Thanks, that's helpful!
I was hoping to avoid threads, just to keep the machining requirements
low. The current longer-term plan is to just print everything as one
print. We'd added the braze joint to make post-assembly inspection of
the injector possible, but we currently don't think that's needed.
Evan Daniel
On Thu, Aug 8, 2019 at 2:05 PM Peter Fairbrother <peter@xxxxxxxxxx> wrote:
On 08/08/2019 17:08, Evan Daniel wrote:
Thanks, this is very helpful!
I switched to MAPP from propane because the propane torch wasn't
working well, and the MAPP torch worked better. It sounds like you're
saying I should instead get a higher-volume propane torch. I'm using a
small handheld Bernzomatic torch with piezo ignition and disposable
cylinders. Do you have a suggested larger torch? I assume I can keep
using the disposable 14oz cylinders, I don't think I need that much
more heat.
It might be good to use two torches, one for background heat and one for
brazing heat, as suggested by Edward. I too often use a propane/butane
weed burner.
Otherwise, Sievert make good but expensive torches which work well but
with larger cylinders. I don't have any of them myself, I picked mine up
at garage sales and on ebay.
I don't have a source for MAPP, but I do have a couple
ancient cylinders around still. And I can buy MAP-Pro, which I assume
is hotter than propane but not as hot as old fashioned MAPP.
Speaking personally, I would hoard the real MAPP for oxy-gas welding
(and melting alumina for thermocouple covers).
For context, the braze joints I'm trying to make range in size from
fairly small (1/16" OD stainless tube into printed fitting) to
somewhat larger (5/8" OD copper or stainless tube to printed mating
part). Some of the mating parts for the larger joints have significant
metal nearby that may act as a heat sink.
Pictures:
https://imgur.com/a/EK7vntk
Yikes! That failed joint is getting very hot. Perhaps better to thread
it, then braze it to seal - tighten, slack off a quarter turn, then braze.
A note on pretinning, ie heating the piece with flux and covering the
joint area with solder before assembly and final brazing - - it it not a
good idea to do it generally as usual practice, because apart from
taking longer it causes diffusion and oxide formation.
Diffusion can be helpful in some circumstances, but unhelpful in others,
especially it you want to be able to undo or redo the joint - a lot of
diffusion will make that impossible, the metal in the joint will change
in composition until eventually it all, joint and workpieces, becomes
one alloy - eg if you are joining copper with 56% silver/copper
eutectic, the silver in the joint will diffuse out into the surrounding
metal, and copper from the surrounding metal will diffuse into the
joint, decreasing the silver content and increasing the copper content
in the joint metal and thus increasing its melting point to (eventually)
match the workpiece metal - and then the joint can never be undone.
In general, fluxes do not change the oxides on the surface back to bare
metal, they form eg copper borates or oxyfluorides which dissolve in the
melted flux. Incidentally, they have to dissolve in the hot flux, so use
plenty! These salts will always be around after pretinning, even after
cleaning, and will negatively affect the joint.
Best practice is to clean, apply flux, heat strongly so as to minimise
hot time and thus oxide formation [1], apply solder/braze, then let cool
slowly. That way the borates etc in the joint formed from the flux will
be minimised, and hopefully be on the surface, dissolved in the
solidified liquid flux.
[1] but without using a too-hot flame which will increase oxidation on
the surface without heating the base metal, which is necessary for good
flow and joint forming.
That said, and bearing all that in mind, pretinning can be useful.
especially where you only have a few pieces to do and where wetting of
the work by the solder is problematic - you can see whether it is wet.
Peter Fairbrother
