Sabtu, 22 Mei 2010



Nickel and its alloys are used in applications that demand corrosion resistance. The main categories of nickel and its alloys are:
commercially pure nickel
nickel-copper alloys
solid-solution-strengthened superalloys
precipitation-hardening superalloys
oxide-dispersion-strengthened alloys

The first two categories are used in corrosion resistant applications, or in processing equipment where product purity must be maintained. The other three alloys are designed for applications requiring high temperature strength and resistance to corrosion at high temperatures.

Nickel alloys can be brazed by all the common processes available, however, precipitation-hardening superalloys need special consideration.
Filler metals

The selection of brazing filler metals depends on the service conditions of the finished assembly. The form of the filler metals include wire, foil, tape, paste and powder.

For corrosive service environments, high-silver bearing filler metals are preferred, with the final joint strength comparable to the base metal.

High nickel alloys are capable of being brazed with copper; however, care must be taken with filler metal placement as molten copper alloys react with the base nickel, reducing filler flow.

Joints that require corrosion resistance and elevated temperature strength are brazed using nickel based filler metals.

Nickel-based filler metals containing palladium, platinum and/or gold form joints which have good ductility, high strength and oxidation resistance.
Joint performance

Precipitation-hardening superalloys (especially containing >l% aluminium or titanium) require either, plating with nickel, the use of a flux, or vacuum brazing. This will prevent the oxidation of the aluminium or titanium which would inhibit the flow of the filler metal.

            Vacuum brazed nickel alloy pressure transducer
(Courtesy of TransInstruments Ltd)

Consideration must be given to the effect of the brazing thermal cycle on the base metal. Precipitation-hardening alloys (which are brazed at temperatures higher than are required for heat treatment) will have adversely affected mechanical properties.

Liquid metal embrittlement occurs in many precipitation-hardened alloys and can crack when stressed parts are exposed to molten metals, especially silver-copper filler metals.

The oxide-dispersion-strengthened superalloys owe their oxidation resistance to the stable and tenacious oxide film, and their creep strength to the large and directional grain structure. So careful brazing conditions must be maintained in order that the base metal structure is not altered during brazing.

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