How To Choose Right Brazing Alloy?
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 50F to 200F (30C to 110C) 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.
by: Deepa Roy
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