When heat treatment is applied to Aluminum and its alloys, its use frequently is restricted to specific operations employed to increase strength and hardness of the precipitation-hardenable wrought and cast alloys. These usually are referred to as the “heat-treatable” alloys to distinguish them from those alloys in which no significant strengthening can be achieved by heating or cooling. The latter, generally referred to as “non-heat-treatable” alloys, depend primarily on cold work to increase strength. Heating to decrease strength and increase ductility – annealing – is used with alloys of both types; metallurgical reactions may vary with type of alloy and with degree of softening desired. Complete or partial annealing treatments are the only ones used for non-heat-treatable alloys.
Bright annealing
Coiled aluminum alloys and foils were traditionally heat treated in an exothermic generated atmosphere or in air. Aluminum alloys that contain even very small amounts of Mg and/or Mn will form surface oxides unless the atmosphere in the annealing furnace is free of moisture and oxygen.
Another problem that control of the annealing atmosphere helps to overcome or avoid is oil staining by oil-base roll lubricants that do not burn off at lower annealing temperatures. If the oxygen content of the furnace atmosphere is kept very low during such annealing, the oil will not oxidize and stain the work.
Additionally the use of air and exothermic gas can produce explosive conditions in the furnace atmosphere. Depending on the amount of rolling lubricants coating the coils surface, an explosive condition could result in the furnace during vaporization of the coils in an air atmosphere. In addition, the variation in atmosphere composition can produce oil staining unexpectedly.
Furthermore, exothermic generators have high maintenance costs and sometimes cause production losses due to equipment breakdown.
Consequently, nearly all heat treaters of aluminium use a pure nitrogen atmosphere supplied by liquid stored nitrogen tanks.
Nitrogen purges the oxygen out of the furnace, eliminates the risk of explosion due to oil vapours and avoids the formation of flammable atmospheres containing toxic components such as CO, as found in exothermic atmospheres.
Sintering
Definition according to ISO 3252: Sintering is a ”thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles”.
Aluminum powder metal parts sintered in controlled atmospheres result in efficient binder removal, size control, less sooting and bright surface finish.
It is important to understand the significance of introducing gases into the specific furnace areas where they are most effective. Called zoning, these special atmosphere injection techniques control both the flow pattern and the chemistry of the atmosphere.
Depending on the alloys to be sintered, in addition to pure nitrogen, hydrogen/nitrogen mixtures are used.
Brazing
Aluminium brazing is the preferred process for the production of automotive heat exchangers such as radiators, condensers, evaporators and heater cores.
This process, the so-called NOCOLOK® Flux brazing process, is carried out in continuous belt furnaces under pure nitrogen.
Nitrogen is introduced in the critical brazing section of the furnace and flows towards the entrance and exit. This prevents the ingress of contaminants from outside the furnace. As the component enters the critical brazing zone, furnace atmosphere becomes established, i.e. the dew point is ≤ –40 °C and the residual oxygen concentration is < 100 ppm. These conditions are necessary for optimum brazing results.
