It was shown in last week's blog that brazing fillets can be a greatly misunderstood phenomenon. I mentioned that some people insist that big fillets are needed, whereas others say that they are not. It was shown that fillets are the natural external evidence that a brazed joint has been made, but a fillet cannot tell anyone about what actually happened between the faying surfaces inside the joint itself. Internal quality of a brazed joint must be determined by performance testing of the brazement and perhaps, if necessary, by such nondestructive techniques as ultrasonic testing. In some rare cases, such as with thin sheet-metal assemblies, this is done by radiography.

So then, how should someone inspect a brazed fillet to be able to get some clues about the "goodness" of the braze? Let's examine that topic in this week's entry.

How should fillets be inspected?

1. Visual – The best way to check the quality of a fillet is simply to look at it. Maybe that sounds corny, but simply look at the fillet closely, perhaps even using a 10X magnifier. Is the fillet concave in shape? Does it go completely around the joint? Is it clean and smooth or is it filled with porosity or cracks?
  • Lack of fill – “Lack of fill” is not a fillet characteristic that alone is cause to reject a part. It all depends on the design of the braze joint and the end-use service conditions to which the part will be exposed. In a properly designed joint that sees low service stresses, it may be perfectly fine to allow a certain amount of lack of fill to occur. Should that happen, look inside the slight “depression” represented by the lack-of-fill area to see the shape of the BFM surface inside that area. If it is shallow and shows a concave surface down inside, then it may be perfectly OK to allow this lack-of-fill area. If, however, the inner surface is rough or convex in shape, it may indicate poor wetting inside the joint itself and may be cause for concern. In parts that see high stress concentrations at the edge of a joint due to thermal cycling or mechanical cycling, it may be desirable to disallow any lack of fill in a fillet, since it might serve as a stress riser.

    It is also wise in such cases to keep the fillet size small to ensure that no “casting defects” (usually evident in larger fillets) could interfere with service performance. Likewise, if the joint is to be used in food service, medical applications or jewelry, any lack of fill may represent an unacceptable surface condition. Additionally, any lack of fill areas might cause disturbance in fluid flow or airflow and might be cause for concern.

  • Size and quantity of surface voids – Be very, very careful about specifying number of voids per linear inch (cm) or specifying size of each voids, etc. This practice can be a trap, and could result in the rejection of parts that might otherwise be perfectly fine. The fact that a fillet might have three bubbles at its surface in a 1-inch length instead of only two allowed bubbles has nothing to do with the quality of the BFM that has flowed inside of the brazed joint. It also calls, once again, for a lot of extra inspection time to do these external fillet measurements when what's happening inside the brazed joint is actually more important.
2. Fluorescent Penetration Inspection (FPI) is NOT recommended – Many people still use FPI on braze fillets to accept or reject parts. This can be a big mistake. FPI is fine for welds, but it is not really useful for brazed joints for two primary reasons: FPI merely shows that there may be surface imperfections on the outside of the fillet but tells absolutely nothing about the inside of the brazed joint itself; and FPI chemical removal requirements are very different in welding than in brazing. If FPI reveals defects in a weld fillet, the entire fillet needs to be cut out or ground away (thereby completely removing all the FPI chemicals) and a new weld bead is then laid down in place. However, in brazing, fillets are not going to be cut away and replaced. Therefore, any entrapped FPI chemicals have to be completely removed from the fillet itself either by ultrasonic cleaning or fluoride-ion cleaning (FIC) before a rebraze can be effectively done.

Do NOT think that FPI contamination in surface voids, cracks or dendritic porosity will be effectively removed by soaking in a solvent or by either hydrogen or vacuum-furnace cleaning. Although some types of oils used in the FPI might theoretically be removable by such furnace cleaning, those furnace processes may actually turn out to be ineffective at completely removing FPI oils and probably will not even touch any of the solids in the FPI chemicals (such as developers, etc.) even though the FPI chemicals are claimed to be volatile or water soluble.

AWS C3.6 Specification for Furnace Brazing specifically discourages anyone from using FPI in brazing inspection procedures. It clearly states in paragraph 5.4.3.1 of that spec that penetrant-inspection techniques “are not suitable for the inspection of braze fillets because they routinely give false results.”

Next time we will look at how fillets should be specified and conclude our discussion.