Aluminium dip brazing and salt bath brazing.  

Dip Brazing is used in the manufacture of complex structures, with inaccessible joints and those made up of components of vastly different thicknesses. The pre-heating process prior to immersion in the salt bath ensures near zero distortion or stress. 

To braze is to join two or more pieces of metal by means of flowing a filler metal between the joint interfaces at a temperature below the melting point of the base metal but above 900°F. The filler metal, upon cooling to the solid state, forms a strong metallic bond throughout the joined area. In aluminium dip brazing the filler metal is 88% aluminium and 12% silicon.

The parts to be brazed, once they have been chemically cleaned, are assembled with the filler metal placed beforehand as near the joints as possible. The assembly is then preheated in an air furnace to 1,025°F to ensure uniform temperature of dissimilar masses in the assembly. The part is then immersed in a molten salt bath. These salts are actually aluminium brazing flux. The bath is maintained at 1,095°F±5°F in a salt bath furnace. As the assembly is immersed or dipped, the molten flux comes in contact with all surfaces simultaneously. This liquid heat is extremely fast and uniform.

Since the bath is a flux, complete bonding on oxide-free surfaces assures extremely high quality joints. The time of immersion is determined by the mass to be heated but is usually under two minutes duration.

The advantages of salt bath brazing.

Dip brazing can allow high production rates because, depending on their size, several assemblies can be dip brazed at one time. It is not uncommon in dip brazing to braze several hundred joints simultaneously in one assembly. The more complex the assembly, the more advantageous the process becomes in terms of high production rates and economy.

Dip brazing can lower material and tooling costs. Assemblies that would normally require expensive casting, machining and tooling can be fabricated from low cost component parts by brazing.

Theses are both lightweight and complex assemblies. Fabrications and sheet metal assemblies processed in this way make stronger, lighter, more complex assemblies than could be produced by any other method.

Finishing costs are low because the brazed condition of assemblies is smooth and clean and usually requires no other mechanical finishing. 

Allowing as this process does, greater use of aluminium, holds many advantages over other metals, such as weight, corrosion resistance, conductivity. Dip brazing uses the versatility and cost savings possible in aluminium fabrication.

The nature of dip brazing provides oxide-free surfaces for complete bonding. Extremely fast and uniform heating of all metal thicknesses make possible dimensional stability exceeding that of welding in machined parts and sheet metal applications.

When the heat treatable alloys are used in a dip brazed assembly, normal heat treating can follow. It is often possible to solution heat treat by quenching from the dip brazing bath, thereby saving a subsequent heat treating operation. Quenching steel is the act of rapidly cooling the hot steel to harden its properties. The salt residues on the load are entirely removable by water washing. By evaporation the salt can even be recovered from the washing water.