Fillet indications furnace brazing copper tubing to Cu-Ni

Date 04-16-2012 15:15

We are trying to vacuum furnace braze deoxidized copper tubing (ASTM B-75 , C12200 ) to a 70-30 Cu-Ni block braze assembly. The tubes bottom out on counter bores approximately .4” into the block for each joint. There are thru holes on each joint. We are putting braze stop off on the small id right below the counter bores to prevent braze filler (BAg 8, liquidus=1435F) from running out the backside of the joint. The main issue we are having is we are getting small pit looking marks in the fillet of the braze on the top od of the tubes. See pic attached. I was wondering if anyone had any recommendations to eliminate these marks. The furnace cycle we are using is shown below:

Hard Vac
Ramp rate 35F/min

Ramp to 1200F, Hold for 5 min
Ramp to 1435F at 4F/min (visually confirm alloy melt/flow through furnace view port, if seen proceed to furnace cool, if not place on hold until seen)

Furnace cool to <1400F

Nitrogen force cool <250F

Additional Notes
We have also been seeing intergranular penetration of the fm into the copper tubing, which is why we are trying to minimize the time at braze temperature. We are held to a max IGP requirement of fm into the bm of 20% bm thickness.
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Attachment: Filletindications.pdf - fillet indications (113k)

By Matt Pohlman (*)

Date 04-18-2012 17:37

Difficult to tell from the photos. Looks like solidification shrinkage (interdendritic shrinkage) at the centerline of the fillet. Depending on the joint requirements it could be rejectable or could be just cosmetic. Try using less filler metal so the fillets are smaller. You only need enough filler to fill the joint unless your customer has add a requirement for fillet size.

By bjbercaw (*)

Date 04-19-2012 11:49

Hey matt thanks for the reply. I was thinking the same thing. We are going to try one ring instead of two. We do have to have a 100% visual fillet but it doesn't specify a size. thanks again for your input.

Brett

By jwright650 (*****)

Date 04-19-2012 11:52

Hi Brett
I've been following along because I didn't have anything to add, but give us an update when you get it figured out. I'm curious what you find is causing the centerline crack or discontinuity in the fillet.