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Guest jim s-w

Soldering Die Cast metal

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Guest jim s-w

Hi All

 

As it says above can you solder die cast metal and what are the techniques and fluxes involved?

 

TIA

 

Jim

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Isn't the recently reported problem of chassis failure/swelling etc. down to lead contamination?

 

Andi

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Depends on the die-cast metal grade and type, but assuming Mazak, or Zamak, (zamac), then, yes, it will solder with ordinary tin lead solder, and an acid flux.

 

But, most people have really extreme difficulty getting it to work, because not enough heat is used, and not the right type of flux.

 

Because a relatively large casting is normally involved a really big capacity soldering iron is needed, not just high wattage, but high heat storage as well, in other words, lots of copper.

 

The items to be soldered must be totally clean, and brushed with a stainless steel brush, to clean and mildly abrade it. It has to be super clean.

 

The flux used has to be acid in action, Bakers fluid will do it, or an acid with zinc added will do the job as a flux, (killed spirits). Most other fluxes are simply not active enough to work, and grease, rosin, and tallow based ones do not work. C&L,(Carrs) say that the Grey Label flux with work with mazak, and they recommend the 179 degree solder.

 

All Zinc based casting material has a tough oxide coating, and needs the acid, and heat to allow the alloy to be in good contact with the solder for it to bond properly.

 

Once tinned, the surfaces can be soldered together as usual. Sometimes difficult Mazak will take lower melting point solder as the tinning solder, and then normal solder to finish.

Lead free solder will also work, but the temperature needed is high, and approaching some of the cast melt temperature, so be very careful, do not apply a gas torch in the hope of reaching the temperature.

 

Lead free solder also bypasses any chance of corrosion from lead in contact with Zinc, but being on the surface solder does not causes Zinc rot etc, that is due to lead in the casting mix.

 

The one type of break in a casting that will not solder is a crystalline structure break, where the surface is broken crystal in appearance, really this should be filed away and the resulting larger crack filled with solder.

 

Some casting will not solder well, Hornby, Farish and older US makes like Varney, the Zamak was different.

 

Modern Chinese Zinc alloy seems to solder all right most of the time. Some castings are not Zamak and have more aluminium, and these are more difficult and may need specialist eutectic solders used. These are so called "flux free" and are applied at approx 400 degree, so very difficult to handle, as the casting may melt.

 

If the casting has been electroplated black or chemically blackened this may stop the soldering taking, it needs all filing away first, total removal near the joint.

 

Hope these notes help,

 

Stephen.

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Isn't the recently reported problem of chassis failure/swelling etc. down to lead contamination?

 

Andi

The problem of castings with Zinc Rot is not new, it dates from the origins of the Zinc aluminium alloys, in the 1920/30's, which were highly patented brands at first.

 

At first pure enough Zinc,(99.99%), was not widely available and it was found that contaminates caused oxidization and surface marks etc, but later it was found that lead added to the castings could be a disaster.

 

Pure zinc,(99.95%) was a UK produced item, from specialist processors Morris Ashby, who made Mazak. US makers generally only used the pure zinc(99.99%) widely after the war.(Zamak, or Zamac).

 

Lead oxide occupies more space than the Zinc and it stresses the metal structure and it all disintegrates. It is not curable in any way, if it starts, it continues.

 

This problem was cured pre-war, but it did affect 1950's post war Hornby and Farish, due to lead scrap in the melt. In the US Varney was affected badly, as they made lead castings in the same workshops as the Zamak, and it got cross contaminated.

 

No modern model railway stuff should be affected, but the Chinese made castings recently have been suspect, as all patents on the material have now long lapsed, and standards may have slipped in not using absolutely purest zinc.

 

It is also suspected that unknowingly they have added scrap lead solder to the cast material, the same old problem Hornby had. However most Factories use lead free solder these days in the west, and the Chinese should be the same.

 

Stephen.

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Since posting my querie I have found Alu Flux which works very well on Dinky supertoys.  It is readily available on Ebay.

 

Peter

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