Torch brazing is one of the most versatile of all brazing processes and is used in a variety of industrial applications. Using either air or oxygen in conjunction with flammable gases, such as propane or acetylene, torch brazing uses heat from a flame to melt a filler metal to join two or more metal parts. This type of brazing can be used to join two pieces of metal with different properties as the process melts only the filler metal and does not melt the base metals. While torch brazing can be used for manufacturing processes, it is most commonly associated with repair work.
In either acetylene or propane torch brazing, the metals used must either be protected by flux — an agent that removes oxidation from the metals — or a vacuum chamber, unless the metal does not oxidize in the temperature range required for the brazing process. The direct flame of the brazing torch is applied to the metal and the pre-set filler metal in a heating process that warms the base metals and the flux. The filler metal — which may be a copper zinc alloy, silver-base alloy, or aluminum-silicon alloy — is melted by the heat, and capillary action causes the filler metal to flow between the two pieces to be joined. Upon cooling, the filler metal acts as an adhesive between the joined pieces, permanently fixing them together.
The most common method of torch brazing is manual torch brazing. This method is widely used because the equipment is easily portable and relatively low in cost as compared to other welding methods. Requiring a relatively small setup of hoses and gauges, along with a torch and two portable tanks, this method requires less gas and creates less thermal stress on metal parts than traditional welding, but it is not appropriate for every situation. Torch brazing is useful for light work as it can be used to quickly make suitable, watertight repairs. It is not a recommended method of repair for projects that require the ability to withstand high amounts of mechanical stress.
In manufacturing situations, automatic torch brazing is sometimes used to seal lap joints. In this process, the product is assembled with bits of the filler metal and flux placed strategically in the joints. The whole product is then heated as it passes through the various workstations, melting the filler metal and allowing it to flow into the joints and bond the metal parts together. The result is a cleanly welded joint with a uniform appearance and a watertight bond.