As has been discussed in previous articles, brazing filler metal (BFM) does not like to bond to, or flow over, oils, dirts, greases, lubricants, or oxides. If any of those contaminants are present on either of the joint surfaces to be brazed, it can render that joint non-brazeable.
Any of these surface contaminants, as described above, could greatly weaken the integrity (strength) of any braze, and could also greatly increase the number of voids between the joint surfaces.
Thus, it should be the goal of all brazers to keep any such contaminants off the surfaces to be brazed!
Remember — Whatever you blast with will be on the surface of the parts!
So, looking at the list of potential blasting media available to most brazing shops, are any of them potentially bad for brazing? Yes.
Any non-metallics used for grit blasting can leave a residue that will not alloy with the metals in the joint area (i.e., with the base metals and the liquid BFM). Such non-metallic residues can remain as hard inclusions in a joint, or might outgas at braze temp,etc., thus, forming voids in the brazed joint. Items 1 through 4 in the list above are non-metallics, and can cause such negative effects inside a brazed joint. Thus, aluminum oxide grit, silicon-carbide grit, sand, and glass beads are all non-preferred grits for preparing surfaces for brazing, since they can, and will, leave surface residues that will negatively affect brazing.
Recommendation: Always use a metallic grit, if possible, to blast any surfaces for brazing, since any metallic grit residue will dissolve into, and alloy with, the molten filler metals, thus minimizing any negative effect from any such residue.
Note: Grit blasting is generally not done on any aluminum components to be brazed, so it should not be an issue. But, if grit blasting were desired, then consideration might be given to dry ice grit blasting (as discussed further down in this article).
The metallic grits (stainless steel, chilled cast iron, nickel-alloy grit) are very effective for use in blasting cabinets used to prepare many industrial and aerospace components that are made from a wide range of steel alloys and other hard aerospace metals.
PRESSURE. Be careful that the pressures used in any grit blasting operation will not damage the metal being blasted. This is especially true for thin metal structures. Blasting thin surfaces will warp those surfaces, potentially damaging them to the point that they cannot be used in production.