Not technically brazing, but I have some recent experience with
copper-to-brass water pipe soldering that may be relevant.
Sort version, that used to be easy, using paste flux, lead-tin solder,
and a propane torch. Sand all mating surfaces to bright metal, flux
thoroughly, assemble, heat, apply solder and watch it wick right in.
Piece o cake. Even I used to be able to reliably do mechanically sound
watertight joints.
Only, well, the authorities decided any lead at all in potable water
systems is a bad idea. Then I had to solder together some water pipes
last winter using only the new approved materials. Disaster. Exactly
what you describe: Everything seemed to go OK, but the solder didn't
wick into the slip joint region at all well. After some reading, short
version of the problem: The flux only has a few-degree effective window
between melting temp of the new high-silver higher-temp solders and
burnoff temp of the flux. Skilled pros with high-temp torches learn to
hit that window. Not me though in the time available.
The solution turned out to be, a mildly expensive metal-bearing flux.
Essentially flux with finely powdered silver solder in it. So you coat a
joint with the flux and heat it, the powder melts and tins the inside of
the joint, pre-wetting it, and even after you've hamhandedly burned off
the flux,for a reasonable interval the externally applied solder will
wick into that joint thoroughly and easily.
I don't know what braze alloy you're using or how high a finished-part
operating temperature you require, but there may be some analogous metal
bearing flux for that braze. Or you may just be able to get away with
using water-pipe metal bearing flux from the hardware store for the
initial wetting and count on your braze material diffusing down to the
mating surfaces. (Or you could be looking at operating temps just fine
for current water-pipe solders, in which case you're home free.)
Good luck!
Henry
On 8/8/2019 10:19 AM, Evan Daniel wrote:
We used a paste type flux. I applied the flux to all surfaces to be
joined before assembly. The solder appeared to my unpracticed eye to
wick in and around the joint during brazing. On inspection after the
failure, it seemed like I had good wicking action on most of the butt
joint contact area, but not on any of the slip joint area. I have no
good way to inspect the opposite side of the joint after assembly,
since that's inside the tiny chamber.
I added a fourth photo to the gallery, I think it shows that behavior
best. Here it is:
https://imgur.com/PHK2GMp
Evan
On Thu, Aug 8, 2019 at 1:05 PM Uwe Klein <uwe@xxxxxxxxxxxxxxxxxxx> wrote:
Am 07.08.2019 um 19:06 schrieb Evan Daniel:
Does anyone have good resources to suggest for braze joint design guidelines?Did you use some flux assist?
Good idea is to have visual check
for the solder wicking being successful.
Uwe
