metals together?
Filler metal atoms have a stronger attraction to the base metal’s atoms than to their own. This force between two surfaces is called adhesion. This preference for atoms other than its own causes the wetting action of the base metal. When wetting action takes place in a small diameter tube or between closely spaced parallel plates as in a joint, capillary action occurs. Capillary attraction is so strong it readily opposes gravity and works to the welder’s advantage by bringing filler material into the joint and distributing it evenly. A soldered joint’s strength comes from the same forces. See Figure 3–2.
Figure 3–2Capillary attraction
On what metals can brazing be performed?
Most common metals can be brazed or soldered including:
•Aluminum
•Bronze
•Brass
•Cast iron
•Copper
•Stainless Steel
•Steel
•Titanium
•Tool steels (some)
•Tungsten carbide (tool bits)
What metals can be soldered?
Most metals may be soldered.
What are the advantages of the brazing and soldering processes over other joining methods?
•Ability to join dissimilar metals—Steel is easily joined to copper, cast iron to stainless steel, and brass to aluminum. Many combinations of metals are readily joined.
•Ability to join nonmetals to metals—Ceramics are easily joined to metals, or each other.
•Ability to join parts of widely different thicknesses—either thin-to-thin or thin-to-thick parts may be joined without burn-through or overheating.
•Excellent stress distribution—Many of the distortion problems of fusion welding are eliminated because of the lower process temperature and the even distribution of heat with more gradual temperature changes.
•Low temperature process—the components being joined like semiconductors are less likely to be damaged, since the base metals are not subjected to melting temperatures.
•Economical for complex assemblies—many parts can be joined in a single process step.
•Joins precision parts well—with proper jigs and fixtures, parts may be very accurately positioned.
•Parts may be temporarily joined, subjected to other manufacturing processes, and then separated without damage.
•Parts can be assembled rapidly—processes are readily adaptable to batch and automatic assembly operations.
•Mistakes are readily fixed—a misaligned part can be repositioned without damage.
•Ability to make leak-proof and vacuum-tight joints—many tanks are soldered or brazed; high-power radio transmitter vacuum tubes and integrated circuits with metal to ceramic joints are brazed.
•Joints require little or no finishing—with proper process design the brazed or soldered joint can be nearly invisible.
•Combined brazing and heat treatment cycles—when protective atmosphere brazing is used, the brazing process may be incorporated into the heat treatment cycle.
What are some disadvantages of the brazing process?
•While brazing processes can produce high-strength joints, they are rarely as strong as a fusion-welded joint.
•The brazed parts and the filler metal may lack a color match.
Important Processes Detailed
How do each of the commercially important brazing and soldering processes work and what are their advantages and applications?
Dip Brazing
There are two types. Molten-metal bath dip brazing uses a pot of molten filler metal, usually temperature controlled and heated by electricity, oil, or gas. The cleaned, fluxed parts are immersed into the molten filler metal that enters the joints by capillary attraction. When the assembly is withdrawn and cooled, the brazing is complete. The parts are usually self-jigging. A layer of flux usually covers the molten metal to retard oxidation.
The other method is molten chemical bath dip brazing. Here the parts are cleaned, filler metal is placed between the joints, the parts to be joined are then assembled with filler in place, preheated, then dipped into a pot of molten chemicals serving as a flux.
The advantage of dip brazing over torch brazing is that even heating of the part reduces distortion. Dip brazing may be manual or automated; it is used on small to medium parts. See Figure 3–3.
Dip soldering very much resembles molten metal dip brazing using a molten metal bath but at a lower temperature.
Figure 3–3Chemical bath dip brazing or soldering
Furnace Brazing
The parts are cleaned, brazing filler metal is placed inside the joints, and the parts assembled using fixtures to hold them in proper position. The pre-placed brazing filler metal can be in the form of filings, foil, paste, powder, tape, or special shapes called preforms that fit the joint. Both batch and continuous conveyor furnaces are used. Furnaces may have multiple heat zones for preheat, brazing, and cool-down.
Flux is used in furnaces with an air atmosphere, but air can be eliminated by using a special atmosphere (argon or helium) or a vacuum. If flux is not used in the brazing process, it will not have to be removed in a later step—a significant advantage. Furnace brazing offers lower distortion than torch brazing, and may also perform heat-treating. See Figure 3–4.
Figure 3–4Furnace brazing
Furnace soldering is similar to furnace brazing, but at a lower temperature and normally in an air atmosphere.
Induction Brazing
This process depends on inducing an alternating current in the part. As this induced current flows around inside the part, it generates heat from the resistance of the part itself, and brings it up to brazing temperature. A solid-state, or vacuum-tube oscillator generates alternating current from 10 to 500 kHz. This current is fed to a coil of copper tubing that
