The practice of MIG Brazing has been in use for a long time. Developed in the late 1960s, it immediately found applications in the automotive industry, primarily cosmetic at first. With this long history, why is there so much discussion about it now? Like aluminum, the renewed interest is due to new applications and increased usage. Manufacturers like Honda, Chrysler, JLR, Toyota and Mercedes-Benz all have repair procedures calling for MIG brazing. As the use of silicon bronze increases, it is important for technicians to be aware of the differences between welding and MIG brazing regarding materials, process, equipment and technique.
So first let’s look at the material. Copper is combined with other elements, such as tin, silicon or aluminum, to create bronze. The added components give the alloy strength, ductility, corrosion resistance, etc. In the automotive industry we primarily use the silicon-based alloy, referred to as silicon bronze. You may also see it referred to as CuSi3 or the AWS designation of ER CuSi-A.
In North America, we tend to refer to all wire-fed welding as MIG welding, when in actuality most of it is MAG welding. The difference is in the type of shielding gas used — Metal Active Gas (MAG) or Metal Inert Gas (MIG). The CO2 or CO2/argon mix gas used with steel is an active gas. The CO2 reacts to the arc releasing oxygen. Inert gases do not breakdown during the weld process. As the name suggests, MIG brazing uses an inert gas — 100 percent argon. Having the correct type of gas and proper flow rate are critical to protecting the weld and the stability of the arc. The typical flow rate is 25 CFH. It is important to verify that the gas regulator (flow gauge) has a separate scale for argon (Ar).
The main distinction between welding and brazing is that MIG brazing is adhesion, not fusion. By definition, welding is the process of bringing the base pieces to a molten state where they are combined or fused, often with a filler material. In MIG brazing, the base material remains intact; it does not melt. Instead, the silicon bronze melts, flowing around and between the base pieces to create the joint. This is possible because silicon bronze melts at approximately 1940° F. By comparison, MIG/MAG welding generates temperatures in excess of 3000° F. This dramatic difference in temperature is one of the biggest benefits of MIG brazing and what makes it ideal for new vehicle construction. Even though bronze is softer than steel, with the proper technique, the repair has a tensile strength very close to that of a steel MAG weld.
|Cross section of MIG braze (left) and MAG weld (right) showing adhesion versus fusion|