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 usually surrounds the part. This copper coil acts as the primary of a transformer and the parts themselves act as the secondary. The copper tubing coil itself is kept from melting by cooling water flowing through the tubing’s interior. Power from one to several hundred kilowatts is used. Induction coils are designed in shapes that maximize the transfer of current to the assembly being brazed and take many shapes and sizes. See Figures 3–5 and 3–6.
Figure 3–5Different shapes of induction brazing coils
Figure 3–6Coils in place on brazing work
Cleaning the parts, inserting brazing filler metal and flux into the joints, assembling the parts and heating them with an induction coil, perform induction brazing. The process is very fast, usually measured in seconds, and used to make consumer and military parts. It is often automated. Some processes utilize a vacuum and use no flux. Induction Soldering is similar to induction brazing, but at a lower temperature.
Infrared Brazing
Infrared brazing is a form of furnace brazing. High-intensity quartz lamps supply long-wave heat of up to 5 kW each. Concentrating reflectors focus heat on the parts. This process is sometimes performed in a vacuum and is usually employed in a conveyor-fed production process. Infrared Soldering is similar to infrared brazing, but at a lower temperature.
Resistance Brazing
Electric current flowing through the joint to be brazed provides heat for this process. The joint is cleaned, fluxed, and braze filler material is placed inside the joint in the form of wire, washers, shims, powder, or paste. Then the joint is placed between two electrodes, squeezed together, and electricity is applied. The source of electricity is usually a step-down transformer providing from 2 to 25 volts. Current runs from 50 amperes for small jobs to thousands of amperes for large ones. The electrodes are high-resistance electrical conductors like carbon or graphite blocks, or tungsten or molybdenum rods. Most of the heat is produced in the electrodes raising them to incandescence. This heat flows into the joint completing the braze joint. The resistance of the joint alone is not usually an adequate heat source. The cycle time varies from one second to several minutes depending on part size. See Figure 3–7.
Figure 3–7Resistance brazing or soldering method
Torch Brazing
Heating is done with one or more gas torches. Depending on the size of the parts and the melting point of the filler metal, a variety of torch fuels (acetylene, propane, methylacetylene-propadiene stabilized, or natural gas) may be burned in oxygen, compressed air, or atmospheric air. A neutral or oxidizing flame will usually produce excellent results.
Flux is required with most braze filler metals and can usually be applied to the joint ahead of brazing. The filler metal can be preplaced in the joint or face fed. Manual torch brazing is successfully used on assemblies involving components of unequal mass. It is frequently used in the repair of castings in the field and is often automated for high-production of small and medium-sized parts. It is a versatile process and probably the most popular brazing technique.
Torch Soldering
This is very much like torch brazing but at a lower temperature. Usually propane, methylacetylene-propadiene gas, or natural gas burning in air supplies the heat.
The joint is cleaned to shiny metal with emery cloth, wire brushes, steel wool, or commercial abrasive pads. The flux (used for wetting the joining surfaces) is usually applied in liquid or paste form, or may be alloyed inside the solder wire.
While widely used in manufacturing and maintenance, it is also used in plumbing to join copper tubing for potable water. See Chapter 15 for a detailed procedure for the torch soldering of copper tubing.
Iron Soldering
Traditional soldering irons contain a copper tip on a heat-resistant handle. They are heated electrically or in a gas, oil, or coke furnace. The copper tip stores and carries heat to the solder joint. This transfer is made possible by heat being transferred from the heated tip of the iron to the part to be soldered; when the joint is raised to soldering temperature, solder is applied to the joint itself, and wets the entire joint. Flux core solder is used for electronic work. This solder has been formed concentrically around a core of one or more strands of flux. In sheet metal and other non-electrical work, the flux may also be in the solder core, or applied as a paste or a liquid.
Today most soldering irons are heated electrically and are available from just a few watts for electronic work to 1250 watts for roofing and heavy sheet- metal work. Many irons for electronic work have temperature-controlled tips to avoid damage to the sensitive components. See Figure 3–8.
Figure 3–8Electrically-heated soldering irons
Wave Soldering
This process is used to solder electronic components onto printed circuit boards. A conveyor belt draws a printed circuit board with components over a fountain (or wave) of solder. This solders all the components in place in a single step. Wave soldering machines are available which can flux, dry, preheat, solder, and clean the flux off a finished board on a single conveyor line. See Figure 3–9.
Figure 3–9Wave soldering
Joint Design for Brazing
Why is joint design an important part of brazing process design?
Well-designed braze joints start with fundamental butt and lap joints. Figures 3–10A shows a butt joint angled to provide more surface area for the brazed or soldered material to bond; this angles joint is called a scarf joint. Figure 3-10B shows both a lap joint and a square edged butt joint. Good joint design insures a reliable, repeatable production brazing process that will provide a strong joint.
Figure