Soldering white metal - first impressions

[RobjUK]

Hi all,

I've just completed my first session of soldering white metal parts and I thought my results may be helpful to others doing this for the first time.

 

Supplies:

A bar of 70'C solder from ebay.

 

Flux - 6% phosphoric acid. 

Having looked at "White metal flux" and various articles on modelling sites I bought a litre of 75% industrial phosphoric acid rust remover for little more than a tiny bottle of flux.

 

I used a small glass jar, approx. 28ml of deionised water and 2.5ml of the acid, slowly dripping the acid in to the water while gently swilling the jar around (and wearing disposable gloves).

I used disposable 3ml polythene pipettes for measuring and transferring the liquids; 11 x 2.5mm deionised water then one 2.5ml measure of acid.

(Using two separate pipettes, to avoid any water-in-acid thermal reactions).

 

That gives pretty close to the apparently ideal 6% solution.

[Note - I needed some phosphoric acid rust remover for an unrelated job. If you only ever want some for flux, it's probably simpler just to get ready made].

 

The heat source was my normal bench iron used for electronics, a 50W Antex temperature controlled type left at at its standard heat setting.

I thoroughly wiped the tip on a damp sponge immediately before the first white metal joint.

 

The solder bar is far too large to use directly in small parts, so I cut small chips off it with electrical sidecutters.

 

I adjusted the parts to get an exact fit first, so there was no stress on the joints.

After holding together in the correct place, I used a cotton bud to wet the joint area with the acid flux then dropped one or two solder chip over it.

[See the photos].

 

Then just holding the tip of the iron on the solder fragments, they melted and the surrounding metal got hot enough for it to flow well within about five seconds or so.

The point the solder starts to flow and wet the white metal is quite obvious, at least with the relatively large parts I've done so far.

Small items that would melt quickly would need a much reduced heat source.

 

I used a rubber band to try and hold the parts in place for the first joint, but I there was so little heat transfer through the parts that I could just hold them together in one hand and apply the flux, solder fragments and iron with the other, without anything getting unpleasantly hot.

 

 

The very intense, localised heat seems to work well - and the flux works perfectly as long as you solder before it dries out.

I tried to do two joints one after the other, fluxing and adding solder to both before heating the first joint. The second did not flow well, I had to re-flux and reheat it for it to "take" properly.

 

Photos:

Iron type, cut solder chips then two examples of joints, showing the chips in place after fluxing then the joint after flowing it.

And finally one of the bogie sides with the upper and lower castings fully joined.

 

I hope this info is some use to someone!

Robert.

 

 

Edited February 20, 2019 by RobjUK
Typo / trivia

[michl080]

nice stuff, isn't it?

 

I prepare the solder bar by melting a large chunk on a heat resistant surface and dropping my vice onto the liquid blob.

It spreads out very nicely and forms sheet of metal about 0.5mm thick. Much easier to handle that way.

 

Michael

[34theletterbetweenB&D]

You can solder reasonably heavy parts together with ordinary rosin cored electrical solder used in the same style on the inside of the joint, directly applying a regular iron to the solder. I still have whitemetal kits assembled this way in my early teens, when I was completely ignorant of the existence of low melt solder intended for this task. It is amazing what is possible when no one has told you it shouldn't be done that way. Not that it would be my first choice method now...