Question: We want to use our vacuum furnace to braze a 6061 aluminum part to a 400-series stainless steel but are concerned about whether that can be done in our furnace, which only is used for aluminum brazing. What are your thoughts?

Answer: Yes, you should be able to join those two metals together via brazing, but some significant precautions are in order. One involves expansion of metals, and the other involves base-metal oxidation.


Thermal Expansion

As shown in Table 1, you can see that aluminum grows faster than all other metals when it is heated: about 50% more than copper and about double that of stainless steels. This must be taken into account when brazing.

At brazing temperature, the brazing filler metal (BFM) will melt and flow into the joint (or melt in place if the BFM was placed as a sheet or foil inside the joint). Then, when the joint has solidified as the metals enter the cooling portion of the brazing cycle, stresses will be set up in the joint as the aluminum wants to “shrink” much more than the stainless metal to which it is being brazed. Because of that huge difference in thermal expansion between the two metals, severe shrinkage stresses in the joint area might rupture the joint or cause significant deformation or cracking of one (or both) of the base metals being joined. How can this be handled?

  • Keep the parts small. If the parts being brazed are small, then the stresses from contraction during cooling may not be overly strong. If that’s the case, physical deformation or cracking might not occur.
  • Use a ductile core metal between the two base metals. If the parts are large, it may be advantageous to introduce an intermediate layer of ductile material as a fairly thick core (perhaps up to one-sixteenth of an inch, or 1 mm) between the two metals being brazed (Fig. 1). Copper would be an excellent choice, since copper is very brazeable to aluminum alloys and to stainless steels. The thickness of the copper core, as mentioned above, should be such that the BFM on each side of that core will only diffuse slightly into the face of the core on either side, leaving a thicker, non-diffused core-layer in the middle to act as the “shock absorber” that can stretch (expand/contract) as needed to take up the differential expansion stresses between the two base metals being brazed.

More next time in part 2.