942 Brazing

Brazing is the process of joining metals by fusion using filler metals with a melting point above 840°F (450°C) but lower than the melting point of the base metals being joined [1]. Soldering also joins metals by fusion, but filler metals for soldering have a melting point below 840°F (450°C). Brazing and soldering differ from welding in that no significant amount of base metal is melted during the fusion process. Ranking the temperature of the process and the strength and the corrosion resistance of the assembly, from highest to lowest, are welding, brazing, and then soldering.

Brazing's advantage is that it is very useful for making complex and smoothly blended joints, using capillary action to draw the filler into the joint. A disadvantage is that it requires that the base metal be heated to a temperature near the melting point; since yield strength decreases drastically at such temperatures, parts must often be supported to prevent sagging under their own weight. Another disadvantage is the corrosive effect of flux residues, which can be overcome by using vacuum brazing or chloride-free fluxes.

Brazing can be used on lap, flange, lock-seam, and tee joints to form smooth fillets on both sides of the joint. Joint clearances are small, ranging from 0.003 in. (0.08 mm) to 0.025 in. (0.6 mm) and depend on the type of joint and the brazing process.

Non-heat-treatable alloys 1100, 3003, 3004, and 5005, heat-treatable alloys 6061, 6063, and 6951, and casting alloys 356.0, A356.0, 357.0, 359.0, 443.0, 710.0, 711.0, and 712.0 are the most commonly brazed of their respective categories. The melting points of 2011, 2014, 2017, 2024, and 7075 alloys are too low to be brazed, and 5xxx alloys with more than 2% magnesium are not very practically brazed because fluxes are ineffective in removing their tightly adhering oxides. Brazing alloys are shown in Table 9.29, and brazing sheet (cladding on sheet) parameters are given in Table 9.30.

TABLE 9.29 Common Brazing Filler Alloys and Forms

Brazing Alloy

AWS Classification

Nominal

Composition (%)

Melting Range °F

Normal Brazing °F

Available As:

Brazing Process

TABLE 9.29 Common Brazing Filler Alloys and Forms

Brazing Alloy

AWS Classification

Melting Range °F

Normal Brazing °F

Available As:

Brazing Process

Designation

Number

Si

Cu

Mg

(°C)

(°C)

Rod

Sheet

Clad" Powder Torch

Furnace

Dip

Remarks

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