from parts after soldering. They are usually non-corrosive; available as powders, pastes, liquids, and as a core within soldering wire. They are used in electrical and electronics applications.
How are brazing fluxes applied?
•Flux can be applied by spraying, brushing, or dipping.
•Flux is adhered to the end of a brazing rod by heating of the rod’s end with the torch and dipping it into the flux. The flux is a dry powder.
•Some brazing (and braze welding) rod comes from the factory with flux already applied to the outside.
•Sheets, rings, and washers of flux can be inserted in the joint before assembly.
•Special guns can inject flux (or mixtures of flux and filler metal) directly into the joint.
•Flux can be dissolved in alcohol and supplied within the fuel gas stream directly to the brazing joint eliminating the manual operation of adding flux. This process automatically controls the amount applied. See Figure 3–14.
Figure 3–14Gas fluxing unit for oxyfuel brazing
How are soldering fluxes applied?
Soldering fluxes are brushed, rolled, or sprayed. Many solders have flux cores, so no separate fluxing step is needed.
Brazing Filler Materials and Soldering Alloys
What properties must brazing filler materials have?
They must have the ability to make joints with mechanical and chemical properties for the application.
•Melting point below that of the base metals being joined and with the right flow properties to wet the base metals and fill joints by capillary attraction. See Figure 3–15.
•Composition that will not allow it to separate into its components (liquation) during brazing.
Figure 3–15Melting points of braze filler metals and solders fall well below most base metals
What do the terms solidus and liquidus mean?
Solidus is the highest temperature at which a metal is completely solid. Liquidus is the lowest temperature at which a metal is completely liquid.
In general what can be said about the solidus and liquidus temperatures of pure metal? What can be said of alloys of two metals?
Because pure metals have an abrupt melting point, their solidus and liquidus temperatures are the same. However, in an alloy of two metals there is both a range of temperatures and a range of compositions at which both solid and liquid phases of the alloy can exist.
What is the best way to show how the melting and freezing properties of an alloy of two metals changes as its composition changes from all one base metal to all of the other base metal?
A constitutional diagram shows how the changing alloy’s mix of composition affects its melt properties. See silver-copper constitutional diagram in Figure 3–16.
Figure 3–16Silver-Copper constitutional diagram
What does the above constitutional diagram show?
•The solidus line ADEB indicates the temperature at which the alloy begins to melt for compositions of copper and silver.
•The liquidus line ACB indicates the temperature above which the alloy is completely melted.
•For a particular mix of the two metals, the melting point is lower than the melting point of either pure metal making up the alloy, or of any other mixture of the alloy. For a silver-copper alloy, the minimum melting point is 1435°F (779°C) and occurs at 72% silver - 28% copper (point C). This is called the eutectic temperature and eutectic composition. Note that copper melts at 1481°F (805°C) and silver at 1761°F (961°C), both well above the 1435°F eutectic temperature.
•For alloy mixtures other than the eutectic, there is a range of temperatures in which both solid and liquid phases of the alloy can exist together (area ADC and area CEB). In these areas of temperature and composition the alloy is mushy or slushy, while at the eutectic it has a sharp melting point and is as fluid as a pure metal.
How can the information in a constitutional diagram suggest brazing and soldering alloys for specific applications?
•By using a eutectic alloy, we can minimize the temperature at which we perform the brazing (or soldering in the case of tin-lead or tin-antimony alloys) and still have a fluid composition that can easily make its way into the brazing joints.
•By using a non-eutectic alloy, we can achieve an alloy that is slushy or less fluid than at the eutectic. The wider the difference between the solidus and liquidus lines in the constitutional diagram, the more sluggish the alloy is in this temperature range. This would be helpful where too fluid an alloy would not stay in place in an inverted joint and where we want capillary attraction to prevail over gravity. A good example of this is soldering a fitting or seam upside down.
Do other soldering and brazing two-metal alloys have the same general pattern as the silver-copper constitutional diagram?
Yes, see the tin-lead constitutional diagram in Figure 3–17.
Figure 3–17Tin-lead constitutional diagram
What are the most common brazing filler metals?
Filler materials covered by AWS specifications are grouped as:
•Aluminum-silicone
•Copper
•Copper-phosphorus
•Copper-zinc
•Heat-resisting material
•Magnesium
•Nickel-gold
•Silver
What determines the choice of braze filler metal?
The filler choice is determined by base materials or metals.
What is the purpose of shims or washers of filler metal preplaced in the work?
Preplacing
