(Fusion welding, cont.) As mentioned earlier, metal exposed to air at reaction temperatures acquires a relatively thick, adherent oxide film which is extremely difficult to remove. Upon vacuum annealing, the oxide film is diffused rapidly into the metal which results in hardening of the weld bead and the heat affected zone. An equilibrium oxide film is present on the metal at room temperature, but it is relatively thin and causes no significant hardening of the metal upon annealing except in the case of extremely fine wire or foil.

Niobium is amenable to machine welding. Faster speeds ensure minimum weld contamination from the atmosphere. If sheet greater that 0.050" thick is to be welded, the current required for full penetration becomes high enough to cause a spread of the molten pool outside the protection of the argon shield. Similarly, when welding with filler rod, the molten pool is too large. Further complications arise when the filler rod becomes heated at a distance from the arc and outside the protection of the argon shield. If it oxidizes, this contaminated metal is melted into the weld bead. The solution is to provide complete protection by using an argon-filled box. The box is constructed so that the leads of the torch are sealed through one wallÑallowing the torch to be operated from the outside of the box, using rubber gloves. The rubber gloves are blanked-off and the air is evacuated from the box and back filled with positive pressure argon or helium. The box is vacuum tight and contains a vacuum pump to assist in argon flushing.

Contamination-free welds can be produced under totally inert atmospheres as compared to welds produced employing inert shielding only.

In a typical niobium fusion weld, there is a gradual transition from a work-hardened sheet structure to the annealed structure of the heat affected zone and, finally, to the large grains typical of the cast metal.

Niobium's corrosion resistance has proved beneficial in applications where welded material is used. Simple tests, carried out under conditions known to cause attack on the parent metal, have shown the weld to be as resistant to corrosion as the parent metal.

The TIG spot welding technique is suitable for spot welding niobium. This torch design employs an argon shield which provides complete protection of the molten zone. Because complete penetration is not desirable, no backup argon is required. For niobium, the applications for this technique are somewhat limited.

RESISTANCE WELDING
While TIG welding is the preferred method for welding niobium sheet in thicknesses of 0.020" or greater, the method is somewhat limited for thinner sheet. It is possible to weld sheet as thin as 0.012" or even thinner, but special attention must be paid to the shape of the electrode tip. Extremely careful jigging is essential for accurate alignment and prevention of distortion and misalignment during welding. For sheet thinner than 0.020", it is generally better to use the resistance method of welding. The problem of contamination during resistance welding is not as great as in TIG welding, because the duration of the weld can be kept short. Spot welding may be carried out in air providing the weld time is restricted to one or two cycles, but welding should be carried out under water. The water does not protect the weld from contamination in the same way as the argon shield does during arc welding. Its function is to remove heat from the weld as quickly as possibleÑthus keeping the time that the metal is hot to a minimum. Either transformer or stored-energy type equipment can be used, but the welding heads should be of the low-inertia type so that, as the welds are made, proper pressure will be maintained throughout the welding cycle. The surfaces to be welded should be cleaned and degreased before welding. Any copper pick-up from the electrode, which contaminates the sheet after welding, may be removed by pickling in nitric acid.

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INDEX  •  CORROSION RESISTANCE PROPERTIES  •  FABRICATION   
NIOBIUM PRODUCTS   •  SPECIFICATIONS of NIOBIUM   
NIOBIUM ELECTRONICS APPLICATIONS