Some braze joints are made with filler material pre-placed in the
joint at assembly time, often in the form of a foil or ring. This
sounds a lot like that approach. I know that approach is used a lot in
furnace and dip brazing (how else do you get the filler material in
there?), but that mostly exhausts my knowledge of the technique. I
asked about that approach further up the thread and was told not to
bother ;)
Evan
On Thu, Aug 8, 2019 at 6:44 PM Henry Vanderbilt
<hvanderbilt@xxxxxxxxxxxxxx> wrote:
Wild thought: If you have a way to finely powder your braze material, you
might try mixing some with your flux, then experiment a bit. It's possible
this may have the same amateur-enabling effect it has in plumbing
applications, making thoroughly wetting the joint surfaces a lot less
dependent on finicky details of tools, materials, and technique.
It's possible too you'll (re)discover why this isn't usually done... Good
luck!
Henry
On 8/8/2019 12:50 PM, Evan Daniel wrote:
I've had somewhat similar experience with electronics soldering. My
experience with that has been that, whereas tin-led rosin-core
electronics solder is basically all the same, lead-free solders most
emphatically are not. Choosing a good lead-free solder made things
vastly easier. For plumbing solders, I've not had as much trouble;
liberal application of flux was mostly adequate.
The problem with the flux boiling off doesn't match my (very limited)
experience with brazing. There, instead of organic fluxes that melt
and then boil away, you're using molten salt fluxes. Getting the flux
to do the right thing definitely seems like a big part of the problem,
just not in that *precise* way.
I'm not entirely sure how hot a working temperature I need, either.
The early versions of the igniter were a fully printed single-piece
head end, chamber, nozzle assembly. There were three small braze
joints for fuel, oxidizer, and pressure sense. The first two should
stay cool, the third at least had no flow past it. We redesigned it
and added a braze joint (2, actually; head end to chamber, and chamber
to nozzle) in an attempt to be able to inspect the injector element
after assembling the injector. The chamber-nozzle joint is the one
that failed. I caught the failure on camera; it was fairly dramatic,
as expected for flame-to-world seals. Further testing is suggesting
that maybe injector assembly wasn't the issue at all, and we just
needed to throw more L\* at the problem; we've removed that joint in
the next version of the full injector assembly. (The igniter,
including nozzle, chamber, and head end, are all part of the same
print as the injector bulkhead.) As for what we were hoping for, in
testing of prior versions of the injector we saw no erosion of the
printed nozzle throat on 100+ runs of 1.0s length, but when we tried
increasing the runtime we started seeing sparks and visible though
slight erosion at 1.3s run time. I was hoping the braze joint would
not be the limiting factor.
So right now the short-term problem is that I have some new injector
parts that need a high-temperature joint that I'd like to assemble and
collect data on. But the long-term problem is just that I thought I
had a braze joint that worked well, and it looked good during brazing
(solder appearing to wick in), but it turned out not to be good on
post-failure inspection. So I'm worrying about my brazing technique in
a more general sense.
Evan Daniel
On Thu, Aug 8, 2019 at 2:03 PM Henry Vanderbilt
<hvanderbilt@xxxxxxxxxxxxxx> wrote:
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
