Questions with Mike Moyer

1. Can I use vacuum brazing to join 304 stainless steel to copper?

Yes. 304 stainless steel may be effectively vacuum brazed to copper using various braze filler metal (BFM) types and chemistries. Filler metals based on gold, silver, and nickel can work. Careful attention must be given to the joint configuration since copper expands slightly more than 304 stainless steel. The copper will be very low in strength afterward, so it may yield to match the size of the stainless steel without notable distortion.

2. Will the braze joint survive cryogenic service?

Brazed assemblies are routinely placed into service as low as 4° Kelvin. There are design considerations and limitations, however gold and silver-based filler metals are commonly used in this application.

3. I need to braze a complex assembly but can’t figure out how to braze it all at once. Is there an option to multi-step braze the assembly?

Yes! An expert braze supplier can organize a multi-step brazing process. Consider the base materials and BFMs so the original braze joints don’t remelt during subsequent cycles. Typically, the first cycle runs hotter than a subsequent cycle and with a BFM that won’t remelt during a subsequent cycle. Sometimes the BFM is so active in diffusing a constituent into the base material(s) that returning to the same temperature may not cause a remelt. Multi-step brazing can be a convenient and effective tool in the manufacturing of high value medical components.

This challenge can be solved! There are methods to prevent it, the most effective is using just the right amount of BFM. If the joint is small and area low, it may be surprising how little BFM is needed to effectively braze the joint. Calculate the cubic area of the joint and try using slightly more BFM than the calculated area. A joint design prone to blockage is a counterbore receptacle with the same ID as the tubing, allowing the BFM to travel right to the tubing ID by capillary action. Use a space at the end of the tube to create a break in the capillary action or design the joint so the tubing may protrude slightly beyond the joint area. These methods create a more difficult path for the BFM to travel to the tube end, reducing the risk of blockage.

5. My customer wants a big braze fillet similar to a weld fillet. Is that the right approach?

This subject arises from time-to-time, and must be questioned. Unlike a weld fillet which creates strength in the joint, a big braze fillet doesn’t, wastes BFM, and can be detrimental. It’s what’s on the inside that counts. Certain BFMs are brittle in a large fillet due to a concentration of undiffused low-melt constituent. In this case, the fillet could crack under even mild fatigue and propagate into catastrophic failure. In brazing, a slight continuous witness of the BFM at the joint interface is typically the most appropriate visual inspection criteria.

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