View Single Post
  #3   Report Post  
Gary Wooding
 
Posts: n/a
Default

rich brenz wrote:
I have some small pieces of aluminum I wish to join, and have Al solder
and flux. The flux has no directions but does say to apply to parts and
do not overheat. Should the flux be wiped off before attempting to
solder? When I try to leave the flux in place, it turns dark brown,
smokes a lot, and seems to prevent the solder from adhering? Any useful
suggestions?(Other than "try brazing" or "get a welder"...)

Rich


It sounds as if you are heating the flux too much.

The process of soldering is essentially the same for all metals. The
solder is a metal (usually an alloy) that has a lower melting point than
the stuff to be joined. Basically, you just heat the work (the bits you
want to join) until its hot enough to melt the solder which turns to
liquid and wets the work. When you remove the heat, the solder freezes
and the joint is made. The important part is that the liquid solder
_wets_ the work. Just like water won't wet a greasy or dirty surface,
the solder won't wet a dirty surface, that's why you must ensure that
both the joint and the solder are clean. That's the theory bit.

In practise its a bit more complicated, 'cos what happens is that the
heat causes both the work and the solder to combine with the oxygen in
the air to create oxides. The oxides act as a barrier, rather like dirt,
that prevents the solder from wetting the joint. The trick is to stop
the oxygen from getting to the joint. That is the job of the flux. As
you heat the joint the flux flows over it to shield it from the oxygen.
When the joint reaches the proper temperature the solder melts and the
joint is made.

Some metals, aluminium and stainless steel for example, oxidise very
rapidly (in seconds) at normal room temperature, so flux for these
metals has the extra job of getting rid of existing oxide as well as
preventing new stuff from forming. This is the reason that these metals
are, traditionally, difficult to solder.

The final problem is that no flux is perfect. If you heat them too much
they lose their properties and the solder refuses to stick. If you heat
the solder and flux directly you will almost certainly destroy the flux.
The secret is to clean the joint and flux it, then heat the _joint_
until its hot enough to melt the solder. If you just heat the solder it
will melt and turn into a ball, the tendency is then to apply more heat
which tends to burn the flux. If the _joint_ is hot enough to melt the
solder it will flow into the joint all by itself.

In summary then. Use the correct flux for the solder and metal you are
using. Clean the joint properly, apply the flux and heat the joint until
the solder flows. A good trick is to place a little bit of solder on the
joint before applying the heat, then, when you heat the joint enough to
melt the solder you can apply more solder if required.

I hope this helps.

--

Regards, Gary Wooding
(To reply by email, change feet to foot in my address)