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(593-613)

aThis product is Clad with 4044 on one side and 7072 on the other side for resistance to corrosion.

aThis product is Clad with 4044 on one side and 7072 on the other side for resistance to corrosion.

Brazing fluxes are powders that are mixed with water or alcohol to make a paste that removes the oxide film from the base metal upon heating. Chloride fluxes have traditionally been used, but their residue is corrosive to aluminum. More recently, fluoride fluxes, which are not corrosive and thus do not require removal, have come into use. They are useful where flux removal is difficult, such as in automobile radiators.

Brazing can be done by several processes. Torch brazing uses heat from an oxyfuel flame and can be manual or automatic. Furnace brazing is most common, and is used for complex parts like heat exchangers where torch access is difficult. Assemblies are cleaned, fluxed, and sent through a furnace on a conveyor. Dip brazing is used for complicated assemblies with internal joints. The assemblies are immersed in molten chloride flux; the coating on brazing sheet or preplaced brazing wire, shims, or powder supply the filler. Vacuum brazing does not require fluxes and is done in a furnace; it's especially useful for small matrix heat exchangers, which are difficult to clean after fluxing.

Upon completion of brazing, the assembly is usually water quenched to provide the equivalent of solution heat treatment and to assist in flux removal and subsequently may be naturally or artificially aged to gain strength.

Minimum requirements for fabrication, equipment, material, procedure, and quality for brazing aluminum are given in the American Welding Society's publication C3.7 Specification for Aluminum Brazing [20].

9.4.3 Soldering

Soldering is the process of joining metals by fusion, but filler metals for soldering have a melting point below 840°F (450°C) [4]. (Brazing, described in Section 9.4.2, uses filler metals with a melting point above 840°F (450°C) but lower than the melting point of the base metals being joined.)

Soldering is much like brazing but conducted at lower temperatures. Soldering is limited to aluminum alloys with no more than 1% magnesium or 4% silicon, because higher levels produce alloys that have poor flux wetting characteristics. Alloys 1100 and 3003 are suitable for soldering, as are clad alloys of the 2xxx and 7xxx series. Alloys of zinc, tin, cadmium, and lead are used to solder aluminum; they are classified by melting temperature and described in Table 9.31.

Soldering fluxes are classified as organic and inorganic. Organic fluxes are used for low temperature [300-500°F (150-260°C)] soldering and usually need not be removed, being only mildly corrosive. Inorganic fluxes are used for intermediate [500-700°F (260-370°C)] and high-temperature [700-840°F (370-450°C)] soldering. Inorganic flux must be removed since it is very corrosive to aluminum. Both fluxes produce obnoxious fumes that must be properly ventilated.

Like brazing, soldering can be performed by several processes. Soldering with a hot iron can be done on small wires and sheet less than in. (1.6 mm) thick. Torch soldering can be performed in a much wider variety of cases, including automatic processes used to make automobile air-conditioning condensers. Torch soldering can also be done without flux by removing the aluminum oxide from the work by rubbing with the solder rod, called abrasion soldering. Abrasion soldering can also be performed with ultrasonic means. Furnace and dip soldering are much like their brazing counterparts. Resistance soldering is well suited to spot or tack soldering; flux is painted on the base metal, the solder is placed, and current is passed through the joint to melt the solder.

Soldered joint shear strengths vary from 6 to 40 ksi (40 to 280 MPa) depending on the solder used. Corrosion resistance is poor if chloride containing flux residue remains and the joint is exposed to moisture. Zinc solders have demonstrated good corrosion resistance, even for outdoor exposure.

TABLE 9.31 Classification of Aluminum Solders

(°C)

Common Constituents

Ease of Application

Wetting of Aluminum

Relative Strength

Relative Corrosion Resistance

Low Temp.

300-500 (149-260)

Tin or lead plus zinc and/or cadmium

Best

Poor to fair

Low

Low

Intermediate Temp.

500-700 (260-371)

Zinc base plus cadmium or zinc-tin

Moderate

Good to excellent

Moderate

Moderate

High Temp.

700-840 (371-449)

Zinc base plus aluminum, copper, etc.

Most difficult

Good to excellent

High

Good

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