002-0.005” | Low Temperature |
| Silver with Lithium | 1300° F | 0.002-0.005” | Low Temperature, Self-Fluxing |
| Nickel | 1900° F | 0.002-0.008” | Joint Strength, Resistance to Corrosion |
| Aluminum | Column 21080° F | 0.002-0.01” | Only braze for aluminum |
| Gold | 1800° F | 0.002-0.008” | Resistance to oxidation and corrosion |
Most of the above mentioned brazing alloys are generally available in various forms like wire, foil, tape, powder and paste. Know about all such forms of brazing alloys.
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