Heating techniques

The method of heating depends on the type of application and the materials to be joined. Resistance heating, dip braz­ing, flame brazing, induction brazing, salt or flux bath brazing all have their place, although for high quality work, wherever possible, the trend since the early 1960s has been for cold wall vacuum furnaces, fig 9.

While it is well-established practice to join a number of materials in air with chemical fluxes protecting the joint from attack during brazing, this approach is totally inadequate where fine tubes are involved, due to subsequent difficulties in removing flux residues. In such cases, depending on the materials involved, brazing may take place in cracked ammo­nia, forming gas, argon or hydrogen after passing it over P₂O₅, heated Cu, reacti­vated Al₂O₃, and through a liquid air trap to achieve a dew point of -50°C as

Joining by brazing: High quality alloys are essential, by E.R.Perry

in the case of stainless steels. Metals such as titanium or zirconium are reac­tive to H₂ and N₂ and vacuum is prefer­red, sometimes to expedite cooling, back-filled with argon or helium to approximately 40mm pressure.

The effect of the brazing cycle will probably affect the properties of the parts being joined and with higher brazing temperatures grain growth can arise. However, there are many instances where higher strengths are required with the heat-treatable alloys when a brazing alloy may be selected in order to allow the joining process to coincide with solution heat treatment. Age-hardening can then take place in a second heat treatment. Age-hardenable aluminium alloys, the Nimonic series and stainless steels of the FV520-type typify materials that are treated by this method.

A major problem is caused by stress relief leading to dimensional changes during the brazing cycle. This may be overcome by machining wrought stock to 0.5-lmm oversize. After an anneal close to brazing temperature, machining is finished to the correct dimension and the component is ready for joining. In inst­ances involving sheet or foil materials, the recommended approach is to make fabricated parts to the required final dimension and then to anneal near the brazing temperatures. The parts are then resized in their dies prior to applying the brazing alloy.

With dissimilar sections, particularly careful attention is needed due to the ability of thin sections to absorb heat quicker than thicker ones. Sound prac­tice is to arrest the heating cycle about 50°C below the brazing temperature and dwell at that temperature for a sufficient time to allow equalisation of heat throughout the furnace charge, otherwise distortion is to be expected together with altered joint clearances.

Stress can be introduced into a joint when brazing parent materials of diffe­rent coefficients of thermal expansion unless suitable provisions are made at the drawing board stage. The stress induced might cause the outer member of a tubular joint to expand away from the inner member at the brazing tempera­ture so that the selected filler will not fill the gap. Conversely if the joint is arranged so that the metal of higher expansion is innermost, hoop stresses generated in the outer member can bring about cracking. (Welding and Metal Fabrication)

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