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Kaiser Aluminum & Chemical Corporation, in cooperation with Ravenswood Aluminum Corporation, conducted a program to determine the optimum Mg level in aluminum-silicon vacuum brazing filler alloys. Most users of vacuum brazing sheet operate at one extreme or the other, i.e., Mg levels around 0.2% or near 1.5%. Both alloy families have problems. We fabricated and brazed several brazing sheet materials with K319 cores (similar to AA3005) and 4045 type filler metals with Mg levels ranging from 0.2% up to 2.0%. The optimum magnesium level in Al-10Si filler metals typically used in vacuum brazing applications was determined to be in the range from 0.6% to 1.0%. Filler metal Mg levels outside the optimum range result in smaller fillet sizes. Empirical data suggest that for filler metal Mg levels about 1.00%, fillet sizes are about twice those for filler metals containing approximately 0.2% Mg.