MIG Brazing and Silicone Bronze Applications
MIG brazing joins steel with a copper-silicon filler without melting the base metal. The exam tests when a procedure calls for braze rather than fusion weld, and why substituting one for the other damages the repair.
The process
The filler is CuSi3 (ERCuSi-A), melting near 1,000 °C - roughly 500 °C below steel. Because the base metal never melts, the joint is a braze, not a weld. It runs as short-circuit or pulsed transfer on GMAW equipment with argon-rich shielding gas, and OEMs specify it for outer panels, roof ditches, and joints on AHSS and boron steel where fusion-weld heat would exceed the metal's tempering range.
Why heat matters
AHSS grades get strength from microstructure. A fusion weld's heat-affected zone can soften dual-phase steel or harden and crack a boron part. Braze keeps heat input low enough to protect both the substrate and the zinc coating. It is not a substitute for a structural fusion weld unless the OEM states it is.
Worked example
A procedure specifies MIG brazing on a boron-steel roof joint. A tech substitutes ER70S-6 because a fusion weld "must be stronger." Reasoning: the wire change converts a low-heat braze into a fusion weld, pushing the HAZ past the OEM's validated range and risking a hard, crack-prone zone; the OEM validated the braze joint for that location. The deviation invalidates the repair.
Traps
Calling it "bronze welding"; using CO2-rich gas instead of an argon-rich mix; preheating the joint; heavy grinding that removes the braze; using braze where a fusion weld is specified.