Terms of Use - Contact Us - Make a Donation

Brazing

Brazing is the process of joining metal by heating the base metal to a temperature above 800°F and adding a nonferrous filler metal that melts below the base metal. Brazing should not be confused with braze welding, even though these two terms are often interchanged. In brazing, the filler metal is drawn into the joint by capillary action and in braze welding it is distributed by tinning. Brazing is sometimes called hard soldering or silver soldering because the filler metals are either hard solders or silver-based alloys. Both processes require distinct joint designs.

Brazing offers important advantages over other metal-joining processes. It does not affect the heat treatment of the original metal as much as welding does, nor does it warp the metal as much. The primary advantage of brazing is that it allows you to join dissimilar metals.

EQUIPMENT

Brazing requires three basic items. You need a source of heat, filler metals, and flux. In the following paragraphs these items are discussed.

Heating Devices T
he source of heat depends on the type and amount of brazing required. If you are doing production work and the pieces are small enough, they can be put into a furnace and brazed all at once. Individual torches can be mounted in groups for assembly line work, or you can use individual oxyacetylene or Mapp-oxygen torches to braze individual items.

Filler Metals
Filler metals used in brazing are nonferrous metals or alloys that have a melting temperature below the adjoining base metal, but above 800°F. Filler metals must have the ability to wet and bond with the base metal, have stability, and not be excessively volatile.

The most commonly used filler metals are the silver-based alloys. Brazing filler metal is available in rod, wire, preformed, and powder form.

Brazing filler metals include the following eight groups:

• Silver-base alloys
• Aluminum-silicon alloys
• Copper
• Copper-zinc (brass) alloys
• Copper-phosphorus alloys
• Gold alloys
• Nickel alloys
• Magnesium alloys

Fluxes
Brazing processes require the use of a flux. Flux is the substance added to the metal surface to stop the formation of any oxides or similar contaminants that are formed during the brazing process. The flux increases both the flow of the brazing filler metal and its ability to stick to the base metal. It forms a strong joint by bringing the brazing filler metal into immediate contact with the adjoining base metals and permits the filler to penetrate the pores of the metal.

You should carefully select the flux for each brazing operation. Usually the manufacturer’s label specifies the type of metal to be brazed with the flux. The following factors must be considered when you are using a flux:

• Base metal or metals used
• Brazing filler metal used
• Source of heat used

Flux is available in powder, liquid, and paste form. One method of applying the flux in powdered form is to dip the heated end of a brazing rod into the container of the powdered flux, allowing the flux to stick to the brazing rod. Another method is to heat the base metal slightly and sprinkle the powdered flux over the joint, allowing the flux to partly melt and stick to the base metal. Sometimes, it is desirable to mix powdered flux with clean water (distilled water) to form a paste.

Flux in either the paste or liquid form can be applied with a brush to the joint. Better results occur when the filler metal is also given a coat.

The most common type of flux used is borax or a mixture of borax with other chemicals. Some of the commercial fluxes contain small amounts of phosphorus and halogen salts of either iodine, bromine, fluorine, chlorine, or astatine. When a prepared flux is not available, a mixture of 12 parts of borax and 1 part boric acid may be used.

WARNING

Nearly all fluxes give off fumes that may be toxic. Use them only in WELL-VENTILATED spaces.

JOINT DESIGN

In brazing, the filler metal is distributed by capillary action. This requires the joints to have close tolerances and a good fit to produce a strong bond. Brazing has three basic joint designs (fig. 6-13): lap, butt, and scarf. These joints can be found in flat, round, tubular, or irregular shapes.

Lap Joints
The lap joint is one of the strongest and most frequently used joint in brazing, especially in pipe work The primary disadvantage of the lap joint is the increase in thickness of the final product. For maximum strength, the overlap should be at least three times the thickness of the metal. A 0.001-inch to 0.003-inch clearance between the joint members provides the greatest strength with silver-based brazing filler metals. You should take precautions to prevent heat expansion from closing joints that have initial close tolerances.

Butt Joints
Butt joints are limited in size to that of the thinnest section so maximum joint strength is impossible. Butt joint strength can be maximized by maintaining a joint clearance of 0.001 to 0.003 of an inch in the finished braze. The edges of the joint must be perfectly square to maintain a uniform clearance between all parts of the joint. Butt joints are usually used where the double thickness of a lap joint is undesirable. When double-metal thickness is objectionable and you need more strength, the scarf joint is a good choice.

Scarf Joints
A scarf joint provides an increased area of bond without increasing the thickness of the joint. The area of bond depends on the scarf angle cut for the joint. Usually, an area of bond two to three times that of a butt joint is desirable. A scarf angle of 30 degrees gives a bond area twice that of a 90-degree butt joint, and an angle of 19 1/2 degrees increases the bond area three times.

Figure 6-14 shows some variations of butt and lap joints designed to produce good brazing results. A comparison of good and bad designed joints is shown in figure 6-15.

BRAZING PROCEDURES

The procedure for brazing is very similar to braze and oxyacetylene welding. The metal needs to be cleaned by either mechanical, chemical, or a combination of both methods to ensure good bonding. The two pieces must befitted properly and supported to prevent voids in the joint or accidental movement during brazing and cooling operations.

Surface Preparation
The surfaces of the metal must be cleaned for capillary action to take place. When necessary, chemically clean the surface by dipping it in acid. Remove the acid by washing the surface with warm water. For mechanical cleaning, you can use steel wool, a file, or abrasive paper. Do not use an emery wheel or emery cloth, because abrasive particles or oil might become embedded in the metal.

Work Support
Mount the work in position on firebricks or other suitable means of support, and if necessary, clamp it. This is important because if the joint moves during the brazing process, the finished bond will be weak and subject to failure.

Fluxing
The method of application varies, depending upon the form of flux being used and the type of metal you are brazing. Refer to the material on fluxes previously described. It is extremely important that the flux is suitable for your job.

Brazing
The next step is to heat the parts to the correct brazing temperature. Adjust the torch flame (oxygas) to a neutral flame because this flame gives the best results under normal conditions. A reducing flame produces an exceptionally neat-looking joint, but strength is sacrificed. An oxidizing flame will produce a strong joint but it has a rough-looking surface.

The best way to determine the temperature of the joint, as you heat it, is by watching the behavior of the flux. The flux first dries out as the moisture (water) boils off at 212°F. Then the flux turns milky in color and starts to bubble at about 600°F. Finally, it turns into a clear liquid at about 1100°F. That is just short of the brazing temperature. The clear appearance of the flux indicates that it is time to start adding the filler metal. The heat of the joint, not the flame, should melt the filler metal. When the temperature and alignment are proper, the filler metal spreads over the metal surface and into the joint by capillary attraction. For good bonding, ensure the filler metal penetrates the complete thickness of the metal. Figure 6-16 shows a good position for the torch and filler metal when brazing a butt joint.

Stop heating as soon as the filler metal has completely covered the surface of the joint, and let the joint cool slowly. Do not remove the supports or clamps or move the joint in any way until the surface is cool and the filler metal has completely solidified.

Finally, clean the joint after it has cooled sufficiently. This can be done with hot water. Be sure to remove all traces of the flux because it can corrode the metal. Excess metal left on the joint can be filed smooth.

The above described procedure is a general one, but it applies to the three major types of brazing: silver, copper alloy, and aluminum. The differences being the base metals joined and the composition of the filler metals.

Silver Brazing

Often, you will be called on to do a silver brazing job. Table 6-2 lists different types of silver brazing alloys and their characteristics. A popular way to apply silver brazing metal on a tubing is to use silver alloy rings, as shown in figure 6-17. This is a practical and economical way to add silver alloy when using a production line system. Another method of brazing by using preplaced brazing shims is shown in figure 6-18. The requirements of each job varies; however, through experience you can become capable of selecting the proper procedure to produce quality brazing.

Copyright © 2001-2006 SweetHaven Publishing Services
All rights reserved