A.  The question about surface quality of fillets is one that comes up frequently.  The solution requires the understanding of many variables, one or more of which can cause the rough porous fillet.

    1) Rough porous fillets have caused considerable confusion when penetrant inspecting. Many times, porosity is taken for cracks, as the bleeding fluid from the pores comes together to form a line. For this reason, AWS C3.6 Specification for Furnace Brazing includes the following; “These inspection techniques are not suitable for inspection of brazed fillets because they routinely give false results.”

    2) Fillet size vs. porosity.  In general, the larger the fillet size the more porosity and roughness occurs. For this reason, the smaller the fillet, the less porosity and roughness will form.

    3) Brazing temperature vs. porosity. Generally, higher brazing temperatures yield  more porosity and roughness. This is caused by the brazing filler metal dissolving more of the base metal, resulting in the liquid filler metal at the brazing temperature having a wider melting range. For best results, the brazing temperature should be as low as practical.

    4) Time at heat vs. porosity. The longer the time at heat the more base metal is dissolved into the filler metal resulting in more porosity.

    5) Location of the filler metal vs. porosity. Apply the filler metal on the opposite side of the joint from where the smooth fillet is desired, preferably on the inside or lower part of the joint.

    6) Atmosphere vs. porosity.  BNi-5 is one of the filler metals more sensitive to atmosphere quality.  Vacuum pressure is only one of many things that makes up atmosphere quality.  Therefore, a very good atmosphere quality is required to minimize porosity.  The application of the T-specimen in the furnace is a good indicator of atmosphere quality

    7) Filler metal solidus-to-liquidus range vs. porosity. In general, the wider the melting range, the more porosity can be expected.

    8) Solubility of base metal vs. porosity.  As a general rule, the more base metal that dissolves into the filler metal, the wider the melting range of the filler metal.  Thus, more porosity is present.

    9) Cooling rate vs. porosity.  Slow cooling allows dendrites to form first, and on further cooling, the remaining liquid portion pulls back into the joint exposing the rough, porous fillet.

    10) Eutectic filler metals vs. porosity.  Eutectic filler metals are better, and have less porosity, assuming that one or more of the above variables do not enter into the brazing cycle.

    11) An element that has a single melting point will dissolve other elements from the base metal during brazing, and can result in fillet porosity and roughness, depending on the variables listed above.

    To produce fillets with little or no porosity will require exercising very careful control of the variables mentioned above.®