Reaction and Brazeability of Magnesium containing Aluminum Alloys with Non-corrosive Flux.

Brazeability and the reaction between base metal containing magnesium and non-corrosive flux have been investigated in relations to the magnesium content in base metal, brazing temperature and holding time. The flux used was KAIF4-K2A1F5⋅H20 system non-corroxive flux. Brazeability was evaluated by the fillet formation test on tee type specimen consisted of vertical member of brazing sheet and horizontal member of base metal. Brazing was conducted in high purity argon gas atmosphere and maintained at 873 K for 300 s. The increase in magnesium content in base metal drastically lowered the fillet formability, moreover, the flux penetrated into magnesium containing base metal preferentially along grain boundaries. The flux reacted with magnesium in base metal and formed compounds of KMgF3 and MgF2. The poor brazeability of the alloys with magnesium is attributable to the reaction between flux and magnesium that consumes the amount of supplied flux resulted in the lower efficacy of flux. The penetration depth of flux into base metal depended on the holding time and temperature, espcially the depth was remarkably affected by the spread area of flux, i.e., the amount of supplied flux per unit area. The wide spreading at higher temperature reduced the penetration, whereas the small spread area at lower temperature enhanced the penetration depth. Accordingly, the plots between the penetration depth and the square root of holding time showed parabolic relations that indicates the consumption of flux. The consumption of flux resulted in the combination of oxygen with compounds.